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Galling between acetal and HDPE? Other material choice?

Galling between acetal and HDPE? Other material choice?

Galling between acetal and HDPE? Other material choice?

(OP)
Hello,

We have an application the involves sliding doors that are spread apart by a linear actuator mechanism. The spreader mechanism works by engaging two toggle arms to the doors and spreading them apart by 5.5 inches. The doors slide on stainless steel rods in the door frame and also ride against a rub strip made from HDPE. The rub strip is mounted to a cross beam of the door. Becuase of the angle of the toggle arms, there is a force component that presses the doors against the rub strip during the initial stage of movement until the toggle arms spread out. Having said that, we originally used extruded aluminum doors that were anodized but in the past few years switched to injection molded doors made from acetal (Celcon M270).

We are experiencing some issues with doors not sliding freely and jamming when the mechanism operates and testing is narrowing down the problem to the rub strip. Typically, the doors can be slid around with finger pressure. In addition to exploring other possible mechanical issues, I would like to find out if there are possibilities for the acetal doors to gall or otherwise not slide freely when pressed against HDPE. I am considering other meterials for the rub strip including metals. I have made prototype rub strips from stainless steel, but they did not provide as smooth movement as the HDPE. Another option would be to use an anodized aluminum strip.

Thanks,

Kyle

RE: Galling between acetal and HDPE? Other material choice?

Maybe you can wrap the rub strip in thin polished stainless.
Or use rigidized stainless (rigidize.com).

Mike Halloran
Pembroke Pines, FL, USA

RE: Galling between acetal and HDPE? Other material choice?

(OP)
Hello,

The rub strip is 3/32 HDPE and is 1" wide x 44" long. We rivet the strip to the cross tube in our door weldment using flat head rivets - making sure the heads are below flush with the surface of the strip. The protoype stainless rub strip I made had a brushed finish from a Time Saver but the grain was not as fine as what you see with the factory brushed finish like a #2 etc.

It's a very strange problem and I hope I am providing enough information. We do lubricate the rub strip with a dry silicone lubricant such as Lubricone, but to be conservative, I am considering the interface to be dry.

Kyle

RE: Galling between acetal and HDPE? Other material choice?

I would suggest trying without the silicone. They can interact with plastics causing major problems. For example, it could be softening (plasticizing) your HDPE causing it to gum up. It can also cause environmental stress cracking, i.e. brittle failure.

Dr. Chris DeArmitt

Plastics consultant to the Fortune 500: www.phantomplastics.com

Webinars on plastics, fillers & impact modification: www.plastictraining.com

RE: Galling between acetal and HDPE? Other material choice?

(OP)
Hello,

I have ordered some UHMW strips and once installed in one of our R&D machines, I will run an automated cycle with the doors. Our system can store trouble codes when the doors do not open properly so we can accumulate data as to whether an alternate material is effective or not. In researching the MSDS of the dry silicone lubricant, I found that one chemical may adversely affect HDPE so an exposure test of HDPE to the lubricant could also reveal a source of trouble.

Kyle

RE: Galling between acetal and HDPE? Other material choice?

Please keep us informed of the outcome. I think you may well have a solution!

Dr. Chris DeArmitt

Plastics consultant to the Fortune 500: www.phantomplastics.com

Webinars on plastics, fillers & impact modification: www.plastictraining.com

RE: Galling between acetal and HDPE? Other material choice?

(OP)
Good morning,

I apologize for not getting back sooner. I have installed a UHMW rub strip in one of our test machines and will be running an automated cycle to monitor wear and other potential sources of trouble. Also, I will be able to test one machine that was causing trouble and use it as a control for comparison between the two materials. Another possibility is that the adjustment procedure was not carried out properly and that could be the main cause and not the materials at all. The tests should provide answers either way. I appreciate all of your help and will definitely let you know what I find.

Kyle

RE: Galling between acetal and HDPE? Other material choice?

(OP)
Hello,

Just an update. I'be been testing a machine and found that an incorrect size timing pulley was installed in the linear actuator. This reduced the force generated by the mechanism and could be the culprit for the problems. I'm still interested in finding out if there can be galling or other wear issues between acetal and either HDPE or UHMW.

Kyle

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