Cracking in PC/ABS (Chi Mei PC540) injection molded parts

[IgorPev]

Hi guys,

I would like to consult with the group regarding a failure I am experiencing in one of my injection molded parts.

The part is plastic shell enclosure made with injection molded PC/ABS (Chi Mei PC540), the screw socket is cracking in both of the bottom bosses. The cracks appear between 1-3 month after assembly, they may occur in storage or in the field.

I have tried to simulate the cracks myself without any successes, I have identified that there is a small gap (~0.2mm) between the two enclosures so some tensile stress may be applied.

The screw is a self-tapping type, the bosses does not show any singes of cracking, only the socket does.

Does anyone know how to test injection molded parts (not transparent) for stress / how to expedite cracking?
Does anyone experienced PC/ABS cracking this way before? (No oily / other solvents are present in the assembly line).

Thanks in advanced.

 

[simi1004]

One way is to cook it. Place them in an environmental chamber and run thermal cycle.

 

[MikeHalloran]

You have to check for solvent/oil contamination all the way back to the molder. ... including the hand lotion used by the girls who degate and deburr the parts.



Mike Halloran
Pembroke Pines, FL, USA

 

[IgorPev]

Simi1004: Can you recommend environmental cycle? will a Halt/Hass be sufficient or just cooking with humidity?

MikeHalloran: If the material so sensitive that it will crack with hand lotion, there is no way to guarantee that it wont cracks in use if the customer applies hand lotion while using the device right?

 

[Screwman1]

It looks like the tensile load is bending the part though the socket.
You have to get rid of the gap to prevent the problem from coming back. The purpose of using a screw is to create tensile load and over time the weakest link in the boss/ socket chain will be likely to fail. In your case it is the socket- increase the thickness at the bottom by 0.2 to close up the gap and increase the load capacity of the socket. Make sure there are chamfers on all inside corners to reduce stress concentration.

 

[chicopee]

Hand lotion should not affect ABS. ABS is used in a lot od kitchen appliances such as refrigerators. I agree with screwman1 about a potential gap that bends the plastic as you tighten the screw. It seems that section needs to be "beefed" up by increasing thickness around that problem section and/or, as proposed above. chamfering or filetting would also help.

 

[IgorPev]

Should the enclose be able to deal with 0.2 mm gap, usually when you design plastic enclosures you keep a gap of 0.2mm in advance.
I did not found any residue / solvents on the part or in the process.

Is there a test (environmental possibly) that can simulate cracking?
Is there a test which can test Yield / Break Tensile to validate the material?

 

[MikeHalloran]

You can cut arbitrarily small test coupons from the as-molded material, then normalize the test results for the reduced area. The statistics will probably be better if you measure the cross section of every sample and use that area for normalizing that sample.




Mike Halloran
Pembroke Pines, FL, USA

 

[Screwman1]

I have not heard of designing in a gap on molded parts. The new picture looks like a crack from flexing the material due to the assembly pressure. Both cracks are in the same general area- to me this indicates a stress concentration in that area. Is there a design feature on the backside- ribbing or something that could cause that.
i have not seen cracking problems with PC-ABS; that is one of the reasons that it is used so much in consumer electronics, it's hard to screw it up in processing.

 

[3DDave]

Those photos show where the crack ends, but not where it starts.

 

[dwallace1971]

How is the part loaded during assembly and operation? What are the environmental conditions in which it is used? What about UV and thermal cycling?

"On the human scale, the laws of Newtonian Physics are non-negotiable"

 

[IgorPev]

This part is an enclosure of a hand held devise (300mm x 75mm x 35mm) which has been manufactured for the last two years. The cracks started to appear about 2 month ago in the lower two bosses of the part, their geometry and starting point are random but they always appear in the lower two sockets.

After assembly there is no sign of stress/cracks, but after 1-3 month they are beginning to appear to the naked eye.

The device is HHD and it is no exposed not in use and not in our storage to any extremes.

The part is molded in Taiwan. (the manufacturer claims that he does not changed resin and use only virgin material)

 

[chicopee]

Well that explains made in Taiwan; have the part made in the USA.

 

[IgorPev]

Unfortunately you can't allays mold in north america / Europe especially not with big quantities.
It should not be the reason for cracking.

 

[cowski]

Is it cracking along a knit line?

"A hand held device" - have you performed drop tests on the finished assembly?

 

[IgorPev]

There are no knit lines in the parts nor in the mold flow simulation.
The device successfully performed drop test when it was initially released 2 years ago.

 

[IgorPev]

I meant to say there are no knit lines in this area.....

 

[cowski]

If you have a hole in the part, there is a knit line close by somewhere...

 

[dwallace1971]

Have you tried drop-testing them since you've discovered the problem? Have you checked the fracture toughness of any of the damaged components?

"On the human scale, the laws of Newtonian Physics are non-negotiable"

 

[IgorPev]

I have tried dropping them unfortunately it did no resulted in cracking. That's why i'm looking for environmental test to expiate the crack development.