Strange surface defect when packing ASA with high pressure

[EduardoC]

This defect happens mostly when working with ASA in molds with satin surface finishes.

When we raise the packing pressure in order to eliminate sink marks there comes a point where marks start to appear in the seen face. The marks appear just where the ejector pins, slides and logos are, and have the same size and shape of the pins, slides, etc.

We have left the logos out of the question because there is a change in the thickness, a very small one though. The pins have been checked and they are flush with the cavity wall.
We thought that maybe it was due to a different heat transfer coefficient of the pins and slides. Changed one of the pins for a copper plug and the defect disappeared in that area.

Anybody knows of the name of this defect? We are puzzled about the origin of this but cannot find any information about it because we do not know the name of it.

I’m sorry I can’t provide photos of this, due to the difficulty of capturing them in a photo and confidentiality issues.

Thanks in advance.

Eduardo

 

[Demon3]

I know a lot about ASA the material and would like to help but cannot really understand what you're seeing. Sorry.

Chris DeArmitt

Expert consulting & training

http://www.phantomplastics.com

 

[btrueblood]

Are you seeing flash (material being forced into the gap between ejector pin and mold)? From the description of the problem (occurs at higher pressure) it sounds like this might be the problem... Maybe a photo would help?

 

[Pud]

Do the marks disappear when heated with a hot air gun? If they do then the part is overpacked and getting stress marks from the ejection system (which would explain your copper slug results but not where the slides are).

A light spray of release agent on the ejection side cavity for a few shots may also give clues if it is the above.

H

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[EduardoC]

Hello to all,

thank you for your answers.

It is very difficult to capture the defect in a photo as you have to turn the part around to properly see the defect. I'll try to take a photo that I can share.

The marks appear in the front side while the ejectors and slides are on the back side of the part. As if the marks were some kind of "reflection" of the opposite face.

Eduardo

 

[Pud]

Have you tried heating the marks as I suggested?
H

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[btrueblood]

Ah, now I understand what you mean, Pud. The higher pack means more friction in the mold, = higher ejection force required, the pins are putting more bending stress into the part around the edges of the pins, and the marks are stress witnesses - like the crazing seen in styrene when it's bent?

 

[EduardoC]

Thanks for the help.

This is the best photo that I could get, not very good though.

I still have to try heating them. I'll let you know of my findings.

 

[Pud]

btrueblood: spot on there, young Sir!

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[EduardoC]

Hello,

I think that the ejector stressed can be ruled out. A colleague inspected a part just opening the mold, no ejection, and the marks were already there. :-/

 

[EduardoC]

Also, the slides and logos also produce the same defect.

 

[Pud]

Has it got a metal flake pigment in it?

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.