Foam/sponge rubber "expansion rate"?

[tom1953]

I have a customer asking me for a foam rubber formulation recommendation, and they want an "expansion rate" of 1.45-1.5. I don't work with sponge, so I'm not familiar with the term. Does this refer to the increase in volume after expansion (by blowing agents)? Or the speed of expansion?

If it's a volume change, their requirement for a Shore A hardness of ~75 Shore A seems like it might be difficult for a rubber sponge with a 50% increase in volume from unblown.


On a related topic, I've recently learned about "Expancel" materials for making foamed materials, including rubber, it seems. Has anybody worked with these materials?

http://www.expancel.com/

Thanks,
tom

 

[GrahamBennett]

Tom, the hardness quoted will be the unblown form of the compound. That is, use a compound that has a known hardness of 75 Shore A and add a suitable blowing agent. I suggest you talk to your customer regarding his/her definition of "expansion rate".

To me Expancel looks like an attempt to move away from azo-based blowing agents, which have been tenuously linked to health problems.

 

[tom1953]

Graham,
I did get a clarification of this from the customer, and they do want 65-85 Shore A AFTER foaming, and the expansion rate means the volume is ~1.5x bigger after foaming. Presumably, if I started with a compound w/specific gravity of 1.2 (g/cc), then 1.2g/1.5cc = 0.8 g/cc density.

It seems to me like it'd be easier to just specify the needed specific gravity/density of the finished sponge, without worrying about the unblown sg, but that's what the customer wants.

Any thoughts on what hardness I'd have to start with, unblown, to get 65-85 blown? I'm thinking it'd have to be ~95A.

tom

 

[Compositepro]

I think that in order to get that high a hardness the foam would have to have unblown, high-density skins over a lower density core. Is that allowed by your customer?

 

[GrahamBennett]

From my information, you need a button of at least 8mm thickness on which to perform a hardness test. This would preclude having a high hardness skin on the part. It is more usual to determine the hardness on a cured, unblown compound and then add the blowing agent, as I said in my first post. I have never, ever, heard of a foamed component having such a high hardness. I suggest you read the relevant parts of ISO48 and ASTM, the standards covering hardness measurements.

Regarding the amount of blow, you need to add 1/3 less compound than that needed to fill the mould cavity for a solid part. In other words, if, for example, the part weights 100g then you need to add approximately 67g of compound containing a suitable agent to achieve 50% blow.