Stress Concerns When Fastening Plastic to Metal
Stress Concerns When Fastening Plastic to Metal
(OP)
I've uploaded a couple of pictures of a SolidWorks assembly. The captive fastener shown is only used to hold the plastic to the metal securely, but the torque requirements are very low. The fastener is a #6-32 captive fastener. The plastic (thinner piece) is 3/16" thick Acrylic DS, and the upper part is die-cast aluminum A380. A co-worker is concerned about stress concentrations causing cracking in the plastic when the fastener is secured.
1) is this a concern?
2) how might I verify we won't see stress related cracking in the part?
Any input on this is greatly appreciated.
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RE: Stress Concerns When Fastening Plastic to Metal
Without that part I can't really form a specific opinion but my gut says that at 3/16" you aren't working with namby pamby sheet acrylic, I'd be more worried about cracking it drilling the hole or cantilever forces against the bolt head.
RE: Stress Concerns When Fastening Plastic to Metal
Why is the die casting not flush against the plastic immediately under the fastener head?
Do you have to have a socket head cap screw with a small head, such as what you have modeled currently?
What is the magnitude of the forces trying to separate the plastic from the Al once the assembly is " in use"?
In general, the current design looks problematic if any significant preload is generated. If however, the fasteners are literally finger tight, and no appreciable forces exist at the joint surfaces, then cracking is unlikely, assuming no UV radiation, exposure to alternating high heat/humidity, etc.
RE: Stress Concerns When Fastening Plastic to Metal
It won't.
Since you state that the torque requirements are "very low", it's highly probably that the screws will loosen in use, and your plastic will not be held securely to the metal.
If you tighten the screws initially tight enough then the plastic will cold flow over time, and again the screws will be loose.
RE: Stress Concerns When Fastening Plastic to Metal
RE: Stress Concerns When Fastening Plastic to Metal
RE: Stress Concerns When Fastening Plastic to Metal
RE: Stress Concerns When Fastening Plastic to Metal
TVP: The metal part has a counterbore to allow the captive fastener to spin free and release so when the part is serviced a contractor can unscrew the fasteners without losing them. The actual head style will most likely be a philips pan head. How do I calculate the magnitude of the forces that each screw will see? An 0-ring type seal will be pushing back against the plastic (held in place by 6 #6-32 fasteners) as well as the weight of the plastic part.
MintJulep: Do designers not attach plastic to metal? Can I use a lock washer to handle your concerns?
dvd: How would I allow for thermal expansion when threading through both the plastic and metal materials?
asimpson: I've got some of these laying around, I'll have a chat with them.
CoryPad: Thanks for the recommendation. Can I use a lockwasher to keep the fastener from loosening?
RE: Stress Concerns When Fastening Plastic to Metal
RE: Stress Concerns When Fastening Plastic to Metal
What kind of seal do you need? 6 6-32 screws won't seal air to vacuum, water to air, etc. (unless it's a very small area)...
V
RE: Stress Concerns When Fastening Plastic to Metal
vc66: The bolt circle is roughly 16in in diameter. We are looking to keep water from entering the assembly. We have already tested with a pressure washer following the proper UL test and had no water penetration.
RE: Stress Concerns When Fastening Plastic to Metal