Heat transfer in over molding plastics

Question

Heat transfer in over molding plastics

Maxokay (Mechanical)

(OP)

23 Feb 18 23:51

This should be relatively straight forward but can't wrap my head around it. I have an electronic enclosure that will be covered in silicone at 120C. There are sensitive electronics that can't get too hot and I want to know if the molding will heat them up too much. Assume that the electronics have just 2mm thick of silicone protecting them from the overmolding. The overmold is 2mm of liquid silicone at 120C for 3 minutes. How hot will the electronics get(or, how hot will the inner wall of silicone that they touch get)

Thanks

Compositepro (Chemical) · Answer 1 · 2018-02-24 00:39:12

The surface of the part you are molding over must be at 120C for 3 minutes in order for the silicone to cure correctly. Silicone is a thermoset material, not thermoplasic. Your question would make sense in the case of a thermoplastic where you inject molten resin that then freezes.

chicopee (Mechanical) · Answer 2 · 2018-02-26 14:53:17

Since it could be a silicone mixture which is a liquid at 120dC, and assuming that the enclosure can be opened once the silicon film is applied, I am thinking that by placing a cold heat absorbing material such as a copper block within the enclosure could minimize damage.

racookpe1978 (Nuclear) · Answer 3 · 2018-02-26 22:48:23

120 C is well over the boiling point of water (Keep that in mind for mental reference.)

Your electronics start out within some sort of enclosure (metal? Non-conducting Plastic? Thermally-conductive plastic? Wood?) at room temperature. That assembly has some mass - which you have not told us. It has some shape - WHich you have not described.

The "container" is then surrounded completely by the molten silicon (whose mass and external shape you have not defined!), which then MUST cool down to room temperature, right?

You now know (or can calculate) the thermal mass (inertia) of this undefined silicon-enclosed assembly. First, find the missing data. Determine "exactly" what is the maximum temperature your electronics inside the glob can tolerate, and for how long they can tolerate that temperature. Set a margin below that temperature.

The maximum temperature on the interior of the "glob" can be controlled by cooling the outside of the glob. To reduce the interior temperature with least effort and contamination, blow cool air on the outside of the silicon as soon as possible after the silicon sets. Theoretically, you could even dunk it in water, but that's too difficult to dry, is messy, allows spills and probably will kill the electronics.

Run several test cases before committing to the process in the production line. Trust the test results, ANY transient heat exchange calculation of the interior of the glob will lucky to be within 5-10%.

IRstuff (Aerospace) · Answer 4 · 2018-02-27 05:39:20

Are you heating the assembly to 120C for 3 minutes, or is that simply the process time?

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Heat transfer in over molding plastics