Fro another project I need custom shaped plastic parts. Since my 3D printer is only in the design stage right now (I just finished software prototyping) I decided to try my hand at casting. This is my first attempt

The piece is sculpted with Plasticine and cast from fiberglass resin. These are not the best mateials to use but they are cheap and get the job done.

I was inspired by Sylvia's Super Awesome Show, she made it look easy.
http://www.youtube.com/watch?feature=player_detailpage&v=1uNopuOBWoA

Step 1: What You Need

The main ingridient here is fiber glass resin, you should find it in hardware shops. the shop near me also had casting resin, wich is better for making big casts, but it was twice the price. Fiberglass resin will work but when it cures it gets hotter than casting resin, in a large cast it can get so hot that it cracks or burns.

Plasticine (oil clay)is used for making the master shape. Real Plasticine is meant to melt hen it gets to about 100C which would make it easy to get out of the mold. Unfortunately mine was a knock off that just caught fire.

Plaster of Paris is for making the mold. you pour it around the Plasticine sculpture.

Not pictured is Lego, for making a box, and plastic wrap or aluminium foil for lining the box.

Step 2: Make the Piece

Since this was only a test I didnt have a plan for what i was going to make but i sugest to have a diagram to refer to.

I rolled the clay flat an trimed the excess. Then I made a right angle shape. In the join i added a small sausage to make sure there was proper contact between the pieces. I did not want a mold with gaps.

I cut out a small cylinder and attached it to the top. This created an overhang in the mold that gave me trouble later.

Since i was trying to create a right angle I molded it onto the corner of the kitchen counter.

Cutting the clay is much easier if your tools are lightly oiled.

Step 3: Make the Mold

Make a box out of lego the aproximate shape of your piece. You could just use a bowl or jar ut using lego means less waste.
Line the box/bowl with plastic wrap as this will make it easier to remove. It will also make a Lego box waterproof and keep it clean. I had a problem getting the wrap into the box and this shows in the mold. It is best if you find a way to hold the wrap down.

To mix the plaster of paris measure how much water it takes to fill the box, that is how much water you should use. Add the plaster to the water a bit at a time and stir until it forms little islands. it should be the consistency of thick paint.

Because I knew i couldn't melt the clay out of the mold I put two tooth picks halfway up the piece so that i could cut the mold in half later. Not pictured I pushed one pick right through and left it there, this caused problems.

Place the piece in the box, as centrally as possibly. Mine was off centre so the mold wall was very thin in some places. Now pour in the plaster. You should do this entire step outside or some where that can get messy, not the kitchen.

Plaster of Paris mixed with water forms a chemical reaction that makes it hard, it doesn't just dry out like clay. This means that if it hardens on you counter or floor you have yo chip it away with a chisel. Think cement. By the same token, don't throw excess plaster down the drain it will harden under water and block you pipes.

Step 4: Extract the Mold

My mould had hardened in a couple of hours but was still damp so I left it over night to dry.

Take apart the lego box and peel away the plastic wrap. Where the plastic wrap didn't get into the corners of the box the mold had rounded edges, this weakened it and made the next step harder. Try to do better than me.

Step 5: Getting the Plasticine Out

With some light sanding i found the tooth picks. with this as my guide i cut the mold in half using a dremmel, a fine hack saw would do it better but slower.
Once I had cut all the way around I gave it a gentle pull and it cam apart. From the picture you can see how off centre I was.

I dunked the porous mold into water hoping that a wet mold would make getting the Plasticine out easier.

Use a piece of wire or a small tool to scrape out the clay. Be careful not to damage the mold though. Since the Plasticine piece was resting on the bottom of the box there was a hole in the bottom of the mold. I broke one half in two and the other half had a hole in it where the wall was thin.

In the last picture you can see the embedded tooth pick.

Step 6: Resin Casting.

Rebuild the lego box. This will hold all the pieces of your mold while the resin sets.

Since the Plasticine piece was resting on the bottom of the box there was a hole in the bottom of the mold. I used this hole to pour the resin in.

Before putting in all the pieces spray them with a release agent, in this case: cooking spray. a friend suggested spraying with hair spray to seal the plaster and then the release. I shall try this next time.

Again I lined the box with plastic to stop sticking and mess. This was a mistake. The solvent in the resin soaked through the plastic and started eating into the lego, as in the last pic. In the future i will use aluminium foil.

I mixed 25ml of resin with five drops of hardener in a disposable cup. It took hours to cure so i think i need more hardener. 25ml was too much so i made a poor mold of a lego brick as well.

Using resin is an outside job. It will make a mess a a chissel wont help this time. It also gives off fumes that a better left outside.

Step 7: Remove the Casting From the Mold.

After the resin has set gently remove the mold. In my case the top had hardened but the whole piece was sti;l a bit floppy so I left it in the sun for a while. UV rays in the sun help to harden.
Once I had removed all the mold i left it in the sun a bit longer.

The second picture shows what it looked like once it was hard.

Step 8: Result!

The piece that came out of the mold was a bit rough so I trimmed it and sanded it with a dremmel. The shape it a bit off from the right angle section i started with. It is also full of bubbles, this is a common problem with resin casting and experienced modelers know how to avoid it. I font.

A flaw in the casting also made the stub only semi circular.

I am really encouraged by these results and my next attempt will be better.

Thanks for reading, please comment if you have any questions or help.