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How to imitate molded power cord strain relief?

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EPAIII:
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.

rcbuck:
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.

coppercone2:
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

ajb:
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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