Everyone seems to say that polyester resin is no good for casting, but good for filling pieces before grinding them down etc. I'm just wondering why? What goes wrong if you try to use polyester resin for casting rather than polyurethane?

Thanks,
Rob.

I have cast with both. You can get castable polyester resins. As far as I can see it has one advantage, its cheap, and several disadvantages.

It smells nasty and the fumes make you high! I don't even use a mask with polyurethane, it hardly smells at all. The rep for the company reckons is so non toxic you could drink it! Don't try this!!!

It is harder to mix, polyurethane is generally 1:1 or 2:1 resin:catalyst, polyester is 100:2. It is difficult to measure 2% of say 6 ml accurately! If you get the measurements wrong, you can end up with a plastic with a tacky surface, but it is tolerant of adding too much catalyst, it just hardens quicker.

The plastic it produces is less like a thermoplastic. It is more brittle, thermoplastics and good polyurethane have a nice "flex" to them. You can add chopped glass fibre or mineral dust to polyester to overcome this to some degree.

I havent't used it a lot, but these are the only problems I have identified.

Max

Have you encountered any shrinkage? Polyester is supposed to shrink a considerable percentage, around 7%.

The microdol all but stops shrinkage. But think about even 7%, say your piece is 2cm at its thickest (bigger then most), 7% equates to 1.4mm. Thats only 0.7mm from each side and you could probably take that into account when making your prototype and with a little sanding of the finished piece. The only problems that this sort of distortion could cause would be if you needed very accurate mechanical pieces, but these would be smaller so the distortion would be smaller. You would struggle to get better than 0.2mm accuracy, for your prototypes, unless you had CNC.

I do prefer polyurethane, but I do think polyester can be cast in some situations.

Max

7% shrinkage is probably a little exagerated. 1% is more like it. But yes, polyester smells dreadful and is VERY toxic. It will make you sick if you don't have the air vented properly.

As a product, I actually completed a whole magic octahedron with it. For puzzle piece casting it is completely useless.

Once the octahedron was finished, if it was left for a few hours, the parts would stick together even though to touch on the surface they weren't tacky.

You could twisty the puzzle and break the fusion and it would be usable again. Then I left it in the car for about 30 minutes on a hot day. The Magic octahedron became VERY fused. Basically turned into a paperweight. Useless.

Don't bother using it for anything but a cheap filler. A puzzle done in polyester does not have a "rubik-plastic" or "thermoplastic" feel.

But a polyurethane one does. Save yourself the hassle, polyurethane is the best to use.

Never had tacky pieces after cure unless I got the catalyst wrong. Maybe the microdol sorts it out, but I doubt it. Probably more to do with how much hotter Australia is than the UK ;) or a different grade of polyester. Also I was advised to put pieces in the oven at 100 deg C for a couple of hours after they come out of the moulds. This was to strengthen them, maybe it also helps stop this.

I will put some pieces under the lamps for a day and see if I can get them to fuse.

I've not done a full puzzle in polyester, only test pieces. I am about to try it for a full puzzle.

Do you add any mineral filler to your polyester, I was told this was almost essential for good results. It cheapens the mix, hardens the set, keeps big pieces cooler during cure (not really a problem with puzzle pieces), reduces brittleness and improves the finish.

Max

I used a very good polyester (one with a good reputation) however, it didn't matter what the ratios were. The tackiness even though it couldn't be felt was still there. You don't need to leave the pieces together under a lamp. Just sit one piece on top of another and leave them for a couple of days.

They should stick lightly together.

I have heard about adding a wax to the polyester so that it cures, moving the wax to the outside as it cures to stop the tackiness, but it all got too complex and easier just to go with the better polyurethane.

Just rehashing an old thread a bit...

Have you tried using a product like this to see if it eliminates the tackiness? I haven't tried it myself yet but I might have to give it a shot.

I had another -ve point to add about polyester.

The resin contains styrene, this eats silicone! This means the moulds don't last very long :(

Max

You could try this and try that to remove the tackiness, but at the end of the day, just use polyurethane. It's cheaper in the long run.