Does this compliant material exist? 1

[rfdesign1]

I am not a materials engineer. I am looking for a compliant material with the following qualities:

1. Shore 'A' hardness of 30 - 50, preferably 35-40
2. Density of 0.5 oz/cu in (0.86gram/cc)
3. Able to be produced in a hemispherical shape of radius 0.85" inch, with a hollowed out hemispherical core of radius ~0.5"

Two of these put together form a 1.7" dia. sphere that can encapsulate a smaller 1" sphere in the hollowed out portion.

Is there a moldable rubber, silcone, sponge rubber, or other material that fits the hardness and density spec'd above and can be economically produced in the shape described? Where should I be looking to search for a suitable material?

Thank you,
Dave

 

[Metten]

1. possible
2. quite low, but possible
3. shape is not an issue for elastic materials

Economically possible: depends on the quantity of spheres. Does the type of rubber matter?

 

[rfdesign1]

The type of material does not matter. Hardness, density, shape, and cost matter.
Thank you

 

[btrueblood]

Just about any rubber would work, with perhaps some degree of foaming or other void formation to reach the density you want. Urethanes would be where I would start, if only because they can be easily mixed and poured into cheap molds in the R+D lab; but there are other materials that might mold more readily in a production shop, and your quantity will dictate price in the end.

 

[btrueblood]

Oh, and urethanes are good at post-cure adhesion, i.e. if you are planning to glue two pieces together, urethane will do that better than, say, silicone.

 

[Tocani]

Additionaly you have to see material compatibility on your specific application
i.e Urethane is not recommended if the piece is going to be exposed to denaturated alcohol
Butyl is not recommended if it is going to be exposed to fatty acids and so on

 

[rfdesign1]

Thank you btrueblood.

Is the foaming for achieving desired density in urethane expensive?
Is foaming finely consistent throughout the part (no large voids)?

How do you manipulate the Shore 'A' hardness of urethane or rubber?

Is rubber typically poured, injected, or stamped into a mold for low cost pieces?

For a cost sensitive application which compounds are appropriate and inappropriate? (natural rubber, synthetic rubber, urethane, silicone rubber, sponge rubber, others...?)

Quantity will be 1000 to 5000 per production run, most likely around 2000. What does that say about the direction?

Thank you!
Dave

 

[Metten]

The production run is ok. You better contact a few producers to get a price idea for your mold and per article.

 

[btrueblood]

Most molders can guarantee a pore size within reasonable limits (say plus or minus 50%) for foamed rubber. Any rubber can be foamed, some more easily than others, and the expense varies accordingly.

You can buy two-part urethane foam mixes to test from places like McMaster-Carr, or from boat-builders/marine supply shops, they come in varying densities, and the cost per part goes up with density (more material in the part); the density also correlates to hardness, so the low shore value 30-50 in a dense foam might not be do-able with off-the-shelf mixes. It might be easier to just design in some void space in the parts and run straight rubber, but you'd need to talk to a molder to be sure.

Standard rubbers vary the durometer by adding plasticizers (typically oils) or reducing filler contents.

Cheapest way to make rubber parts is by compression molding - preheat mold, weigh out a slug of premixed ("milled"), uncured rubber compound, open hot mold and toss in the slug, then close the mold (typ. in a hydraulic press) letting excess rubber squeeze out into purpose-made spill channels, and then let it bake - the rubber cures and hardens. Open the mold, extract part, chuck in the next slug, etc. etc. Small parts in small quantities (prototyping) can be done in a fairly cheap press, with heating controlled by cartridge heaters. Big runs like 1000 parts, might push towards a larger press, multi-cavity mold with more elaborate temperature controls or even steam heating. A rubber chemist I work with described the process as like baking a cake - anybody can make a cake from a packaged mix with the recipe steps and time/temperature written out for you. The trickiest part about new designs is in selecting the ingredients: rubber type, filler compounds, plasticizers, and curing agents, and then milling/mixing the components and testing to find the cure time/temperature (not to hot or the rubber will "scorch")... to get the right result after curing. Thus starting with a single cavity mold until you've figured out the details might be the better route to start.

Like tocani said, you need to determine the environment your ball is going to live in - it almost sounds like it's going to float on water? For long durations? Outdoors? You might want an EPDM elastomer for that kind of thing, but figuring out a watertight joint for the two halves will be the big trick. Urethane will work for a short duration, but because it is fairly permeable to water, the foam, or void space, will tend to fill with water over time.

 

[rfdesign1]

Thank you btrueblood for all the information. There is no exposure to water, heat, or chemicals except for exposure to curing polyurethane.
I've been distracted lately evaluating Thermoplastic Elastomers (TPE) options, but finding that the 0.350" thickness of the part is problematic in terms of requiring manual removal of the relatively thick part from molds.