Polymeric based thermal accelerometer microsensor

Abstract

A polymeric microinjected 3-axis thermal accelerometer is presented in this paper. The device has two main components: a polymer encapsulation body composed by two pairs of microparts and polymeric membranes. The membranes are supporting the active elements (temperature sensors and heater). The accelerometer requires detailed specifications for the design, mould machining and polymer selection. The encapsulation body is made of polystyrene due to the high MFI (melt flow index) and low coefficient of thermal expansion required for microinjection moulding. The final design consists in a set of two different mouldings, enabling an easy coupling and dimensions bellow 500 micrometer. A conventional type of mould was entirely designed for the device. The mould structure is made of steel 1.1730 while the inserts were machined in steel 1.2311 indicated for compression. A draft angle of 2% was machined for the machined for the runner system while the moulding zones have no draft angle but they were over dimensioned (0.45%) to account for the material shrinkage in order to obtain the micro parts with the desired dimensions. Microparts were moulded using a Boy 12A/M micromoulding machine (mould temperature of 60ºC, polymer temperature ranges between 190- 240ºC and a cycle time of 42 seconds) and the manufactured microparts are within the tolerances required for the application.