Shiny spots/glossy spots on GF Nylon part

[MoldCrasher]

We've been molding gunstocks for a few years now flaw free. All of a sudden we are faced with glossy/shiny spots where walls intersect. We had the tool reblasted yesterday and by the end of the day the shiny spots were starting to reappear. The material has not changed nor any of the parameters and is in the same machine it has been.

Any ideas?

http://i42.tinypic.com/2jdgqhf.jpg

[Chris]

hows the water temp/ flow ?

have you cleaned the water circuits with an acid ?

[Suhas]

Agreed with Chris. Can you also check the actual mold temp? can you compare - 'good part' weight vs 'bad part' weight? Monitor the cushion to see if there is a bad check ring.
Suhas

[Chris]

what percent of glass is this ?

It is more than possible to have screw/tip wear as Suhas has mentioned

[MoldCrasher]

We have had some flow issues in the past. We checked the flow this morning, everything seems ok. I wish I had a surface temp thermometer, to monitor actual mold temp, but we dont. I was thinking along the same lines.

We just had the screw replaced about a month ago along with the check ring.

Its a 33 % glass filled nylon with impact modifier.

We are using a general purpose nozzle.

Cushion seems to be holding steady.

[Chris]

i would definitely be getting on PPE or IMS website and ordering a laser thermometer.

they are worth the money. It stops the guessing game.
I would also start checking for a weak or burned out heater band.

Checking the Ohms on each band will require you to disconnect the wires from each accordingly.

Chris

[MoldCrasher]

Thanks for the recommendations Chris. Laser thermometer sounds like a good investment.

[Chris]

no problem, let us know your findings.Its always interesting to hear and help others with problems like this.

[MoldCrasher]

We sandblasted the tool again today. We lowered the material melt and increased injection speed which seems to be doing the trick so far. The variation we have with this process on this job has really run the gamut. It changes majorly about every month. I guess maybe you can boil it down to material lot changes perhaps? If thats the case, its pretty alarming how the material ranges shipment to shipment.

[Chris]

yes, which is why you i hope are documenting the process and also staying Lot code tracked so you have traceability.

your data sheets should tell you the melt flow index from lot to lot or shipment to shipment.

[rickbatey]

My first guess is that the mold may have had a surface treatment due to the glass filler causing wear and gloss issues on the part. Now the coating is gone in certain areas, typically they use Dyna Blue to protect the mold steel for varying gloss.
Last point is do you know for sure, that the resin is completely dry? When was last check? When was the drier desiccant replaced? Return air filter cleaned?
Resin varies lot to lot! Robust machines, molds, and processes run more good parts with less adjustments!
Rick.

[brentb]

Might want a contact electronic thermometer infrared sometimes doesn't work well on shiny surface, dont have experience with laser though

KOM
brent

[MoldCrasher]

Well, it turns out it was a bad lot of material. The supplier sent us some material that had not been mixed properly. We received a new batch of it this morning and we are making good parts. I think there were other small factors related to this problem as well. But thanks for all the responses. All good points and questions.