Halftone Expose / Wash Out

[upperhandprinting]

I'm preparing to *try to do a 4 color design on a dark shirt with a white underbase. The design will have half tones on every colored printer. I will be using 230 mesh screens and coating them 1/1. Just wanted to ask advice on exposure times and tips on washing it out.

I've heard that when doing half tones, your exposure time can drop to as little as 20 seconds. I'm guessing this has to do with wanting to cure the needed areas, but not be long enough to cure the areas you want to wash out. For spot colors, using the same emulsion, I've been exposing them for 1.5 minutes with a 1/1 coat. That has seemed to work fine and I've only had 1 blow out which I'm guessing was caused by too high of water pressure. Does anyone do half tones frequently and have any recommendations on exposure times?

As far as washing out the stencil, I'm assuming I should use the lowest water pressure possible that will still wash it out but not harm the halftone. If there are any other tips and trips, like which side to wash it out on or any other ideas you guys might have, I'd like to hear them all.

Thanks so much for the help!

[Saati]

The exposure time for halftone printing should be the same as any other exposure time. The proper exposure should yeild acceptable detalil and durability for line work and halftone work. Now with that being said it is common practice to underexpose halftone screens to achieve less undercutting. When this is doen we would recomend underexposing slightly to do this since it will leave the stencil open to solvent attack and may give you issues reclaiming. We always recomend using a 21 step exposure guide to dial in the exposure so that it is repeatabel and the performance of the screen will be predicitable.

[ApeShirt]

Could you please explain what undercutting is? I'm going to be doing my 1st halftone print soon

[Saati]

Could you please explain what undercutting is? I'm going to be doing my 1st halftone print soon

undercutting by screen printing definition is the reduction of size of an image due to the light traveling beneath the film positive.

Undercutting can be caused by a couple of factors:
poor vacum (the film positive is not in good contact with the emulsion)
the light source not perpendicular to the screen
over exposure (too long of an exposure)

a way to check to see if you are haveing undercutting is to expose your screen, wash it out, dry it, and then lay the film back onthe screen and look at it with a loop to see if the dots ont e screen are smaller than the film positive.

Like I said above, it is very common for people to cut back their exposure slightly to achieve a true representation of the dot but if everything is done correcctly this is not entirely necessary.

I have kept the above short since this thread go go on for days with 4color process and halftone theory but lets keep it simple for this instance.

[ApeShirt]

Thank you Saati. That helps a lot. Seems like a strange phenomena

[upperhandprinting]

So. I'm using the saati textile pv emulsion. I did a 1/1 coat on a 160 mesh screen. I have an image with halftones set at 45 lpi. I tried burning the screen for 30 seconds to start. Most of it looks really good, but a section of it wouldn't wash out. After probably too long, I finally blew out the screen. So, would you suggest burning it for less time so it's easier to wash out, or more time so it won't blow out? Thanks for your help.

[Saati]

So. I'm using the saati textile pv emulsion. I did a 1/1 coat on a 160 mesh screen. I have an image with halftones set at 45 lpi. I tried burning the screen for 30 seconds to start. Most of it looks really good, but a section of it wouldn't wash out. After probably too long, I finally blew out the screen. So, would you suggest burning it for less time so it's easier to wash out, or more time so it won't blow out? Thanks for your help.

The entire image should wash out the same. If there is an area that is not washing out well I would be suspect that it is getting pre exposed somewhere, the thickness of the emulsion is not consistant across the scree, or your film positive has some issues with density in that area.