Cheaper Mould alternatives?

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Cheaper Mould alternatives?

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Junior	Junior posted 8 Years Ago HOT Topic Details
Group: Forum Members Posts: 73, Visits: 473	I want to make a 2-part mould roughly covering 3.5sqmtrs but the overall cost is quite large. Would it be possible to use a cheaper alternative to the Uni-mould Tooling resin (which appears to be the most expensive part) such as a cheap Polyester resin to bond all the layers of chopped strand mat? Or is there another cheaper alternative to the uni-mould system that would give adequate results? I don't intend on pulling hundreds of releases, at most I would say 5 in total?
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Hanaldo	Hanaldo posted 8 Years Ago HOT Post Details
Group: Forum Members Posts: 2.5K, Visits: 28K	FLD (21/04/2017) Regarding the whole warping issue, I've used the materials you linked for a 1/2 car sized mould without loss of shape or dimension. In my head I consider the resin to exotherm as it cures so it turns solid in a hot state. Cooling that solid then makes it shrink. If you can do individual layers and allow them to cure you stop it getting hot so it doesn't shrink. I suspect warping comes about from hotter areas as they've been made thicker so any contraction is then uneven. A little care is all that's required. Not quite as simple as that; effectively all you are doing is controlling the shrinkage to its minimum. Resins shrink due to the presence of uncross-linked free radicals after the polymerization stage (read: after the resin has turned to a solid). Regular unsaturated polyester resins typically have the highest percentage of uncross-linked free radicals, vinyl esters and epoxies have slightly less due to the presence of the more reactive epoxy groups. Polyesters can shrink as much as 7% by volume, vinyl esters are closer to 3%, and epoxies are >1%. The reason you get more shrinkage when the exotherm gets too high or you have too much catalyst is because the free radicals are actually produced before the resin gels. Cross-linking takes place between the gelled stage and the solid stage, and it requires the free radicals to be able to move around. If the exotherm gets too hot or there is too much catalyst, then the movement of the free radicals is slowed and the reaction stops before many of the free radicals can cross-link, leaving you with a higher percentage of uncross-linked monomers and thus a higher rate of shrinkage. In short, ALL resins shrink, it's just how polymerization works. Polyester tooling resins actually shrink just as much as regular polyester resins, but they are filled with a thermoplastic additive that expands at the same rate that the resin shrinks, thus giving almost truly zero shrinkage (0.00-0.1% volume). I think the important thing is how much shrinkage you can actually tolerate. Especially at hobby level, the tolerance for shrinkage is generally pretty high, you aren't working to thousandths of a mm and for the most part you probably aren't going to care if there's a bit of print-through. Apologies for the chemistry lecture, I hope others find it as interesting as I do!
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