Plastic Choice For Toroid Mounts

Plastics used in moulding toroid mounts and headers come in two broad categories, thermoset and thermoplastic.

Thermoset plastics include epoxies, phenolics and diallyl phthalate, which are known for their environmental stability and their ability to tolerate over 750 degrees Fahrenheit without melting. Thermoplastics include nylon, polypropylene, polycarbonate, polyester such as Valox and Rynite, LCP such as Vectra and PPS such as Ryton, all of which begin to melt if they are held much above 500 degrees Fahrenheit for an extended period. The chemistry that gives thermoplastics a lower melting point also makes them less expensive to mould, which gives them a cost advantage over thermoset plastics.

Thermoplastics are widely used in applications that do not see temperatures above 500 degrees Fahrenheit, except for a few seconds during the winding lead to terminal and component to PCB soldering process.

Thermoset plastics on the other hand are popular in magnetic applications when they are used together with self stripping magnet wire. The unstripped and untinned wire is wrapped around a terminal moulded into a thermoset header or toroid mount and then dipped into a 750 degree Fahrenheit solder pot. The high temperature solder burns off the wire insulation, tins the wire and solders it to the terminal in a cost effective way, without melting the mount.

There are trade offs between the two plastic types that have to be considered. The mount will be less expensive if it is moulded from thermoplastic, but it will require pre-tinning the winding leads and careful heat management while soldering the leads to the mount and the mount to the circuit board. The thermoset mount will be more expensive, but with self stripping magnet wire several terminations can be soldered at once and the wire will not need to be stripped or pre-tinned, possibly making the overall cost of producing the component lower.

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