HDPE Rotary Hinge Pins 1

[U Offor]

I am trying to make a simple plastic crate where all four side walls will be connected to the bottom face via hinges that allow the walls to rotate to and away (think of the sides of a cube with no top falling down outward and vice versa).

The thing is these hinges need to be purely plastic (HDPE) and look like the hinges of this box cover where there is an extrusion/pin fitted inside a hole, allowing the cover to rotate.
Link

Should I be worried about friction since the pin and hole are both HDPE? Will this be a transition fit? Can't find tolerance charts for this type of arrangement.

 

[IRstuff]

I think this would be a much better box, which folds very neatly, but uses metal hinge pins. Metal pins are more robust, as well.

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[U Offor]

@IRstuff It's true metal will be best for the hinges rather than plastic. I am limited on fabrication options and just want molds for all the pieces, then attach them together. This idea I have seems to make sense to me.

Adding metals will introduce other manufacturing headaches haha.

 

[Demon3]

Usually plastic hinges are made of PP. Look up the term "living hinge". Think of shampoo bottle caps that hinge. When properly designed, they have very long lifetimes.

If you have the type of hinge where two parts actually rub together, then add a lubricant like UHMW silicone to reduce friction, sound and wear.

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[chicopee]

You have not said how the sidewalls of the crate will stay up when the crate is fully loaded. If you expect the crate to have a long life (circa 10 years) then plastic will have oozed some of its plasticizer making the plastic brittle although that may not be apparent. A good example was the dashboards of Japanese cars which cracked after a few years on the road and you could see the plasticizer vapor deposit on the inside of the windshields. Another example is about my downhill ski boots. Those boots were 10 years old and the plastic fronts of the boots that held the two top buckles sheared recently just by skiing downhill. So think how these plastic crates will be used before embarking on plastic hinges.

 

[rickfischer51]

I doubt that HDPE has plasticizer. Flexible PVC is plasticized, and that is likely the material used on the Japanese dashboards. I think stiffness of PE is usually controlled by degree of crystalization. Dont know about the ski boots, but plastics crack for many reasons other than plasticzer loss, such as contact with solvents, hydrolysis, UV degradation, being cooled below the ductile/brittle transition temperature, poor molding practices, and bad part design. One or more of these may have contributed to the cracked dashboards. The cracked dashboards I've seen are usually cracked on the top where they get lots of UV exposure.

The plastic PP hinge idea is cool, but has the drawback of now you have one piece and a much larger mold. Would definitely put a dent in the budget. How are those made? Blow molded? Rotomolded?

As far as a plastic pin, dont mold a solid pin. It will likely be too thick and sink and not work well. Use a cored out design (see pic). The coring keeps the walls reasonable, the cost down, and may act to clean crud out of the joint. Part it on the ZX plane so the parting line is on the small flat on the side, and gate it on the pointy end so its strait and gate embrittlement is not at the head.

Rick Fischer
Principal Engineer
Argonne National Laboratory

 

[U Offor]

@Chicopee

On two walls of the crate, I will have two pins on each side (four total) which interlock into corresponding slots (class LT2 transition fit to account for heat, tolerance). That way, the load that the crates will carry and the load from other crates stacking up on it can be transferred symmetrically. At least, that's how I imagine it would work.

Sure the compression stresses will be higher on the pins, but I can get a decent life out of the box before it goes the way of the dodo haha.

 

[U Offor]

@Rick

I don't see the cored out design picture or a link to it. This is new information for me because right now, all my pins are solid and not cored out (I assume you mean slightly hollowed out).

 

[rickfischer51]

My bad. I think I didnt hit the link to actually do the upload.

Not sure how big of a pin diameter you are looking at. Ive seen stuff with larger pins that were molded solid and they didnt work well.

Rick Fischer
Principal Engineer
Argonne National Laboratory

 

[U Offor]

@ Rick

Thanks for the picture. My pin is definitely not like that but a simple cylindrical solid extrusion formed with the crate with I believe a 1/8" diameter (too lazy to check now).

 

[Cushionpak]

thinks for your pos picture.

and add a lubricant like UHMW silicone to reduce friction.

Air cushion machine

http://www.cushionpak.com