Mold Texture and Part Shininess

This e-mailer had a long question about texture and shininess. I have shortened it to include the critical portions…

The largest part quality issue is shininess in the textured portion of the part. A particular part is molded out of PC/ABS C6600, we have performed some studies and came up with the following conclusion: 

The draft angle on the texture should be modified to allow the gas freely from the texture grains. This would allow the melt to attain the shape on the texture depth. Even though proper venting is done, the gas must escape from the texture grains is so important than others.

Does this sound correct?
My Response
You are correct in believing that the melt is not properly attaining the shape of the texture. Basically, proper surface texture reproduction is the result of good interaction between the polymer melt and the mold surface during mold filling. If the melt cannot properly reproduce the mold texture, it obtains a shiny appearance. As a result, the primary factors which improve this are an increase in melt temperature or mold temperature at the point of contact.
Regarding mold temperature: There is really only one way to increase mold temperature… by increasing the coolant temperature, or reducing the coolant flow.
Regarding the melt temperature: Since we are only concerned with the temperature of the melt as it contacts the mold surface, there are five ways to improve this: 1) increase barrel temperature 2) increase screw recovery speed 3) increase back pressure 4) increase injection speed and 5) decrease the gate size.
Options 1-3 are most helpful if your melt temperature is too low when measured by a melt pyrometer.
Options 4 & 5 are often the most helpful because they avoid excessive heating and degradation inside the barrel, and only provide heating through shear at the time of injection.
Additional Thoughts
If you strongly believe venting is the cause, you can always seal the parting line and add a vacuum pump to investigate that prospect further.

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