HackSpace magazine

Moulding and casting tips for makers

By Gareth Branwyn. Posted

Moulding and casting can often seem intimidating to those not versed in the art. But it doesn’t have to be. These days, there are all sorts of sophisticated materials for making everything from simple and complex moulds, and an even greater variety of materials available for casting what you wish to duplicate. In a pinch, you can even do respectable moulding and casting with common household and shop material, such as hot glue, decorator's caulk, and even jelly/jello.

How thick should your mould be? You don’t want the sidewalls of your mould (the area around the object to be cast) to be too thin, but if you make them too thick, you’re just wasting moulding material (and money). Your moulds should be about 1⁄2” to 1” away from the piece you’re moulding. It’s best to make it a little thicker, closer to 1”, if you plan to use a zipper cut (also called a jeweller’s cut) when removing the original object from the mould. This will provide more stability, but otherwise, 1⁄2” to 3⁄4” or so is fine. A zipper cut is used in a one-part block mould where you remove the object inside by making zigzag cuts through the silicon moulding rubber. This allows the mould to close up tighter when you are ready to pour your casting materials.

How many are you going to make? Take into consideration the number of copies you wish to make to determine the cost of your mould-making materials. One-offs can be made with much cheaper moulding materials.

Using Lego for mould boxes If you’ve looked at any moulding and casting how-tos online, or done any yourself, you likely already know this trick, but it’s still worth mentioning. Lego bricks make for a perfect, reusable, and resizable mould box, and nearly every hobbyist (and pro) who does casting uses them. They can be used for one-part, two-part, and block moulds.

Glycerine and gelatine work as a mould (youtu.be/AS7dlRPryP8)

Reinforcing foam core mould boxes Frank Ippolito, of Tested.com, recommends reinforcing pinned-together foam core mould boxes by wrapping them in plaster-infused bandage material. This adds strength and stability and holds the box together tightly so that the weight of the mould material doesn’t warp and 'blow out' your mould box.

Don’t forget the mould release agent! You can buy commercial release agents to spray or dust on, and you can use things like cooking oil, petroleum jelly, cooking spray, and talcum powder. There are tons of recipes online for homemade release agents.

Quick, easy, reusable moulding material You can make decent-quality moulds using a material called Oyumaru, sometimes sold as 'Blue Stuff'. You heat it in water until pliable and then mould it around your object. When you’re through with the mould, you can reheat it and reuse.

Use marbles for casting 'keys' Two-part moulds use something called casting keys. These are protrusions on one side of the mould that go into indents on the other side. This ensures that the two parts line up properly and lock into each other for the pour. Marbles make easy positive keys that you can press into clay that you’re using to create support around the object you’re moulding.

Allow space around an armature When making moulds for latex rubber casts with armature wires inside, make sure your wireframe 'floats' in the mould so that the armature does not protrude out of the final casting.

Recycle old, failed moulds Failed/obsolete moulds can be reused with the same material type if you cut it into small chunks and use it to fill big spaces in your moulds. This will save you money on mould material.

Casting your own knobs If you’re working on, say an electronics project, and you want to finish it off with your own custom-designed control knobs, doing so is easy. You can find silicone moulds in all sorts of shapes and sizes (used for making sweets). All you have to do is position your threaded hardware in the centre of the mould and fill with casting resin or other casting material. The YouTube channel Stuff I Made has a video on the process (hsmag.cc/dkrvML)

If you're casting your own knobs, make sure you add the threaded part to the mould

Mould levelling When pouring a cast into a mould with a lot of real estate, getting the pour level can be an issue. To help with this, place the mould on a board and sink four screws into the board like table legs. By adjusting the screws, you can make sure the casting resin is perfectly level in the mould.

Elongate your pour When pouring silicone rubber into a mould, hold up the bucket, or cup of your rubber mix, as high above the mould as possible. This will help break bubbles that might be in the mixture.

Getting bubbles out of epoxy To get the bubbles out of epoxy (if you don’t have a vacuum chamber), use a heat gun or hair-dryer.

Use a vacuum chamber/pressure pot To remove bubbles from resin casts as they cure, use a vacuum chamber or pressure pot. There are dozens of videos on YouTube on how to make pressure pots from cheap tanks used in paint spraying (such as the Harbor Freight Air Pressure Paint Tank) and ones on making smaller pots using little more than some PVC piping, a pressure gauge, and a safety valve. Here’s a good video on making a PVC pot (hsmag.cc/hcjBFn).

Calculate how much casting material you will need To calculate volume of solid casts, if possible, immerse the object in a bucket sitting in a pan that’s filled to the rim with water, and push the object down until it is fully immersed. Measure the volume of the overspill, and that should be quite close to the volume of your object. Add 10% volume to your projected amount of casting material to compensate for leaks or spills.


From HackSpace magazine store