Can silicone Rubber be rotationally molded?

I have a product in mind using silicone rubber. I cand’t use injection molding because it has an undercut that’s why I was wondering if it can be rotationally molded. ie. a cone with a small hole on top. I would very much appraciate any input. Thanks.

One of the coollest things about molding elastomers is that you can have undercuts and interior cavities.

I just got back from the rotomolder’s this morning…didn’t see or hear anything about elastomers…my guess would be ‘no’ as the temps are usually lower than what is needed to unlink, heat, and re-link them polly-mers. But what do I know. Listen to Bryan. :wink:

It doesn’t even have to be that flexible. One of our vendors injects polycarbonate into molds with an undercut of about .007" depth. It doesn’t seem like much, but the very the fact that the feature doesn’t get sheared off is something the mfg is very proud of.


By the sounds of things, your part is small (because you have considered injection moulding) and requires no detailed internal shape (because you are considering rotomoulding)

You may be better looking at some of the dipping or blowing techniques. They will poduce similar or better part quality, but much cheaper as the cycle times will be a lot less.

Thanks to all for the infos. I will have about 10- 15 degree draft inward similar to a hollow cone that’s why injection molding may not work. The part is about 3" high and around 2" wide so it’s a small part and no details inside. If it is injection molded, it will need to have a core (will add to the cost) and having the part pull out from the core is the problem that’s why rotomolding
came to mind. I guess blow moldding is also a possibility like a photographers blower brushes that cleans the lenses. Thanks again to all. This has been a big help. Any more inputs?

as far as i know you can’t do silicone in rot moulds. I have just finished a couple of silicone tools for injection moulding and I doubt that you could get the rot mould up to the required temperature. Even though its a undercut it doesn’t sound to severe for inj moulding. good luck

you can also take into account the 5% shrinkage that happens when the part is cooling. Roto experts say this allows you to do something similar to a 5-7 degree draft, and still be able to pull it out of the mold after it shrinks.

If your talking about TPE like santoprene (sp?) and not silicone?

I’m pretty sure you don’t injection mould silicone, you compression mould it. I have never seen the process in action, nor used it much, so I can’t really tell you much other than that.

If the undercut is designed correctly, you could use a slide or core on the IM tool. This adds some cost to the tool, but the advantage lies in the cycle times and overall reduction in cost per part. Take a look at the inside of most consumer electronic enclosures, slides everywhere.

I’m definitely talking about Silicone rubber because of its abiltiy to handle high temperatures. Yes you can injection mould liquid silicone rubber.
The silicone rubber on the baby bottle nipple is injection moulded.

I’m trying to minimize cost so any additional action is undesirable.
I’ve seen compression moulding done when I went to see a factory in China last year. They compression mould those silicone trivets you see in stores such as bed bath and beyond and others.

The best way I can describe compression moulding is like the waffle maker you buy for the kitchen. You have a clam shell type of mould. You put the solid silicone rubber (cut and weight to the right amount) and place it in the
bottom half of the mould. Close the mould and wait until the rubber melts. Pull up the mould and then remove the moulded trivet. Then they place the moulded part in an oven to bake for a few hours ( I forgot the actual hours). I think they clean the part first before putting it in the oven (remove flashing).

They order thick sheets of clear silicone rubber (the size of a twin bedsheet) and knead it mechanically. They have this 2 big steel rollers that they put the sheet through for color mixing and removing bubbles. As the sheet is put through the rollers many times, color paste is added. They then put it through again to thoroughly mix the rubber. For the final step, they roll flat the silicone again to the desired thickness. They then cut it into cubes to the desired size and weight.

Thanks again for the inputs. I hope my input also helps.