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In-situ density measurement for plastic injection molding via ultrasonic technology

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Abstract

Density variation during the injection molding process directly reflects the state of plastic melt and contains valuable information for process monitoring and optimization. Therefore, in-situ density measurement is of great interest and has significant application value. The existing methods, such as pressure—volume—temperature (PVT) method, have the shortages of time-delay and high cost of sensors. This study is the first to propose an in-situ density measurement method using ultrasonic technology. The analyses of the time-domain and frequency-domain signals are combined in the proposed method. The ultrasonic velocity is obtained from the time-domain signals, and the acoustic impedance is computed through a full-spectral analysis of the frequency-domain signals. Experiments with different process conditions are conducted, including different melt temperature, injection speed, material, and mold structure. Results show that the proposed method has good agreement with the PVT method. The proposed method has the advantages of in-situ measurement, non-destructive, high accuracy, low cost, and is of great application value for the injection molding industry.

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Abbreviations

2s + 1:

Filter window size

b :

Intercept of the linear fitting

c :

Ultrasonic velocity

f :

Frequency

f c :

Central frequency of the transducer

h :

Thickness of plastic melt

H(f):

Transfer function of the echo signals

j:

Imaginary unit

k :

Slope of the linear fitting

K :

Proportionality propagation coefficient

m :

Coefficient that convert the unit of damping coefficient from Np/cm to dB/cm

P :

Melt pressure

R 0, R 1 :

Correlation function of u1 and u2

R 0, R 1 :

Reflection coefficients of the Material 1/Material 2 surface and Material 2/Material 3 surface, respectively

Δt :

Time delay between u1(t) and u2(t)

T o, T′0 :

Transmission coefficients of the ultrasonic waves passing forward and backward through the Material 1/Material 2 surface, respectively

T :

Melt temperature

u(t):

Time-domain signals

U o :

Original ultrasonic signal generated ultrasonic transducer

U 1, U 2 :

First and second echo signals reflected from the two surfaces of Material 2, respectively

U(f):

Amplitude spectrum of signals

U 1(f), U 2(f):

Amplitude spectrum of U1 and U2, respectively

V :

Specific volume

Z 0, Z1, Z 2 :

Acoustic impedances of Materials 1, 2, and 3, respectively

α :

Damping coefficient

ρ :

Density

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Acknowledgements

This work was supported by the “Pionerr” and “Leading Goose” R&D Program of Zhejiang, China (Grant No. 2022C01069), the National Natural Science Foundation of China (Grant No. 51875519), the Key Project of Science and Technology Innovation 2025 of Ningbo City, China (Grant No. 2021Z044), and the Project of Innovation Enterprises Union of Ningbo City, China (Grant No. 2021H002). The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Authors and Affiliations

  1. State Key Laboratory of Fluid Power and Mechatronic Systems, College of Mechanical Engineering, Zhejiang University, Hangzhou, 310027, China

    Zhengyang Dong, Peng Zhao, Kaipeng Ji & Jianzhong Fu

  2. Key Laboratory of 3D Printing Process and Equipment of Zhejiang Province, College of Mechanical Engineering, Zhejiang University, Hangzhou, 310027, China

    Zhengyang Dong, Peng Zhao, Kaipeng Ji & Jianzhong Fu

  3. Beijing Institute of Aeronautical Materials, Beijing, 100095, China

    Yuhong Chen

  4. Haitian Plastics Machinery Group Co., Ltd., Ningbo, 315801, China

    Shiquan Gao

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  1. Zhengyang Dong
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  2. Peng Zhao
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Correspondence to Peng Zhao.

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Dong, Z., Zhao, P., Ji, K. et al. In-situ density measurement for plastic injection molding via ultrasonic technology. Front. Mech. Eng. 17, 58 (2022). https://doi.org/10.1007/s11465-022-0714-2

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