Introduction: Molding Plastic With Polystyrene Paste. Cold Molding Process

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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. 

Pure acetone do not dissolve polystyrene, but turns it into a white and opac paste. This paste may be filled in a mold.

When dryed and hardened into a mold, this paste turns into an plastic object .

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

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!

Supplies

An object to mold.

In my case , my model was an instructable robot .

For the mold:

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)

For the plastic to be mold:

a glass jar or a metallic box + polystyrene foam and acetone

For unmolding :

tooth brush, metalic brush, cutter, files, a screw driver...

Decoration:

paint, varnish...

Process duration

several days (depending on the thickness of your object)

Step 1: Preparing the Model

In order to test the principle of this molding method, I decided to mold an instructable robot 8cm x 5,5cm x 3cm .

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.

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 !

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.

When the model is done, I put WD40 on it (in order to facilitate the future unmolding) .

Step 2: Preparing the Mold

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

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) .

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

Mix the plaster of Paris + water and pour the mix in a box.

Push the model face down into the plaster, and put a weight on it. Let it dry the plaster .

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.

Drop out the model. You have now a female mold

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.

When the mold is clean and smooth, put WD40 in it. You are ready for the molding step.

Step 3: Preparing the Polystyrene Paste

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

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.

As you can see, a lot of bubbles remain. It is normal

Step 4: Molding

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.

Step 5: Unmolding

Yes, you read correctly : several days. Not less because the acetone must be completely evaporated.

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.

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.

After this drying time, the material must be hard.

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

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.

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

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.

Finally, the obtained object does not seem seems robust.

Step 6: Finitions

Now the object is dry : you may paint and varnish it it . In my project I used acrylic paint.

Et voilà! You're done!

Step 7: Conclusion

This process is easy to do and does not require any special tools. Just avoid acetone fumes.

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

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

Greetings from France. Bruno