I needed to make a mould of a 50 cal shell casing to make replicas, I'm in the UK so they are expense to get hold of here.
As I was making the mould I thought I would document the process so that others can learn how to do the process.
I will be making a very basic 2 part block mould out of RTV silicone rubber.
This is a mould for use on my Life Sized ED-209 Project, from Robocop check out the Facebook page
the shells will be used as set dressing when its complete.
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Step 1: What You Need
Silicone Release Spray
RTV Silicone and catalyst , this comes in a variety of hardnesses and working speeds, I like to use a slower working speed version, gives me a pot life of 40 minutes to work with it. Its probably best to use a slower silicone if your not used to it.
scrap wood to make a box around what you would like to mould.
Oil based modelling clay.
tongue depressors for mixing.
Acorn nuts or something with a round end.
Masking tape or duct tape to hold the box mould together.
Step 2: Preparing and Making the Box.
depending on what object you are moulding while depend on where you put your seam line, its possible that a thin line will show up here or some flashing from the casting material which would have to be removed from the final piece.
I have put a red line on the picture of the shell where i want my seam line to be.
using oil based clay build up to the seam line. if you have a hollow object you may want to plug up the holes, i did this with my shell casing but left enough to get the shape. you also want to push something in to the clay which is round this will make registration keys which will allow the mould to be put back together the say way it came apart. you can use acorn nuts or the end of a paint brush or clay tool.
using tape and pieces of scrap wood build a bod around the piece making it as water tight as you can, its best to make it on a base that you can move around.
Step 3: Mixing a Pouring the Silicone
you will have to read the guides for your silicone and different silicones come in different colours depending on the brand. I am using the Tomps.com value range which is great.
The tomps value silicone mixes at a ratio of 20:1 that's 20 parts silicone to 1 part catalyst by weight.
I mix it up in small batches if i can, for the first part i need a fair bit so i am using 200g of silicone plus the catalyst, mixed in a pot using a tongue depressor. be sure not to whisk air in to it and use the depressor along the sides of the pot to ensure you mix everything fully.
because I have a longer pot life once I have a consistent colour i leave it to stand for 5 - 10 minutes before I pour to allow some of the air bubbles to surface. if you have a vacuum chamber you can put the tub in there and force the air out slowly. I don't have one right now so this is the best I can do.
when pouring do not pour directly over the item you want to mould, pour in to a corner and allow the silicone to flow over it, i like to put the whole mould box on an angle so i prop it up with a block of wood. when pouring try and create a very thin stream, the idea of this is to break up some of the air bubbles as it falls in to the mould box.
make sure you fill well over the top of the item being moulded.
for larger moulds I like to make sure that most of the item is covered and if need be used chopped up bits of old silicone moulds to pack out the new mould. this just saves on wasting new material.
check your silicone for how long you need to leave it, but i would say 12 - 24 hours is best.
Step 4: First Demoulding and Reboxing
remove the tape and peel off any over spill and keep it for use on a later mould.
move the clay, then rebuild the box around the mould again with the unmoulded part facing up, making sure the box is tight around the mould, you may have to put some tension on the tape to do this.
you need to allow a point to pour the casting material in, you can just make a cone out of clay and sick it to your part, mine is too small for that so I super glued a couple of rods to the casing, one to pour through and one to allow air to escape.
give a liberal spray of silicone mould release, this will stop the new silicone sticking to the existing stuff.
I then mixed up another 100g batch and poured that in and left it over night.
Step 5: Finishing
So just debox the same way as before, once the wood has been removed you should be able to pull the two parts of the mould apart, they should come apart easily.
using a sharp hobby knife I cut a funnel in one of the holes where the bars were.
to use the mould just firmly tape the 2 halves together and fill with your casting medium.