Resin Starvation

[brakemeister]

Other than cold cavity surface (less than 150 degrees F), what kind of molding conditions would contribute to the lack of a resin-rich surface of a gear molded out of Celcon GC25A?

 

[patprimmer]

180 deg F would be ideal for good surface finish, but I don't think that is your problem.

Good surface finish with glass filled crystalline materials depends on a number of factors. If any one is insufficient, a glassy or silver streak type of finish occurs.

The causes are glass suck back or out-gassing. Both cause tiny voids at or near the surface which gives a silver streak type of finish.

To avoid this, dry the material. Yes I know Acetal doesn't need drying, but you get a better finish and less mould deposit if you do dry it.

Use high injection speed. This might be limited by jetting, then it becomes a trade off as to which fault you can tolerate, or you need to change the gate.

The highest practical mould temperature. This allows the mould to fill and pack before the surface of the plastic skins or gels.

Enough injection pressure to fully pack the mould.

Enough holding pressure to hold the pack stable, so the material below the skin does not move and disturb the fibres in the skin.

You can get a feel for this by watching the cushion while the moulding freezes.

A big enough gate and sufficient hold up time to keep the pack on the material until it is frozen enough to avoid resin movement just below the surface.

These conditions might also effect mould shrinkage and material strength.



Regards

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[Demon3]

Just wanted to write and agree with Pat. That was an excellent reply.

 

[darin69]

Excellent reply by pat, i agree. Also, don't be afraid to raise mold temp. (230-250 F) or so and maybe try slow to medium inj. speed. Also, I would be very cautious of high screw speeds, and excessive regrind rate if applicable. Definitely dry mat'l (180 F). Good luck!