Problems with paintable ABS

[Ryan King]

We have been having a problem with our painted parts for quite some time now. It comes and goes at random and we are unable to root cause the problem. It is across the board on all molding machines and nearly every painted part molded in ABS. The primer and paint is "soaking" into the substrate in the gate area leaving a very noticeable defect after the painting process. After a very extensive DOE I have still been unable to determine the root cause is in the molded part. It’s obvious that there is molded-in stress in the gate area but that is impossible to eliminate. I believe that the defect is being caused in the painting process by over aggressive primers and/or solvents that bring the molded-in stress to the surface. I am a molder and by no means a paint expert. Does anyone have any experience with this problem and do you have any advice to offer?:confused:

[rickbatey]

WHat is the gating method for these parts? Hot drops direct into the part are probably going to cause more issue due to the fact that they never close. Have you tried dropping the hold psi towards the end of the hold time? What are the melt temps, water temps, and process settings? Also include how the mold is watered. That will help in determining the best long term solution.
Have you checked the part temp at the gate with a contact probe? You may have a much higher ejection temperature than you know, causing this issue. Have you tried to run extremely long cooling times and test those parts? You may even have hot spots around the gate, and need to improve the cooling or switch these circuits to a seperate water source.
We did figure out a test for parts prior to paint: Spray the parts with mold cleaner. If there is high molded in stress, the cleaner really attacks the part.
Rick.

[Ryan King]

Thank you for your response.

They have a variety of gates but none are dropped directly noto the part. They are all hot to cold runners either edge, cashew, or subgated to a tab gates. Melt temps are all between 450-500F. Water temps vary from 70F-160F (complex part design, wall, boss, and rib thickness force us to run cold water for sink and cycle times). These parts are VERY sensitive to sink and our quality standards are so high that dropping hold pressures is not an option.

I have not yet tested part temp at the gate.

I ran an extensive DOE on many variables:
mold temp moving
mold temp stationary
melt temp
hot runner temp
fill speeds
hold psi

I was unable to root cause the defect with the DOE. The only other factors I can think of is raw material and the painting process. We are currently looking into lot variations in the raw material and are working with the paint dept. to get a DOE done on thier end. I too found that spraying mold cleaner on the parts helps reveal molded in stress. It's much easier then rubbing them with acetic acid.

[rickbatey]

Thank you for your response.



I have not yet tested part temp at the gate.

I ran an extensive DOE on many variables:
mold temp moving
mold temp stationary
melt temp
hot runner temp
fill speeds
hold psi

.

I see a couple of variables left off: Pack or hold time! You should run this variable and see what happens. Lastly decompression speed and distance.

Also I am curious as to the actual steel temps near the gates and defects.

Also, has any one noticed the machine hesistating when it starts to inject? Perhaps this is a cold slug from the nozzle.
Rick.