Surface Finish On Polycarbonate

[bassnut]

My injection molding supplier is having a hard time getting a good finish on the critical face of my parts. The finish appears grainy and with magnifaction small, almost microscopic craters are visable on the surface. The surface of the die is polished smooth however. Are these being created by bubbles? Trapped gas?
What's the remedy?
The mold is vented but I don't know the details.
The material is virgin black GE Lexan.

 

[patprimmer]

Polycarbonate is hygroscopic.

If it is not properly dried it out gasses water vapour during moulding. This normally results in tiny bubbles that are often described as silver streaks.

There are many grades of Lexan. Please supply details re melt viscosity and additive package.

If we could see an image of the surface, that might help.

Polycarbonate also shows an effect often called fingerprints or record lines. This is from the mould and or melt being to cold.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[bassnut]

Thanks patprimer. I'll try to post a photo link soon.
There was a problem with silver streaking (splay) but that seems to have been resolved.
I'll try to find out more about the material. It did come pre-dyed from the supplier. Nothing else was added during the molding process.

 

[Pud]

Polycarbonate is not pre-dried from suppliers as there is not a bag available (used to come in metal tins with soldered lids many, many years ago!) with sufficient barrier properties to prevent moisture take up.

PC requires drying at 120 deg C for 4 hours minimum in a dessicant or vacuum dryer.

After drying, much more than 4hrs before moulding it will require drying again.

Rgds

harry

 

[patprimmer]

Polycarbonate cannot reliably be used as supplied, as it only takes a very small amount of moisture to cause problems, and it can pick up that much between the bag being opened and the material being used if it is not in a hopper that contains dry air.

I am starting to suspect out gassing from a flame retardant or mineral additive.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[injectionmolder]

Hi bassnut

Ask your molding supplier what happens if the following are increased:
melt temperature
Tool temperature
Injection pressure
Injection velocity
At the same time be care full not to over pack the cavity, but see sounds to me like this has to do more with the way they are injection the polymer. As a second question ask them if any mold release is being used in the process, that could also give you that type of finish.
Ask also GE what percentage of moisture there is in the resin at the time when you get it, I have seen Lexan (PC) in plastic bags but never pre-dried.

My best regards