Silicon molds

[Jimmy]

Hi all,

1st post here!

I have a question re casting: I am producing a 2 part silicon mold to reproduce a couple of parts. I need to create some very thin tunnels for air to escape from certain areas and was wondering if anyone has any tips on tools for this job?

I was thinking that I could sharpen the end of some brass tube and will probably try this but any tips would be appreciated while i'm waiting for my silicon to turn up.

Thanks in advance,
Jim

[Rocketeer]

There are a couple ways to do this.

1. Make the vents part of your master; glue thin styrene rods onto the master.

2. Cut the vents into the silicone after it's cured; a sharp knife (I've used my scalpel) does this easily enough.

3. Reorient the part so the bubble-catching features are oriented upward, so the bubbles don't collect there. For instance, if casting a head, you'd orient it face-down, with the pour stub at the back of the head. That way, bubbles won't collect in the nose and chin; and if they do, a little poking with a toothpick will dislodge them.

4. Also, if you can manage it, you should arrange your mold and your pouring passage so that you're filling the mold from the bottom; that way bubbles will be less likely to form. For instance, if you were casting a chair, you'd want the resin to enter the mold at the bottom of each chair leg, and you'd have a vent at the top of the chair back. So you'd have a funnel-shaped opening at the top of the mold to pour the resin into, with a passage leading down to the bottom of the mold, connecting with the chair legs.

[Joseph C. Brown]

If you scroll 2/3 down this linked page, there are some nice images of what you describe wanting:

http://www.dakkadakka.com/dakkaforum/po ... 01577.page

Also, bring up Google, type in 'silicone mold pour sprue' and click search. Then hit the Images button, and you'll see many examples of what you are looking for.

[Jimmy]

Thanks for the replies.

1. Make the vents part of your master; glue thin styrene rods onto the master.

Sometimes these things are so obvious you cant see them! I will definitely do this. Positioning of the master is difficult - whatever way i do it Im gonna get bubbles.

Also the reason I want a tool for this is that I want to add air channels to existing molds. I have seen these before but cant seem to find them again.

[Umi_Ryuzuki]

A brass tube that has been sharpened at one end will cut the sprue lines.
Be sure to use a tube that is slightly larger than the sprue diameter you want.
Experiment a little in some cured scrap RTV. The tube will cut a slight flare at the starting of the cut.