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I'd like to produce prototype parts by molding a silicon sheet (0.6mm thickness) to a shape that looks roughly like below:

Hollow silicon part

If split in two, this part looks like this:

enter image description here

I have attempted to print this part with a couple of 3d printers using flex resins, but without much success at maintaining a decent accuracy on the thickness. I have also tried to make 3d-printed molds for liquid silicone but getting working parts out of these molds proved a real challenge (I suck a degasing, and these parts have really thin walls).

So, I am looking in buying a flat 0.6mm silicone sheet with fairly accurate constant thickness and then mold it with a vacuum bag under controlled temperature. A couple of questions before I go down that path:

  1. is the general idea sound ? (shaping an existing flat sheet of silicone). I am asking because I can't find any youtube video of people who produce parts like this and everyone knows that if it does not exist on youtube, it does not exist. i.e., I can't make myself believe that no one has done that or documented that before.

  2. Is temperature needed ? (would a vacuum bag be sufficient to mold a sheet ?) If not, how much temperature (compared to the specified max temperature of the silicon sheet) ?

  3. Should I try an outer mold or an inner mold ?

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  • $\begingroup$ I can trivially 3d-print a mold for the inner space or the outer space of this part so, I do not see what you mean by a form I am unable to make. Regarding the different types of silicone, thanks, I had no idea about it. I will look into it. $\endgroup$
    – mathieu
    Dec 12, 2022 at 23:07
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    $\begingroup$ Actually I think there are no thermoplastic silicones. Only thermoset or chemically curing. A form is a mold. Or a plug. So the tolerance issues are in the printing flex materials, but not rigid materials? It's just that with the rigid mold your casting process doesn't go well? Be aware that molds and casts have features such as feeders to accommodate for shrinkage and the like. Thin walls might requires doing it under pressure. Silicone is also known as a high temperature material, probably higher than your vacuum bag material. $\endgroup$
    – DKNguyen
    Dec 12, 2022 at 23:14
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    $\begingroup$ This might give you ideas should you need to turn your resin forms into metal forms so you can work with higher temperatures which might help somehow: youtube.com/watch?v=GJOOLH9ZP2I $\endgroup$
    – DKNguyen
    Dec 13, 2022 at 1:29
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    $\begingroup$ Have you considered blow molding? $\endgroup$
    – DKNguyen
    Dec 13, 2022 at 1:34
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    $\begingroup$ Don't forget that you can make sacrificial molds. You might be better off melting your mold out rather than trying to mechanically pry it off. Dissolving filament might be an easy way. $\endgroup$
    – Abel
    Dec 13, 2022 at 3:23

2 Answers 2

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Don't forget that you can make sacrificial molds. You might be better off melting your mold out rather than trying to mechanically pry it off. Dissolving filament might be an easy way.

It has advantages with silicone RTV (and several other materials as well). Dissolving in water will also apply moisture to cure RTV- moisture that couldn't get through because of the mold.

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Based on the comments by @DKNguyen, the answer is that the general idea is unsound.

The reason is that silicon is a thermoset plastic so, it is not possible to thermoform it.

Now, for the record, I looked into alternatives:

  1. thermoforming thin sheets of another type of thermoplastic plastic. This did not work because the sheets ended being sheared during forming.

  2. Changing the shape to make it more friendly to thermoforming so that shearing is minimized. This did not work because, either the sheet still sheared or the result was not functional anymore (the shape could not do what it was supposed to do)

In the end, I:

  1. redesigned the part so that ot could be molded with a two-part mold
  2. built 2-part molds from 3d-printed resin
  3. built an aluminium overmold to hold the resin mold
  4. did plastic injection of with a desktop plastic injection Holipress and flex pellets

The final product works and, in can anyone is interested, source code can be found here

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