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Need help selecting plastic material for injection molding

Need help selecting plastic material for injection molding

(OP)
Hello all,
I'm new to this forum; seeking help as an inexperienced product designer on the selection of a plastic material for injection molding.

I have a small device that acts very much as a clamp for a cylindrical device that could range from 7.2mm to 9.5mm diameter. I have attached a simple representation to illustrate. The opening in the clamping area is around 7mm in diameter. The cylindrical device will be inserted and removed repeatedly, in the 1000s of cycles. Could someone provide possible material candidates that could support this function? I need a material that can within the stress cycling without losing (to much) the clamping/retention force over time. The insertion force for the cyclinder is about 1-3 or 4 lbf. Thanks in advance.

Howard

RE: Need help selecting plastic material for injection molding

Looks like you need a material with:

Moderate stiffness to provide the clamping force
Moderate elongation to break so it won't break when you flex it once
Very good fatigue resistance so that it stands up to 1000s of cycles

I would start with polypropylene and as it's inexpensive and versatile. Then, if that does not take the number of cycles then switch to POM/acetal as that has great fatigue resistance. Injection molds are really expensive so perhaps machine the part from a solid block and use that prototype for testing.

Does it need outdoor stability?

Chris DeArmitt PhD
President - Phantom Plastics LLC

Consulting, ideas and training on plastic materials
www.phantomplastics.com

RE: Need help selecting plastic material for injection molding

(OP)
Dr. DeArmitt,
Thank-you for your quick reply. This device could be used outdoor, but it depends very much on the user. I do believe the device would experience much more to the mechanical stress cycling as described than environmental/thermal stresses. This material would be used on a wearable device, so it's conceivable that it would be outdoor or left on a car dashboard for extended amount time.

What do you think about high-impact ABS such as LG Chem HI-121H? This was suggested by a plastic supplier, but I came to this forum to get other opinions. Much appreciation for your input.

-Howard

RE: Need help selecting plastic material for injection molding

On what are you clamping? A clamp with a 7 mm diameter used on cylinders of 7.2 to 9.5 mm dia will have have a clamp load at the large diameter 12.5 ((9.5-7)/(7.2-7)= 2.5/0.2 = 12.5) times the load at the small diameter, unless the cylinder is soft and compresses. Is the clamp left in place for extended periods of time? If so, creep is an issue.

PP is the material of choice for plastic hinges, and this is essentially a plastic hinge. But all a hinge has to do is resist fracture. It applies no force. A clamp must apply a force and sustain it, and I don't think PP is a good choice for that. Olefins tend to be creepy. ABS is prone to UV degradation and oxidizing. Unless properly stabilized, it would not last long outside.

I'm concerned about the shape of your clamp. The opening is very small compared to the nominal diameter, so it will see much higher stress during installation than during use. That will be your alternating stress component for fatigue calculations. Also, the sides are thick, so all the flexibility comes from the thin section at the bottom. That will concentrate the stress there, and cause creep and fatigue issues. The thinnest sections are at the clamp surfaces. Not sure you want flexibility there. And, it doesn't look like a very moldable part. Thickness is varying all over the map, and that a plastic part design no-no. It could be nolded, but it is far from ideal. If you have access to finite element analysis, you should run some simple models and look at your stresses. Don't worry about material yet. Try and get a moldable shape that gives you low uniform stresses with the clamping loads you need. Use a simple linear elastic material model with E = 300 ksi and Poisson's ratio = 0.4.

Rick Fischer
Principal Engineer
Argonne National Laboratory

RE: Need help selecting plastic material for injection molding

(OP)
Rick,
Thanks for your input. Great questions.

*The device would be clamping a rigid (aluminum) cylinder.
*Yes, the cylinder could be left in place, depending on end user.
*The simplified drawing provided was for illustrative purpose only to communicate the concept and does not depict all features or relative proportions of the final design. I ran a quick modeling in Solidworks to see where the stress concentration would be, and as you indicated it's mainly at the bottom where we want the flex to occur. Also, I 3D-printed a few units using ABS and they performed somewhat close to what I'm after, but due to imperfections associated with 3D printing, they all break where we would expect them to break after 1 or 2 cycles.

Because of various contours in the design, it was indicated by our injection molder that it could not be molded in any practical way. With that in mind, our strategy is to mold half, put two halves together and sonic weld. I'm not too worried about the UV degradation and oxidation of ABS. To a certain extent, I would want the device to fail after so many hundreds or (low) thousands of cycles and be disposed of (planned obsolescence). I'll see if there is an ABS blend, or other types, that has the flexibility needed and low creep. Thanks again.

Howard

RE: Need help selecting plastic material for injection molding

A weatherable version of ABS is called ASA i.e. Luran S or similar. Will resist oxidation and UV. Same mechanical properties as ABS.

Your plan to sonic weld may place the weak spot right where the max stress is.

Chris DeArmitt PhD
President - Phantom Plastics LLC

Consulting, ideas and training on plastic materials
www.phantomplastics.com

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