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

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

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

(OP)
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.

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

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

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

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

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

(OP)
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?

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

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.

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

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.

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

(OP)
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?

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

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

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

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.

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

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

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

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"

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

(OP)
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)

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

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

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

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

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

Is it cracking along a knit line?

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

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

(OP)
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.

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

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

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

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

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

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"

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

(OP)
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.

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

ABS may or may not be susceptible to chemical attack from hand lotions etc, but polycarbonate sure is. I have seen mass failures of parts with similar symptoms to yours made of a PC based material due to the wrong additives for chemical resistance being used in the blend. For the record, investigation found the material was susceptible to a lot of chemcials used in the production line, including the hand soap. This being a hand tool, it could be exposed to the same. Some pointers that may help you diagnose:

1) Is it a constant stress or constant strain part? Constant strain parts are difficult to make crack in a delayed manner (i.e. 2 months away) without chemical attack, and thus indicate chemical attack. Constant stress parts can of course creep.
2) I would suggest opening up a crack on a part that has failed, and looking at the profile of the crack. A brittle looking crack or crazing in the area may be indicative of chemical attack. A ductile failure not so much.
3) A lab/the material supplier should be able to look at the crack profile and get at least an indication of if chemical attack has taken place
4) If there isn't any tensile stress in the area, chemical attack won't really be a big issue - but even moulded in stresses can provide the tensile stress necessary for chemical attack to proceed.
5) Get some translucent samples of the material made up if you can (remove the coloured master batch if possible, for example), you will be able to see crazing and so on much easier in them.

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

Taking another look at the first picture, it appears that there are a couple of strands, since normally ABS has no reinforcement, is this plastic fiber reinforced?; also it appears by the appearance of the width of the crack that the crack originated somewhere else but not in the boss. What is the application of this part and is it exposed to low temperatures?

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

(OP)
Hi guys thanks for your help, it is highly appreciated.

These are enclosures of a HHD and its is (not in storage nor in the field) not subjected to any temp extremes.

PC/ABS is wide spread bland (mostly in consumer electronics) so i find it hard to believe that it can be so easily attacked by hand lotion.

I am still investigating the issue and i have found that the shells are in constant tensile strength due to 0.2-0.3 mm gap between the bosses so the part may creep, hard to imagine that such a gap may couse failures in plastic molded parts.

The crack area is brittle so i'm thinking about bad batch of material, i'm molding now parts from Sabic C6600 which will be later compared to the current Chi Mei PC540, plus i am supposed to get to transparent samples for internal stress check using two polarized films.



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

It might not be hand lotion, that's just an example - many chemicals can cause this issue. Chemicals (paints, thinners, cutting fluid and oil, etc) can literally evaporate and deposit themselves from airborne onto PC and crack it. Although that would be an extreme case.

The 0.2-0.3mm gap is closed by the screws I assume, in which case since it's constant strain it's difficult for it to cause an eventual crack, since stress relaxation will take care of it.

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

Nereth1,

Is the reference to constant stress vs. constant strain language peculiar to plastics? If so, what is the way that stress is divorced from strain? I prefer linear solutions where strain = K*stress, but somehow this does not apply here, so I'm guessing it's a domain specific reference. I did try to look, but don't know how to differentiate the search terms to get what's described here.

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

3DDave,

It's the result of viscoelasticity in plastics. Somewhat similar to creep at higher temperatures in metals. Constant stress and constant strain loading cases are ideals but most cases are a mix.

In any case, plastic slowly stretches over time.

Constant strain: If you take a 100mm rod of plastic and stretch it to 105mm and clamp it there, in addition to any classical plastic deformation from overstress, it will also slowly molecularity reorganise itself and take a permanent set somewhere between 100 and 105mm. The longer you leave it at 105mm the closer to 105mm its' length will be when it relaxes after you remove the clamps holding it at 105mm. While it is clamped, if you strain gauge up the clamps, you will see the force on them exhibited by the plastic slowly drop over time. It won't likely break in this situation without something like chemical attack happening simultaneously, because neither stress or strain are rising.

Constant stress: Hang a weight on another 100mm rod of equal weight to the force your clamps that were holding at 105mm, and the plastic will immediately stretch to 105mm (as would any material). But wait a while, and the plastic will slowly flow and stretch beyond 105mm. The mechanism is the same as the relaxation in the constant strain situation, except the force never decreases so the effect we observe is a bit different. This will more often cause an actual fracture, as the plastic just keeps stretching until it goes, although the time period can be extreme in some cases if load is low enough.

If he is screwing two plastic parts together to a set position (gap=0 from gap=0.2mm) then that is mostly a constant strain situation, we wouldn't expect to see a delayed failure in that case without something like chemical attack or other service loads (e.g. dropping it one too many times).

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

Nereth1,

Thanks. Makes sense.

When I looked at the OP pictures it looked to me as if the crack terminates in the bore of the screw recess and starts elsewhere. I usually prefer an overall picture, in this case of both sides of the part to see where the fractures begin and end. For certain the fractures look very brittle.

I see from the IDES material page
Tensile Elongation (Break, 73°F) 100% ASTM D638
Tensile Strain (Break) 95% ISO 527-2/50

So a little deflection should not be hurting this unless there is a significant stress riser or these specs define elongation differently than for metals. Also, IDES copy protects their page. http://catalog.ides.com/Datasheet.aspx?I=62673&...

The original resin maker page, http://fjt.chimeicorp.com/gate/gb/www.chimeicorp.c...={510F54DB-0F81-4AE4-9F78-EF28BEF26551} has different values than IDES, but not very different.

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

That's exactly right, 3DDave, and that's why I'm thinking about chemical attack or similar rather than pure mechanical failure. Chemical attack makes things fail below normal breaking stresses and strains, and tends makes them fail after a significant time delay, in a brittle manner.

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

There is a standardized method for testing for chemical compatibility using test coupons. See the attachment. Also, sharp corners should be avoided as they cause stress concentrations that lead to cracking. The mold maker should be able to go back and add slight radii to the sharp corners that are on or near the load path.

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