阅读说明：本技术 一种聚合物熔体温度的在线测量方法及装置 (Online measurement method and device for polymer melt temperature ) 是由 赵朋 张剑锋 纪凯鹏 董正阳 夏能 周宏伟 傅建中 于 2020-04-09 设计创作，主要内容包括：本发明公开了一种聚合物熔体温度的在线测量方法,包括：在线测量超声波在注射成形过程中熔体中的超声声速c,在线测量注射成形过程中熔体的压力P,利用公式(1)得到注射成形过程中熔体的温度T。本发明还公开了一种聚合物熔体温度的在线测量装置。通过本发明的方法和装置,可以实现在线熔体密度的原位表征,进而实现熔体质量的在线定量化测量。这种方法成本相较红外方法显著下降,对结晶过程,剪切发热等理论研究有重要的意义。(The invention discloses an on-line measuring method of polymer melt temperature, which comprises the following steps: and (3) measuring the ultrasonic sound velocity c of ultrasonic waves in the melt in the injection molding process on line, measuring the pressure P of the melt in the injection molding process on line, and obtaining the temperature T of the melt in the injection molding process by using a formula (1). The invention also discloses an on-line measuring device for the temperature of the polymer melt. By the method and the device, the in-situ characterization of the linear melt density can be realized, and the online quantitative measurement of the melt quality can be further realized. Compared with an infrared method, the method has the advantages that the cost is obviously reduced, and the method has important significance on theoretical researches such as a crystallization process, shear heating and the like.)

1. A method for on-line measurement of polymer melt temperature, comprising: the ultrasonic sound velocity c of ultrasonic waves in the melt in the injection molding process is measured on line, the pressure P of the melt in the injection molding process is measured on line, and the temperature T of the melt in the injection molding process is obtained by using the following formula:

P＝c²(f(T,P)-f(T,P₀)) (1)

f(T,P)、f(T,P₀) Respectively as follows:

wherein: p₀At 1 atm, C, b_1m、b_2m、b_3m、b_4m、b₅Are constant coefficients.

2. The method of claim 1, wherein the temperature T is determined by Newton's iterative numerical method.

3. The method of claim 2, wherein the number of iterations is set to 4 to 10.

4. The method of claim 1, wherein the ultrasonic sound velocity c and the pressure P are measured by an ultrasonic probe and a pressure sensor disposed at the same cross section of the melt.

5. The method of claim 4, wherein the pressure or ultrasonic sampling frequency is higher than 250MH and the signal retention rate is higher than 20 Sa/s.

6. An apparatus for on-line measurement of polymer melt temperature, comprising:

the ultrasonic probe is used for measuring the ultrasonic sound velocity c of ultrasonic waves in the melt in the injection molding process on line;

a pressure sensor for measuring the pressure P of the melt in the injection molding process on line;

and the data processing unit receives signals of the ultrasonic probe and the pressure sensor to obtain ultrasonic sound velocity c and pressure P values, and the polymer melt temperature is obtained by using the formula (1) in claim 1.

7. The apparatus of claim 6, wherein the ultrasonic probe and the pressure sensor are disposed at the same cross-section of the melt.

8. The apparatus of claim 6, wherein a plurality of sets of ultrasonic probes and pressure sensors are arranged along the melt flow direction to obtain the temperature distribution of the melt in the whole cavity by online measurement.

9. The device for on-line measurement of the temperature of the polymer melt according to claim 6, wherein the ultrasonic probe is in close contact with the measured surface of the front mold through a coupling agent, and the other end of the ultrasonic probe is tightly pressed and fixed in the mold; the pressure sensor is arranged in a mounting hole on the rear die, and the measuring surface and the surface of the cavity are on the same plane.

10. The on-line measuring device for the temperature of the polymer melt according to claim 9, wherein the couplant is an ultrasonic high-temperature couplant.

Technical Field

The invention belongs to the technical field of ultrasonic testing and material forming, and relates to an online measuring method and device for the temperature of a polymer melt.

Background

Injection molding is the most widely used means of producing high performance polymer products, typically a dynamically complex batch process. In the whole injection molding process, the polymer undergoes huge pressure and temperature changes from high temperature and high pressure to normal temperature and pressure, and is a complex thermodynamic process. The evolution process of the mesoscopic morphological structure and the macroscopic physical properties of the melt in the process is related to the properties of thermodynamics and the like of a final product. Therefore, by the method for online measurement in the forming process, the evolution process information of the physical properties and the mesostructure of the melt in the forming process is known and acquired, and the method is an important basis for regulating and controlling the product performance. The injection molding process is a typical thermodynamic process, with melt temperature and pressure being the most important variables in the process. Pressure measurement has become mature, but accurate measurement of melt temperature has been a great challenge due to limitations of the temperature measurement principle.

The traditional thermocouple is a contact temperature measurement method, relates to temperature conduction, and is difficult to characterize the rapid temperature change condition in the measurement forming process due to the slow response speed. The contact detection method results in that it can only measure the mold surface temperature or the melt surface temperature. The use of a thermocouple to measure the melt temperature in the mold resulted in far from actual melt temperature variations. The infrared optical fiber temperature sensor is the sensor with the best temperature measuring accuracy in the current mold, but the high price thereof causes the serious limitation of the application thereof. In addition, some scholars have researched ultrasonic measurement methods of melt temperature, but the methods cannot be separated from soft measurement methods based on data linear regression, and effective quantitative measurement methods are not provided.

Disclosure of Invention

The invention aims to provide an online melt temperature measuring method in an injection molding process by combining die cavity pressure information and ultrasonic signals.

A method of on-line measurement of polymer melt temperature comprising: measuring the ultrasonic sound velocity c of ultrasonic waves in the melt in the injection molding process, measuring the pressure P of the melt in the injection molding process, and obtaining the temperature T of the melt in the injection molding process by using the following formula:

P＝c²(f(T,P)-f(T,P₀)) (1)

f(T,P)、f(T,P₀) Respectively as follows:

wherein: p₀At 1 atm, C, b_1m、b_2m、b_3m、b_4m、b₅Are constant coefficients.

In the calculation process, P, c, P₀Are known numbers, and f (T, P), f (T, P) can be expressed theoretically₀) Substituting equation (1), realizing the real-time measurement of the melt temperature by solving equation (1), and converting the problem into the solution of a unitary equation, namely:

g(T)＝P-c²(f(T,P)-f(T,P₀))＝0 (2)

the formula (2) is a complex equation, no analytic solution exists, and a certain numerical calculation method is needed for calculating the result of obtaining the temperature T. The invention adopts a Newton iteration numerical method, and the iterative computation process is as follows:

by giving an initial T₀Then according to the above-mentioned iterative formula, through several iterations, obtain T_nCan be combined with T_nAs a solution to the equation, the resulting melt temperature is calculated. Since the convergence efficiency of the newton iteration method is high, the number of iterations is preferably 4 to 10, and 5 is further preferable. T obtained by iterative solution₅The temperature value obtained by measurement and calculation is used for realizing the measurement of the melt temperature in the forming process.

Preferably, the online measurement of the ultrasonic sound velocity c is realized by using a non-contact ultrasonic probe; and realizing online measurement of the pressure P by using the pressure sensor. The ultrasonic sound velocity c and the pressure P are measured by an ultrasonic probe and a pressure sensor which are arranged at the same cross section of the melt.

During actual installation, the ultrasonic probe is in contact with the measured surface of the front mold through a coupling agent in a fitting manner, and the other end of the probe is pressed and fixed inside the mold through mechanical means such as a spring. Preferably, the couplant is an ultrasonic high-temperature couplant, so that the coupling efficiency of the probe is improved, and the effective coupling duration is prolonged. The pressure sensor is arranged in a mounting hole on the rear die, and the measuring surface and the surface of the cavity are on the same plane. Preferably, the ultrasonic probe and the pressure sensor are arranged at the same cross section of the melt.

The ultrasonic probe is connected with the ultrasonic signal generation and acquisition equipment through a cable, the equipment has the functions of waveform modulation, echo acquisition, display and continuous recording of the ultrasonic signals, and can finish continuous acquisition of the ultrasonic signals in a certain time period. Preferably, the sampling frequency of the device is higher than 250 MHz. The signal preservation rate in the experiment is 100Sa/s, namely the waveform of 100 echoes is preserved every second, and the signal preservation rate is preferably more than 20 Sa/s. The pressure sensor is connected with the data acquisition system, and the sampling frequency of the pressure signal is consistent with the ultrasonic sampling frequency. The experimental measurements were started after the equipment was commissioned successfully.

Starting sampling of the equipment when a certain cycle in the injection molding process starts, finishing signal acquisition and storing signals to be transmitted to the operation equipment when the process is finished, extracting real-time change conditions of pressure P and sound velocity c in the process, extracting the pressure and the sound velocity at the same moment, and calculating to obtain the temperature T of the melt in the real-time process according to a measurement model deduced by the invention.

The invention also provides an online melt temperature measuring device in the injection molding process based on ultrasonic sound velocity and melt pressure or the invention provides an online polymer melt temperature measuring device, which comprises:

the ultrasonic probe is used for measuring the ultrasonic sound velocity c of ultrasonic waves in the melt in the injection molding process on line;

a pressure sensor for measuring the pressure P of the melt in the injection molding process on line;

and the data processing unit receives signals of the ultrasonic probe and the pressure sensor, obtains ultrasonic sound velocity c and pressure P values, and obtains the temperature of the melt in the injection molding process or the polymer melt by using the formula (1) in claim 1.

The data processing unit can adopt a microprocessor, an industrial computer, a control chip or an integrated circuit board and the like.

Preferably, the ultrasonic probe and the pressure sensor are arranged at the same cross-section of the melt when mounted.

Preferably, a plurality of groups of ultrasonic probes and pressure sensors are arranged along the flow direction of the melt, so as to obtain the temperature distribution of the melt in the whole cavity through online measurement. By utilizing the method, when a plurality of groups of ultrasonic probes and pressure sensors are arranged in the die cavity of the die, the distribution condition of the temperature of the melt in the whole die cavity can be measured on line.

Preferably, the ultrasonic probe is in contact with the measured surface of the front mold through a coupling agent in a fitting manner, and the other end of the ultrasonic probe is pressed and fixed in the mold; the pressure sensor is arranged in a mounting hole on the rear die, and the measuring surface and the surface of the cavity are on the same plane.

Preferably, the couplant is an ultrasonic high-temperature couplant.

The method realizes synchronous online measurement of sound velocity and pressure of a melt at a certain local position in the forming process by respectively installing the ultrasonic probes and the pressure sensors at the front side and the rear side of the cavity of the injection forming die. The incidence relation of melt pressure, sound velocity and temperature can be obtained through deduction by combining the sound velocity equation, the volume modulus equation and the PVT equation of the melt, and therefore the measurement of the melt temperature in the forming process is achieved through obtaining the sound velocity and the pressure according to the measurement. If the signals are continuously collected in one period of the forming process, the change curve of the melt temperature at a certain measuring point in the cavity in the forming process can be obtained. The method can accurately reflect the evolution process of the temperature under different process conditions. The method has great popularization potential in industrial application and experimental research, and has important significance on theoretical researches such as crystallization process, shear heating and the like.

The invention provides a direct measurement method of the temperature of the melt in the die cavity for the first time, which is different from other existing research methods, and mainly aims at measuring the temperature of the non-isothermal melt in the die in the forming process rather than the temperature of the isothermal melt in a charging barrel of an injection molding machine. The invention comprehensively utilizes two physical information of sound velocity and pressure, and creatively provides a physical model among the sound velocity, the pressure and the temperature. The method can realize low-cost measurement of the melt temperature, can further realize online measurement of the melt density through a pressure-density-temperature correlation model of the material, and is favorable for realizing online quality control of a formed product.

Principle of measurement

According to the ultrasonic echo waveform obtained by the measurement of the ultrasonic probe, the ultrasonic propagation sound velocity c in the melt can be calculated, namely:

wherein d is the thickness of the mold cavity at the measuring point, and Δ t is the echo time difference of the ultrasonic waves on two different surfaces of the melt. Meanwhile, the pressure P of the melt at the measured point can be directly measured by the pressure sensor in the mold. Further, the ultrasonic sound velocity c and the pressure P can be analyzed and derived.

According to the ultrasonic propagation theory, under a certain temperature and pressure, the relationship between the bulk elastic modulus and density of the polymer melt and the sound velocity is as follows:

ρ_mc²＝K_m(4)

in the formula, ρ_mIs the density of the polymer melt. Melt K_mIs the bulk modulus of elasticity of the polymer melt, c is the ultrasonic sound velocity; at the same time, the bulk modulus of elasticity of the polymer melt is related to the pressure to which it is subjected:

wherein P is the pressure to which the polymer melt is subjected.

Assuming an initial density of ρ₀Of a polymer melt of mass m₀，ρ₀Can be expressed as:

initial volume of melt V₀And Δ V is the volumetric compression of the polymer melt. The density ρ of the polymer melt when the melt is subjected to a pressure P_mComprises the following steps:

the following compounds (5) to (7) can be obtained:

the combination of (4) and (8) can obtain:

P＝c²(ρ_m-ρ₀) (9)

it should be noted that the condition for satisfying the above model formula is that the polymer melt is at the same temperature, i.e., at a certain temperature, the polymerPressure P, speed of sound c, density rho of melt_mThe expression of the formula (9) is satisfied. Rho₀For this temperature, the density at which the pressure is a gauge pressure (1 atm) is a standard value. At the time of actual measurement, ρ₀Will vary with temperature and is not a fixed constant at p₀And rho_mWhen all are simultaneously changing variables, the melt density or temperature cannot be calculated by directly measuring P, c.

The invention further converts the formula (9) by means of a correlation equation of 'pressure-density-temperature' of the polymer, namely a PVT characteristic equation. The PVT property equation of a polymer is an essential property of a polymer, and for a specific volume V (i.e. the reciprocal of the density) of a specific polymer in any state, a certain corresponding relation is always satisfied with the pressure P and the temperature T of the specific volume V.

The PVT equation mainly comprises two expression forms of a Tait equation and a Spencer equation, the precision of the Tait state equation is relatively higher, and the Tait state equation is used as a tool for calculating the density of the material. The Tait equation can be expressed as:

when the polymer is in the solid state, T<b₅+b₆When the content is P, the content is,

when the polymer is in the molten state, T>b₅+b₆When the content is P, the content is,

here, in equations (10), (11) and (12), all parameters except T, P and V are constant coefficients of the equation, and correspond directly to a specific polymer material, and specific numerical values can be obtained by experiment or by referring to literature.

T is always satisfied when the polymer is a melt in an injection molding process>b₅+b₆Condition of P, and thus, density of melt ρ_mAnd rho₀Can be expressed as:

where T and P are the temperature and pressure at a given time, P₀Is 1 atm, i.e. 0.1 MPa. The belt (9) of the formulas (13) and (14) comprises:

P＝c²(f(T,P)-f(T,P₀)) (1)

in the calculation process, P, c, P₀Are known numbers and, theoretically, real-time measurement of melt temperature can be achieved by solving equation (1).

During actual on-line measurement, an ultrasonic probe is arranged on the back of a front mold cavity of the injection mold, and an ultrasonic detection coupling agent is used for connecting two measurement surfaces. The ultrasonic probe is installed and fixed in the injection molding die by means of certain external force (such as a mechanical spring) and a mechanical limit. Similarly, the pressure sensor is arranged in the mounting hole at one side of the rear die of the injection die, the pressure sensor is in contact type measurement, and the measuring surface of the sensor and the surface of the cavity are positioned on the same plane. One end of the ultrasonic detection cable is connected with the ultrasonic probe, the other end of the ultrasonic detection cable is connected with the ultrasonic acquisition card, one section of the pressure sensor cable is connected with the sensor, and the other end of the pressure sensor cable is connected with the data acquisition system. And (4) switching on the power supply of the equipment, and debugging the detection equipment until the signals of the ultrasonic echoes and the stable pressure signals can be observed and continuously recorded.

The injection molding machine is characterized in that an installed injection mold is installed on an injection molding machine, injection molding raw materials dried in advance are added into a hopper of the injection molding machine, the plasticizing temperature of a screw is set, after the temperature reaches a set value, a motor of the injection molding machine is started, appropriate technological parameters such as injection, pressure maintaining and cooling are set, and after a plurality of injection cycles, the injection molding process can be started after the system is stable. Firstly, starting an acquisition recording command of ultrasonic waves and a pressure sensor, then closing a mold, injecting, maintaining pressure, cooling, storing materials, opening the mold, ejecting, then stopping signal acquisition of equipment, storing the recorded signals in one batch to the local for subsequent further analysis and processing, and then carrying out the next cycle of production and measurement. And finally, processing the obtained signal data to obtain a change curve of the ultrasonic sound velocity and the melt pressure in each batch process, substituting the sound velocity c and the pressure P obtained by real-time measurement into the formula (2), iteratively solving the temperature T by an iteration method of the formula (2'), thereby obtaining melt temperature data on a measurement point, and finally obtaining the temperature change condition of the polymer melt on an ultrasonic propagation path in the forming process by data integration.

In the whole process, technological parameters in the production process, such as plasticizing temperature, injection pressure, holding pressure and the like, can be adjusted, which influence the change condition of melt temperature in the forming process and the performance of a final product, and the online measurement can be realized by the method provided by the invention.

Based on the method provided by the invention, the in-situ characterization of the melt density on line can be realized through the derivation of PVT equation and the device and the model, and further the quantitative measurement of the melt quality on line can be realized. Compared with an infrared method, the method has the advantages that the cost is obviously reduced, and the method has important significance on theoretical researches such as a crystallization process, shear heating and the like.

Drawings

FIG. 1 is a schematic structural diagram of a laboratory testing apparatus according to the present invention.

Fig. 2 is a schematic view of a mold cavity pressure sensor mounting location and a mold cavity shape as used in the examples.

Fig. 3 shows a temperature change curve measured by an infrared optical fiber temperature sensor in a certain injection molding process and a temperature change curve measured by the method.

FIG. 4 shows the measurement results of the method and the infrared method during the complete process of filling, pressure maintaining and cooling during the injection process.

Detailed Description

The forming mold of this embodiment adopts a direct glue feeding form, and a schematic view of a measuring device (experimental mold) is shown in fig. 1, wherein 5 and 6 are respectively a rear mold and a front mold of the mold, 2 is a glue feeding port of a polymer melt, and the melt enters a mold cavity 4 through the glue feeding port, flows in the cavity, is cooled and is solidified into a final product. In the figure, 1 and 3 are respectively a melt pressure sensor and an ultrasonic probe, wherein the ultrasonic probe is used for non-contact measurement and is therefore a certain distance away from a cavity.

Specifically, the cavity in the experimental mold is a sheet structure with the length of 200mm, the width of 30mm and the thickness of 2mm, and the installation position of the cavity pressure sensor and the shape of the cavity used in the embodiment are shown in fig. 2. The ultrasonic probe is contacted with the surface of the die through a coupling agent, and is pressed and fixed inside the die at the other end through a spring by a mechanical means. And installing a pressure sensor in an installation hole at one side of a rear mold of the injection mold, wherein the pressure sensor is used for contact measurement, and a measurement surface of the sensor and the surface of the cavity are positioned on the same plane. One end of the ultrasonic detection cable is connected with the ultrasonic probe, the other end of the ultrasonic detection cable is connected with the ultrasonic acquisition card, one section of the pressure sensor cable is connected with the sensor, and the other end of the pressure sensor cable is connected with the data acquisition system. And (4) switching on the power supply of the equipment, and debugging the detection equipment until the signals of the ultrasonic echoes and the stable pressure signals can be observed and continuously recorded. The sampling frequency of the ultrasonic acquisition card used in the experiment is 250MHz, the storage rate of signals in the experiment is 100Sa/s, namely the waveforms of 100 echoes are stored every second.

The injection molding machine is characterized in that an installed injection mold is installed on an injection molding machine, injection molding raw materials dried in advance are added into a hopper of the injection molding machine, the plasticizing temperature of a screw is set, after the temperature reaches a set value, a motor of the injection molding machine is started, appropriate technological parameters such as injection, pressure maintaining and cooling are set, and after a plurality of injection cycles, the injection molding process can be started after the system is stable. Firstly, starting an acquisition recording command of ultrasonic waves and a pressure sensor, then closing a mold, injecting, maintaining pressure, cooling, storing materials, opening the mold, ejecting, then stopping signal acquisition of equipment, storing the recorded signals in one batch to the local for subsequent further analysis and processing, and then carrying out the next cycle of production and measurement. And finally, processing the obtained signal data, calculating the ultrasonic sound velocity c by recording the time difference between the ultrasonic emission wave and the ultrasonic echo and the thickness of the die cavity, and directly measuring the pressure P by using a pressure sensor. The melt temperature can be obtained by using the formula (2) and the iterative formula (2').

In order to verify the accuracy of the measurement method provided by the invention, the experimental result of the method is compared with the measurement result of the infrared optical fiber sensor under the same condition.

Fig. 3 shows a temperature change curve measured by an infrared optical fiber temperature sensor in a certain injection molding process and a temperature change curve measured by the method. The method can be used for replacing an infrared method, and realizes accurate, rapid and low-cost measurement of the melt temperature.

We selected several sets of data under several different sets of process parameters to validate the method. The results of the experiment are shown in table 1.

TABLE 1

In addition, fig. 4 shows the measurement results of the method and the infrared method during the complete process of filling, pressure maintaining and cooling during the injection process. Because the temperature is influenced by various forming parameters, the method can realize the on-line detection and diagnosis of various parameters influencing the quality of the final product, such as the injection speed, the pressure maintaining pressure and the like.