Development of point-probe capacitive transducer for thickness measurement in injection molding

Abstract

In the past decades, the requirement of online monitoring of injection molding process has significantly increased. A parallel-plate capacitive transducer had been developed and applied in globally monitoring material status. However, the parallel-plate capacitive transducer cannot be applied to molds of complex surfaces or to monitor specific locations of product. Considering the shortcomings of existed hardware sensors, a point-probe capacitive transducer has been developed to satisfy demand. The proposed sensor configuration consists of point-electrode, nest, screw bolt and insulation layer. This configuration has been designed to contact with injected material inside mold to detect material status change, and simultaneously sustain high pressure from inside mold. Insulation layer of zirconia ceramic is coated to the side surface of electrode by plasma spray. Then, the capacitance equation and sensitivity of proposed sensor is derived and analyzed. Stress analyses of sensor components are conducted by simulation software to demonstrate feasibility of this sensor configuration. After measurement scheme characterization of measurement circuit based on Pcap chip, experiments have been conducted on a mold to obtain corresponding CT (capacitance/time) output. The results shows proposed capacitive transducer can effectively online measure the thickness at specific locations where sensor is installed. The maximum relative error is about 3%, less than 5%, thus the developed sensor is accurate in measuring the thickness. Besides, the sensor has other functions of monitoring some process variables. The melt flow can be detected once the melt flow reaches the sensors’ location. By the capacitance signal’s time interval of two sensors, the average flow rate between two installed sensors can be derived.

Date of Award	2015
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology