Method for on-line measurement of polymer melt temperature and apparatus thereof

Abstract

The present disclosure discloses a method for on-line measurement of the polymer melt temperature, comprising: on-line measurement of ultrasonic sound velocity c of melt in an injection molding process, on-line measurement of melt pressure P in the injection molding process, and obtaining melt temperature T in the injection molding process by formula (1). The present disclosure also discloses an apparatus for on-line measurement of the polymer melt temperature. The method and the apparatus provided in the present disclosure may enable on-line and in-situ characterization of the melt density and further enable on-line quantitative measurement of the melt quality. Compared with infrared measurement methods, the method provided herein is significantly reduced in cost, which is of great significance to theoretical researches of crystallization process and shear heating.

Claims

1. A method for on-line measurement of polymer melt temperature, comprising: measuring, by an ultrasonic probe, ultrasonic sound velocity c of melt during injection molding on line; measuring, by a pressure sensor, melt pressure P during injection molding on line, wherein the ultrasonic probe and the pressure sensor are arranged at a same cross section of the melt, wherein a sampling frequency of pressure or ultrasonic is higher than 250 MHz, and a signal preservation rate is higher than 20Sa/s, wherein an end of the ultrasonic probe fits and contacts a surface to be measured of a front mold by using a couplant, and another end of the ultrasonic probe is pressed tightly against and fixed onto inside of the mold, and wherein the pressure sensor is mounted into a mounting hole on a rear mold to measure a surface coplanar with a cavity surface; and calculating, by a microprocessor, melt temperature T during injection molding by using a Newtonian-iterative numerical method according to a formula (1) as follows, and controlling, by the microprocessor, on-line quality of a molded product based on the melt temperature:
P=c.sup.2(f(T,P)−f(T,P.sub.0))  (1) wherein f (T, P) and f (T, P.sub.0) are respectively: $f\left(T,P\right)=\frac{1}{\left[b_{1m}+b_{2m}\left(T-b_5\right)\right]\left\{1-C\ln \left[1+\frac{P}{b_{3m}{e}^{\left[-b_{4m}\left(T-b_5\right)\right]}}\right]\right\}}$ $f\left(T,P_0\right)=\frac{1}{\left[b_{1m}+b_{2m}\left(T-b_5\right)\right]\left\{1-C\ln \left[1+\frac{P}{b_{3m}{e}^{\left[-b_{4m}\left(T-b_5\right)\right]}}\right]\right\}}$ wherein: P.sub.0 is 1 standard atmospheric pressure, while C, b.sub.1m, b.sub.2m, b.sub.3m, b.sub.4m, and b.sub.5 are constant coefficients.

2. The method for on-line measurement of the polymer melt temperature according to claim 1, wherein a number of iteration times is set to 4-10.

3. An apparatus for on-line measurement of the polymer melt temperature, comprising: an ultrasonic probe, configured to perform on-line measurement of ultrasonic sound velocity c of melt during the injection molding process; a pressure sensor, configured to perform on-line measurement of pressure P of the melt during the injection molding process, wherein the ultrasonic probe and the pressure sensor are arranged at a same cross section of the melt, wherein a sampling frequency of pressure or ultrasonic is higher than 250 MHz, and wherein a signal preservation rate is higher than 20Sa/s, wherein an end of the ultrasonic probe fits and contacts a surface to be measured of a front mold by using a couplant, and another end of the ultrasonic probe is pressed tightly against and fixed onto inside of the mold, and wherein the pressure sensor is mounted into a mounting hole on a rear mold to measure a surface coplanar with a cavity surface; and a microprocessor configured to receive signals from the ultrasonic probe and the pressure sensor to output numeral values of the ultrasonic sound velocity c and the pressure P, obtain the polymer melt temperature T according to a formula: P=c.sup.2(f (T, P.sub.0)−f(T, P.sub.0)), and control on-line quality of a molded product based on the polymer melt temperature; wherein f (T, P) and f (T, P.sub.0) are respectively: $f\left(T,P\right)=\frac{1}{\left[b_{1m}+b_{2m}\left(T-b_5\right)\right]\left\{1-C\ln \left[1+\frac{P}{b_{3m}{e}^{\left[-b_{4m}\left(T-b_5\right)\right]}}\right]\right\}}$ $f\left(T,P_0\right)=\frac{1}{\left[b_{1m}+b_{2m}\left(T-b_5\right)\right]\left\{1-C\ln \left[1+\frac{P_0}{b_{3m}{e}^{\left[-b_{4m}\left(T-b_5\right)\right]}}\right]\right\}}$ wherein: P.sub.0 is 1 standard atmospheric pressure, while C, b.sub.1m, b.sub.2m, b.sub.3m, b.sub.4m, and b.sub.5 are constant coefficients.

4. The apparatus of on-line measurement of the polymer melt temperature according to claim 3, wherein the ultrasonic probe and the pressure sensor are arranged at a same cross section of the melt.

5. The apparatus of on-line measurement of the polymer melt temperature according to claim 3, wherein a plurality of sets of ultrasonic probes and pressure sensors are arranged along a direction that the melt flows in, so as to measure a melt temperature distribution across a cavity on-line.

6. The apparatus of on-line measurement of the polymer melt temperature according to claim 3, wherein a high-temperature ultrasonic couplant is selected as the couplant.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a structure schematic view of a verification apparatus used in the laboratory according to the present disclosure.

(2) FIG. 2 is a schematic view of a mounting position of the cavity pressure sensor used in the example and of the cavity shape.

(3) FIG. 3 shows temperature variation curves measured by an infrared optic-fiber temperature sensor in a certain injection molding process, and temperature variation curves measured by this method.

(4) FIG. 4 shows the measurement results of this method and infrared measurement method across the process of filling, pressure holding and cooling down phases in the injection process.

DETAILED DESCRIPTION

(5) The mold used in this embodiment adopts direct plastic feeding, and the schematic diagram of the measurement apparatus (experimental mold) is as shown in FIG. 1, wherein 5 and 6 represent a rear mold and a front mold of the mold respectively, and 2 is the plastic inlet of polymer melt, through which the melt enters a mold cavity 4, and flows, cools down inside the cavity and cures into a final product. In the drawing, 1 and 3 represent a melt pressure sensor and an ultrasonic probe respectively, wherein the ultrasonic probe works by non-contact measurement, so it keeps a certain distance from the cavity. Specifically, the cavity in the experimental mold is of a sheet structure with a length of 200 mm, a width of 30 mm and a thickness of 2 mm. The mounting position of the cavity pressure sensor used in the embodiment and the cavity shape are as shown in FIG. 2. The ultrasonic probe contacts the mold surface by using the couplant, and the other end of the probe is pressed tightly against and fixed onto the inside of the mold by mechanical means and a spring. The pressure sensor, which adopts contact measurement, is mounted into a mounting hole on the rear mold side of the injection mold to measure a surface coplanar with a cavity surface. An ultrasonic detection cable is connected with ultrasonic probe at one end, and connected with an ultrasonic acquisition card at the other end; a pressure sensor cable is connected with the sensor at one end, and connected with a data acquisition system at the other end. The equipment is powered on, debugged, and detected until ultrasonic echo signals and stable pressure signals can be observed and recorded continuously. In the experiment, the ultrasonic acquisition card in use has a sampling frequency of 250 MHz, the signal preservation rate is 100 Sa/s, which means 100 echo waveforms are preserved per second.

(6) The mounted injection mold is installed onto the injection molding machine. Pre-dried raw materials for injection are added into the hopper of the injection molding machine. A plasticizing temperature is set for the screw, so that when the temperature reaches the set value, a motor of the injection molding machine is turned on. Appropriate process parameters such as injection pressure holding and cooling parameters. After several cycles of injection, the system turns to stable, and the injection molding process may start then. Firstly, acquisition and recording commands are enabled for ultrasonic and pressure sensors, then the injection molding machine closes the mold, injects, holds the pressure, cools down, stores the material, opens the mold, and ejects the product. After that, the equipment stops signal acquisition, locally saves the signals recorded for one batch for further analysis and processing, and then proceeds with the next production and measurement cycle. At last, the obtained signal data are processed, so that the ultrasonic sound velocity c can be calculated by recording the time difference between the transmitted ultrasonic and the ultrasonic echo as well as the cavity thickness, and the pressure P can be directly measured by the pressure sensor. The melt temperature can be obtained by formula (2) and iterative formula (2′).

(7) In order to verify the accuracy of the measurement method proposed in the present disclosure, we compare the experimental results obtained by this method with those measured by an infrared optic-fiber sensor under the same condition.

(8) FIG. 3 shows temperature variation curves measured by an infrared optic-fiber temperature sensor in a certain injection molding process, and temperature variation curves measured by this method. It can be seen that both of the results hold good consistency in terms of absolute values and change trends, indicating that the method proposed in the present disclosure may replace the infrared method and enable accurate and rapid measurement of melt temperature at a low cost.

(9) We selected several groups of data under different process parameters to verify the method. Experimental results are as shown in Table 1.

(10) TABLE-US-00001 TABLE 1 FIR temperature Pressure Ultrasonic Melt sensor sensor sound velocity temperature measurement measurement measurement calculation Measurement Group results results results results errors 1 244.40° C. 19.67 MPa  995.61 m/s 251.05° C. 2.72% 2 238.14° C. 19.72 MPa 1038.01 m/s 225.36° C. 5.36% 3 245.82° C. 20.33 MPa 1020.31 m/s 236.87° C. 3.64%

(11) In addition, FIG. 4 shows the measurement results of this method and infrared measurement method across the process of filling, pressure holding and cooling down phases in the injection process. Because the temperature is affected by various molding parameters, this method may realize on-line detection and diagnosis of a variety of parameters that affect the final product quality, such as the injection speed and holding pressure.