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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