Screen Exposure: Bake the Cake

Drawn from working printers’ conversations on The Shirt Board, 2011–2017. Direct quotes link to the exact post. Other references link to the thread. TBT: Screen Printing Exposure: Bake the Cake

Exposure is the step where a coated screen becomes a stencil — or fails to. It’s also the most misunderstood step in the shop. The failure is invisible. An underexposed screen looks fine. It washes out beautifully and holds gorgeous detail. Then it falls apart a hundred prints into a discharge run.

The framing that anchors this article comes from Murakami’s Al Buffington, in a post worth reading in full. Exposing a screen is baking a cake. A baker never pulls the cake out early because the top looks done. “90–95% of the emulsion strength comes from the exposure. Everything else is a band-aid to make it stronger, from post exposure to hardeners.”

What exposure actually does

UV light crosslinks the emulsion. It cures the stencil through its full thickness — from the print side, which faces the light, up through the squeegee side, which gets the least. A stencil is fully exposed when the squeegee side is as cured as the print side.

That’s why the classic underexposure test happens there. Wipe the squeegee side with a clean white rag right after washout. If emulsion color transfers, the screen is undercured. Slime on the inside of the screen at rinse is the same verdict in wetter form.

The underexposure epidemic

The forum’s elders are blunt about which direction most shops err. DouglasGrigar: “Underexposure is the MAIN problem for new people in this industry.” He adds that many suppliers promote it, and it directly harms new printers.

Why would anyone promote it? Because underexposed screens wash out fast and hold stunning detail. That demos well. Buffington described the pattern from the seminar podium. The screen-room worker underexposes to keep every dot. He’s thrilled with the image. Then he “uses up rolls of tape and drives the production manager nuts with pinholes appearing all the time… or needing a new discharge screen in the first hundred prints.”

The bill for underexposure, collected across threads: pinholes and tape. Stencil breakdown on press — worst with discharge and waterbase, which attack soft emulsion. Delamination. And, counterintuitively, harder reclaim. Buffington notes that underexposed photopolymer plus screen-opener spray flash-cures the leftover sensitizer into the mesh.

Overexposure is real too — the detail tension

The honest complication: you can overdo it. squeezee supplied the physics: “If you over-expose you will get more under-cutting, burn-through and light scatter. These affect small elements disproportionately.” In other words, fine lines and halftone dots close up first. This is the eternal tension. Full cure wants more light. Fine detail wants less. Shops like alan802’s split the difference in practice. They back off a light unit or two, and only on very fine halftone work.

Before you blame the exposure time for lost detail, check the film. A thin inkjet positive that isn’t truly opaque to UV lets light sneak through. It cures the areas that were supposed to wash out. As yorkie put it in the same thread, “the need for especially dark film is a symptom of an out of control exposure process.” Blue moon’s microscope comparison is memorable. Imagesetter dots are perfect circles. Inkjet dots “look like potatoes… or blackberries.” Good D-max on your positives buys you the latitude to expose fully.

Calibrate — but don’t trust the calculator blindly

The standard tool is a step test. Expose a screen through a calculator film, or in timed increments, and read the result. Then came the catch. Rockers ran two brands’ calculators on the same screen. The Chromaline film said cut his time by 75%. The Saati 21-step guide said change nothing. Calculators are guides, not verdicts. Several optimize for image, not cure.

The community’s synthesis is blue moon’s method, described in the same thread. Step up your exposure “until there is no slime on the back, add a little bit, and note what step you are on the Saati scale. Then leave the strip on the glass” — under a scrap of film. The strip sees the same light path as a real screen. It becomes a permanent control. As your bulb ages or seasons change, it tells you when your calibrated time has drifted. Belt-and-suspenders shops like Evo’s cross-check. They burn the same calculator at 1.5×, 2×, and 2.5× the expected time and compare.

Never copy another shop’s time

The forum’s LED threads are full of printers swapping exposure times. Five seconds. Thirteen. Twenty-five. Fifty. Buffington’s warning lands directly on that habit. LED units have a narrow exposure latitude — that’s why good ones adjust in tenths of a second. The correct time depends on your humidity, your screen’s internal moisture, your emulsion temperature, your film density, your coating thickness. His example: the same emulsion that needs 14 seconds in the Arizona desert may need 16 on the foggy California coast. That’s a 12% swing from weather alone.

Two corollaries. First, a screen that isn’t fully dry can’t be fully exposed, no matter the time. “Dry to the touch” means nothing. A moisture meter is the only way to know, and 30–35% humidity is his optimal screen-room range. Second, thick stencils need recalibration, because (as Colin found) “LED bulbs do not like thick stencils much at all.” Emulsions also simply differ. Dottonedan notes one can fully cure in a fifth the time of another.

Post exposure: the band-aid, examined

Can you fix an underexposed screen by exposing it again after washout? A whole thread argued it. The chemistry answer came from Buffington: pure photopolymer (SBQ) emulsion stays light-sensitive after development — even with diazo added. So post exposure genuinely adds crosslinking. Diazo-based emulsion is different. It loses most of its exposure capability once it’s been wet. Sunning those screens mostly just dries them. That’s still worthwhile before a discharge run.

But every authority in the thread converged on the same hierarchy. Kiwo’s tech sheet, quoted by StuJohnston: post-exposing takes four times the original exposure to improve resistance 10–15%. “Exposing the screen fully with the primary exposure offers better resistance than underexposing initially, then post-exposing.” The Ulano rep who spent three hours in dirkdiggler’s shop said it flatly: expose correctly the first time. Post exposure is not necessary. And crucially, post exposure cannot reverse the dot gain or ragged edges an underexposure already baked in.

Where it earns its keep: squeegee-side-to-the-sun before long waterbase and discharge runs. Tonypep post-exposes any discharge job over 200 pieces. It strengthens exactly the side that got the weakest light, and it dries the screen for free. As ebscreen put it: “Hell, it’s free insurance.”

The takeaway

Fully dry the screen. Expose it fully. Prove it — no slime, no color on the rag, crisp washout, and a step-wedge strip living on the exposure glass as your drift alarm. Give up a light unit only when a halftone job truly demands it. Treat post exposure as insurance on long water-based runs, never as a correction. The cake gets its full time in the oven.

Threads quoted span 2011–2017 on theshirtboard.com. Direct quotes are linked to the original posts — the full threads are worth the read.