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POM - properties, experiences and useful links

Polyoxymethylene, often called Acetal, is one of the key engineering plastics. Among plastic materials, it is one with the most crystalline structure. POM is known for its good fatigue/creep resistance, low friction and good performance in cold temperature. But the key characteristics of POM have been posted in several web pages already, so instead of posting them yet again I rather recommend a few good links. As a source of information for the strengths and limitations of POM I recommend:

 

Practical experiences of POM:

(For confidentiality reasons I can seldom post pictures of my own projects)

  • POM felt like a suitable material for a tool handle that needed to slide over an aluminum bar. It served this purpose well, but the handle itself was so slippery that it was very difficult to have a good grip of it.
  • POM is often used for different types of pulleys and wheels. In one case the pulley worked fine until winter came and the temperature sunk below zero. Because of the high thermal expansion tendency of POM, the pulley shrunk around the metal-bolt that was used as an axis. Even though POM retains its performance in cold conditions, the thermal expansion is clearly important to take into account.    
  • The functional properties of POM seemed perfect for an industrial application but due to the product specific flammability standard classification requirement UL94-V0, POM was not chosen. If the requirement is higher than HB, the suitability of POM is worth evaluating in an early phase.
  • POM was selected as the material for a product that was subjected to cyclic loading because of its good fatigue resistance. The products however appeared to wear down quite rapidly, so the material was changed to PBT. As a result the lifetime of the product doubled. The fatigue resistance of PBT was sufficient for the application but wear resistance considerably better than before.    

I’ll add more links to the page every time I run into something useful. If you have a good link, case or story in mind, please email me at markus@plasticprop.com or add it below in comments.

To fully grasp the look and feel of POM order Plasticprop Essentials sample kit including 20 of the most common plastics. It has been carefully designed to convey and explain the characteristics of each material. Read here what the sample tells about POM.

Pictures:

Comments

harshad
POM means nylon am i right?
Markus
Harshad, NO, POM does not mean nylon. Nylon is originally a DuPont trademark for polyamide, PA. POM is an abbrevation for Polyoxymethylene. It is commonly called "acetal" Markus
Mohi
What is the standard compactness of POM? Dose it differ with the type of material produced?
Markus
Mohi, Do you mean the density of the material? It should be around 1,42 on both, homopolymer and copolymer. Thermal expansion of POM is high, so theoretically you can measure slightly different density in cold and warm conditions. Markus
Gopinath
hi. POM can flexible for removing under. For example, removing internal thread by jump off method.
Markus
Gopinath, Yes, as long as the shape is not so sharp that it will cut itself off rather than jump over the undercut. And there should be room on the other side of the wall. When ejected, semi-crystalline materials are typically still in quite rubbery stage. Markus
vivek singh
In what conditions POM get brittle?
Ben
Thank you for the great resource! For a clip application requiring a « jaw » where the two jaws must remain about 2mm apart and the overall clip thickness is 2mm, AND the application requires the clip to be usable in temperatures that range from 10 Celsius down to -30 Celsius, will the thermal expansion of POM result in the 2mm jaw separation shifting significantly?
Markus
Ben, I would be surpised if it did. Perhaps you can find some quick release clips and explore how they behave in cold. Markus
Markus
Vivek, Sorry for late reply. Basicly it would be long term exposure to high temperature (thermal degradition), UV-light or chemicals (acids mainly). POM Tg is very low, -40°C or so. Going below that is not very likely? Compared to amorphous materials the risk is fairly low. In harsh conditions it may happen. I hope this helped, Markus
rajiv Joshi
I'm producing micro components using POM, where moulded parts are easily getting broken. Can you please inform probable reason to look for such brittleness of POM components?
rajiv Joshi
In connection to my previous question on brittleness issue of POM components, pl. note that we are pre-drying it at 85 deg C for 2 hrs. Also our moulding machine barrel temp. is around 180-200 deg C. Pl. revert. if we need to change our process or pre-drying temp. / time.
Markus
Rajiv, I'm not the greatest expert when it comes to process paramaters. I suggest you place the question to one of the LinkedIn groups specialized in injection molding process trouble shooting. That usually works very well. Good luck, Markus
Noel Tellis
I'm using POM rod (white) for a certain product. After machining on the Lathe (facing) the part is clean with porosity in the center of the rod. But after cleaning the part in Ultrasound cleaner the center develops a black spot. The black spot is not consistent and few faces are clean as white. Could u suggest me a way to either remove the black spot or to somehow avoid it without changing the material. Thank you.
Markus
Noel, I believe the porosity on the center of the rod is due to shrinkage on the extrusion process. Perhaps you could leave some extra material in this area and melt it to fill the holes. And then clean it in one way or another. Huuh, secondary operations. Sorry, I can't come up with any better suggestion for now. Good luck getting this solved, Markus

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