Wednesday, 10 July 2013

Learning to cast resin

While I'm on a roll resurrecting old tutorials, here's some notes from my ventures into casting. I'll go back and revisit this sometime in the future and flesh it out a bit more, but it may be of use to someone in it's current state.

There's a shed load of stuff you need, all of them pretty cheap to get hold of and useful. I kept having to do trips into town at the weekends to pick up something else :

- RTV rubber
- the fast cast resin
- plasticine
- wooden spatulas (either buy lots of cheap ice lollies from Asda and feel refreshed each time you make a mold or cast, or Tomps sell boxes of 100)
- syringes
- lots of lego blocks, preferably the 2x4 or longer, plus a large, thin base
- plastic party cups (50p for 100 from Asda)
- Vaseline.
- a cheap brush
- something to melt vaseline in, I've got a cheap curry bowl from Poundland
- some decent scales; the more accurate the better.
- lots of old sprues to make the pouring channels in the molds
- Talc

JB's casting tutorial in the Articles section of DakkaDakka.com is pretty good, with a few modifications as noted below. I think most of the variations are because I couldn't get hold of the molding putty he used in the UK without paying a fortune to import it. In the UK, the best place to get resin and the RTV rubber is http://www.tomps.com , they were quick to respond to my questions and are planning on putting together a beginner's pack sometime soon. Just don't be fooled into buying the casting manual they have... the useful bits are already on their website; the rest you really don't need.

1. Why the Vaseline?
A number of tutorials suggest putting a layer of olive oil on the first half of the mould. this may work if you're using putty that JB uses. This doesn't seem to stop the RTV rubber from sticking together though, so a thin layer of melted vaseline painted over the rubber (not the models!) will help you not to make a large blue rubber block. Painting over the models as well will obscure details, making the casting process pretty pointless. You can use a spray can of release agent, but Tomps didn't think it was necessary while I was learning as I'm unlikely to want the early molds to last too long.

2. Why the Talc?
The first test cast I did came out minus most of the smaller details. There were no visible air bubbles in the casts, it just didn't seem to have made it into all the corners so everything looked a little melty. A layer of talc on the two halves of the rubber before casting (with the excess shaken off of course) seems to help the resin flow into some more details; though it's still not 100% crisp, but this may have been to talc being missing in some placed.

3. Why the syringe?
Leaving gravity to do it all may work if you're only doing one or two large bits, but if you're trying to do smaller things like rifles with fiddly bits, you're going to need some pressure to get it flowing through the vents. Be careful not to push the syringe into the channels when injecting the resin though as you'll push the two halves apart and cause leakage/air bubbles.

4. where's the elastic bands mentioned in the Article?
The first time I tried casting, I did have elastic bands and card around the molds, but it didn't seem to be working very well. The molds were deforming and I was getting a lot of leakage and air bubbles. Luckily the two molds that I'd made, side by side, can be secured between lego blocks on the base. This allows nice even pressure across everything apart from the top. It's not stopped the flash from forming, but it's now significantly less than with the elastic band. I think I just need to put more bits of card in between the molds and the lego, to increase the pressure slightly.

5. Think about the placement of your pieces in the mold
As the title says... think carefully and make sure there aren't any bits that are going to trap air.you are injecting the resin in, so you'll need somewhere for the air to come out. I found it best to have the channel going down the side and along the bottom with some air vents coming out of the top. Always try to angle any pieces so there aren't any corners that bubbles will get trapped in.

6. Indentations and a simple solution
When it came to creating my first mold I noticed that I'd sculpted some recesses that were going to cause a massive problem, then I had a brainwave. Using a bit of green stuff I created some little plugs that are molded to the recessed area and a hooked area at the top. When you create the mold halves, make sure this piece is sitting in the recessed area properly. When you go to create an actual cast, carefully place the plugs back into the now empty mold so that they will allow you to create recessed areas that a 2 part mold wouldn't normally like.  They easily pop out and leave you with a recessed area. *tada!* I wouldn't suggest using too many things like this though as you're bound to forget to put them in (like I did for test cast 2) and you end up with an odd hooked lump on your piece. You also need to remember to put a little bit of vaseline on this, otherwise the resin can stick to the greenstuff plug.

7. What about all these rubbish test molds?
You don't need to throw them away. Keep them; slice them up into little bits and put them around the edges when pouring in your RTV to create a new mold. As long as there's no release agent or vaseline on them they will stick to the new rubber, bulking out the mold and saving you some precious rubber

What else?

A number of people recommend vacuum chambers or pressure pots. If you're only doing basic, fairly plain things I don't think they are necessary. They may, however, be the answer to me getting more detailed casting... though it's rather expensive to shell out a couple of hundred pounds on a theory. A vibrating plate that you sit the molds onto would help to bump some of the bubbles out of your mold. Poundland occasionally has "personal massagers" that would be good for making something like this

2 comments:

  1. Thats another useful article, Paul! I was planning to dip my toes in resin castnig as well, but I'll put it off for now. It is planned and has to be done eventually, so I better save this article! :D

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  2. At some point I need to go back to the casting and try for more detail. I've also found that differen people find different things that work for them... These are what worked for me
    When you do giv it a go, feel free to ask questions and I will try to help; though I cannot claim to be an expert yet :)

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