Any experience with inert gas purge or evacuated molds for rubber

[btrueblood]

Has anyone had any experience with the subject matter, and could you point me towards a reference that talks about this matter? We are having trouble with air inhibition of cure in molding and bonding (to primered bronze substrate) of a peroxide-cured EPDM. I have seen one referenced paper (abstract only) in a google search, apparently in German, that talks about using inert gas purging and mold cavity evacuation (vacuum pumping) in rubber injection molding. Wondering if this technique is useful or not before spending time and resources tracking down that lone paper, and then paying to have it translated.

 

[tdd10]

I have used bumping to degas the mold... but that was using the controls of the press. It acts like vaccuum pumping to evacuate the trapped gas in the mold. Now is this in the bonding as well (shows up in a tear test)? If it's on the bonding layer of your primer I would say you are possibly mixing the bonding agent too fast creating tiny bubbles in the coating. Wherever there are bubbles will be little to no adhesion of the brass to rubber surface. Especially if you are brush coating these parts. I hope this helps.

 

[btrueblood]

No, not bubbles in the primer, this is big patches on a complex molded part, with lots of places that don't necessarily see a good flow path from the transfer pot sprues to the vents. Yes, we currently bump and allow extrusion of the rubber from the mold to help expel air...but air entrapment has been a problem on some parts. We may have fixed things for now by better controlling the insert dimensions to ensure a repeatable gap (testing yet to confirm this). In the end, the manufacturer we are using also has some molds for FKM elastomers that use a vacuum assist, just needed to ask them the question - and they said "oh yeah, that might work..."

Thanks anyway, tdd10.

 

[acspain]

Couple of ideas:

All the presses at my previous employer used vacuum chambers surrounding the molds and the vacuum would be on full tilt during the molding process (compression, injection, and transfer), this prevented trapped air/gases in most molds.

Unbonded patches usually indicated that the rubber wiped away the bonding agent or indicated poor coverage in the first place. If wiped away, a mold design change would be needed.

Difficult to bond combinations of metal and rubber utilized a special bonding agent and/or a roughed up insert surface similar to sand blasting but in your case probably with a less aggressive media, thus creating a physical bond along with the chemical one.

If you are still having problems try roughening the inserts, then different different bonding agent.