Loctite threadlocker alternative for plastic screw

[Managuensis]

Hello,

In Loctite threadlocker TDS, it is stated that it is not usable on plastic parts. See below.

We have a metallic screw, plastic hole pair which is exposed to a lot of vibration during usage in warehouse robotics. Hole is M5 and tightening torque is 0.5 Nm. What we'd like to know are:

[ol 1]
[li]Are there any threadlocker product that is suitable for plastic threads?[/li]
[li]Is there a way to strengthten this fixation without using any threadlocker? I think we cannot increase the tightening torque as our female threads are made of PVDF material (thermoplastic). Which type of washer would work with 0.5 Nm?[/li]
[/ol]

 

[MintJulep]

See Loctite 425.

If that doesn't work they have other products for plastic that might.

 

[3DDave]

Typically one uses a large metal insert into the plastic to increase the bearing area for the threads. It sounds like there is a huge elastic mis-match between the metal screw and the plastic so nearly no elastic stretch is developed in the fastener.

Alternatively one could use a plastic screw or add some other spring element to make up for the difference.

 

[drawoh]

Managuensis,

Threadlockers have two problems with plastics. Some plastics are so chemically stable that the threadlocker will not bind to them. Other plastics dissolve in contact with Loctite[ ]242, I suspect including PVDF.

Cheap, shoddily made plastic screws are self locking!

--
JHG

 

[EdStainless]

A metal insert would be the best route. And there are special adhesives to use on the insert depending exactly on the plastic.

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P.E. Metallurgy, consulting work welcomed

 

[TugboatEng]

PVDF is a special plastic for high temperature and corrosive chemical exposure. The Loctite series thread lockers are all acrylic plastic. Is acrylic compatible with your process?

Also consider using countersunk screws as the positive locating effect will help prevent loosening due to vibration as it prevents relative motion between the parts. I assume there is more than one screw?

Finishing the plastic hole undersized will provide some positive retention.