Screen Printing Defect Atlas
Twenty-seven failure modes: morphological identification, mechanism attribution and corrective action — each documented with a structural figure
Abstract
The efficiency of defect disposition is governed by the accuracy of attribution, not by the speed of parameter adjustment. A single surface symptom may correspond to several distinct mechanisms: a white spot on the printed area may originate from a plugged mesh opening or from particulate contamination on the substrate; a directional streak may originate from a nicked squeegee edge or from a local protrusion on the platen. Misattribution directs adjustment to the wrong variable; the process window is then compressed by successive trials until the operation becomes unrecoverable.
This manual classifies field defects of screen printing into six causal domains comprising twenty-seven failure modes: screen and stencil, ink film and transfer, registration and geometry, curing and appearance, substrate and downstream processes, and process-systemic effects. Each entry follows a fixed six-part structure: phenomenon, structural figure, mechanism of failure, criteria and localization, corrective action, and recurrence control. Figures are two-panel: the left panel is a magnified specimen representing the morphology of the defect; the right panel is a mechanism section or causal schematic representing its generation path. Morphology supports grading and disposition; the generation path determines the object to which corrective action is applied.
All numerical values are industry experience ranges, intended to establish direction only. Production parameters must be fixed after first-article inspection (FAI) and calibration against in-house measurement. Chapter 8 provides the four-step disposition sequence (contain, localize, correct, prevent) and a twenty-seven-row lookup matrix, suitable for posting at the machine as an annex to the work instruction.
Figure convention: circled numerals identify locations within the drawing and correspond one-to-one to the ① ② ③ markers in the caption. No Chinese or English text is set inside the figures, so that no label falls below the minimum type size on mobile displays; the caption serves as the legend and as the basis for interpretation. The access log at the foot of this page is retained in Chinese to preserve record continuity across both language versions.
1. Basis of judgement: deviation, defect and variation
Without a defined basis of judgement, the term “defect” is not operational. The same colour deviation is acceptable on an advertising banner and constitutes a critical defect on a medical device fascia. The first chapter of a defect manual must therefore establish the acceptance basis rather than enumerate symptoms.
1.1 Three classes of departure and their disposition routes
Process deviation denotes a controlled parameter outside its validated window — temperature, tension, viscosity, speed. Its route is parameter restoration; product disposition is not involved. Product defect denotes output that fails the acceptance criteria — voids, misregistration, adhesion failure. Its route is containment, rework or concession, the latter requiring approval. Process variation denotes natural dispersion within a controlled state, such as ±2 μm film thickness or ΔE 0.5; its route is monitoring and capability assessment, and it is not a legitimate object of corrective action.
Conflation of the three produces two systematic losses. Treating variation as defect inflates inspection and rework cost. Treating defect as variation releases nonconforming product to the customer. Treating process deviation as product defect removes the output without removing the cause, and the same mechanism recurs in subsequent lots.
1.2 Three fixed conditions of inspection
Disputes over colour and appearance arise predominantly from inconsistent viewing conditions rather than from the product. Three conditions shall be fixed. Illumination: a standard light source with controlled correlated colour temperature and colour rendering index; shop lighting or side-window daylight shall not be used as the basis of colour judgement. Geometry: colour difference is normally assessed at 45° incidence and 90° observation; gloss and orange-peel are assessed at a fixed low grazing angle. Distance and duration: constant viewing distance and constant observation time, to avoid drift in judgement as observation is prolonged.
Where these conditions are not fixed, the result is not reproducible and the inspection record loses traceability.
1.3 Grading of defects
A four-level scheme is recommended. Critical: affecting safety, function or regulatory marking; zero acceptance. Major: discernible within the customer’s normal field of view, or affecting assembly and downstream processes. Minor: discernible only at close range, without functional effect. Cosmetic: appearance only; concession may be granted by area. Grading converts concession from a negotiated favour into a governed decision.
1.4 Mandatory elements of a defect record
A defect record shall contain five elements: (1) location — artwork coordinates, lot, machine, shift; (2) morphology — size, density, spatial distribution (continuous, periodic, random); (3) onset — first-article failure or mid-lot excursion, together with any change in the interval (ink, screen, squeegee, substrate lot); (4) eliminations — substitution trials performed and their conclusions; (5) disposition — parameters changed, sample size verified, approver.
Only with all five elements can a defect be closed. Where any element is absent, the same mechanism will recur in a later lot.
2. Screen and stencil failures
The stencil process sets the upper bound of printing accuracy. Failures in this domain share a common signature: the failure originates in stencil making and manifests during printing. Attribution should therefore begin at the screen and proceed towards the machine.
2.1 Mesh plugging: complete loss of ink transfer, mesh mark retained
Phenomenon A local area of the print shows no ink, with an irregular boundary. Backlit inspection of the screen shows the openings filled and opaque.
Mechanism Resin and filler in the ink form a film within the opening and close it; alternatively, incomplete reclaim leaves residue accumulating on the mesh wall. Drying rate, idle time and ambient temperature are the dominant factors.
Criteria and localization (1) Inspect the opening under magnification: a filled opening retaining mesh structure is mesh plugging, distinguished from a pinhole by the presence of mesh residue. (2) Wipe locally with reclaim solvent and reprint; disappearance confirms dried ink rather than emulsion delamination. (3) Retrieve idle-time and reclaim records to separate retention drying from incomplete cleaning.
Corrective action Clean with reclaim solvent and a soft brush, working along the mesh wall; hard tools are prohibited as they damage the emulsion. For persistent plugging, soak in a dedicated screen cleaner and rinse at high pressure. Where the plugged area exceeds 5% of the image area and cleaning fails, strip and re-make the screen rather than running a degraded stencil in production.
Recurrence control Return ink or seal the screen after a defined idle interval (commonly 10–15 min); perform the three-step reclaim sequence (solvent flush, brush, dryness check) at every colour change; filter ink before use to control pigment agglomerates and skin.
2.2 Pinholes: sharp-edged white spots, no mesh mark
Phenomenon Fine white spots scattered across the printed area, typically 0.2–2 mm, with clean edges and substrate visible at the bottom.
Mechanism Three independent sources. Stencil voids: bubbles or particles introduced during coating leave the emulsion uncrosslinked, and development opens a through-channel. Substrate contamination: silicone oil, release agent or dust lowers local surface energy, preventing wetting. Ink entrainment: gas introduced by circulation or agitation is not de-aerated before printing, and the ruptured bubble does not level.
Criteria and localization (1) Reprint the same artwork with a newly made screen; disappearance attributes the fault to the stencil. (2) Reprint on cleaned substrate from the same lot; disappearance attributes the fault to the substrate. (3) Allow the ink to stand for de-aeration and re-test; disappearance attributes the fault to the ink.
Recurrence control Allow coated screens to stand before exposure; fit filtration (commonly 100–150 mesh) to the ink circuit and limit return-ink drop height; remove dust before printing and apply corona, flame or plasma treatment to low-energy substrates.
2.3 Edge serration: periodic loss at the image boundary
Phenomenon Regular serration along line and text edges, most pronounced on fine lines; under magnification the boundary recedes into the image.
Mechanism During exposure, UV diffracts in the air gap between film and emulsion. The marginal emulsion receives a partial dose and remains under-crosslinked; development removes it, producing an undercut edge. The root cause is poor contact: insufficient vacuum, film curl, or particles on the mesh surface.
Criteria and localization (1) Verify vacuum gauge reading and evacuation dwell against the process specification. (2) Re-expose with flat film and additional weighting; observe whether serration converges. (3) Verify development pressure; excessive pressure removes partially crosslinked margins and amplifies serration.
Corrective action Increase vacuum and extend evacuation dwell; for line widths below 0.15 mm use a collimated or point-source exposure unit; develop with reduced pressure and shorter duration, adopting the conservative principle of under-development rather than over-development.
2.4 Emulsion delamination: sheet peeling with exposed mesh
Phenomenon After a number of impressions, a large area of the image abruptly loses ink; backlit inspection shows exposed mesh with a sheet-like fracture edge.
Mechanism Insufficient anchorage between emulsion and mesh. Contributing causes include excessive tension causing mesh deformation, insufficient coating thickness, underexposure producing low crosslink density, and continuous swelling of the emulsion by ink solvent. The location of delamination is diagnostic: separation along the squeegee path indicates excessive squeegee pressure or solvent attack; separation at the corners indicates excessive tension.
Criteria and localization (1) Re-measure screen tension against the tensioning record. (2) Verify coating passes and dry-film thickness. (3) Verify the solvent system of the ink lot against the solvent resistance class of the emulsion.
Corrective action For immediate containment, apply an additional coating and re-expose. The fundamental remedy is selection of a solvent-resistant emulsion, tension control within the elastic range of the mesh, and exposure dose sufficient for the required crosslink density.
2.5 Tension decay: registration drift accumulating with impressions
Phenomenon The first article registers correctly; deviation increases monotonically with impression count. Deviation is greater when a screen has been stored for several days before use.
Mechanism Polymeric mesh creeps under load. The higher the initial tension and the closer to the elastic limit, the faster the stress relaxation. Thin-wall aluminium frames, aged stretching adhesive and humidity fluctuation accelerate decay. Falling tension elongates the screen; image dimensions increase and registration acquires a systematic bias.
Criteria and localization (1) Re-measure tension at the same point and temperature and compare with the tensioning record. (2) Monotonic increase with impressions indicates tension decay; irregular jumps indicate mechanical or locating-system faults. (3) Verify screen age and storage conditions.
Corrective and preventive action Tension with a settle-and-retension cycle: reach the set value, allow to settle, retension on relaxation, repeat two to three times, then fix with adhesive. Maintain a screen ledger recording every re-measurement; downgrade or scrap on exceeding tolerance. Avoid stacking and direct light exposure in storage.
3. Ink film and transfer failures
Ink film formation in screen printing is the forced transfer of an ink body through the mesh openings under squeegee pressure. Failures in this domain therefore converge on three governing variables: theoretical ink release (open area and emulsion thickness), transfer efficiency (squeegee angle, pressure, snap-off distance and release speed), and levelling behaviour (rheology and ambient conditions). Adjustment shall follow the same order: confirm the release capacity of the screen, then the transfer condition, then the rheology. Reversing the order allows the variables to mask each other, and the effect of each adjustment cannot be attributed.
3.1 Thin ink film: insufficient colour density, reduced opacity
Phenomenon Hue appears lighter and saturation lower; substrate colour shows through on dark stock; local areas fall below the surrounding density within the same lot.
Mechanism Ink release per unit area falls below the lower process limit. Theoretical release is approximately proportional to open area multiplied by mesh thickness, and actual transfer is that value multiplied by transfer efficiency. Excessive mesh count, excessive emulsion thickness (reducing effective opening depth), small squeegee angle or excessive pressure (ink removed rather than forced through), excessive snap-off (ink retracts before release), and high viscosity all reduce deposited film.
Criteria and localization (1) Determine ink weight per unit area gravimetrically or film thickness by gauge, and compare with the reference sample to separate global from local thinning. (2) Global thinning attributes to mesh count, squeegee angle and pressure. (3) Local thinning requires verification of local plugging and platen flatness.
Corrective action Reduce mesh count or reduce emulsion thickness to raise open area; restore squeegee angle to the conventional range (approximately 60°–75°); adjust viscosity in accordance with the ink supplier’s technical data sheet, and do not use thinner as a routine measure.
3.2 Heavy ink film: edge build-up with incomplete cure
Phenomenon Film thickness exceeds the upper limit, with a raised bead at the image boundary; blocking occurs after stacking or under load; sectioning reveals uncured ink within the film.
Mechanism Release and transfer efficiency are both excessive: low mesh count, large open area, low viscosity, small snap-off, low squeegee hardness or small squeegee angle. When deposited volume exceeds drying capacity, the surface skins over and solvent escape is blocked, producing a dry surface over a wet interior.
Criteria and localization (1) Quantify the excess by film gauge or cross-section. (2) Verify that mesh count and emulsion thickness match the accuracy class of the product. (3) Verify drying capacity against the deposited volume.
Corrective action Increase mesh count, increase viscosity, increase squeegee angle, increase snap-off moderately. Extended baking shall not be used as a remedy for finished parts: the surface is already sealed, and further heating traps solvent, raising the risk of cracking and blistering downstream.
3.3 Directional streaking: density bands parallel to the squeegee stroke
Phenomenon One or more bands parallel to the print direction, of constant width and stable position, appearing as light or dark bands.
Mechanism A nick, burr or trapped dried ink on the squeegee edge prevents effective compaction at that location, locally reducing transfer and producing a skip band. Secondary causes are out-of-flatness of the blade holder, local loss of screen tension, and local protrusion on the platen.
Criteria and localization (1) Assess positional stability: a fixed band attributes to the squeegee or screen; a band that migrates with impression count attributes to substrate or platen. (2) Substitute a squeegee and reprint; disappearance confirms an edge defect. (3) Inspect the edge under oblique light for nicks or discontinuous reflection.
Corrective action Grind or replace the blade; re-level the holder; remove trapped ink with solvent. Increasing pressure is not a corrective action: higher pressure makes the skip at the nick more pronounced.
3.4 Orange peel: micro-topography of the ink surface
Phenomenon A fine stippled surface resembling orange peel; inconspicuous at normal incidence but pronounced under low-angle grazing light.
Mechanism Mismatch between surface tension and internal flow of the wet film. When solvent evaporates too rapidly, the surface skins over while the interior continues to contract, producing regular undulation. High viscosity, elevated ambient temperature, direct air impingement on the printed surface and thin films all accelerate the effect.
Criteria and localization (1) Trial print with a slower solvent. (2) Reprint with the part removed from the air stream. (3) Increase film thickness moderately and observe improvement.
Corrective action Introduce a slow solvent and add a levelling aid per supplier recommendation; control air velocity and temperature; where necessary select an ink system with superior levelling.
3.5 Bubbles and craters: volcano-like depressions
Phenomenon Circular depressions with raised rims and exposed substrate at the centre; dense occurrence produces a matte, pitted area.
Mechanism Two independent mechanisms. Bubble rupture: entrained gas escapes, and the ink does not flow back in time; depressions are randomly distributed with a thickened rim. Poor wetting: local substrate surface energy is below the ink surface tension and the ink contracts; depressions cluster in one region and correlate with substrate treatment condition.
Criteria and localization (1) Classify by spatial distribution: random indicates entrained gas; clustered indicates contamination or non-uniform treatment. (2) De-aerate the ink and re-test. (3) Clean or corona-treat the substrate locally and re-test.
Corrective action Filter and de-aerate the ink; avoid large drop heights in the return circuit; clean and de-oil substrates and apply corona, flame or plasma treatment to low-energy materials; add defoamer within the recommended ratio, since excess induces cratering.
3.6 Bleeding and wicking: feathered, widened edges
Phenomenon Edge sharpness is lost and a feather-like spread appears outside the image; fine lines thicken and small text loses legibility.
Mechanism Capillary penetration of ink into or across the substrate. Low viscosity, small snap-off, excessive squeegee pressure (ink forced beyond the stencil boundary), porous substrates (paper, textile, untreated plastics) and slow drying all amplify the effect.
Criteria and localization (1) Reprint with a higher mesh count to reduce deposit and observe edge recovery. (2) Trial print at increased viscosity. (3) Compare with a denser substrate of the same material.
Corrective action Increase viscosity, reduce squeegee pressure and increase snap-off moderately; seal porous substrates with a primer; for fine-line work use a high mesh count combined with a fast-drying ink.
4. Registration and geometry failures
Registration error is commonly attributed to insufficient machine accuracy, yet statistically this is the least frequent cause. Most registration failures originate in inconsistency between screens (tension, deformation, artwork compensation) and in dimensional stability of the substrate. Repeat positioning accuracy of the machine is typically in the micrometre to ten-micrometre range and is the most stable element of the process; it should not be the first object of investigation.
4.1 Misregistration: geometric offset between colour layers
Phenomenon A later colour is offset relative to an earlier one and registration marks do not coincide; the offset is uniform across the sheet or varies as a gradient.
Mechanism Four sources. Inconsistent screen condition: differing tension produces differing elongation. Worn locating datum: clearance increases at pins and stops. Substrate dimensional change: paper absorbs moisture or film is heated. Machine repeatability: guide clearances and servo reversal error.
Criteria and localization (1) Classify by offset pattern: uniform and unidirectional indicates datum or machine; gradient indicates screen deformation or substrate stretch. (2) Print registration marks with each colour separately and measure actual coordinates to build a compensation matrix. (3) Compare with a new screen of the same specification to identify screen-to-screen variation.
Corrective action Standardize and record tension across colour screens; replace worn locating components; pre-condition or thermally stabilize the substrate; apply reverse compensation at the artwork stage and fix the compensation values in the film data.
4.2 Ghosting: secondary image adjacent to the primary
Phenomenon An equispaced pale duplicate appears beside the primary image; the spacing is either constant or random.
Mechanism At snap-off, the ink body is drawn into filaments; on rupture part of the ink returns and deposits beside the primary image. Excessive snap-off, an unsuitable release-speed profile, insufficient ink cohesion (low viscosity), squeegee angle mismatch and repeated strokes at the same position are the inducing conditions.
Criteria and localization (1) Measure the spacing between secondary and primary image: constant spacing indicates a mechanical condition (snap-off, release angle); random spacing indicates filament rupture. (2) Trial print with a single-direction stroke. (3) Trial print at increased viscosity.
Corrective action Reduce snap-off, optimize the release-speed profile, increase ink cohesion; avoid repeated strokes at the same position; reduce print speed where necessary to improve transfer.
4.3 Geometric distortion: stretch and trapezoidal deformation
Phenomenon Circles become ellipses and rectangles become trapezoids; web material elongates in the machine direction and exhibits wavy edges across the width.
Mechanism Elastic or plastic deformation of the material under tension or heat. Excessive unwinding tension, elevated dryer temperature, high thermal shrinkage of the material and insufficient compensation for curved-surface development are the principal causes.
Criteria and localization (1) Measure the same datum before and after printing and compute the strain and its direction. (2) Reduce tension or temperature and re-measure: reversible deformation is elastic, irreversible is plastic. (3) Compare material lots to assess incoming stability.
Corrective action Apply closed-loop zone tension control; reduce dryer temperature or residence time; apply pre-compensation at the artwork stage based on measured strain; retain lot samples and test shrinkage on receipt.
4.4 Moiré: interference pattern in halftone areas
Phenomenon A periodic light and dark pattern in halftone areas, appearing as scales or waves, with an apparent shift as the viewing angle changes.
Mechanism Interference occurs when two periodic structures have closely spaced spatial frequencies. The warp and weft of the mesh form one fixed frequency; halftone angle and line ruling form the other. Interference is greatest when mesh count and line ruling are integer multiples, or when the halftone angle approaches 0° or 90°.
Criteria and localization (1) Reprint with a different mesh count; a change in pattern confirms interference. (2) Adjust the halftone angle and re-output the film. (3) Determine whether the effect is confined to one colour, which has the highest probability of conflict with the mesh direction.
Corrective action Angle the halftone away from the warp and weft (approximately 45° is conventional in screen printing, with separation between colours); maintain a non-integer relationship between mesh count and ruling, with ruling empirically not exceeding one quarter of the mesh count; use frequency-modulated screening for high-definition work.
5. Curing and appearance failures
Curing is a window of energy rather than a single set point. Below the lower limit, crosslink density is insufficient and the outcome is adhesion failure and blocking. Above the upper limit, crosslink density becomes excessive or the resin degrades, producing embrittlement, yellowing and a reduction in adhesion strength. The width of the window is determined jointly by the ink system, substrate heat tolerance and equipment capability, and can only be established by a gradient trial; parameters from another plant are not transferable.
5.1 Adhesion failure: interfacial separation
Phenomenon Cross-cut or tape peel removes the film in sheets; the fracture surface is clean and the substrate is exposed.
Mechanism Adhesion strength is governed by interfacial bonding. Low substrate surface energy (PP, PE, untreated PET), residual oil or release agent, incompatible ink system, undercure, or excessive film thickness generating internal stress can all reduce interfacial strength. The fracture location is diagnostic: separation at the ink–substrate interface indicates wetting and interfacial bonding; separation within the film indicates cohesion and degree of cure.
Criteria and localization (1) Perform cross-cut plus tape peel; record fracture location and percentage removed. (2) Re-test after corona, flame or plasma treatment. (3) Compare with an ink system specified for the substrate.
Corrective and preventive action Surface treatment to raise surface energy and cleanliness; select a matching ink system and confirm operation at the mid-point of the curing window; hold film thickness within the supplier’s recommended range; complete adhesion, flex and solvent resistance verification before series production.
5.2 Undercure: dry surface over an uncured interior
Phenomenon The surface is touch-dry but smears on wiping; blocking occurs after stacking; residual solvent odour is detectable in section; tape testing removes ink.
Mechanism Energy or time is insufficient for complete crosslinking and solvent evolution. Excessive film thickness, low oven temperature, short residence time, insufficient UV energy density or lamp ageing, and shadowing on three-dimensional parts all produce a skinned surface over an uncured interior.
Criteria and localization (1) Finger pressure combined with a solvent-moistened swab; assess loss of gloss and colour removal. (2) Record the oven profile with a data-logging probe, or measure UV energy density with a radiometer. (3) Reduce film thickness or line speed and re-test, to distinguish insufficient energy from excessive deposit.
Corrective action Increase energy or extend residence time, but only after a gradient trial confirms the upper limit has not been approached; measure UV energy at defined intervals and replace aged lamps; apply separate power or zoned drying to heavy-ink areas.
5.3 Overcure: embrittlement, cracking and yellowing
Phenomenon The film becomes brittle and cracks in fine lines on folding or die-cutting; light colours yellow; adhesion strength decreases.
Mechanism Excessive crosslink density or thermal degradation of the resin, caused by excessive temperature, prolonged dwell, UV overexposure or excess catalyst. Over-crosslinking removes the capacity for deformation, and cracking initiates at stress concentrations, with the fracture mode changing from ductile to brittle.
Criteria and localization (1) Perform fold and die-cut tests and observe whether cracks follow stress lines. (2) Compare samples cured at different settings within the same lot. (3) Review recent changes to temperature, line speed and lamp condition.
Corrective action Return the process point to the mid-range of the curing window and avoid sustained operation at the upper limit; select a more flexible ink system for parts requiring subsequent forming; construct a curing-window chart (energy density against adhesion and flexibility) and mark the process point in the central region.
5.4 Colour difference: quantified assessment in place of visual judgement
Phenomenon Hue, lightness or chroma deviate within a lot or between lots; the deviation is visually discernible and is rejected by the customer as lot-to-lot inconsistency.
Mechanism Five governing variables: ink lot (pigment concentration and dispersion), film thickness (directly controlling density), curing temperature (heat-sensitive pigments shift colour), substrate colour and opacity, and measurement conditions (illuminant, geometry, background).
Criteria and localization (1) Measure ΔE with a spectrophotometer under fixed illuminant and geometry, against the reference sample. (2) Where ΔE is acceptable but the customer does not accept, the cause is viewing conditions; standardize the comparison environment. (3) Where film thickness is normal but hue is offset, verify ink lot and curing temperature.
Corrective action Establish ink lot retention samples and incoming inspection; bring film thickness under process control, since density is governed primarily by thickness rather than pigment addition; perform colour assessment in a standard viewing booth, never against window daylight.
ΔE tolerance shall be written into the acceptance standard and shall not be negotiated at the machine. A workable grading: strict appearance parts ≤1.5, general industrial parts ≤2.5, non-appearance functional areas may extend to 4 or above. The specific value is set jointly by product positioning and customer requirement.
5.5 Non-uniform gloss: localised specular variation
Phenomenon Under low-angle grazing light, patchy gloss differences appear on a single plane; the effect is inconspicuous at normal incidence.
Mechanism Gloss is governed by surface micro-roughness. Non-uniform film thickness, insufficient levelling, uneven dryer air distribution, local mesh plugging, and uneven calendering or lamination tension all alter local roughness and therefore the specular component.
Criteria and localization (1) Measure gloss at multiple points at a fixed angle and plot the distribution. (2) Verify local plugging and platen flatness at the corresponding positions. (3) Re-test with the air supply reduced or reconfigured.
Corrective action Equalize film thickness (screen and squeegee), improve levelling (slow solvent or additive), optimize the dryer air field to avoid direct impingement; where matt and gloss parts are produced in sequence, fix the viewing angle for first-article approval.
6. Substrate and downstream failures
Failures in this domain share a common signature: the root cause lies outside the printing process while the symptom appears within it. The physical state of the substrate determines whether ink can adhere, and the conditions of downstream processes determine whether the printed result survives. Investigation must therefore extend to incoming inspection and downstream parameters, and shall not be confined to the press.
6.1 Substrate distortion: paper cockle and film curl
Phenomenon Paper edges undulate and film curls; registration offset follows the distortion direction as a whole.
Mechanism Paper fibres swell on absorption or contract on desorption; internal stress in film releases on heating; stacking pressure and local humidity or temperature gradients produce non-uniform deformation.
Criteria and localization (1) Measure flatness and moisture content of the lot before printing. (2) Record workshop temperature and humidity, and the storage position of the material (against a wall, near a window, on the floor). (3) Pre-condition or flatten the stock before the trial print and observe whether distortion converges.
Corrective action Bring material into the workshop environment at least 24 hours before use; hold temperature and humidity within the process range (paper commonly 50%–60% RH); reduce dryer temperature to the material’s tolerance; avoid single-point suction that locally stretches thin material.
6.2 Scratching and scuffing: film cut through
Phenomenon Linear bright marks appear, with a V-shaped groove in section, exposing the substrate at greater depths; when occurring in batches, the direction of the marks is highly consistent.
Mechanism Friction in handling, hard contamination on platens or guides, relative sliding between stacked sheets, and mechanical contact in cutting or die-cutting. The direction of the marks indicates process flow and is a reliable traceability clue.
Criteria and localization (1) Sample progressively along the process sequence to identify the first station at which marks appear. (2) Examine the groove under magnification for embedded foreign matter. (3) Inspect contacting components at that station (brushes, stops, belts).
Corrective action Line contacting surfaces with soft material, remove contamination, interleave sheets or apply protective film; grade scratched product against the acceptance criteria; solvent wiping shall not be used to conceal surface damage.
6.3 Static attraction: particulate pickup and discharge damage
Phenomenon Fine white spots and fibrous contamination scattered randomly across the print; on film substrates, double feeds, clinging and registration drift also occur.
Mechanism Friction or separation charging under low humidity creates an electrostatic field that attracts airborne particles; accumulated charge discharges and may perforate a thin ink film, producing pinholes.
Criteria and localization (1) Measure relative humidity and substrate surface resistance. (2) Operate the static eliminator and re-test to see whether the spots disappear. (3) Verify the effectiveness of the dust removal device.
Corrective action Install ionizing bars or static eliminators with periodic verification; hold relative humidity at 50%–60% RH; ensure reliable machine earthing; fit a dust-removal roller at unwind.
6.4 Downstream cascade: four recurring disputes
Phenomenon The printing process passes full inspection, yet the downstream process fails in batches, and responsibility is contested between processes.
Mechanism and action (1) Foil adhesion failure (marker 1): excessive film thickness or low surface energy prevents the foil adhesive from keying — reduce film thickness, raise surface energy, and run a gradient trial on temperature, pressure and dwell. (2) Emboss cracking (marker 2): brittle film and short paper fibre — select a flexible ink, keep cure below the upper limit, and route the crease away from the thickest ink. (3) Die-cut burr (marker 3): blunt blade, insufficient pressure, low paper moisture — grind the blade, add make-ready, recondition the stock. (4) Lamination blistering (marker 4): residual solvent in the film releases after lamination — dry fully before laminating, or change to a solvent-free adhesive system.
Recurrence control Downstream parameters shall be confirmed jointly with printing parameters. Whenever the ink system or substrate changes, complete a combined print-and-finish trial before scheduling production.
7. Process-systemic problems
The failures described above are persistent and their attribution path is definite. The problems that genuinely consume engineering resource are low-frequency and non-reproducible: the root cause is not a component but a drift in process capability. Disposition does not rely on part replacement but on data — records indexed by time, by position and by parameter.
7.1 Intermittent defects: distinguishing random scatter from periodic recurrence
Phenomenon Defects occur intermittently within a lot, without fixed position; conventional investigation yields no conclusion.
Criteria and localization Plot defective units against production sequence (the abscissa in the figure is time or piece index). Random scatter (left) shows no fixed interval and points to incoming-material variation, environmental change or operator difference. Periodic recurrence (right) points to a mechanical cycle. For periodic failure, compute the period as interval count multiplied by cycle time and compare it with the circumference of each rotating element — roller circumference, screen circumference, index-table pitch, squeegee stroke — which usually localizes the object immediately.
Disposition route For random patterns, investigate by layer: man, machine, material, method, environment. For periodic patterns, compute the period first and then examine the corresponding rotating element, avoiding undirected part replacement.
7.2 Mid-lot drift: first article acceptable, process out of limit
Phenomenon The first article passes all criteria; after several hundred impressions a key characteristic exceeds its limit, and the offset continues in one direction.
Mechanism Four cumulative effects: tension decay enlarges image dimensions progressively; viscosity rise from solvent loss thins the film and reduces density; squeegee wear rounds the edge and increases transfer; thermal distortion of frame and guides under continuous operation. The resulting offset is monotonic (left), as distinct from the step change produced by a lot change (right).
Corrective action Establish a process control chart, sampling key dimensions and colour difference at fixed intervals and plotting the trend; apply parameter compensation before the trend reaches the control limit (retension, replenish solvent, rotate the blade); record the drift rate in the process card so that the compensation interval is consistent across shifts.
7.3 Lot-change failure: incoming properties change, parameters do not follow
Phenomenon With the same machine and the same parameters, a change of material lot produces a level shift across the entire batch.
Mechanism Key incoming properties have changed: ink absorption and moisture content of paper, surface energy and thickness of film, pigment concentration and viscosity of ink. Existing parameters constitute the answer to the previous lot, not a process guarantee for the current one.
Corrective action Incoming inspection shall verify key properties (surface tension, thickness, moisture content, viscosity, hue); each lot change requires first-article approval and a pre-production trial; retain a sample of every lot so that disputes are resolved by reproduction rather than recollection.
8. Disposition sequence and lookup matrix
8.1 Four-step sequence: contain, localize, correct, prevent
- Contain Define the suspect scope, covering all work in progress and stock between the last confirmed acceptance and the discovery of the defect, and identify it physically. Control outflow before investigating cause; reversing the order allows nonconforming product to continue to escape.
- Localize Narrow the scope by substitution trials to one of six elements: screen, ink, substrate, machine, environment, operator. Change one variable at a time and reprint immediately; changing several variables simultaneously makes the result non-attributable.
- Correct Apply action to the confirmed root cause and verify: continuous production of not fewer than 30 pieces, fully acceptable, before the action is deemed effective.
- Prevent Fix the effective action in three places: the parameter clauses of the work instruction, the items of the check sheet, and the inspection items of first-article approval. Corrective action that is not institutionalised does not remain effective.
8.2 Lookup matrix
| Symptom | Primary attribution | Rapid verification | Entry |
|---|---|---|---|
| Local total loss of ink, mesh mark in opening | Mesh plugging | Local wipe with reclaim solvent, reprint | 2.1 |
| Fine sharp white spots, no mesh residue | Pinhole (void, dust, gas) | Reprint with a new screen | 2.2 |
| Periodic serration on line edges | Poor film–emulsion contact | Verify vacuum and evacuation dwell | 2.3 |
| Sheet delamination after hundreds of prints | Emulsion delamination | Re-measure tension, check coating passes | 2.4 |
| Registration offset increasing with prints | Tension decay | Re-measure tension at same point and temperature | 2.5 |
| Low density, substrate showing through | Thin ink film | Gravimetric ink weight per unit area | 3.1 |
| Edge bead, blocking on stacking | Heavy ink film | Section film thickness and cure state | 3.2 |
| Bands parallel to print direction | Squeegee edge defect | Substitute squeegee and reprint | 3.3 |
| Fine stipple on ink surface | Orange peel (poor levelling) | Trial print with slow solvent | 3.4 |
| Circular craters with raised rims | Entrained gas or poor wetting | Classify distribution: random or clustered | 3.5 |
| Feathered, widened edges | Bleeding (low viscosity) | Trial print at increased viscosity | 3.6 |
| Geometric offset between colours | Screen inconsistency or worn datum | Classify offset: uniform or gradient | 4.1 |
| Secondary image beside primary | Filament rupture at release | Measure stability of image spacing | 4.2 |
| Stretched or trapezoidal image | Tension or thermal shrinkage | Measure datum before and after printing | 4.3 |
| Periodic pattern in halftone areas | Moiré (angle interference) | Reprint with a different mesh count | 4.4 |
| Tape peel removes film in sheets | Adhesion failure | Determine fracture location | 5.1 |
| Smears on wiping, solvent odour | Undercure | Solvent-moistened swab test | 5.2 |
| Cracking on folding | Overcure (embrittlement) | Compare cure settings | 5.3 |
| Colour shift between lots | Ink lot or film thickness | Spectrophotometric ΔE | 5.4 |
| Localised gloss variation | Film thickness or air field | Multi-point gloss measurement | 5.5 |
| Paper cockle, film curl | Moisture or heat distortion | Measure moisture and flatness before printing | 6.1 |
| Linear bright marks, consistent direction | Handling scratch | Progressive sampling along the process | 6.2 |
| Particulate pickup, double feeds | Static attraction | Re-test with ionizer in operation | 6.3 |
| Print passes, finishing fails | Downstream cascade | Combined print-and-finish trial | 6.4 |
| Intermittent, no fixed position | Random or periodic pattern | Plot against production sequence | 7.1 |
| First article good, drift over the lot | Cumulative drift | Trend chart against control limits | 7.2 |
| Level shift on lot change | Incoming property change | Incoming verification of key properties | 7.3 |
8.3 Five recurring misjudgements
- Treating a pinhole as mesh plugging. Both present as white spots, but one requires reclaim and the other requires environmental and ink control; the criterion is whether mesh structure remains within the opening.
- Increasing pigment in response to low density. Density is governed primarily by film thickness; adjusting the formulation before thickness is under control destroys traceability of subsequent lots.
- Attributing registration error to machine accuracy. The offset pattern must be classified first: uniform and unidirectional indicates a datum problem, gradient indicates deformation. Machine repeatability is usually the most stable element of the process.
- Raising squeegee pressure in response to directional streaking. Higher pressure makes the skip at the edge defect more pronounced; the correct action is grinding or replacement.
- Extending baking in response to heavy film. The surface is already sealed; further heating retains solvent within the film and markedly increases the probability of cracking and blistering downstream.
8.4 Daily check items (recommended)
From the twenty-seven failure modes, the control points with preventive value can be compressed into six items completed within five minutes before start-up: (1) screen tension spot check, same point and temperature, value recorded; (2) oblique-light inspection of the squeegee edge; (3) ink viscosity and temperature; (4) workshop temperature and humidity, and static-eliminator status; (5) full first-article approval including a downstream trial piece; (6) verification of the previous lot’s retention sample and the state of the containment area.
All values stated in this manual are industry experience ranges, intended to establish direction. Before any product is released to production, the equipment manual, the ink and substrate suppliers’ technical data sheets (TDS) and the first-article approval result shall be the sole basis. Terminology follows the definitions of the current stencil printing terminology standard (GB/T 9851.6-2026).