Polyurethane resin casting is flaking

[christ]

Hi all,

I'm trying to slush-cast a few parts of a Ghostbusters proton pack in polyurethane using RTV silicone moulds, but I'm having trouble with the built-up layers of PU resin not adhering to one another.

Below is the setup I'm using for another part of the proton pack. I think the silicone was flexing a little as the PU cured, which meant subsequent layers were moving relative to one another, so I've since increased the thickness of the silicone and made a new mother mould out of fibreglass.

I've yet to try casting this in PU resin - I was going to try gelcoat followed by polyester resin and fibreglass, but I'd prefer to use PU if possible.

Below is a pour of PU in the original mould.

I'm using Fastcast polyurethane resin with black pigment added to it. I tried casting the item in the first picture without adding any pigment and it still came out a bit flaky.

Any suggestions are gratefully received!

 

[BrundelFly]

You're probably letting the resin completely kick in between layers. Try doing the second coast while it is still tack. You might also consider a thing first coat, then using a thixotropic additive, apply second coats. With a thicker resin, you will have to apply it in with a brush. Silpack makes a material called TROWEL ON -60 that is used to make mother molds. Its basically SILKAST with the thixotrpic already in it. I used it for X WING bodies and it worked well. I primered my mold, did a resin "beauty coat" then brushed in the Trowel On. Worked great. Very light weight.

However, the material to use for your project is probably fiberglass and cloth.
Use a Bondo-resin mix for your first coat. Then apply the cloth and resin before that completely kicks.

 

[CowardlyLot]

Also, how old is the resin? It does have a shelf life...I learned that the hard way.

 

[christ]

Thanks very much for the replies guys (sorry for the delay).

BrundelFly:

I think I have been letting it harden between layers. I tried to get the second coat on as soon as the first one had stopped being tacky, not whilst it was tacky.

I will investigate the thixo additive for PU resin. It's tricky getting the resin to stay on the silicone for the first coat so that sounds like a good idea.

The resin is pretty thin (runny) so I have been finding as I try to slush round subsequent coats it suddenly 'goes off' and starts to form huge thick lumps mid-flow.

When you say "primered my mold", what did you prime it with?

Also, this "bondo resin mix" you refer to - do you mean car body filler mixed with polyester resin? Followed by a second coat of polyester resin + fibreglass cloth? What sort of ratio am I looking for in the mix for the first coat - 50:50?

CowardlyLot:

The resin isn't that old - a few months now at most. When I initially tried the casting the resin was brand new from the supplier.

I've noticed the 'B' component of the PU resin starts to crystallise and go cloudy after it's been sat for a long time, so I always check it's clear before using it.