Hi Willy,
No direct experience buy have been involved with a number of flexing plastic parts (the flexing was part of the function) and years ago in a true hinge but that was before this bottle top type was born.
If I was starting from scratch I would get and existing cap that you like or more than one and I would cross section them to look in detail at the profile and sizes involved as these are very important. If you were very keen you could use some sort of edge measure or CMM to pick up the geometry. Obviously there may be some designs covered by patent so you would have to check that aspect.
As far as I know this type closures are all moulded in Polypropylene which has a particular molecular structure that makes it great for spring and hing type applications BUT with the proviso that you need to ensure several things in the part.
- Material flow direction. Polypropylene has a spiral molecular structure and you can think of these as coil springs. If these coils lie alongside each other lengthwise then you can pretty much flex this as much as you want (we tested into the millions for one hinge) but if they lie across the flex/hinge direction then breakage is only a matter of time.
- Plastic flow in the mold. You can end up with problems in the molding operation by getting weld lines or hesitation that greatly affect the operation of the flex/hinge. Means things like gating position and flow paths need to be well thought out. My first work on such a hinge involved shifting the gate position from where it was easiest for the tool to where it properly filled the tool (also went from a single to double gate. With no change to part geometry it went from failure to success
Flow analysis such as Moldflow is a very good idea for this sort of design. Again if you look at your samples you will get an idea of this flow. - Some hinges use a post molding stamping operation at the local hinge point as this forces the molecules to lie normal to the hinge area and it allows a thicker flow path. Used to be very common but used far less now as better understanding of plastic flow and the availability of flow analysis tools is more widespread.
There are other materials such as Nylon sometimes used for a single hinge closures but despite its strength the much shorter molecular chains and hydroscopic tendency make it less suitable in my mind.
Not sure if this helps but I hope so.
Regards,
Brent Drysdale
