plastic & rubber molding

[Keveo]

When designing a rubber part do you use the same guidelines/general practices as plastic parts or is there a huge differance between them? I know designs in rubber may be more feasible than plastic due to the fact that rubber is flexible.

 

[MikeHalloran]

Uh 'rubber' includes a lot of very different materials.

Thermoplastic elastomers can be shot in molds that are similar to those for plastics, and the part designs are usually similar.

Natural rubber is so stretchy that greatly simplified molds can be used. Best example of that I've seen is some French rectangular foglights I bought years ago. The gasket was stretched over a rounded rectangle of glass, so you'd expect a cylindrical form ... but the as molded shape was a flat annulus with slightly thickened edges. Of course, natural rubber has other problems. I guess they didn't have much ozone in France at that time, or the gaskets were a commodity item; they became brittle, checked, cracked and failed completely in six months.

Many rubber parts are shot in transfer molds, which demand simpler part geometry than injection molds.

You need to narrow down your choice of materials a little, soon.


Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

Like Mike says.

As well as one heck of a lot of types of rubber there are also a heck of a lot of types of plastic.

Also both come in thermoplastic and thermoset versions and some as thermoset in a thermoplastic matrix. This further complictes the potential answer.

An indication of your application might help.



Regards

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[Keveo]

Well, I don't have the material selected yet...im shooting for a 60-70 Shore A durometer and the part will be submersed in hydraulic oil at a standard operation temp of 130-150 degrees f with a max of 180f. My biggest concern is how to address undercuts and draft and if this part is manufacturable or how much grief it would cause and what would be a better approach. See pics.

http://www.imageviper.com/dip/93786/isolator.jpg

http://www.imageviper.com/dip/93787/isolator_2.jpg

http://www.imageviper.com/dip/93788/isolator_3.jpg

 

[Pud]

Hi Keveo,

Shouldn't be much of a challenge to a half-decent toolmaker, although the tool cost will be a bit higher than an open/shut tool.

Not sure about the hydraulic oil immersion though - I note Hytrel (Du Pont) is supposed to be good with hydrocarbons according to Du Pont - whether this includes hyd oil suggest you talk to Dupont (Or any of the other manufacturers of polyester elastomers.)

This link will give you an overview of some of the elastomerics available.

http://www.distrupol.com/downloads/flexible.pdf

Cheers

Harry

 

[patprimmer]

The two main manufacturers of Polyester based elastomers are DuPont and DSM.

Presuming thermoplastics and injection moulding, they would be my first choice for this job from what you have said so far. They are reasonably expensive, but will cycle fast and have good overall balance of properties.

PEBAX is another possibility, but quite expensive. From vague memories, some polyurethanes may be OK.

They will jump over that small undercut so long as the opposite side of the mould is opened first to avoid compressing the material to much. It can be bent or stretched a lot to eject, but can't be compressed to much if there is no room to yield.

Regards

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