Introduction: Cast a Polymorph Keyring

This instructable covers work I have done while being a Maker in Residence at Derby Silk Mill ( ) in the UK.

Derby has an amazing history of metal casting - especially signs for the railway industry. It is still the site of the QualCast brand (Quality Castings).

I wanted an acitivity which shows the process of casting these metal signs, but without the danger of building a whole forge (but maybe in the future....).

I started to play with using Polymorph (a low temeprature melting point thermosetting plastic, which can be melted in hot water and handled while still flexible). I thought to replace the metal with this plastic and to make an activity for visitors to the museum which they can make and take away with them.

I decided to make a small keyring with the museum logo using a mould process.

After quite a bit of trial and error, this is the process I came up with to do simple plastic items using a layered, laser cut mould unit.

Step 1: Parts and Tools Required

You will need the following parts:

You will need the following tools:

  • Kettle (to boil water)
  • Plastic container
  • Spoon
  • Single handed clamp
  • Drill and 6mm drill bit

Step 2: Make the Mould

This can be done in many ways, but I used a laser cutter to convert a logo into the design required.

The mould is comprised of a number of layers, each with different detail. 6mm holes are put in every layer for guidance. They will eventually fit onto the M6 bolt which clamps the whole unit.

For my logo of Derby Silk Mill, this required 3 main layers, along with a front and back piece.

The main three layers of the logo are:

  • The shape of the back of the keyring (a rounded-off rectangle) - cut in 2mm acrylic
  • The shape of the outline of the mill - cut in 2mm acrylic
  • The windows/detail of the mill - cut in 0.8mm polyproylene

The front and back pieces were cut from 5mm acrylic, so it was strong. I used an additional layer on the back, as the 6mm nuts were getting in the way. Check the photos for more detail on this.

The .dxf files are available here, please adjust for your logo/design.

I labelled the layers with numbers 1-6, so that they are easy to put on in order.

The M6 bolt goes through the back piece, with another piece of 5mm acrylic to level the back for the M6 nuts. These nuts hold the M6 bolt in place so it does not keep slipping out.

The wingnuts go on the top and clamp it all closed, when we leave the mould to cool.

Step 3: Prepare the Mould

If you just plonk the polymorph into the mould then it will stick to the acrylic. I know as I have been there.

We need to use some form of release agent.

After quite a few tests I came up with a system that worked reliably for me.

For larger flat areas I used a piece of greaseproof paper placed into the mould.

For the detail levels of the mould I rubbed candle wax onto the mold to try and cover every surface. This works as a great release agent for the polymorph.

You dont want too much, but enough to ensure the cast item releases.

Step 4: Prepare the Polymorph

Polymorph is a very interesting plastic which has a low melting point (around 30-60C). At normal room temperature the plastic is hard and machinable, but put it into just boiled water and it will go flexible and mouldable.

Polymorph comes as white granules. I found it always best to work with new polymorph, but there is no reason why a mis-formed cast cannot be re-melted and re-cast.

I set the kettle to boil some water.

While that is happening I measured out 1/2 tablespoon of white polymorph granules and put these into a measuring jug. This will depend on the size of your mould.

You can add colourant at this point, which makes the end result much more interesting. The colourant also comes in granules - I used 5 or 6 granules for each cast. Try it out and see if you are happy with the colour.

Pour on quite a lot of just-boiled water.

If working with younger folk then be VERY carfeul at this point.

Leave it for a minute or two. The granules should go transparent in the hot water.

Move the blob of plastic around with a spoon and bring it out of the hot water. It might be too hot to touch - if it is then wait a while, but ensure it is still flexible.

If it is OK to touch (although it will still be a bit hot) mix in the colourant so that you get a nice even colour (unless you want some different designs).

Squidge the polymorph into a thin layer (this helps it heat up and go flexible again).

Place it back into the hot water to keep it nice and flexible.

Step 5: Put the Plastic Into the Mould

Ensure the mould has been prepared correctly, with greaseproof paper and candle wax as release agents.

Take the hot blob of polymorph and push it a bit into your mould.

Put the top onto the mould and clamp down with the single hand clamp. This should squidge the polymorph into all the corners of the mould, with the excess pushing through the excess holes in the mould.

Tighten up the wingnuts.

Leave to cool.

Step 6: Remove From the Mould

When the polymorph is totally cool you can undo the wingnuts. Polymorph takes longer to cool in the middle than you think, so leave it quite a while.

Take the top layer off - this should come off easily.

Next take the bottom layer off.

Then remove the polypropylene layer - this is the detailed layer and it is quite flexible so should come of quite easily.

The cast should then pop out of the mould, if you are lucky.

You might need to prise the layers apart with a flat headed screwdriver, but be very careful if you do this.

You should be left with a cast unit with some additional polymorph excess.

Step 7: Finish the Unit and Add Keyring

I uses a pair of scissors to cut off the excess polymorph.

I then drilled a 6mm hole in the top left hand corner of the cast piece.

If there are any blemishes or small pieces of polymorph still on the cast piece, then you can quickly dip into hot water, which will re-melt the outer layer. Be careful if you do this to not ruin any of the detailled parts of the cast.

Add a keyring loop through the hole and voila! Your own cast keyring.

Thanks to Derby Museums and Derby Silk Mill Museum for giving me the oportunity to work on this project.

They have an anmazing collection of cast metal signs and other items.

Please drop by the Silk Mill museum if you can:

Some of my work at the Silk Mill is blogged here: