Hello Rubsky and all the other great posts here. We have been doing tests at various facilities using the Starlight or STE and here are some tips that will help>
Coating: Keep coating thin. 1:1 dull maximum, 1:2 sharp a little less emulsion, 1:1 sharp even less emulsion. This is with an American style coater with a thick 1/8th inch edge and a thin 1/16th inch edge. In Japan we use much sharper scoop coaters than the US so that is why they recommend more coats. So the edge profile should be looked at when talking about the number of coats since different coaters, speed and angle can all yield very different emulsion thicknesses.
Exposure: Humidity is the main reason times from one shop to another could be different, along with different thicknesses of emulsion coated, different scoop coaters, etc. So times can vary shop to shop. I can shoot faster in SoCal on a hot offshore day when humidity is at 6% than I can on a rainy day with humidity at 100%. We don't see the differences much on a Metal Halide since it has a wider latitude or window of good exposures. But on an LED with shorter times, the window for a good strong expsosure and details can be much more narrow. Humidity or the moisture in the air is absorbed by emulsions, like a sponge, and it affects exposure. Also dry your screens well before exposure. Drying overnight, or using a dehumidifier is recommmended in your screen room. Separate the sink, reclaiming and developing from your screen storage area. It will be hard to dial in the correct exposure if the humidity jumps up and down due to reclaiming, wet reclaimed screens, or overspray entering your screen room area.
LED vs Metal Halide this is one of user preference. I have a new appreciation for the productivity of LED and if you don't overcoat the emulsion they work very well and have cost/production benefits. They do produce strong screens, but a pure photopolymer emulsion like HVP or Murakami T9 have an advantage with post exposure. This really helps with discharge, water base or HSA printing to obtain a 'stronger' screen through post exposure.
Step Test, Hardness Scale, Exposure Calculator for LED the Step test is the way to go. The 2 pieces of film used in a calculator will block a substantial amount of LED light. My tests here in the lab gave higher exposure times than my step test. The step test method did help get details and durability. Using a hardness scale with the resulting times and hitting 6-7 will validate the time.
Rubsky: Use Murakami MS-Degreaser - this is our go to product for good emulsion adhesion. If exposed well the emulsion will hold better with a good degreaser. Dishwashing soap should be avoided due to oils in there to prevent dry hands.
Don't be surprised at the softness of the exposure on an LED. Once dried it will print plastisol fine. For waterbase and discharge try Murakami T9. You can post expose in sun, or on a 5k MH, or back on the LED itself for post exposure. Post expose the squeegee side. Pure photopolmers and LED allows for post exposure. M&R's new Gemini LED exposure unit does exposure on one side and post exposure on both sides very efficiently.
Rubsky you mentioned SP emulsion, This is SP-1400? I have many, many SP emulsions, please clarify.
Admiral's times of
28 seconds for 225S
40 for 150S
for SP1400 match closely to my tests here. Note he coated 1:1 sharp. Less coating is better for LED exposure. If you coat more or use a thick edge your times will go up. I would use these times on halftone work, for vector I would bump up exposure by 5-10 seconds just to cook it more.
Bake the Cake, if you dry well, expose as much as you can while still retaining details, dry well in a hot box or the sun, you will have baked the cake and have a durable screen.
I will be presenting at Totally T-shirts in April showing how to use LED with Murakami T9 to print any ink without hardeners. Contact SGIA for reservations.
Al
