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Can somebody break it down to me what the different types of emulsions mean? For example, dual cure, photo emulsion, diazo, presensitized? What does one do that the others can
 

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Emulsion is basically a poly-resin glue like Elmer's Glue except, when it dries, it will still dissolve with water.

Sensitizer reacts when bombarded with UV energy and cross-links the emulsion components in to the mesh so it won't dissolve with water.

Areas that you cover with a UV opaque positive, don't get exposed and will dissolve with water. That opening made in the stencil is where the ink transfer through the mesh.

Sensitizers
SBQ sensitized emulsion is pre-sensitized at the factory and has a shelf life of up to 18 months. These are very fast, but fragile. Strong solvents used on SBQ emulsion that is under exposed can freeze the stencil by penetrating and crosslink the un-crosslinked molecules in a way that can't be broken apart with stencil remover.

SBQ is a great emulsion for companies that have weak light sources. They don't have strong resistance to solvent inks, and little resistance to water unless it is a 'special' modern emulsion like the new KIWO Polycol Discharge.

Dual-Cure is an emulsion that has a combination of sensitizers, usually SBQ and Diazo. Because of the nature of Dual-Cure, they have the best copy properties. You need to add the diazo sensitizer and stir it into the emulsion, then let it sit for an hour to let the bubbles that form on the diazo salts when water touches them - degass. The only dual-cure I know of that doesn't use diazo and is completely pre-sensitized at the factory is the hybrid Ulano QX-1. It has traditional exposure speed, but dual-cure properties and 18 month shelf life.

Diazo
Diazo sensitizer is old school and slowly being pushed from the market by the new trend toward pre-sensitized emulsions, but at this time, if you want the ultimate in water or solvent resistance, diazo is the ticket. Not fast, but strong.

All emulsions (especially the fast exposing SBQ) should be coated and stored in yellow safe light. Actually turn off the lights with SBQ when you're not in the room.
 

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Richard,

Thanks for the very detailed explanation! I learned a lot from that post.

I used to use dual-cure when I was doing detailed work, and presensitized for everything else. Ever since I read your multiple post on the 21 step test and finally getting a hold of a Stoufers strip and really dialing in my times, I have had great results developing halftones and cmyk screens using the presensitized stuff.

I am currently printing mostly platisol. If I don't do waterbased, is there any reasons/advantages to dual cure if I am getting good results with the presensitized?

Thanks!
Nick
 

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You will get better resolution from a dual-cure, but at the expense of speed.

You will also get a color change that's always reassuring after you expose.

There is no reason to fear plastisol, it's like salad dressing and wouldn't hurt any stencil. If you can print the halftones you need, I would stick with the SBQ stencil.

Where did you actually buy your Stouffer scale?
 

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Believe me, I don't fear plastisol, I love it!

I got my stouffer strip at: 21 Step Stouffer Scale

Really fast service, California to Cleveland OH in a couple of days! I wish I would have known about this test years ago. I love how you can make calculated time adjustments, unlike other time test I've done in the past. It is also about zero additional work. Thanks for the recommendations (and sorry for going off topic).
 

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Thanks.

I have them on my list of web vendors. I'm always looking for more, because I don't want to go into the T-2115 selling business.

Closer to you are Stouffer themselves and Nazdar SourceOneOnline Cincinnati where Dave Durbin is the only branch manager to stock the Stouffer T-2115.

Any of SourceOneOnline branches can pull from Cincinnati by searching for Nazdar part number "21step".

Printer Information and Training for Screen Printers
 

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The best exposure test for stencil hardness is a US$10 Stouffer 21 Step Transmission Gray Scale.
Transmission Step Wedges
Exposure FAQ Screen Making Products how to measure exposure

A transmission gray scale is a small 5 inch film positive with darker and darker filters next to each other in steps. This is a standard photographic darkroom test positive that's been used since the 1930's.


When you tape one on the stencil as you expose, you'll get a simulation of 21 different exposures to the stencil and you get visual feedback that shows how well your stencil is cured when you develop the stencil.





Areas that didn’t get enough exposure to harden and adhere to the mesh will dissolve with water and rinse down the drain. Aim for a minimum of a Solid Step 7 that adheres to the mesh and survives development. More exposure will make your stencil more durable and less will make the stencil less durable but light scatter could start to choke fine lines or halftones.

Properly used it should last a lifetime, so I want you to put it on every screen you expose for the rest of your life.

Exposure Math for Stouffer Transmission Gray Scale
As Nick wrote above, you can easily use math to calculate an exposure correction because the amount of each step is a known amount.

You want to aim for a solid step 7. Each numbered 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's easy.

 
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Discussion Starter #9
This is very useful. Thanks. I do have one question.

lets say I ahve exposed my screen for 3 mins. I take it and wash it out and find that the screen isnt exposed at all. Is it possible to throw the screen back on the exposure unit and expose longer...or is it one and done.
 

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If you wash it out and it isn't exposed, it will be washed out (or soaked and deteriated) so there won't be anything left to expose. It's kind of a one and done thing. When I was figuring out my times, I always coated a second screen, assuming the worste for the first one.

Good luck!
Nick
 
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I havent any equipment yet...but it is on the way. So im sure I will learn a thing or two to begin with...good to know though.
 

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Failure during development

lets say I ahve exposed my screen for 3 mins. I take it and wash it out and find that the screen isnt exposed at all.

Is it possible to throw the screen back on the exposure unit and expose longer...or is it one and done.
If your stencil fails during development, it won't adhere to the mesh. Letters will dissolve or peel off.

If it survives development, after it dries, it's ready to print. This will be very clear in real time, when you are developing your own screens.

So my answer is no.
 

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if you are lacking a stouffer scale you can always do a traditional step test by covering the screen and moving your cover back at set intervals, such as every minute or two minutes you move your cover back a 1/2 inch or inch. Just keep track of how many intervals you have done. If you wanted to start with a base exposure time of 5 minutes you would expose your last step, the one with the cover moved entirely off, for five minutes. Yes, you will have an un-usable screen because only part of it will be exposed properly, but until you change your exposure set up, the time reading you get will be good to go.
 
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