Breakage of Sunvisor Axle (Plastic PA)

Some nylon has very low impact strength, you might consider adding an appropriate filler to add some ductility, heating alone probably not a factor, but you must know the type of PA and any fillers or additives that might play a role...then you must know the stresses involved that the piece will endure as well as any environmental conditions that could affect the material...

https://www.creativemechanisms.com/blog/3d-printing-injection-molding-cnc-nylon-plastic-pa

I am looking for a test for prooving the degrading of the plastic material. Only a few broke axles broke which are produced with the degraded material that stayed for a long time in the screw of the machine.

Take a good sample, and a suspected bad sample, and send them to a testing facility for comparison...

https://www.inspecta.com/en/Our-Services/Testing/Destructive-Testing/

Discard them if there's any doubt that they are from a good run!

This sounds trivial in the injection molding process.

Depends on too much you have not told us.

Check these.

Laboratory Test Methods to Determine the Degradation of Plastics in ...

Thermal Degradation of Plastics

"Material of the plastic axle is PA" - what kind of nylon?
If it's nylon 6 or nylon 6,6 (or a few other variations) then it will need to gain its natural water content before it has substantial impact strength. Not just excessive residence time in the molding machine, but any time at all, will deplete the water whose molecules form an essential link in the PA.
Depending on its geometry, the axle may need from hours to days in a humid environment, to condition the part. Soaking the parts in ordinary tap water will completely alter the breakage rate.

https://knowledge.ulprospector.com/1489/pe-dry-vs-conditioned-polyamide-nylon/

explains this.

We had to boil our plastic items to stop them from cracking.
Nylon is a very nasty plastic. It will crack for no reason after a few years. We had to stop using nylon and now it is banned in tap washers.

While you investigate the shaft/axle material, you might also like to perform some ESC (Environmental Stress Cracking) checks on your virgin parts.

Get samples of EVERY lubricant used on your assembly line for the equipment AND the product, get some sample product pieces and liberally coat them with the lubricant and seal in individual press seal bags and set aside for around 2 weeks.

Regardless of what claims the lubricant suppliers make about compatibility, there have been instances when interactions have caused significant embarrassment.

Also, examine whether the failed parts are from the same cavity of the tool. If you do not have cavity ID, then find tooling marks on the surfaces or even the ends of ejector pins and see if the problem is isolated to a single cavity.

Where ON THE PART is the failure happening? If it's always in the same place, then maybe there is a "notch" (Sharp corner) that is stress concentrating force in that place.

Finally, send someone to the customer site and watch the operators installing the product from at least 30m away. You might observe them using a rubber mallet or some other tool to encourage the part into place that is causing the breaks.

Good luck.

Any solvents, adhesives or other chemicals that are used anywhere near the part could be effecting it.
Considering the leverage on a sunvisor (if it's what I assume it to be) I'd suggest a metal axle is needed. It could well save money in the long run.
Del

Perhaps metal would be a better option?

Raman Spectroscopy or Flat-Plate X-ray may be useful to look at molecule structure after heating.

I've had some experience with Nylon 6 in the past where upon heating a 2 mil film of nylon to temperatures above 320°F, the amorphous/crystalline structure of the polymer shifted from being more on the amorphous side at room temperature as a flexible film to becoming more on the crystalline side, causing the film to become very brittle and shatter into a million pieces in a 200 foot long process oven. I've just read that this phenomenon is understood and referred to as a Brill transition.

A buddy of mine had an old Ford Mustang from the period when Ford was using a nylon toothed cam gear in their motors. With age, oil exposure, and perhaps overheating (coupled with hard acceleration when trying to do a burnout), the teeth on his timing gear shattered and end up in pieces in the oil pan.

Check out the peer reviewed article starting on page 44 in the July 2016 issue of Spectroscopy magazine:

http://images2.advanstar.com/PixelMags/spectroscopy/pdf/2016-07.pdf

Here's a link to a Raman Spectrometer offered by JASCO, a highly regarded Japanese based company:

https://jascoinc.com/products/spectroscopy/probe-raman-spectrometer/

Usually the processing temperature in the injection molder is not sufficiently high to significantly degrade the material.

Best answer is to test the questionable parts vs known good parts to see if there really is a difference.

Second point, mentioned by someone else in the posts is that nylon is hygroscopic and can absorb up to 20% of it's weight in water. I used to work as design and manufacturing engineer for a landscape equipment company and a fact of life is that nylon is more brittle when dry. We recommended landscapers keep their string trimmer line and trimmer heads soaking in water when not in use. Wet nylon is much more ductile and shock resistant. It is possible that the time in the injection molder barrel drove off moisture, resulting in a loss of ductility. Soaking in water will restore that ductility. You could confirm this by weighing a sample of parts, soaking them in water for 48 hours and reweighing them. If there is a significant weight gain, then the parts went in with low water content. We saw most of our problems in parts that were stored in a non-climate-controlled warehouse in Arizona where atmospheric humidity was 5% and ambient was up to 120 deg F. Our parts were shipped in polyethylene bags, but after 6 months they would start breaking. Water soaking eliminated the breakage.

On trimmer heads we went to a copolymer with higher ductility and impact strength when dry to avoid the "Arizona Problem".