Author Topic: How to imitate molded power cord strain relief?  (Read 1362 times)

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Offline rcbuck

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How to imitate molded power cord strain relief?
« on: October 15, 2021, 12:28:01 am »
Is there a way to somewhat imitate the flexible strain relief that is used on the connector end of a walwart? Pix1 is typical walwart. Pix2 and pix3 are the DC connectors that are available from DK and Mouser (CUI PP3-050A ).

The best I have been able to do is squeeze silicone glue into the end of the plug. It works but looks kind of cheesy. Maybe there is a way to eliminate the plastic cap and mold some type of rubber around the metal part after attaching the wires?
 

Offline james_s

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Re: How to imitate molded power cord strain relief?
« Reply #1 on: October 15, 2021, 12:43:20 am »
I typically use a piece of heatshrink tubing over the wire. If I want to get fancy I'll sometimes use multiple layers with each being a little shorter than the one under it. Shrink it over the metal structure that clamps the wire and it will normally be fairly secure.
 
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Offline rcbuck

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Re: How to imitate molded power cord strain relief?
« Reply #2 on: October 15, 2021, 06:08:34 am »
I guess that would work but heat shrink tends to be fairly stiff after it shrinks.

Silicone rubber would be better but I don't think you can get it in small quantities. For a few uses per year the large quantity kits are expensive and I sure it has limited shelf life.
 

Offline tooki

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Re: How to imitate molded power cord strain relief?
« Reply #3 on: October 15, 2021, 06:03:07 pm »
I typically use a piece of heatshrink tubing over the wire. If I want to get fancy I'll sometimes use multiple layers with each being a little shorter than the one under it. Shrink it over the metal structure that clamps the wire and it will normally be fairly secure.
I do it exactly the same way! :)

I guess that would work but heat shrink tends to be fairly stiff after it shrinks.
Heatshrink tubing varies wildly in flexibility. You can get heatshrink that remains highly flexible.
 

Online TERRA Operative

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Re: How to imitate molded power cord strain relief?
« Reply #4 on: October 15, 2021, 06:11:43 pm »
I also do the heatshrink thing.

I wonder if you could 3D print a mould to surround the end of the plug and wire and inject black silicone caulking inside?
I've had good success with 3D printing moulds to make silicone sleeves for the 3D printer hot end using high-temp automotive silicone gasket caulking.
Any circuit design must contain at least one part which is obsolete, two parts which are unobtainable, and three parts which are still under development.

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Offline MadScientist

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Re: How to imitate molded power cord strain relief?
« Reply #5 on: October 15, 2021, 06:13:40 pm »
You cAn 3D print some very nice “ rubber “ strain reliefs from materials like TPU and other materials.
EE's: We use silicon to make things  smaller!
 

Offline jpanhalt

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Re: How to imitate molded power cord strain relief?
« Reply #6 on: October 15, 2021, 06:24:13 pm »
If flexibility is not an issue, I use ordinary heat shrink.  When it is an issue (as with heavier cables) in use neoprene rubber heat shrink.  McMaster Carr  and, it seems, ordinary electronics distributors carry it (https://www.mouser.com/datasheet/2/418/3/NG_CS_1654025_Sec3_NTFR_0313-794624.pdf ).  Usually, is use two pieces, a longer inner piece and a shorter outer piece.

EDIT: Also for larger cords, e.g., power cords, pre-made strain reliefs are available.  Some of my tools are quite old, and when I replace their power cords, I used a pre-made, virtually identical relief.
« Last Edit: October 15, 2021, 06:28:55 pm by jpanhalt »
 

Offline tooki

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Re: How to imitate molded power cord strain relief?
« Reply #7 on: October 16, 2021, 01:49:47 pm »
If flexibility is not an issue, I use ordinary heat shrink.  When it is an issue (as with heavier cables) in use neoprene rubber heat shrink.
While that may be even softer still, one doesn’t need to resort to neoprene. Even among standard polyolefin, there are huge, huge differences in flexibility.

I picked up silicone heatshrink on AliExpress. It’s nice, but the 1:1.5ish shrink ratio limits its applications.
 

Offline Terry Bites

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Re: How to imitate molded power cord strain relief?
« Reply #8 on: October 16, 2021, 04:52:55 pm »
 

Offline Buk

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Re: How to imitate molded power cord strain relief?
« Reply #9 on: October 16, 2021, 08:03:09 pm »
Get yourself a roll of this.
(Note: the 'self-amalgamating' bit; its not adhesive tape.)

Cut a piece roughly this shape:


Start wrapping with the thin end close to the plug.

The very flexible rubber bonds to itself and, over time, becomes a single piece.

The tape is sold for plumbing repairs, but it is ideal for this.
It is cheap -- much cheaper that Sugru (and doesn't go off) -- readily available (B&Q, Homebase, Wicks in the UK) and easy to use.

 

Offline rcbuck

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Re: How to imitate molded power cord strain relief?
« Reply #10 on: October 16, 2021, 09:08:14 pm »
TERRA Operative, your mold idea sounds good. You may be able to create the mold with slick cardboard wrapped into a tapering shape and inject the caulking.

MadScientist, 3D printing with TPU also "looks" viable but I don't have a 3D printer. I am planning on getting one someday. I don't know how you would print something around a connector though. My understanding of the printers is that they print everything in reference to the bed.

Buk, the self-amalgamating tape may be the way to go. I will have to go to the home repair big box store and buy a roll. After your post I did some research and saw a couple of posts that said it remains flexible after self fusing.

Thanks everyone for the suggestions.
 

Online TERRA Operative

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Re: How to imitate molded power cord strain relief?
« Reply #11 on: October 17, 2021, 05:11:15 am »
One little trick regarding using the caulking. If you mix a little cornflour into it, it will go off in the mould much quicker.
With a larger or well seaed mould, the caulking could take weeks or never go off in the centre. The cornflour doesn't affect durability but it does carry just enough moisture in with it to set the curing process off.
Any circuit design must contain at least one part which is obsolete, two parts which are unobtainable, and three parts which are still under development.

https://www.youtube.com/user/NearFarMedia/
 
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Offline Terry Bites

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Re: How to imitate molded power cord strain relief?
« Reply #12 on: October 17, 2021, 11:11:44 am »
mmm[attachimg=1]
 

Online Mechatrommer

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Re: How to imitate molded power cord strain relief?
« Reply #13 on: October 17, 2021, 02:10:18 pm »
It's extremely difficult to start life.. one features of nature.. physical laws are mathematical theory of great beauty... You may wonder Why? our knowledge shows that nature is so constructed. We simply have to accept it. One could describe the situation by saying that... (Paul Dirac)
 
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Offline rcbuck

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Re: How to imitate molded power cord strain relief?
« Reply #14 on: October 17, 2021, 06:12:58 pm »
Mechatrommer,

Perfect! I found the size I needed and parts have been ordered.
 

Offline EPAIII

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Re: How to imitate molded power cord strain relief?
« Reply #15 on: October 20, 2021, 07:29:07 am »
They make strain reliefs that are designed to fit in a hole in sheet metal or plastic. They do not necessarily grip the wire/cable like a molded one would, but a simple knot in the wire/cable on the inside of the case will prevent it from pulling out.

OK, I looked for an example and this type does grip the cable:

https://www.mcmaster.com/cable-strain-relief/continuous-flex-snap-in-plastic-cord-grips/

Check your local electronic supply places, this is a very standard type of component. The types that do not grip the cable could be attached to a connector's shell with silicon adhesive or whatever.

In a pinch I have used heat shrink. It comes in different types and some are more flexible than others. Sometimes I have used progressively longer lengths of different diameters to make it more flexible as you get further from the connector: longer piece of small diameter with a second, slightly shorter piece with a larger diameter, and then a third piece, shorter yet with a larger diameter again. It can look very nice and some connectors will have a piece that will grip the triple layers for mechanical attachment.
« Last Edit: October 20, 2021, 07:36:10 am by EPAIII »
 

Offline rcbuck

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Re: How to imitate molded power cord strain relief?
« Reply #16 on: November 03, 2021, 02:25:46 am »
Well, the parts I ordered and received do not fit over the outside of the connector very easily. You can force them over the connector with some effort but I don't like having to do that.

With some additional research today on the fleabay site I found what is probably a good solution.
https://www.ebay.com/itm/402845498458?hash=item5dcb767c5a:g:0O0AAOSwqCJckdWx

Unscrew the rubber cap, solder the wires, and screw the cap back in place. Then fill the flexible part with black silicone caulking.
 

Offline coppercone2

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Re: How to imitate molded power cord strain relief?
« Reply #17 on: November 04, 2021, 04:19:51 pm »
I am interested in doing this right. The way is likely to make a piece on a 3d printer, make a silicone mold, then cast the part in silicone or best suitable polymer and glue it on. Or you can do a vacuum casting directly onto the cable, if the resin is sticky enough.

Silicone molds are the magic for making complex parts, if you chose the durometers right, you can pull one part out of another part , in the fashion of 'birth', that is non-damaging deformations occur in the mold during extraction, allowing you to cast something along the lines of 'fake grape bushel'. If the mold was stiff, you would never extract such a structure.

You can also use a spring, possibly in combination with the taper you built.

The 'direct cast on' option would require someone to give a known good source of a castable material that is suitable for this, sticks to cable too. Otherwise.. its made with a plastic extruder and thats too hard core
« Last Edit: November 04, 2021, 04:23:31 pm by coppercone2 »
 

Offline ajb

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Re: How to imitate molded power cord strain relief?
« Reply #18 on: November 04, 2021, 05:08:02 pm »
I've seen some barrel plugs and 3.5mm/1/4" plugs that come with that sort of feature molded into the back shell.  They're not as common as the plain type, unfortunately, so mostly I do a layer or three of good polyelfin heatshrink as others have described.

If you were doing this commercial, Henkel has a low-pressure molding system that might be worth looking at.  I'm not sure if it would be able to hit the pressures required to do something as intricate as the conventional notched strain relief you see on a lot of plugs, but with a suitably flexible material you might be able to get away with a simpler tapered design.  I'm sure it won't be cheap, but probably easier and less stressful to get right than conventional injection molding.  https://www.henkel-adhesives.com/us/en/products/encapsulants/low-pressure-molding.html 

The real trick with that, or any other molding process is going to be designing and fabricating the mold.  Getting small parts with intricate details molded accurately generally requires high pressures, which means a very precisely machined and rigid mold.  If you've ever gotten a cheap plastic part with lots of little flaps of plastic hanging off of it you've seen the consequences of high pressure molding using a poorly made or worn out tool.  With a low viscosity material you might be able to use gravity or very low pressure but it really depends on the part.  It's all tradeoffs just like any other kind of engineering.
 


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