My 18AWG stranded wire continues to come out of the terminal block it's screwed into. What are some ways that I can more reliably secure this in an industrial environment?
(This is info already mentioned in the comments on the question — but nobody wrote an actual answer, so I'm doing that. It does also agree with my small experience.)
Do not use any solder at all. Use a wire ferrule, which is a metal sleeve that slides over the (straight) strands and is crimped in place, making a solid end that can be clamped securely by a screw terminal of the type you are using.
Caveats:
Use the proper crimp tool, which will leave a textured surface that is readily gripped by the terminal and resists sliding out more.
Adding the ferrule will increase the size of the wire end, possibly too large to fit in the screw terminal. (This should not be a problem if one of the options you have considered is doubling over the wire.)
This picture shows several sizes of ferrules, two ferrules crimped on 22-gauge stranded wire, and the crimp tool I used ($21 when I bought it).
The plastic part of the ferrule guides the strands in, covers any exposed metal, provides some strain relief, and identifies the size of the ferrule. Caution: ferrule makers do not all use the same color scheme!
Twist it together and double it back on itself. That is, strip twice the terminal depth, twist and bend over. Ensure that the wire then approximately fills the screw terminal. If it doesn't, you need a smaller terminal. There should not be any bare wire (stripped insulation) beyond the terminal itself. Good termination practise should be second nature.
Bad terminations may well initially work, but are liable to fail, heat up and perhaps cause fires. A good termination will last a lifetime.
Basic electrician termination training, this. At least it was when I did it about 30 years ago :)
You MUST NOT solder tin the whole portion that goes ino the connector. Tinning the VERY END is OK to stop it unravelling. See my prior answer on this here
A method of "strain relief" that works well but which is "very naughty" and which people will criticise is:
The screw or clamp both makes contact with the conductor and also clamps the insulation rather than just the conductor. I'll add a diagram if this does not get run out of town on a rail.
I was shown this method decades ago by a man who designed/built/installed taxi meters in taxi fleet cars. I tried it. It worked.
This method is contrary to a number of things you'll have been taught. It does not seem to be an utterly terrible idea in theory and proves to be a good one in practice.
A more conventional method is to either add a sleeve which can be clamped to the cable end or run the insulated part of the cable just before the outer ends under a clamp which is screwed down onto the outer.
Crimping a ferrule to the bare copper tips is the best to prevent some strands from not being part of the conductor full gauge and to prevent lose strands to touch other terminals when that can be the case. A ferrule crimper that squeezes with 4 or 6 dices all around is what I recommended. I do not recommend any kind of solder as it will damage insulation, cause copper oxidization later, and it is not a "one metal" rule for conductivity. Copper cables should be well crimped to copper lugs or copper ferrules for the best conductivity and longevity of the connection.
It depends on if vibration exists and causes the wire to move how rugged the attachment must be. Normally a 5 to 10 lb. pull test is the criterion for any connection, whether it is twist wire clamped or crimp lug attached with proper two-stage crimp on insulation and conductor.
Even molded connector junctions can and often do fail due to designers under-estimating how much strain users actually apply to their DC power cords on laptops or iPads (I have had to replace mine often from grandkids and my frequent use).
If this describes your situation, then do not tin but neatly twist, shape (with tool or needle-nose or plastic barrier wall) and clamp under screw head for an air-tight pressure contact with sufficient but not excessive force to pass pull test. This will translate into a fixed screw torque level with calibrated production tools or training.Better terminal strips have barriers and crimp washer under screw head.
However in my own practice, I prefer a thin solder dip or coating so that it wicks under the insulation and a star washer. The solder wicking adds strain relief. The star washer exerts more contact pressure and the solder mitigates loose strands.
Careless loose strands can only be avoided by proxy inspection or a better design with crimped terminals and routine pull tests.
If rugged strain relief is needed to prevent frayed wire breakage, then additional cable plastic clamp strain relief is needed such that the stiffness on strain relief is about <5x stress/strain of the wire for a graduated relief. This can be a moulded jacket as well.
