Polypropylene mix

we have a part which is 220mm long x 100mm wide, it incorporates 2 living hinges along the 220mm length, the part has varying thicknesses, from 1mm thick to 3mm.

problem, we have some depression in the thicker areas and the length is out by 2mm, not shorter though longer, the width of the part is perfect.

the living hinges are ok though a little stiff, hinge thickness is 250um.

material we have used is a polypropylene, it is a bit to rigid, we have changed the mould following suggestions which is working

can anyone make some suggestion on the best Polymer mix to use,

I have read many articles about mixing PE with PP etc.

Michael

PS or supplier that can give us the required material spec

Mix PE and PP homopolymer to hope for copolymer characteristics. Consult your resin supplier for proper PP grade.

KOM
brent

Here is the recipe we use for a living hinge. In this blend the living hinge doesn't even have to be flexed. The hinge is for a door that does not get cycled more than a couple hundred times in it's life though. This particular recipe also reduced the stress whitening of the part that can happen with dark materials. In your case though it looks like the part is white already?Attachment 275

I require a white shade Joe, and matt finish, so what change should I make to your recipe please. what do I replace black with, is it the obvious white and is it the PP or HDPE,

I was thinking only 5% alathon. and the black is ???


many thanks

Replace the black with whatever % concentrate required and up the percentage of HDPE to cover the rest. If the material is pre-colored I would go with a 93%/7% mix.

Living hinges are tricky when you're dealing with fit form function of mating components in an assembly or whatnot.

Typically thickness of the hinge determines the stiffness, though 250um is already on the thin side. (thin enough it may also be contributing to your sink areas you experience.)

Does the hinge span across the entire length of the part? Can you reduce the length of the hinge to help stiffness?
The hinge design itself has a lot to do as well.. Really you want the hinge to be able to swivel in two separate areas not just one. Like if you just had a sharp "triangle" type feature cut into a small strip of plastic it just won't swivel properly because you'd be stretching material from either side of the strip of plastic. If that makes any sense.. Also anything you can do to soften up rough edges will help prevent shear (brittleness and lack of strength) and encourage polymer orientation for improved strength and longevity.

Also it helps a lot of actuate the hinge while it's still hot and then letting it cool afterwards
As for material I wouldn't know the best recommendation I'd imagine any PP would probably work? Definitely consult material expert but maybe you can play with HDPE + PP mix ratios to gt your desired stiffness in the mean time..