Composite Bronze Question

[Stevem]

I am doing a composite bronze cast and I need to ask a question. When I am casting the mold I am painting in a bronze coat of Polyester resin and then coming back and putting on a layer of thickend resin before I start my mat layers. The problem I am having is even when the first layer has completely dried the second layer is wetting the first layer too much and it seems to wrinkle the finish on the bronze side. This isn't a huge problem because I can repair it after it is demolded but I can't seem to find a cure for the problem. I have tried kicking the resin a little hotter so it would set up quicker but it still has the same effect ever now and again. I've tried adding marble dust to take away some of the liquidness of the resin as well as Aerosil to thicken and try to keep the second layer from melting the first layer. Anyone else have this problem? If so, have you found a correction? I'm tired of having to repair all these casts. I don't mind the chasing but the rebuilding is getting old. Can someone offer some help?

Thanks, Steve

[Stevem]

Anyone?

[Denis]

Hi Steve,

I am just setting up for some casting myself so I don't know the answer to you question but I have had a lot of experience with polymers. The resin gets it's strength from cross linking. You can add chain modifiers that cause more chains (weaker) or longer chains (stronger). So one of the critical factors in the strength of the first layer is the set up time, the longer it is the stronger and more crosslinking you are going to get. This is because the monomers are mobile even after the set so I would recommend to 1) slow down the setup time. and 2) wait 48 hours or longer for complete cure before adding second coat.

Hope this helps,
Denis

"When bankers get together for dinner, they discuss art. When artists get together for dinner, they discuss money."
- Oscar Wilde

[Denis]

PS
I would also speed up the second coat set up time. This leaves less time for the Monomers to penetrate the first coat. You could also add filler to the second coat and use a thinner coat thus reducing the shrinkage stress on the first coat.
D

[Daniel]

What kind of mold are you using? Are you brushing or spraying the gel-coat/ bronze powder onto the surface? I wonder if the problem is that you are using a silicone mold and brushing the resin onto the surface. Perhaps the first layer is very thin, and when you brush the second layer on the force of brushing is causing the first layer to pull away somewhat from the surface and create wrinkles. Maybe you could try spraying the gel-coat onto the surface.

Just a side-note, it is really best to laminate the fiberglass just after "tack-off" when the gel-coat has solidified but hasn't fully cured. You get a better bond between the gel-coat layer and the fiberglass laminate this way, rather than waiting a long time after the gel-coat has completely cured.

[Stevem]

Thanks for the information, I am using a polyurethane mold and I am brushing the gel coat layer, I have tried coating the second coat after the first has dried and after tack off. sometimes it works and sometimes it doesn't, I am always able to repair the cast I just want to know what the variable is that is causing this wrinkling to occur. On the last cast I even tried putting on two bronze layers before backing and I still experienced a bit of wrinkles.

[ahirschman]

Wish I could add something, but I use polyurethanes and never have this problem. My initial bronze coats are any where from about 1mm to a good 10mm or more. I try to have at least 2 mm or more throughout before applying backup coats of polyurethanes with very close shrinking characteristics. I have had sculptures deform a little if I add too much polyurethane, and it is of a type that has high exothermic temp, but not usually a problem.

[Denis]

Steve,

I just finished my first full sized torso and head.

I used a silicone clam shell type mold with a silicone mold release.
I used smooth cast 325 from these guys.

http://www.sculpt.com/

It sets up fast so I then placed fiberglass matt in the shell while the first coat was tacky and poured more resin inside. Then I put the two pieces together and poured more resin into the shell to set the seam. It was complete in a afternoon.
If you check out the last segment here they show you how to cast a torso

http://www.youtube.com/watch?v=tSE07pGD0oQ

Anyway the results are great with no wrinkles. there is a little finish work to do on the seam but that's all. so I didn't have the backing coat shrinking against the primary coat I mentioned earlier, I got both coats to shrink together. If you try silicone and the 325 then areas to look at may be the silicone mold thickness and the firmness of the mother mold. I average between 1/8 to 1/4" . fairly stiff silicone with a epoxy fiberglass backup mold. Someone mentioned hard brushing as a source also... so you may be pulling the resin away from the mold, that happened to me. the last thing that occurred to me is that you may be applying the first resin coat too thick. Check out how much they use in the video.

Hope this helps.

Denis
PS Post a picture of your work.