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Hey guys i was casting alot lately and need some help. With items 2or more feet i was using smoothon oomoo, but it gets costly, so i am trying rtc rubber in pressure cooker. But again i need to make a large reuseable mold. In canada btw thx in advance.

Need more details;
1)what is it you are trying to cast?
2)Is this just a rubber question?
3) Or is it just tips on making a larger mould?

Slipstream wrote:
Need more details;
1)what is it you are trying to cast?
2)Is this just a rubber question?
3) Or is it just tips on making a larger mould?

Polymer clay type meterial "doesn't harden when cooked and melting temp of 85 degres.
Any type of molding meterial as long as it can be reused
Any tips on large mold making would be loved.

Thx

If you are trying to make a huge, but cheap mold, you could use silicon sealant with a plaster/fiberglass shell around it.

Silicone sealant, cures with moisture. If you mix some corn starch into it, you get a gummy mess that eventually takes on silly putty consistantcy and will cure all the way through, Keep your hands/tools wet while working with it to make it easier to handle and give it a little extra moisture it needs to cure. Once it's sured, you can create a plaster or fiberglass hell around it to help it keep it's shape.

Some petroleum jelly to keep the halves from sticking together, and you can repeat the process for the other side of your mold.

It's ugly, and involves more labor, but it works, and it's cheaper.

---

Some people just squeeze out a whole tube of silicone sealent into a bowl of water and knead it until it's a ball of less tacky silicone putty, instead of the corn starch. That works as well.

The easiest and cheapest way to make a large mould is the following;

1) Place model on a piece of formica board. The wood that has the shiny surface on it?Ideal stuff for mould making.

2) Secure model to the board, I use double sided tape for this as you don't need a permanent fixing,just long enough for the silicone to set.

3) Grab some cardboard boxes. Measure how high the model is and cut the cardboard at least 4 inches higher. For example you have a model of a brick(!) that measures 4x8 and 4 inch high. What you are doing is making a case for casting. Cut four lengths of cardboard, 2 at 5 inch and 2 at 9 inch and all at 6 inch in height.

4) Grab a glue gun from a hobbyshop/art shop and plenty of glue sticks!

5) A thing you will need to bear in mind. The longer the model the MORE silicone you will have to use. If (in the example of the brick) you cast the length way thinly it will collapse inwards during casting. If you allow for this and increase the gap between the cardboard and the model the silicone will be more rigid and should stay in place during casting.

6) Setting up the model case. PLace any one of the cardboard surrounds beside the model and hold while you draw a line of glue(your gluegun)along the bottom of the card and stick to the formica. Hold in place until the glue starts to set. Get the other of the same length and do the opposite side. Then do the remaining 2 pieces. After it has cooled and dried use the gluegun again to run up the connecting edges of cardboard. This has 2 functions;1) It stops any leaks during casting and 2) It strengthens the mould.

7) Before you start casting draw horizontal line along the inside of the mould .This is your 'stop filling!' point. Usually you'll only need upwards of half an inch.

8) It is always better to fill from a corner of a mould;NEVER pour silicone directly onto a model! Also slowly,ok?

9) Get into the habit of making a case for each time you are going to cast;It will stop a mould bulging outwards when filling it with resin.

10) As for the silicone; all types of mould making silicone will work but it depends on how intricate the model is? The harder silicone tends to be the red stuff (sorry can't remember the terms at the moment,it's been ten years since I worked with silicone!). It is very robust but unless the model is very basic I'd stay away from it because (as it is robust) it is more likely to tear quickly. The white silicone is more your likely option. It comes in various grades ranging from very soft to almost robust! ask the supplier for something in the medium area. Soft tears very easily in more detailed models,medium will last a bit longer.

11) Use a model release spray each time you cast. More importantly,use it on the master model before you cast it for the first time.

That's as much as I can remember on mould making for the moment! As I said it has been a long time since I did any and I've forgotten a lot! If you have any questions pm me it may jog my memory!

Reusable?
http://www.composimold.com/make-your-own-unique-molds-and-casting.html

You microwave it into liquid, then you can use it as a mold.

Warning though it gets REALLY hot if you are not careful. possibly get a thermometer and figure out the ideal poring time.

Iv had the ABS black bases completely melt on me once.

Thanks for all the tips guys I will be using most if nto all of them. Sorry for the slow reply.... life gets busy ....real busy lol.