Troubleshooting guide

Solvent Based Flexo Inks

Faults, causes and fixes for solvent flexo surface print on PE, woven PP and kraft sacks, with the method behind every number.

Uniform Industries · Greater Noida, India · ufindustries.com
Figures are typical for the class and carry their test method. The values that govern a supply are those on the Technical Data Sheet for the grade.

Before you start

How to use this guide

Work down the page. Cheap checks before expensive ones, and measure before you change anything.

Measure the surface before you measure anything else: dyne on the reel you are about to run, at three points across the width and one at each edge, because in this category the substrate causes more stopped jobs than the ink does.
Quote the cup with every viscosity figure and take the sample at 25 plus or minus 1 C. A DIN 4 second to DIN 53211 is not an ISO 2431 4 mm second and neither is a Ford B4 second; the three cups have different reservoirs and different orifice lengths and they do not give the same seconds on the same ink. An uncorrected press-hall reading is the commonest reason a shade drifts across a shift.
Change one thing at a time and write down what it was, because on a solvent flexo press treatment, blend, dryer and impression all produce the same-looking print fault.
Correct the dryer profile and the extraction before you correct the solvent blend, and correct the blend before you correct the ink. A rising profile with genuine fresh-air turnover and enough dwell fixes more drying, blocking and odour faults than raising the setpoint ever will, and turning the temperature up on a saturated tunnel makes all three worse. That is not a reason to leave a genuinely cold dryer alone: retained solvent is governed by dwell, film weight, air velocity at the web, air temperature and fresh-air turnover together.
Never judge a two-component 250PU film at the delivery: it needs 5 to 7 days at 25 C to develop, so a resistance test on day one is testing an unfinished film.
Test adhesion, rub, friction and light fastness on the customer's own film or kraft, at 23 C and 50 % RH, at 24 h and again at 7 days, because every one of those properties belongs to the interface and not to the drum.
Treat the things you handle every hour as the hazards they are. The ISO 8296 dyne liquids are reproductive toxicants absorbed through the skin, UniFlex-259HD is an isocyanate adduct and a respiratory sensitiser, and dried nitrocellulose residue is a self-heating fire risk. Gloves, extraction, closed and solvent-wet waste bins, and bonded and earthed drums are part of the method, not an afterthought at the end of it.

Solvent Based Flexo InksHow to use this guide

The process

What happens, in order, and the one variable held at each step.

The process

What is controlled at each step

Solvent Based Flexo InksThe process

Control variables

What to hold, and how to measure it

Variable	Target	How it is measured	How it drifts
Press viscosity	16 to 18 s process and tone on 360 to 500 l/cm; 18 to 20 s line and solid on 180 to 300 l/cm; 20 to 22 s kraft and woven sack on 100 to 160 l/cm; 18 to 22 s overprint varnish	DIN 4 mm flow cup to DIN 53211, which is withdrawn as a standard but remains the working method for that cup. If an ISO 2431 4 mm cup is used instead, say so on the job card and never interchange the two figures; the geometries differ and the seconds differ on the same ink. Sample and cup both at 25 plus or minus 1 C, which is a house condition set for our press halls and not the 23 C of ISO 2431 or the 20 C of DIN 53211, so the temperature is quoted with every figure. Orifice checked against a calibration oil or a certified reference cup on a stated interval.	Climbs steadily through a shift as the fast fraction leaves an open pan. A straight climb is evaporation; a step is somebody's addition. Reading it at press-hall temperature instead of 25 C hides both.
Surface energy at the print station	38 mN/m minimum on corona-treated PE and woven PP for surface print; 40 to 44 mN/m where the print is later laminated. Do not chase treatment above about 44 mN/m: over-treatment generates low molecular weight oxidised material, which is a weak boundary layer in its own right and fails in store exactly as bloom does, and on thin film in a nip it treats the reverse face as well, which then blocks. The real control is watt density and dose at the treater, not the dyne reading alone.	ISO 8296 test liquids or dyne pens on the reel about to run, read at 2 s, three points across the width plus both edges; ASTM D2578 in the laboratory. The liquids are formamide and 2-ethoxyethanol mixtures, both reproductive toxicants absorbed through the skin: nitrile gloves, no skin contact, cap immediately, swabs to solvent waste. Prefer a formamide-free set where one is qualified against ISO 8296. Keep the set in date, because a set that has aged or taken up water reads the film low and gets a good reel rejected.	Falls in store, typically to 34 to 36 mN/m over four to six weeks on a high-slip PE, faster in a warm store. It falls fastest at the edges and on the face that carried the antiblock. Confirm ozone extraction on the treater is running whenever treatment is raised.
Ink temperature in the tray and in the loop	25 to 30 C measured in the ink itself at the tray or chamber, and below 32 C anywhere in the circulation loop including the return	Probe thermometer in the ink, not an air reading, taken at the tray or chamber and again at the return into the drum	Rises through the shift from dryer return air and pump work. Above about 32 C an ethanol-led 210PA blend is losing its fast fraction continuously, so the ink is gone before the plate reaches impression and the plate starts to fill.
Anilox volume against ruling	Volume is meaningless without the ruling it belongs to, and both go on the job card. About 2.5 to 5 cm3/m2 on 360 to 500 l/cm for process and tone; about 6 to 12 cm3/m2 on 180 to 300 l/cm for line and solid on film; about 14 to 25 cm3/m2 on 100 to 160 l/cm for kraft, opaque white and woven sack coverage. Exact volume varies with cell angle and depth to opening ratio, so take the roll maker's figure for the specific engraving.	Measured volume on the roll, by the roll maker's method or an in-house volumetric check, not the engraving figure on the end cap	Cells wear and plug. A roll that has lost 30 % of its volume prints as a weak ink at correct viscosity, which invites thinning and makes it worse.
Dry deposit	0.5 to 1 g/m2 on process and tone from a 360 to 500 l/cm roll; 1 to 2 g/m2 on a film solid from a 180 to 300 l/cm roll; 2.5 to 3.5 g/m2 on a kraft or woven sack solid or an opaque white from a 100 to 160 l/cm roll, where holdout is being bought with weight	Weigh a cut area of printed against unprinted substrate on an analytical balance and divide by the area. On kraft and woven PP the basis weight varies from place to place in the same reel by more than the ink weighs, so a single pair is inside its own noise and is not a result: cut at least five printed and five unprinted pairs adjacent to each other at the same position across the web and report the mean with its spread. Stripping the ink with solvent and reweighing to constant weight is an option on smooth film with a one-component ink only. It is not an option on kraft, which loses fibre and sizing to the wipe, nor on woven PP tape, which sheds fines, nor on a cured 250PU film, which is built not to dissolve.	Follows anilox condition and impression. It creeps up as impression is added to recover a weak solid, and the extra film is then what the dryer fails to empty.
Dryer profile and extraction	Rising through the tunnel, typically 40 to 50 C at entry to 60 to 80 C at exit, with genuine air turnover and vapour below 25 % of the lower explosive limit. Operation above 25 % and up to 50 % LEL is permitted by EN 1539 only on a dryer designed, certified and documented for it, with a fail-safe continuous LEL monitoring and interlock system of defined response time and a defined purge. That is a machine specification, not a press-floor setting. The line's own safety file says which case applies, and where the file does not say, run below 25 %.	Air temperature read deck by deck at the nozzles, not from the setpoint; extraction volume measured, not fan hours; LEL from the line's own monitor with its calibration in date	Filters load and extraction falls quietly. The setpoint stays where it was while the air going into the tunnel gets more saturated every week, and the first sign is a reel that blocks.
Reducer identity	UniFlex-295RD matched to the series; UniFlex-296SL only while the hall is above 35 C; ester-only reducer on the 250PU series, no alcohol once hardener is in	Read the label on the can at the press and log it on the job card at every addition. On toluene-free and ketone-free work, take the reducer from a sealed drum with a certificate of analysis for that batch, never from recovered solvent and never from an unlabelled gantry can.	The slow blend goes in at 11 am when the hall is hot and is still in the pan at 11 pm when it is not, and the reel then goes to the rewind with solvent still in the film.

Solvent Based Flexo InksControl variables

Control variables

What to hold, and how to measure it

Variable	Target	How it is measured	How it drifts
Two-component dose and pot life	UniFlex-259HD at 3 to 5 % w/w on ink; pot life 6 to 8 h at 30 C and 4 to 5 h at 35 C	Weighed addition on the pan quantity, mixed at the press; pot life taken as the point at which press viscosity has risen 5 s on DIN 4. UniFlex-259HD is an isocyanate adduct and a respiratory and skin sensitiser: weigh and add it with nitrile gloves, eye protection and local extraction at the press, and never where there is no ventilation.	Viscosity creeps, then climbs away in the last hour. Activated ink and moisture-struck hardener generate carbon dioxide, so a sealed container of either can pressurise and burst: never reseal activated ink or a gassed can into a closed drum, vent it and dispose of it as hazardous waste, and keep opened cans away from water and heat. Hardened ink returned to stock gels the drum, so a pan mixed too large is a hazard as well as money in the waste bin.
Impression	Kiss impression, set from zero on every deck and marked	Back off until the print drops out, bring on the minimum that gives a full solid, mark the setting and log it	Creeps on through the run as operators chase a weak solid that is really a plugged anilox. It shows as halo on the solid edge and shadow dots filling.
Winding tension and reel temperature	Winding tension recorded in N per metre of web width at unwind, print and rewind, with the taper stated as a percentage from core to outside, typically 10 to 25 %. The figure belongs to the film gauge and width and comes from the film supplier's winding recommendation, goes onto the job card at make-ready and is logged at the end of the reel. Reel surface below 30 C at the rewind.	Press tension readout in N/m against the job card; reel hardness checked with a hardness tester or by a consistent hand check at the same three positions across the width; infra-red thermometer on the reel face at the rewind	Rises with speed and with dryer exit temperature. A warm reel wound tight blocks in the core days after the press hall has signed the job off.
Press hall humidity and water in the ink	Below about 65 % RH at the press for alcohol-led NC and PA blends; water content of the pan ink within the house limit for the series, typically a few per cent and always below the point at which the resin shocks out	Thermohygrometer at the press, read at the time the fault appears rather than at shift start; Karl Fischer water content to ISO 760 on the pan ink and on the reducer drum, compared with a fresh sealed drum	Climbs through a humid day and through the run, because an open pan of ethanol-led ink takes up water from the hall all shift. The haze arrives in the last two hours of a monsoon run and not at the start, which is why it gets reported as a bad batch.
Press hall solvent inventory	On toluene-free and ketone-free work: dedicated and labelled wash solvent, wash tank, hoses, pumps, filters and chambers; every drum in the hall traceable to a certificate of analysis; no recovered or reclaimed solvent on the job	Written inventory walked and signed at make-ready, listing each drum, each gantry can, the wash tank and the recovered solvent tank, with the certificate of analysis reference for each, plus headspace GC-MS on the finished pack where the customer requires evidence	Degrades quietly through shared equipment. A hose or a filter that has been on an aromatic job puts a trace back into the first pan of the clean job that follows it, and no ink certificate will ever explain that result.

Solvent Based Flexo InksControl variables

The mechanism

Why the tape test passes on Monday and fails on Friday

Surface print on polyethylene fails at the film surface, not in the ink. Corona treatment decays, slip amide keeps arriving from inside the polymer, and over-treatment leaves loose oxidised material of its own, so the ink ends up keyed to a weak boundary layer 20 to 50 nm thick that shears away with the print still attached. An ethanol wipe that restores adhesion is evidence of that layer, not proof of which one it is, and in-line corona held between 38 and 44 mN/m at the press is the fix.

Solvent Based Flexo InksThe mechanism

02 | 20 faults

Faults

Symptom, cause, check, change, confirm. The trap is the move to avoid.

Fault 1 of 20

Ink lifts with the tape on treated PE

WHAT YOU SEE

The solid prints and looks right, but a straight tape pull across it takes ink off, worst on the white and on the heaviest laydown. The failure is usually patchy across the web rather than uniform, and often worst at the edges.

WHAT IT LOOKS LIKE

CAUSES, MOST LIKELY FIRST

Surface energy below 38 mN/m on the reel actually being runThe corona-oxidised layer relaxes in store and the polar groups rotate back into the bulk polymer, so the film that was certificated at 42 mN/m at the extruder is no longer that at the press.
Slip additive bloomed to the surfaceErucamide and oleamide migrate out of the polyethylene over days and weeks and build a low-energy weak boundary layer; the ink keys to the bloom rather than to the film, and the bloom shears. Antiblock is a different thing: silica, talc or diatomaceous earth particles that do not migrate but sit proud of the surface and can be shed mechanically onto the print face in the wound reel.
Treater set for the wrong web width or watt densityThe untreated margin either side gets no corona at all, so adhesion fails at the edges while the centre passes and the operator reports it as random.
Over-treatment at the extruder or at the pressPast about 44 mN/m corona keeps oxidising but the product is low molecular weight oxidised material lying loose on the surface. It reads as a high dyne number and behaves as a weak boundary layer, so the print keys to something that shears.
Wrong series for the film, or ink over-reducedA straight polyamide 210PA on a high-slip PE or on PET has no resin route to the surface; over-reduction takes out the binder that was carrying what little key there was.

Solvent Based Flexo InksInk lifts with the tape on treated PE

Fault 1 of 20 · continued

Ink lifts with the tape on treated PE

CHECK, IN THIS ORDER

Test with ISO 8296 test liquids or dyne pens on the reel you are about to run, three points across the width plus both edges, read at 2 s and not on the wet stripe. Gloves on: the liquids are formamide and 2-ethoxyethanol mixtures and both are reproductive toxicants that go through skin.
Dyne both faces of the film to confirm you are printing the treated side; on a co-extruded PE the two faces can read 8 mN/m apart.
Straight pull with 3M 610 or equivalent tape across a solid 24 h after printing, reported as percentage of ink retained. Do not use ISO 2409 cross-cut on a 2 um film on PE, the blade cuts the substrate and the result is meaningless.
Wipe a test patch with ethanol on a lint-free cloth, let it flash off, then draw down or re-print. If adhesion comes back, you have evidence of a weak boundary layer, not proof that the layer is slip bloom: ethanol takes off amide, transferred antiblock fines, roller and compressor oil, mould release and low molecular weight oxidised material alike. Dyne before and after the wipe, check the rollers and the compressed air line, and where it matters run ATR-FTIR on the untouched surface for the amide band.
Read the extrusion date on the reel and how long it has been in store, and at what temperature.

THEN CHANGE

Raise in-line corona at the press to 38 to 42 mN/m measured at the print station, and stop relying on the film supplier's certificate. Do not go past about 44 mN/m; over-treatment builds its own weak boundary layer and on thin film it treats the reverse face, which then blocks in the reel. Confirm the ozone extraction on the treater is running.
Where treatment cannot be raised, move from UniFlex-210PA to the UniFlex-230NP nitrocellulose and polyamide blend, which has a wider substrate window, or put a primer down.
Bring press viscosity back into 16 to 20 s DIN 4 at 25 C and set the anilox for the dry deposit the job needs rather than flooding the plate to force coverage.
For a permanently low-energy or high-slip film, move to UniFlex-250PU with 3 to 5 % w/w UniFlex-259HD and allow 5 to 7 days at 25 C for full property development before testing.
Reject the reel if it reads below 34 mN/m across the width and does not respond to in-line treatment; no ink corrects a surface that is not there.

CONFIRM IT IS FIXED

Tape pull on a solid 24 h after printing at 23 C and 50 % RH showing 95 % or more ink retained, repeated on a retained sample at 7 days, with the dyne reading taken at the print station recorded on the job card.

THE TRAP

Tipping adhesion promoter or a dose of hardener into a one-component duct. It does nothing when the failure is at the film surface, it shortens the working life of the whole pan, and it buries the one number nobody took: what the treatment actually was.

Solvent Based Flexo InksInk lifts with the tape on treated PE (continued)

Fault 2 of 20

Print blocks in the reel, face to back

WHAT YOU SEE

The reel runs clean and feels dry at the rewind, then the print picks off onto the back of the film when the reel is unwound at the bag line, or the finished bags stick in the stack after a day on a warm pallet. Worst in the core and where the solid is heaviest.

WHAT IT LOOKS LIKE

CAUSES, MOST LIKELY FIRST

Retained solvent in the filmA flat, hot dryer profile skins the surface and the depth of the film never empties; the solvent left behind keeps the film soft and it cold-flows under the pressure of the wound reel.
Winding tension too high on a warm reelHeat plus pressure gives intimate contact between the print face and the back of the film, and the ink film transfers to the face it is pressed against.
Slow reducer still in the blend after the hall cooledUniFlex-296SL is formulated to hold the film open in a press hall above 35 C; left in the pan at night it simply leaves solvent in the film at the same line speed.
Overprint varnish missing or too thinOn a surface print the varnish is the block barrier as well as the wear layer, and a starved film gives neither.
Slip and antiblock on the reverse faceThe reverse of the film is the surface the print is pressed against, and it carries the slip amide and the antiblock. That is the pair that blocks, which is why a print-to-print test can pass while the reel still fails.

Solvent Based Flexo InksPrint blocks in the reel, face to back

Fault 2 of 20 · continued

Print blocks in the reel, face to back

CHECK, IN THIS ORDER

Cut a sample from the finished reel and run static headspace gas chromatography for total and individual residual solvent. Do not judge it by smelling the core.
Read the dryer air temperature at the nozzles deck by deck and confirm it rises through the tunnel rather than sitting flat and hot, and confirm extraction is genuine air turnover and not recirculated saturated air.
Block test in the pair the reel actually makes: printed face against the reverse face of the same film, under 100 g/cm2 for 24 h, at 40 C for general work and at 50 C where the pack is palletised warm or shipped in a closed container. Run print to print as a second pair only where the converted bag stacks face to face. Peel at a controlled rate and report both transfer and peel resistance.
Measure reel surface temperature at the rewind with an infra-red thermometer, and read the winding tension in N/m against the job card.
Check which reducer is actually in the pan, 295RD or 296SL, against what the job card says.

THEN CHANGE

Correct the dryer to a rising profile with real extraction before touching the ink at all.
Drop and taper winding tension, and let the reel stand and cool before slitting or bag making.
Move the blend faster: UniFlex-295RD in place of 296SL once the hall is below 35 C.
Apply or increase UniFlex-290OG overprint varnish at 1.0 to 2.0 g/m2 dry on film, run at 18 to 22 s DIN 4 at 25 C.
Back the treater off where the reverse face is reading treated; the print needs 38 to 42 mN/m, the back of the film does not.

CONFIRM IT IS FIXED

Block test print face to reverse face at 40 C for 24 h under 100 g/cm2 with no transfer, and at 50 C where the pack is shipped warm, with residual solvent on a cut reel sample within the house acceptance for the construction and reel surface below 30 C at the rewind.

THE TRAP

Turning the dryer up. A hotter flat profile skins the surface faster and locks more solvent underneath it, so the reel blocks worse, and the extra heat is now also distorting a PE web that was running in register.

Solvent Based Flexo InksPrint blocks in the reel, face to back (continued)

Fault 3 of 20

Viscosity climbs and the shade drifts

WHAT YOU SEE

The cup reading goes up every hour, the operator thins, and by the end of the shift the solid is weaker than the approved draw-down although the viscosity is back on number. Worst on the open pans at the far end of the press.

WHAT IT LOOKS LIKE

CAUSES, MOST LIKELY FIRST

Preferential loss of the fast fraction from an open panEthyl acetate leaves first by a wide margin. Ethanol and n-propyl acetate follow at broadly similar rates, so the pan enriches in the slowest component, n-propanol, which is also held back by the NC and PA binders more than its pure evaporation rate predicts. The blend gets progressively slower, and that changes release rate and pigment wetting as well as the efflux reading.
Thinning with whatever single solvent is on the gantryAn arbitrary solvent corrects the cup reading and moves the blend away from the balance the ink was designed around, so gloss, transfer and drying all shift with it.
Cup and sample not at 25 CThe viscometer is reporting press-hall temperature as much as ink condition; a 10 C swing across a shift is several seconds on DIN 4 with no change in the ink.
Returned ink of unknown blend charged into the panInk recovered from an earlier job already carries its own letdown, so the starting point is not the supply viscosity the letdown calculation assumes.
Anilox cells pluggingA plugged roll prints weak at correct viscosity, the operator reads the weak print as thick ink, thins, and the pan ends up over-reduced.

Solvent Based Flexo InksViscosity climbs and the shade drifts

Fault 3 of 20 · continued

Viscosity climbs and the shade drifts

CHECK, IN THIS ORDER

Read DIN 4 to DIN 53211 with the sample and the cup both at 25 plus or minus 1 C, taking the ink temperature in the ink itself rather than the air temperature, and record which cup was used.
Log viscosity every 30 min against the clock. A straight climb is evaporation; a step is an addition somebody did not record.
Look in the thinner can at the deck: matched 295RD, the 296SL slow blend, or a drum of ethyl acetate.
Measure the anilox volume rather than trusting the engraving figure on the end cap, and compare it with the band for its ruling.
Draw down the pan ink against the standard on the same substrate and read density on a densitometer, not by eye under press-hall light.

THEN CHANGE

Cover the pans, keep the ink recirculating and stop the return line splashing into the tray.
Dose only with the matched UniFlex-295RD, or 296SL where the hall is above 35 C, and fit automatic viscosity control reading every 30 to 60 s.
Get the ink temperature down: move the tray out of the dryer return, or jacket it.
Dump and recharge a pan whose blend has drifted beyond correction rather than chasing it with toner.
Clean or replace a plugged anilox before changing ink strength at all.

CONFIRM IT IS FIXED

Viscosity holding within plus or minus 1 s DIN 4 at 25 C across two hours with automatic dosing running, and solid density within the agreed tolerance of the approved draw-down at both the start and the end of the reel.

THE TRAP

Correcting colour with strength or toner while the blend is still drifting. It moves the shade back for twenty minutes and leaves a pan that matches nothing by the end of the run, and the next shift inherits it as their starting ink.

Solvent Based Flexo InksViscosity climbs and the shade drifts (continued)

Fault 4 of 20

Plate fills in and ink dries on the anilox

WHAT YOU SEE

Fine text, reverse type and shadow dots close up within an hour and the solid goes grainy. A ring of dried ink builds on the anilox at the ends, and the print goes light across the width.

WHAT IT LOOKS LIKE

CAUSES, MOST LIKELY FIRST

Blend too fast for the hall temperatureAn ethanol-led 210PA blend at 40 C in the press hall is releasing solvent between the anilox and the impression, so the ink is drying on the plate shoulder instead of transferring.
Chamber end seals worn, or the ink not reaching the roll endsA leaking or worn end seal lets ink weep and dry at the roll ends, and a chamber that is not flooded right across its width, or an open pan that does not carry ink to the full face, leaves the end cells un-wetted between revolutions. That is why the deposit builds exactly where the symptom shows it, at the ends and not across the whole roll. Check the seals and the chamber fill before touching the blend.
Dryer set-back over the deckAn over-pressured tunnel pushes hot air back across the plate and dries ink at exactly the place it must stay wet.
Ink running warm in the tray and the loopAbove about 32 C the alcohol fraction is leaving the ink continuously, so the ink arriving at the plate is already short of its fast solvent.
Viscosity too high for the anilox in useA thick ink does not release cleanly from a fine cell, so a film is left on the land and in the cell and it dries there.

Solvent Based Flexo InksPlate fills in and ink dries on the anilox

Fault 4 of 20 · continued

Plate fills in and ink dries on the anilox

CHECK, IN THIS ORDER

Look at where the deposit is. A ring at the ends with a clean centre is a seal and chamber-fill fault; an even deposit across the face is solvent balance, temperature or blade.
Pull and inspect the chamber end seals for wear, hardening and correct fitting, and confirm the chamber is flooded across the full width at running speed.
Measure air temperature at the plate with a probe, and hold a paper strip at the deck to find dryer back-flow.
Take the ink temperature in the tray and at the return with a probe rather than an air reading.
Measure viscosity at running temperature and not at shift start.

THEN CHANGE

Replace worn end seals and set the chamber to flood right across the roll face before changing anything in the ink.
Move a fraction of the blend to the slower component with UniFlex-296SL, which is exactly what it is formulated for.
Balance the dryer so it pulls rather than pushes, and stop the back-flow at the deck.
Bring viscosity down into 16 to 18 s DIN 4 at 25 C for tone work on a 360 to 500 l/cm anilox.
Wash the anilox properly, and ultrasonically where it has been left, using a cleaner the roll maker approves for that ceramic and for the ceramic to base bond, with gloves, eye protection and extraction; hot caustic and amine cleaners attack skin and can lift the ceramic on some rolls. Never let nitrocellulose residues, filters or sludge dry out: NC skins, wipes and used filter elements are a self-heating fire risk and go into a closed lidded metal bin kept solvent-wet and cleared every shift. Bond and earth drums and containers before decanting or pumping solvent. Set a standing wash routine for press stops.

CONFIRM IT IS FIXED

One hour at running speed with no build on the plate shoulder and no ring at the roll ends, reverse type holding open at the smallest size on the job, and anilox volume measured clean before and after the run.

THE TRAP

Flooding the plate by raising viscosity or backing the blade off. It fills the shoulders faster, moves the fill-in out into the solid, and adds ink the dryer then has to take back out at the other end of the press.

Solvent Based Flexo InksPlate fills in and ink dries on the anilox (continued)

Fault 5 of 20

Solvent smell in the bag at the customer

WHAT YOU SEE

The reel passed at the press and the bags were made without incident. Weeks later the customer opens a carton and reports a smell, or the packed product picks up taint. Nothing on the job record shows anything wrong.

WHAT IT LOOKS LIKE

CAUSES, MOST LIKELY FIRST

Retained solvent in the filmA flat hot profile on recirculated saturated air skins the film; the depth never empties and the residue is the slowest part of the blend, the n-propanol and n-propyl acetate, which are also the ones that smell.
Film weight too heavy for the line speed and the dryerA 3 g/m2 sack solid does not empty on the dwell that empties a 1.5 g/m2 film solid, and dwell is tunnel length divided by web speed, nothing else.
Slow blend left in the pan after the hall cooled296SL is designed to hold the film open above 35 C; run at night at the same speed it simply leaves more solvent in the film.
Reel wound warm and immediately wrappedWhat little solvent is still leaving the film has nowhere to go once the reel is tight and shrink-wrapped, so it stays in the roll and comes out at the customer.
Extraction quietly degradedLoaded filters and a tired fan drop air turnover without changing the setpoint, so the tunnel reads the same temperature and does a fraction of the work.

Solvent Based Flexo InksSolvent smell in the bag at the customer

Fault 5 of 20 · continued

Solvent smell in the bag at the customer

CHECK, IN THIS ORDER

Static headspace gas chromatography on a cut sample of the finished reel, reporting total and individual species in mg/m2 against prepared standards, with the conditioning stated. A nose test in a solvent-saturated press hall is not a result.
Run the same headspace GC on an unprinted control cut from the same reel and sealed at the same time. If the odour species is on the plain film, stop looking at the dryer.
Read the dryer profile deck by deck at the nozzles, not the setpoint on the last tunnel, and measure extraction volume rather than counting fan hours.
Calculate actual dwell, tunnel length divided by web speed, against the film weight being asked to empty.
Sample from the reel core as well as the outside; the core is where retention shows.

THEN CHANGE

Correct the dryer to a rising profile with genuine extraction first, before changing anything in the ink.
Reduce line speed or film weight until the dwell matches, and take the speed loss rather than releasing the reel.
Move the blend faster with UniFlex-295RD as soon as the hall allows it.
Let the reel stand and cool in ventilated storage before slitting and bag making, rather than wrapping it hot, and keep finished goods away from the solvent store.
Where the species tracks to the substrate rather than to the print, take it up with the film supplier with the unprinted control result in hand.

CONFIRM IT IS FIXED

Static headspace GC on the released reel within the agreed house acceptance for that construction, reported in mg/m2 by species, with the unprinted control run alongside and any brand-owner limit applied on top, and the figure recorded against that reel number.

THE TRAP

Airing the bags out on the floor for a day, or deciding the reel is fine because nobody in the press hall can smell it. The press hall has been breathing that blend all shift, so the operator's nose is the least sensitive instrument on the floor, and airing a stack only empties the outside of it.

Solvent Based Flexo InksSolvent smell in the bag at the customer (continued)

Fault 6 of 20

Two-component PU job fails its resistance test

WHAT YOU SEE

A UniFlex-250PU job prints well and the film looks right, but the MEK rub or the product resistance test is no better than a one-component ink and sometimes worse. Often the first sign is the customer's own test, not ours.

WHAT IT LOOKS LIKE

CAUSES, MOST LIKELY FIRST

Alcohol reducer added at the pressHydroxyl reacts with the isocyanate and consumes it, so the crosslink never forms. This is the usual reason a two-component flexo job fails, and it is invisible in the print.
Hardener under-dosed or badly mixedUniFlex-259HD at 3 to 5 % w/w on ink is a weighed addition on the pan quantity; eyeballed into a moving pan it goes in short, and short means partly crosslinked.
Tested too earlyFull property development takes 5 to 7 days at 25 C, so a resistance test at the delivery is testing a physically dried film with the chemistry still running.
Hardener stale or moisture struckThe adduct takes up atmospheric moisture, which consumes isocyanate and generates carbon dioxide, so the can gasses and can pressurise; shelf life is 6 months sealed, and a part-used can decanted back is finished.
Pot life exceededPast 6 to 8 h at 30 C, or 4 to 5 h at 35 C, the isocyanate has largely reacted in the pan rather than in the film.

Solvent Based Flexo InksTwo-component PU job fails its resistance test

Fault 6 of 20 · continued

Two-component PU job fails its resistance test

CHECK, IN THIS ORDER

Look in the reducer can at the deck. If it is the alcohol-bearing 295RD rather than the ester-only version for the 250PU series, stop there; the rest of the investigation is academic.
Recalculate the dose as 3 to 5 % w/w on the weight of ink actually in the pan, and check it was weighed.
Note the time the hardener went in and the ink temperature, and compare with 6 to 8 h at 30 C and 4 to 5 h at 35 C, taken as a 5 s rise on DIN 4.
Check the hardener can date and whether it had been opened before; a gassed, cloudy or crystallised can is scrap and is vented and disposed of as hazardous waste, not resealed.
ASTM D5402 MEK double rubs on the heaviest solid at 24 h and again at 7 days, same pad and same load, and compare the two figures.

THEN CHANGE

Change to the ester-only reducer for the 250PU series and dump the contaminated pan. Alcohol cannot be corrected out of a two-component ink.
Weigh the hardener on the pan quantity, mix it properly and add it at the press with nitrile gloves, eye protection and local extraction, never in the ink store and never where there is no ventilation; it is a respiratory and skin sensitiser.
Re-test at 5 to 7 days at 25 C before judging the film or the grade.
Open a fresh 1 kg or 5 kg hardener pack, sized so it is finished within the shift, and never decant activated ink or a part-used can back into a closed drum, which can pressurise.
Mix smaller pan quantities more often rather than carrying one pan across a whole shift.

CONFIRM IT IS FIXED

ASTM D5402 MEK double rubs on the heaviest solid at 7 days meeting the count agreed for the job, with the 24 h reading logged alongside so the development between the two is visible on the record.

THE TRAP

Doubling the hardener when the first test fails. Excess isocyanate does not crosslink harder or faster. It raises the reaction rate roughly in proportion, so it roughly halves the working life: a 6 to 8 h pan at 30 C becomes 3 to 4 h, climbs away without warning and gels the pan, and the surplus isocyanate can leave a film brittle enough to crack on the fold of the finished bag.

Solvent Based Flexo InksTwo-component PU job fails its resistance test (continued)

Fault 7 of 20

Print rubs off the filled woven PP sack

WHAT YOU SEE

The bag is perfect off the press. After filling, stacking and a lorry journey the print is scuffed grey at the fold and along the top seam, and the brand name on the outer bags of the pallet is unreadable.

WHAT IT LOOKS LIKE

CAUSES, MOST LIKELY FIRST

No overprint varnish, or a starved oneOn a filled sack the varnish is the wear layer and the ink is not; without it the abrasion is taken directly by a 2 to 3 g/m2 pigmented film.
Print on a lightly treated tape surfaceWoven polypropylene tape is a low-energy, drawn surface; below 38 mN/m the key was never there, so there is nothing for the abrasion to work against.
Retained solvent leaving the film soft at conversionA film that is still carrying solvent has not reached its final hardness, and it scuffs at loads it would survive a week later.
Wrong series in the panUniFlex-210PA is a film surface ink; the abrasion duty on woven sack belongs to UniFlex-280WP, which is built for adhesion to a tape surface and for rub.
Slip too low on the finished surfaceBags that drag against each other instead of sliding convert the whole handling load into abrasion at the contact points.

Solvent Based Flexo InksPrint rubs off the filled woven PP sack

Fault 7 of 20 · continued

Print rubs off the filled woven PP sack

CHECK, IN THIS ORDER

Sutherland rub to ASTM D5264, 4 lb block, 100 cycles print to print at 23 C and 50 % RH, receptor and print mounted as the method specifies, on a cut sample and again on a filled bag.
Weigh printed and varnished against unvarnished stock for varnish coat weight, five adjacent pairs on woven PP, and check press viscosity of the 290OG at 18 to 22 s DIN 4 at 25 C.
Dyne the woven PP at the print station with ISO 8296 test liquids, gloves on; 38 mN/m is the minimum on the tape surface.
ASTM D1894 coefficient of friction, static and kinetic, on the varnished surface.
Confirm the grade actually in the pan is 280WP and not a film surface ink drawn from the wrong shelf.

THEN CHANGE

Apply UniFlex-290OG at 2 to 3 g/m2 dry on woven PP and kraft, which on a dedicated varnish anilox means roughly 10 to 16 cm3/m2, or UniFlex-291OM where a matt finish is specified, running the matt at the top of the press window because the matting agent raises the effective viscosity. Confirm the coat weight by cut-and-weigh on the run and adjust it to the bag line's measured rub and COF result.
Raise in-line corona to 38 to 42 mN/m on the woven PP ahead of the first print station, and no higher.
Move to UniFlex-280WP for woven sack work.
Empty the film properly before winding, as for the blocking and retention faults above.
Where the sack is handled hard, take the varnish to a two-component route and allow 5 to 7 days at 25 C before judging it.

CONFIRM IT IS FIXED

Sutherland rub to ASTM D5264, 4 lb block, 100 cycles at 23 C and 50 % RH with no significant transfer on the varnished print, repeated on a filled and stacked bag after 48 h, with the varnish coat weight recorded.

THE TRAP

Stirring wax or slip compound into the ink duct to stop the rubbing. It lowers the surface energy of the printed surface, so the varnish then crawls off the very areas that needed protecting, and any colour trapping onto it beads as well.

Solvent Based Flexo InksPrint rubs off the filled woven PP sack (continued)

Fault 8 of 20

Adhesion was fine at press, gone a week later

WHAT YOU SEE

Tape pull at 24 h was clean and the job was released. The converter runs the same stock a fortnight later and the ink lifts, or the print flakes along the fold on the finished bag.

WHAT IT LOOKS LIKE

CAUSES, MOST LIKELY FIRST

Slip additive bloom continuing in the reelErucamide and oleamide keep migrating out of the polyethylene after printing and build a weak boundary layer underneath the ink, which then shears off with the ink attached to it.
Corona treatment decaying in storeThe oxidised surface relaxes over weeks, fastest on high-additive grades and in a warm store, so a print that had a marginal key loses it entirely.
Keyed to a marginal surface in the first placeInk printed onto 36 to 37 mN/m adheres on day one and has no margin left to lose, so any decay at all takes it below the threshold.
Over-treated film, where the key was never to the polymerCorona past about 44 mN/m leaves low molecular weight oxidised material lying on the surface. It reads high on the dyne pen, prints and tapes clean on day one, and shears in store exactly as bloom does.
Reel stored warmBoth bloom rate and treatment decay rise with temperature, so a pallet by a wall in the sun ages the film several times faster than the store average.

Solvent Based Flexo InksAdhesion was fine at press, gone a week later

Fault 8 of 20 · continued

Adhesion was fine at press, gone a week later

CHECK, IN THIS ORDER

Dyne the stored reel again and compare directly with the reading taken at the press, and record both with dates.
Wipe a patch with ethanol on a lint-free cloth, let it flash, then re-test adhesion. Recovery is evidence of a weak boundary layer, not proof of bloom: it removes oil, mould release and low molecular weight oxidised material just as readily. Dyne before and after the wipe, rule out roller and compressed air contamination, and where it matters confirm the amide by ATR-FTIR on the untouched surface.
Tape pull on retained samples at 24 h, 7 days and 28 days, not only at 24 h.
Ask the film supplier for the slip type and level and the antiblock type separately, the rest of the additive package, and the extrusion date on the reel; they are different lines on the specification and they fail in different ways.
Take the temperature of the film store and of the printed stock store, at the warmest point rather than the average.

THEN CHANGE

Print film as close to extrusion as the schedule allows, and treat in line at the press regardless of what the certificate says, to 38 to 42 mN/m and not beyond about 44.
Specify a medium or low slip film for print work and carry the slip in the UniFlex-290OG varnish instead, where the level can be set and measured.
Move to UniFlex-230NP, or to UniFlex-250PU with 259HD, where the film will not hold a key on its own.
Store film and printed stock below 30 C and out of direct sun.
Where the customer's film cannot be changed, qualify the job on retained samples at 28 days before committing a production run.

CONFIRM IT IS FIXED

Tape pull on retained samples at 24 h, 7 days and 28 days, all at 95 % or more ink retained, with the dyne reading at print and at 28 days recorded alongside on the same record.

THE TRAP

Re-running the job on a fresh reel and calling it fixed. The fresh reel has simply not bloomed yet, the fault returns with the next batch, and by then the ink has been blamed twice and the film has never been measured once.

Solvent Based Flexo InksAdhesion was fine at press, gone a week later (continued)

Fault 9 of 20

Pinholes, fisheyes and crawling in the solid

WHAT YOU SEE

Round clear spots in the solid, sometimes with a raised rim, and ink pulling away from a point rather than covering it. They repeat across the web but not at the cylinder repeat.

WHAT IT LOOKS LIKE

CAUSES, MOST LIKELY FIRST

Silicone or oil contamination on the filmA trace of mould release, roller oil or compressor oil is enough to drop local surface energy well below the ink's wetting threshold, and the ink retracts from it.
Foam and entrained airAn oversized or fast-running pump, an air leak on the suction side and a return line splashing into the pan all carry air into the ink; the bubbles break at the film surface and leave a crater the drying film cannot close.
Over-dosed defoamer or slip compoundThis is silicone repellence rather than foam, and it comes from the correction rather than from the press.
Surface energy too lowBelow about 38 mN/m the ink beads rather than wets, and on a poorly treated patch it beads locally and prints as pinholes.
Blend too fast to levelThe film sets before surface tension has pulled it flat, so every small defect is frozen into the print.

Solvent Based Flexo InksPinholes, fisheyes and crawling in the solid

Fault 9 of 20 · continued

Pinholes, fisheyes and crawling in the solid

CHECK, IN THIS ORDER

Draw the pan ink down on clean film with a bar before anything else. It takes two minutes and it splits the investigation in half: if the drawdown craters the fault is in the ink and the press is innocent; if it is clean the fault is on the press or on the film, and only then start wiping rollers and dismantling air lines.
Wipe each suspect roller with a clean cloth and look for oil, and check the compressed air line for a working water and oil trap.
Look at the pan for standing foam and at the return for splashing; drop the pump speed and watch whether the foam decays, and check the suction side of the pump for an air leak.
Dyne the film at the station, 38 mN/m minimum.
Filter a sample of the pan ink and look at what the filter catches, and run Karl Fischer water content on the pan where the hall is humid.

THEN CHANGE

Reduce pump speed, seal the suction side, stop the splash and let the entrained air out before considering a defoamer at all; size the pump to the deck rather than running one setting for the whole press.
Where defoamer or slip has already been over-dosed, replace the pan ink; silicone repellence cannot be corrected by adding anything.
Clean or change the contaminated roller and fit a coalescing filter on the air line.
Raise treatment to 38 to 42 mN/m at the station.
Slow the blend slightly with UniFlex-296SL so the film has time to level before it sets.

CONFIRM IT IS FIXED

A 500 mm solid across the full web at running speed with no pinholes visible at a 10x loupe, and a clean bar drawdown of the same pan ink on the same film.

THE TRAP

Adding more defoamer. If the craters are silicone repellence, more silicone widens them, and the pan is then unusable for every colour that follows it through the same wash-up.

Solvent Based Flexo InksPinholes, fisheyes and crawling in the solid (continued)

Fault 10 of 20

Colour weak and chalky on kraft cement bag

WHAT YOU SEE

The solid looks flat and pale against the draw-down, the sheet feels dry to the touch immediately, and the colour shows through on the reverse more than it should. All five causes below fail the same way: the vehicle is drawn into the absorbent multiwall kraft and carries the pigment in with it, so the pigment that should be on the surface scattering light ends up inside the sheet.

WHAT IT LOOKS LIKE

CAUSES, MOST LIKELY FIRST

Anilox too fine for a kraft solidSack coverage needs a 100 to 160 l/cm roll measuring about 14 to 25 cm3/m2. A process roll at 2.5 to 5 cm3/m2 cannot deliver the film weight an absorbent stock consumes before it starts to build colour, so the pigment that arrives is all inside the fibre.
Wrong series in the panUniFlex-270KS has its alcohol balance set deliberately for holdout on kraft; a film surface ink has no such control and simply sinks into the sheet.
Viscosity too lowOver-reduced ink has less body and more free solvent, so it penetrates faster and further, which is the opposite of what holdout needs.
Blend wrong for the stockToo fast and the film has no body when it meets the sheet; too slow and it has all the time it needs to travel into the fibre. Both read as chalky.
An exceptionally open or absorbent kraftSome multiwall grades will take whatever film weight is offered, and no ink adjustment holds pigment on a surface that is effectively a sponge.

Solvent Based Flexo InksColour weak and chalky on kraft cement bag

Fault 10 of 20 · continued

Colour weak and chalky on kraft cement bag

CHECK, IN THIS ORDER

Measure the anilox volume and compare with about 14 to 25 cm3/m2 for a 100 to 160 l/cm sack roll, not with the process band.
Weigh printed against unprinted for the dry deposit, at least five adjacent pairs because kraft basis weight varies more than the ink weighs; a kraft solid where holdout is being bought with weight sits at 2.5 to 3.5 g/m2.
Check press viscosity on DIN 4 at 25 C: kraft and woven sack solids run at 20 to 22 s, not at 16 s.
Confirm the grade in the pan is UniFlex-270KS.
Look at the reverse of the sheet and at a torn cross-section under a loupe for strike-through.

THEN CHANGE

Move to UniFlex-270KS, whose solvent balance is set to keep pigment on the surface of an absorbent kraft.
Open the anilox to 100 to 160 l/cm, which will measure roughly 14 to 25 cm3/m2, and set press viscosity to 20 to 22 s DIN 4 at 25 C.
Stop reducing. Where strength genuinely needs to come down, use UniFlex-297EX extender varnish, which drops colour without thinning the film or changing the solvent balance.
Add a fraction of the slower alcohol with UniFlex-296SL to hold the film on the surface at speed.
On an exceptionally open kraft, print a base white or a size coat first and accept the extra station.

CONFIRM IT IS FIXED

Density on the solid matching the approved draw-down on the customer's own kraft, with dry deposit weighed at 2.5 to 3.5 g/m2 as the mean of five adjacent pairs and no visible strike-through on the reverse of the sheet.

THE TRAP

Thinning further to make it transfer. A thinner ink travels deeper into the fibre, so the colour gets weaker still, and the operator thins again. On absorbent stock the correction is anilox volume and holdout, never viscosity downwards.

Solvent Based Flexo InksColour weak and chalky on kraft cement bag (continued)

Fault 11 of 20

Second colour will not lie on the first

WHAT YOU SEE

The overprinted colour breaks up, beads or mottles wherever it crosses the first colour, and is perfectly clean where it sits on bare film. Reverse type in the second colour closes or fills.

WHAT IT LOOKS LIKE

CAUSES, MOST LIKELY FIRST

First-down film not dry at the next stationThe second colour's solvent redissolves the first and the two films mix at the interface, which prints as mottle and as trapping loss.
Slip or wax in the first-down inkAnything that lowers the surface energy of the printed film makes it a poor substrate for the next colour, exactly as it would for a varnish.
Interdeck drying too aggressive at deck oneA hard, glazed first-down surface gives the second colour nothing to bite into, so it beads on its own dried predecessor.
Second colour viscosity too lowA thin second colour has no body to cover a printed surface and simply retracts into the low points.
Wrong sequence for the jobA heavy opaque white or a dense solid placed late in the sequence is asking the most difficult film to trap onto the most difficult surface.

Solvent Based Flexo InksSecond colour will not lie on the first

Fault 11 of 20 · continued

Second colour will not lie on the first

CHECK, IN THIS ORDER

Touch test and tape pull on the first colour at the second station with the second deck lifted.
Dyne the printed first-down solid and compare with the bare film; a large drop points at slip or wax in the ink.
Read interdeck air temperature and volume deck by deck.
Check press viscosity of both colours at 25 C, and the anilox volumes against their rulings and against the sequence.
Run a short trial with the sequence reversed and compare the trap directly.

THEN CHANGE

Balance interdeck drying so the first colour is dry but not baked: rising through the press, not maximum at deck one.
Take wax and slip out of the first-down ink and carry slip in the UniFlex-290OG varnish at the end of the sequence, where it belongs.
Lift the second colour into the 18 to 20 s DIN 4 band for line and solid work.
Reorder the sequence so opaque white and heavy solids go down where they can be properly dried.
Where two heavy solids must trap, move the first down to UniFlex-230NP for its harder film.

CONFIRM IT IS FIXED

A clean trap across a 100 mm overlap panel with no beading at a 10x loupe, and reverse type in the second colour holding open at the smallest size on the job, held for a full reel.

THE TRAP

Drying the first colour harder to stop the mixing. It glazes the surface, so the trap gets worse rather than better, and the extra heat is now also driving the web shrinkage that shows up as register drift on PE.

Solvent Based Flexo InksSecond colour will not lie on the first (continued)

Fault 12 of 20

Streaks and score lines in the solid

WHAT YOU SEE

Fine lines running in the web direction through the solid, in one colour only. They do not move when the plate is changed and they do move when the blade is stepped across.

WHAT IT LOOKS LIKE

CAUSES, MOST LIKELY FIRST

Nicked or worn doctor bladeA notch in the edge leaves a ribbon of ink on the land at that point, and it prints as a continuous line down the web.
Dried ink or agglomerate in the panAn unfiltered return re-feeds the pan's own dried skin, and a hard particle trapped under the blade cuts a line until it clears or scores the roll.
Blade pressure and angle wrongA blade set outside its working angle chatters instead of wiping, so the wipe is intermittent and prints as fine parallel lines.
Anilox scoredA groove in the roll surface holds ink through the wipe and delivers it to the plate every revolution.
Viscosity too high for the wipeA thick ink is not cleared cleanly from the land at speed, so the blade leaves a film that prints as a broad soft streak.

Solvent Based Flexo InksStreaks and score lines in the solid

Fault 12 of 20 · continued

Streaks and score lines in the solid

CHECK, IN THIS ORDER

Step the blade across and watch the line. If it moves, the fault is the blade; if it stays, look at the roll.
Run the pan ink through a 100 micron filter and inspect the residue.
Inspect the anilox surface under a loupe with raking light for a score.
Check blade angle, pressure and wear pattern against the press maker's setting.
Measure press viscosity at running temperature.

THEN CHANGE

Change the blade, and fit an in-line filter on the return so the pan stops re-feeding its own skin.
Filter or replace the pan ink and keep the pan covered. Used filter elements and dried skins are nitrocellulose waste: into the closed, solvent-wet metal bin, never left to dry out in an open bin.
Reset blade angle and pressure to specification.
Re-engrave or replace a scored anilox; no blade setting prints past a groove in the roll.
Bring viscosity into the band for the anilox in use.

CONFIRM IT IS FIXED

A full-width solid at running speed with no lines visible at arm's length and none at a 10x loupe, holding clean for a complete reel.

THE TRAP

Winding blade pressure on to wipe the line away. It wears a flat on the blade within minutes, scores the anilox permanently, and converts a twenty minute blade change into a re-engraving and a new roll.

Solvent Based Flexo InksStreaks and score lines in the solid (continued)

Fault 13 of 20

Halo round the solid and heavy dot gain

WHAT YOU SEE

A dark rim on the leading or trailing edge of solids and type, dots printing visibly larger than the file, and shadow tones filling to solid. The job looks heavy and dirty rather than weak.

WHAT IT LOOKS LIKE

CAUSES, MOST LIKELY FIRST

Too much impressionThe plate is squeezed into the substrate and the excess ink is pushed out at the edge of every image element, where it prints as a rim.
Anilox volume too high for the plateA fine plate fed by a coarse roll has more ink than the image can carry, and the surplus has nowhere to go but sideways.
Plate durometer or relief wrong for the jobA soft plate deforms under impression and spreads the image; a shallow relief lets the shoulder carry ink into the non-image area.
Viscosity too lowA thin ink flows at the nip under pressure, so it spreads further for the same impression.
Mounting tape too soft under a solidThe plate beds into the tape at the nip and the contact area grows, which is mechanically identical to adding impression.

Solvent Based Flexo InksHalo round the solid and heavy dot gain

Fault 13 of 20 · continued

Halo round the solid and heavy dot gain

CHECK, IN THIS ORDER

Set impression by kiss: back off until the print drops out, then bring on the minimum that gives a full solid, and mark the setting.
Measure anilox volume against the plate ruling: 360 to 500 l/cm at about 2.5 to 5 cm3/m2 for process and tone, 180 to 300 l/cm at about 6 to 12 cm3/m2 for line and solid on film.
Check plate durometer and relief depth against the job specification.
Measure press viscosity at 25 C on the DIN 4 cup.
Check the mounting tape grade against the duty of each plate.

THEN CHANGE

Reset impression to kiss on every deck, starting from zero rather than from where the last job left it.
Match the anilox to the plate ruling instead of running one roll for the whole press.
Use harder mounting tape under solids and softer under tone.
Bring viscosity up into the band for the anilox in use.
Where a design carries both a heavy solid and a fine highlight, split them across two plates and two rolls.

CONFIRM IT IS FIXED

A printed control strip showing dot gain within the agreed curve for that plate and anilox pair, with solids full at the marked kiss impression setting, held at running speed.

THE TRAP

Reducing anilox volume to kill the halo while leaving the impression heavy. The solid goes weak, impression gets wound on again to recover it, and the halo comes back with a lighter solid than the job started with.

Solvent Based Flexo InksHalo round the solid and heavy dot gain (continued)

Fault 14 of 20

Bags will not run on the bagging line

WHAT YOU SEE

The filling or bagging line jams, or the opposite happens and the filled stack slides apart on the pallet. The print is correct and the complaint is entirely about handling.

WHAT IT LOOKS LIKE

CAUSES, MOST LIKELY FIRST

Coefficient of friction outside the line's windowSlip is a setting, not a quality: too little and the bags drag and jam, too much and a stacked pallet loses its interlock and collapses.
Slip carried in the ink rather than in the varnishInk covers only the image area, so friction then varies between printed and unprinted panels across the same bag.
Film's own slip blooming on top of the varnish levelErucamide continues to arrive at the surface in store, so COF measured at 24 h keeps falling for weeks afterwards.
Matt varnish used where the line was set on glossThe matting agent changes surface texture and therefore friction, not only gloss, so 291OM and 290OG do not run the same on the same line.
Retained solvent leaving the surface tackyA film that is not empty reads as high friction on the instrument and as a sticky bag on the line.

Solvent Based Flexo InksBags will not run on the bagging line

Fault 14 of 20 · continued

Bags will not run on the bagging line

CHECK, IN THIS ORDER

ASTM D1894 coefficient of friction, static and kinetic, print to print and print to reverse, at 23 C and 50 % RH, at 24 h and again at 7 days.
Ask the bagging line for its actual working window rather than accepting a single target number. Where they cannot state one, start from kinetic COF of about 0.20 to 0.35 print to print by ASTM D1894: below about 0.15 a stacked pallet loses its interlock, above about 0.4 the bags drag on the chutes and grippers. Narrow it on a trial run and record the agreed window on the job card.
Find out whether anyone has added slip compound into the ink duct.
Dyne the varnished surface; a very low reading says the slip level is high.
Check the varnish grade and its coat weight by weighing five adjacent cut pairs.

THEN CHANGE

Set the slip level in UniFlex-290OG to the line's measured window and leave the ink alone.
Where COF drifts down in store, reduce the slip in the varnish to allow for the film's own bloom, qualified on retained samples first.
Run UniFlex-291OM matt only where the line has been set up for matt, at the top of the press window.
Empty the film properly so tack is not being measured as friction.
Where the line needs two behaviours, leave a slip-free panel where the grippers run.

CONFIRM IT IS FIXED

ASTM D1894 static and kinetic COF inside the bagging line's stated window, or inside the agreed 0.20 to 0.35 kinetic starting window where the line has none, at 24 h and again at 7 days, measured on retained samples of the customer's own film.

THE TRAP

Tipping slip compound into the duct to free a jamming line. It fixes the friction that shift and destroys the trap and the varnish adhesion in the same move, and the next job through that pan inherits the contamination.

Solvent Based Flexo InksBags will not run on the bagging line (continued)

Fault 15 of 20

Gloss is patchy and the varnish mottles

WHAT YOU SEE

The overprint varnish looks glossy in places and dull in others, usually following the heavy ink areas, and the difference between printed and bare film is worse than it was on the proof.

WHAT IT LOOKS LIKE

CAUSES, MOST LIKELY FIRST

Varnish applied over an ink film that is not emptySolvent still leaving the ink passes up through the varnish while it sets and disrupts the surface as it goes.
Coat weight too low or unevenA starved varnish film follows every irregularity underneath it instead of levelling over them, so the gloss maps the ink.
Matting agent settled in the matt panThe matting agent is denser than the vehicle, so a pan left standing runs glossy at first and matt later, or the reverse.
Anilox too fine for the varnishIncomplete transfer leaves a discontinuous film, which reads as mottle rather than as low gloss.
Surface energy uneven across the webThe varnish wets the well-treated areas and retracts slightly on the rest, so gloss follows the treatment pattern.

Solvent Based Flexo InksGloss is patchy and the varnish mottles

Fault 15 of 20 · continued

Gloss is patchy and the varnish mottles

CHECK, IN THIS ORDER

ISO 2813 gloss meter at 60 degrees on printed and unprinted areas and at five points across the web, rather than judging under press-hall light. Re-read at 20 degrees where the 60 degree value is above about 70 gloss units, as it will be on UniFlex-290OG, and at 85 degrees where it falls below about 10, as it will on UniFlex-291OM matt; 60 degrees alone does not discriminate at either end and that is the difference between finding a mottle and missing it.
Weigh five adjacent varnished and unvarnished pairs for coat weight; 1.0 to 2.0 g/m2 dry on film and 2 to 3 g/m2 dry on sack.
Check varnish viscosity: UniFlex-290OG supply 30 plus or minus 5 s, press 18 to 22 s DIN 4 at 25 C.
Stir the matt pan thoroughly and re-read gloss; a change after stirring says the matting agent had settled.
Read the interdeck drying ahead of the varnish station.

THEN CHANGE

Dry the ink properly before the varnish station by balancing the interdeck, rather than adding heat at the varnish itself.
Open the varnish anilox to roughly 6 to 10 cm3/m2 on film or 10 to 16 cm3/m2 on sack and raise the coat weight to a continuous film.
Keep the matt varnish recirculating and stirred, and run it at the top of the press window instead of thinning it in.
Bring press viscosity into the band; thinning a varnish gives less film, not more flow.
Raise in-line corona ahead of the varnish station where the film surface is uneven, to 38 to 42 mN/m and no higher.

CONFIRM IT IS FIXED

ISO 2813 gloss within the agreed band at five points across the web and at the start, middle and end of the reel, on the customer's own film, read at 60 degrees and re-read at 20 or 85 degrees where the 60 degree figure falls outside about 10 to 70 units.

THE TRAP

Thinning the varnish to make it flow out. Less solids means less film, so the mottle gets worse, and the coat weight is now also too low to take the scuff on the bagging line, which turns one complaint into two.

Solvent Based Flexo InksGloss is patchy and the varnish mottles (continued)

Fault 16 of 20

White is grey and short of opacity

WHAT YOU SEE

The opaque white looks grey against the standard, especially where it goes over a coloured or dark film, and adding more white at the mixer makes no difference at all.

WHAT IT LOOKS LIKE

CAUSES, MOST LIKELY FIRST

White over-reducedOpacity comes from film thickness and pigment volume together; thinning removes both, and titanium dioxide cannot hide anything it is not thick enough to cover.
Anilox too fine for a whiteWhites and sack solids need a 100 to 160 l/cm roll at about 14 to 25 cm3/m2, and a process roll at 2.5 to 5 cm3/m2 simply cannot lay down the film weight required.
Titanium dioxide settled, or the white is a tinting whiteAt about 1.35 to 1.6 kg/l, which is roughly 40 to 55 % rutile TiO2 by weight, a genuinely opaque white is the heaviest ink on the press, and a drum or pan left standing separates, so what is drawn off the top is short of pigment. If the drum measures below about 1.35 kg/l it is a tinting white at roughly 30 to 38 % TiO2, and no anilox will make it opaque.
Single hit where the job needs twoOne station can only carry the film weight its anilox delivers; past that point the only route to opacity is a second deck.
Judged in the wrong placeWhite read on a dark film under press-hall light looks grey whatever the opacity, so the operator chases a fault that is in the viewing and not the print.

Solvent Based Flexo InksWhite is grey and short of opacity

Fault 16 of 20 · continued

White is grey and short of opacity

CHECK, IN THIS ORDER

Weigh five adjacent printed and unprinted pairs for dry deposit; an opaque white sack solid typically needs 2.5 to 3.5 g/m2.
Measure anilox volume and compare with about 14 to 25 cm3/m2 on a 100 to 160 l/cm roll for whites and sack solids.
Measure the density of the white in the drum with a density cup and compare with about 1.35 to 1.6 kg/l; below that you have a tinting white, not an opaque one.
Check press viscosity; opaque white and sack solids run at the top of the window, not the bottom. Stir the drum and the pan thoroughly and re-read viscosity and density; a settled white reads thin and prints grey.
Print over a black and white contrast chart on the actual substrate and read contrast ratio, rather than assessing it by eye.

THEN CHANGE

Stop reducing and raise the anilox volume.
Run a double hit where one station cannot carry the film weight.
Keep the white recirculating and stir the drum before charging the pan.
Hold supply viscosity at 70 plus or minus 15 s DIN 4 at 25 C for opaque white and sack solids, and let down no further than the job needs.
Where the drum is a tinting white, change the grade; opacity is bought at the mill in pigment volume, not at the press.

CONFIRM IT IS FIXED

Contrast ratio over a black and white chart on the customer's own substrate meeting the agreed figure, with the dry deposit weighed as the mean of five adjacent pairs and recorded against the reel.

THE TRAP

Adding more white pigment at the mixer, or a toner to correct the grey cast. Where the shortfall is film thickness, more pigment in the same thin film changes nothing measurable, and the toner has now turned the white into a colour that will not match the next batch. Measure the drum density first so you know whether you are short of film or short of pigment.

Solvent Based Flexo InksWhite is grey and short of opacity (continued)

Fault 17 of 20

Colour fades on the cement bag in the yard

WHAT YOU SEE

The bags looked right at delivery. After weeks stacked outdoors the reds and oranges have gone pale and the shade has moved, worst on the sun-facing bags and worst of all in the reduced and tinted areas.

WHAT IT LOOKS LIKE

CAUSES, MOST LIKELY FIRST

Pigment chosen for the process set rather than for outdoor lifeDiarylide yellows rate about ISO 105-B02 Blue Wool 4 to 6 in full tone depending on the grade, and the lake reds in general use on sack work rate lower still: PR53:1 about 2 to 3 and PR57:1 about 3 to 5, both dropping sharply in reduction. That is indoor and short-life performance, and a cement bag yard is neither. The rating belongs to the specific pigment index number, not to the family.
Shade built by heavy reductionA 1:25 reduction with extender or white costs one to two Blue Wool steps, because there is far less pigment per unit area to absorb the same exposure.
No overprint varnishA varnish carrying a UV absorber is worth up to one Blue Wool step and is also the physical wear layer; without it the pigment takes the full exposure.
A brand shade matched outside the sack pigment setMatching a corporate colour closely at the light booth can mean leaving the high-rated pigment grades, and the match is then exact for a fortnight.
Film weight below specificationLess pigment per square metre means a smaller reserve to lose before the shade visibly shifts.

Solvent Based Flexo InksColour fades on the cement bag in the yard

Fault 17 of 20 · continued

Colour fades on the cement bag in the yard

CHECK, IN THIS ORDER

Read the pigment list, by colour index number, on the Technical Data Sheet for the shade actually supplied, not the range statement on the page.
Filtered xenon arc exposure rated against the ISO 105-B02 Blue Wool references 1 to 8, with the irradiance, the filter, the black standard temperature and the relative humidity recorded, on the full tone and on the working reduction, on the customer's own substrate. Without those conditions two laboratories can return different ratings on the same print and both be right.
Compare a retained delivery sample against a bag returned from the yard, in a light booth and not in daylight.
Weigh the film weight of the faded area and compare with the specification.
Confirm whether the overprint varnish was actually applied on that run.

THEN CHANGE

Rebuild the shade on the sack pigment set, phthalocyanines, benzimidazolones, quinacridones, DPP and carbon black, selecting the grades individually. Ratings within these families run from about 6 to 8 in full tone and the rating belongs to the pigment index number, not to the family, so name the chosen pigments and their measured ratings on that shade's own data sheet.
Where a brand shade cannot be matched on grades rating 7 to 8, say so in writing and name the rating on that product's own data sheet before the job runs.
Print at the specified film weight instead of reducing to make the drum go further.
Apply UniFlex-290OG carrying a UV absorber where the exposure warrants it, and specify it rather than assuming it.
Where the reduction is costing the rating, formulate a lighter shade at full tone rather than cutting a strong pigment 1:25.

CONFIRM IT IS FIXED

Filtered xenon arc exposure against the Blue Wool references, with the exposure conditions recorded, showing 7 to 8 in full tone and, at the working reduction, not less than 6 where the full tone rates 8 and not less than 5 where it rates 7, since one to two steps are lost between full tone and a 1:25 reduction. The reduction ratio is recorded alongside the rating and the work is done on the customer's own substrate.

THE TRAP

Promising outdoor life for a shade that was matched on the process set because it matched better in the booth. The match is perfect for a fortnight, the complaint arrives at the end of the monsoon, and by then the whole print run is in the field.

Solvent Based Flexo InksColour fades on the cement bag in the yard (continued)

Fault 18 of 20

Print blushes and goes hazy in humid weather

WHAT YOU SEE

The solid goes milky and the gloss drops, worst on the deep shades and on the varnish, worst at the end of a fast run and worst in the monsoon. Fine pinholes usually come with it. The same ink ran clean in February, so it gets reported as a contamination fault or a bad batch.

WHAT IT LOOKS LIKE

CAUSES, MOST LIKELY FIRST

Water taken up from the hall air by an alcohol-led blendEthanol and n-propanol are fully miscible with water and pull it out of the air in an open pan at 70 to 90 % RH. All shift, in at the top of the pan and never out again.
Evaporative cooling at the plate taking the film below the dew pointA fast blend leaving the film absorbs its latent heat from the film, which drops below the dew point of a humid hall and condenses water into itself before it has set.
NC and PA resin shocked out of solutionNitrocellulose and polyamide tolerate only a few per cent water. Past that the resin comes out of solution as a fine haze, so the film dries cloudy, weak in gloss and weak in strength, and no amount of drying clears it.
Wet reducer or a drum left open in the hallA part-used drum of ethanol standing open in a monsoon hall takes up water on its own, so the fault arrives with the addition rather than with the ink.
Compressed air carrying waterA failed water trap blows wet air straight at a film that is already cold from evaporation, which is the worst possible combination.

Solvent Based Flexo InksPrint blushes and goes hazy in humid weather

Fault 18 of 20 · continued

Print blushes and goes hazy in humid weather

CHECK, IN THIS ORDER

Read hall temperature and RH at the press at the time the fault appears, not at shift start, and note where in the run it started. A haze that arrives in the last two hours is water accumulating, not a bad batch.
Karl Fischer water content to ISO 760 on the pan ink and on the reducer drum, compared with a fresh sealed drum of each.
Draw the pan ink down on clean film in an air-conditioned room. If it dries clear there and hazy at the press, the fault is the hall and not the ink.
Check the compressed air water and oil trap, and blow a sample onto a cold mirror.
Read gloss to ISO 2813 on the hazy area and on a clean area of the same reel, rather than judging milkiness by eye.

THEN CHANGE

Know which blush you have before you correct it. A condensation blush at the plate is corrected by moving a fraction of the blend to UniFlex-296SL, so the film cools less as it dries and stays above the dew point. A pan that has taken up bulk water is corrected by keeping water out, and by dumping the pan; the two corrections are not interchangeable and the slow blend makes the second one worse.
Cover the pans, close the drums, and keep reducer out of the hall and sealed between additions.
Dump a pan whose Karl Fischer water content is above the house limit for the series. Water cannot be driven out of a pan on a running press.
Raise hall dehumidification where the press has it, or drop line speed so the evaporative cooling at the plate is less severe.
Fit and maintain a coalescing water trap on the compressed air, and confirm the dryer supply air is not drawing unfiltered monsoon air straight into the tunnel.

CONFIRM IT IS FIXED

Gloss to ISO 2813 back inside the agreed band and a clean bar drawdown of the same pan ink on the same film, with hall RH, hall temperature and the Karl Fischer water content of the pan recorded against the reel.

THE TRAP

Chasing it with a fresh drum of ink. The water is arriving from the hall, the open pan and the reducer, so the new drum blushes within the hour and the job is two drums down with the hall unchanged. Measure the water before you change the ink.

Solvent Based Flexo InksPrint blushes and goes hazy in humid weather (continued)

Fault 19 of 20

Toluene or a ketone turns up on a toluene-free job

WHAT YOU SEE

The ink was supplied as a toluene-free and ketone-free grade and the customer's own headspace GC on the finished pack reports toluene or MEK at a few mg/m2. Nothing on the ink certificate explains it and the press log shows the correct grade in the pan.

WHAT IT LOOKS LIKE

CAUSES, MOST LIKELY FIRST

Carry-over in the wash-up systemA shared wash tank, recovered wash solvent and a wash pump that has cleaned toluene-bearing jobs put a trace back into the first pan of every clean job that follows.
Hoses, pumps, filters and chambers moved between jobsInk circulation parts hold residue in dead legs and under seals, and a rinse does not empty them. The trace then leaves slowly over the first hour of the next job.
Reducer bought locally against a specification nobody checkedA drum of ethyl acetate or ethanol from a general supplier can carry toluene or acetone as a residue from the supplier's own tankage, and no ink control catches it.
Recovered or reclaimed solvent used as a thinnerDistillation recovers a mixture. What comes back is not what went out, and on a declared-free job it is the one thing that must not go into the pan.
The substrate or the construction, not the inkReprocessed PP tape, printed reclaim in the film, an adhesive tape on the reel, a laminating adhesive or contamination in the finished goods store will all put an aromatic into a pack that was printed cleanly.

Solvent Based Flexo InksToluene or a ketone turns up on a toluene-free job

Fault 19 of 20 · continued

Toluene or a ketone turns up on a toluene-free job

CHECK, IN THIS ORDER

Headspace GC-MS on the finished pack, reporting the species and the quantity in mg/m2 against prepared standards, with the reporting limit of the method stated. A total figure answers nothing here.
Run the same analysis on an unprinted control cut from the same reel and sealed at the same time. If the species is on the plain film, the ink is not the source.
Walk the solvent inventory: every drum in the hall, every gantry can, the wash tank, the recovered solvent tank and the reducer actually in the pan, with a certificate of analysis for each.
Take a swab or a rinse sample from the hose, pump, filter housing and chamber used on the job and run those on GC as well.
Read the ink certificate of analysis and the raw material declaration for the batch that was in the pan.

THEN CHANGE

Dedicate and label wash solvent, wash tanks, hoses, pumps, filters and chambers for the toluene-free and ketone-free work, and stop rotating them through aromatic jobs.
Buy thinners to a written specification with a certificate of analysis for each batch, and refuse recovered or reclaimed solvent on these jobs entirely.
Flush a circulation set that has been on an aromatic job and then run a sacrificial charge through it before the clean job starts.
Where the species tracks to the plain film, take it up with the film supplier with the unprinted control result in hand rather than reformulating the ink.
Keep the claim where it belongs. We can state that a grade is formulated without toluene and without ketones and that its raw materials are selected to the EuPIA Charter. What ends up in the finished pack is a property of the whole construction and the whole press hall, and it is assessed by headspace and migration work on the customer's own pack on request.

CONFIRM IT IS FIXED

Headspace GC-MS on the finished pack showing no toluene and no ketone above the stated reporting limit of the method in mg/m2, with the unprinted control run alongside and the signed solvent inventory for that shift on the record.

THE TRAP

Treating a toluene-free ink as a toluene-free pack. The ink is a formulation; the pack is a construction printed in a hall with its own solvent inventory, its own wash-up and its own thinner supply. An ink certificate on its own answers none of that, and a customer is right to ask for the pack to be tested rather than the drum.

Solvent Based Flexo InksToluene or a ketone turns up on a toluene-free job (continued)

Fault 20 of 20

Print breaks up across the woven PP fabric

WHAT YOU SEE

The print is solid in places and broken in others in a pattern that follows the weave, worst over the tape overlaps and at the selvedge. Reverse type closes on one part of the width and prints thin on another. On tubular fabric the print is sometimes found on the wrong face, and only after conversion.

WHAT IT LOOKS LIKE

CAUSES, MOST LIKELY FIRST

Printing the wrong face of a tubular fabricOnly one face is treated and only one face becomes the outside of the finished sack, and on the reel the two are not always the same face. Nothing on the print tells you until the bag is made up.
Tape overlaps and warp variationThe fabric is not flat. Crossing points sit high and the interstices sit low, so a plate set to kiss on the high points never reaches the low ones and the solid breaks in the gaps.
Treatment applied at the loom days or weeks earlierCorona on drawn PP tape decays like corona on PE. Fabric treated at the loom and then stored is frequently below 38 mN/m by the time it reaches the press, whatever the certificate says.
Fibrillation of the tape surfaceOver-drawn or abraded tape splits into fibrils that stand off the surface. Ink sits on the fibrils rather than on the tape and comes away with them at the first handling.
Anilox and viscosity set for film rather than for fabricA fabric solid needs the film weight of a 100 to 160 l/cm roll at about 14 to 25 cm3/m2 and 20 to 22 s DIN 4 at 25 C. A film roll cannot bridge the weave at any impression.

Solvent Based Flexo InksPrint breaks up across the woven PP fabric

Fault 20 of 20 · continued

Print breaks up across the woven PP fabric

CHECK, IN THIS ORDER

Dyne both faces of the fabric at the press with ISO 8296 test liquids, gloves on, and identify which face becomes the outside of the sack before the first impression is set.
Ask the fabric supplier when the tape was treated and when the fabric was woven, and read the date on the reel.
Look at the fabric under a loupe with raking light for fibrils and for the height of the tape overlaps.
Measure the anilox volume against about 14 to 25 cm3/m2 for fabric coverage, and check press viscosity at 20 to 22 s DIN 4 at 25 C.
Set impression by kiss from zero, then look at where the break-up is. Broken interstices say film weight; broken or haloed overlaps say impression and plate hardness.

THEN CHANGE

Treat in line at the press to 38 to 42 mN/m regardless of what was done at the loom, and confirm the reading on the face you are actually printing.
Move to UniFlex-280WP on a 100 to 160 l/cm roll and run at the top of the viscosity window; fabric takes film weight, not thin ink.
Use a softer plate and a softer mounting tape for a fabric solid so the plate can follow the weave, and keep the hard tape for film work.
Reject fabric that is visibly fibrillated. No treatment and no ink holds onto a surface that is already coming apart.
Where fine reverse type is specified on fabric, agree the minimum size the weave will hold with the designer before origination rather than arguing it at the press.

CONFIRM IT IS FIXED

A full-width fabric solid at running speed with the weave covered and no break-up at the overlaps at a 10x loupe, reverse type open at the agreed minimum size, and the dyne reading on the printed face recorded at the print station.

THE TRAP

Winding impression on to force the ink into the weave. It flattens the tape, spreads the image, prints a halo at every overlap and can emboss the weave into the plate on a thin fabric. The answer is film weight from a coarser anilox and a softer plate, not pressure.

Solvent Based Flexo InksPrint breaks up across the woven PP fabric (continued)

At the press

One page to work from, and the methods behind every number.

Quick reference

One page at the press

Symptom	Check this first	Change this first
Ink lifts with tape on PE	Dyne the reel at the station, edges included	In-line corona to 38 to 42 mN/m, not past 44
Adhesion fine at press, gone in store	Ethanol wipe and re-test the same reel	Low-slip film, or carry slip in the varnish
Reel blocks, face to back	Reel surface temperature and tension in N/m	Rising dryer profile and lower tension
Solvent smell at the customer	Headspace GC on core sample plus plain film control	Extraction and dwell, before the blend
Viscosity climbs all shift	Ink temperature in the tray, cup at 25 C	Cover the pans, dose with 295RD only
Plate fills in, dried ring at the roll ends	Chamber end seals and chamber fill across the width	Replace the seals, then slow the blend with 296SL
2K PU fails resistance	Look in the reducer can for alcohol	Ester-only reducer, dump the pan
Print rubs off the filled sack	Varnish coat weight, five adjacent cut pairs	290OG at 2 to 3 g/m2 dry on sack

Solvent Based Flexo InksQuick reference

Quick reference

One page at the press

Symptom	Check this first	Change this first
Pinholes and fisheyes	Bar drawdown of the pan ink on clean film, first	Drop pump speed, seal the suction, stop the splash
Weak chalky colour on kraft	Anilox volume against 14 to 25 cm3/m2 at 100 to 160 l/cm	270KS on a coarser roll at 20 to 22 s
Second colour will not trap	Dyne the printed first-down solid	Balance interdeck, take slip out of the ink
Streaks down the solid	Step the blade and see if the line moves	Change the blade, filter the return
Halo and heavy dot gain	Reset impression to kiss from zero	Match anilox volume and ruling to the plate
Bags jam or the pallet slides	COF to ASTM D1894 at 24 h and 7 days	Set the slip in the varnish to the line's window
Gloss patchy, varnish mottles	Gloss at 60 degrees, five points across the web	Dry the ink before the varnish, raise coat weight
White prints grey	Drum density against 1.35 to 1.6 kg/l, then dry deposit	Coarser anilox or a double hit, stop thinning

Solvent Based Flexo InksQuick reference

Quick reference

One page at the press

Symptom	Check this first	Change this first
Shade faded in the yard	Pigment index numbers on the TDS for that shade	Rebuild on individually selected 7 to 8 grades
Print blushes and goes hazy	Hall RH now, and Karl Fischer water on the pan	Close the drums and cover the pans; then 296SL
Toluene or ketone on a declared-free job	GC the pack and the plain film control together	Dedicate the wash-up, hoses, pumps and filters
Print breaks up on woven fabric	Which face is treated and which becomes the outside	280WP on 100 to 160 l/cm, softer plate and tape

Solvent Based Flexo InksQuick reference

Test methods

The measurements this guide relies on

Measurement	Method	What it tells you, and the usual acceptance
Flow cup viscosity	DIN 4 mm flow cup to DIN 53211, or the ISO 2431 4 mm cup where stated explicitly	The efflux time of the ink as it will run. The DIN 4 and ISO 2431 4 mm cups are different geometries, with different reservoirs and orifice lengths, and they do not give the same seconds on the same ink, so the cup is named with every figure and the two are never interchanged. Supply 50 plus or minus 10 s for process and line shades, 70 plus or minus 15 s for opaque white and sack solids, 30 plus or minus 5 s for varnishes. Press 16 to 22 s depending on anilox. Sample and cup both at 25 plus or minus 1 C, a house condition set for our press halls rather than the 23 C of ISO 2431 or the 20 C of DIN 53211, so the temperature is quoted with the figure. Orifice checked against a calibration oil or a certified reference cup on a stated interval.
Wetting tension	ISO 8296 test liquids, or ASTM D2578 in the laboratory	Surface energy of the substrate at the moment of printing. 38 mN/m minimum on corona-treated PE and woven PP for surface print, 40 to 44 mN/m where the print will be laminated, and not chased above about 44 because over-treatment builds its own weak boundary layer. Read on the reel about to run, at three points across the width and at both edges. The standard liquids are formamide and 2-ethoxyethanol mixtures and both components are reproductive toxicants absorbed through the skin: nitrile gloves, no skin contact, cap immediately, swabs to solvent waste, and prefer a formamide-free set where one is qualified against ISO 8296. Keep the set in date, because liquids that have aged or taken up water read the film low.
Tape adhesion, straight pull	In-house method using 3M 610 or equivalent tape	Percentage of ink retained after a firm straight pull across a solid, 24 h after printing at 23 C and 50 % RH. Acceptance 95 % or more, repeated at 7 and 28 days on retained samples. ISO 2409 cross-cut is deliberately not used on a 2 um film on PE, because the blade cuts the substrate.
Solvent rub resistance	ASTM D5402, MEK double rubs	Degree of crosslink on a two-component UniFlex-250PU film. Read on the heaviest solid at 24 h and again at 7 days, same pad and same load, because the film develops over 5 to 7 days at 25 C. The difference between the two readings is the evidence that the hardener worked.
Rub and scuff resistance	Sutherland rub to ASTM D5264, 4 lb block, 100 cycles print to print	Abrasion resistance of the finished surface at 23 C and 50 % RH, with the receptor and the print mounted as the method specifies and the result reported both as transfer to the receptor and as change to the printed surface. A varnished surface print typically shows no significant transfer at 100 cycles; an unvarnished one starts to transfer at 25 to 50. Repeat on a filled bag, because a cut sample is not a loaded sack. TAPPI T 830 uses the same tester but is written for containerboard and is not interchangeable with D5264 on film or on woven tape.
Coefficient of friction	ASTM D1894	Static and kinetic friction of the finished surface, print to print and print to reverse, at 23 C and 50 % RH. Measured at 24 h and again at 7 days, because the film's own slip additive keeps arriving at the surface. The acceptance is the bagging line's own working window. Where the line cannot state one, start from kinetic COF of about 0.20 to 0.35 print to print, since below about 0.15 a stacked pallet loses its interlock and above about 0.4 the bags drag on chutes and grippers, then narrow it on a trial and record the agreed window.
Gloss	ISO 2813, 60 degree geometry as the reference	Gloss of the overprint varnish, read at five points across the web and at the start, middle and end of the reel, on printed and unprinted areas separately. Re-read at 20 degrees where the 60 degree value exceeds about 70 gloss units, as on UniFlex-290OG, and at 85 degrees where it falls below about 10, as on UniFlex-291OM matt; 60 degrees does not discriminate outside that middle range, which is where both of our varnishes sit. It is the only reliable way to separate a real mottle from press-hall lighting.

Solvent Based Flexo InksTest methods

Test methods

The measurements this guide relies on

Measurement	Method	What it tells you, and the usual acceptance
Light fastness	Filtered xenon arc exposure, rated against the ISO 105-B02 blue wool references 1 to 8	Outdoor life of the shade. A blue wool rating means nothing without the exposure conditions attached, so the irradiance, the filter, the black standard temperature and the relative humidity are stated with every rating, and the samples are exposed alongside the blue wool references on the same rack. Sack and cement bag shades are built on grades rating 7 to 8 in full tone, selected by pigment index number rather than by family, and a 1:25 reduction costs one to two steps. Rated on the full tone and on the working reduction, on the customer's own substrate, unvarnished unless the varnish is specified.
Residual solvent	Static headspace gas chromatography on a cut sample of the finished reel	Total and individual retained solvent, which is what causes odour, taint and blocking. Reported in mg/m2 against prepared standards and not as area percent, with the conditioning stated, typically 30 to 60 min at 80 to 120 C in a sealed vial. As an order of magnitude to judge a first result against: total retained solvent of the order of 5 mg/m2 or less on food-related constructions and 10 to 20 mg/m2 on non-food sacks, with any individual species limit set by the brand owner applied on top. Sample from the core as well as the outside, and run an unprinted control from the same reel. The binding acceptance belongs to the construction, the line speed and the dryer, so it is confirmed on the customer's own job before it becomes a specification.
Water content of the ink	Karl Fischer titration to ISO 760	Water carried in an alcohol-led NC or PA blend, run on the pan ink and on the reducer drum and compared against a fresh sealed drum. It is the one measurement that separates a monsoon blush from a contamination fault, and it is taken at the time the haze appears rather than at shift start, because an open pan takes up water all shift. The house limit belongs to the series and is a few per cent at most; past it the resin shocks out and the pan is finished.
Solvent identity in the finished pack	Static headspace GC-MS on the finished pack, with an unprinted control from the same reel	Identification and quantification of individual species in mg/m2 against prepared standards, with the reporting limit of the method stated. It is how a toluene-free or ketone-free formulation statement is supported on a real pack, and the plain film control is what separates the ink from the substrate, the adhesive and the store. The statement we make is about the formulation we supply; what is in the pack belongs to the construction and to the press hall, and is assessed on the customer's own pack on request.

Solvent Based Flexo InksTest methods

When the press-floor fixes have not worked

What to send us

A 300 mm square of the unprinted substrate from the same reel, with the reel number, the extrusion or weaving date and which face is treated, plus the dyne readings you took at the press and the date you took them.
A printed sample showing the fault and a printed sample of the same job running correctly, both cut from the reel and both labelled with reel number, deck and time.
The press log for the job: line speed, anilox volume and ruling per deck, plate ruling and durometer, impression settings, viscosity readings against the clock with the cup named, winding tension in N/m, and the dryer temperature at each deck with the extraction setting.
Which reducer was in each pan and every addition made during the run, with times. For a two-component job, the hardener batch number, the weighed dose, the time it went in and the ink temperature.
The ink batch numbers and about 500 g of the actual pan ink, sealed and labelled, taken at the time the fault was running rather than from a fresh drum.
Hall temperature and relative humidity at the press at the time the fault appeared, where the complaint is haze, blushing, pinholing or gloss loss, and the reading at shift start alongside it.
For a toluene-free or ketone-free complaint, the signed press-hall solvent inventory for that shift: every drum, gantry can, the wash tank and the recovered solvent tank, with certificate of analysis references, plus a sample of the reducer actually in the pan.
For an adhesion, rub, friction or fade complaint, retained samples at 24 h, 7 days and 28 days where they exist, plus the failed material returned from the customer, and a statement of how the pack is filled, stacked, stored and transported.
For an odour, taint or blocking complaint, a sealed cut sample from the reel core in a clean glass jar with a foil-lined cap, taken before the reel is disturbed, together with an unprinted control from the same reel sealed in the same way at the same time, so headspace GC can be run on material that has not been aired and the substrate can be ruled in or out.

Technical servicesales@ufindustries.com | +91 8750051884 | E105, GNEPIP, Industrial Area Site-5, Greater Noida, Uttar Pradesh 201306, India

Solvent Based Flexo InksTechnical service