Machining accuracy with silicone rubber 3

[Finglas]

Anyone got experience of CNC machining silicone. Using it as a gasket and we're not sure what sort of accuracy can be achieved? Would it be in the region of +/- 0.5mm? Will be using it as a gasket and it will be 15mm thick.

 

[btrueblood]

Dunno why you wouldn't machine a mold and mold that. You could try freezing the stuff to make it stiffen up, it will machine a little easier that way. Alternatively, use a grinding burr.

 

[Finglas]

I imagine a mold would be expensive for 1 custom made piece of silicone?

 

[btrueblood]

Doesn't look complicated enough to be expensive - if you have a CNC mill already. Machine the mold in two pieces of aluminum with a couple of bolt holes to clamp it together, or one piece if you don't care about having the thickness held to a very tight tolerance. Cut a couple of vent holes, fill the cavity with RTV*, bolt it up and let it cure... The nice thing about silicone is you really don't need to worry about draft in the mold, nor release compounds, just have a fairly smooth mold surface and you should be good.

*

https://www.mcmaster.com/rtv-silicone-casting-compounds

- there are a lot more and probably cheaper sources out there, google/amazon etc.

 

[enginesrus]

Machining soft materials usually requires freezing them.

 

[iainuts]

Try water jet cutting.

 

[Finglas]

@iainuts - Cool, thanks for that.

 

[MikeHalloran]

You can't cut the illustrated step with a water-jet; you need to cut entirely through the sheet.
You could water-jet two flat pieces and bond them together.

... but it's MUCH easier to machine a stepped cavity in a block of aluminum, polish it, pour in RTV, and let it cure.
Self-leveling RTV probably wouldn't even need a cap/cover on the mold. ... and would cure more slowly if a cap is present to restrict uptake of atmospheric moisture for curing.

Two-part RTVs intended for molding would probably need a two-piece mold, and will cure more quickly than single-part RTV.



Mike Halloran
Stratford, CT, USA

 

[badservo]

I made a few special seals using acetal for the mold and a 2 part silicone with good results. Didn't measure it post bake, but it seemed like a perfect fit.