Repair of compression molded rubber 1

[awol]

We have nitrile and neoprene parts compression molded under high pressure. Once the rubber product is complete, we machine parts of the molded rubber part to various configurations. Sometimes voids below the surface are discovered during the machining process. Most of the time these voids do not impact the performance of the molded part, but the voids are visually unacceptable. We have tried RTV to repair and they look worse. We have used MEK to dissolve rubber 'dust' and create a paste to fill the voids. Not great success. Is there a method to repair molded rubber products and create a good bond with the parent rubber and blend the repair visually? We have confirmed the function is not compromised with the voids but we cannot send these blemished parts to our customers. -Awol

 

[GrahamBennett]

Voids in compression molded parts are usually due to trapped air or moisture in the compound. To avoid trapped air, make sure your blank weights are = part weight + 10%. Use 3 or 4 "bumps"* of the platens before applying full pressure to the moulds. Both of these are effective. To avoid moisture, make sure your ingredients are kept as dry as possible (that's assuming you do your own mixing) or your compound in a warm, dry atmosphere (about 30°C or so).

* A "bump" is the application of low pressure on the mould for a few seconds followed by slightly opening the press before re-applying low pressure.

 

[GrahamBennett]

Sorry - forgot to mention this. You can repair the voids by buffing the void surface with a small buffing tool then fill the hole with an amount of uncured rubber. This can be cured using a hot-shoe clamp mould (similar to that used in the retread industry for making spot repairs).