Repairing plastic hinges

[Grindstone CowboyParticipant @grindstonecowboy]

Hi

Does anyone have any good tips for repairing plastic hinges? I mean the type that are moulded into the box and flex as you open and close the box. My Mitutoyo dial caliper box is starting to show definite signs of failure and I'd like to catch it before they part company completely.

As an aside, I'm also going to replace the foam inserts which have completely disintegrated – any tips for what to replace those with?

Many thanks,

Rob

[Grindstone CowboyParticipant @grindstonecowboy]

Plastic hinges are fatigued – how can I fix them?

[John Doe 2Participant @johndoe2]

Look up "Shadow foam" for the latter, no connection etc.

For the former, I don't know but you might try a glue gun, or carefully applied strips of gaffer tape, having thoroughly cleaned the hinge surface first of course.

[Michael GilliganParticipant @michaelgilligan61133]

Foam inserts are best replaced with Plastazote

[ trade name … ‘clones’ appear to be available ]

.

To the best of my knowledge there is no way of repairing a failed Polypropylene hinge … just use tape or add a proper mechanical hinge.

MichaelG.

[Bill Davies 2Participant @billdavies2]

I've used the green polyester felt from Hobbycraft to line an old steel micrometer case. It's very thin, but cushions the old micrometer. I glued it using contact adhesive.

Bill

[Dave HalfordParticipant @davehalford22513]

You used to be able to get plastic hinges for model aircraft control surfaces maybe they still use them.

[HollowpointParticipant @hollowpoint]

Perfect opportunity to make yourself a nice wooden box. Oak or walnut, brass hinges and a custom cutout for it to sit in lined with felt. 😎

Edited By Hollowpoint on 03/04/2023 13:49:11

[Ady1Participant @ady1]

A decent duck tape does those funny ones, cut to shape

[duncan webster 1Participant @duncanwebster1]

I thought oak caused steel to rust?

[DalboyParticipant @dalboy]

I made this insert to fit into one of my draws, it would be so easy to add hinges and a lid. This was made from mahogany and then lined. You can buy sticky backed felt to line it with to protect the instrument.

It was marked out and then a router used to take out material for the micrometer to fit

[roy entwistleParticipant @royentwistle24699]

Do NOT use oak. The tannin will stain the caliper

Roy

I noticed Duncan typed faster than me

[BazParticipant @baz89810]

Dave Halford is correct about the hinges, there are many types, proper miniature hinges, moulded plastic ones and strips of Mylar film about 20mm wide. Mrs Google will find your nearest stockist,

[Martin KyteParticipant @martinkyte99762]

I think it’s the oxalic acid in oak that induces corrosion.

regards Martin

Some plywoods do to.

Edited By Martin Kyte on 03/04/2023 20:34:31

[Tim StevensParticipant @timstevens64731]

There is no cure for fatigue. Once the failure has started, there is very little chance of repair – except renewal, which is not repair sensu stricto ( or should it be senso strictu?). The problem is that fatigue works by extending a crack – and the crack is the source of further failure. It may be possible, in some cases, to remove the crack completely including the material on both sides, and filling the space with new material which is integrated (not just glued or stuck) into the original. Perhaps possible with a steel component restored by welding, but even then there are likely to be future problems with the weld not being the same as the original metal, or the heat of the weld affecting the condition of the surroundings in various ways.

A cloth hinge, if you can manage it, is likely to last at least as long as the original plastic. But the glue might not.

Cheers, Tim

[Grindstone CowboyParticipant @grindstonecowboy]

Thanks chaps, all useful info as usual. I think I've found an equivalent type of foam and some wing hinges on Amazon. Might just try some Gorilla tape in the interim and see how that goes on the hinges.

The wooden box idea will go on the "to do" list, though!

Thanks again.

Rob

[John Doe 2Participant @johndoe2]

Plastazote looks good, but you will need special tools to cut it cleanly and uniformly at varying depths. Shadowfoam is designed for the person without such tooling, yet still provide a professional looking result. It is made from many layers of material that are bonded together but cleanly separate at the depth you require, once you have cut the outline of the item into the foam.

[EmgeeParticipant @emgee]

If you have any old and unused 3.5 inch floppy discs you can open them up to use the disc material as a hinge, work a treat.

Emgee

[John HaineParticipant @johnhaine32865]

I made a collet box for ER16 collets by CNC machining in a block of MDF, with a matching block that sits on top to "close" it. To reinforce the material and proof it I soaked it in several coats of Ronseal wet rot treatment – this is a penetrating "lacquer" that soaks into the material and dries and hardens. Not sign of any corrosion after 10 years. May work on other woods too.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by John Doe 2 on 03/04/2023 22:10:22:

Plastazote looks good, but you will need special tools to cut it cleanly and uniformly at varying depths. Shadowfoam is designed for the person without such tooling, yet still provide a professional looking result. It is made from many layers of material that are bonded together but cleanly separate at the depth you require, once you have cut the outline of the item into the foam.

.

For what it’s worth … Plastazote cuts very cleanly with a knife, or with a sharp-edged tube

[but yes, to make a flat-bottomed recess, you would need to glue layers together]

The reason I particularly recommend it though, is its widespread use in modern military packaging …From personal experience testing such stuff; I think it is probably the available material that is least-likely to degrade with age or climatic cycling.

MichaelG.

[martin haysomParticipant @martinhaysom48469]

i have the same problem. one day i will make a new box. from pallet wood

[Anonymous]

Posted by Grindstone Cowboy on 03/04/2023 12:07:03:

Does anyone have any good tips for repairing plastic hinges? I mean the type that are moulded into the box and flex as you open and close the box. My Mitutoyo dial caliper box is starting to show definite signs of failure and I'd like to catch it before they part company completely.

You're the one staring at the actual article of course, Rob, but it might not be as bad as you think. I have a daily pill reminder with 7 compartments that I've used continually since 1986. I expect many here have too. I remember wondering at the time how long the 'living hinges' (what the artsy designers called them at the time) could last. But none of the seven has failed in all that time though they've "showed signs of fatigue" for years.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Peter Greene 🇨🇦 on 05/04/2023 18:16:06:

You're the one staring at the actual article of course, Rob, but it might not be as bad as you think. I have a daily pill reminder with 7 compartments that I've used continually since 1986. I expect many here have too. I remember wondering at the time how long the 'living hinges' (what the artsy designers called them at the time) could last. But none of the seven has failed in all that time though they've "showed signs of fatigue" for years.

.

I worked at KODAK in the early-mid 1970s, and we were using such hinges on the backs of the little 110 cameras: It was soon convincingly demonstrated that it matters A LOT how soon after ejection from the moulding machine the hinge is first flexed … Get it right and they last for ages, but let it cool too much before that first flex and the hinge is doomed to fail.

Trouble is … the end user doesn’t know whether they have a good ‘un or a bad ‘un.

MichaelG.

[Anonymous]

Interesting, Michael, and makes sense to exercise the hinge while the material is still partly plastic. For small volumes the hinge could probably be closed by the operator upon ejection from the core, but I wonder how they could automate it for large volumes such as pill containers.

[Grindstone CowboyParticipant @grindstonecowboy]

Hi chaps

Sorry for the delay in replying, been away on holiday.

It's a genuine Mitutoyo, but unsure of the age as I bought it at a car show many years ago. One of the three hinges is definitely parting company. Not sure if age or storage conditions have a bearing on it, but I can see that the box has bowed slightly so the axis of the hinge pins (for want of a better description) is not a straight line. I guess that puts extra stress on it.

Rob

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