A method for quickly determining the quality of ultrasonic welded joints in plastic sheets without energy-conducting ribs

Abstract

This invention discloses a method for rapidly judging the quality of ultrasonic welded joints of plastic sheets without energy-conducting ribs, comprising the following steps: setting a high-frequency displacement sensor on the top of the pneumatic shaft of an ultrasonic welding machine; performing ultrasonic welding using the ultrasonic welding machine, with the high-frequency displacement sensor collecting displacement data of the welding head; processing the collected data to obtain the vibration and downward displacement of the welding head during the welding process, and determining five stages in the welding process; calculating the joint quality judgment coefficient X based on the duration of the steady-state melting stage and the average amplitude of the welding head; when the welding does not exhibit a collapse stage and the joint quality judgment coefficient X is greater than a set strength threshold, it is considered a qualified welded joint. This invention can predict whether the quality of the welded joint meets the standards by processing the data collected by the high-frequency displacement sensor during the welding process. Testing has shown that the prediction accuracy of this method exceeds 99.8%, indicating that welding quality can be controlled in real time during the welding process.

Description

Technical Field

[0001] This invention relates to a method for rapidly determining the quality of ultrasonic welded joints in plastic sheets without energy-conducting ribs. Background Technology

[0002] Ultrasonic welding is characterized by high efficiency, cleanliness, low cost, flexible operation, and ease of automation, making it considered the most suitable method for welding thermoplastic materials. While ultrasonic welding without energy guides can minimize production costs and improve efficiency, it presents two major problems: ① The surface finish of the unprocessed sheet material is lower, leading to instability in the welding process and joint quality; ② Without the guidance of energy guides, welding energy is easily dissipated within the workpiece, causing overheating. When the temperature reaches the material's thermal decomposition temperature, thermal decomposition occurs, forming porous regions and resulting in decreased joint strength.

[0003] To ensure that the strength of the welded joints of thermoplastic material components meets the requirements after welding, a method must be adopted for judgment.

[0004] The current method for determining this is:

[0005] (1) Tensile test. Each specimen is stretched on a tensile testing machine until the joint breaks, and the tensile strength is used to evaluate the performance of the welded joint. The above methods are time-consuming and require more manpower and resources; moreover, they are destructive and difficult to apply to the welding quality judgment of large batches and complex parts.

[0006] (2) Based on the principle of sound propagation, Li et al. derived expressions for the energy absorbed by the workpiece / workpiece interface in four stages, and then correlated them with the welding process signal curve to propose a method for predicting weld quality. Li et al. used an Artificial Neural Network (ANN) algorithm to predict the quality of CFRTP ultrasonic welding without energy-conducting ribs. SUN et al. used a Bayesian regularized neural network (BRNN) to predict the quality level of CF / PA6 ultrasonic welded joints. Although the above methods can determine whether the joint is qualified, their accuracy is not high. Summary of the Invention

[0007] To address the aforementioned technical problems, this invention provides a method for rapidly determining the quality of ultrasonic welded joints in plastic sheets without energy-conducting ribs, characterized by a simple algorithm and high accuracy.

[0008] The technical solution of this invention to solve the above problems is: a method for rapidly judging the quality of ultrasonic welding joints of plastic sheets without energy-conducting ribs, comprising the following steps:

[0009] (1) Install a 400KHz high-frequency displacement sensor at the top of the pneumatic shaft of the ultrasonic welding machine and connect the signal output end of the high-frequency displacement sensor to the computer to measure the real-time vibration data of the welding head during the subsequent welding process.

[0010] (2) Place the carbon fiber reinforced nylon 66 composite sheet to be welded on the mold of the ultrasonic welding machine;

[0011] (3) Set welding parameters and use an ultrasonic welding machine to perform ultrasonic welding of carbon fiber reinforced nylon 66 composite material. At the same time, record the displacement data of the welding head during the welding process using a high-frequency displacement sensor and a computer.

[0012] (4) After welding is completed, the collected data is processed to obtain the vibration and downward displacement of the welding head during the welding process, and the five stages T1 to T5 in the ultrasonic welding process are determined.

[0013] (5) Process the data to obtain the duration of the T3 stage and the average amplitude of the welding head during the welding process;

[0014] (6) Substitute the obtained data into the formula for the joint quality judgment coefficient X to calculate the result;

[0015] (7) Set a strength threshold based on the relationship between the joint quality judgment coefficient X and the tensile strength of the welded joint, and compare the calculation results with the strength threshold. When the welding has not entered the collapse stage and the joint quality judgment coefficient X is greater than the set strength threshold, it is a qualified welded joint.

[0016] In the above method for quickly judging the quality of ultrasonic welded joints of plastic sheets without energy-conducting ribs, in step (1), the ultrasonic welding machine model is KZH-2026, the rated power of the ultrasonic welding machine is 2.6kW, the rated frequency is 20kHz, and the rated amplitude is 25μm; the welding mode is selected as time mode, and the preset delay time and holding pressure time are 2s and 3s respectively before welding; 7075 aluminum alloy welding heads with diameters of 9mm, 12mm, and 18mm are used for welding.

[0017] In the above method for quickly determining the quality of ultrasonic welded joints of plastic sheets without energy-conducting ribs, in step (2), the carbon fiber reinforced nylon 66 composite material is 4mm thick, and the overlap size of the upper and lower workpieces is 25×38mm. 2 .

[0018] The method for quickly judging the quality of ultrasonic welded joints of plastic sheets without energy-conducting ribs, in step (4), the determination of the five stages in the ultrasonic welding process is as follows:

[0019] T1 stage, also known as the Coulomb friction stage: At the beginning of welding, the plate begins to vibrate under the action of ultrasonic waves from the welding head, and the amplitude gradually increases. The heat source mainly comes from the Coulomb friction between the two plates, and the plate does not melt.

[0020] T2 stage, i.e., the unsteady melting stage: Under the action of ultrasound, the plate at the welding surface heats up and begins to soften, some micro-protrusions melt, viscoelastic heat dissipation gradually replaces Coulomb friction as the heat source, the amplitude of the welding head begins to decrease, and the displacement shows unstable changes.

[0021] The T3 stage, also known as the steady-state melting stage, is when the micro-protrusions on the welding surface are completely melted, and the welding enters a stage where the welding area steadily increases. At this time, the amplitude of the welding head is stable, and the displacement of the welding head increases steadily over time. The steady-state melting stage is the key stage that determines the welding quality.

[0022] T4 stage, also known as the collapse stage: As welding progresses, the plate temperature continues to rise, and the plate is damaged and collapses under the action of the welding head. The amplitude of the welding head is stable, but the displacement increases at a faster rate than in the T3 stage. This stage will damage the welded joint, and welding time should be avoided to prevent it from entering this stage.

[0023] T5 stage, also known as the solidification stage: After welding stops, the welding head stops vibrating, and the plate temperature begins to drop.

[0024] In the above method for quickly judging the quality of ultrasonic welded joints of plastic sheets without energy-conducting ribs, the formula for calculating the average amplitude A of the weld head during the welding process in step (5) is as follows:

[0025]

[0026] In the formula A i =(Y imax -Y imin ) / 2 represents the amplitude of the welding head during a certain vibration period, Y i For a given vibration cycle, the welding head displacement, Y imax and Y imin These are the maximum and minimum values ​​of the weld head displacement for a certain vibration cycle, respectively, and n is the number of vibration cycles in stage T3.

[0027] The above-mentioned method for quickly judging the quality of ultrasonic welded joints of non-conductive plastic sheets is based on the heat generation mechanism of ultrasonic welding of polymer materials. When the welding enters the T3 stage, the heat generation Q of the welded joint is determined by viscoelastic dissipation. α is the hammer impact coefficient, ε0 is the strain amplitude, ω is the angular frequency, and E″ is the material loss modulus; it can be seen that the viscoelastic dissipation is proportional to the square of the welding head amplitude, and the amount of energy during welding determines the welding area; therefore, in step (6), the formula for calculating the joint quality judgment coefficient X is:

[0028]

[0029] In the formula, t is the duration of stage T3, and a is the average amplitude of the welding head vibration during ultrasonic welding.

[0030] The beneficial effects of this invention are as follows:

[0031] 1. Rapid assessment of weld joint quality. By processing data collected by high-frequency sensors during the welding process, it is possible to predict or detect whether the strength of the weld joint meets the standards. This allows for timely control of welding quality and prompt detection and resolution of defects.

[0032] 2. High acquisition frequency ensures high accuracy. The ultrasonic welding machine is equipped with a high-frequency sensor with an acquisition frequency of up to 400kHz. It can collect multiple sets of data during the welding process, thereby improving the accuracy of the present invention in predicting the strength of ultrasonic welded joints without energy-conducting ribs.

[0033] 3. High degree of automation. This invention uses high-frequency sensors to collect real-time displacement data of the welding head during the welding process. The data collection process is fully automated and requires no human intervention, enabling rapid judgment and recording of welding process data. This facilitates automated welding, improves production efficiency, and contributes to the lightweighting of automobiles in my country.

[0034] 4. The detection principle is simple and easy to understand, and the operation process is convenient and quick. This invention proposes a joint quality judgment coefficient X based on whether a T4 stage occurs during the welding process and the duration of the average amplitude of the weld head during the T3 stage. When the X value of the ultrasonically welded joint reaches a certain range based on the collected data, the welded joint is judged to meet the quality standards. Compared to existing methods, this invention requires no complex operations, is more convenient and quick, and eliminates the need for destructive strength testing of the joint, facilitating widespread adoption. Attached Figure Description

[0035] Figure 1 This is the overall flowchart of the present invention.

[0036] Figure 2 This is a schematic diagram of the welding apparatus of the present invention.

[0037] Figure 3 This is a schematic diagram showing the displacement of the welding head in five stages during the ultrasonic welding process of carbon fiber reinforced nylon 66 composite material.

[0038] Figure 4 The graph shows the effect of welding time on the tensile strength and weld area of ​​the ultrasonic welded joint of carbon fiber reinforced nylon 66.

[0039] Figure 5 This is a schematic diagram showing the relationship between the tensile strength of a welded joint and the joint quality judgment coefficient X. Detailed Implementation

[0040] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0041] like Figure 1 As shown, a method for quickly determining the quality of ultrasonic welded joints of plastic sheets without energy-conducting ribs includes the following steps:

[0042] (1) Install a 400kHz high-frequency displacement sensor on the top of the pneumatic shaft of the ultrasonic welding machine of model KZH-2026, and connect the signal output end of the high-frequency displacement sensor to the computer to measure the real-time vibration data of the welding head during the subsequent welding process.

[0043] (2) Place the plastic sheet without pre-placed energy-conducting ribs to be welded—a carbon fiber reinforced nylon 66 composite sheet—on the mold of the ultrasonic welding machine. The dimensions of the carbon fiber reinforced nylon 66 composite sheet are 132×38×4mm. 3 The overlap size of the upper and lower workpieces is 25×38mm. 2 .

[0044] (3) Set welding parameters and use an ultrasonic welding machine to perform ultrasonic welding of carbon fiber reinforced nylon 66 composite material. The welding head is a 12mm 7075 aluminum alloy welding head. The rated power of the ultrasonic welding machine is 2.6kW, the rated frequency is 20kHz, and the rated amplitude is 25μm. Select the welding mode as time mode and preset the delay time and holding pressure time to 2s and 3s respectively before welding. Taking 4mm thick carbon fiber reinforced nylon 66 composite material as an example, the welding time is 2.7s. After the welding starts, the welding head displacement data is recorded by a high-frequency displacement sensor and a computer during the welding process.

[0045] (4) After welding is completed, the collected data is processed to obtain the vibration and downward displacement of the welding head during the welding process, and the five stages T1 to T5 of the ultrasonic welding process are determined, such as... Figure 3 As shown:

[0046] The T1 stage, also known as the Coulomb friction stage, begins when the plate material starts to vibrate under the ultrasonic waves of the welding head, with the amplitude gradually increasing. The heat source mainly comes from the Coulomb friction between the two plates, and the plate material does not melt. This stage is characterized by large and unstable amplitude on the welding head displacement curve.

[0047] In stage T2, the unsteady melting stage, under the action of ultrasound, the plate at the welding surface heats up and begins to soften, and some micro-protrusions melt. The viscoelastic heat dissipation gradually replaces Coulomb friction as the heat source, the amplitude of the welding head begins to decrease, and the displacement shows unstable changes. In this stage, the welding head displacement curve shows that the amplitude begins to decrease, but it is still unstable.

[0048] The T3 stage, or steady-state melting stage, is characterized by the complete melting of the micro-protrusions on the welding surface, marking the beginning of a stable increase in the welding area. At this stage, the weld head amplitude stabilizes, and the weld head displacement increases steadily over time. The steady-state melting stage is crucial in determining weld quality. This stage is reflected in the weld head displacement curve as a relatively stable amplitude, with the smooth curve showing a steady upward slope.

[0049] Stage T4, also known as the collapse stage: As welding progresses and the plate temperature continues to rise, the plate is damaged and collapses under the action of the welding head. The amplitude of the welding head is stable, but the rate of increase in displacement is greater than that in stage T3. This stage can damage the welded joint, and welding time should be avoided to prevent it from entering this stage. In this stage, the amplitude of the welding head displacement curve is relatively stable, but the displacement of the smooth curve rises rapidly compared to stage T3.

[0050] T5 stage, also known as the solidification stage: After welding stops, the welding head stops vibrating, and the plate temperature begins to drop. This stage is represented by the cessation of vibration on the welding head displacement curve.

[0051] like Figure 3 As shown, Figure 3 The darker gray curve in the figure is the welding head displacement curve, which is obtained by smoothing the real-time vibration curve of the welding head using the 200 adjacent value averaging method.

[0052] Depend on Figure 3 , Figure 4 It can be seen that the welded joint has the highest tensile strength in the T3 steady-state melting stage. This stage is characterized by a stable welding process and a continuously increasing weld area. This stage is most closely related to the joint strength. However, once the welding enters the T4 stage, the joint collapses, and the strength decreases significantly. Therefore, the optimal welding time for ultrasonic welding of thermoplastic materials (using 4mm thick carbon fiber reinforced nylon 66 composite material as an example) should be within the T3 stage.

[0053] (5) Determine the T3 process, namely the duration of steady-state melting process and the average amplitude of the welding head during the welding process, based on the recorded data.

[0054] The formula for calculating the average amplitude 'a' of the welding head during welding is:

[0055]

[0056] In the formula a i =(y imax -y imin ) / 2 represents the amplitude of the welding head during a certain vibration period, y i For a given vibration cycle, the welding head displacement, y imax and y imin These are the maximum and minimum values ​​of the weld head displacement for a certain vibration cycle, respectively, and n is the number of vibration cycles in stage T3.

[0057] (6) Substitute the obtained data into the formula for the joint quality judgment coefficient X to calculate the result.

[0058] According to the heat generation mechanism of ultrasonic welding of polymer materials, when the welding enters the T3 stage, the heat generation Q of the weld joint is determined by viscoelastic dissipation. α is the hammer impact coefficient, ε0 is the strain amplitude, ω is the angular frequency, and E″ is the material loss modulus; it can be seen that the viscoelastic dissipation is proportional to the square of the welding head amplitude, and the amount of energy during welding determines the welding area; therefore, in step (6), the formula for calculating the joint quality judgment coefficient X is:

[0059]

[0060] In the formula, t is the duration of stage T3, and a is the average amplitude of the welding head vibration during ultrasonic welding.

[0061] (7) Set a strength threshold based on the relationship between the joint quality judgment coefficient X and the tensile strength of the welded joint, and compare the calculation results with the strength threshold. When the welding has not entered the collapse stage and the joint quality judgment coefficient X is greater than the set strength threshold, it is a qualified welded joint.

[0062] Extensive experiments have determined the relationship between the tensile strength of welded joints and the joint quality judgment coefficient X. Figure 5 As shown. There is generally a linear correlation between the tensile strength of a welded joint and the joint quality assessment coefficient X. According to the requirements of automobile manufacturers, the acceptable tensile strength for thermoplastic material parts during assembly is 4.5 kN; therefore, the strength threshold is set at 4.5 kN. This relationship can quickly detect the tensile strength of ultrasonically welded joints of 4 mm thick carbon fiber reinforced nylon 66 composite materials. The overall idea is to propose a joint quality assessment coefficient X and experimentally determine the relationship between X and the tensile strength of the welded joint, thereby determining the range of the joint quality assessment coefficient X to predict the tensile strength of the welded joint. This theory can also be used to predict the tensile strength of ultrasonically welded joints of other thermoplastic materials. (Refer to...) Figure 5 It can be concluded that when the joint quality judgment coefficient X of one of the welded joints is greater than 0.028, it is a qualified welded joint.

Claims

1. A method for rapidly determining the quality of ultrasonically welded joints of plastic sheets without energy-conducting ribs, characterized in that... It can quickly and accurately predict whether the quality of welded joints meets the standards, including the following steps: (1) Install a 400KHz high-frequency displacement sensor at the top of the pneumatic shaft of the ultrasonic welding machine and connect the signal output end of the high-frequency displacement sensor to the computer to measure the real-time vibration data of the welding head during the subsequent welding process. (2) Place the carbon fiber reinforced nylon 66 composite plate to be welded on the mold of the ultrasonic welding machine; (3) Set welding parameters and use an ultrasonic welding machine to perform ultrasonic welding of carbon fiber reinforced nylon 66 composite material. At the same time, record the displacement data of the welding head during the welding process using a high-frequency displacement sensor and a computer. (4) After welding is completed, the collected data is processed to obtain the vibration and downward displacement of the welding head during the welding process, and the five stages T1 to T5 in the ultrasonic welding process are determined. (5) Process the data to obtain the duration of the T3 stage and the average amplitude of the welding head during the welding process; (6) Substitute the obtained data into the formula for the joint quality judgment coefficient X to calculate the result; The formula for calculating the joint quality judgment coefficient X is: ； In the formula, t is the duration of stage T3. This represents the average amplitude of the welding head vibration during ultrasonic welding. (7) Set a strength threshold based on the relationship between the joint quality judgment coefficient X and the tensile strength of the welded joint, and compare the calculation results with the strength threshold. When the welding has not entered the collapse stage and the joint quality judgment coefficient X is greater than the set strength threshold, it is a qualified welded joint.

2. The method for rapidly determining the quality of ultrasonic welded joints of non-conductive plastic sheets according to claim 1, characterized in that: In step (1), the ultrasonic welding machine model is KZH-2026, the rated power of the ultrasonic welding machine is 2.6 kW, the rated frequency is 20 kHz, and the rated amplitude is 25 μm; the welding mode is selected as time mode, and the preset delay time and holding time are 2 s and 3 s respectively before welding; 7075 aluminum alloy welding heads with diameters of 9 mm, 12 mm, and 18 mm are used for welding.

3. The method for rapidly determining the quality of ultrasonic welded joints of non-conductive plastic sheets according to claim 1, characterized in that: In step (2), the carbon fiber reinforced nylon 66 composite material is 4 mm thick, and the overlap size of the upper and lower workpieces is 25 × 38 mm. 2 .

4. The method for rapidly determining the quality of ultrasonic welded joints of non-conductive plastic sheets according to claim 1, characterized in that: In step (4), the five stages of the ultrasonic welding process are determined as follows: T1 stage, also known as the Coulomb friction stage: At the beginning of welding, the plate begins to vibrate under the action of ultrasonic waves from the welding head, and the amplitude gradually increases. The heat source mainly comes from the Coulomb friction between the two plates, and the plate does not melt. T2 stage, i.e., the unsteady melting stage: Under the action of ultrasound, the plate at the welding surface heats up and begins to soften, some micro-protrusions melt, viscoelastic heat dissipation gradually replaces Coulomb friction as the heat source, the amplitude of the welding head begins to decrease, and the displacement shows unstable changes. The T3 stage, also known as the steady-state melting stage, is when the micro-protrusions on the welding surface are completely melted, and the welding enters a stage where the welding area steadily increases. At this time, the amplitude of the welding head is stable, and the displacement of the welding head increases steadily over time. The steady-state melting stage is the key stage that determines the welding quality. T4 stage, also known as the collapse stage: As welding progresses, the plate temperature continues to rise, and the plate is damaged and collapses under the action of the welding head. The amplitude of the welding head is stable, but the displacement increases at a faster rate than in the T3 stage. This stage will damage the welded joint, and welding time should be avoided to prevent it from entering this stage. T5 stage, also known as the solidification stage: After welding stops, the welding head stops vibrating, and the plate temperature begins to drop.

5. The method for rapidly determining the quality of ultrasonic welded joints of non-conductive plastic sheets according to claim 4, characterized in that: In step (5), the average amplitude of the welding head during the welding process The calculation formula is: ； In the formula The amplitude of the welding head during a certain vibration cycle. For a given vibration cycle, the welding head displacement is... and These represent the maximum and minimum values ​​of the welding head displacement, respectively, for a given vibration cycle. This represents the number of oscillation cycles in stage T3.

6. The method for rapidly determining the quality of ultrasonic welded joints of non-conductive plastic sheets according to claim 4, characterized in that: According to the heat generation mechanism of ultrasonic welding of polymer materials, when the welding enters the T3 stage, the heat generation Q of the weld joint is determined by viscoelastic dissipation. , ε is the impact coefficient, ε0 is the strain amplitude, and ω is the angular frequency. Let be the material loss modulus; from this, we can see that the viscoelastic dissipation is proportional to the square of the welding head amplitude, and the amount of energy during the welding process determines the welding area.

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