As you can imagine, when we work with heating yarns connections, the risk of Hot spots is a major issue, even in SMART textiles. On course the risk is even more critical as the power involved is higher , as in composites molds heating by example!
(see the page Hot spots of our web site!)
In electro heating applications, most of the resistive based alloy yarns are note weldable. therefore we recommend to use connections by crimping
When you talk of flexible conductive yarns, and if you want a fair fatigue withstanding, you must work with a yarn made out of an high number of multi filaments.
As you can see on the drawing, a single connection by screw will only directly connect a few number of the fibers through the screw or the bottom of the connecting device. As we are working with resistive fibers, all the current will have to pass through this few number of fibers before the load can spread through all the cross section of the wire!
This will cause over heating and a very dangerous hot spot locally next to the connection point!
To have a good connection, we see that , optimally, all the fibers present in the cross section should be in direct contact with the input conductive material of the connection. (in fact the connection must be a "cold" spot!)<
this may not be possible in rather thick yarns as some fibers will remain in the core for a while without being directly in contact with the external circumference of the cross section.
We consider that fair connection is obtained when all the fibers present on the external layer of a yarn are in direct contact with the crimp.
As explained before the connection must be a "cold spot"; to achieve this we need to have a good conductor material(as copper or aluminium by example) n a form of a small tube just fitting the diameter of the heating wire, plus the diameter of the conductive yarn.
many product exists in the market, and most of time with an outside thermo-shrinlable insulating material. The problem, in case of very fine and flexible wires, will be to find a diameter small enough!
the interface between a rigid part, as a welding or a crimp, and a flexible heating yarn is the place where the strain applied to the heating wires is the highest!
If there is a wide angle of movement , we recommend to reduce it as much as possible at the place of the crimp and to report it a bit more along the heating wire.