A thick-film ceramic capacitive sensor for embedding in polymer extrusion die

A key component for self-actuating polymer extrusion dies, the in situ gap sensor, was designed. The authors describe the preliminary design and tests of a new thick film ceramic capacitive sensor which is intended to improve in situ gap measurements. The sensor consists of three layers of ceramic tape. The sensor and guard geometries are silk-screened onto two of the layers while in the green state. The third layer is used to protect the geometries from the molten polyester and the cleaning operations. Tests with three different sensor geometries indicate that the sensor output is linearly proportional to gap size for 0 to 3 mm gaps. The gaps of interest are in the 1 to 1.5 mm range. The accuracy and repeatability of the output voltage were within acceptable limits. Sensor performance was not affected by light grinding and polishing. Although the output DC voltage varies nonlinearly with temperature, this is not expected to hinder its use in this application