Ultrasound Sensors for Process Monitoring in Injection Moulding

Abstract

Injection moulding is an extremely important industrial process, being one of the most commonly-used plastic formation techniques. However, the industry faces many current challenges associated with demands for greater product customisation, higher precision and, most urgently, a shift towards more sustainable materials and processing. Accurate real-time sensing of the material and part properties during processing is key to achieving rapid process optimisation and set-up, reducing down-times, and reducing waste material and energy in the production of defective products. While most commercial processes rely on point measurements of pressure and temperature, ultrasound transducers represent a non-invasive and non-destructive source of rich information on the mould, the cavity and the polymer melt, and its morphology, which affect critical quality parameters such as shrinkage and warpage. In this paper the relationship between polymer properties and the propagation of ultrasonic waves is described and the application of ultrasound measurements in injection moulding is evaluated. The principles and operation of both conventional and high temperature ultrasound transducers (HTUTs) are reviewed together with their impact on improving the efficiency of the injection moulding process. The benefits and challenges associated with the recent development of sol-gel methods for HTUT fabrication are described together with a synopsis of further research and development needed to ensure a greater industrial uptake of ultrasonic sensing in injection moulding.

Keywords: injection moulding; lead-based chemicals; lead-free chemicals; process monitoring; sol-gel; ultrasound sensors.

Figure 1

Monitored parameters with ultrasonic sensors at the different locations of injection moulding.

Figure 2

Sound velocity based on pressure and temperature in PP [22].

Figure 3

(a) Ultrasound propagation; (b) Ultrasound echo signals [18].

Figure 4

(a) Location of the thermocouples, pressure sensors, and ultrasonic sensors at the moulded part; (b) Comparison of Ultrasound echoes and pressure sensor [29].

Figure 5

(a) Fan-shaped array of the ultrasonic probe at the barrel, and (b) ultrasonic probes between the nozzle and the barrel [22].

Figure 6

Fabricated sol-gel ultrasound sensors in the micromoulding process (a) on the barrel and (b) at the mould insert [50].

Figure 7

(a) Fabricated sol-gel UTs at the nozzle and Mould insert; (b) The velocity and amplitude of ultrasonic sensor vs. Process time [52].

Figure 8

Fabricated sol-gel ultrasound sensor at the nozzle; (a) cross-section of the nozzle with the probes; (b) the measured temperature for PP by a thermocouple [56].

Figure 9

Velocity of ultrasonic signal at the nozzle during injection moulding process [56].

Figure 10

Fabricated sol-gel ultrasound sensor at the extension nozzle; (a) the T-shaped extension nozzle and data acquisition system for ultrasound sensor; (b) ultrasonic velocity vs. process time for ABS#1 & ABS#2 with different injection speed [52].

Figure 11

Tensile test for PP and ABS moulded parts with different injection speed [52].