Bonding/Cracking problem with ABS
Bonding/Cracking problem with ABS
(OP)
I am bonding a 1 m long black ABS molding (air-mold) with PU glue on a steel surface using primer and activator. The glue and the system are approved by glue manufacturer. The application is on a vehicle. The ABS is cracking after a period of several months. The material is acc. to scans ok. Does anyone have experience of the possible factors or materials which could cause the kind of cracks.
Thanks in advance
Thanks in advance





RE: Bonding/Cracking problem with ABS
Regards
Pat
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RE: Bonding/Cracking problem with ABS
Regards,
Cory
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RE: Bonding/Cracking problem with ABS
RE: Bonding/Cracking problem with ABS
Regards,
Cory
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
RE: Bonding/Cracking problem with ABS
The more acrylonitrile, the better the chemical resistance and not all ABS types are the same. I would suggest that you ask your supplier for an ABS grade with high acrylonitrile levels.
ABS is intrinsically very unstable to UV light, the polybutadiene rubber gets degraded and the material becomes brittle. If they accidentally sent you regular ABS without UV stabilizer then that would explain what you're seeing. If they did UV stabilize it (using additives) then you either need a better stabilizer or to use ASA instread of ABS. ASA retains properties and color for decades outdoors but is more expensive than ABS.
Chris DeArmitt
RE: Bonding/Cracking problem with ABS
Does stress increase susceptibility to UV degradation. I imagine it might.
Just because it does not crack wit one thermal cycle fresh after assembly does not mean it will not stress crack after repeated cycles, and exposure to chemicals and UV light
Regards
Pat
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RE: Bonding/Cracking problem with ABS
As you know, the ASA will have much better UV resistance than any ABS because ASA has a rubber that is more or less impervious to UV (polybutylacrylate).
Chris DeArmitt
RE: Bonding/Cracking problem with ABS
RE: Bonding/Cracking problem with ABS
Regards,
Cory
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RE: Bonding/Cracking problem with ABS
It's not by any chance multi-point gated and this is at a weld line? Just a thought at 1m long...
Cheers
Harry
RE: Bonding/Cracking problem with ABS
It's also right on the edge of the date stamp which may be a stress riser.
Do those adheives contain any solvents. Are the surfaces prepared with solvents.
Are there any plasticised plastics or rubbers in the vacinity.
Regards
Pat
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RE: Bonding/Cracking problem with ABS
Agree re stress raiser (although this would suggest marginal design criteria)- also solvents. Many years ago we made some hollow ABS balls with the two halves glued. Tried some activator (Loctite) which we sprayed on the joint. Cured off instantly, but the balls fell apart about a month later! Not on the glue line - the rest of it, due to residual solvents! Activator "approved" by Loctite!!!
I wonder if the op is a bit misguided using adhesive "approved" by the adhesive supplier. They would probably approve anything!. If the curing agents are not water based, and after extended inhalation you get "high", they are no good for ABS!
Could the op indicate whether the cracking is in generally the same position, or if it's random along the length? Pat may have a very valid point. ABS, like most amorphous polymers are very notch sensitive, and the sharp edges at the bottom of the date stamp may be the cause.
Also, is the ABS part glued so that stresses are "built in" - e.g. straight ABS part glued to a curved steel part?
Cheers
Harry
RE: Bonding/Cracking problem with ABS
RE: Bonding/Cracking problem with ABS
Plastics typically have about 10 times the co-efficient of thermal expansion of steel.
Car bodies go through considerable thermal cycling.
I have actually seen a car where the OEM engineers attached a glass filled PBT cowl grill with 3 self tapping screws. One screw in the middle and one at each end. The screws went through the moulding directly into the sheet metal on the firewall.The thermal expansio in normal use sheared the outboard off or pulled them out of the sheet metal.
To solve the problem, they still used a fixed screw in the middle, but moulded in slots and used shoulder bolts on the outboard attachments. These shoulder bolts and slots allowed about 3mm or 1/8" movement at each end, or 6mm or 1/4inch over the entire length which was about 1 mere.
Glass filled PBT has about half the coefficient of expansion of ABS. 6mm of strain over 1 metre in ABS equates to considerable stress.
Regards
Pat
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