Improving thermal conductivity of RTV silicone for hard candy
Improving thermal conductivity of RTV silicone for hard candy
(OP)
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.





RE: Improving thermal conductivity of RTV silicone for hard candy
RE: Improving thermal conductivity of RTV silicone for hard candy
RE: Improving thermal conductivity of RTV silicone for hard candy
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RE: Improving thermal conductivity of RTV silicone for hard candy
RE: Improving thermal conductivity of RTV silicone for hard candy
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RE: Improving thermal conductivity of RTV silicone for hard candy
What is your hypothesized reason that thermal conductivity is causing your problem?
Yes, metal molds have better conductivity. They also do not absorb moisture. I have worked many void problems, and moisture is the most common cause at 300F with no pressure.
RE: Improving thermal conductivity of RTV silicone for hard candy
RE: Improving thermal conductivity of RTV silicone for hard candy
RE: Improving thermal conductivity of RTV silicone for hard candy
It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.
RE: Improving thermal conductivity of RTV silicone for hard candy
RE: Improving thermal conductivity of RTV silicone for hard candy
RE: Improving thermal conductivity of RTV silicone for hard candy
The bubbles you see in the sugar are mainly superheated steam and will collapse when cooled. The temperature you cook sugar to is a measure of the residual water content. If you try to melt sugar without mixing with water the sugar will burn (decompose from the high temperature).
RE: Improving thermal conductivity of RTV silicone for hard candy
1 regular silicone without drying
2 regular silicone dried in an oven at 300 degrees F. for about 20 minutes
3 Silicone matrix consisting of 50% silicone and 50% silica (sand) dried at 300 degree F. for 20 minutes.
The first sample returned the usual result full of bubbles on the casting surface (see pic 1)
The second sample was a big improvement, the dried silicone cut the bubbles dramatically (see second pic)
The third sample was even better; the casting was almost perfect; it's hard to tell, but most of the bubbles are on the back side of the casting.
Obviously moisture plays a part here, Compositepro I thank you for that. (I struggled with this for weeks) Not quite sure why the matrix with silica showed the best results unless thermal conduction is playing a part.
RE: Improving thermal conductivity of RTV silicone for hard candy
If the heated molds did better, by being dryer, there was also a cooling rate difference. Does the mold with the 50% silica in the matrix cool slower than the one without the 50% silica? or faster? What is the thermal mass effect of replacing RTV Silicone with Silica?
I have a feeling that there is a critical cooling rate that will allow bubbles to nucleate and grow in the sugar/water solution. If you cool to below where the partial pressure of water vapor in sugar is low quickly, then slowly cool through the solidification of the sugar, the bubbles should rise out. Opened back molds correct?
Nick
RE: Improving thermal conductivity of RTV silicone for hard candy
Thanks for the feedback, westrock. That helps us all learn.
RE: Improving thermal conductivity of RTV silicone for hard candy
In my application it appears that the critical factor affecting casting quality is moisture absorption into the mold material. I think Compositepro is on the right track by stating that the silica in the matrix is allowing less moisture into the mold material, (basically by displacing silicone). If I can eliminate as much silicone bulk from the side walls and other parts of the mold, cut silicone by 50% by substituting silica (which is basically inert) and dry the molds properly, the castings should be close to perfect. At 50% the silica does not seem to affect the qualities of silicone that make it a great mold material, although shear strength, and durability my be compromised in the long run.
RE: Improving thermal conductivity of RTV silicone for hard candy
If this is a commercial venture, the problems associated with temporary tooling seem like a moot point. If it is extremely critical to provide aesthetically perfect samples, presumably for sales or tradeshow samples, perhaps the metal tooling is not a bad investment.
I did have one other thought on this. Years ago I was involved in a urethane casting project in which we had to put all material into a bell jar and apply high vacuum to de-bubblize the pour. Our molds were, in fact, metal, and the purpose of vacuuming was solely to address porosity in the finished casting. My memory fails, but it seems to me we had to exceed 28" in order to get a clean pour. With the shorter work time of a hardball or hardcrack sugar mixture, this idea may prove not to be viable, but thought I'd mention it.
Good luck!
It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.
RE: Improving thermal conductivity of RTV silicone for hard candy
I did try molten sugar in a vacuum; that was scary :) Molten sugar reacts violently in a vacuum chamber.
RE: Improving thermal conductivity of RTV silicone for hard candy
Thanks guys; things were learned, results were posted... very cool.
Charlie
www.facsco.com