I am having irregular random failures in a part with a living hinge molded out of Delrin 500T. Some parts coming out of the tool work great, some don't. Those that don't break on the first 3-10 bends seem to last forever.

The part does need to be delrin instead of PP and Nylon for some of the the other functions of the part. The hinges need to last about 40 to 50 +/- bends.

I have created a chain of 25 cylinders,
-Wall thickness of the cylinders is 0.55mm.
-Between the cylinders are the hinges which are 0.3mm thick, I have tried both a 0.2mm radius leading to the hinge, and a 0.4mm radius, and have not seen a difference in the living hinge behavior.
-gating: sub gate into every other cylinder to make the knit line be in the middle of the middle ungated cup instead of the hinge between adjacent cups. gate diameter .028" or 0.7mm diameter
-hinge length is 1.6mm. each hinge in the chain is flexed 64 degrees max both positively and negatively from the normal straight line chain which results in about a 0.9mm to 1.3mm radius at the neutral axis of the hinge.

using the calcs listed in the web page referenced elsewhere in this forum, the minimum hinge length for a 64 degree bend (1.117 radians) is 0.8mm, which I have effectively doubled.

engr.bd.psu.edu/pkoch/plasticdesign/living_hinge.htm

Thanks for your help.
Doug

Imho, I would investigate moulding conditions - particularly tool temperature. Acetals are usually run at well below the optimum 80-100ºC which will give maximum crystalline properties and minimal moulded in stress.

www.tynevalleyplastics.co.uk

Why be happy when you can be normal?

I agree with Harry. Acetal is very often moulded at well below its optimum mould temperature.

A few things I would look at.

1) Dry the graduals. Yes I know acetal is not supposed to need drying, but drying does help obtain optimum properties.

2) Use a mould temperature of 80 deg C at least.

3) Use a high molecular weight grade.

4) Use a non nucleated grade.

5) Use a grade with minimal mould release.

6) Do you have different colours. Can you do them all in natural.

7) The temperature of the moulding as it is bent through the first cycle will be quite critical. Is a bending of the hinge built into the opening mechanism of the mould. This pulls the molecules into a parallel arrangement across the hinge.

Regards
Pat
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If the Delrin you are using is homopolymer, a co-polymer might give you more elongation before break. Check data sheets from the myriad of producers out there...

Drying as Pat suggested is another good point.

H

www.tynevalleyplastics.co.uk

Why be happy when you can be normal?

Thanks for the advice.
@Pud: the Delrin 500T is a copolymer as far as I understand. We tried a homopolymer which behaved worse.
@Pat:
1)The plastic is dried prior to molding.
2)This is a hot manifold system, running at between 380-390F at the tips, barrel temp about the same, We've tried mold temps from 155F to 200F, and have gotten slightly better results at 155F. But pressure increases at this temperature to 700 to 800 PSI.
4)What is a non-nucleated grade and how would that affect the performance?
5)The parts are not run with a mold release sprayed into the mold. We have tried an additive in the Dupont Delrin 500T before, Axel INT-38DHT at 0.5% which showed a slight improvement, but even though the molding parameters didn't change between subsequent runs, the limited success could not be repeated the next run. Higher percentages of the additive didn't improve it either. We also tried a Sabic Lubricomp* Compound KI001A and Lubriloy* Compound K2000 without success.
6)We have been running these in natural. Colorants didn't affect performance.

7) Warm out of the tool, the parts can be flexed forever, and are quite strong. After sitting for a day, they become brittle, fracture is typically an audible "crack" when it breaks. "Annealing" by soaking them at an elevated temperature (50C) for a time causes increased breakage.

The maddening part about this project is that we made a mud mold first with 4 cylinders instead of 25, to verify that the design would work, and it worked fine (except for the intermittent knit line that ended up on the hinge) which we fixed by gating every other cup on the final mold. The other difference was that the mud mold was a cold runner system.

I would appreciate any other advice that anyone can offer.
(Changing careers to cleaning portable toilets has already been considered. )

Thanks for your help.
Doug

Delrin® 500T NC010 is a toughened, medium viscosity acetal homopolymer resin with impact resistance similar to
Delrin® 100. It can be used in parts requiring noise reduction.

From the second hit on Google.

It is a homopolymer alloy with a rubber modifier of some type, so quite a different animal to Acetal..

Being worse on annealing means You are forming non linear crystals in the hinge area.

Nucleating agents are small sharp particles added in small quantities. The accelerate the formation of crystals so the material sets faster for faster cycle times. This creates a large number of small spherical or lamina crystals typically of random orientation.

For a live hinge you need a crystal structure where the molecules pack tight and PARALLEL across the hinge. That is the purpose of the bending operation while the part is still very hot. This draws the material like the stretching in fibre production. PP is by far the best material for this. Even nylon is dodgy at live hinges. They can work a bit, but not in the same class as PP.

A toughened grade is unlikely to be nucleated.

Have you talked to DuPont about this.

Look at their data sheets and choose a straight homopolymer non toughened with the highest possible elongation at break.

Regards
Pat
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