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Improving thermal conductivity of RTV silicone for hard candy

  1. Mar 24, 2013 #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. Mar 24, 2013 #2

    Q_Goest

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    You could mix copper filings in.
     
  4. Mar 24, 2013 #3
    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. Mar 24, 2013 #4

    Q_Goest

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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. Mar 24, 2013 #5

    Q_Goest

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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. Mar 24, 2013 #6
    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.
     
  8. Aug 24, 2016 at 3:26 PM #7
    Heat transfers two different ways. Electrons, and Phonons. Electricity, and Sound. Phonons are vibration of the crystal structure. Generally rubber will not transfer heat well because it isn't electrically conductive, and doesn't transfer sound well.

    The trick is to increase the particle load by an enormous amount (better than 90%, this is how arctic silver works, high particle loading, and mixed particulate sizes, so the small particles fit in the gaps of the large particles). To keep the mixture from cracking try to find a spherical particulate, so shot instead of powder. Silver may corrode less than copper. Industrial grade diamond would be best, but isn't spherical so you might have to keep the particulate loading low, depending on how the mold is handled (more flexing = more tearing). It isn't linear, and even highly loaded molds will be no where the solid particulate's thermal conductivity.

    Diamond transfers heat so well, because it is hard, and therefore transfers sound well. The last idea would be to bridge the entire thickness of the mold using spheres of the same size. You can get a pack of 100 surgical stainless bearings for ~$5 (62 thousandths of an inch in diameter). Just go for the size bearing that your mold is thick. Since they are smooth, the candy may release from the bearings well. This should keep everything food grade (depending on what silicone you are using):

    https://www.amazon.com/Stainless-Sphere-Mirror-Like-Precision-Tolerance/dp/B00CNM03D2
     
    Last edited: Aug 24, 2016 at 3:37 PM
  9. Aug 24, 2016 at 4:13 PM #8

    Nidum

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    Cast ducts into the rubber and use circulating water for cooling . Keep the walls of the actual sweet cavity as thin as practical .
     
  10. Aug 24, 2016 at 9:20 PM #9
    Since Copper is considered a "heavy metal", if you haven't already done so, please investigate its toxicity before using it in contact with food. Ceramic spheres could be safer because they are inert, just don't crush any of them (ground glass equivalent).

    A good place to start searching for materials:
    https://www.google.com/?gws_rd=ssl#q=food+grade+material+site:gov

    To eliminate bubbles, de-gas the liquid either before or after pouring into molds. This is done by briefly reducing the air pressure around the product, causing the dissolved gasses to leave the solution. Put the stuff in a strong container and connect a vacuum pump. Alternatively, you could try a vacuum cleaner as a vacuum pump if it is powerful enough, although that's not likely considering the viscosity of your sugar solution.

    Let us know what you settle on.
     
    Last edited: Aug 24, 2016 at 9:33 PM
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