This company grinds rubber all the time, their main business is grinding rollers for printing presses, printers, and other precision type roller work.
The grinder was just an old Cincinnati centerless grinder and they had a way of feeding the rubber through it. They can hold tight tolerances on it too, I didn’t stick around to watch them run extruded cord stock through one but it is a main part of their business. Since I build their molds and urethane centrifugal tools the grinding part only effects my work in the amount of excess material they want to be removed during the grinding operation.
Parker Seals couldn’t extrude anywhere close enough to the tolerances they needed evidently, which I found of interest simply because I’ve built extrusion dies that can hold pretty damn close sizes, especially if the material is exactly consistent batch after batch like that would have to be.
I think what the issue is that they have to post-cure them in an over or autoclave and that is where the biggest shrink variation can happen on really tight parts.
Parker must have had a very good QC department on them, but they would have cheerfully chucked my customer under the bus if it could have been shown they were negligible in any way.
Perhaps a Cincinnati #2, or even larger. Thanks for your replies. It’s truly fascinating.