Fracture in HDPE Living Hinge

[5Fish]

I have a problem with a HDPE living hinge. When I bend the hinge it breaks.
It's a tiny hinge, only 0.005" thick and 0.030" wide. I have made it for years without a problem, but now the molders give me parts that crack and they can't seem to figure out why.
I am NOT a polymer engineer. I would appreciate any help.

 

[insideman]

Why polyethylene? Polypro is waht you should be using.

Other than that, have the molding conditions or the material changed? How much regrind? And other nitty gritty details.

 

[5Fish]

We are using HDPE instead of PP because the hinge is required to break under strain. A PP hinge would require significantly more strain to break it. (Yeah, I know it's a tough application; we want the hinge to be strong, but not too strong.)
There is NO regrind allowed in the mix.
As far as I know, there has been no CONSCIOUS change in the molding parameters. We rely on contract molders so I don't have any direct control over the process.
They tell me that they haven't changed anything.
And yet, the parts have changed.

 

[patprimmer]

There may have been a change to the molecular weight or melt flow index of the PE.

The density (degree of crystalisation) of the PE may have changed.

The moulding may have moulded in stress.

The hinge may have been worked once while still hot from moulding, and that no longer happens.

The gates may have been changed so there is a weld line on the hinge.

Regards
pat

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[plasgears]

Try visiting the molding facility and hammering on the subject of regrind. I suspect we lost a large account because of a growing number of failures after not having even infant mortality. The prime suspect was 100% regrind or forwign sourcing of materials. Automotive business is ruthless on future parts costs. The price of cars keeps going up, but the pricing of components keeps going down. Now, that's illogical!

 

[sean01]

5Fish,

I think I would start by having the material samples analysed - I know this is a case of starting from square one - but if I had a nickel for everytime something I designed had gone through a sneaky material change (most normally for cost reasons - but sometimes through material obsolesence) I would be a rich and happy man !!!

This is a relatively simple test for any lab, and should highlight the probable grade of material, mech properties and give an insight into whether or not it contained any regrind or not. Once you have this data, you can then pursue things through the manufacturing process. Given the parts have been ok previously, if you have one, you could compare it to a 'master sample' for any clues. Comparing parts in this way under ultra violet light for example can give you an insight into material flow differences, which can point you in the direction of gate and venting problems. It's a good idea to measure the part thoroughly as well - not only in terms of dimensions, but also mass. Another good insight into gate and vent problems is to look around the outside edge (split line) of the component. Gassing, poor material filling and overpacking can show up here. These symptons are all good indicators of a change in moulding parameters which may be affecting your component.

Lastly, it's always worth asking the moulding company whether or not they keep documentation for what they are doing !! Most technical moulders I've come across (particularly those that supply into the automotive sector) will keep records of mould machine set-up parameters on a disc - since very often the machines are computer controlled and this allows them to set them up rapidly. If you ask for these - be careful not to ask just for mould set-up parameters - ask for details of any of their inspection results and also things like tool maintenance details. As pat has said previously, you may find they are testing the hinge now - before you get it and straining it ? I'm not sure about the moulding of HDPE - but some materials (like acetal) can have a by-product (formaldehyde) which if not regularly cleaned out of the tool can give symptons similar to brittle fracture.

Lastly, don't ignore the way things are packed !!! One company I worked at moved production of moulded components to a new company, who took all set-up data, tools and in some cases machines from the previous company - and still managed to to be supplied defective parts. After weeks of investigating and material testing we found that they had saved themselves some money by collecting the parts in heat-shrink bags (sealed when a certain quantity was reached)instead of cardboard boxes. It seems obvious now - that when the bag was sealed - it shrunk, putting a load on the components we weren't expecting which affected performance. I know its less likely than a processing change - but it shows you what can happen in the real world !!

It's usually a good idea to investigate these sort of things in conjunction with a design of experiments test (taguchi or linear) so that you get some information about the effect of changing mould parameters (if applicable) such a thing not only contributes to your knowledge base, but also acts as a justification for any judgements if things have changed.

sean

 

[usjbh]

If the molder is trustworthy and is using the same HDPE from the same supplier, I would begin my investigation with the material supplier.

The ideal situation would be to take some of your previously acceptable parts and your current parts that are failing and have them analysed using Waters High Temeperature Gel Permeattion Chromatography to see if there a a significant shift in the molecular weight distributions. This is a common technique used to tell bad performing polyethylene from that which works well. Both samples may have the same molecular weight average but their mw distribution is in all probability quite different. Further analysis using TA Instruments melt rehology techniques may also be warranted to further characterize the properties of your HDPE. You will need to find a service lab in your area to perform these tests and provide the analysis of results.

I had a similar problem years ago with a lower density grade of Polyethylene that met all the molecular weight and melt index requirements of the material specifications only to discover that the supplier changed their basic condensation polymerization process as a cost competative move. This resulted in changing the characteristics of the thin membrane we relied on for its non tearing characteristics when pierced with a needle.

I assume you are not using "bare bones" polyethylene. This being the case, the supplier adds either heat stablizers or oleomide slip agents to improve moldability. A change in these seemingly benign additives may also have an impact on performance if the molecular weight distributions are siimilar.

 

[5Fish]

Thank You Insideman, patprimmer, sean, plasgears, and usjbh. You have given me a lot to think about and do. So far we have no definitive answer, but we are exploring a lot of directions simultaneously.

 

[TheAnswerGuy]

5Fish,

All of the answers you have been given thus far are excellent scenarios.

What I think, should also be checked out is if your molder has started to use a mold-release product or has changed brands. Most of these products are very good, but I have found some that will actually plug-up the vents on the cavities
-or-
if this tool has been running for a while the vents may actually be worn away from the parting line and may need to be re-cut.



theanswerguy@tr-usa.com

 

[Chris1001]

I think the design of the hinge & gate position might need review before looking at the material. For example, the hinge should have a land area on both sides to orientate the polymer flow try efunda.com for an example. Another tip is to gate away from the hinge so that material flows through the hinge uniformly. Cold flow at one eng of the hinge can lead to crack initiation.

I find that HDPE properties depend a lot on processing conditions. A moulder might be naturally tempted to use low processing temperatures (tool & melt) which might not do you any favours.

 

[smltwnboy]

I haven't worked with HDPE, but I have found that several polymer mechanical properties are sensitive to premold drying... If not done properly, you literally can lose as much as 50% of some mechanical properties. The polymer manufacturer should be able to tell you if HDPE is sensitive to drying and what properties would be affected. They can also test the finished part and tell if the material was properly dried.

 

[patprimmer]

HDPE is not sensitive to water and absorbs virtually none.

It is so insensitive to water, that foaming agents used for it produce water vapor as a by product, and I have seen water diliberately added as a foaming agent, with some success.

Regards
pat

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[Like2Strike]

Pat,

You are also reminding me of another valid point where "5Fish" could be experiencing failure from.
If the material (PE) is being stored in a damp place it could allow excess moisture to accumulate on the material and cause vapor to be trapped within the hinge area during molding.
Whereas briefly drying the material prior to molding should help solve that problem.

To all posters:
Please lets not forget that "5Fish" has been moling these components for years without problems. He is looking for solutions as to why it is all of a sudden not working!

Livin Aloha
(AKA "The Answer Guy")

Frank M.
Tradewind Resources

http://www.tr-usa.com

 

[JoelMyers]

I agree with Like2Strike. Something just happened that changed the part that had functioned in the past. I don't know much about HDPE, but I had a similar issue with a nylon part. The material was over dried and this caused a loss in properties. All of the analyses showed that this material was not degraded and should have been good material. What is interesting was that the same drying conditions were used as before, but in the winter months the atmosphere is drier than summer and this reduced the initial moisture content in the nylon. So when the material was dried as usual the final material was too dry causing an increase in viscosity. The remedy was to use different drying parameters in the winter vs. summer.

I hope this helps.

Joel Myers

 

[5Fish]

I'm definitely going to look into the moisture issue. I understand that HDPE is normally insensitive to moisture content but this is an abnormal molding application (a living hinge made from HDPE instead of PP). It seems possible that moisture in the mix could end up in the hinge area.
Keep those ideas coming. We still haven't solved the problem and I am poring over your responses.

 

[patprimmer]

If you want to solve this problem, you will need to focus on some facts, and discard the fiction.

Water does not react with PE, and therefore does not degrade it.

Water is absorbed by nylon, and if water is absorbed BEFORE MOULDING,it does react with nylon during the moulding process. Every molecule of water present can break a molecule of nylon, significantly reducing it's molecular weight, and thereby significantly reducing it's properties.

Water absorbed by nylon AFTER moulding, also acts as a plasticiser, significantly changing it's properties.

THESE HINGES ARE NOT NYLON

Back to the material currently being used and failing.

This material and process was used for some time with success, but now fails. SOMETHING HAS CHANGED.

WE NEED TO IDENTIFY THESE CHANGES.

We DO NOT NEED TO CHANGE THE ORIGINAL PROCESS OR MATERIAL, but we do need to identify the deviation from this and rectify it.

I must say, I never considered condensation on the granules, as I live in Sydney Australia, and our climate is such that I have never seen it never happen here.

In moulding PE in the presence of water, water vapour generated by heating water present during the moulding process will form bubbles of gas. These bubbles will weaken the structure, but will be visible via silver streaks on the surface.

The strength of a live hinge is effected significantly by being flexed during cooling, just after the material is frozen, but before the crystals fully form.

This flex stretches the hinge, and orients the molecules across the hinge, giving a linear crystal structure at the hinge, which is much stronger than the random crystals typically formed without stretching. This information is readily available from promotional material for use of PP in live hinges.

In the manufacture of man made fibres, stretching is a critical part of the process to obtain suitable tensile strength. As both PP and PE are suitable for making fibres, the linear orientation of molecules on stretching is applicable to both.

The strength of live hinges is also dependant on a number of material properties that might vary from batch to batch or from grade to grade, or from source to source, or by the inclusion of regrind.

These are:-

1) Molecular weight (melt viscosity, melt flow index MFI, melt strength, hard or soft melt or whatever you want to call it)

2) Degree of crystallisation, and when it occurs in the process.

3) Type of crystals, and when and how they are formed.

4) Contamination of the material with foreign bodies such as other polymers and dirt or additives such as nucleating agents, colour, mould release, flame retardants, fillers, etc.

Live hinge performance is also dependant on a number of processing factors.

These are:-

1) The material should be injected into a relatively thick section, on one side of the hinge, so that the cavity fills on one side only, then the material is pushed through the hinge to fill the other side. If material gets to the other side without passing through the hinge, you will surely get a weld line on the hinge that will dramatically reduce it's performance.

2) To develop good hinge properties, the hinge must be flexed through its full range on ejection. Many cap moulds with live hinges in the cap, have a built in device to flex the hinge as the mould opens.

3) The mould surface temperature AT ALL POINTS must be acceptable. Simply attaching cooling water hoses the wrong way, or rust build up in a waterway, or a change in water supply temp can create problems.

4) The mould must be properly vented. Vents are often neglected, and DO BLOCK.

5) The melt temperature must be acceptable.

6) The injection speed must be acceptable.

7) The effective hold pressure must be acceptable. This is dependant on the condition of the check valve and the cushion allowed for in the shot size setting.

8) The thickness of the hinge can change if the parting line on the mould is repaired or cleaned up in maintenance.
I once saw a snap closing cap (closing mechanism Incorporated a live hinge) that mysteriously started to change when the mould was polished as routine maintenance. The toolmaker removed a few microns from some pins. This changed the concentric radii in the hinge area, and changed section thickness a few microns. This was enough in %age change terms on the very thin area, to create a stress riser next to the curved part of the hinge. They all failed. It took quite some time to find a change of a few microns. It was eventually identified by following QC procedures and identifying the start of the failures with an event.

End of rave.

The points I would check first are:-

1) Has the material changed in any way, i.e. new grade, new supplier to the moulder, new factory for the manufacturer, new source of additive, by the manufacturer or the moulder, new source of colour or new batch of colour.

2) Has the mould changed in any way, i.e. has it been repaired or had a major maintenance process.

3) Has the machine it is moulded on changed.

4) Has the staff changed at the moulders, and therefore has the process changed.

5) Do a time line on problems vs changes.

When they say nothing has changed. Don't believe them. Things are constantly changing, from ambient temp, to wear and tear on equipment, from batch to batch variation in material, to even if someone now leaves a door open that used to be closed.

Regards
pat

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