Silicone + Plaster of Paris = cheap molds?

[resin_curious]

I'm new to resin casting and have already had some fun casting with silicone molds. My issue is the silicone is so expensive, especially in Australia I'm paying about $70 a kg.

I have a pretty ambitious project that requires 14+ molds but I calculated the minimum volume and it comes out to 8kg of silicone or $560!

I was wondering if I could substitute the majority of the volume with plaster of paris?

Example I would paint on the silicone which would be 2-5mm thick capturing the detail and then cover the rest in plaster of paris. Please let me know if anyone has tried this and if it is possible?

[mangozac]

This is absolutely do-able and is very common for larger moulds like in the prop making industry. You will ideally want to add a thixotropic additive to the resin so that it can be applied to the part and stay in place instead of all running off.

This technique is a lot of extra work though. What size items are you wanting to mould?

[resin_curious]

[resin_curious]

I've bought my second batch of silicone I'm going to try this technique, slow pour over each master, then paint on silicone after trying to get an even thickness. Then use plaster straight over the cured silicone still in the mold.

At 30mm thickness my average mold volume is 0.4 cubic centimetres equating to about 500 grams of silicone.

Painting on a thin layer of silicone I'm hoping to get this down to 200 grams of silicone per mold.

If anyone already knows this wont work please tell me now.

[mangozac]

I'm not convinced this is the best way to do it - while it will work it's very seldom done for miniatures. You will need to have lots of locating keys in the silicone so it grabs the plaster properly. 500g of silicone is a very big mould - what scale miniatures are you doing? I would expect the average mould for a 28mm figure to be around 100g.

What casting techniques will you be using? If no vacuum is involved I'd say stop now and forget about he plaster. You need to be able to squeeze the moulds a bit as they are being filled with resin in order to help dislodge and eject air pockets.

[resin_curious]

[resin_curious]

Oh I should also mention I'm not vacuuming anything as I don't have one (maybe one day).

[mangozac]

Silicones for casting generally have a specific gravity of around 1.1, so weigh about 1.1g per cubic centimetre. Not too far off what you've calculated. Based on your indicated mould side I doubt it would use any more than 400g. Remember the cavities for the parts will remove a few cubic centimetres of volume.

If your miniatures are pretty flat the mould doesn't need to be 30mm thick. You could probably get away with 20mm.

2kg is not a lot of silicone. I think you're setting yourself too high an expectation. And without any vacuum equipment I'd say forget about the plaster (and definitely not screwing around with plastecine between the silicone and plaster - that definitely will not work).

[resin_curious]

Okay that is valuable information I will try to reduce my total thickness to 20mm saving my 33% on total material use. Not bad.

I already tried a 30mm thickness and had major leaking issues, a guy at my university workshop suggested the plaster backing.. again he vacuums his and I wont, although I think I will try a thin layer of plaster to help when I rubber band the mold, avoid deformity.

I've just gone back and recalculated esssential only parts at 20mm and got 2.1kg of silicone which pretty good, I found a new supplier who will give me $35/kg.

My only concern now is that a 30mm thick mold leaked like crazy, how am I to secure a 20mm mold without plaster?

Thanks for your feedback so far, I'm sure it's saving me a lot of time and money.

[mangozac]

Leaking has absolutely nothing to do with the thickness of the mould: it means you haven't got your two mould halves clamped together firmly enough. Forget about plaster: it has no use for the kind of casting you want to do and none of us on here use it for moulding/casting. Just get some 3mm MDF (or if you want to do things properly get 3mm polyethylene as the resin won't stick to it) and cut it to the same shape as two sides of your mould. Stick the mould in the middle and a nice, firm rubber band around the whole lot.

Note you don't want the rubber bands to squeeze too tightly or you'll get mould slip and distortion.

$35 a kg for silicone is very cheap. I'd be quite wary of it. My current recommended resin products are Pinkysil and Ultrasil, both from Barnes and both about double the price you're suggesting.

I want to make sure you understand that resin casting is by no means a cheap hobby. You will screw up and waste both silicone and resin, especially early in the learning process!

[resin_curious]

Hmm I used cardboard the first time I banded it together. Probably the reason for leaks.

I was concerned about the silicone too I had a long chat with the supplier and they where convincing, time will tell.

Yes it is expensive I was exploring resin casting as a means of low volume production for a start up company. I'm an industrial designer, so it could be a way to get started.

I think I've had all my answers questioned I will try for a 20mm thickness mould and hopefully it works.

[mangozac]

OK keep us posted!