Make Plastic Parts From Scratch




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.

Teacher Notes

Teachers! Did you use this instructable in your classroom?
Add a Teacher Note to share how you incorporated it into your lesson.

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.

ShopBot Challenge

Participated in the
ShopBot Challenge

Be the First to Share


    • Made with Math Contest

      Made with Math Contest
    • Multi-Discipline Contest

      Multi-Discipline Contest
    • CNC Contest

      CNC Contest

    21 Discussions


    6 years ago on Introduction

    I've done some casting before as well... you might want to try creating your sample with candle wax. That way you could just heat up your mold (once it's completely solid and dry) and the wax will melt out. :)


    7 years ago on Step 8

    Did you think about pouring the plaster of paris in 2 stages? You could pour half, let it set, apply some sort of barrier then pour the other half. It would save having to cut the mould as it should just pull apart. (I have absolutely no experience in this so could be talking rubbish - it's just the first thing that occurred to me!)

    4 replies

    Yep, that's what I meant to finish typing about the silly putty comment below. You fill the bottom half of your box with silly putty, coat with talc or a very thin layer of vaseline, and press your part into the putty. It also helps to press in a couple of dowels or pins to make alignment pins for your mold. Then you pour the plaster, let it completely set, and remove the half. Flip it over, put it back in the box, and recoat with your lubricant and pour the other half. Works like a charm, especially for pieces that have an obvious and easy two-part seam.


    Reply 7 years ago on Step 8

    We can make various plastic products according to your requirements, design drawing or sample, customization and OEM are welcomed.
    If you have any questions or need our help you can contact us at any moment. Our E-mail is:

    That would be a much better way of doing it.When i was pouring the plaster i was excited so i just rushed through and didnt think very far ahead.

    There are a lot of great tips on making molds of all kinds on YouTube. Look for username "SmoothOnInc" for some nicely detailed ones. Being a company, most of their videos are pushing their own products, but a lot of it should extend to competing brands or DIY solutions.


    7 years ago on Introduction

    LOVE the Lego mold idea! kudos!

    There is also Shapelock (also goes by another name) Put in 150F water and its mold-able and reusable.


    7 years ago on Step 8

    Very nice for a first casting attempt, reminds me of my first stabs at it years ago. I've made tons of miniature and action figure pieces this way since then. If I may,
    a few tips :)

    To avoid having the piece sitting on the floor of your box, add a straightened paper clip or wire protruding from the top of the model, in this case it would stick out from the cylinder you added. Then place a wooden dowel across the top of the box and wrap the wire around it to suspend the part in the middle of the mold. This also creates a channel to pour into and makes is easier to lift and remove.

    For your box, Legos are definitely a good way to go, to avoid the little pockets at the bottom, you can get the flat cap pieces and cover the bottom with them for a smoother base. Another tip the guy I learned from showed me is to fill half of your box with Silly Putty, keeping it as level as possible, and then lightly dusting it with as fine a talc powder as you can get to keep it from sticking together.

    To avoid bubbles in the resin, mix thoroughly, pour slowly and evenly, and when you are finished, take an object and tap lightly against the sides of the mold a few times, this helps break up the air bubbles before the piece sets.

    Anyway, this looks great, and I look forward to a Part II soon!

    1 reply

    Thanks for the feed back. As I did this I realised i was making noob mistakes that i could skip on the next attempt. The bubbles were worrying me though, thanks again.


    7 years ago on Step 8

    Great project!

    What was the total cost of materials? Just wondering if this is a better option then a low temp thermal plastic like ShapeLock. In about 5 minutes time i could have a chunk heated and shaped and ready for use about 10 minutes later.

    This reminded me. I need to make some shelf supports of my own.

    3 replies

    nice stimulating project . What about silicon for the mold, interesting material, inexpensive and hot temperatures resistant

    Total materials cost was about R70, which works out at around $6 US but i have no idea how much it would cost in other countries. I still have enough material to make many more molds, so it is realy cheap.

    I looked at shapelock and home-made bio-plastics but this still was cheaper than shapelock and, i suspect, stronger than both.

    Thanks for you interest


    7 years ago on Step 6

    It was more likely the hardener, which is a pretty strong solvent. Resin should just be resin, a bunch of short-chain polymers, just kinda standing there.


    7 years ago on Introduction

    Have you ever considered making your own plastic? Dissolving styrofoam packing with acetone converts the styrofoam to a hard styrene plastic. The liquid hardens within minutes, and it don't get much cheaper than that.


    7 years ago on Introduction

    "Unfortunately mine was a knock off that just caught fire..."

    LOL Love that line! Been there!

    Nice instructable, I've worked with fiberglass half my life when I had my autobody shop, and cast some blocks and ground them out with an air dremel before but never realized or thought of looking for something to make molds from.


    7 years ago on Introduction

    Look for a material called PCL (Polycaprolactone). Also cheap, and would have been ideal for this project.


    7 years ago on Introduction

    Nice idea. When I saw the title and the picture, I thought you might suggest just heating and bending plastic, which might also serve in some situations.


    7 years ago on Step 8

    Good post! That's how we learn to do stuff! You just do it and you learn from your mistakes. A lot of people wouldn't try because "they don't know how." You're going to be a great do-it-yourselfer because you just do it.