mold making

[GTRfan01]

hello,
I have a simple question for anyone out there who has had experience making a silicone mold to cast resin parts.

my question regards the use of rubber to rubber release agent or similar used on the master part to make the mold.

i have read the how to on this forum and it says to use it, others tell me i dont need to as silicone wont stick to the master.

does it change if the part is more intricate?

if i do have to use it, does it have to be the brush on rubber to rubber one or will other release agents work?

I am using 'pinkysil' which is a platinum based silicone and i have been told its a bit more sensitive to some products.

thanks very much in advance

[kitbash]

I've done a lot of mold making, and I always use a parting agent. I use RTV silicone for my molds, and believe me, I've learned the hard way that rubber sticks VERY well to rubber!!
As for a parting compound, I've had great luck with simple vaseline, brushed lightly onto a mold-half. You only need enough to create a slight, oily-looking sheen on the rubber. I've never had a problem using this method. Alumalite also makes a purpose-made liquid that you brush on, and I'd assume it's a petroleum based compound, although I've never used it.

Don't try to pour the second-half dry, dude...not worth the risk!!

[GTRfan01]

Originally Posted by kitbash

I've done a lot of mold making, and I always use a parting agent. I use RTV silicone for my molds, and believe me, I've learned the hard way that rubber sticks VERY well to rubber!!
As for a parting compound, I've had great luck with simple vaseline, brushed lightly onto a mold-half. You only need enough to create a slight, oily-looking sheen on the rubber. I've never had a problem using this method. Alumalite also makes a purpose-made liquid that you brush on, and I'd assume it's a petroleum based compound, although I've never used it.

Don't try to pour the second-half dry, dude...not worth the risk!!

hi kitbash,
thanks for the reply
is yours a platinum based RTV? i was told that vaseline might react with a platinum based silicone??

oh, and do you use the vaseline on the original part to make the mold in the first place?

[rgriffs]

good advice, do not pour rubber onto rubber without a release agent, it will become as one. with no evidence of a join line. i use a release agent bought from a resin stockist, i have used vaseline, but it reacted with the particular rtv rubber i was using at the time. you can always test it first. i only use the release for making the mould, very rarely for producing castings, again, depends on your rubber type. as with most new skills, they need to be explored by yourself, start by moulding easy bits to gain experience, plus to reduce the scrap costs. the most important thing about making the mould is to remember that when you pour the resin into the mould, the air has to be displaced. avoid steps which trap air, use talcum powder to coat the mould, this helps 'the resin to 'wet' the mould, and swirl the mould to coat everywhere. good luck, you will not look back once you have got it right.

[OutaFocus]

I never use a release agent on the master when creating a mold, but I always use it when casting duplicates. I use Vaseline on the RTV where it will touch the second half of the RTV mold,

I always use a tin cure RTV for the reasons you stated. Platinum cure RTV has the advantage of better stability, it won't shrink during curing like tin cure will, but the shrinkage is negligible and the downside to platinum cure RTV is too much trouble. It is veeery sensitive to what comes into contact with it during curing.

If you are going to use the platinum cure and use clay when making the mold, be SURE you use a clay with no sulfur in it. Sulfur will inhibit the curing of the RTV leaving you with a real mess.

I didn't catch where you are, but I use a clay named Kleen Klay, it has no sulfur and is safe for use with platinum cure RTV.

My final advice is this, if you haven't already purchased the RTV, buy a tin cure RTV, you will be much better off especially if you are new to casting. Trying to troubleshoot problems will be much harder if you use platinum cure RTV.

Scott

[GTRfan01]

thanks heaps for your advice guys, ive already bought the silicone so ill stick with it, i know its the best type so i'll just learn to use it.
i know i have to be careful thats why im just making sure i have it all down in my head before i jump at it and possibly waste material by not using it properly, i think im all good to go now, so thanks everyone for the advice.

[rgriffs]

lots of informative videos on you tube.

[cinqster]

Yep, I only use Vaseline on my rtv moulds. Not heard about platinum rtv, so no idea, I'm afraid.

If you find the Vaseline too thick to use as a release agent, you can thin it with white spirit (mineral spirit for those overseas). I cut it about 1:5 vaseline/spirit ratio and still works well. Apply it thinly with a brush and you can see it 'wick off' as it goes on, but still does the job...good luck and let us see the results!