Living hinge advice

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,

I was hoping someone would have the knowledge to help us remedy the over length and depressions we are getting.

any advice on a appropriate material would also be appreciated.

Attachment 265Attachment 266

Where is the part gated? What are the gate dimensions?
Rick

Hi Rick,

on the end, let me find exact dimension, though is very small possibly 1mm x 12mm.

Thanks

mike

Hi Rick, the images will show all I hope, the length is reduced to a more tolerable dimension, since changing plastic, still not 100%, should be 200 long. still have problem with depressions, which will be something intolerable.

the gate it self is 10mm x10mm, I can get the drawings to you if that helps.

BTW thanks for helping I didn't know where to turn.

regards

Mike


Attachment 267 Attachment 268

Attachment 269 here is the other half of the mould Rick

Hello!
If the part is too long, try reduce cooling time. i had same problem on similar tool, and i had to reduce cycle time for almost half, from 50s to 33 s. open the mold as soon it is possible, so the part start shrinking when its hot. And reduce holding pressure time, but you will have biger sink marks, i dont know how much it is aceptable. Boris

Hello Boris, the sink marks are very visible - to a point its not acceptable, they have got the length close enough by changing the plastic. Still trying to resolve, not sure about the outcome, can only persist


thanks for suggestions


Michael

BTW, I have read an article, where they have overfilled the mould by leaving it a little open, then whilst still soft (plasticised) closing the mould.

Try running a warmer mold temp to increase material shrinkage.
Increasing gate size will help a lot, but you can also try increasing pack pressure/time to reduce sinks.
Also make sure your nozzle ID isn't too small; larger nozzle ID will help

I would challenge the part design, a long part like that with varying wall thicknesses is expected to have some sink.
From a processing stand point, turn up back pressure and slow fill speed down to get as much volume in the cavity as you can on first stage fill. Go like 99.999% full then into pack and hold.

Not sure how far you are in your development, or if there's room to modify the design, but a properly designed part will have a uniform wall thickness throughout.
See example of how that thick boss section was done:

Attachment 270Attachment 271

I will advise of your suggestions, thanks and here is hoping for some good outcomes.

BTW changing the shape is not a thing we can really do

Mike

I appreciate the help, thanks

anyone have any ideas on a composite PP PE mix, or has anyone tried PE on living hinges ???

Mike

It looks to me like it is gated in the thick middle section, then flows through the living hinge and into the thick sections. Packing out a part through a living hinge is not easy at all. Thick to thin to thick. Being the mold is not designed/gated properly for a living hinge anyway I would recommend adding 2 more gates on either side of the living hinge.

For the longest life to a living hinge you would want to try and flow the material perpendicular thru the hinge it will last much longer that way. Flexing the hinge while hot is a common practice to help orientation in the hinge area, but many experts in the hinge business run the mold temperature through the hinge area low in to hold the hinge in a more amorphous state which is more flexible. The more crystalline structure through the hinge area the stiffer and more brittle it will be.

I appreciate the help mate, I will let them know and lets the change.