We mold a U shaped component in ABS with a clear PC insert [for indicator lamp]. At the time of molding we don’t see any problem in the component.

But when it comes out of painting shop we see cracks in the ABS part The crack start from the meeting point of PC and ABS and proceed in the ABS. At least 10% of components are rejected this way.

Greetings, please define the exact materials, paint and painting process. One the face, this sounds like a chemical reaction affecting the base resins causing chain relaxation. Thus weakens the materials ability to resist internal, molded in stresses, at crazing locations.
The molding process could be inducing high internal stresses so evaluation of this may help. Typical molding yields should be 96% or better.

Iempandi, Ed brings out some good points and key questions. If you are "Insert Molding" your Clear PC Lens----rather than in an automatic 2-Shot tool/press, you will have a greater Shrink factor Delta between the 2 parts resulting in higher hoop stresses when the outer ABS is prevented from obtaining it's normal shrink size. Higher residual stresses in molded parts are typically problematic with paint applications due to chemical interaction with the solvents/thinners in most paint formulations. Since PC and ABS are Hydrascopic----the tendency for these resins to absorb these solvents (before they are allowed to fully evaporate during the paint cure) will usually act upon the higher stress regions of the part----resulting in cracks and crazing. As Ed suggests, you may be able to reduce the stresses by optimizing the molding process----note: if you were to mold the PC Lens as the second shot into the ABS, you would greatly reduce the shrinkage Delta between the 2 parts and reduce the stresses. If the PC Lens is hand loaded into the ABS mold, you might consider preheating the Lenses prior to loading in the mold----this may require your operators to wear gloves but it will contribute to some incremental benefit to reduction of stresses (as compared to Cold/Room Temperature Lenses). You must also investigate your paint formulation and the solvents/thinners used since PC and ABS have a low rating for general "Chemical Resistance".

Thank you mark As you have observed we hand load the PC part.I will first try the preheating of the PC part then mold the component and give the feedback.

S.Iyempandi

Dear Mr Lyempandi,

Did you think to use EXJECTION for your part? With this process, you can produce your part without stress in the melt and use your PC insert and the painting in on step. This process could certainly help you to solve your problem in potimizing your molded part.

I invite you to have a look on the website of Exjection and contact me by email for more information if you are interested in.

Best regards

Sebastien Prud'Homme

Don't paint the part hot (pack them off and let them cool sufficiently), if you are using a mask make sure the mask conforms to the part and vice versa. Does the cracking come in waves or is it spread out evenly throughout every production run?

Stress-Make sure you are maintaining your moisture content as this will change your fill characteristics and could result in stress.

PC has low chemical resistant and ABS has a moderate chemical resistance if during painting process both materials are in contact with solvents or if the paint used is solvent based it can be part of the cracks problem. may be the usage of water base paint can help.

Regards.....F.Gallegos

Are you annealing the parts pre paint? Sometimes this helps to alleviate molded in stress. Reduction of stress reduces the ability of chemical attack.

Hi... I am working in automotive industry. We are doing painting for a pcabs part named roof rack. Facing solvent attack issues after painting.We are doing matt black color. Our yield is only 50%. Before painting the parts have no traces of crack in that surface. After painting during flash off itself crack visible. Struggling a lot with this issue.