Three-Colour Screen Run: Fringe, Broken Line and Chalky Red
A workshop must screen-print 250 A2 posters (420 × 594 mm) in three colours - a deep red ground, a black line drawing and an opaque white headline - on a manual four-colour carousel. Only 40 spare sheets of the 170 gsm uncoated cartridge paper are in stock. The artwork is supplied as vector with the white headline knocked out of the red ground, and the black drawing includes lines 0.3 mm thick. After 30 pulls, the white letters show a 1 mm red fringe on one side, the black lines have broken up and read as grey, and the red solid is mottling and looks chalky. The supervisor blames the squeegee and wants to add 10% ink thinner. Deadline is four days; an offset run would cost three times the budget and would not be ready in time.
1. Separate the three faults into artwork, screen/mesh and press/ink causes, and rank the fixes by cost and by how fast they can be applied.
Read the worked answer
Fault 1 - 1 mm red fringe on the white headline
- Cause: artwork. The white is knocked out of the red ground, so any registration drift exposes paper and lets the red edge creep into the letter. Screen printing on a manual carousel cannot hold registration tighter than about 0.2–0.3 mm across an A2 sheet.
- Fix: make the red a solid ground printed full-sheet, with the white overprinted on top, and add a 0.2 mm spread of red under the white edge. Cost: minutes in the vector file.
Fault 2 - 0.3 mm black line breaking up and reading grey
- Cause: mesh and stencil, not the squeegee. A 0.3 mm stroke is narrower than the emulsion bridge can reliably hold; the stencil edges crumble and the line prints as a dotted grey. The mesh chosen for the red solid is also too coarse to hold fine line.
- Fix: thicken strokes to a 0.5 mm minimum in artwork, and print the black on a 120–140 threads/cm mesh with a sharp capillary-film stencil.
Fault 3 - red mottling and chalky on uncoated 170 gsm
- Cause: press/ink and stock. Thinned ink on an absorbent uncoated sheet sinks in; the binder separates and the solid looks chalky.
- Fix: do not add thinner - that makes it worse. Use a heavier-bodied ink, a 43–49 threads/cm mesh for the solid, a harder squeegee (about 70 shore) at 45–60° with 2–3 mm off-contact, and a consistent pull speed.
Ranking
- Artwork changes - free, 30 minutes, no materials.
- Mesh/stencil change - one screen reclaim and remake, half a day.
- Ink and squeegee adjustment - free but needs a 10-sheet test.
- Paper change - effectively impossible (40 spare sheets). So the fix must come from the first three.
2. Write the revised production plan: print order, trapping, mesh per colour, and how you use the 40 spare sheets.
Read the worked answer
Revised plan
- Artwork first (0.5 day)
- Delete the knockout. Red prints as a full-bleed solid ground; the white headline overprints on it.
- Add a 0.2 mm spread (trap) of red beneath all white edges, and confirm the black line drawing sits on the red with a 0.2 mm choke so misregistration cannot open a paper gap.
- Minimum stroke weight raised from 0.3 mm to 0.5 mm; the 0.3 mm hairlines go
- Keep white as 100% opaque white, not a tint.
- Screens
- Red ground: 43–49 threads/cm, thick emulsion, for a dense solid.
- Black line: 120–140 threads/cm with capillary film, for edge sharpness.
- White: 62–77 threads/cm, enough ink deposit for opacity without squeezing out.
- Print order
- Red ground → flash/air dry → black → white last. White last keeps the headline bright; if white is printed early it picks up red from the following screens.
- Press discipline
- Off-contact 2–3 mm, squeegee 70 shore at about 60°, one consistent stroke, no 10% thinner.
- Register on crosshairs outside the trim, not on the image.
- Using the 40 spare sheets
- Run a 10-sheet setup per colour for registration and ink density; dry, inspect at 1 m, then run the 250. Reserve 10 for re-pulls and keep 5 dry for the client's approval. 250 + 10 + 10 + 5 = 275, inside the 290 available.
- If the black still breaks after the 0.5 mm change, stop and re-make the stencil - never keep pulling and hope.
3. The supervisor wants to add 10% thinner and keep pulling. How do you argue against it with evidence from the sheet itself, without a confrontation?
Read the worked answer
Argue from the print, not from authority.
- Show the mechanism. Put a good pull and a thinned-ink pull side by side under a loupe. Thinner lowers viscosity, so the ink flows into the paper's fibres and the binder separates: the solid lightens, the edges halo and the 0.5 mm line spreads and fills. The 'chalky' look gets worse, not better.
- Point at what thinner cannot fix. The 1 mm fringe is a registration/trapping problem - thinner cannot move the white back into register. The broken line is a stencil/mesh problem - thinner will make the line bleed, not close it. Two of the three faults are untouched by the proposal, which is the strongest argument.
- Use the sheet count. There are 40 spare sheets. One 10-sheet thinner trial consumes a quarter of the buffer and proves nothing. Instead, spend those 10 sheets on the 0.5 mm and overprint test.
- Offer a shared test. 'Let us run ten with the changes, dry them, and judge both at 1 m. If mine are worse, we go back to your way.' That converts a status conflict into a measurement.
- Flag the deadline arithmetic. A four-day window allows one screen remake (half a day) plus drying time. Arguing costs hours; testing costs 20 minutes.
Close on the shared goal: 250 posters that read correctly from a corridor, delivered in four days.