Crumb Rubber and Regrind Compression Molding

Anyone have war stories to share about compression molding rubber, either sheet form or “regrind”? Specifically looking for more durable products, less so laminated fabric + foam cases. For example: bumper plate using Nike regrind.

I don’t know too much about the topic but I did see a video on IG where they cut up that Nike plate. It’s only a thin layer of the recycled rubber. It’s metal inside like a typical plate.

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This is the standard construction. Cast iron core with compression molded rubber or PU encasement. I would be pleasantly surprised if the usage of reground gave a more durable product. This is a demanding application.

I’ve spec’d compression molding to manufacture protective boots for scuba gauges before. The benefits are very low tooling cost, but cycle times can be long and there can be substantial labor at the end of the cycle to get parts off the tool and trim them.

Unlike injection molding, undercuts are perfectly legal in compression molding. But again, there is labor at the end of the cycle. Elastomers like silicone and other types of rubber are the most common materials of choice for this process. But I’ve utilized compression molded sheet ABS and styrene before with medical enclosures and rebreather cases.

The drawback to compression molded rubber is that it does not have the precision detailing ability of injection molding and will not hold dimensional stability without a substrate (i.e. the bumper plate example). We would regularly reject gaylords full of gauge boot blanks from Taiwan when ambient heat from the journey across the pacific was too high causing all matter of deflection and deformation issues. We tried moving final assembly to the factory in Taipei, but it proved to complex, costly and logistically impossible.

In order to get that flecked texture look on the Nike bumper plate, the process is designed to utilize sheet regrind or a crumb rubber mix that comes off a molded billet cut to various thicknesses and used on short draw/high draft parts like the plate.

I always thought combining crumb rubber floor sheets with a compressive process that could indicate assembly direction, locking or precision placements of objects on top of the panel once installed could be quite useful.

I haven’t worked in situations that are as high temperature or high pressure as something like that Nike plate, but I’ve worked on a ton of footwear outsoles that utilize regrind. Sometimes it was for the distinct visual “fleck,” but often it was a way to make use of at least some of the factory-rejected outsoles that occur naturally during production and would otherwise just get thrown in the trash. In that scenario adding regrind is unfortunately making the rubber substantially less durable, not more, and we would have to be careful how much we added so as not to compromise the performance of the outsoles.

The issue is that the regrind rubber has already been cured, and depending on your rubber type and moulding temperature and/or pressure it may or may not revert back to it’s pre-cured state during a second moulding. If it doesn’t, the regrind rubber won’t fully bond with the newly cured rubber, remaining instead as “floating” chunks throughout the product that separate from the rest of the rubber pretty easily.

A good visual example of this is the Nike Space Hippie line, which liberally uses reground rubber and EVA in its outsoles and midsoles. They can get away with it there because it’s not a performance product, but just running your finger along those midsoles will cause numerous flecks of regrind to fall out almost immediately.