Hopefully this little section will expand as I chart and photograph more problems during the plastic mixing and casting processes. But here are a few to start.
Mixing and Pouring Too Slowly
Please note, this issue only applies to polyurethanes that cure in the 2 - 4 minutes range like Smooth-on's 305 and 310, and some of the Synair products. Other products such as Conathane UC-30 cure in 10 minutes.
With the faster curing products, it's an interesting chemical process to watch when used without black pigment. Once mixed, in around 2.5 minutes, you will notice a slight gelling of the fluid, but within another 10 seconds a cloud will bloom from the the centre of the mix, outwards and will become completely which in 10 seconds.
The above picture shows two different pieces that I have tried to cast and have spent too long mixing. I managed to pour the mix into the two halves of the molds but as I closed one half on the other, it literally freezed at that exact moment. This means that the plastic went hard a fraction of a second before the mold closed. Basically I cut it too fine.
Mind you, with fresh polyurethane products, the cure time is a little faster than it says on the packet. 3 minutes could actually go off in 2 minutes. Also, if you live in a hot dry climate (like Australia in the summer), expect the cure time to be even quicker again. As it ages a few days, the cure time will be more like it says on the packet. And over time, as the product becomes older and usused for a while, it can take up to three times longer.
With products around the 7 to 10 minute cure time, the above issue does not really occur. By the time the mix starts to gel, there is ample time to close the molds. Having a freeze trying to close the molds really isn't possible. There are advantages and disadvantages to 3 minute or 10 minute cure times. Personally, I'm going back to the slower times. I enjoyed the speed of three minute turn-arounds but with 10 minutes, you can load up more molds.
Incorrect Mixing Ratios of Parts A and B
Most products are 1:1 mixing or 1:2. Just means, if you pour out 100mls of A, you pour 100mls of B. Conathane UC-30 is 1:2 which means that 100mls of A, and then 50mls of B. Whatever the ration on the packet really is of no consequence to the user provided that you stick with the ratio EXACTLY!
In the case of polyester plastic resin, you can take many liberties with the curing agent. In this case, estimating how many drops will only affect the speed of the cure. Not so in the case of polyurethane. If it's 100mls of one, it must be 100mls of the other. Measure carefully because it makes a difference.
The above photo, shows two pieces, the white being the good piece and the one on the right being the result of slightly off ratios. Basically what I got was a jelly-like result and the colour curing orange rather than white. Interestingly, it stayed rubbery jelly for many days until finally becoming firmer. I kept it around to see if it would come good and while it did become much firmer, it was never fully hard.
This is only one effect of incorrect ratios. You'll come across many more wonderful and weird experiments going wrong. A tip here, get some of those floggy white film containers that your local Fuji photo store throws out. They are brilliant for measuring the amounts, restricting stirring and bubbles, and the smooth plastic makes it easy to slide the remaining cured junk plastic out and into the rubbish.
Silly Oven Accidents
When making molds for your pieces, remove your original piece from the mold if you intend to put it in the oven. If you don't, you will become what is technically known as a goose. In fact, don't even bother putting a curing silicone mold into the oven to speed it up, it's not worth the bother. Leave it alone.
But by way of explaination, I put it in the oven because I was having a dreadful time with a silicone rubber product from Synair, the PlatSil® 7130. You can recognise it from it's blue colour in the curing agent. It is very unforgiving if you don't add enough curing agent, and you will end up with a mold that is either way to soft, or nothing more than a hard putty mess. Having said that, the product does work if the correct amount of agent is added, and it was my own fault if I was too scummy trying to put less agent in.
I did this because the amount of curing agent was not really enough for the 500g amount especially in a cold environment. I knew I would run out. So I wasted more than I should have, and it probably would have been better to run out of agent than scum and ruin so many molds. Takes me a while to learn. Anyway, my two favourites are still Dow Corning 3110 and Fiberlay's RTV Silicone #1 Kit.
Therefore, because it wouldn't cure, I tried it in the oven, forgetting that the original piece was still inside. The piece melted and the mold didn't improve. Oh well.
Not Enough Pigment
Our piece on the right is a nice black but the one on the left is a bit grey. It'll stick out on the puzzle like a sore thumb. With the Conathane and Synair products, the pigment used with them is a paste. Too little gives grey effects. With the Smooth-on products, the pigment is not a paste, but aa dye product in a squirty bottle called "So Strong Tint". This bottle is a fantastic method for applying the black dye. As opposed to the pastes, too little of this in your mix, leaves your piece purple instead of grey, a little like "black" pen ink which is usually really a dark purple anyway.
I found that the Smooth-on dye is compatible with the Synair polyurethane. I don't know about the Conathane, but the paste is fine. One note here, Conathane is very good with it's mixing of black paste. The Synair product, however, is not very tolerant. In fact it's the worst of the three for taking pigment. You have to add a LOT of black paste, and mix it very very well.
Warping Pieces / Pour Mixing / Wet Spots
The three pieces here are from an octahedron that was in progress. What has happened here is a case of pigment paste not being mixed in properly. The mixture of the A and B parts is probably ok, but what I may have done here is use the same straw instead of changing it, leaving unmixed dye paste in the pot. When the plastic has started it's cure, the unmixed paste has created 'web spots'. Wet spots are a common problem but this is probably the worst possible case.
The way to avoid this, is to use on straw to mix pigment into pot B very well. Then use a second clean straw to mix the contents of A into the B pot.