Custom cable boots

Commercially manufactured cables typically come with moulded boots. These are very useful because they protect any internal solder or crimp joints from flexing (which is typically a very weak point) and act as a strain relief. The strain releiving is so good that normally the cable itself is the weak point and will snap before the connector breaks. There are plenty of companies that will make any custom cables you want at a reasonable price per cable, provided that you want to order them by the palette load. If you want a few tens or fewer, one’s options are somewhat limited.

I needed to make a cable that could hook into an EMG (electro myography) machine. It turns out that surface EMG electrodes use a somewhat standardised connector that closely resembles snap buttons on used on a lot of clothing. It further transpires that the “Prym”/”Dritz” brand size 3 sewing snaps (note, not size 3/0 which is much smaller) are exactly the right size. There’s also a metric size one that fits that’s available in the UK which I believe is 12mm. Either way, the sewing snaps fit, so that’s what I used.

The connection between the wire and snap was simply made by soldering with a small amount of extra, non-corrosive flux added. If the cable in the pictures below looks suspiciously like an earthing cable, that’s because it is :).

The cable boot itself is made from polycaprolactone (specifically the instamorph brand) which is a plastic which melts at 60 degrees C. The plastic has a few very nice properties.

First it melts at 60 degrees, so not only is is mouldable at easily achieved temperatures, when heated in boiling water, it stays mouldable for quite a long time. Secondly it’s quite sticky and bonds very well to other plastics and a little bit to metal. This makes it ideal for cable boots as the good bonding means it takes the strain.

The temperature is low enough that you can actually just blob some plastic over the cable by hand and make a respactably robust if rather rough looking cable boot. For a somewhat more professional look, you need a mould. Here’s what the mould looks like:

mould

It was printed on a Form 1 printer. For these purposes, the Form 1 is rather superior to an FDM printer. It’s a small piece, so we really need the high resolution. Also, the Form 1 resin is impervious to many glues and doesn’t stick to polycaprolacetone at all, so de-moulding is very easy, whereas polycaprolacetone sticks very well to the more common FDM plastics (ABS, PLA and particularly HIPS).

To use, you solder a wire to a snap stud and push the stud into the hole in the left hand mould, making sure the wire goes between the two alignment cones. You then put a blob of hot plastic into the cavity on the right hand mould and clamp the two halves together very hard in a vice. The alignment cones and holes make the moulds self-aligning in practice to around a tenth of a milimeter on these peices.

You can then lever the moulds apart (the chamfered edges make that easy to do with a screwdriver) and trim any flashing and risers. Note that the right hand mold has a hole as well which goes through to the outside and provides an exit path for excess material, much like a riser. The results look very nice:

BOOT

The detail reproduction is remarkable. It’s not possible to see it in the photo, but careful examination of the piece reveals that it’s actually reproduced the very fine faceting that you can see in the 3D rendering of the mould.

They are also strong. Destructively testing them shows that the cable itself breaks before the connector.

The main downside is that the air coming out of a beefy laptop’s side vent can easily reach 60 degrees, so these can melt if you leave them in the wrong place…

One thought on “Custom cable boots

  1. Pingback: Neutrogena light mask part 2: down the rabbit hole | Death and the penguin

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