Ultrasonic vibration auxiliary spreading chopped fiber composite compression molding method and device

Abstract

The invention belongs to the technical field of hot press molding of chopped fiber reinforced composite materials, and discloses a compression molding method and device for a chopped fiber composite material with auxiliary spreading by ultrasonic vibration. In order to solve the problem that in the compression molding process of a chopped fiber reinforced thermoplastic composite material, chopped fiber sheets are unevenly laid, so that a prepared molded product is poor in mechanical property and unstable, the compression molding method and device for ultrasonic-assisted vibration laying of the chopped fiber reinforced thermoplastic prepreg are designed. Ultrasonic vibration in the in-plane direction is introduced, the sheet non-uniformity caused by the material scattering process is eliminated through the resonance effect of the ultrasonic vibration, and the compression molding exhaust process designed in the invention is subsequently matched, so that redundant gas in the material is eliminated, and the pore defect caused by pressure loss is made up; and a chopped fiber reinforced composite material compression molding product with good mechanical properties and high stability is prepared.

Description

Technical Field

[0001] The invention relates to the field of hot pressing molding of chopped fiber composite materials, and in particular to a method and device for molding chopped fiber composite materials with ultrasonic vibration-assisted laying. Background Art

[0002] Fiber-reinforced thermoplastic resin-based composites continue to replace metal components in aerospace, automotive manufacturing and other fields due to their advantages such as light weight, high strength, short molding cycle and good recyclability. They have become the preferred material for lightweight design in aerospace, automotive and sports fields. However, many metal components with complex geometric features have not been replaced due to the difficulty of molding. As a new type of thermoplastic composite material, short-fiber reinforced thermoplastic resin-based composites provide sufficient formability for creating parts with complex features due to the high fluidity and deformation ability brought by the reduced fiber length compared to continuous fibers. This greatly improves production efficiency and process flexibility, and reduces production costs.

[0003] Compression molding, the mainstream molding technology for chopped fiber reinforced thermoplastic resin-based composite materials, is a process in which prepreg (continuous fiber or chopped fiber) is placed in a molding mold and molded into special-shaped products under a certain temperature and pressure. It has the advantages of high production efficiency, high product dimensional accuracy, and excellent product performance. However, during the compression molding process of chopped fiber reinforced thermoplastic prepreg, the uniformity of the placement of the chopped fiber prepreg sheet directly determines the stability of the mechanical properties of the molded component. Chopped fiber reinforced thermoplastic resin-based composite materials prepared by traditional compression molding have difficulty in effectively controlling the distribution state of the prepreg sheet during the molding process, resulting in unstable quality and low mechanical properties of the molded products.

[0004] In response to the problems of unstable quality and low mechanical properties of existing compression molded products, some scholars have conducted research. The invention patent number of Zhao Yingnan et al. is CN 117698168A, and the invention name is "A method for hot pressing of carbon fiber composite materials based on ultrasonic assistance". The patent invented a method for compression molding of thermoplastic composite materials with the assistance of ultrasonic vibration. Although this method can improve the mechanical properties of thermoplastic resin-based composite molded components, it is only applicable to continuous fiber thermoplastic resin-based composite materials. In addition, the ultrasonic vibration is directly applied to the mold, and the application range of the ultrasonic vibration cannot be freely controlled, which also increases the service life of the mold and equipment. The article "Effects of fabrication processes and tape thickness on tensile properties of chopped carbon fiber tape reinforced thermoplastics" published by Yi Wan et al. in "Composites Communications" No. 22, 2020 describes a wet papermaking process to prepare a carbon fiber sheet molding material with a more uniform short chip structure distribution. The mechanical properties of the products molded with this sheet are improved. However, this method requires redistributing the chopped carbon fiber prepreg sheets in water, which can reduce the service life of the thermoplastic resin to a certain extent, damage the mechanical properties of the formed sheet, and the long initial preparation time affects the efficiency of product formation. Therefore, it is necessary to develop a compression molding laying method and device suitable for chopped fiber reinforced thermoplastic resin-based composite materials that comprehensively considers the service life of the molding mold and thermoplastic prepreg, as well as the compression molding efficiency, to achieve the production of molded products with high mechanical properties and high stability. Summary of the Invention

[0005] The purpose of the present invention is to solve the problem of uneven laying of chopped fiber prepregs during the current compression molding of chopped fiber reinforced thermoplastic resin-based composite materials, which leads to poor and unstable mechanical properties of the prepared molded products. A method and device for compression molding of chopped fiber composite materials with ultrasonic vibration-assisted laying is invented. By applying ultrasonic vibration in the in-plane direction to the chopped fiber reinforced thermoplastic resin-based prepreg inside the mold, the unevenness of the sheet caused by the laying process of the chopped prepreg is eliminated. In conjunction with the exhaust process in the subsequent compression molding process, most of the gas inside the molded product is eliminated, further reducing the defects of the molded product. This method comprehensively considers the service life of the compression molding mold and the thermoplastic prepreg and the influence of the adaptability of the chopped fiber reinforced thermoplastic resin-based composite material, thereby improving the mechanical properties and stability of the molded product.

[0006] The technical solution of the present invention:

[0007] A method for compression molding short-fiber composite materials with ultrasonic vibration-assisted paving comprises the following steps:

[0008] (1) Determine and weigh the required weight of the chopped fiber reinforced thermoplastic resin prepreg according to the volume of the expected molded product, divide the chopped fiber reinforced thermoplastic resin prepreg into several batches of ultrasonic vibration units of equal mass, and control the thickness of a single ultrasonic vibration unit to be between 10% and 50% of the thickness of the product;

[0009] (2) Clean the mold of the molded product and apply a release agent to its surface. A total of three coats of release agent are required, with an interval of 5 minutes between each coat.

[0010] (3) After the release agent is applied, a single ultrasonic vibration unit is manually laid flat in the mold of the molded product, and after determining the prepreg laying height, the corresponding ultrasonic vibration parameters and ultrasonic vibration path are selected; the short-cut fiber composite material molding device with ultrasonic vibration auxiliary laying is fixed on the work surface, the motor is started, the working plane of the ultrasonic vibration auxiliary device C is determined, and the ultrasonic vibration auxiliary device C is turned on to vibrate the uneven short-cut fiber reinforced thermoplastic resin prepreg in the mold of the molded product to be uniform;

[0011] (4) Repeat step (3) and place the ultrasonic vibration unit in batches and perform ultrasonic vibration until all batches of ultrasonic vibration units are placed in the mold of the molded product and vibrate evenly;

[0012] (5) Turn off the motor and remove the short fiber composite material molding device with ultrasonic vibration assisted laying, set the molding process temperature, pressure and holding time, and add exhaust action at the appropriate temperature;

[0013] (6) Start the molding machine and start executing the set process plan. After the process plan is completed, the molded product is demolded and post-processed to obtain a short-cut fiber reinforced thermoplastic resin composite product with low defects and high stability.

[0014] The selection principle of ultrasonic vibration parameters and ultrasonic vibration path in step (3) is:

[0015] ① When the prepreg placement height is ≤5mm, the ultrasonic vibration parameters are selected in the range of 35-40kHz high frequency and 10-20μm weak amplitude, the ultrasonic vibrator moving speed is 40-60mm / s, and the ultrasonic vibration path adopts a single "return" path;

[0016] ② When the prepreg placement height is 5mm-10mm, the ultrasonic vibration parameters are selected in the range of 25-35kHz medium frequency and 20-30μm medium amplitude, the ultrasonic vibration head movement speed is selected in the range of 30-40mm / s, and the ultrasonic vibration path adopts two "loop" paths connected end to end;

[0017] ③ When the prepreg placement height is greater than 10mm, the ultrasonic vibration parameters are selected in the range of 15-25kHz low frequency and 30-50μm strong amplitude, the ultrasonic vibration head moving speed is selected in the range of 20-30mm / s, and the ultrasonic vibration path adopts a zigzag cycle path.

[0018] The ultrasonic vibration path needs to be scanned twice repeatedly, and the two ultrasonic vibration paths are executed in opposite directions. After the "U"-shaped ultrasonic vibration path is executed, it needs to stay and vibrate at the center point of the plane for 3-5 seconds.

[0019] In step (4), each batch of ultrasonic vibration units must be left to stand for 10-20 seconds after vibration is completed before the next batch of ultrasonic vibration units can be laid.

[0020] The specific implementation method of adding the exhaust action in step (5) is as follows: after the cavity temperature of the mold of the product to be molded is heated from room temperature to the starting end of the melting temperature range of the thermoplastic resin at a heating rate of 5-10°C / min, a temperature node is set every 10°C. After the temperature of the molding machine reaches the temperature node, the molding machine opens the molding mold to a position where the upper and lower molds are 5-15mm apart and maintains this position for 5-10s, then closes the molding mold and re-pressurizes it to the set pressure value to complete an exhaust action.

[0021] The exhaust action in step (5) is set to be performed every 10°C within the range from the melting temperature range of the selected thermoplastic resin to the end of the holding temperature range.

[0022] A chopped fiber composite material compression molding device with ultrasonic vibration-assisted material placement, comprising a support frame A, an XYZ three-axis linear sliding unit B provided on the support frame A, and an ultrasonic vibration assisting device C for ultrasonically vibrating the chopped fiber reinforced thermoplastic prepreg inside the mold;

[0023] The XYZ three-axis linear sliding unit B includes an X-direction synchronous belt linear module B1, a Y-direction synchronous belt linear module B2 and a Z-direction ball screw linear module B3. The X-direction synchronous belt linear module B1 is fixed to the upper end surfaces of the support frames A on both sides and is connected to maintain balance through a light rod. The slide is driven by a linear motor to move on the guide rail to realize the movement of the ultrasonic vibration auxiliary device C in the X direction; the Y-direction synchronous belt linear module B2 is vertically fixed to the slide in the X-direction synchronous belt linear module B1 through an L-shaped connector, and the slide is driven by a linear motor to move on the guide rail to realize the movement of the ultrasonic vibration auxiliary device C in the Y direction; the Z-direction ball screw linear module B3 is vertically fixed to the slide in the Y-direction synchronous belt linear module B2 through a slide rail, and the slide is driven by a linear electric drive to move on the guide rail to realize the movement of the ultrasonic vibration auxiliary device C in the Z direction; the XYZ three-axis linear sliding unit B operates synchronously to realize regular and uniform vibration of the ultrasonic vibration auxiliary device C;

[0024] The ultrasonic vibration auxiliary device C includes an ultrasonic generator C1, a transducer C2, a horn C3 and a replaceable ultrasonic vibrator C4. The transducer C2, the horn C3 and the replaceable ultrasonic vibrator C4 together constitute the end of the ultrasonic vibration auxiliary device C, and the end of the ultrasonic vibration auxiliary device C is fixed on the XYZ three-axis linear sliding unit B; the horn C3 is arranged at the bottom of the transducer C2 and above the replaceable ultrasonic vibrator C4, and the ultrasonic generator C1 is arranged outside the frame of the XYZ three-axis linear sliding unit B and is connected to the transducer C2.

[0025] The ultrasonic vibration auxiliary device C also includes a fixed bracket C5 that supports the XYZ three-axis linear sliding unit B and a flange C6 that connects and fastens. The fixed bracket C5 is connected and fixed to the slide of the Z-direction ball screw linear module B3, and the flange C6 is fixed on the upper surface of the fixed bracket C5 and is connected to the end clamping part of the ultrasonic vibration auxiliary device C.

[0026] Beneficial effects of the present invention: The compression molding exhaust process method designed by the present invention eliminates excess gas inside the material, compensates for pore defects caused by pressure loss, and prepares short-fiber reinforced composite molded products with good mechanical properties and high stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a flow chart of the implementation of the ultrasonic vibration assisted paving method for chopped fiber composite material compression molding of the present invention;

[0028] Figure 2 Schematic diagrams of ultrasonic vibration paths in three cases of the chopped fiber composite material compression molding method with ultrasonic vibration assisted paving in the present invention;

[0029] Figure 3Schematic diagram of the exhaust-assisted compression molding process of the chopped fiber thermoplastic PA6 resin composite material of the present invention;

[0030] Figure 4 This is a schematic diagram of the overall structure of a chopped fiber composite material compression molding device with ultrasonic assisted laying of the present invention, which is suitable for chopped fibers and thermoplastic resins;

[0031] Figure 5 for Figure 4 Schematic diagram of the structure of the XYZ three-axis linear sliding unit;

[0032] Figure 6 for Figure 4 Schematic diagram of the structure of the ultrasonic vibration auxiliary device;

[0033] In the figure: B1-X direction synchronous belt linear module, B2-Y direction synchronous belt linear module, B3-Z direction ball screw linear module; C1-ultrasonic generator, C2-transducer, C3-amplifier, C4-interchangeable ultrasonic vibrator, C5-fixing bracket, C6-flange. DETAILED DESCRIPTION

[0034] The specific implementation of the present invention is further described below in conjunction with the accompanying drawings and technical solutions.

[0035] Example 1

[0036] Taking compression molding of chopped fiber-reinforced thermoplastic resin composites as an example, prepreg made from fiber grade T700 and PA6 resin was selected as the raw material. The prepreg was cut into short slices measuring 6.35mm×13mm×0.08mm, and then compression molding of the chopped fiber-reinforced PA6 resin composite was performed. The target component was a 300mm×300mm×2mm chopped carbon fiber-reinforced thermoplastic composite sheet. Based on the material properties, appropriate process parameters were selected to carry out this example.

[0037] First, before the experiment begins, the weight (500g) of the required chopped fiber reinforced PA6 resin prepreg is determined and weighed according to the volume of the molded board, and the load-bearing chopped fiber prepreg is evenly divided into five batches of ultrasonic vibration units of equal mass (100g). The mold of the molded board is cleaned and the mold surface is coated with a release agent. The release agent is applied three times in total, and the interval between each coating is 5 minutes. After the release agent coating is completed, the compression molding paving device described in the present invention is fixed, and then the chopped prepreg vibration units are placed in batches and ultrasonic vibration is performed. The motor is started, and the working plane of the ultrasonic vibration auxiliary device is determined according to the paving height to start vibration until all batches of chopped prepregs are placed in the mold and vibrate evenly. Then the motor is turned off and the compression molding paving device is removed. According to the principles of ultrasonic vibration parameter and vibration path selection, the ultrasonic vibration parameters used in this laying are: when the prepreg laying height is ≤5mm, the frequency is 40kHz, the amplitude is 15μm and the moving speed is 50mm / s; when the prepreg laying height is 5mm-10mm, the frequency is 25kHz, the amplitude is 30μm and the moving speed is 30mm / s.

[0038] Determine the short fiber reinforced PA6 resin prepreg molding process plan, in which the heating rate and cooling rate are 10℃ / min, the molding temperature is 260℃, the holding time is 20min, the preload pressure is 2MPa, the molding pressure is 10MPa, the exhaust action temperature range is 220℃-260℃, the exhaust action upper and lower mold interval is selected as 10mm and maintained for 5s. After the plan is set, start the molding machine and start to execute the set process plan. After the plan is executed, the molded plate is demoulded and post-processed. Subsequently, the short fiber PA6 composite plate is processed according to ASTM D The 3039 test standard was cut into tensile parts and subjected to tensile performance tests. The results showed that after ultrasonic vibration-assisted paving and exhaust treatment, the tensile strength increased by 48.6%, the porosity decreased from 2.21% to 0.82%, and the mechanical performance stability was significantly improved. Subsequently, the PA6 sheet was cut, cured and polished, and then observed under an ultra-depth-of-field microscope. Compared with the sheet that had not been treated with ultrasonic vibration, it can be observed that the short-cut carbon fiber sheet structure inside the sheet is more uniform, and there are fewer out-of-plane corrugation defects. Therefore, the more effective the stress transfer, the more significantly reduced the early damage caused by defects, and the better and more stable the mechanical properties of the sheet.

Claims

1. A method for compression molding of chopped fiber composite materials with ultrasonic vibration-assisted laying, characterized in that: The following steps are involved: (1) Determine and weigh the required weight of the chopped fiber reinforced thermoplastic resin prepreg according to the volume of the expected molded product, divide the chopped fiber reinforced thermoplastic resin prepreg into several batches of ultrasonic vibration units of equal mass, and control the thickness of a single ultrasonic vibration unit to be between 10% and 50% of the thickness of the product; (2) Clean the mold of the molded product and apply a release agent to its surface. A total of three coats of release agent are required, with an interval of 5 minutes between each coat. (3) After the release agent is applied, a single ultrasonic vibration unit is manually laid flat in the mold of the molded product, and after determining the prepreg laying height, the corresponding ultrasonic vibration parameters and ultrasonic vibration path are selected; the short-cut fiber composite material molding device with ultrasonic vibration auxiliary laying is fixed on the work surface, the motor is started, the working plane of the ultrasonic vibration auxiliary device (C) is determined, and the ultrasonic vibration auxiliary device (C) is turned on to vibrate the non-uniform short-cut fiber reinforced thermoplastic resin prepreg in the mold of the molded product uniformly; (4) Repeat step (3) and place the ultrasonic vibration unit in batches and perform ultrasonic vibration until all batches of ultrasonic vibration units are placed in the mold of the molded product and vibrate evenly; (5) Turn off the motor and remove the short fiber composite material molding device with ultrasonic vibration assisted laying, set the molding process temperature, pressure and holding time, and add exhaust action at the appropriate temperature; (6) Start the molding machine and start executing the set process plan. After the process plan is completed, the molded product is demolded and post-processed to obtain a short-cut fiber reinforced thermoplastic resin composite product with low defects and high stability.

2. The compression molding method of a chopped fiber composite material according to claim 1, characterized in that: The selection principle of ultrasonic vibration parameters and ultrasonic vibration path in step (3) is: ① When the prepreg placement height is ≤5mm, the ultrasonic vibration parameters are selected in the range of 35-40kHz high frequency and 10-20μm weak amplitude, the ultrasonic vibrator moving speed is 40-60mm / s, and the ultrasonic vibration path adopts a single "loop" path; ② When the prepreg placement height is 5mm-10mm, the ultrasonic vibration parameters are selected in the range of 25-35kHz medium frequency and 20-30μm medium amplitude, the ultrasonic vibration head moving speed is selected in the range of 30-40mm / s, and the ultrasonic vibration path adopts two "loop" paths connected end to end; ③ When the prepreg placement height is greater than 10mm, the ultrasonic vibration parameters are selected in the range of 15-25kHz low frequency and 30-50μm strong amplitude, the ultrasonic vibration head moving speed is selected in the range of 20-30mm / s, and the ultrasonic vibration path adopts a zigzag circulation path.

3. The ultrasonic vibration path according to claim 2, characterized in that The ultrasonic vibration path needs to be scanned twice and the two ultrasonic vibration paths are executed in opposite directions. After the "U"-shaped ultrasonic vibration path is executed, it needs to stay and vibrate at the center point of the plane for 3-5 seconds.

4. The compression molding method of a chopped fiber composite material according to claim 1, characterized in that: In step (4), each batch of ultrasonic vibration units must be left to stand for 10-20 seconds after vibration is completed before the next batch of ultrasonic vibration units can be laid.

5. The compression molding method of chopped fiber composite material according to claim 1, characterized in that: The specific implementation method of adding the exhaust action in step (5) is as follows: after the cavity temperature of the mold of the product to be molded is heated from room temperature to the starting end of the melting temperature range of the thermoplastic resin at a heating rate of 5-10°C / min, a temperature node is set every 10°C. After the temperature of the molding machine reaches the temperature node, the molding machine opens the molding mold to a position where the upper and lower molds are 5-15mm apart and maintains this position for 5-10s, then closes the molding mold and re-pressurizes it to the set pressure value to complete an exhaust action.

6. The compression molding method of a chopped fiber composite material according to claim 1, characterized in that: The exhaust action in step (5) is set to be performed every 10°C within the range from the melting temperature range of the selected thermoplastic resin to the end of the holding temperature range.

7. A short fiber composite material compression molding device with ultrasonic vibration assisted laying, characterized in that: The ultrasonic vibration-assisted paving short-fiber composite material compression molding device comprises a support frame (A), an XYZ three-axis linear sliding unit (B) arranged on the support frame (A), and an ultrasonic vibration auxiliary device (C) for performing ultrasonic vibration treatment on the short-fiber reinforced thermoplastic prepreg inside the mold; The XYZ three-axis linear sliding unit (B) includes an X-direction synchronous belt linear module (B1), a Y-direction synchronous belt linear module (B2) and a Z-direction ball screw linear module (B3). The X-direction synchronous belt linear module (B1) is fixed to the upper end surface of the support frame (A) on both sides and is connected to maintain balance through a light rod. The linear motor drives the slide to move on the guide rail to realize the movement of the ultrasonic vibration auxiliary device (C) in the X direction; the Y-direction synchronous belt linear module (B2) is vertically fixed to the X-direction synchronous belt by an L-shaped connector. On the slide in the linear module (B1), the slide is driven by a linear motor to move on the guide rail, thereby realizing the movement of the ultrasonic vibration auxiliary device (C) in the Y direction; the Z-direction ball screw linear module (B3) is vertically fixed to the slide in the Y-direction synchronous belt linear module (B2) through the slide rail, and the slide is driven by a linear electric drive to move on the guide rail, thereby realizing the movement of the ultrasonic vibration auxiliary device (C) in the Z direction; the XYZ three-axis linear sliding unit (B) operates synchronously to realize the regular and uniform vibration of the ultrasonic vibration auxiliary device (C); The ultrasonic vibration auxiliary device (C) comprises an ultrasonic generator (C1), a transducer (C2), a horn (C3) and a replaceable ultrasonic vibrator (C4); the transducer (C2), the horn (C3) and the replaceable ultrasonic vibrator (C4) together constitute the end of the ultrasonic vibration auxiliary device (C); the end of the ultrasonic vibration auxiliary device (C) is fixed on an XYZ three-axis linear sliding unit (B); the horn (C3) is arranged at the bottom of the transducer (C2) and above the replaceable ultrasonic vibrator (C4); the ultrasonic generator (C1) is arranged outside the frame of the XYZ three-axis linear sliding unit (B) and is connected to the transducer (C2).

8. The device for molding and laying out chopped fiber reinforced thermoplastic resin-based prepreg according to claim 7, characterized in that: The ultrasonic vibration auxiliary device (C) also includes a fixed bracket (C5) that supports the XYZ three-axis linear sliding unit (B) and a flange (C6) that connects and fastens the fixed bracket (C5). The fixed bracket (C5) is connected and fixed to the slide table of the Z-direction ball screw linear module (B3). The flange (C6) is fixed to the upper surface of the fixed bracket (C5) and is connected to the end clamping part of the ultrasonic vibration auxiliary device (C).

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