Inspecting Plastic Injection Molded Threads

[TWeber]

I am looking for information on best practices of measuring internal and external molded threads.

I currently have a impact modified nylon (as well as some additional proprietary modifiers) injection molded (USA made) part with a nominal wall of .100" and a external thread of 1.660-16 UN-2A RH. The pellets are properly dried and the majority of the 200+ dimensions are in tolerance. The pack pressure is high (I do not know specific parameter). The part is about 8.3" long x 1.8 diameter and hollow. It is about .007 out of round. It is an A class tool and I would say that it was properly molded.

The accompanying face cap is Zytel 101 which is over-molded on to lexan.

Both the body and the face cap thread together very well.

Unfortunately, metal go/no-go thread gauges were purchased before the parts were tooled. The parts do not fit the purchased gauges. The body will not fit into either ring gauge, and the face cap fits on to both male gauges. Regardless, the parts form, fit and function are excellent.

Upon further investigation it seems that my company often purchase gauges that do not fit the parts and tosses them into a drawer and forgets about them. No one has an answer as to how to QC inspect threads, but they insist that my product must be inspected. This has been hot button issue as of late, but no one want to commit to a solution. This is not made easier by the fact that the nylon changes dimensionally based on humidity.

What have people here found to be a good method of QC inspecting plastic threads? Custom thread gauges? Optical comparator? Push on plug and ring gauges accompanied by thread pitch gauges? Any suggestions would be helpful. Thanks!

 

[Demon3]

First let me preface this with a warning that I am a materials guy, not a machine / mechanical person.

In your shoes I'd probably be interested in the force required to tighten the screw and the strength of the joint once made. That's what I'd test, i.e. torque to turn it and then how much force is needed to rip the screw out of the socket.

After all, once can measure a lot but who cares what the thread looks like as long as it works?

Chris DeArmitt - PhD FRSC CChem
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[MikeHalloran]

On my planet, if the parts fit together well enough, and do so consistently, we call it a day.

If you must inspect the threads, then the logical thing to do is measure a decent sample of them as well as you can, change the drawing so the tolerances cover the normal size range, and have special gages made to inspect to that bastard size, whatever it is.

Try to ensure that the cost of the special gages comes out of the budget of whoever is insisting on inspection.


Mike Halloran
Pembroke Pines, FL, USA

 

[TWeber]

I should have also specified that the user will be screwing and un-screwing the face cap on a daily basis. The amount of force to removed the face cap laterally would be extreme(tons), not something I need to test. The face cap is hand tightened. We have had problems in the past of receiving parts after a few years that required pipe wrench installation. This is very undesirable.

Mike, I agree. Unfortunately others don't. And they have a good point. How to you ensure that the parts coming in 5 years from now are the same ones arriving today? Apparently retained samples are not good enough. How do you reject a vendor's parts when they are out of spec and don't work when they didn't meet the spec in the first place? We use China tooling and molding a lot and they often try to see what they can sneak past our QC department. Not my choice.

If you look at the thread call out you'll see we already purchased special gauges, and two sets of them at that! This was my exact suggestion, measure the threads to define a new call out and make new gauges, and don't jump the gun next time.

Any other thoughts out there?

 

[KENAT]

In terms of the 'how to measure', for larger threads - especially female - a former employer would use Dental mold compound to take an impression and then use an optical comparator on that.

When I talk about dental mold compound I mean the stuff they use to take an impression of your teeth for crowns etc.

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[patprimmer]

1) If making precise measurements on nylon, you need to accurately moisture condition.
2) If it is modified nylon, you need to ensure the compound is consistent as variations can influence shrinkage and moisture uptake.
3) Investigate what clearance between the threads functions correctly and draw your spec to that. The drawing and gauges and QC specs/methods should always reflect what works, not the other way round.

It's pretty simple at the end of the day. What is the basic purpose of QC. It is to ensure parts are suitable for purpose at minimum cost, not that parts fit a gauge or match a drawing.

Regards
Pat
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[MikeHalloran]

>>> How to you ensure that the parts coming in 5 years from now are the same ones arriving today? Apparently retained samples are not good enough. <<<

Okay, where were these standards proponents when the current parts failed to meet the chosen standards? Will they have grown a backbone by the time you change vendors again?

In order for inspection to be effective in getting what you want, you need to be prepared to send back an entire lot, or dump it in a landfill.

Once you have done that, word gets around, and first article inspections go a lot smoother.



Mike Halloran
Pembroke Pines, FL, USA

 

[Pud]

Gauges made for metallic threads will usually fail threads designed for metal made in plastic. Nylon is especially difficult due to the ambient conditions. (e.g. what temp to measure? what moisture content?, etc)

If you have a shadow graph, you will see that the threads shrink more at the root than at the tips due to to differential cooling rates (varying sections), so a full thread form inspection will result in rejection anyway. (So called "A" class tool* notwithstanding!)

In a previous life, working for multi-nationals, plastic threads were always fit and function, as MikeHalloran suggested. By all means keep "Master Samples", but you will need to know material moisture content, blah blah blah before checking.

Note to designers: Never specify a thread developed for metal to be made in plastic.

Cheers

H

* Whats this then?

http://www.tynevalleyplastics.co.uk

Why be happy when you can be normal?

 

[TWeber]

KENAT
I really like the denial impression method for first piece. I will be using that in the future.

Patprimmer
I completely agree. I made all of the same arguments in multiple meetings, especially how ambient conditions dimensional change nylon. I may use your last line there in the next chain of emails. Thanks!

Mike
Standards didn't exist back then.
Unfortunately we do reject and throw out about 10 thousand dollars of parts on a monthly basis. Doesn't change our vendor's habits, and it doesn't change purchasing's mind about which vendors to use.
Often, even with first inspections our molders will just take first shots off the mold, without looking at them, throw them in a box and send them, just to meet their date. Parts come in missing major features and every processing issue you can think of. And we keep going back for more.

Good to know I am not alone in my thoughts and suggestions. I'll update on what management decides to do.

 

[TWeber]

We ended up waiting for a humid day (65%) and making bastardized gauges to the plastic body and face cap. That seemed to appease management. I wonder if this will become policy, or if they will forget about measuring threads in the future. My guess is that they will forget.