In a bowl or a metal can, pour a few tablespoons of acetone, and put polystyrene in it. The polystyrene will dissolve and form a white, opaque paste. DO NOT BREATHE the vapors. Add as much acetone and polystyrene as you need to get the required volume of your object.

Clean the object with a toothbrush, a metallic wire brush, a cutter, a file, screwdriver, a sandpaper, a sausage, a dream catcher, whatever you want.

Rotational moulding is a low pressure, high temperature manufacturing method for producing hollow one-piece plastic parts. The moulding process dates back hundreds of years to the Swiss use of the method to make hollow chocolate eggs.

I filed the sides in order to smooth the polystyrene of my model. The smoother the model in polystyrene is, the finest will be the result. In my case, I did not shaped the eyes and the printer of my robot. I was right: It is really easier to dig holes later in the plaster in order to get later the salients parts of the model.

The process is cold. It means that the mold may be in in wood, or concrete, or (silicone in soap), or chocolate if you want, but not any plastic wich may be dissolved with acetone (ABS plastic for example) .

When the plaster is dry, put the mold + model in an oven at 80°C during several hours. Don't do that in the oven you cook because of the possible remaining toxic fumes. The polystyrene model will reduce. It will be easy to push it out.

The possible objects we can obtain have the shape of the model, are light (float over the water) but dimensions are not precise and I don't think the obtained object can stand a big mechanical constraint

Polystyrene foam may be dissolved in a mix acetone (20%) + unleaded petrol (80%). In this case, the mix is liquid (Depends on the quantity of dissolved polystyrene) , and it can be reused as glue.

Go outside or open the window. The acetone is toxic and very bad for lungs . Don't breathe it. Never..... Unless you want holes in your lungs

Rotational moulding is a very competitive alternative to blow moulding, thermoforming and injection moulding for the manufacture of hollow plastic parts. It offers designers the chance to produce stress-free articles, with near uniform wall thickness and moderately complex shapes. Typical moulded parts include bulk containers, tanks, canoes, toys, medical equipment, automotive parts and ducts.

Plaster of Paris filled in a plastic box+ WD40 + screw driver , needle, sand paper, files (in order to dig and smooth the edges of the mold)

I roughly shaped the robot in polystyrene, with a hot wire device (there are a lot of instructables on this subject) , because this method is easy and quick to do.

Then, the result remains quite rough, the sides are not perfectly straight, my robot's belly swelled up, and I saw air bubbles inside the material.

First of all, the molded object is shorter than the original: it measures now 77mm x 53mm x29mm whereas the model was 80mm x 55mm x 30mm

First of all, remember : the moded object you'll get at the end will never be better than the mold. Be watchful and careful.

I choosed to make a plaster mold . There are a lot of instructables showing this process. The mold will be used once only.

Caution: Do not put the assembly ( mold + molded part) in your kitchen oven at 80°C in order to accelerate evaporation because of the toxic fumes of acetone.

Smooth the edges in the mold and finish the details: Dig the details missing in the female mold ( in my case, the eyes, the buttons, and the printer of my robot) with a screw driver, a needle, a chisel point, etc.... Remember that the holes you do in the mold will be turned in relief when you will pour the liquid polystyrene paste.

This is the start point of this instructable : I wanted to investigate this process , realize an object, and evaluate the positive and negative points.

Of course, it is possible to use any other object in wood, plastic or metal or silicone or chocolate as a model.... but don't forget one thing: you have to think that this object will later be plounged in the plaster, and then it must be unmold without breaking the mold when the plaster is dry !

But I think it is surely possible to make Ears rings , toys , keyrings or decoration objects for examples . Please post me your realizations!

Then break the plaster with a screwdriver in order to extract the molded part (the robot, in my case) . The mold will be lost.

When you have the required volume of polystyrene paste (step 2) , put this paste in the mold, put a weight on it and let it dry for several days.

My vercict is : if you are not in hurry (it last some days) , molding polystyrene with this cold process is possible, but please...don't expect miracles!

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In fact, if the evaporation surface is small, and if the object is thick, the acetone may not be completely evaporated, and the molded object may still be soft. In this case, let the molded part into the mold and let it dry for a few more days at room temperature. In my case, my robot was 30mm thick. After one week, it was still soft. It needed 10 days for drying.