WO2020057601A1 - 通过超声波手段间接测量注塑机型腔压力的方法及装置 - Google Patents

通过超声波手段间接测量注塑机型腔压力的方法及装置 Download PDF

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WO2020057601A1
WO2020057601A1 PCT/CN2019/106683 CN2019106683W WO2020057601A1 WO 2020057601 A1 WO2020057601 A1 WO 2020057601A1 CN 2019106683 W CN2019106683 W CN 2019106683W WO 2020057601 A1 WO2020057601 A1 WO 2020057601A1
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ultrasonic
injection molding
tie rod
molding machine
cavity
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PCT/CN2019/106683
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English (en)
French (fr)
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赵朋
张剑锋
赵耀
顾复
黄俊业
夏能
傅建中
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浙江大学
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Priority to US17/256,524 priority Critical patent/US11573146B2/en
Publication of WO2020057601A1 publication Critical patent/WO2020057601A1/zh

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
    • G01L11/00—Measuring steady or quasi-steady pressure of a fluid or a fluent solid material by means not provided for in group G01L7/00 or G01L9/00
    • G01L11/04—Measuring steady or quasi-steady pressure of a fluid or a fluent solid material by means not provided for in group G01L7/00 or G01L9/00 by acoustic means
    • G01L11/06—Ultrasonic means
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
    • B29C45/00—Injection moulding, i.e. forcing the required volume of moulding material through a nozzle into a closed mould; Apparatus therefor
    • B29C45/0082—Reciprocating the moulding material inside the mould cavity, e.g. push-pull injection moulding
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
    • B29C45/00—Injection moulding, i.e. forcing the required volume of moulding material through a nozzle into a closed mould; Apparatus therefor
    • B29C45/17—Component parts, details or accessories; Auxiliary operations
    • B29C45/76—Measuring, controlling or regulating
    • B29C45/77—Measuring, controlling or regulating of velocity or pressure of moulding material
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
    • B29C2945/00—Indexing scheme relating to injection moulding, i.e. forcing the required volume of moulding material through a nozzle into a closed mould
    • B29C2945/76—Measuring, controlling or regulating
    • B29C2945/76003—Measured parameter
    • B29C2945/76006—Pressure
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
    • B29C2945/00—Indexing scheme relating to injection moulding, i.e. forcing the required volume of moulding material through a nozzle into a closed mould
    • B29C2945/76—Measuring, controlling or regulating
    • B29C2945/76003—Measured parameter
    • B29C2945/76066—Time
    • B29C2945/76076—Time duration
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
    • B29C2945/00—Indexing scheme relating to injection moulding, i.e. forcing the required volume of moulding material through a nozzle into a closed mould
    • B29C2945/76—Measuring, controlling or regulating
    • B29C2945/76177—Location of measurement
    • B29C2945/76224—Closure or clamping unit
    • B29C2945/76234—Closure or clamping unit tie-bars
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
    • B29C2945/00—Indexing scheme relating to injection moulding, i.e. forcing the required volume of moulding material through a nozzle into a closed mould
    • B29C2945/76—Measuring, controlling or regulating
    • B29C2945/76177—Location of measurement
    • B29C2945/76287—Moulding material
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
    • B29C2945/00—Indexing scheme relating to injection moulding, i.e. forcing the required volume of moulding material through a nozzle into a closed mould
    • B29C2945/76—Measuring, controlling or regulating
    • B29C2945/76451—Measurement means
    • B29C2945/76471—Acoustic

Definitions

  • the invention belongs to the technical field of ultrasonic testing and material molding, and in particular relates to a method for indirectly measuring cavity pressure in an injection molding process by ultrasonic means.
  • Injection molding is the most widely used molding method for producing plastic products.
  • the polymer melt is injected into the mold cavity.
  • the entire process time is very short and accompanied by drastic temperature and pressure changes. Different pressure changes will directly affect the quality of the final product, so the pressure curve of the mold cavity is considered as the fingerprint of the molding process, used to characterize and diagnose the molding process, and can be used to further guide the optimization of the injection molding process. Therefore, for the molding process, the measurement of the cavity pressure curve is very important, and it is also a key step to realize the intelligent injection molding process.
  • the current measurement method for the cavity pressure in the mold is mainly through a cavity pressure sensor, which can accurately measure the pressure change of the melt inside the mold cavity in real time. However, this method has certain defects.
  • This method can only measure the pressure change at a certain measuring point in the interior, and the sensor is installed on the surface of the mold cavity, which damages the mold and will leave defects on the surface of the injection molded product, plus the high price of the sensor It is difficult to popularize on each mold, which brings some resistance to the online detection of the injection molding process.
  • the purpose of the present invention is to provide a method and a device for ultrasonically nondestructively detecting the pressure of a mold cavity in an injection molding process.
  • the ultrasonic method is used to measure the force of a rod of an injection molding machine, thereby indirectly measuring the pressure change inside the mold cavity.
  • a method for indirectly measuring cavity pressure of an injection molding machine by ultrasonic means including: simultaneously transmitting ultrasonic waves to each tie rod along the axial direction of the tie rods, and detecting the time difference between the ultrasonic echo of each tie rod and the ultrasonic echo when the tie rods are free.
  • the average pressure P inside the cavity of the injection molding machine is obtained by the following formula:
  • R is the cross-sectional radius of the tie rod
  • ⁇ t total is the sum of the echo time differences received on all the tie rods
  • W is a constant related to the material of the tie rod
  • A is the projected area of the cavity in a plane perpendicular to the axial direction of the tie rod.
  • a method for measuring the pressure of a tie rod of an injection molding machine by ultrasonic means includes: transmitting ultrasonic waves to each tie rod along the tie rod axis at the same time, and detecting the time difference between the ultrasonic echo on each tie rod and the ultrasonic echo when the tie rod is free, according to the following The formula gives the total pulling force F rod of the injection molding machine rod :
  • R is the cross-sectional radius of the tie rod
  • ⁇ t total is the sum of the time difference of the echoes received on all the tie rods
  • W is a constant related to the material of the tie rod.
  • a method for measuring the pressure of a tie rod of an injection molding machine by ultrasonic means includes: transmitting ultrasonic waves to each tie rod along the tie rod axis at the same time, and detecting the time difference between the ultrasonic echo on each tie rod and the ultrasonic echo when the tie rod is free, according to the following The formula obtains the pull force F of any tie rod of the injection molding machine:
  • R is the cross-sectional radius of the tie rod
  • ⁇ t is the time difference of echoes received on the tie rod
  • W is a constant related to the material of the tie rod.
  • the invention can be used for real-time detection of the pressure in the cavity of an online injection molding machine, and can also be used for detecting the pressure in the cavity of an injection molding machine in a certain state.
  • ultrasonic waves are continuously transmitted to each tie rod during the injection molding process, and the average pressure inside the cavity of the injection molding machine is displayed in real time during the injection molding process.
  • the clamping force sensor is used to detect multiple sets of ⁇ - ⁇ t data, and the obtained multiple sets of ⁇ - ⁇ t data are used to fit.
  • a ⁇ - ⁇ t straight line passing through the origin, and the slope of the straight line is obtained to obtain the W, where ⁇ is the stress on the tie rod, which is the ordinate; ⁇ t is the time difference of the ultrasonic echo, which is the abscissa.
  • the stress ⁇ on the tie rod can be directly detected by the clamping force sensor; the time difference ⁇ t of the ultrasonic echo can be acquired by an ultrasonic acquisition unit.
  • tie rods there are four tie rods.
  • an ultrasonic probe is provided at one end of each tie rod.
  • the real-time output time and the average pressure curve inside the cavity of the injection molding machine are output.
  • the recording frequency of the ultrasonic echo signal is higher than or equal to 20 Hz.
  • a device for indirectly measuring cavity pressure of an injection molding machine by ultrasonic means including:
  • An ultrasonic probe installed at one end of the pull rod, the ultrasonic probe is axially installed for the pull rod;
  • An ultrasonic acquisition unit for controlling ultrasonic signals and receiving ultrasonic echoes
  • the calculation and output unit processes the collected ultrasonic echo signals and outputs the average pressure result inside the injection molding machine cavity.
  • the calculation and output unit uses the following formula to obtain the average pressure P inside the injection molding machine cavity:
  • R is the cross-sectional radius of the tie rod
  • ⁇ t total is the sum of the echo time differences received on all the tie rods
  • W is a constant related to the material of the tie rod
  • A is the projected area of the cavity in a plane perpendicular to the axial direction of the tie rod.
  • the ultrasonic acquisition unit has four ultrasonic acquisition channels; and the calculation and output unit is a computer.
  • the ultrasonic acquisition unit is an ultrasonic acquisition card; the computer is connected to a display for displaying the average pressure result inside the cavity of the injection molding machine in real time.
  • the invention uses the change of the echo waveform of the ultrasonic signal and is based on the theory of material acoustic elasticity.
  • the pressure of the material during the forming process is measured online and used for process monitoring and diagnosis.
  • Ultrasound is a common non-destructive testing method, which can directly detect a large number of physical quantities, including the deformation of materials under stress.
  • the pressure of the melt cavity on the mold cavity is transmitted to the injection molding motorized template in the form of force, and then transmitted to the pull rod of the injection molding machine, and finally reflects the force deformation of the pull rod.
  • the method of measuring the force extension of the rod position of the injection molding machine by ultrasonic is used to characterize the melt pressure inside the cavity during the injection molding process. This method is stable, non-destructive, online, and low cost.
  • Injection molding is an intermittent, cyclic batch process. Before the injection process begins, the mold is closed. At this time, the injection molding machine cylinder gives the mold a certain clamping force. Due to the reaction force, the clamping force will also pass through the injection molding machine. The fixed mold and the movable mold are transferred to the pull rod of the injection molding machine. At this time, the pull rod of the injection molding machine is subjected to a certain pulling force. This pulling force can be measured by an ultrasonic detection method. Similarly, when the injection process begins, the high temperature and high pressure polymer melt is injected into the cavity of the mold.
  • the front mold and the back mold of the mold are also subjected to the extrusion force from the melt, and this extrusion force and The clamping force is the same and will eventually be transmitted to the tie rod of the injection molding machine.
  • This method analyzes the displacement of the ultrasonic echo passing through the tie rod of the injection molding machine, and obtains the force on the tie rod based on the principle of sonoelastic effect, and converts it into the pressure inside the mold cavity according to the physical force model.
  • the ultrasonic signals are continuously collected and analyzed within a certain period of time (usually a cycle of the injection molding process), the change curve of the pressure in the mold cavity during the molding process can be obtained. Further analysis of the obtained curves can be used to characterize various information of the injection process and can provide a basis for further process optimization.
  • the measurement point of the present invention is not selected on the injection mold, and it is creatively proposed to realize the indirect characterization of the mold cavity pressure by detecting the stress of the injection molding machine rod.
  • This method not only avoids the damage to the mold by the measurement itself and the cost of processing, but because the measurement is installed on the injection molding machine frame, it can be adapted to almost all molds, which is conducive to the popularization of the method.
  • the pressure in the cavity can be detected under a certain state, and the pressure in the cavity can be detected in real time during the injection molding process, and the detection process is simple and highly accurate.
  • Figure 1 is a schematic diagram of a measuring device
  • FIG. 2a and FIG. 2b are respectively the pressure curve of the melt at a measuring point inside the cavity during a certain injection molding process and the force change curve of the tie rod measured by an ultrasonic probe on a tie rod;
  • FIG. 3 is a schematic diagram of a cavity structure inside a mold used in a specific example
  • FIG. 4 is a schematic diagram of different curve shapes corresponding to different V / P switching points during the injection process.
  • the measurement method involved in the present invention can be applied to the detection of cavity pressure in an injection molding machine for producing plastic products as shown in FIG. 1, which is mainly composed of an injection unit and a mold unit, and the injection unit is mainly an injection cylinder 7
  • the polymer is melt-plasticized by the screw inside the injection barrel and injected into the cavity 5 inside the mold.
  • the mold unit consists of key parts such as fixed template 6, movable template 3, and mold 4.
  • the injection molding machine provides certain power to push the movable template to close the front and back parts of the mold, and a cavity is formed between the movable template and the fixed template. And then proceed with the injection process of the molten polymer.
  • the device In order to measure the change of the force of the pull rod during the molding process, the device is provided with an ultrasonic probe 1 at the end of the pull rod, and at the same time, it is ensured that the ultrasonic probe 1 is axially disposed directly opposite the pull rod, and can transmit ultrasonic waves along the axial direction of the pull rod.
  • an injection molding machine has four tie rods, which are distributed at the four corners of the machine which are symmetrical to each other. For the accuracy of the measurement, it is preferred that, when the measurement is specifically carried out, four ultrasonic probes are installed at the ends of the four pull rods away from the fixed die.
  • a commonly used ultrasound coupling agent such as glycerin
  • glycerin can be used as a coupling medium and smeared between the probe surface and the measured object, and a certain mechanical method is used to fix and press the ultrasound probe.
  • a multi-channel ultrasonic acquisition device ie, an ultrasonic acquisition unit
  • the device should have the functions of multi-channel adjustment, control, display, and continuous recording of ultrasonic signals, which can be completed in a certain period of time. Realize continuous acquisition of ultrasound signals.
  • the frequency of continuous acquisition should be higher than 20 Hz.
  • the device should preferably have four ultrasonic acquisition channels to meet the real-time synchronized signal acquisition of the four lever positions.
  • the ultrasonic acquisition equipment can use the existing four-channel ultrasonic acquisition card.
  • the principle of this measurement method is to analyze the ultrasonic echoes (or ultrasonic echoes) passing through the inside of the tie rod. It can be obtained that the echoes at different time points have different time shifts. According to the theory of material acoustic elasticity, The displacement of the wave (or the time difference of receiving the ultrasonic echo) is calculated to obtain the force situation of the material.
  • ⁇ 0 , ⁇ and C ⁇ are the density of the tie rod, the stress of the tie rod, and the propagation speed of the ultrasonic wave under the stress.
  • l, m and n are the Murnaghan constants of the material, and ⁇ and ⁇ are the Lamé constants of the material.
  • Equation (2) the initial velocity of the ultrasonic wave can be obtained by equation (2). Calculate by bringing in the constant It can be known that when ⁇ is within the experimental range (that is, the range of stresses on the tie rod in the experiment), The value of is approximately unchanged, and Equation (1) can be reduced to a linear function
  • the simultaneous formulas (2) and (3) include:
  • K [2l ⁇ + ⁇ + ( ⁇ + ⁇ ) (4m + 4 ⁇ + 10 ⁇ )] / [2 ⁇ (3 ⁇ + 2 ⁇ ) ( ⁇ + 2 ⁇ )] is the acoustic elastic coefficient of the material, which is an inherent material parameter. Acoustic elasticity of the drawbar.
  • t 0 is the propagation time of the ultrasonic wave when the tie rod is not stressed. Similar to t 0 , t ⁇ represents the propagation time when the tie rod is under stress ⁇ . E is the elastic modulus of the tie rod. When there is no pressure, the natural length of the tie rod is l 0 , and the relative time change (t ⁇ -t 0 ) / t 0 is set . From the formulas (4)-(6), we can get
  • the stress on the tie rod can be calculated by formula (8), but the coefficient K is very complicated and difficult to obtain due to the five material constants l, m, n, ⁇ and ⁇ . Therefore, as a preference we try to avoid calculating the coefficient K directly and define:
  • Equation (9) can be simplified as:
  • the injection molding machine provides clamping force to the mold before the injection phase begins. This clamping force is applied to the tie rods of the injection molding machine and the injection mold, and thus strain is generated on the tie rods.
  • the plastic melt will exert a force F on the cavity of the mold. As F increases, this force F will gradually offset the initial clamping force, and then with the clamping force on the tie rod The force increases and reaches a peak.
  • the force F rod on the pull rod can be calculated:
  • ⁇ total is the total strain of the four tie rods, which is measured and calculated by four ultrasonic probes installed at the ends of the tie rods, R is the cross-sectional radius of the tie rods; ⁇ t total is the ultrasonic echo when all the tie rods are under stress and when the tie rods are free
  • the sum of the time differences of the ultrasonic echoes that is, the sum of the time differences of the echoes on the four tie rods for a group of ultrasounds transmitted simultaneously at a time; ks is the coefficient obtained in the previous step.
  • the average pressure inside the mold cavity is represented by P. Due to the relationship between the acting force and the reaction, F rod is equal to the acting force F cavity of the melt in the cavity on the mold. Therefore, the calculation formula of the average pressure P inside the cavity is shown in formula (13):
  • A is the projected area of the cavity in a plane perpendicular to the axial direction of the tie rod.
  • ultrasonic probes are installed at the ends of the four tie rods of the injection molding machine.
  • the installation method can be selected by using glue or coupling agent for ultrasonic testing, and by using a certain external force (such as a magnetic table base).
  • the ultrasound probe is pressed on the end of the injection molding machine rod. Connect one end of the signal cable to the ultrasonic probe and the other end to the ultrasonic acquisition card. Turn on the power and adjust the displayed waveform until you can observe and continuously record the echo signal.
  • the injection molding machine installs the mold on the injection molding machine, add the pre-dried injection molding material to the injection molding bucket, set the plasticizing temperature of the injection screw, and after the temperature reaches the set value, turn on the injection molding machine motor and set the appropriate injection
  • several injection cycles such as holding pressure and cooling, after the system is stable, it is ready to start the injection molding process.
  • the injection molding machine closes the mold, injects, maintains pressure, cools, stores the material, opens the mold, ejects, then stops the ultrasonic acquisition, and saves the recorded echo signal to the local area for a period of time. For further analysis and processing, the next production and measurement cycle.
  • the obtained echo signal data is processed to obtain the time difference (or ultrasonic time shift ⁇ t) between each ultrasonic echo and the reference echo (first echo).
  • each time measurement point is calculated.
  • process parameters such as injection pressure, holding pressure, and clamping force can be adjusted during production. This will affect the performance and quality of the final product, and will also be reflected in the process pressure curve obtained by ultrasonic measurement.
  • This example adopts the method of direct feeding, and installs a mold core with a cavity structure of 1 mm thick sheet.
  • Fig. 2a shows the pressure curve of the melt at the measuring point inside the cavity during the injection molding process using an existing sensor
  • Fig. 2b is an ultrasonic probe measured on a pull rod of the present invention.
  • Force change curve of tie rod We can see that the change in the force on the tie rod has a good agreement with the change in the cavity pressure measured by the cavity pressure sensor, which also shows that this method of ultrasonic force measurement on the tie rod can be used to measure the mold during the injection process. Changes in cavity pressure.
  • the sampling frequency of the ultrasonic acquisition card used in the experiment is 100 MHz, and the minimum time interval ⁇ t is 10 ns, that is, the time resolution is 10 ns. When converted to the force on the tie rod, the resolution is about 1.25 MPa. When the curve is small, a jagged outline appears. You can improve the measurement accuracy by increasing the sampling frequency of the device. As a preference, we recommend that the sampling frequency of the device should not be less than 500MHz.
  • F cavity and F rod should be the same.
  • a sheet-shaped cavity mold with a length of 200mm, a width of 30mm, and a thickness of 1mm as the experimental mold, and verified our model.
  • the cavity used in the experiment The installation position of the pressure sensor and the shape of the cavity are shown in Figure 3.
  • the average pressure in the mold cavity can be simplified as:
  • FIG. 4 shows different curve shapes corresponding to different V / P switching points (holding pressure switching points) during the injection process.
  • the change of the curve due to the V / P switching time can be used to detect the premature V / P switching during the online process.
  • this method can also be used for on-line detection and diagnosis of various parameters that affect the quality of the final product, such as dwell time, dwell pressure, and injection speed.

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  • Physics & Mathematics (AREA)
  • Acoustics & Sound (AREA)
  • Engineering & Computer Science (AREA)
  • Manufacturing & Machinery (AREA)
  • Mechanical Engineering (AREA)
  • General Physics & Mathematics (AREA)
  • Injection Moulding Of Plastics Or The Like (AREA)
  • Moulds For Moulding Plastics Or The Like (AREA)

Abstract

一种通过超声波手段间接测量注塑机型腔（5）压力的方法，包括：沿拉杆（2）轴向分别同时向每个拉杆（2）发射超声波，检测每个拉杆（2）超声波回波时间差，得到注塑机型腔（5）内部平均压力。可实现某一状态下型腔（5）内压力的检测，也可以实现注塑过程中型腔（5）中压力的实时检测，检测过程简单，精确度高。

Description

通过超声波手段间接测量注塑机型腔压力的方法及装置 技术领域
本发明属于超声测试与材料成型技术领域，具体涉及一种通过超声波手段间接测量注塑成型过程中型腔压力的方法。
背景技术
注射成形是应用最广泛的用于生产塑料制品的成型方法。在注射成形过程中，聚合物熔体被注射进模具型腔，整个过程时间非常短暂并伴随着剧烈的温度压力变化。不同的压力变化情况将直接影响到最终产品的质量，从而模具型腔的压力曲线被认为是成型过程的指纹，被用来表征和诊断成型的过程，并可以用于进一步指导注塑工艺的优化。因此，对于成型过程而言，型腔压力曲线的测量是至关重要的，也是实现智能注塑成形过程的关键步骤。当前对于模具内部型腔压力的测量手段主要是通过模腔压力传感器，这种测量手段能够实时精确地测量模具型腔内部熔体的压力变化情况。然而这种方法存在一定的缺陷。该方法只能够测量内部某一个测点位置的压力变化情况，并且，传感器安装在模具型腔的表面，对于模具有损坏，也会使得注塑产品的表面留下缺陷，再加上传感器高昂的价格，很难普及在每一个模具上面，这给注塑成形过程的在线检测带来了一定的阻力。
此前，有相关研究人员提出过一种型腔压力的软测量手段，但是这种手段尽管是一种无损的方法，却涉及了成型过程中熔体密度，温度，螺杆压力等多项参数，具体实施起来不仅复杂而且缺少参数之间物理联系和理论支持，必将损失一部分准确性。
发明内容
本发明目的是提出一种超声无损检测注射成形过程模具型腔压力的方法和装置，通过使用超声波方法测量注塑机拉杆的受力，从而间接测量模具型腔内部的压力变化情况。
一种通过超声波手段间接测量注塑机型腔压力的方法，包括：沿拉杆轴向，分别同时向每个拉杆发射超声波，检测每个拉杆超声波回波与拉杆自由时的超声回波的时间差，根据如下公式得到注塑机型腔内部平均压力P：
P＝πR 2·W·Δt total/A
其中：R为拉杆的截面半径；Δt total为所有拉杆上接收的回波时间差总和；W为与拉杆材质相关的常量；A为型腔在垂直于拉杆轴向的平面的投影面积。
一种通过超声波手段测量注塑机拉杆压力的方法，包括：沿拉杆轴向，分别同时向每个拉杆发射超声波，检测每个拉杆上超声波回波与拉杆自由时的超声回波的时间差，根据如下公式得到注塑机拉杆的总的拉力F rod：
F rod＝πR 2·W·Δt total
其中：R为拉杆的截面半径；Δt total为所有拉杆上接收的回波时间差总和；W为与拉杆材质相关的常量。
一种通过超声波手段测量注塑机拉杆压力的方法，包括：沿拉杆轴向，分别同时向每个拉杆发射超声波，检测每个拉杆上超声波回波与拉杆自由时的超声回波的时间差，根据如下公式得到注塑机任一拉杆的拉力F：
F＝πR 2·W·Δt
其中：R为该拉杆的截面半径；Δt为该拉杆上接收的回波时间差；W为与拉杆材质相关的常量。
本发明即可以用于在线注塑机型腔内压力的实时检测，也可以用于某一状态下注塑机型腔内压力的检测。作为优选，在注塑成型过程中连续向每个拉杆发射超声波，实时显示注塑成型过程中注塑机型腔内部平均压力。采用该技术方案，可以针对在线的注塑过程对腔内压力进行检测。
作为一种选择，在检测前，针对同种材质的拉杆，在不同锁模力下，利用锁模力传感器检测得到多组σ-Δt数据，利用得到的多组σ-Δt数据，拟合出一条过原点的σ-Δt直线，求得该直线的斜率，得到所述的W，其中σ为拉杆上的应力，为纵坐标；Δt为超声波回波的时间差，为横坐标。其中拉杆上的应力σ可由锁模力传感器直接检测得到；所述超声波回波的 时间差Δt可由超声波采集单元采集得到。
作为优选，所述拉杆为四个。
作为优选，每个拉杆的一端均设有一个超声探头。
作为优选，实时输出时间与注塑机型腔内部平均压力曲线。
作为优选，超声波回波信号记录频率高于或等于20Hz。
一种通过超声波手段间接测量注塑机型腔压力的装置，包括：
安装于拉杆一端的超声探头，该超声探头针对拉杆轴向安装；
用于控制超声信号并接收超声回波的超声波采集单元；
计算和输出单元，对采集的超声回波信号进行处理，输出注塑机型腔内部平均压力结果，所述计算和输出单元采用如下公式得到注塑机型腔内部平均压力P：
P＝πR 2·W·Δt total/A
其中：R为拉杆的截面半径；Δt total为所有拉杆上接收的回波时间差总和；W为与拉杆材质相关的常量；A为型腔在垂直于拉杆轴向的平面的投影面积。
作为优选，所述的超声探头为四个；所述的超声波采集单元具有四个超声波采集通道；所述计算和输出单元为计算机。
作为优选，所述的超声波采集单元为超声波采集卡；所述计算机与一显示器相连用于实时显示所述注塑机型腔内部平均压力结果。
本发明利用超声波信号回波波形的变动，并基于材料声弹性理论的基础，通过分析超声信号，实现对材料成型过程压力的在线测量并用以过程的监测和诊断。
超声波是一种常见的无损检测的手段，可以直接检测众多的物理量，这其中就包括了材料的受力变形。注塑成形过程中，熔体对模腔的压力会以力的形式传递到注塑机动模板上，进而传递到注塑机的拉杆上，最终反应在拉杆的受力变形上。本发明通过超声波测量注塑机拉杆位置受力伸长的方法，进而表征注射成形过程中型腔内部的熔体压力。该方法具有稳定，无损，在线，低成本等诸多特点。
注射成形是一个间歇，循环的批次过程，在注射过程开始前，先是模 具被合上，此时注塑机油缸给模具一定的锁模力，由于反作用力的原因，锁模力也将通过注塑机的定模与动模，被传递到注塑机的拉杆上，此时注塑机的拉杆受到了一定的拉力，这个拉力可以通过超声检测方法测量得到。同理地，当注射过程开始后，高温高压的聚合物熔体被注射进入模具的型腔，此时模具的前模与后模也受到来自熔体的挤压力，并且这个挤压力与锁模力同理，最终也会传递到注塑机的拉杆上。本方法通过分析经过注塑机拉杆的超声波回波的位移情况，并基于声弹性效应的原理，获得拉杆上的受力，并依据物理受力模型，转换为模具型腔内部的压力。显然的，若在某一段时间内(通常是注射成形过程的一个周期)连续对超声波信号进行采集并且分析，便可以获得成型过程中模具型腔内压力的变化曲线。进一步分析获得的曲线，可以用于表征注射过程的各种信息，并能够为进一步的工艺优化提供依据。
区别于其他已有的研究方法，本发明的测量点未选择在注塑模具上，并独创性地提出通过检测注塑机拉杆受力实现对模具型腔压力间接表征。该方法不仅避免了测量本身对于模具的破坏和加工的成本，并且由于测量安装在注塑机机架上，因而可以适配几乎所有的模具，有利于该方法的普及。
利用本发明的检测方法和检测装置，即可实现某一状态下，型腔内压力的检测，也可以实现注塑过程中型腔中压力的实时检测，且检测过程简单，精确度高。
附图说明
图1为测量装置示意图；
图2a和图2b分别为某一次注射成形过程中型腔内部测点熔体的压力曲线与某一拉杆上超声波探头测得的拉杆受力变化曲线；
图3为具体实例中用到的模具内部的型腔结构示意图；
图4为注射过程中，使用不同V/P切换点所对应的不同曲线形状的示意图。
具体实施方式
本发明涉及到的测量方法可以应用于如图1所示的用于生产塑料产品 的注塑机中的型腔压力的检测，其主要由注射单元和模具单元组成，注射单元主要为注射料筒7，聚合物在注射料筒内部经螺杆熔融塑化，并被注射到模具内部型腔5。模具单元由定模板6、动模板3、模具4等关键部分组成，在注射开始之前，注塑机提供一定的动力，推动动模板使模具前后两部分闭合，动模板与定模板之间形成型腔，然后进行熔融聚合物的注射过程。
注塑过程中，由于注塑机的动模板受到推力，这也就意味着注塑机的另一部分即后模板8将受到相反的反推力，这部分反推力最终作用在注塑机拉杆2上(后模板8与拉杆一端固定，并置于机架的导轨上；拉杆另一端与定模板6相互固定，定模板6同时与机架固定)。
为了测量成型过程中拉杆受力的变化情况，装置在拉杆的末端安装了超声探头1，同时保证超声探头1正对拉杆轴向设置，能够沿拉杆轴向发射超声波。通常，一台注塑机往往有四根拉杆，分布在机器互相对称的四个角的位置上。为了测量的准确性，作为优选，在具体实施测量时，将安装四个超声探头分别在四根拉杆远离定模扳的末端。
超声探头的具体安装与耦合方式，可以使用常用的超声波耦合剂，例如甘油等，作为耦合介质，涂抹在探头表面和被测物之间，并另外采用一定的机械方法固定按压超声探头。此外，也可以使用胶水将探头直接粘合在被测物表面，既可实现固定也作为耦合剂使用。
超声探头的另一边，连接的是多通道的超声波采集设备(即超声波采集单元)，该设备应当具备对于超声信号多通道调整，控制，显示，并连续记录的功能，能够完成在某一时间段内对超声信号实现连续的采集。作为优选，连续采集的频率应当高于20Hz。作为优选，设备应当最好拥有四个超声波采集通道，满足四根拉杆位置实时同步的信号采集。超声波采集设备可选用现有的四通道超声波采集卡。
该测量方法的原理是通过对经过拉杆内部的超声波回波(或称作超声回波)进行分析，可以得到，不同时间点的回波存在不同的时移，依据材料声弹性理论，可以根据回波的位移(或者接收到超声波回波的时间差)，计算得到材料的受力情况。
在t＝0s时从超声波探头发出的超声波在注塑机的拉杆内部沿轴向以一定速度传播，在拉杆的另一端反射，最后超声回波被同一探头在t＝t 1接 收。当拉杆受到注塑机的锁模力时，拉杆由于受力而伸展。基于声弹性理论，超声波在连杆内的传播速度C σ发生变化，如公式1所述。这种情况下，回波在t＝t σ时刻被探头接收，满足：
Figure PCTCN2019106683-appb-000001
转化后得到：
Figure PCTCN2019106683-appb-000002
其中ρ 0，σ和C σ分别是拉杆的密度，拉杆的应力和该应力下超声波的传播速度。l，m和n是材料的Murnaghan常数，λ和μ是材料的Lamé常数。显然，假设σ＝0，公式(1)变成：
Figure PCTCN2019106683-appb-000003
转化后得到：
Figure PCTCN2019106683-appb-000004
因此，超声波的初始速度可以通过方程式(2)获得。通过将常数带入，计算
Figure PCTCN2019106683-appb-000005
可知，当σ在实验范围内(即实验中拉杆受到的应力的范围)时，
Figure PCTCN2019106683-appb-000006
的数值近似不变，可将式(1)简化为一次函数
Figure PCTCN2019106683-appb-000007
其中
Figure PCTCN2019106683-appb-000008
进一步的，联立式(2)和(3)，有：
(C 0-C σ)/C 0＝K·σ(4)
其中：
K＝[2lμ+λμ+(λ+μ)(4m+4λ+10μ)]/[2μ(3λ+2μ)(λ+2μ)]是材料的声弹性系数，是一种固有的材料参数，表征了拉杆的声学弹性。
另一方面，式(4)中C 0和C σ还存在以下的关系式：
C 0＝2l 0/t 0             (5)
C σ＝2l 0(1+σ/E)/t σ          (6)
其中t 0是在拉杆没有受到应力的情况下超声波的传播时间。与t 0类似，t σ表示在拉杆处于应力σ的情况下的传播时间。E为拉杆的弹性模量。没有压力时拉杆的自然长度是l 0，设定相对时间变化(t σ-t 0)/t 0，由式(4)-(6)，可得：
Figure PCTCN2019106683-appb-000009
因为Kσ＜＜1,因此公式可被简化为:
Figure PCTCN2019106683-appb-000010
其中
Figure PCTCN2019106683-appb-000011
是材料的属性常数.
拉杆上的应力可以通过公式(8)计算，但是系数K非常复杂并且由于五个材料常数l，m，n，λ和μ难以获得。因此，作为优选我们试图避免直接计算系数K，并定义：
Figure PCTCN2019106683-appb-000012
式(9)可以被简化为:
σ＝ks·Δt     (10)
然后，在锁模力测试计的帮助下，可以根据在不同锁模力情况下测量的数据得到系数ks(即常数W＝ks)。若我们在一系列的锁模力下，通过锁模力传感器测得了拉杆上的应力σ与超声波回波之间的Δt，将这些数据做散点图，拟合一条过零点的直线，这条直线的斜率便是系数ks。获得ks之后，就可以通过超声方法测量锁模力和型腔压力曲线。
在得到拉杆的受力之后，需要通过一定的物理模型，将拉杆上的受力转换为模具型腔内的压力。如图1所示，首先，在注射阶段开始之前，注塑机向模具提供锁模力。该锁模力施加在注塑机和注塑模具的拉杆上，因此在拉杆上产生应变。当注射阶段开始时，在某一时刻，塑料熔体将在模具的腔体上施加力F.当F增加时，该力F将逐渐抵消初始锁模力，然 后随着拉杆上的锁模力力增加并达到峰值。此时，一方面，可以通过计算拉杆上的力F rod：
F rod＝πR 2·σ total    (11)
同时根据式(10)，得到：
σ total＝ks·Δt total  (12)
其中σ total是四个拉杆的总应变，由安装在拉杆末端的四个超声波探头共同测量并计算得到，R为拉杆的截面半径；Δt total是所有拉杆受应力作用时超声波回波与拉杆自由时的超声回波的时间差的总和，即针对某一时同时发射的一组超声波，四个拉杆上的回波时间差总和；ks为上一步中得到系数。另一方面，模具型腔内部的平均压力用P表示。由于作用力与反作用的关系，F rod与型腔内熔体对模具的作用力F cavity相等，因此，型腔内部的平均压力P的计算公式如公式(13)所示：
P＝F cavity/A(13)
其中，A为型腔在垂直于拉杆轴向的平面的投影面积。
实际检测时，是将四个超声探头分别安装在注塑机四根拉杆的末端，安装的方法可以选择用胶水粘合或者用超声检测的耦合剂，并借助一定的外力(例如磁力表座)将超声探头按压在注塑机拉杆的末端。将信号线一端链接超声波探头，另一端链接超声波采集卡，接通电源，调整显示的波形，直到能够观察并连续记录到回波的信号。
在注塑机上安装好模具，将预先干燥好的注塑原料添加到注塑料斗中，设定好注塑螺杆的塑化温度，待温度达到设定值之后，打开注塑机电机，设定好合适的注射保压冷却等工艺参数，经过几次注射循环之后，待系统稳定之后，准备开始注射成形过程。首先开启超声波的采集记录命令，接着注塑机合模，注射，保压，冷却，储料，开模，顶出，然后停止超声波采集，并将记录的一段时间内的回波信号保存到本地，以便后续进一步分析处理，然后进行下一个生产与测量的循环。最后，将得到的回波信号数据进行处理，得到每个超声回波与参考回波(首个回波)的时间差(或超声波时移Δt)，结合声弹性理论，计算得到每个时间测点上对应拉杆的受力情况，并进一步整合，计算，最终得到模具型腔内部熔体的平均压力- 时间曲线。
整个过程中，可以调整生产过程中的工艺参数，例如注射压力，保压压力，锁模力等。这将影响到最终产品的性能质量，也将同时体现在超声波测量得到过程压力曲线。
具体实例：
该实例采用了直接进胶的方式，安装型腔结构为1mm厚的薄片的模芯。
为了直观体现测量方法的合理性，图2a给出了采用现有的传感器检测某一次注射成形过程中型腔内部测点熔体的压力曲线，图2b为本发明某一拉杆上超声波探头测得的拉杆受力变化曲线。我们可以看到，拉杆上的受力变化与模腔压力传感器测得的模腔压力变化有很好的吻合性，这也说明了超声波测量拉杆受力的这种方法可以用来测量注射过程模具型腔的压力变化情况。
实验中使用的超声波采集卡的采样频率是100MHz，最小的时间间隔Δt为10ns，即时间分辨率为10ns，换算到拉杆上的受力，其分辨率为1.25MPa左右，这对于拉杆受力较小时，曲线会出现锯齿状的轮廓，可以通过提高设备的采样频率，提高测量的精度。作为优选，我们建议设备的采样频率应不低于500MHz。
显然，若将拉杆上的受力转换到模具型腔内部的受力，F cavity与F rod应该是相同的。为了验证我们的测量方法和理论模型的准确性，我们选择了长度为200mm，宽度30mm，厚度1mm的片状型腔模具作为实验模具，对我们的模型进行了验证，实验中用到的模腔压力传感器安装位置以及模腔的形状如图3所示，
根据流体力学的知识，模具型腔内的平均压力可以简化为：
Figure PCTCN2019106683-appb-000013
我们选择了不同注塑参数下的两组数据来验证该模型。这两组数据分别通过两种不同材料LDPE和PP的注塑工艺获得。计算结果如表1所示。
表1
Figure PCTCN2019106683-appb-000014
表1中实验结果表明，测量的模腔受力和拉杆受力，相对误差小于5％。因此通过对比LDPE与PP测量结果的两组数据中模腔压力与拉杆受力的数值，证明了可以通过拉杆端受力的测量，实现模腔内平均压力的测量。
进一步的，我们提出的这种方法可以在合适的锁模力下表征注塑成形过程模腔压力的变化情况，进而实现注射成形过程的在线，并能够像模腔压力传感器一样在一定程度上预测注射产品的性能，同时这种方法成本相对较低。图4给出了注射过程中，不同V/P切换点(保压切换点)所对应的不同曲线形状。
显然，由于V/P切换时间导致的曲线的变化可以用于在线过程中V/P切换过早的检测。同理，这种方法还可以用于保压时间，保压压力，注射速度等多种影响到最终产品质量的参数的在线检测与诊断。

Claims (13)

  1. 一种通过超声波手段间接测量注塑机型腔压力的方法，其特征在于，包括：沿拉杆轴向，分别同时向每个拉杆发射超声波，检测每个拉杆上超声波回波与拉杆自由时的超声回波的时间差，根据如下公式得到注塑机型腔内部平均压力P：
    P＝πR 2·W·Δt total/A
    其中：R为拉杆的截面半径；Δt total为所有拉杆上接收的回波时间差总和；W为与拉杆材质相关的常量；A为型腔在垂直于拉杆轴向的平面的投影面积。
  2. 根据权利要求1所述的通过超声波手段间接测量注塑机型腔压力的方法，其特征在于，在注塑成型过程中连续向每个拉杆发射超声波，实时显示注塑成型过程中注塑机型腔内部平均压力。
  3. 根据权利要求1所述的通过超声波手段间接测量注塑机型腔压力的方法，其特征在于，其中所述W可由下述方法求得：在检测前，针对同种材质的拉杆，在不同锁模力下，利用锁模力传感器检测得到多组σ-Δt数据，利用得到的多组σ-Δt数据，拟合出一条过原点的σ-Δt直线，求得该直线的斜率，得到所述的W，其中σ为拉杆上的应力，为纵坐标；Δt为超声波回波的时间差，为横坐标。
  4. 根据权利要求1所述的通过超声波手段间接测量注塑机型腔压力的方法，其特征在于，所述拉杆为四个。
  5. 根据权利要求1所述的通过超声波手段间接测量注塑机型腔压力的方法，其特征在于，每个拉杆的末端均设有一个超声探头。
  6. 根据权利要求1所述的通过超声波手段间接测量注塑机型腔压力的方法，其特征在于，实时输出时间与注塑机型腔内部平均压力曲线。
  7. 根据权利要求1所述的通过超声波手段间接测量注塑机型腔压力的方法，其特征在于，超声波回波信号记录频率高于或等于20Hz。
  8. 一种通过超声波手段间接测量注塑机型腔压力的装置，其特征在于，包括：
    安装于拉杆一端的超声探头，该超声探头针对拉杆轴向安装；
    用于控制超声信号并接收超声回波的超声波采集单元；
    计算和输出单元，对采集的超声回波信号进行处理，输出注塑机型腔内部平均压力结果；
    所述计算和输出单元采用如下公式得到注塑机型腔内部平均压力P：
    P＝πR 2·W·Δt total/A
    其中：R为拉杆的截面半径；Δt total为所有拉杆上接收的回波时间差总和；W为与拉杆材质相关的常量；A为型腔在垂直于拉杆轴向的平面的投影面积。
  9. 根据权利要求8所述的通过超声波手段间接测量注塑机型腔压力的装置，其特征在于，所述的超声探头为四个；所述的超声波采集单元具有四个超声波采集通道；所述计算和输出单元为计算机。
  10. 根据权利要求8所述的通过超声波手段间接测量注塑机型腔压力的装置，其特征在于，所述的超声波采集单元为超声波采集卡；所述计算机与一显示器相连用于实时显示所述注塑机型腔内部平均压力结果。
  11. 一种通过超声波手段测量注塑机拉杆压力的方法，其特征在于，包括：沿拉杆轴向，分别同时向每个拉杆发射超声波，检测每个拉杆上超声波回波与拉杆自由时的超声回波的时间差，根据如下公式得到注塑机拉杆的总的拉力F rod：
    F rod＝πR 2·W·Δt total
    其中：R为拉杆的截面半径；Δt total为所有拉杆上接收的回波时间差总和；W为与拉杆材质相关的常量。
  12. 一种通过超声波手段测量注塑机拉杆压力的方法，其特征在于，包括：沿拉杆轴向，分别同时向每个拉杆发射超声波，检测每个拉杆上超声波回波与拉杆自由时的超声回波的时间差，根据如下公式得到注塑机任一拉杆的拉力F：
    F＝πR 2·W·Δt
    其中：R为该拉杆的截面半径；Δt为该拉杆上接收的回波时间差；W为与拉杆材质相关的常量。
  13. 根据权利要求11或12所述的通过超声波手段测量注塑机拉杆压力的方法，其特征在于，其中所述W可由下述方法求得：在检测前，针对同种材质的拉杆，在不同锁模力下，利用锁模力传感器检测得到多组σ-Δt数据，利用得到的多组σ-Δt数据，拟合出一条过原点的σ-Δt直线，求得该直线的斜率，得到所述的W，其中σ为拉杆上的应力，为纵坐标；Δt为超声波回波的时间差，为横坐标。
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