Living Hinge Design GUide

[Ama-fessional Molder]

Would it not do a better job of aligning the molecules if you used a bigger nozzle and moved fast to stretch the extrusion?

Nope.

You need to force the polymer through a constriction for that to happen. That's why the ideal thickness for a living hinge is no more than about .020" in molding. The very act of injecting it through the thin section of the mold is what straightens the polymers.

[Mjolinor]

I just wondered if it is the same as those beer can plastic rings. They align when you stretch them that's why they get incredibly strong.

[Davo]

Ama, Are you saying something can be technically correct but not _really_ correct? I don't seem to follow your reasoning.

There have been videos of drill press drive belts printed in ninjaflex. It is very durable material. Tens of millions? No idea. What are the requirements of the item being designed? If I want a flexible hinge for a watertight container for my phone, so I don't lose the lid, I don't need it to last tens of millions of cycles. I might never open it 1000 times.

[Ama-fessional Molder]

A living hinge, by definition, is made from the same material as the two pieces it connects. I actually made that statement backwards. It technically isn't, but basically is... if that makes sense. Since it isn't the same material it's just a *regular* hinge.

As I have said, if it only needs to survive a hundred or so cycles then it's perfectly fine. If you need more than that AND a rigid material for the rest of the case, then a living hinge isn't the answer for a 3d printed part.

[curious aardvark]

Nope you are completely wrong on this point. Like I said use the RIGHT filament and there is absolutely no reason this can't be done.

The colorfab xt would easily give you thousands of flexes. I haven't got any left or I'd prove it.

Also polypropylene comes in a3d printable filament, that too would just keep on working.

And if you want a brilliant example of a lifetime living hinge (well dozens of them actually) look no further than the one piece grabbing hand:


You need to wait till you've got your 3d printer and have used some of these filaments before making such sweeping statements :-)

[Ama-fessional Molder]

I never said it can't be done. I said you aren't going to get the lifespan of a molded hinge out of it, by several orders of magnitude.

A molded hinge lasts millions of cycles.

I also said that you are not going to get that lifespan out of anything that requires a specific material other than flexible ones.

If you need to make something out of nylon, then you are stuck using nylon for the living hinge. If you print with a secondary material, then it is no longer a living hinge. By definition a living hinge is made with the same material as the rest of the part.

So it's all well and good to make toys with it, but if you need a working component that requires specific properties that the above mentioned polymer doesn't offer, then you need to print it with two materials, or live with the greatly reduced lifespan.

[Mjolinor]

Which brings me to a question. Shower screen seals and other stuff like that has a hard part for clipping on the door and a soft part for sealing against whatever.

How do they do that. They look like all the same material and thicknesses don't vary enough but they are quite soft on the seal and hard on the clip.

[Ama-fessional Molder]

2 shot molding, or overmolding. 2 shot usually involves molding both materials at once, in two steps, while overmolding uses a part from an external process and inserts it into the tool.

They mold one part of it first, then move it into a second cavity and shoot a secondary material over the top of the first.

For something like a long seal, it's a 2 part extrusion process. The first is extruded through a die, then it is ran almost immediately into a second die. Sometimes they punch out holes along the length of the first step in order to give the second material something to bind to.

Careful color matching takes care of the rest.

This is a 3 color/3-shot mold.

[Ama-fessional Molder]

Here's a better example, toothbrushes. These have the hard and soft materials.

https://www.youtube.com/watch?v=7PWJGW6v7s0


There is another type of this process that I recently got to see in action on 650 ton machines at Aptar in Mukwanago WI. The die has three parts, with a central "cube" that has cavities on 2 sides. The front half is injected, then the mold opens, turns 180 degrees, flips the caps closed, then ejects the finished parts that were previously in the back side, then it closes again. The process overmolds another color on top of the first.

The extruder for the second color is mounted on top of the machine, vertically.

96 cavities, two colors, and closed caps - 14 seconds per cycle.

I am trying to find a video of a similar process...

EDIT:

Here you go. This is a more advanced one, and obviously significantly more expensive, but the same idea as what Aptar was using.
These appear to have an HDPE or LDPE first shot, with the red second shot over the top. Of course I could be wrong about the material entirely.



Edit again... holy...

[Mjolinor]

But those are two materials. I am talking about the ones that seem to use only one material.