TPU Boot Cover Interference Fit

[goldenfab]

I'm finishing up a design for a consumer product that someone else started. Can't share the specific details unfortunately. I have a nylon part that has a 35mmx49mm footprint with a radial brim flush with the bottom that sticks out 1mm and is 4mm tall (from the bottom). All the corners have a generous radius. There is a boot that fits over the bottom of the box and over this brim. The boot is about 2mm thick and extends just over the brim (~2mm). The material of the boot specified is TPU Shore A95. I want the boot to be removable by hand but not sloppy or wiggle or any change of falling off. Currently the cavity of the boot is exactly the same dimensions as the nylon bot it goes over. Should I make the this an interference fit? If so in the radial direction, height or both? What would be a good starting point? Are there any rules of thumb or books, reading material to get up to speed on this sort of thing?

Thanks,

Adam

 

[btrueblood]

Hmm. A sketch would certainly help here.

TPU at shore 95 is pretty hard stuff (think skate wheels), so it sounds like you need a snap-fit design. There are a lot of resources if you google the underlined term. If, however, this is really intended to be a soft, easily removable feature with a simple friction fit (no "bump" or latch), then either you need very tight tolerance control or softer material, possibly both. Again, hard to visualize what you describe. In the end, it sounds like you need to do some experiments - you can buy castable urethanes in a variety of cured durometers (though the highest I found on McMaster is 94A), and mold some boots in machined aluminum molds. Machining blocks of urethane is tricky...there is a u-tube video where the guy is using brake cleaner to cut the greasiness of the urethane in order to get his cutting tool to "bite", not something I'd do in an enclosed space...or using my own lathe.

 

[goldenfab]

I added a graphic to the original post, its just a crude mach up that is similar to the design I am working on. Let me know if its not clear and I'll clarify with a better drawing.

Now that you mention it I think you are right. 70 shore A is probably way to hard for this to slip on. As far as I have gathered on this project the boot was an afterthought and the brim was added after one of the first prototypes to help keep it on. I think the change adding the brim did not account to the hardness of the material.

 

[Pud]

I would be concerned about getting the boot off the tool with 70A.
The inner lip needs an angle/rad on to get it off (bump)as a minimum.


www.tynevalleyplastics.co.uk

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[btrueblood]

Ah ok, there is a "lip" or bump, which forms a positive mechanical retention of the cap. Agree with Pud, add a taper and radius on the inner edge of the cap to let it more easily be popped off the core (inner) tool, and from the finished part. With pieces like this, having a slit or a hanging tab, to give the user a place to start the boot over the retaining lip can make things easier, though you maybe don't want that. For the size you are talking about, i'd probably start down around 40A shore and work upwards if needed. You were talking about Shore 95 originally, which would be very tough to pull over the lip shown, and probably require a multi-piece core tool*. At 40-50A durometer, there should be enough flexibility in the urethane to stretch over the bump to make it easy to install, yet give enough tension to not easily be dislodged.

Regarding tolerances, I would make the tool "at size", and count on the typical 1.5% shrinkage of the urethane during curing to make it a snug fit.

*But realize that tools like that make molding shops like Pud's charge you extra for the hand-work required to eject parts from the mold tooling.