How do I test exposure? By Richard Grieves
Submitted By: Dottonedan Date: May 26, 2014, 01:44:44 PM Views: 1288
Summary: Written by Richard Greives describing measuring UV Exposure.

Q1: How do I test exposure?

Measuring UV Exposure
It is the primary job of the screen maker to learn how to judge exposure. Invisible UV energy reacts with emulsion sensitizer and hardens the stencil so it won't dissolve with water and rinse down the drain. You block UV energy with a positive during exposure so the image area dissolves when you develop the stencil with water.

Screen makers can only expose one side of the stencil, so it is important for the UV energy to move through the stencil, all the way to the back, where the squeegee blade will rub the stencil where it is weakest.
When the stencil doesn't hold in the mesh, it didn't get enough exposure. Start by checking the suggested exposure time on the product
Technical Data Sheet. These estimated time charts are based on known commercial screen printing UV light sources on 305 mesh. You can measure your lamp with two exposures using a Stouffer 21 Step Scale to measure the stencil hardness, then refine your exposure with an exposure calculator.Stouffer T2115 21 StepStouffer T2115 21 Step Transmission Test Positive - Visual Feedback

The best exposure test for basic exposure hardness is a Stouffer 21 Step Transmission Gray Scale, a 5.5" camera film positive opaque at one end, and clear at the other, with 21 steps in between. The 21 steps simulate 21 different exposures and you get visual feedback to make clear judgements without guessing. Put a 21 Step on every stencil you expose for the rest of your life.
Stouffer 21 Step Transmission Guide distributorsDIRECT EMULSIONS and CDF DIRECT-FILMS - Solid Step 7When you develop the stencil, areas that didn’t get enough exposure to survive development will dissolve with water and rinse down the drain. You want a minimum of a  Solid  Step 7 that survives and holds in the mesh. More exposure will make your stencil more durable and less will make the stencil less durable.

Stouffer 21 Step for establishing a MINIMUM exposure with a  Solid  Step 7 on direct stencils exposed on the mesh.
The guide numbers are clear on the film so they get 100% exposure. You will know you have a Solid Step, because you won't see a difference between the guide number, (7), and the patch of stencil around it. If you have a 'crusty' step, you know that unexposed stencil on the inside of the screen has broken down and washed away with water. Not Solid!

The great thing about a Stouffer 21 Step Transmission Gray Scale is that it only costs from (roughly) US$6.00 to US$12.00 and should last a lifetime. The 21 Step is a real photographic positive with 21 steps of opacity that simulates 21 different exposures - with one exposure. Much faster than than a manual step test, where you have to move a piece of cardboard or Rubylith to block UV energy. Exposure Math for Stouffer Transmission GrayScale You can use math to calculate an exposure correction because the amount of each step is a known amout. Each step to increase or decrease exposure has a value of 1.414. If you need to adjust more than one step, you have to multiply the steps. It's logarithmic.

1 step is 1.414 exposure
2 steps 1.414 * 1.414 = 1.9999 so you should double exposure.
3 steps 1.414 * 1.414 * 1.414 = 2.827 means about 3x exposure.
4 steps 1.414 * 1.414 * 1.414 * 1.414 = 3.99758 which means 4X.

See, it makes sense.
Use a 21 Step on every screen for visual feedback You will get visual feedback of the invisible, UV energy that cross links your stencil when you put a transmssion gray scale on every stencil. You will notice when it washes out differently and you will know your lamp or stencil has changed. You will know when to change your exposure to get back to a minimum Step 7.

For visual feedback like a Step Speedometer, put a 21 Step on every screen you expose for the rest of your life.
Stouffer 21 Step Transmission Guide distributors
INDIRECT FILMS - Surviving Step 6 Indirect films are processed off the mesh, so they require a balance of hard and soft so they will adhere to the mesh after development. Too much exposure and they will harden, but not stick. Not enough and the will dissolve and rinse down the drain. Each step on a Stouffer 21 Step scale will be a different color.

Aim for a 'surviving' Step 6. No 7 step, and darker steps as the guide number decrease.

Underexposed: Look for a lighter color. Stencil will be thin (and look thin) resulting in poor press endurance. Underexposing for resolution sacrifices durability.

Overexposed: Stencil will be thick (and look thick) resulting in poor adhesion due to over hardening.
Ink and Clean Up Solvent ResistanceSolvent resistance and run length are improved as exposure is increased. Select the highest exposure that will give you the resolution you actually need for the job you will print. Forget the smallest lines and letters on test positives unless your work really involves lines or letters of that size. For calibrating exposure and artwork for fine line printing please read "Why do fine lines close up?"
Underexposing for resolution much finer than you need, can sacrifice durability. Remember to make a new test whenever you change your mesh count, mesh color, or coating procedure.

Q2: Why do fine lines close up?

Before an Exposure Calculator You have to find an acceptable exposure before you can use an exposure calculator that brackets that exposure with 2 steps under and 2 steps over exposure, so you can judge the fineness of line you can print. The minimum exposure to cross link emulsion so water won't dissolve it and it holds in the mesh, is a Solid Step 7. More exposure will make the stencil more durable (harder), but light scatter in the mesh and diffused light from a longer exposure, can under cut the positive, and choke the final image size. Undercutting UV energy from any lamp, but especially from banks of fluorescent lamps comes at your fine line positive from the left and right. It hardens your stencil at an angle which chokes the size of your printed dot.CompensateThe problem shows up with halftones and fine lines under 0.030". It may turn out you have to make a line 0.040" to that you actually print a line 0.030" and you have couquered Mother Nature's attempt to frustrate you.

Don't under expose, so you can hang on to fine lines that close up, because of undercutting, because you will have a stencil that is not completely cross linked and will break down on the press. You must outwit Mother Nature and calibrate all halftones and finelines to compensate for this choking or stencil failure

Add Known Controls to your Positive
Add simple lines of a known width to your positive and use that positive to MEASURE the result on your final screen print.

Large block letters - Who cares?
The right side was exposed to more UV energy than the left. Notice how the lines are choked.  Line choked when exposed to different amounts of UV energy

Q3: What distance should my lamp be?

Inverse square law
A certain volume of UV energy falls on a specific square foot - one foot from the lamp That same amount of light has to cover hot spots

Q4: Why doesn't my image wash out?

Doesn't stay in the mesh
Emulsion is easy. If the stencil washes out - it wasn't exposed with enough UV-A energy, the sensitizer is not crosslinked and the stencil dissolves with water and rinses down the drain - just like it is supposed to.

Image area doesn't wash out
If the image area doesn't dissolve and rinse down the drain, the sensitizer was somehow crosslinked with heat or UV-A energy. This usually means your positive failed to stop UV energy from reaching the stencil like a bad umbrella. I often get calls from people that wash outside or actually apply the positive outside, and aren't thinking about the tremendous invisible UV energy of the sun.

Dime Complete Opacity Test
To judge if your positive completely stops UV energy, tape a dime, different thicknesses of wire or a piece of aluminum foil to the stencil and after exposure compare how the area covered by the dime washes out compared to the dark areas of your positive. If the area covered by the dime doesn't wash out, you have exposed the stencil to UV energy or heat energy before you exposed with a positive 'crusted' the stencil.

Positive Failure
Test to make sure the emulsion actually will dissolve
Wash fresh stencil with no exposure

Crust - A little bit of exposure, but not enough penetration to cross link the entire thickness.
Positive Failure and Fine Line Test Pattern

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