Low pressure injection molding - material performance for hinge design 1

[pf26]

We try to use low pressure injection molding to produce small current sensors (including sensitive windings). The parts have a hinge to make it openable. Using Overtec 5FR, we have quality issues, especially for the hinge. The material is very soft, so we can make the hinge as thick as 4mm, but after a few hundreds openings, it starts crack where the material elongates most - we fear it is even worse at lower temperature.
Does anyone have experience to share with the Overtec 5FR, or would advise an alternative material (we inject at 200Celsius, around 5bars - manually, with a hotmelt-glue-like gun.)
Thks, Pierre

 

[btrueblood]

Polypropylene is the usual go-to material for living hinges, but even it has limitations. And I don't think it would process very well with a low tech glue gun type injection molding.

You might try making the hinge as thin as possible...but polyesters (your hot glue stick) don't form strain-hardened hinges like PP does, so it might not be any advantage.

You might try purchasing a PP living hinge and overmolding it with the polyester.

 

[pf26]

Thanks. We noticed that the hinge does not even need hundreds of openings to fail. Just keeping it open (=elongating the material) for enough time will do. This is quite strange, because some time ago, we tested that material against creeping, it elongated a few percent, then did not extend further even for extended periods of time, even at higher temperature (60C). But possibly it was injected in different conditions - we need to double check this point.

Overmolding with PP raises the issue of gluing PP to our material (I think 5FR is polyamide resin with additives); We have tryed to incorporate several materials into our hinge and had issue with gluing.
We already overmold a fine net of glass fiber, which makes the hinge resist event when the 5FR has cracks - but this does not seem satisfactory.
Note that there is a compression spring in our design, to make the hinge open on one side and close the other side (similar as in flat pliers). Possibly this is the root cause of our trouble, and we should have a traction spring to keep the part closed instead - removing permanent elongation strain on the material.

Would anybody have more info on the 5FR we use, for instance, could a relatively high injection temperature (210C) affect its mechanical performance afin injection ? Thks,

 

[collner]

If you want to use low pressure I thank you just can heat the mould to a temperature were you material have low viscosity, basically the same as at the ingot.
To use that method without an extensive heating and cooling arrangement, you will have to accept long cycle times.

 

[pf26]

We finally reduced the injection temperature to around 180C, and now got much better results. We also had to increase the gate size to make sure we can fill the mold (which we don't need to preheat). Thanks.