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Old 05-02-2017, 12:09 PM
Rich Arentzen Rich Arentzen is offline
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Casting

We have been casting some blocks that are 1" high by 3"x5". We notice that they tend to curl up some lengthwise in the annealer. Thoughts? We are casting a Glasma glass in graphite forms and trying our best to equalize the temps. Anyone experienced in this type of work willing to talk through observations?
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Old 05-02-2017, 01:14 PM
Eben Horton Eben Horton is online now
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When you pour into graphite, the areas that are in contact with the graphite lock solid before the top surface and as it is cooling in the mold, the contraction sucks the top down because the top is softer than th chilled areas that touch the graphite.

How's that for a run on sentence ?
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Old 05-02-2017, 01:52 PM
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Pete VanderLaan Pete VanderLaan is offline
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Don't forget that glass has a linear expansion coefficient. in other words, it will shrink. Over a four inch run, it should shrink about 96 ten thousandths from 300C to 20C which may not sound like much but you can see it quite visibly. In this case, it's shrinking from about 1300C to 20C. That will really be visible. It is linear after all. Figure roughly about one ten thousandth for every three degrees Centigrade . Division, a simple math.
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Old 05-02-2017, 03:21 PM
Rich Arentzen Rich Arentzen is offline
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So why is the piece bowing then? Not the middle collapsing but the ends of the rectangle turning up?
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Old 05-02-2017, 03:39 PM
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Pete VanderLaan Pete VanderLaan is offline
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As Eben said. Stick and shrink. It doesn't just shrink sideways. It also shrinks up and down. Have you ever wondered why you can't fill your pot to the brim no matter what ( unless you overflow it with a miniscus)
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Old 05-02-2017, 04:01 PM
Art Freas Art Freas is online now
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Would a hotter and longer annealing help fix this? My thought is that a longer time further above the strain point may result in a more even contraction on the cool down. Not sure that is a right thought though
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Old 05-02-2017, 05:08 PM
Charles Friedman Charles Friedman is offline
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Torch the perimeter of the casting the most and let the interior cool more before boxing.
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Old 05-02-2017, 07:08 PM
Rich Arentzen Rich Arentzen is offline
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Why is it curling up though? I cant believe that it is too much to ask to cast a bar with a parallel top and bottom.
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Old 05-02-2017, 08:52 PM
Steven O'Day Steven O'Day is online now
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I agree with Art on the longer times. If you are using a refractory kiln shelf you will need to take this into account for the thickness, the shelf will hold the heat at the bottom of the piece and you might not get even cooling without longer anneal times. I think Bullseye recommends twice the time for uneven cooling.
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Old 05-02-2017, 09:00 PM
Brice Turnbull Brice Turnbull is offline
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It is curling because when you put it in the annealer the top center is much hotter and expanded due to heat than the side areas and bottom area (area, 3 dimensional). As the top cools in the annealer, it shrinks and pulls the sides closer together. You need to cool the top faster, to more closely match the pace of the temperature change in the sides and bottom. I'm supposing, and this is what other folks have said above here.

I may not understand the physics accurately, but the solution should be the same...

The way I make tiles is to drop the glass in the mold, then cover it with a steel plate. I'd suggest a graphite plate to avoid scale getting in the glass, but mainly, try pulling the heat out of the top at something like the same rate as the rest of the piece by laying a graphite block on it.

1" is really thick, and the center will likely still be hot and somewhat fluid even with top cooling, so you may still get a concave surface in the center, but this is my first suggestion. Anneal longer than normal, because you have a much greater heat differential than putting in a blown piece.

Best Wishes
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Old 05-02-2017, 09:13 PM
Eben Horton Eben Horton is online now
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A good option would be to call East Bay Batch and ask if they could sell you some of the batch that they make for John Lewis. It has a super low expansion for this very reason.
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Old 05-03-2017, 06:08 PM
Rich Arentzen Rich Arentzen is offline
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Good Suggestions. Ill keep you posted. We have even tied a 30 hour annealing cycle. I bet John Lewis would know the answer!
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Old 05-03-2017, 09:47 PM
Cecil McKenzie Cecil McKenzie is offline
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Rich, There was an artist on the board a few years ago that was casting pieces in graphite to be used in the old style sidewalks. He seemed very high energy and I think got put off by the boards reaction to some of the things he said. I don't recall his name but maybe someone from the board remembers him.

He talked some about making a very large coke bottle for someones installation piece that was related to the movie" The Gods Must Be CRazy "

I would guess he knows something about casting in graphite. I think he was maybe from Philadelphia.Some one on the board might know who I am talking about. Good luck with your project.
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Old 05-03-2017, 09:48 PM
Eben Horton Eben Horton is online now
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Quote:
Originally Posted by Rich Arentzen View Post
Good Suggestions. Ill keep you posted. We have even tied a 30 hour annealing cycle. I bet John Lewis would know the answer!
He does. That is why he has a batch formulated for casting that doesn't "shrink".

Have you tried de molding them upside down and torching the colder sides a little bit before annealing them?
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