Stress Concerns When Fastening Plastic to Metal

[uwbsme]

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.

http://files.engineering.com/getfil...1ac26946c9e&file=Example_Captive_Fastener.jpg

http://files.engineering.com/getfil...2e01dd53b&file=Example_Captive_Fastener_2.jpg

 

[BiPolarMoment]

What exactly is "very low"? I'm assuming you don't have clamping requirements otherwise you would be starting there. How is it being torqued in the first place?

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.

 

[TVP]

A couple of questions:

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.

 

[MintJulep]

The captive fastener shown is only used to hold the plastic to the metal securely

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.

 

[dvd]

Plastic can grow ten times as much as steel through the difference in thermal expansion coefficients. You have no allowance visible for thermal growth. I don't think that you are going to have much luck with this design.

 

[asimpson]

Have a word with some self-tapping screw manufacturers like PLASTITE. Should give you recommendations on screw for plastic.

 

[CoryPad]

If you don't tighten fastener sufficiently, they will self loosen. If you tighten fasteners to prevent loosening, you can overload polymers. It is best to add metallic compression limiters to clamped polymeric parts. Take a look at Spirol.

 

[uwbsme]

BiPolarMoment: Very low torque requirements mean that it is being fastened down with a handheld cordless screwdriver on the lowest setting. I haven't calculated the torque that equates to that. The fastener head is actually a philips pan head. The part does press against a seal. There are 6 of these fasteners that hold the plastic against the metal. They can't be overtightened or the seal will become wavy and there will be gaps.

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?

 

[CoryPad]

I would not add a helical split washer nor a conical washer. I would fix the overhang condition of the hole in the plastic part, add a compression limiter, and tighten the fastener to create enough preload to prevent the parts from separating.

 

[vc66]

Lock washers won't do anything. What size is your bolt circle (I assume it's a cylindrical shape)?

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

 

[uwbsme]

CoryPad: I think I'm going to use a threaded insert that will be molded directly into the plastic part. This should also act as a compression limiter, no? The overhang condition (counterbore) you reference is part of my captive fastener feature. I need the fastener to spin free once it disengages from the aluminum die casting. The threads in the plastic part is what will retain the fastener. Do you know how I would calculate the required preload?

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.

 

[CoryPad]

You calculate required preload by analyzing the applied loads (mechanical, thermal) and establishing a preload level that resists those. VDI 2230 provides simple and complex calculation methods, I recommend obtaining this document. Regarding the overhang condition, perhaps things will change with the metallic insert, but as the parts are shown in your attached images, the acrylic sheet in unsupported from the fastener contact stress and the part will creep/craze/crack/fail.