Shrinkage

A concept for a heated mold.

Injection molded and sand cast parts require a high rate of flow and the maintenance of temperatures above the glass transition to fill their molds. If the material cools before the mold is filled, shrink cavities and distortions of the part can occur. The most common way to deal with this is to keep parts small, with thin walls and oriented such that the material doesn’t have far to travel. Might there be a way around this? Could it be possible to heat the walls of the mold to maintain high internal temperatures until the material has filled the mold entirely? I don’t have a foundry to try this myself, but I would love to see someone experiment with heating a sand mold while casting aluminum to see if it reduces or eliminates problems with shrink cavities. Any additional control that we can get over the cooling process can only help, right?

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3 thoughts on “Shrinkage

  1. I don’t know about injection mold glass, but one of the geo professors build a heated diamond forge but it’s kinda hard to have a cavity rather than a flat surface in the diamond. You could probably build something similar at a lower temperature&pressure as that was designed to simulate the p/t of moho and mantle interactions.

    I’ve actually melted glass in a pottery kiln, it flowed around cone 10 (2300º F). I used several colors of glass, and you can still see individual colors (rather than just mud) but it smoothly transitions between them and has a slight meniscus. You could use a setup similar to thermite welding a rail with a solid collected in a hopper and have it warm with the kiln and melt down into the mold below.

    • When I say “glass transition”, I’m talking about something different from silica-based glass like in windows; I’m talking about the phase transition where a solid becomes molten. When you’re casting metal or plastic parts, the material relies on the internal energy retained from being melted in a crucible or other device to stay molten while it fills the mold. If the material cools before this happens, you can have portions of the mold which are unfilled or shrink prematurely. I think that if we were able to heat the mold to near the glass transition temperature for the desired material, we could fill the mold more easily and then cool it at our leisure. I suggested using a sand mold because that’s generally what’s used to cast metal parts; but it could be any kind of mold, really… This would be significantly easier for thermoplastic injection molds, as they are generally made of a material which is thermally conductive and has a much higher glass transition temperature.

      Still, it would be pretty cool to be able to melt glass for custom parts. About how much do those kilns run for?

      • Well if your creative and willing to deal with uncertain results and temperatures you can get a cone 10 level fire out of an earth kiln, I’ve got a couple of buddies that have done that some.

        Otherwise your looking at 5 bucks a brick building your own + whatever your heating system is. My favorite is wood, I believe the most common is gas followed by electric. With a wood kiln it would take about 18 hours of continuously feeding it wood, about a log every 30 seconds a peak or something ridiculous to reach and hold cone 10.

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