The Lying Down Foal's Pimples

Several years ago I worked at an injection molding firm; I already knew quite a bit about the injection molding process by the time I got there (the production manager was quite impressed that I actually knew was Tenite was) but I also managed to pick up quite a bit more information about injection molding – the kind that has proven quite helpful in my model horse research.

(It was car parts, not horses or toys, BTW. At one point I tried to convince this particular company that horses might help with their “cash flow” problem, but that was a nonstarter. And it is also, now, a noncompany.)

During some downtime I actually skimmed some of the reference material in the engineering and design studio, and in the process ended up solving a minor Breyer mystery: why the Traditional Lying Down Foal has “pimples.”

On most of the earlier Lying Down Foals, the pimples are fairly prominent – to the touch, if not the eye – and cover a large portion of the righthand side of the mold. Most of the more recent releases of this mold don't sport the pimples anymore, but I don't think it has anything to do with any alterations or corrections made to it: I think they're just being polished off or removed with an acetone rubdown – after assembly, but before painting. This may explain why they haven't released many Lying Down Foals recently – it requires too much prep work!

What probably happened to the mold was something called “orange peeling.” Here's a concise explanation of “orange peel” from the book Tool and Die Making Troubleshooter, by Richard M. Leeds (published by SME, 2002.)

During the polishing process, exerted pressures have a burnishing rather than a cutting effect that can be the principal cause of a localized plastic deformation known as orange peel. Orange peel is characterized by a rippled appearance, which develops when polishing pressures exceed the yield strength of the steel at its surface. Once orange peel appears, there is tendency to apply more pressure to eliminate the rippled appearance, and doing so often results in severe pitting of the steel.

Pits are small depressions that may form when small abrasive particles are torn away from the surface where polishing pressures exceed tensile strength. The appearance of pits during polishing is frequently blamed on defects in the steel, but this is often not the case. A clue to the source of pits may be their orientation. If the pits are the result of nonmetallic inclusions present in the steel, they will usually be randomly oriented and few in number. However, if pitting is the resulting of overpolishing, they will tend to be numerous and spread over most of the polished surface.

Those little depressions would turn into bumps when the parts are molded. They're numerous, and spread over most of the righthand side of the mold. It seems a reasonable assumption: the Lying Down Foal mold was overpolished. There's some evidence that the Lying Down Foal mold was extensively retooled prior to full production, so it's likely that the orange peeling and pitting occurred sometime during that phase. I know it had to have happened at least before 1971: all of my early Lying Down Foals with Blue Ribbon Stickers - including the little one at the top of my post - have pimples.