Injection molding speed and pressure real-time monitoring method based on input electric energy

Abstract

The invention discloses an injection molding speed and pressure real-time monitoring method based on input electric energy. The injection molding speed and pressure real-time monitoring method comprises the following steps that 1, calculation parameters are determined according to process setting and injection parts; 2) measuring the power consumption in the injection molding process; (3) in the injection molding process, the injection action and standby energy consumption are ignored, that is, the electric energy consumption is equal to the energy consumption in the injection process; (4) based on the law of conservation of energy, the energy of all parts in the injection molding process is superposed, and the injection speed v in the injection molding process is obtained; 5) calculating injection pressure p according to the energy consumption and the injection speed v; compared with the prior art, the method has the advantages that the number of needed devices is small, operation is easy, measurement and real-time feedback are easy, and digitization and automation are easy to achieve.

Description

Technical Field

[0001] The invention relates to the technical field of injection molding monitoring, and in particular to a real-time monitoring method for injection molding speed and pressure based on input electrical energy. Background Art

[0002] Injection molding is currently the most effective manufacturing method for polymer products, occupying a large market share. Its principle is to inject molten polymer materials into the mold under pressure, and then cool and shape the product. During the injection process, the pressure, speed and temperature of the polymer melt have a huge impact on the final molding quality.

[0003] Among them, speed and pressure are two mutually coupled but relatively independent process parameters. Different combinations of injection speed and injection pressure have a great impact on the shape, strength and stability of the product. However, the production of injection molding equipment is a black box process, and it is impossible to directly obtain process changes and fluctuations in the production process, and it is impossible to timely discover and deal with abnormal situations in the production process. At the same time, in the mold adjustment stage in the early stage of production, the difficulty of mold adjustment, long cycle and poor effect often occur in actual production because of the lack of understanding of changes in the production process.

[0004] Injection molding production status monitoring, especially speed and pressure monitoring during the production process, is one of the key technologies to assist injection molding production. Therefore, real-time monitoring of injection speed and pressure of injection molding plays an important role in improving product quality, stabilizing product production process, improving initial investment difficulty and optimizing production abnormality adjustment. If it is further controlled on this basis, the entire production process can be better optimized, which has important practical significance in engineering.

[0005] At present, the commonly used method for monitoring the production process is mainly obtained by adding various sensors to the equipment and the mold. Displacement sensors and pressure sensors are installed at the tail end of the injection screw, or corresponding sensors and ultrasonic sensors are installed on the mold side to analyze the process changes and molding status during the molding process. This method is the most direct method. However, the installation of sensors is usually accompanied by the disassembly and secondary processing of the equipment. At the same time, in order to measure the status in the mold cavity, it is very likely to affect the surface of the mold cavity, seriously affecting the surface quality of the product. Moreover, some sensors have strict usage standards and rules, which increases the learning cost of production practitioners. While the change in electrical energy reflects the changes of the entire equipment during the production cycle, it also meets the two requirements of equipment external connection and easy operation, solves the difficulties caused by the above monitoring methods, and is an ideal process monitoring signal.

[0006] In the prior art, for example, Diao Simian (Mold Industry, Injection Molding Process Detection Method Based on Injection Molding Machine Screw Position and Pressure Curve, 2022, 48(2): 1-7) disclosed a method based on the injection molding machine screw position and pressure signal, using a nonlinear data dimensionality reduction method and a neural network to measure the process changes of the molding process and predict the quality of the molded product; this method is simple and rapid, but relatively difficult.

[0007] Li Dong (Journal of Fujian Engineering, Research on Injection Molding Process Parameters Based on Detection Technology, 2014, 12(3): 246-249) disclosed a method for monitoring the molding process and analyzing the influence of process parameters. The maximum temperature in the mold is measured by a temperature sensor, and the order of influence of process parameters on the maximum temperature of the mold cavity is analyzed through orthogonal experiments, thereby analyzing the influence of process parameters. This method has poor accuracy. Summary of the invention

[0008] The present invention aims at the problems existing in various sensor methods for real-time monitoring of speed and pressure changes in the injection molding production process, and proposes a real-time monitoring method for injection molding speed and pressure based on input electric energy based on the law of conservation of energy and electric energy as a medium. The present invention is suitable for production process monitoring of electric injection molding machines.

[0009] The technical solution of the present invention is as follows:

[0010] A method for real-time monitoring of injection molding speed and pressure based on input electrical energy comprises the following steps:

[0011] Step 1) determining calculation parameters according to process settings and injection parts; wherein the calculation parameters related to process settings include melt temperature and material property parameters; the calculation parameters related to injection parts include various equipment parameters (i.e., machine parameters in the following calculation process);

[0012] Step 2) measuring the power consumption during the injection molding process;

[0013] Step 3) During the injection molding process, the energy consumption for injection action and standby is ignored, that is, the power consumption is equal to the energy consumption of the injection process;

[0014] Step 4) Based on the law of conservation of energy, the energy of each part in the injection molding process is superimposed to obtain the injection speed v in the injection molding process;

[0015] Pdt=E inj +E melt +E s +Q inj +Q hf +Q hj

[0016] Among them, E inj、E melt 、E s , Q inj , Q hf and Q hj They are injection kinetic energy, melt kinetic energy, screw kinetic energy, injection temperature rise loss, along-the-line loss and local loss;

[0017] Step 5) Calculate the injection pressure p from the energy consumption and the injection speed v;

[0018] Pdt=v·p

[0019] Wherein, v is the injection speed, m / s; p is the injection pressure.

[0020] Furthermore, during the injection molding process, only the energy in the injection direction is considered and the work done by gravity during the production process is ignored; the injection kinetic energy E is obtained inj , melt kinetic energy E melt and screw kinetic energy E s The calculation formula is as follows:

[0021]

[0022] In the formula, ρ m is the density of molten polymer, V i is the volume of polymer melt at the moment of injection, v inj is the velocity of the polymer melt at the nozzle, ρ s is the screw density, V s is the screw volume.

[0023] Furthermore, according to the pressure change formula and temperature rise formula of the tapered circular pipe flow channel, the energy consumption caused by temperature rise is Q inj The calculation formula is as follows:

[0024]

[0025] Where n is the non-Newtonian index, η is the melt viscosity, v is the injection speed, l is the length of the pipe section, and d is l is the diameter of the large end of the tapered tube, d s is the diameter of the small end of the tapered circular tube, and d is the equivalent diameter of the nozzle;

[0026] According to the selected material parameters and the melt temperature of the injection section, the melt viscosity η is calculated as follows;

[0027]

[0028] Where η is the melt viscosity, η0 is the zero shear viscosity, is the shear rate, τ* is the critical stress level when it turns into shear thinning, n is the non-Newtonian index, T is the temperature, T* is the glass transition temperature, A1, A2 and D1 are material coefficients.

[0029] Furthermore, according to the loss along the way and the local loss, the energy consumption of the loss along the way and the energy consumption of the local loss are calculated;

[0030] For the loss along the way and local loss, only the instantaneous energy of the melt passing through the nozzle is considered;

[0031] Loss along the way f The calculation formula is as follows;

[0032]

[0033] In the formula, h f is the loss along the way, λ is the resistance coefficient along the way, l is the length of the pipe section, d is the nozzle diameter, and g is the gravitational acceleration;

[0034] Local loss h j The calculation formula is as follows;

[0035]

[0036] In the formula, h j is the local loss, ζ is the local loss coefficient, v inj is the nozzle melt velocity, g is the gravitational acceleration;

[0037] Energy loss along the way Q hf and local energy loss Q hj The calculation formula is as follows:

[0038]

[0039] Where D is the screw diameter, h is f is the loss along the way, h v For local loss.

[0040] Compared with the prior art, the beneficial effects of the present invention are as follows: the present invention provides a new method for monitoring injection speed and pressure during injection molding, which requires fewer devices, is simple to operate, is easy to measure and provide real-time feedback, and is easy to realize digitization and automation. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 is a device connection diagram of the present invention;

[0042] Figure 2 It is a comparison curve diagram of the calculated power and the measured power in the present invention;

[0043] Figure 3It is a comparison curve diagram of the calculated injection speed and the measured injection speed in the present invention. DETAILED DESCRIPTION

[0044] In order to make the present invention more clearly understood, the present invention is further described below based on specific examples and drawings of the present invention.

[0045] A method for real-time monitoring of injection molding speed and pressure based on input electrical energy comprises the following steps:

[0046] Step 1) determining calculation parameters according to process settings and injection parts;

[0047] Step 2) Calculate the temperature compressibility coefficient β according to the selected material parameters according to formula (III): T , according to the material parameters and the melt temperature of the injection section, the melt viscosity η is calculated according to formula (IV);

[0048] Step 3) measuring the power consumption during the injection molding production process;

[0049] Step 4) According to the injection molding process parameters and equipment size parameters, calculate the loss along the way h according to formula (V) f According to formula (VI), the local loss h j ;

[0050] Step 5) Based on the law of energy conservation, the electrical energy consumption is equal to the energy consumption of the injection process, and the injection speed v is calculated according to formula (I);

[0051] Step 6) Calculate the injection pressure p according to the energy consumption and the injection speed v according to formula (II);

[0052] in:

[0053] Pdt=E inj +E melt +E s +Q inj +Q hf +Q hj (Ⅰ)

[0054] Pdt=v·p (Ⅱ)

[0055] Where P is the equipment power measured during the injection process, W; E inj 、E melt 、E s , Q inj , Q hf and Q hj They are injection kinetic energy, melt kinetic energy, screw kinetic energy, injection temperature rise loss, along-the-path loss and local loss, J; v is injection speed, m / s; p is injection pressure, Pa.

[0056] Among them, when actually calculating the electric energy and energy consumption, it is necessary to add two energy conversion efficiency coefficients k m and k s The two power conversion coefficients are the energy conversion efficiency of the motor and the energy conversion efficiency of the screw pushing the melt, which are determined by experience and are 0.8 and 0.7 respectively.

[0057] The isothermal compression coefficient of the molding process is calculated as follows:

[0058]

[0059] In the formula, β T is the isothermal compressibility coefficient, Pa -1 ; V is the volume of the polymer in the screw, m 3 ; V0 is the volume under reference pressure (standard atmospheric pressure), Pa; B and C are temperature-related material coefficients, dimensionless; the material coefficients are obtained by experimental fitting and determined by querying the process card.

[0060] The calculation results show that for conventional injection molding materials and their appropriate process parameter range, the isothermal compression coefficient is on the order of 10 -6 to 10 -8 Therefore, in actual calculations, the effects of pressure and temperature on the compression of polymer melts are ignored.

[0061] The viscosity of polymer melt at injection temperature is calculated as follows:

[0062]

[0063] Wherein, η is melt viscosity, Pa·s; η0 is zero shear viscosity, Pa·s; is the shear rate, s -1 ; τ* is the critical stress level when it turns into shear thinning, dimensionless; n is the non-Newtonian index, dimensionless; T is temperature, K; T* is the glass transition temperature, K; A1, A2 and D1 are material coefficients, dimensionless.

[0064] The calculation formula for the loss along the way caused by the resistance along the way encountered by the polymer melt in the barrel is as follows:

[0065]

[0066] Where λ is the drag coefficient along the way, dimensionless; l is the length of the pipe section, m; d is the nozzle diameter, m; g is the gravitational acceleration, m / s 2 ; v is the injection speed, m / s;.

[0067] Among them, the calculation method of the drag coefficient along the way is λ=64 / Re, Re is the Reynolds number. Since the viscosity of polymer melt is relatively high, when analyzing the flow of polymer fluid in the barrel, the default is laminar flow state. Therefore, the loss along the way can be calculated using formula (V).

[0068] The local loss caused by the local resistance of the polymer melt at the nozzle is calculated as follows:

[0069]

[0070] Where ζ is the local loss coefficient, dimensionless; v inj is the nozzle melt velocity, m / s.

[0071] The local loss coefficient is determined by the shape and size of the nozzle. The nozzle of the injection molding equipment is a tapered circular tube, and the coefficient can be determined by the center angle and the diameter of both sides.

[0072] In this embodiment, the power consumption during the injection molding production process is measured by sampling from external sensors, with an average frequency of 66.7 Hz; the injection molding machine's own sensor samples the injection speed pressure at 1000 Hz.

[0073] In this embodiment, the injection temperature is higher than the glass transition temperature of the melt, which meets the recommended molding process parameters of the material.

[0074] A method for real-time monitoring of injection molding speed and pressure based on input electrical energy, the specific method is as follows:

[0075] Based on the characteristics of the injection molding machine and experiments, it is found that the energy consumption in the production process is basically used for the injection action, while the standby energy consumption is extremely low. Based on the law of conservation of energy, the energy of each part of the production process is superimposed to obtain the injection speed v in the production process.

[0076] The energy in production includes: injection kinetic energy, melt kinetic energy, screw kinetic energy, injection temperature rise loss, along-the-way loss and local loss. Only the energy in the injection direction is considered and the work done by gravity in the production process is ignored.

[0077] The injection kinetic energy E inj , melt kinetic energy E melt and screw kinetic energy E s The calculation formula is as follows:

[0078]

[0079] In the formula, ρ m is the density of molten polymer, kg / m 3 ; V i is the volume of polymer melt at the moment of injection, m 3;v inj is the velocity of the polymer melt at the nozzle, m / s; ρ s is the screw density, kg / m 3 ; V s is the screw volume, m 3 .

[0080] It should be noted that the volume V of the polymer in the screw changes in real time with the injection time, and in the actual calculation, the calculation is realized by updating the screw distance; specifically, in the actual calculation process, it is realized by iterating the injection speed of the previous time period.

[0081] According to the pressure change formula and temperature rise formula of the tapered circular pipe flow channel, the energy consumption caused by temperature rise is Q inj The calculation formula is as follows:

[0082]

[0083] Where n is the non-Newtonian index, dimensionless; η is the melt viscosity, Pa·s; v is the injection speed, m / s; l is the length of the pipe section, m; d l is the diameter of the large end of the tapered circular tube, m; d s is the diameter of the small end of the tapered circular tube, m; d is the equivalent diameter of the nozzle, m.

[0084] According to the loss along the way and the local loss, the energy consumption of the loss along the way and the local loss are calculated; the energy consumption only considers the instantaneous energy of the melt passing through the nozzle, and the length of the pipe section is assumed to be 1mm;

[0085] Since it is difficult to calculate, we calculate this part of the kinetic energy under ideal conditions here and take it as the energy loss. Then the energy loss along the way is Q hf and local energy loss Q hj The calculation formula is as follows:

[0086]

[0087] Where D is the screw diameter, h is f is the loss along the way, h v For local loss.

[0088] By combining the above relationship and multiplying the input power with the motor energy conversion efficiency and the screw push melt energy conversion efficiency, the injection speed v during the molding process can be solved. The relationship between injection speed, injection pressure and power consumption is as follows:

[0089] Pdt=v·p

[0090] The injection pressure p can be calculated from the above formula and the injection speed v.

[0091] Example:

[0092] like Figure 1 As shown, it is a schematic diagram of the equipment connection of the present invention, including an injection molding machine 1, a voltage cable 2, a current cable 3, a power analyzer 4, a data transmission line 5 and a processing terminal 6. Since the power analyzer 4 has limited current processing capability, a current transformer must be added in practical applications.

[0093] The current and voltage of the three-phase power supply of the injection molding machine are measured independently, and the power in the production process is calculated by a power analyzer 4 and transmitted to a processing terminal 6 through a data transmission line 5. The data transmission line 5 adopts the RS-232 protocol.

[0094] The present invention compares the calculated power and the actual power calculated based on the injection pressure and injection speed during the injection molding process. Figure 2 As shown; after decoupling the injection speed, the injection pressure can be obtained at the same time.

[0095] Since the power consumption measured by the power analyzer 4 is from the start of injection molding, the time in the figure is calculated from the start of injection molding as 0s. In order to highlight the similarity of data in the molding process, the start time in the figure is not 0s.

[0096] The measurement method is as follows:

[0097] (1) Produce injection molded products.

[0098] Select appropriate molds, materials and supporting processes to ensure that the entire production process is free of abnormalities.

[0099] When conducting experiments, different combinations of process parameters are set based on uniform analysis to verify the universality and accuracy of the final results.

[0100] (2) Based on the experimental results and the internal data of the injection molding machine, the relationship between the energy consumption of the production process and the injection speed and injection pressure is analyzed.

[0101] (3) The accuracy of the following formula is verified by experimental results.

[0102] Pdt=v·p

[0103] Wherein, P is the equipment power measured during the injection process, W; v is the injection speed, m / s; p is the injection pressure, Pa.

[0104] Among them, the time differential dt is actually the sampling frequency interval.

[0105] The present invention provides a curve for real-time monitoring of the injection molding speed based on the input electrical energy during the injection molding process. Figure 3 As shown in the figure, by the formula Pdt = v·p, the pressure can be calculated while the speed is calculated.

[0106] The measurement method is as follows:

[0107] (1) Produce injection molded products.

[0108] Select appropriate molds, materials and supporting processes to ensure that the entire production process is free of abnormalities.

[0109] When conducting experiments, different combinations of process parameters are set based on uniform analysis to verify the universality and accuracy of the final results.

[0110] (2) Based on the experimental results and the internal data of the injection molding machine, the relationship between the energy consumption of the production process and the injection speed and injection pressure is analyzed.

[0111] (3) The accuracy of the following formula is verified by experimental results.

[0112] Pdt=E inj +E melt +E s +Q inj +Q hf +Q hj

[0113] Where P is the equipment power measured during the injection process, W; E inj 、E melt 、E s , Q inj , Q hf and Q hj They are injection kinetic energy, melt kinetic energy, screw kinetic energy, injection temperature rise loss, along-the-path loss and local loss, J.

[0114] Among them, the time differential dt is actually the sampling frequency interval.

[0115] Due to the relationship between the in-mold pressure and the V / P conversion relationship of the injection molding machine, large deviations occur at the end of the molding process.

[0116] The injection molding experiment used acrylonitrile, butadiene, and styrene terpolymers. After verification in a total of 48 groups of experiments, the accuracy of the injection start stage was on average higher than 90%, and the accuracy of the end stage was about 50%.

[0117] exist Figure 3It can be seen that when the injection time does not exceed 1.3s, the difference between the calculated injection speed and the actual injection speed does not exceed 10%, which shows that this method has high accuracy in the initial stage. The deviation in the latter part mainly comes from the pressure and temperature rise of the melt inside the cavity when the cavity is about to be filled. In actual production, most of the melt will enter the cavity in a very short time, and the latter part of the injection time is mostly used as compensation for the injection of the front part, which is similar to the pressure holding process. Accurate monitoring of the front part of the injection process can basically meet the injection requirements for production.

[0118] The above is only an application example of the present invention, and is not intended to limit the scope of the applicable samples. The materials that can be measured by the present invention do not need to be listed one by one here. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for real-time monitoring of injection molding speed and pressure based on input electrical energy, characterized in that: The steps include: Step 1) determining calculation parameters according to process settings and injection parts; wherein the calculation parameters related to the process settings include melt temperature and material property parameters; and the calculation parameters related to the injection parts include the parameters of the injection molding equipment itself; Step 2) measuring the power consumption during the injection molding process; Step 3) During the injection molding process, the energy consumption for injection action and standby is ignored, that is, the power consumption is equal to the energy consumption of the injection process; Step 4) Based on the law of conservation of energy, the energy of each part in the injection molding process is superimposed to obtain the injection speed v in the injection molding process; Pdt=E inj +E melt +E s +Q inj +Q hf +Q hj Among them, E inj 、E melt 、E s , Q inj , Q hf and Q hj They are injection kinetic energy, melt kinetic energy, screw kinetic energy, injection temperature rise loss, along-the-line loss and local loss; Step 5) Calculate the injection pressure p from the energy consumption and the injection speed v; Pdt=v·p Wherein, v is the injection speed, m / s; p is the injection pressure.

2. A method for real-time monitoring of injection molding speed and pressure based on input electrical energy according to claim 1, characterized in that: During the injection molding process, only the energy in the injection direction is considered and the work done by gravity during the production process is ignored; the injection kinetic energy E is obtained inj , melt kinetic energy E melt and screw kinetic energy E s The calculation formula is as follows: In the formula, ρ m is the density of molten polymer, V i is the volume of polymer melt at the moment of injection, v inj is the velocity of the polymer melt at the nozzle, ρ s is the screw density, V s is the screw volume.

3. The method for real-time monitoring of injection molding speed and pressure based on input electrical energy according to claim 1, characterized in that: According to the pressure change formula and temperature rise formula of the tapered circular pipe flow channel, the energy consumption caused by temperature rise is Q inj The calculation formula is as follows: Where n is the non-Newtonian index, η is the melt viscosity, v is the injection speed, l is the length of the pipe section, and d is l is the diameter of the large end of the tapered tube, d s is the diameter of the small end of the tapered circular tube, and d is the equivalent diameter of the nozzle; According to the selected material parameters and the melt temperature of the injection section, the melt viscosity η is calculated as follows; Where η is the melt viscosity, η0 is the zero shear viscosity, is the shear rate, τ* is the critical stress level when it turns into shear thinning, n is the non-Newtonian index, T is the temperature, T* is the glass transition temperature, A1, A2 and D1 are material coefficients.

4. The method for real-time monitoring of injection molding speed and pressure based on input electrical energy according to claim 1, characterized in that: Calculate the energy consumption of along-the-way loss and local loss based on along-the-way loss and local loss; For the loss along the way and local loss, only the instantaneous energy of the melt passing through the nozzle is considered; Loss along the way f The calculation formula is as follows; In the formula, h f is the loss along the way, λ is the resistance coefficient along the way, l is the length of the pipe section, d is the nozzle diameter, and g is the gravitational acceleration; Local loss h j The calculation formula is as follows; In the formula, h j is the local loss, ζ is the local loss coefficient, v inj is the nozzle melt velocity, g is the gravitational acceleration; Energy loss along the way Q hf and local energy loss Q hj The calculation formula is as follows: Where D is the screw diameter, h is f is the loss along the way, h v For local loss.

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