Air entrapments in closed cell foam rubber

[Bobfoam]

Hello everyone,

We manufacture closed cell foam products at our facility by compression molding. Currently we are experiencing severe air entrapment issues with our SBR/EPDM/Neoprene blend stocks. Could anyone give any ideas why this could be happening all of sudden? Where do entrapments normally occur, extrusion, molding, mixing? When the air traps are cut open there is a cured skin inside, no contamination. Any ideas or solutions will be greatly appreciated! Thanks

 

[berti]

Hi bobfoam,
unfortunately you do not mention size of "hole" in relation to foam cell size. If it is not too big I would suspect some coarse pieces or undispersed aggregates of the blowing agentin the compound. Bigger holes should have "mechanical" reason.

HTH

Berti

 

[GrahamBennett]

Bobfoam - how long do you allow the compound to mature before you use it? You should allow about 24hrs to allow any trapped air caused by mixing to discipate.

 

[Bobfoam]

Thanks for the replys. Our process is faily consistant to an extent. Our closed cell material is mixed thru a Banbury 11D mixer and sheeted onto a skid. The majority of the time the material will sit for approximately 24hrs before entering our extrusion process. Our extrusion process involves remixing our SBR-EPDM blend stock materials thru another 11D mixer, thru (two)two roll mills, then into the extruder. Our pure EPDM materials go thru the same process but do not exhibit the same problem. During the months of October-April we experience great quality, but when Spring time hits our quality goes crazy. We have been looking at raw materials, performed alternative experiments, but have not been able to point out any resolutions.

 

[GrahamBennett]

First of all I question the need for a remix in the Banbury prior to the mill train. The first mill will heat the stock up sufficiently if it is heated to around 60-70°C. From your description can I assume you use a hot feed extruder? If so, do you heat this independently? To what temperatures? (Cold feed extruders are much better at extruding foams and sponges.)

Finally, have you noticed any problems during the summer months when the humidity levels are much higher than in winter? If this is the case, make sure your temperatures are all above due point (I would recommend using 50°C in any case for the mixer, higher on the mills and extrduer) to ensure no condensation occurs.