Exposure Glass Repair...

[Quinine]

We just picked up a used Poly Lite vacuum exposure unit this week. Got it all cleaned and set up and everything is working great on it. Only one fairly significant concern right now - there is a pretty good size chip in the glass. Of course, it's more or less in the center of the glass. I was thinking this afternoon of picking up a glass repair kit and filling the chip. Anyone ever do this? Does it work to fix the potential for exposure issues due to the chip by filling it? I searched the threads and didn't see anything specific to this issue. Any help would be great. Thanks.

[vrooooom]

It's a bit of a safety hazard, IMO, since most exposure unit glass should be untempered. Seems like any repair would further distort the exposure behind it, but Andy or another more knowledgeable than I will have to rule on this.

Whatever you fill it with will distort the same or worse than a crack, unless you can just melt the whole thing down and fabricate a new sheet of plate glass.

[Quinine]

Thanks for the response. That's what I was wondering about. Hopefully, someone can elaborate further one way or another.

[NeroInferno]

Hello,
the only way to solve this is to change the glass. Like James said, be sure it's untempered and of appropriate thickness. Considering however you need to change it you can try with the glass repair kit if they aren't expensive (try with the car glass ones), perhaps you'll solve and save money. Another solution is to use it but pay attntion when positioning the screen on the glass. It's a mess but it works if you have the safety light in theexposure table.

In my opinion I'll change it if it isn't so expesive, saving time to position the screen. Consider also that nick can be a break point in a future.

Fabio

[exxxlonghair]

I agree with everyone above that the glass should be replaced. I disagree about the glass being untempered. If you get tempered glass you just have to make sure they don't add any uv coating (sometimes added for office building windows). If you have a large exposure unit - I built mine 50" x 60" it's hard to trust untempered. I have used 1/4" and 3/8" tempered and have never had any issues with exposure or distortion. Tempered is a bit more expensive but I could drop a f-ing brick in the middle of my glass and still be shooting screens.

[Quinine]

Yeah. I'm not sure where the dogma about tempered glass came from but we have a scratch built unit (besides the Poly Lite) that has tempered glass and we shoot screens through it just fine. Take a look at some of the older forums and threads on here if you have any doubts or questions about tempered glass. You will find that the very subject of tempered vs. untempered glass is dealt w/ directly and any obfuscation about the subject will be quickly cleared up. Not lookin' good for the chipped glass repair so far... Thanks, Guys.

[Andymac]

Re tempered vs untempered. there was a good article on Screenprinting magazine regarding exposure glass a few years back. they found the tempered glass, as long as it was clear, worked ok, after allowing for exposure to be recalibrated. but they also noted some drawbacks or differences from regular plate.

If the tempered glass has no UV filtering or discoloration, the only drawbacks are
(a) longer exposure - although this is also a factor with plate if the thickness changes - thicker glass absorbs more UV, so the exposure takes longer
(b) tempered glass can pick up surface flaws form the rollers during the tempering process (high heat), you need to make sure the surface has no flaws as they will/could show on a stencil
(c) higher cost
(d) actually more brittle. I had a discussion with my glass guys when replacing the glass last time it broke. It can break as easily as regular plate, it's just when it does, you don't get shards.

When purchasing glass, make sure to phone around. the glass business is a game of markup on markup. Depending on where you buy into the chain, especially with larger sheets (80"x 153", like on our large units) the price can range from hundreds to thousands.

[crosshair]

The tempered glass vs untempered glass thing is another case of a situation where there are advantages and disadvantages to both options, and the experienced printers understand this....but inexperienced printers read it one way and repeat it to one another as received, not earned wisdom. And so it becomes fools' dogma on GP.

I've been using tempered glass since always. My stencils come out great. My glass has never broken despite many years of abuse. 45 seconds longer exposure time is not a significant factor in my workflows.
Myth = Busted.

[Quinine]

Thanks, Andy. As usual your thoughts are sound, to the point, and helpful. I guess I need to start looking at glass soon. Thanks for the pointers and clearing things up for me/us.

[vrooooom]

Brg!