Ultrasonic on⁃line measurement of polymer melt information in injection molding

doi:10.19491/j.issn.1001-9278.2024.04.010

Abstract

Abstract:

During the injection molding process， the polymer melt undergoes intricate changes in temperature and density， directly impacting the final product quality. An ultrasonic on⁃line measurement method for monitoring the temperature and density of the polymer melt during molding was proposed in this article， and the results validated the accuracy of this method through comparisons with alternative approaches. A non⁃sealed rheological mold with an open bottom was designed and manufactured. An acquisition platform was established for collecting the ultrasonic signals as well as the temperature and pressure signals. Following with the signal collection， analysis and computations were performed to derive the variation curve of ultrasonic velocity within the polymer melt. Through incorporating the pressure and temperature signals， the computational analysis of melt information during injection molding was conducted. The experimental results demonstrated that the ultrasonic velocity signal could qualitatively reflect the melt evolution process within the mold cavity without a damage. Through leveraging the pressure signal， the iterative calculations of melt temperature were performed， showing an error of less than 6 % compared to the results from infrared fiber⁃optic temperature sensors. This resulted in a quantified analysis of polymer melt temperature with a minimal degradation. Through separating the time/frequency domain analyses of the ultrasonic signal， the changes in the acoustic impedance and sound velocity were obtained， enabling the calculation of the evolving density curve of the melt. This curve closely aligned with the results obtained through the pressure⁃volume⁃temperature method， presenting a mere mean squared deviation of 0.040 3 g/cm³. As a result， the non⁃destructive quantitative measurement of polymer melt density was achieved. This ultrasonic measurement technology enables the real⁃time monitoring of polymer melt information in injection molding， exhibiting vast potential applications in the practical production processes.

Key words: injection molding, melt information, temperature, density, ultrasonic measurement

CLC Number:

TQ320.66⁺2

CHEN Hao, JIAO Xiaolong, ZHU Ningdi, DONG Zhengyang, ZHANG Jianfeng, ZHAO Peng. Ultrasonic on⁃line measurement of polymer melt information in injection molding[J]. China Plastics, 2024, 38(4): 60-66.

Figures/Tables 12

Fig.1. Schematic diagram of ultrasonic wave propagation through mold and melt

Fig.2. Schematic diagram for calculating ultrasonic propagation time using cross correlation method

Fig.3. Schematic diagram of the experimental platform for ultrasonic measurement of melt information

Fig.4. Schematic diagram of experimental mold and sensor

中心频率/

MHz

脉冲电压/

阻尼/

$Ω$

采样频率/

MHz

增益/

中心频率/

MHz

脉冲电压/

阻尼/

$Ω$

采样频率/

MHz

脉冲重复频率/Hz

增益/

300

250

100

Tab.1. Parameter settings for ultrasonic equipment

熔体温度/

℃

注射压力/

MPa

注射速度/

保压压力/

MPa

模具温度/

℃

240

Tab.2. Parameter settings for injection molding equipment

Fig.5. Temperature， pressure and ultrasonic signal changes during injection molding process

Fig.6. Sensor temperature and ultrasonic measurement temperature

时间/s	温度传感器测量结果/℃	压力传感器测量结果/℃	超声声速测量结果/m·s^-1	熔体温度计算结果/℃	测量误差/%
1.10	235.0	19.51	1 038.42	221.54	5.73
1.65	237.3	19.90	1 036.27	224.15	5.54
2.20	235.2	14.20	1 037.34	224.76	4.44

Tab.3. Verification results of the correctness of the melt temperature measurement method

Fig.7. Frequency domain results of 0.3 s and 1.5 s ultrasonic echo signals

Fig.8. Logarithm and linear fitting of frequency domain transfer function for 1.5 s ultrasonic echo signal

Fig.9. Comparison of the evolution process of sound velocity and impedance with density measurement results during injection molding process

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