Creep in 3D printed Nylon

[Tegguy]

I am hoping someone might have some knowledge that can help me out. I am a Mechanical Design engineer tasked with solving a problem with an existing design. We have a set of grown Nylon 12PA parts that get bolted together with an O-ring between them. We have heatsert's in the base with a #6 screw coming through the cover that gets torqued.

Here is the problem: The heatsert's are pulling out of the base. I have done some bench top testing and this is all pointing to creep/coldflow as I will torque a screw and come back an hour later and retorque it and get 10-30 degrees rotation out of the fastener before re-torqueing.

I don't understand creep enough to properly come up with a repair solution so I had a few questions
1) Does creep occur under any stress or only if the stress gets high enough? I see the equation takes into account "activation energy" but I'm not exactly sure what this energy is
2) Are there any good ways to try to predict creep without doing testing to establish information?
3) Can edge distance impact creep?
4) Is there a good way to predict creep in threaded portions (how many threads act as bearing area)?
5) Looking at some material information for various materials I see "Creep Strength" and "Creep Modulus" but I'm not exactly sure what these mean.

 

[SWComposites]

1) Does creep occur under any stress or only if the stress gets high enough? > this is material and temperature dependent; in general creep will occur under any stress, but the rate may be so low to be insignificant
2) Are there any good ways to try to predict creep without doing testing to establish information? > no
3) Can edge distance impact creep? > indirectly, as smaller edge distance will have higher local stresses
4) Is there a good way to predict creep in threaded portions (how many threads act as bearing area)? > you will need specific test data for a threaded connection
5) Looking at some material information for various materials I see "Creep Strength" and "Creep Modulus" but I'm not exactly sure what these mean. > the material info should list a test method used to generate the data; if so obtain a copy of that test method; or go to the ASTM D20 Plastics committee site and look for creep test method documents.

Is your Nylon material unreinforced or reinforced (e.g. with glass fibers)? Unreinforced plastic will have much lower creep resistance.

 

[btrueblood]

What SW said. Also,

"We have a set of grown Nylon 12PA"

What do you mean by "grown"? Is this a 3d printed part? Fused deposition process?

Remelting a part to push in the heatserts always changes the properties (generally weakens) of the plastic. Can you change the design to use molded-in inserts, or captive fasteners with a large washer element to spread the load? Can you increase the size of the inserts and/or add more of them, again to spread the load? What drove the torque spec. that you are using - did you do seal testing to determine minimum required torque to seat/seal the oring?

 

[IRstuff]

I will torque a screw and come back an hour later and retorque it and get 10-30 degrees rotation out of the fastener before re-torqueing.

Does this repeat, i.e., go back another hour later and have to retorque?

grown Nylon 12PA parts

Any non-bulk material will be less dense than bulk material, so it's possible that it's not technically "creep" but simple failure of the matrix, if "grown" means additive manufacturing.


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[Tegguy]

Is your Nylon material unreinforced or reinforced (e.g. with glass fibers)? Unreinforced plastic will have much lower creep resistance.

Un-reinforced being applied with MJF currently

What do you mean by "grown"? Is this a 3d printed part? Fused deposition process?

Yes 3D printed using MJF process.


Long term thinking potentially bolt/nut with a washer but the question is how much bearing area is required to not cold flow over the life of the product (hermetic seal). Trying to find a way to save the current production units on hand to keep deliveries with a possible long term change.

What drove the torque spec. that you are using - did you do seal testing to determine minimum required torque to seat/seal the oring?

Inheriting the problem but from what I'm gathering no testing was done and the creep issue wasn't thought of. We're seeing back out with 3 in-lb of torque on a #6 as well

Does this repeat, i.e., go back another hour later and have to retorque?

Yes I did this 3-4 times yesterday and each time it required the screw to turn to torque properly. We also visibly see the heatserts pull out from the material but it's an time based problem it seems not instantaneous.

 

[IRstuff]

What's the process for inserting the inserts? Is it printed with space claim of the insert or is there post-processing?

What is the density of the printed nylon compared to bulk?

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[Tegguy]

What's the process for inserting the inserts? Is it printed with space claim of the insert or is there post-processing?

Heat them up with a soldering iron and slowly press them in (melted plastic flows into grooves and such). I'll see if I can find a link but they are made by Dodge (Stanley) and are called UltraCert II... The hole is modeled per the spec and printed in place (potential point of problem due to tolerances)

What is the density of the printed nylon compared to bulk?

Density of MJF Nylon 12PA is 1.01 g/cm3

 

[IRstuff]

https://www.stanleyengineeredfasten.../Resources/Docs/Dodge/Dodge_Catalog_rev-F.pdf

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[Tegguy]

Thank you. I do have that document but these are vendor installed so the process isn't super clear to us.

I do know the boss diameter wasn't done per the spec but I don't know if this affects static strength, creep, or both (assuming both). One question I'm having a hard time answering is "what change will make the product good over its life?" (Years and multiple environments) especially if it is creep. I'm currently looking at replacing the Oring with a sealant to remove the stress from compression but there is still internal pressure that will be constant. Goal being to try to save the current units and I think the only way is to get the stresses as low as possible (lower torque, larger bearing area, ECT?

 

[Pud]

Nylon 12, although being great at chemical resistance is probably the lowest nylon for mechanicals and temperature.
I would suggest you've got the worst of both worlds as in choice of manufacture method and material.

Otoh, top marks for being 'eco friendly' as it's made from castor oil!

Politicians like to panic, they need activity. It is their substitute for achievement.

 

[btrueblood]

" Trying to find a way to save the current production units on hand to keep deliveries with a possible long term change."

Ok, the current design in failing in the field? Likely without people re-torquing the fasteners every 4 hours, i.e the failure occurs due to service loads, e.g. sealing pressures and normal handling/use vibration loads etc.

So, without a way to increase the bearing area (change to a flanged insert installed from the back side, or use a washer and nut, larger insert, more inserts) then yeah, use epoxy to glue the sucker together (at which point the fasteners become secondary and the joint is no longer seperable without destruction of the housing). No other choices I can see, and not clear if any of those will satisfy clients.

I'd start focusing on a redesigned part, and getting it done quickly and correctly, so that you have a supply chain to replace the units that are going to start failing at increasing rates in the field. Never a fun problem to have, but if done right you might save the company's reputation.

 

[Pud]

btrueblood/op: nylon 12, being very chemically resistant is almost impossible to glue.
If the part has the usual rough finish, a mechanical hold might be had with glue.

As an aside, some years ago we made parts in unfilled PA12 (injection moulded). Two of them needed a 6mm dia x 12mm long pin inserted. The pins were inserted using pillar drill as a press with a drift instead of a drill. After a day, the pins could be pushed out with a thumb. This didn't matter fortunately as the pins were held captive in the assembly.

Politicians like to panic, they need activity. It is their substitute for achievement.

 

[Tegguy]

Thanks for the replies everyone. I wasn't even thinking about adhesive not bonding to the Nylon.