I have the need to prototype a product out of PP that contains several living hinges. Rapid injection molding this prototype is not a great option due to the large footprint of the part and therefore the large size of tooling it would require. I know PP is difficult to machine and I have read that creating living hinges this way is next to impossible to do. Can this be done? I’m interested to hear your insight on this!

Regards,
Jason

Hi Jason,
To answer your question........it depends. What are the basic dimensions of your part, what are the dimensions of your living hinges. Please include the wall thickness of the main part and the hinges. How many parts are you looking for? Any grade of PP?

Outside of that woefully generic response, I'm curious and was interested in knowing what your hoping to learn from the machined part?

Machining yes depends on dimensions but can usually be done BUT it will only bend a limited number of times before failing compared to an injection moulded hinge. Perhaps injection mould a number of smaller hinges and weld in place.

Jason, when a PP part is injection molded, the gate location is on one side or the other of the hinge. When the part fills, the molecules are oriented "mostly" parallel to the direction of flow creating layers of "molecular strings" perpendicular to the direction of the hinge. This is what allows the hinge to work. When molded parts are hot off the press, flexing the hinge a few times while still hot improves the orientation of the molecules and increases the longevity of the hinge function. In the case of machining the hinge, you will be able to easily create the geometry of a hinge but the block of PP will have been cast so the orientation is really random. This means that yes, you will be able to machine a hinge & it will work, for a few times. Don't expect it to flex a thousand times like a toothpaste cap. Also, heat the part up to 150F after machining and let it sit there for a while. This will relieve some of the induced stress from machining and may give you a few more flexes of the hinge. Look for living hinge geometry on the internet and give it a try. Bill Hentig

Hi Jason. Bill is right. There is no way to create a living hinge with machining process. The strength of living hinge comes from the molecule orientation during the molding process. I suggest you redesign the hinge so that it does not take the geometry of living hinge. Sometime we replace the living hinge portion with a PP sheet (with the right orientation) in prototyping. It works but attaching the PP sheet to the PP part sometime is a challenge.

Jason,

You can create a serviceable prototype integral hinge by coining (cold forming) the thin hinge feature. The process causes cold flow away from the forming tool and results in a degree of the desired molecular orientation across the hinge.

Machining the hinge is pretty much a lost cause I'd say. And however much you explain the difference between a prototype and reality, a broken model always makes a bad impression on somebody. Usually somebody influential.

I've had this prototyping problem and solved it by machining or rapid prototyping the bulk of the part and using a prototype mold for the smaller active element of the living hinge. When fastened together, the assembled hinge action behaved as you would expect from a molded part. Don't know if this will help your particular project.

Thanks everyone for your comments - they are greatly appreciated. Although I have worked in new product development for many years I've only recently begun to work primarily with plastics and injection molding. I have much to learn.

Using properly molded hinge "inserts" is a nice idea although for this project there are 6+ hinges all at varying lengths. I guess one could do an injection run of shorter hinges and place them end to end to get all the desired lengths. Welding PP is not a process our shop is too familiar with either so that would be an added constraint, although it looks to be as simple as using a soldering iron and some filler material. I'll look into these options this week.