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MEISTERICS

Casting Resin

16 posts in this topic

Does anyone know why the Resin expands when creating castings? NOTE, expansion increases as temperature of mold increases. The website states that the resin should actually shrink.

Keep note that i do use MB's & the manufacturer is alumilite.

Thanks,

Jared

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This is a real shot in the dark, but an uneducated guess is that maybe the heat generated by the chemical reaction is what causes the plastic to expand. Metal expands when heated, why not plastic? Then again, maybe it's just one of life's mysteries that we mortals are better off not understanding. Kinda like why the Cubs haven't won the World Series in over a century. wacko.gif

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"Kinda like why the Cubs haven't won the World Series in over a century."

Global warming. :lol:

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1/2% shrinkage is very small and is what casting suppliers are striving for. This would only be a problem if you were trying to cast this material in a 'hard' mold, like PoP or Durhams. The chances are that you will never get the cast out without damaging the mold (been there, done that).

The general rule is 'hard cast - soft mold'. These resin materials are best cast in RTV rubber molds. Using RTV, I doubt you would notice any slight expansion, which makes me want to ask the question; How did you come across or notice this expansion in the first place and is it causing you a problem?

As for the microballoons, they should not affect anything. They do not take part in any chemical reaction, just like adding stone chips to a concrete mix.

Dave

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adding microballoons only creates more expansion, at least for me it does. if it expands just as much with microballoons as it does without.... then i have no idea. i just know, for me, microballoons makes it expand quite a bit more.

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Well i am using a silocone mold. i think this is happening when i am mixing the solutions i am trapping air and not giving it enough time to rise to the surface.

My expansion is more that minimal. 20-40% depending on the temp of the mold. The manufacturer keeps telling me its moisture but i doubt highly thats the entire cause. I say that because i keep my resin bottles closed exception to when mixing batches and they are only exposed to air for NO MORE than 20 min.

i was going to test without MB's and see what happens if there is any expansion.

Irritating to say the least because i think compromising the strength of my castings.

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I'm not sure I follow you, but if you're pouring too fast and trapping air bubbles, you can GENTLY squeeze the mold and kinda "burp" the air up and out the pour hole, than continue pouring. I've used Alumilite Super Plastic in RTV molds a few times, and that's what I've done.

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20 - 40% expansion! This does not sound right.

When you add microballoons, they DO have volume and probably account for the numbers that you are talking about.

Dave

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I suppose I should walk everyone thru my steps.

1. pour both types of resin into 2 differents cups.

2. add equal amount of MB's to each solution.

3. pour equal amount of each part by weight desired into cup.

4. mix vigourously for 20-30 seconds.

5. pour into two part mold to top of casting.

6. let resin cure. this is where the expansion happens, just before it hardens. The resin will push out my pouring whole. The amount of expansion that occurs depends how hot the mold is. Or at least thats the correlation.

The only thing i can think of is that the resin is trapping air when i mix the 2 parts together and the heat is causing the air to expand within the resin in the liquid form and only stopping when the resin is hardened. i might be really reaching with that hypothesis.

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The only thing i can think of is that the resin is trapping air when i mix the 2 parts together and the heat is causing the air to expand within the resin in the liquid form and only stopping when the resin is hardened. i might be really reaching with that hypothesis.

Sounds reasonable to me. The 20 - 40% estimate may be stretching the imagination some though, if the gas expansion theory is to hold good.

When I was working with resin and MB's, after mixing vigorously, I would spread the mix up the sides of the cup. I found this forced the bubbles to the surface and I was able to eliminate more this way. The only true way to eliminate the air is with a high vacuum, but unless you are making 50 casts every day, it is hard to justify the expense.

Dave

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You can also try using a round rod to mix, instead of a paddle shaped stirrer. It will trap less air.

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You can also try using a round rod to mix, instead of a paddle shaped stirrer. It will trap less air.

its the micro balloons. the more you add the more it expands. With mine anyway. I think its the heat of the plastic expanding the air in the microballoons and then it hardens, leaving you with a product larger than your master or mold space. I have not yet found an alternative, except less balloons.

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HUMIDITY

Cast in the winter paint in the summer.

Pretty sure Tater hit the nail on the head w/ the humidity comment. In the winter, my casting seems to very considerably more than in summer (Oregon summers are dry... not humid). And man, trying to get consistent pours in the wet winter months can get irritating! However, I've had excellent results with pouring throughout the summer. I kept thorough details of weight recordings via Excel of my lures for the last 2 years too (really helps to save time and $). By putting the dates in as well, I can definitely see where humidity would be a key factor.

Keep in mind I also do not use alumilite.

Edited by A-Mac

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