[Jildou]

Hello everyone,

Im not jewellery crafter, but a doll maker. Ive been having some really odd trouble with my resin and since none of my regular advisors seemed to be able to pinpoint the problem, I figured I should try a different milieu.

Ive been trying to cast parts of a large Ball Jointed Doll in polyurethane resin in silicone moulds.
Ive started out with Smooth Cast 300, a quick-curing resin (pot life is about 3 minutes). This seemed to work pretty well, until I started refining the first cast to make it into the master cast: When I started sanding, often tiny bubbles would appear that had previously been hidden under de surface.
Odd thing number one: Not every limb had (relatively) the same amount of bubbles.
I adjusted my mixing (slower and more careful) and mixing equipment (switching from wooden to plastic spatulas), thinking that I was either stirring too hard or moist equipment would be the problem.
Didnt help.
I tried working in an as dry as possible environment (air conditioning, heating). Didnt help.
I tried blow-drying and/or heating the moulds up front and during casting.
Didnt help.
I tried placing the mould with the resin inside a vacuum chamber.
Didnt help (quite the contrary, nearly all the resin got sucked out).
I tried de-gassing the mixture before pouring it into the mould (had to be very quick about it, due to the short pot life).
Didnt help.
Switched to a slower curing resin (Smooth cast 305, pod life 10 min), in order to be able to mix slower and de-gas longer.
Didnt help.
I tried triple de-gassing (de-gas separate components, de-gas mixture, de-gas mould with mixture inside).
Didnt help.
REALLY odd thing number two: The faster curing resin actually gives LESS bubbles WITHOUT de-gassing than the slow one (with or without the vacuum).

It happens with new, unopened bottles of resin as well as bottles that have been opened and stored before.
It happens with another brand of polyurethane as well (Axson F31).

I have no clue what is causing this, and anybody who Ive asked is baffled as well.
The mould itself doesnt seem to be the problem: Though I didnt de-gas it during the making due to the fragile and often hollow original parts, there arent a lot of trapped bubbles in the silicone and I didnt notice any popping or deforming, or dents/pimples in the casts.
The bubbles are only INSIDE the resin, rarely showing up at the surface, so its not a matter of air trapped due to shape.
Theres plenty of channels for any trapped air to get away too, and everybody Ive showed them to with any knowledge of mould-making said they looked perfect (were impressed, even).
Ive also always operated at room temperature (as the bottle suggested), or higher (like I said, heating didnt seem to help either).

Im at the end of my wits here, please help!

[Katherine Swift]

Hi,

This is a pretty common occurrence with quick cure polyurethane resins when vacuum casting.  I checked with the Smooth On (the product manufacturer) online directions for this product, and they say vacuum casting is not necessary.

If anything with polyurethane resins, pressure casting is a better choice.  It will make the bubbles very small and keep them that way while curing rather than trying to suck them out.

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