MvA Pumpkin Monster Casting

22 Oct

Halloween is just around the corner and I’m rapidly trying to turn out a few of my pumpkin monsters (Wicked Jack from the Monsters Vs Aliens Halloween special). Here’s a bit of the process I’m going through to create these and the sort of thing you can find in the supporter section.

“There’s quite a few tutorials around about casting small parts, but not much for larger pieces so I took a few photos as I was working on my Halloween project, Wicked Jack, a 12″ pumpkin monster from Monsters Vs Aliens. Things are hectic, so I haven’t had time to pretty this up, so bear with me for a quick and dirty guide to large scale casting…”

Step 1 – Get ready to make a mess. Protect anything that you don’t want covering in sticky or hard resin! I find this large box from a flat pack wardrobe useful as everything stays within it and can’t roll off the worktop.

Step 2 – Small castings frequently use silicone molds, but this tends to be pretty expensive. Those techniques are going to cost you a fortune if you scale them up for this job. Therefore, here’s what I will be using to make mold – latex. It’s cheap and cheerful, and gets the job done. Unfortunately, it does take a while to create the mold in the first place.

Still Step 2 – To create a latex mold you need to apply 15-20 coats of latex, giving each layer enough time to dry (upwards of 3-4 hours for each layer). Over a week I applied coats in the morning, afternoon and evening until I was finally happy with the thickness of the mold, and then left it for 3 days to ensure it was completely cured.

– Ensure that the master has been sealed to protect the paint.
– You can use a latex thickener additive to thicken the latex to the consistency of emulsion paint, however you don’t want to use this for the first couple of latex layers, otherwise it may obscure some of the details in your master.
– For smaller molds you could get away with 10-15 layers.
– For the final few layers, cover the mold in cheesecloth and paint the latex over it. This will help strengthen the mold. I haven’t done this here as the original intention was to do this in one piece and to get the mold off over the tight waist required as much elasticity as possible.

Step 3 – Reinforcing the mold. Ugh. The intial one-piece plan didn’t work (old plaster and it was getting heavy) so I split the mold in half making sure the seam lay along one of the vines running up either side of the body. This resulted in two rather floppy molds which weren’t going to hold up with some weight apllied to them, so the molds were put back on the master, wrapped in clingfilm and mod-roc (plaster bandage) applied to both sides. 3m of this wasn’t enough to make it as rigid as I’d like, so I shot both sides with expanding foam. Now I have a mother mold that the laex mold can sit in and retain its shape.

Step 4 – Resin. I’m now using a 2-part plastic resin. You mix equal weights of both parts together and this hardens enough to demold in 30 minutes.

Step 5 – Additives. I’ve got a couple of extra tricks up my sleeve for this cast which you wouldn’t normally do with the smaller castings. The first of the additives (on the left) are polyfibres, which you add to the mix to make it more thixatropic. Commonly known as thickening. I’m adding this as I need the resin to stick to the sides of the mold. Without it, the resin would be as runny and collect at the bottom of the mold. 5% of the resin weight will thicken it up nicely.

Step 6 – Yes, scales are a must… I usually put the container and spoon on the scale and press the Tare button to reset it to zero.

Step 7 – Extract a little of the resin – I usually used about 20-30g at a time as it will set very fast and I don’t want any unused resin going hard before it’s been applied.

Step 8 – You can add these polyfibres to part A or part B of the resin before they’re mixed. I’m adding them to part B first as it’s thinner than part A and easier to mix together later. You only need 5% by weight (a very heaped teaspoon is approx 2g)

Yum, looks like baby food 😛

Step 9 – Add the other part of the resin and you should now have a thickened liquid. The clock starts ticking now….

Step 10 – Here’s the second additive, to be added to reduce the weight of the resin and bulk up the volume. You can add quite a bit to the resin, though I prefer to stick to a ratio of 2:1 for resin and filler. It makes a horrible grey colour but it’ll get painted and saves a fortune in resin.

Step 9 – Here’s the final mix ready for pouring…

… and in it goes…

For the head, I use a bit less polyfibre so the resin flows better, and then roll the head around so all areas of the mold are covered.

Step 10 – For the larger body shells I use an old brush or spoon to move the poured resin to where it needs to be. The brush will be ruined, so buy lots of cheap ones.

Step 11 – Once the resin is set, the latex if peeled from the cast piece. If you have significant under-cuts it may take a while to get it off as you try to free the latex.

FIN – Tada! Cast pieces all ready for refining (some bubble filling) and then paint.

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Posted by on October 22, 2012 in Uncategorized


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