Thermal Staking

posted in: Miscellaneous | 0

Hot tool staking probably started with a screwdriver or some similar tool heated with a torch and then applied to a tab or post to capture another part. The process does not work much differently today, though the equipment and tooling has become much more sohpisticated. Most thermal staking machines today use electric coil heaters with embedded thermocoules and digital temperature controllers to maintain probe temperature. The tip, attached to the probe, is designed to have low thermal mass, that is to say it can change temperature easily. The tip is heated by the probe, which is mounted to a press that delivers the probe and tip assembly to the work and applies force to deflect the head(s) to be formed. When the tip contacts the work, it transfers heat to the work to melt the material and form the detail. If the press were simply retracted at this stage in the process, most materials would stick to the hot tip and either form strings or the head would actually be pulled off of the tab or post. To prevent this, pressurized (and sometimes cooled) air is delivered to the tip long enough to reduce the tip temperature enough to allow a clean release and leave a well-formed detail with sufficient strength to hold the assembly together. This action is usually called post-cooling. Following retraction, the tip is then reheated by the hot probe. If more than one probe assembly is installed on the machine, they can be individually controlled if there are a sufficient number of heater controllers on the machine, or they can be zoned together, with one thermocouple providing temperature feedback to the heater controller, and other heaters modulating in open-loop fashion. This works better then one might think when identical probe/tip assemblies doing the same work are on the same zone and heaters are closely matched. There is considerable freedom in tooling design for the heat staking process, and multiple size heads can be formed at multiple levels in the assembly. Probes cannot go into tight spaces in parts, though, as they may thermally deform nearby details. Likewise, not all plastics respond well to thermal staking, including matrials with sharply crystalline melting points such as polyamid (nylon), or narrow melt to degradation temperature ranges such as polyvinyl chloride.

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