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Creep in polymers under load
5

Creep in polymers under load

Creep in polymers under load

(OP)
Hi all,

i'm trying to design a product that will be used as a clamp, when the clamping force is applied by using a quick release lever.

I want to choose the correct polymer so the material creep at the contact point with the quick relese will be minimal (as its the smallest area with a high force applied to it).

This is the first time that i'm facing such a problem so i not sure how to approach it.

Thanks.

RE: Creep in polymers under load

All polymers creep. Small, highly stressed parts should be made from metal. Fiber reinforced polymers may be an option.

RE: Creep in polymers under load

(OP)
Yes i realise that metal is the more suitable option, but this time the polymer is a requirement.
I know for a fact that there are polymere based clamping solutions, and i know how to narrow down the list by other parameters and material properties, but i want to know how to correctly calculate a value or a set of them with which i can choose the right polymer for the job, by comparing them with the material properties.

So i'll try to refine my question:
1) Is there a simplified way of calculating an estimated value that can help to narrow down the polymer list for a more in-depth analysis?
2) What are the material properties (in polymers) that define the resistance to creep (e.g. Shor value or some other parameter)?

RE: Creep in polymers under load

2
2) What are the material properties (in polymers) that define the resistance to creep (e.g. Shor value or some other parameter)?

Creep rate data for polymers is hard to find, but some googling can get you some useful information. Because the test is hard to do (special equipment, long duration of test) and the data highly non-linear, I don't think there has ever been a standard methodology to define creep resistance, even in metals. One thing to do is find the melt processing temperature for thermoplastics, and look for ones with higher values, along with higher room temperature ultimate strength, this will narrow the list for you somewhat. Similarly, the heat deflection temperatures (ASTM D648) will point you towards more creep resistant materials, generally. You can also find tables like these http://www.dotmar.com.au/creep-resistance.html to help guide your search.

You also need to figure out how long your clamp is expected to hold a load - this will bound the time limits of your creep data.

In the end, if it's critical, you will need to pay somebody to do testing on your material, do the testing yourself, or ask the material vendors for data. Reinforcements (fibers) in the melt can help, especially if you can get the fibers to align with the tensile stress.

RE: Creep in polymers under load

2
Some ways to avoid creep problems:
High melt point, as was mentioned.
Cross-linked (thermoset) polymers. These also come as molding compounds with fillers and reinforcements.
Highly crystalline polymers, as the crystals act as cross-links.
Keep stresses and contact pressures as low as possible through smart design. Do not use a small "contact point". Matching serrations works well.

RE: Creep in polymers under load

I suggest you do some comprehensive reverse engineering on the Vise-Grip/IRWIN Quick-Grip line of clamps, to include modeling the various parts well enough to estimate the stress levels in the plastic parts.
I think the medium size is rated at 100 lb of clamp force. They represent state of the art, or pretty close, as to what's achievable in real plastics, or more correctly in plastic/metal assemblies.
They are very handy around the house, but they do yield and they do creep. ... like all plastics.

If someone has given you a limited time in which to produce a satisfactory design, especially if there's no budget available for exploration, you might as well just quit now. As hinted, getting reliable data for plastics is not easy.

Wait, it gets worse. The actual properties you will get in the actual part you have molded, depend rather strongly on the design of the mold, and of the skill of the moldmaker, and of the skill of the molder. Unlike with metals, you don't get any guaranteed properties you can rely on. You can probably get 'representative properties', extracted from tests of molded coupons, not actual parts.

Wait, it gets worse. You will find that many of the plastics with the most desirable properties are made only in laboratory quantities or require exotic processing, and are hideously expensive.

Wait, it gets worse. Improving yield and stiffness performance is generally done by adding minerals or glass fibers. ... and the properties change in nonlinear ways with proportions of the additives, and with their size, length, morphology, chemistry, and surface treatment/coatings. For all of that, you are on your own. You can buy resins compounded with however much you want of whatever you care to add, but again, you don't get guaranteed properties, or even typical properties, until you mold the stuff into coupons and test it yourself, or find someone who already has.

You have taken but one small step on what promises to be a long, strange, interesting journey.
Take notes. Measure. Ask. Listen. Learn.
I wish you well.



Mike Halloran
Pembroke Pines, FL, USA

RE: Creep in polymers under load

Great post Mike. But wait, it gets worse. Just when you think it's all worked out, production will under dry or over dry the molding material and compromise the material properties. Then they will ask (if you're lucky) if it's OK to use them.

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RE: Creep in polymers under load

(OP)
Thank you all guys for the helpful tips (great post indeed Mike thumbsup2).
Sound like i got myself a handful with this project.

RE: Creep in polymers under load

(OP)
Not yet, finishing with another project so we can start manufacturing.

But as soon as I'm done with it i'll start working on the clamping device, and post updated in case something interesting pops up for better or wore lol.

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