These clear pieces are actually polyester. Clear polyurethane is more expensive than regular.
When trying to find decent information on the internet regarding plastics and how to do your own, it's very difficult starting out. Yes, there are plastic manufacturers out there with informative sites. Yet, unless you know the difference between a polymer and a polygon, or what "pot life" is, then you'll spend a long time searching for equipment, products, and learning what to do.
This is a guide to using plastics to cast your own puzzle pieces, for beginners, by a beginner, in beginner terms. I'll teach you the terms you'll need to know and what to do.
Thermoplastics and Themoset Plastics
What is the difference? Grab a puzzle. Any will do. Feel the surface. Should be fairly slippery and resonably hard. This plastic is most likely to be thermoplastic. Thermoplastic is heated and melted in a machine, and then injected into a steel mold by an injector. As you can guess, metal molds and injection machinery cost thousands of dollars. So for people like you and me who want to make a few pieces but don't have the gear, we have to resort to using thermoset plastic.
Did that sound negative? Having used only Polyester resin until recently, I would have said that Thermoset plastics couldn't cut it puzzle-wise. However, now that I've finally started using Polyurethane resin, I see things very differently now. There are many different types of thermoset resins, the three most popuar being: Polyester, Epoxy & Polyurethane.
This falls into two categories. Castable and non-castable. The non-castable is used in fibreglass kits for boats etc. You'll be wanting castable resin for making puzzle pieces. The type of plastic produced from polyester resin and its hardening agent, is a very shiny, beautiful plastic. But it is not flexible. It also stinks. Use an industral mask when using this plastic resin as the fumes can make you feel quite awful. It is the cheapest material to buy, and I would only recommend it as a way of learning to use plastic.
It is glassy in its appearance, yet it is also very brittle at thin areas like sharp edges and thin walls.
However, in its favour, it is very very tough wherever on a piece that isn't thin. A common complaint regarding polyester pieces is that they remain a little sticky or tacky. I found this to be a major problem. The re-seller couldn't help me, no site on the net could tell me what was wrong. So I started some trial and error.
The first thing I tried was to put the polyester piece in the oven at 120 degrees for 15 minutes and then letting it cool for another 15 minutes. It was fixed (sort of). Basically, the resin cures to a point in the mold but where it makes contact against the rubber mold surface, there is a reaction that stops the final cure on the surface.
Taking pieces (no matter how old they are) and popping them in the oven for some decent heat, will cure the final surface of the piece. No more stickiness. Well, I really should say that I thought all was ok, but when I actually took a finalised puzzle made from polyester in the car with me. The heat of the day, returned the puzzle back to a tacky state and it could no longer be turned. By the way, "curing" is the chemical reaction between the resin and the hardiner (or catalyst). As a final note if you still can't visualise the properties of polyester plastic, then think of those cheap hard chess pieces that you find in cheap chess sets. Very hard, shiny and kind of brittle.
Again, falls into two categories: Castable and non-castable. I have only seen it available from my dealer in non-castable format. I have never used either but I haven't heard anything positive about Epoxy in casting so avoid it. But as I said, I have no experience with it.
Which is the real octahedron? Answer: None of them.
This stuff is gold. It is also expensive, but worth it in the long run. Running nearly twice the price of polyester, but depending on the grade you buy, it has the exact properties, look and feel of thermo plastics even though it is a thermoset plastic.
Other bonuses with the stuff is that it cures faster than any of the other resins, it is flexible like thermplastics, and it has a long pot-life. Ok, what's a potlife? This is the time between when the resin and hardiner come together, and when the reaction begins. Once reaction begins, you can't work much with it. It starts to get hard. Therefore, mixed resin must get into the molds before the end of the pot life. And urethane has a rather generous pot life.
Warning Regarding the Use of Polyurethane
As opposed to polyester, you cannot simply open the tins, use some, and then re-seal the cans again. Once the cans are exposed to air (moisture), they begin to "go off". To stop this process from occuring, the people who sell you the Part-A and Part-B cans of polyurethane should also offer you a can of moisture purge (Nitrogen) for purchase.
If you do not have this aerosol spray, your expensive resin will be lost eventually after the first use. You have to spray this stuff into each can after each use.
Viscosity... What is it?
You will see this term when dealing with or buying resins or rubbers. And to put it the most simple way, it just means, how runny the liquid is. To get an idea of how runny (viscous) certain liquids are, here's a rough guide:
Water: Low viscosity
Polyurethane resin: Viscos
Polyester resin: High viscosness
Honey: Very high viscosness
So basically, Urethane is very runny, which is fantasic for making sure that the resin runs right down into the cracks and intricate parts of the mold.
The polyurethane that I use is Conap UC-30. If you live in Sydney, Australia, contact ERA Polymers. For the USA and other countries, find your local reseller at www.conap.com.
Doug Mueller suggested I mention this. I forgot all about it. When buying a resin, make sure it has a durometer level of close to 80. This means how hard the plastic will be. This is different to viscousness, which is when the resin is a liquid. The durometer is how hard the plastic will be when it sets (complete cure time). With that being said, when trying to purchase resin, don't bother referring to "durometer" when speaking to a sales guy. Refer to it simply as "hardness".
There are quite a few grades of polyurethane hardness so make sure it's close to 80. Lower values like around 30 (I think) are used for making soft skateboard wheels or rollerblade wheels. And you know that is no good for puzzles.
Unlike polyester resin, where the amount of hardiner affects the amount of time the resin cures, with polyurethane, you CANNOT afford to get the ratio wrong. If you do, there will be no cure, or the mixture will not cure properly. Follow the instructions and be very pedantic about obeying the instruction to the letter.
As a suggestion, when you are mixing part-A and part-B together, do it in a gentle figure-8 instead of going round and round. This later action will introduce more air bubbles into the mix. Why it does this I have no idea. It's a weird science. Just be gentle with your mix and be happy with that. Also, don't use a mixing dish, that needs to be cleaned out ofter each use. It's cheaper and easier to just simply head to your local supermarket and buy those cheap plastic party cups (not polystyrene cups, the'll eat through - so I'm told). Mix in one of those, use it and toss away after.
Remember to use rubber gloves too. I buy those packs of 100 that fit eather hand and have powder inside to help remove them easier. I'm a bit of a scruge with those and I use each one 3 or 4 times. You can imagine that there's not much powder left inside after that and they're a mongerel to get off after a while.
There's more to add to this, but I'll add that later. For now, also check out puzzle rubbers. And email me if there's something I haven't mentioned that you think I should.