Stress Relief for post molded 6/6 parts 2

[ornerynorsk]

Hi Everyone,

Does anyone have experience in heat stress relieving (post mold) of Dupont Zytel 70G33 (6/6 nylon, black, 33% glass) or similar. We've run some trials and gotten satisfactory results, but plastics are not my primary expertise and I want to make sure we are not shooting ourself in the foot. Cooking out essential ingredients, inducing stress, etc.

Please see the attached profile pic. This is a clamping element for 2 parallel 3/4 inch tubes. Within a week of coming out of the tool, our parts pull the clamping legs closed from stresses. We wedge the legs open, bring the part up to 250 deg F over the course of an hour, soak for 2 hours, cool to ambient overnight. I have all the material props, heat deflection, etc, for this material, but want to be cautious before implementing it into production phase. I don't think we're coming close to thermal degradation at that temp, but I want to float this out to the folks who really know polymers.

Strength tests on our treated parts have shown a 15% increase in breaking strength.

Thanks everyone.

 

[ornerynorsk]

For some reason the upload process deleted some of the coring detail. The leg sections are not thick as shown. Avg wall for the entire profile is ~0.150 (3.8 mmm)

 

[Pud]

I knew I had seen it somewhere:

http://www2.dupont.com/Plastics/en_US/assets/downloads/processing/198118E.pdf

Page 29 (actual page number - not PDF number!) - Post Mould Annealing.

According to the above, 250°F is a bit low...

Cheers

Harry

http://www.tynevalleyplastics.co.uk

 

[ornerynorsk]

Thanks Harry,

I have this document as well. I'm thinking metal, but I don't necessarily want to anneal the part, just relieve some of the internal stresses. Or is this one and the same in the world of polymers?

I'm also wondering if water absorption might actually be the culprit, but here in Minnesota we have terrifically dry air during the winter months. Drying without the wedge won't bring the part back to design profile, either, so I give a low probability to this factor.

 

[Demon3]

If you know it will warp on storage why not design the mold to take that into consideration? That way as molded it will be out of spec but after storage it will warp to the desired shape. That avoids post annealing altogether saving time and money.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[Pud]

Annealing plastics is the same process as metals - relieving internal stresses from whatever source.

Water absorption is vital to give toughness to nylons - unless the dewpoint in Minnesota is down to -40°C, the nylon will absorb water.

The higher the tool temperature during moulding, the lower the residual stresses and the lower the post-mould shrinkage. Try an oil heater and get the tool surface temperature up to the top end (+75°C) and see if it's any better. (Unless that's what you're doing!!)

A bit late, but where is the feed point? (Just interested)

Cheers

Harry

Harry

http://www.tynevalleyplastics.co.uk

 

[ornerynorsk]

We designed the tool to have +5 degrees of compensation, wasn't nearly enough. Reworking tool is big $$$, multiple slides and movable cores, there is additional feature which the profile does not show, but does not affect this clamp feature because the gate flow is essentially an edge gate .150 thick by 3 inches wide. Volume is only around 3000 pcs/yr so it is far cheaper for us to thermally process parts than to rework tool. Our thermal eqpt is in-house, while our moldmaking capability is not.

We will find out what mold temp that our molder is running at. That is a factor I had not considered.

Thanks everyone.

 

[Demon3]

Thanks, I understand why you prefer the annealing now.

You can reduce warpage by subtly changing your material and still keep all your mechanicals intact. For example, you can replace some of the glass fiber with mica, or with glass beads to reduce warpage. I saw some recent work using a special silica called NanOsil to reduce warpage in glass filled nylon. You can get information on NanOsil from

http://www.nanosilasd.com/

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[Pud]

Have you tried just leaving a part to see if it actually returna to "as moulded"? I was wondering if it's a case of differing sections conditioning at different rates.

Try boilng (low simmer) a sample for 12hrs and see what happens 24 hrs after that. (Drop one in the office coffee maker) Just a thought.

Chris's suggestion is a good one - assuming you have control over what the material specification. (My personal choice would be one of the partially aromatic nylons from EMS Chemie (Grivory GV3H)) Low water take-up, low warpage, etc etc, and here in the UK anyway, cheaper!!

Cheers

Harry

http://www.tynevalleyplastics.co.uk

 

[MacGyverS2000]

Hmmm, I wondered why my morning tea tasted a bit off... ;-)

Dan - Owner

http://www.Hi-TecDesigns.com

 

[ornerynorsk]

We have first articles and first-from-batch samples going back 5 years, they do not return to normal. The material suggestions and boiling water are suggestions I keep in the ammo belt as we work through this. Thanks guys for your help, greatly appreciated!

No worries, we've got some hot water equipment, as well, so the office coffee will remain safe, and as bad tasting as it always was!