Making Rubber Casts - Help

[Talion]

Ok I have had enough :flame:

I\'m trying to make a rubber cast of some bits I\'ve created. But i always end up with a solid block of silicone rubber. I bought a £30 tin, and only have a little left with nothing to show for it.

This is the process I\'m using

1. Roll Plastacine out, push items in so that they are half covered.

2. Build a lego brick wall around it, and pour in the mixed rubber (rubber and catalyst).

3. Leave for 24 hours

4. Remove plastacine, and cover the half mould in grease (I\'m using Olive oil)

5. Rebuild lego wall and pour in second half of cast

6. Leave for 24 hours

7. End up with a solid lump of rubber!!!!!!

What am I doing wrong?
I don\'t understand why it\'s turning into a solid lump, even when I cut it to pieces to get my bits back out, you cannot see any kind of join.

Someone Help Pleeeassssse

 

[airhead]

should work - couple of questions:

You are making a 2 part mold for a part that is not just a flat back part? If so, why are you removing the part for the second half of the mold?

I once made a mold with ureathane rubber, then cast ureathane plastic resin in it forgetting the mold release. Same problem - solid lump of rubber/plastic.


try www.smooth-on.com - they have some good tutorials. Also try www.hirstarts.com in the tips pages.

 

[Spacemunkie]

Use talcum powder instead of oil. Get some catalyst booster too.

 

[Talion]

If so, why are you removing the part for the second half of the mold?

I\'m not I\'m leaving it in there.

Use talcum powder instead of oil. Get some catalyst booster too.

What will the Talcum powder do that\'s different to the oil (I\'ll give it a go though)

My main problem is I don\'t know why the 2 halves are moulding into 1. I\'ve done 3 moulds so far doing the same thing, and the two halves seem to weld together. When cutting out the peices to get them back the rubber looks like I just poured it all in in one go. It\'s impossible to tell where one half ends and the other starts.

I\'m going to have to invest in some mould releasing agent.

 

[leopardpixie]

try this link(s)
http://www.hirstarts.com/moldmake/moldmaking.html

 

[Einion]

From what I\'ve read it simply sounds like the olive oil is just not working as a release ??? Any reason you chose that specifically or was it just what you had on hand? Oils/greases should work of course, try Vaseline, I know someone who uses this as a release and that definitely works.

One other quick thing: you may have just left it out as you were typing but just in case you\'re not, be sure to key the surface of the plasticine so that the two halves of the mould, when you get them eventually, can be aligned easily.

Einion

 

[minimaker]

To confim what Einion said, I use diluted vaseline/spray vaseline and that works as a mould release.

Bye, Ming-Hua

 

[Talion]

Any reason you chose that specifically or was it just what you had on hand?

Just wa sat hand thought that any type of oil would work. Obviously not.

use diluted vaseline/spray vaseline and that works as a mould release.

What do you use to dilute the vaseline? How thick do you cover it?

 

[Spacemunkie]

Use talc or buy some proper mould release agent. If that doesn\'t work, then it\'s definitely something you are doing that\'s the problem.

Make sure you don\'t disturb the master after making the first half of the mould as the silicon could attach itself to rubber that has no release agent on it.

The only other thing is that you haven\'t left the rubber to set long enough. As I said, get some catalyst booster -it\'ll drop your set time down to around 4 hours....