supergerms

Hi again,
Ah todays problem i have encountered is my emulsion setting practise. I burn in the image with a 500watt halogen light at night for 10 minutes bout 20cm away from screen. The screens dry over night and for about 24hrs in a dark box before being set.

Now when i 'burn' in the image and go to wash up, the image wont completely come through with out a bit of rubbing, brushing or pressure spraying which takes bits of set emulsion out of the screen.

I don't think i'm exposing too long- think!

Also when i set me image its printing onto translucent paper similar to tracing paper, mainly due not being able to work out how to get a positive image onto clear film with only basic means. Could this be the problem?

I am using an emulsion where the sensitizer has been mixed in quite a while ago, and has probably gone bad. Think this could be my problem?

cheers
- super

 

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splathead

How old is old? If your emulsion is older than 6 months, that is probably the issue.

Have you ever exposed a good screen with the settings you use? If yes, then I would buy new emulsion.

If no, then what settings did you lat burn a good screen with?

 

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RichardGreaves

If the stencil is hardening - and hard to wash out, it is still working.

What is failing, is your positive. If your positive allows UV energy to leak through and expose it, it will be weak and only harden a thin crust of stencil at the surface. Like a loaf of bread, the inside will be soft and unexposed.

Dime Complete Opacity Test
To judge if your positive completely stops UV energy, tape a coin or cut a piece of aluminum foil in a decorative shape to the stencil and see if the dark areas of your positive are failing you and letting UV-A energy through to the stencil. If the area covered by the coin doesn't wash out, you have exposed the stencil to UV energy or heat energy and the stencil resists dissolving with water and going down the drain.

Wire Test
To test for fine lines or halftones
Test this by taping different thicknesses of wire to an unused part of your stencil. The metal wire will not allow UV energy to transfer through it like a poor stencil.

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

Stouffer 21 Step Guide
You can test exposure with a manual step test or simulate exposure with a Stouffer 21 Step Guide. It is an old fashioned film positive that is opaque on one end, and transparent on the other. With one exposure, you simulate 21 different exposures. The first function of any screen maker is to learn to judge the hardness of the stencil so exposure isn't wasted on a stencil that won't change anymore and prevent under exposure that will cause pre-mature breakdown or failure when you develop the stencil after exposure.

http://www.ulano.com/FAQ/FAQexposure.htm#Q1

__________________
How are you measuring? retired Ulano Technical Support
Screen printing since 1979 - SGIA Academy Member

 

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supergerms

It may be a combination of both i think, because the emulsion is over 6 months old and the positive could be better, however i have set some good screens using the same process just before the ones i stuffed up.

How do people put there positive on clear film? can print shops do it for you?

cheers -super

 

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splathead

Quote: Originally Posted by supergerms How do people put there positive on clear film? can print shops do it for you?

Usually with a laser or inkjet printer. A print shop can do it for you, but it's not usually necessary.

 

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