Ultrasonic online nondestructive measurement method for melt density during molding

Abstract

The present disclosure discloses an ultrasonic online nondestructive measurement method for a melt density in injection molding, which solves the problems of difficult installation, high cost, influence on product surface quality, and the like existing in an existing density measurement method. According to the present disclosure, an ultrasonic velocity is obtained from a time domain signal with reference to time domain and frequency domain signal analysis of ultrasonic echo signals, an acoustic impedance is calculated by full spectrum analysis of a frequency domain signal, and the melt density is calculated from a correlation of the ultrasonic velocity, the acoustic impedance, and the density. The method has the advantages of having high measurement accuracy and being nondestructive, online and low in cost, and has a great application value in the injection molding industry.

Claims

1. An ultrasonic online nondestructive measurement method for a melt density in injection molding, comprising the following steps: (1) mounting an ultrasonic probe on an outer side wall of a mold cavity, and emitting an ultrasonic wave toward a polymer melt in the mold cavity; (2) collecting reflection echoes of two surfaces of the melt in contact with a mold, wherein the reflection echo of the surface close to the probe is denoted as U.sub.1, and the other reflection echo is denoted as U.sub.2; (3) calculating an ultrasonic propagation velocity in the polymer melt based on time domain signals of the reflection echoes U.sub.1 and U.sub.2; calculating an acoustic impedance of the polymer melt based on frequency domain signal amplitude spectra of the reflection echo U.sub.1 and U.sub.2; and (4) calculating the melt density based on ?=Z/c and the calculated ultrasonic propagation velocity and acoustic impedance, wherein Z is the acoustic impedance of the polymer melt, and c is the ultrasonic propagation velocity in the polymer melt.

2. The ultrasonic online nondestructive measurement method for a melt density in injection molding according to claim 1, wherein a method for calculating the ultrasonic propagation velocity c is: $c=\frac{2h}{?t},$ wherein h is a thickness of the polymer melt in an ultrasonic propagation direction, and ?t is a time difference between the reflection echoes U.sub.1 and U.sub.2, and is calculated by using a cross-correlation method from the time domain signals of the reflection echoes U.sub.1 and U.sub.2.

3. The ultrasonic online nondestructive measurement method for a melt density in injection molding according to claim 1, wherein the acoustic impedance of the polymer melt is obtained by solving an ultrasonic propagation proportionality coefficient, an acoustic impedance coefficient of a back mold material, and an acoustic impedance coefficient of a front mold material.

4. The ultrasonic online nondestructive measurement method for a melt density in injection molding according to claim 3, wherein the acoustic impedance Z=Z.sub.1 of the polymer melt is obtained by solving the following formula: $K=.Math.\frac{4Z_0{Z}_1\left(Z_2-Z_1\right)}{\left(Z_1+Z_2\right)\left(Z_1^2-Z_0^2\right)}.Math.,$ wherein ? is an operation of solving an absolute value; K is the ultrasonic propagation proportionality coefficient; and Z.sub.0, Z.sub.1 and Z.sub.2 are sequentially acoustic impedance coefficients of the back mold material, a melt material and the front mold material in the ultrasonic propagation direction.

5. The ultrasonic online nondestructive measurement method for a melt density in injection molding according to claim 1, wherein the ultrasonic propagation proportionality coefficient is obtained by fitting a relationship between a transfer function and a frequency ? of ultrasonic echo signals.

6. The ultrasonic online nondestructive measurement method for a melt density in injection molding according to claim 5, wherein the relationship is as follows:
ln(|H(?)|)=ln(K)?2mh?(?/?.sub.c), wherein H(?) is the transfer function of the ultrasonic echo signals; K is the ultrasonic propagation proportionality coefficient; m is a proportionality coefficient of a unit of an attenuation coefficient converted from (Np/cm) to (dB/cm); h is a thickness of the melt in the ultrasonic propagation direction, ?.sub.c is a center frequency of the ultrasonic probe, and ? represents the attenuation coefficient.

7. The ultrasonic online nondestructive measurement method for a melt density in injection molding according to claim 1, wherein the ultrasonic probe is arranged perpendicular to a flow direction of the polymer melt, and a side of the polymer melt receiving an ultrasonic signal has a plane structure perpendicular to the ultrasonic signal.

8. The ultrasonic online nondestructive measurement method for a melt density in injection molding according to claim 3, wherein the ultrasonic propagation proportionality coefficient is obtained by fitting a relationship between a transfer function and a frequency ? of ultrasonic echo signals.

9. The ultrasonic online nondestructive measurement method for a melt density in injection molding according to claim 8, wherein the relationship is as follows:
ln(|H(?)|)=ln(K)?2mh?(?/?.sub.c), wherein H(?) is the transfer function of the ultrasonic echo signals; K is the ultrasonic propagation proportionality coefficient; m is a proportionality coefficient of a unit of an attenuation coefficient converted from (Np/cm) to (dB/cm); h is a thickness of the melt in the ultrasonic propagation direction, ?.sub.c is a center frequency of the ultrasonic probe, and ? represents the attenuation coefficient.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] FIG. 1 is a flowchart of an ultrasonic online nondestructive measurement method for a melt density in injection molding according to the present disclosure;

[0034] FIG. 2 is a schematic diagram of an echo formed by an ultrasonic wave passing through a polymer melt in a measurement method according to the present disclosure;

[0035] FIGS. 3A-B show time domain signal of an ultrasonic echo in an embodiment, and FIGS. 3C-D show frequency domain signal of the ultrasonic echo;

[0036] FIG. 4A shows an ultrasonic velocity calculated by analyzing an ultrasonic time domain signal in an embodiment; FIG. 4B shows a process of linear fitting calculation of an ultrasonic frequency domain signal in the embodiment;

[0037] FIG. 5 shows measurement results of an ultrasonic velocity, an acoustic impedance and a density in an embodiment, and a comparison with a PVT method; and

[0038] FIG. 6 shows measurement results in the case of different materials and a comparison with a PVT method.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0039] The present disclosure is described in detail below with reference to a flowchart of the present disclosure. An ultrasonic online nondestructive measurement method for a melt density in injection molding according to the present disclosure is combined with of time domain and frequency domain signal analysis. An ultrasonic propagation velocity is calculated from time domain signal analysis, an acoustic impedance of the melt is calculated by full spectrum analysis of a frequency domain signal, and the melt density is calculated from a correlation of the ultrasonic velocity, the acoustic impedance, and the density.

[0040] The present disclosure is described in detail below with reference to the embodiments.

[0041] As shown in FIG. 1, an ultrasonic online nondestructive measurement method for a melt density in injection molding includes the following steps. [0042] (1): Mount an ultrasonic probe on an outer side wall of a mold cavity, and emit an ultrasonic wave toward a polymer melt in the mold cavity, where an initial ultrasonic signal is denoted as U.sub.0. [0043] (2): Collect reflection echoes of two surfaces of the melt in contact with a mold, where the reflection echo of the surface close to the probe is denoted as U.sub.1, and the other reflection echo is denoted as U.sub.2, as shown in FIG. 2. [0044] (3); Calculate an ultrasonic propagation velocity in the polymer melt by using a transit time method based on time domain signals of the reflection echoes U.sub.1 and U.sub.2, where a method for calculating the ultrasonic propagation velocity c is as follows:

[00004]$c=\frac{2h}{?t},$

[0045] where h is a thickness of the melt in an ultrasonic propagation direction, and ?t is a time difference between the reflection echoes U.sub.1 and U.sub.2, and may be calculated by using a cross-correlation method:

R.sub.u.sub.1.sub.u.sub.2(?)=?.sub.0.sup.t.sup.totalu.sub.1(t)u.sub.2(t+?)dt

R.sub.u.sub.1.sub.u.sub.2(?t)=max(R.sub.u.sub.1.sub.u.sub.2(?),

[0046] where ? is a time delay, and t.sub.total is a total time of echo signals. [0047] (4): Calculate an acoustic impedance of the polymer melt based on frequency domain signal amplitude spectra of the reflection echo U.sub.1 and U.sub.2, where a calculation process is as follows:

[0048] A transfer function of ultrasonic echo signals may be expressed as a ratio of frequency domain signal amplitude spectra:

H(?)=U.sub.2(?)/U.sub.1(?),

[0049] where U(?) is a frequency domain amplitude spectrum of an echo signal, and is obtained from a time domain signal u(t) by fast Fourier transform:

U(?)=?.sub.??.sup.+?u(t)e.sup.?i?tdt.

[0050] U.sub.1(?) and U.sub.2(?) may be obtained by using the above; and then H(?) is obtained.

[0051] Based on the law of ultrasonic propagation, the transfer function H(?) may be calculated by using the following formula:

H(?)=K.Math.exp(?2mh?(?/?.sub.c).sup.n?j(?.sub.1(?)??.sub.2(?))),

[0052] where h is a thickness of the melt in the ultrasonic propagation direction, ?.sub.c is a center frequency of the ultrasonic probe, ? represents an attenuation coefficient, and for the coefficients n=1 of most polymer materials, a function of a frequency ? may be obtained by calculating a logarithm on both sides of the formula:

[00005]$\begin{array}{cc}\ln \left(.Math.H\left(?\right).Math.\right)& =\ln \left(K\right)-2\mathrm{mh}?\left(?/{?}_c\right)\\ & =b+K?\end{array},$ [0053] where b=ln(K) may be regarded as an intercept, and a proportionality coefficient K=e.sup.b may be regarded as a slope. The intercept and the slope may be obtained by linear fitting of data.

[0054] Based on the law of ultrasonic reflection and transmission, the coefficient K is calculated by using the following formula:

[00006]$K=\frac{R_1{T}_0{T}_0^{}}{R_0}=.Math.\frac{4Z_0{Z}_1\left(Z_2-Z_1\right)}{\left(Z_1+Z_2\right)\left(Z_1^2-Z_0^2\right)}.Math.,$

[0055] where R.sub.0 and R.sub.1 are reflection coefficients of a front surface and a rear surface of the melt in contact with a mold material, respectively, T.sub.0 and T.sub.0 are transmission coefficients of ultrasonic signals propagating forward and backward through the front surface of the melt in contact with the mold respectively, and Z.sub.0, Z.sub.1 and Z.sub.2 are sequentially acoustic impedance coefficients of the back mold material, a melt material and the front mold material in the ultrasonic propagation direction, where Z.sub.0 and Z.sub.2 are known.

[0056] When K, Z.sub.0 and Z.sub.2 are known, a cubic equation with one unknown about the acoustic impedance Z.sub.1 of the melt can be seen from the above formula, and the acoustic impedance may be calculated by solving the equation. [0057] (5): Calculate the melt density based on ?=Z/c and the calculated propagation velocity and acoustic impedance, where Z is the acoustic impedance of the polymer melt, and c is the ultrasonic propagation velocity in the polymer melt, which are obtained in steps (4) and (3), respectively.

[0058] The present disclosure is described in detail below with reference to the embodiments.

Embodiment

[0059] A specific implementation of the present disclosure is described with ultrasonic measurement of a melt density of a self-made slit rheological mold as an example. An injection molding machine used in this embodiment was China ONGO Z70JD, an ONGO model. A measuring system is composed of an ultrasonic measuring apparatus and a pressure-temperature measuring apparatus (so that a PVT method is used to measure the density for verification). An ultrasonic signal transmitter/receiver (CTS-8077PR, SIUI, China) is configured to transmit a signal to an ultrasonic probe (5P20, Doppler, China) and receive an ultrasonic echo passing through a polymer melt. The ultrasonic probe is mounted on a back of a mold core. A digital oscilloscope (DSOX-3014 T Keysight Technologies, USA) is configured to display and record received data for further signal processing and analysis. A pressure sensor (SPF04, Futaba, Japan) and an infrared temperature sensor (EPSSZT, Futaba, Japan) are mounted in a mold cavity, and pressure and temperature data is collected by a data acquisition card (MVS08, Futaba, Japan) and displayed on a computer.

[0060] First, an experiment was performed by using a low density polyethylene (LDPE) material, with a total data acquisition time of 4.5 seconds. There were a total of 450 sampling points. Ultrasonic echo signals at a beginning time (0.3 s, left figure) and ultrasonic echo signals at an intermediate time (1.7 s, right figure) of the forming process are shown in FIGS. 3A-D, including time domain signals in FIGS. 3A-B and frequency domain signals in FIGS. 3C-D. It should be noted that an amplitude of an echo signal U.sub.2 increased gradually, indicating that the mold cavity was gradually filled with the polymer melt.

[0061] Curves of a calculated ultrasonic velocity and a measured temperature and pressure in the entire process are shown in FIGS. 4A-B. It can be seen that there is a close correlation between the ultrasonic velocity and the temperature and the pressure, which means that ultrasonic signals can reflect different stages in the injection molding process. A logarithm of a fast Fourier transform (FFT) amplitude spectrum and a transfer function is shown in FIG. 4B. A logarithm near a center frequency of the probe has stronger linearity (R.sup.2=0.9123). Therefore, points in this range are used for linear fitting and for obtaining an intercept. An intercept (ln(K)) at each sampling time was collected in the entire forming period, and then an acoustic impedance was calculated, as shown by the green dashed line in FIG. 5. Online measurement results of a change in density were calculated by using the ultrasonic propagation velocity and the acoustic impedance. A comparison between a density measurement (an ultrasonic method) and a reference value (a PVT method) is shown by red and blue solid lines in FIG. 5. A root mean square error (RMSE) was 0.0245 g/cm.sup.3, and a maximum relative error was 6.48%, indicating that this method can accurately measure the melt density.

[0062] An experiment using a polyvinyl chloride (PVC) material was also performed to test effectiveness of the proposed method in the case of using different materials. Density measurement results are shown in FIG. 6. Compared with an LDPE material, the PVC material has a similar density change process, but has an overall density greater than that of the LDPE. The density measurement RMSE of the PVC material was 0.01409 g/cm.sup.3 with a maximum relative error of 5.59%, which is similar to that of the LDPE. The implementation of the proposed ultrasonic method does not need to involve material characteristic parameters, and thus the method can be applied to density measurement of different materials. Therefore, the proposed ultrasonic method can be applied to other types of polymer materials.