Thermal Insertion

A heat staking machine can be used for hot tool insertion as well as thermal staking or swaging. The most common inserts for hot tool insertion are brass female-threaded fasteners placed in molded holes to provide good purchase for bolts used to secure circuit boards or other components/subassemblies to a chassis or inside a case, or to secure the case halves. Brass inserts are preferred to steel for this operation because the copper content of the brass assures a relatively low thermal mass for the inserts and they will change temperature rapidly. The inserts are typically placed in the holes partially engaged. The press is actuated and the tips come into contact with the inserts. At this point, an insertion delay can be programmed to allow the inserts to warm up to a temperature that allows for melting of the thermoplastic material. After the delay, the press continues to press the inserts into the holes. Depth is usually controlled by a mechanical stop. After contacting the stop, the press can simply retract if the insertion is low precision, or the press can stop while compressed air is used to cool the inserts and tips to prevent the inserts floating back out of the hole on a cushion of expanding hot plastic. Getting the tip temperatures and delay times just right takes a bit of experimenting. Beware of the temptation to simply use press force to jam not-quite-hot-enough inserts into their holes, as holes almost always have suceptibility to cracking at the knit or weld lines when under hoop stress. The pull-out and torque strength of the set inserts can usually be improved by slowing the process down and allowing the heat to soak a little more deeply into the plastic during the process. As with thermal staking, inserts of various sizes can be set on multiple levels simultaneously.