Use of electronics embedded wristbands have increased tremendously in recent markets. One of the biggest challenges with integrating the flexible printed circuit (FPC) within the wristband is the protection against high pressure and temperature during overmolding of thermoplastic polyurethane (TPU).

 

Electronic components were traditionally encapsulated with 2K epoxy. The problem with this method is the prolonged cycle time during production and the brittleness of the material that might shatter due to thermal shock and excessive pressure.

 

However, with our expertise in Low-Pressure Molding, Rocktin Technology have redesigned the entire process using Polyamide (PA). Polyamide is a thermoplastic that have great flexibility when solidified, high flowability at melting temperatures, and the cycle time can be shortened from hours to minutes. This is an excellent material for this particular application. 

The biggest challenge with this technology is that the battery on board can only tolerate up to 60 degrees Celsius. Using 2K epoxy does not generate high temperatures during the process, but Low-Pressure Molding requires the material to be heated to above 200 degrees Celsius. However, our experienced engineers managed to work around this and were able to develop a solution.

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Using our expertise in mold design, we designed a mold with increased cooling abilities around sensitive components. The design included having the gate further away from the battery so that the temperature of PA is lower when reaching the sensitive components. Coolant circulation in the mold helped with rapid temperature reduction to avoid damaging any components. With these added features, the cavity needed to be redesigned in order to have higher flowability and prevent any short shots. A magnesium aluminum alloy material was chosen for molds due to its excellent heat conductivity. 

The other challenge is that due to the high temperature of the TPU overmold, we needed to ensure our material selection has a high melting point in order to prevent remelting. Not all PA material have the same melting temperature. We started with choosing of PA that has high softening temperature, but also a high level of hardness. In this case, Technomelt PA6839S from Henkel is the selected candidate. 

Then through thorough verification and testing, we were able to produce a solution that far exceeded the original method not only in terms of production time, but also its physical robustness.