Improving thermal conductivity of RTV silicone for hard candy

  1. I use RTV silicone rubber to make moulds for hard candy. Hard candy is basically boiled sugar that is poured into moulds at just under 300 degrees F. The castings of hard candy develop bubbles on the surface because the silicone does not dissipate the heat, unlike traditional metal moulds that dissipate the heat quickly. I've tried many different matrix formulations using silicone mixed with silica, and zinc oxide; results improve with silica but not enough to eliminate all bubbles. I know that thermal conductive silicone is used in various industrial applications, I just don't know how they are formulated.
     

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  3. Q_Goest

    Q_Goest 2,976
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    You could mix copper filings in.
     
  4. From what I've read, the thermal conductive properties of materials added to the matrix don't necessarily have a bearing on the overall silicone matrix heat conduction performance. Although I did consider copper.
     
  5. Q_Goest

    Q_Goest 2,976
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    Adding a significant amount of copper filings will increase conductivity. Think of it this way, consider 2 layers, one copper and one silicone, equally thick. The thermal conductivity through each of them is independent of the other. By mixing those 2 layers together, the analysis remains the same. The heat transfer through the copper particles is a function of the copper and the heat transfer through the silicone is a function of that material. To maximize the thermal conductivity, use as little silicone as you can and as much copper as possible.
     
  6. Q_Goest

    Q_Goest 2,976
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    PS: Those bubbles are also decreasing thermal conductivity. The thermal conductivity of silicone is higher than the gas that's in the bubbles.

    Also, try this: http://en.wikipedia.org/wiki/Thermal_grease
     
  7. I'll try copper in the matrix to see what happens; ceramic has also been suggested but I'm not quite sure what type of ceramic to use. The thermal grease factor also plays into the equation as well. I found that a thin film of vegetable oil in the mould does decrease the bubbles, but the film has to be very thin in order not to wreck the sugar casting detail.
     
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