Foaming injection molding process

Abstract

The invention relates to the technical field of polymer injection molding processing, in particular to a foaming injection molding process which comprises the following steps: S1, a self-closing nozzle is self-closed, and a screw rod completes pre-molding under preset back pressure; s2, the self-closing nozzle keeps self-closing, the screw is switched into low-pressure injection so as to keep the compressed state of the pre-plasticized material, the pressure of the screw is larger than the preset back pressure during pre-plasticization of the screw, and the pressure and time are determined according to the diffusion and mixing mass of gas in the melt; s3, the self-closing nozzle keeps self-closing for a short time, injection is converted into high pressure from low pressure, so that pressure is established in the pre-plastic before the nozzle is opened, then the self-closing nozzle is completely opened, and injection mold filling is started; and S4, injection mold filling is completed, and the self-closing nozzle is closed immediately. According to the method, low-pressure injection is used for replacing a traditional pressure maintaining method after pre-molding, so that the pressure of the pre-molding material can be maintained in the foaming injection molding process, the pressure can be increased to high pressure which cannot be achieved by traditional pressure maintaining, the pressure maintaining time is long, and the foaming quality is improved.

Description

Technical Field

[0001] The present application relates to the technical field of polymer co-injection processing, and in particular to a foaming injection molding process. Background Art

[0002] Foam injection molding is the process of introducing gas during the polymer injection molding process, so that a large number of bubble nuclei are formed in the polymer melt and grow into bubbles when the mold filling pressure is released. During the cooling and solidification process of the plastic, the bubbles are shaped into the final size and shape, and finally a product with a foamed structure is formed.

[0003] Reference Figure 1 In a foaming injection molding process in the related art, the polymer raw material is first added into the barrel of the injection molding machine, the raw material is plasticized and conveyed forward by the rotation of the screw, the gas used for physical foaming is injected into the melt at a preset position, and then the melt containing the gas is passed through the self-closing nozzle of the injection molding machine under high pressure and injected into the mold cavity. Among them, the low pressure threshold is 15-100Mpa, and the high pressure threshold is 110-200Mpa.

[0004] With respect to the above-mentioned related technologies, in conventional injection molding control methods, when the screw completes pre-molding, it usually no longer maintains the pressure on the stored material and releases the pressure, or even if a certain back pressure can be maintained, the pressure value is not high enough and the pressure maintenance is not long-lasting. As a result, if it is used in a foaming process, it will foam in advance before injection, resulting in the inability to guarantee the quality of the foam cells. In addition, in conventional foaming injection molding, there is no short-term pressure maintenance before the injection is completed and the stored material between the self-closing nozzle and the end of the screw is instantly decompressed to zero pressure, causing the gas in this section of material to escape and become large bubbles, affecting the foaming quality of the next mold. Therefore, it is necessary to provide a foaming injection molding process that can maintain the pre-molding pressure, thereby improving the foaming quality. Summary of the invention

[0005] In order to maintain the pressure of the pre-plastic during the foaming injection molding process, thereby improving the foaming quality, the present application provides a foaming injection molding process.

[0006] A foaming injection molding process provided in this application adopts the following technical solution: A foaming injection molding process comprises the following steps: S1: The self-closing nozzle closes automatically, and the screw completes pre-molding under the preset back pressure; S2: The self-closing nozzle remains self-closing, and the screw switches to low-pressure injection to keep the pre-plastic in a compressed state. The pressure is greater than the preset back pressure during screw pre-plasticization, and the required pressure and time are determined by the diffusion and mixing quality of the gas in the melt; S3: High-pressure injection starts, the self-closing nozzle remains self-closed for a short time (≥0.5s), and the injection changes from low pressure to high pressure, thereby building up high pressure in the pre-plastic before the nozzle opens, and then the self-closing nozzle is fully opened, and high-pressure injection molding begins; S4: After the injection molding is completed, the self-closing nozzle is closed immediately.

[0007] By adopting the above technical solution, in step S1, the screw completes pre-molding under a preset back pressure, which can provide a higher pressure gradient for the subsequent foaming process, so that the bubble nuclei in the polymer melt are formed more and more evenly. In the subsequent steps, the stable high-pressure environment also helps the bubble nuclei to become more tiny pores during the cooling and solidification process of the plastic, thereby improving the bubble quality of the final product and making the bubble structure of the product more uniform and fine. In step S2, the screw is switched to low-pressure injection to keep the pre-plastic in a compressed state, avoiding instantaneous pressure relief of the stock, and during the injection molding process, it can also effectively reduce gas escape and increase the nucleation rate, ensuring that the gas is reasonably distributed and grows in the melt, which helps to maintain the foaming quality of each mold at a high level and improve the stability of the entire foaming injection molding process.

[0008] Furthermore, in S2, the pre-plastic is kept in a compressed state for a period of T 1 s.

[0009] By adopting the above technical solution, the pressure generated by low-pressure injection causes the gas-containing melt to 1 The gas further diffuses into the melt as quickly as possible within the time, creating good conditions for uniform cell nucleation. During the mold filling process, due to the increase in the pressure drop gradient, more energy can be provided to prompt the gas to form cell nuclei, greatly improving the cell nucleation rate, increasing the number of cells, and laying the foundation for the formation of high-quality cell morphology. Maintaining a high pre-pressure until the injection starts, achieving a pressure drop gradient that cannot be achieved by conventional injection molding, helps to increase the number of cells and control the growth of cells at the same time, effectively increasing the cell density and reducing the cell size. At the same time, the existence of a large pressure drop gradient makes the cells more uniform and small during the growth process, avoiding the situation where the cells are too large or uneven in size.

[0010] Furthermore, in S2, the pressure value of the low pressure injection is X 1 Mpa, X 1 It is greater than the preset back pressure during screw pre-plasticization, and the required pressure and time are determined by the diffusion and mixing quality of the gas in the melt.

[0011] By adopting the above technical solution, a specific range of low pressure injection pressure X is adopted. 1, using pressure to diffuse the gas-containing melt into the melt as quickly as possible. Under the action of pressure, the gas can be more evenly distributed in the polymer melt to avoid the local accumulation of gas to form air pockets. It helps to form a uniform pore structure in the subsequent injection molding process and improve the quality of the foamed product. Suitable X 1 The pressure can keep the pre-plastic in a compressed state, and after the injection begins, it can increase the pressure drop during the filling process. Proper pressure control can effectively control the growth of the pores, ultimately increasing the number of pores, reducing the pore size, and obtaining high-quality pore morphology.

[0012] Furthermore, in S3, the self-closing nozzle is kept open for a short delay, and the delay time lasts at least T 2 s(T 2 ≥0.5), and at the same time the injection changes from low pressure to high pressure until the pressure of high pressure injection is basically established.

[0013] By adopting the above technical solution, when the self-closing nozzle is delayed to open, high-pressure injection is started synchronously and maintained for at least T 2 The self-closing nozzle delay of 1 second can ensure that the self-closing nozzle is delayed until the injection action has started to build up a sufficiently high pressure, effectively preventing the self-closing nozzle from opening too early and the injection action from not responding in time, thereby preventing the gas-containing melt from losing pressure and causing gas to escape, thereby reducing the quality of the bubble. By controlling T 2 Time can avoid this problem and ensure the quality of the foam cells.

[0014] Furthermore, in S3, the pressure value of the high pressure injection is X 2 Mpa.

[0015] By adopting the above technical solution, appropriate X 2 Pressure helps control the growth and distribution of bubbles during the injection process. Under high pressure, the melt's ability to wrap bubbles is enhanced, and gas is not easy to escape, so that the bubbles can grow controllably under uniform pressure, making the bubbles smaller and more uniform, increasing the density of the bubbles, and thus improving the quality of the foamed product. Appropriate high-pressure injection pressure X 2 It can make the gas-containing melt flow and fill better in the mold cavity. Higher pressure can overcome the viscosity resistance of the melt and the resistance of the mold cavity, ensuring that the melt fills the mold quickly and completely, reducing the occurrence of defects such as short shots and lack of material.

[0016] Furthermore, in S4, when the self-closing nozzle is closed, the injection is switched to pressure holding and the start of pre-molding is delayed for a short time, and the delay time lasts at least T 3 s By adopting the above technical solution, when the self-closing nozzle is closed, the pressure is simultaneously maintained and maintained for at least T 3The holding process can provide a stable external pressure environment for the cells, so that the cells can maintain their shape during the cooling and solidification process. Especially for some thin-walled or complex-shaped foam products, the appropriate holding time T 3 It can prevent the bubbles from breaking or collapsing due to changes in internal pressure during the cooling process, thereby ensuring the foaming quality of the product, effectively compensating for the volume shrinkage of the material during the cooling process, preventing the product from having dimensional deviation, deformation or surface shrinkage due to shrinkage, and ensuring the dimensional accuracy and appearance quality of the product.

[0017] Furthermore, in S4, the pressure value during the holding period is X 1 Mpa.

[0018] By adopting the above technical solution, the holding pressure value is X 1 Mpa can provide a stable external pressure environment for the pores. During the cooling and solidification stage of foam injection molding, the gas pressure inside the pores and the external pressure need to be balanced to prevent the pores from rupturing or collapsing. 1 Mpa can ensure that each mold of foam injection molding product can get the same pressure compensation during the cooling and curing process.

[0019] Further, in S2, the screw stays at the pre-plasticization completion position and the preset back pressure and pre-pressing time are maintained by low-pressure injection.

[0020] By adopting the above technical solution, the screw stays at the pre-molding completion position and maintains the preset back pressure, or at least maintains the minimum pressure to prevent pre-foaming, thereby effectively avoiding the pre-foaming phenomenon at this stage. Under a stable back pressure environment, the gas can be stably dissolved in the polymer melt, and premature bubbles will not be formed due to a sudden drop in pressure, which helps to accurately control the timing of bubble formation, making the bubble structure of the next mold of the foaming injection molded product more consistent, and improving the stability and consistency of product quality.

[0021] In summary, the present application includes at least one of the following beneficial technical effects: Precise control of pressure and time parameters in each step, such as the screw completing pre-molding under preset back pressure, low-pressure injection to maintain the pre-plastic compression state and a specific duration, and reasonable setting of pressure and time in high-pressure injection and pressure holding stages, etc., to ensure uniform formation of bubble nuclei, effectively control pore growth, increase pore nucleation rate, reduce pore size, avoid excessive or uneven pore size, make pores smaller, more uniform, and denser, thereby improving the pore quality of the final product and forming high-quality pore morphology; The pressure control of each process stage cooperates with each other to prevent gas escape, melt pressure loss and other problems. For example, low-pressure injection prevents instantaneous pressure relief of stored materials, high-pressure injection ensures good filling of the melt in the mold cavity, and pressure maintenance compensates for material cooling shrinkage, etc., to ensure the stability of the melt in the entire foaming injection molding process, reduce the occurrence of various defects such as short shots, lack of material, shrinkage marks, deformation, etc., and optimize the entire process; In the bubble formation and growth stage, or in the melt flow and cooling molding stage, the controllability of the process is enhanced by precise control of the screw position, back pressure, injection pressure and time of each stage, making the bubble structure and overall quality of each mold foam injection molded product more consistent, improving the stability and consistency of product quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a flow chart of the injection molding process in the related art.

[0023] Figure 2 This is a flowchart of the foaming injection molding process provided in Example 1 of the present application.

[0024] Figure 3 This is a flowchart of the foaming injection molding process provided in Example 2 of the present application.

[0025] Figure 4 It is a schematic diagram of the foaming hole morphology under low pressure in the actual test of this application.

[0026] Figure 5 It is a schematic diagram of the foaming hole morphology under high pressure in the actual test of this application. DETAILED DESCRIPTION

[0027] In order to make the purpose, technical solutions and advantages of this application more clear, the following Figures 2 to 5 , Example 1 and Example 2, further describe the present application in detail. Example

[0028] The present application embodiment discloses a foaming injection molding process. Figure 2 , a foaming injection molding process comprises the following steps: S1: The screw completes pre-plasticization under the preset back pressure; S2: The screw turns to low-pressure injection, and the pressure value is X 1 Mpa, the pre-plastic remains compressed and lasts for T 1 s; S3: When the self-closing nozzle gradually opens, the synchronous starting pressure value is X 2 Mpa high pressure injection and maintain the delay time for at least T 2 After a short delay of s, the self-closing nozzle is fully opened again after the injection pressure is established, and injection and mold filling begin; S4: When the self-closing nozzle gradually closes, the synchronous starting pressure value is X 1 Mpa pressure and maintain the delay time for at least T 3 After a short delay of s, the short pressure holding ends and the self-closing nozzle is completely closed.

[0029] The foaming injection molding process provided in the embodiment of the present application utilizes the pressure generated by low-pressure injection to diffuse the gas into the melt as quickly and completely as possible, and maintains this pressure until the injection begins. As a result, the pressure drop during the mold filling process is increased to a pressure difference that cannot be achieved by conventional injection molding. This can greatly increase the cell nucleation rate, increase the number of cells, reduce the cell size, and ultimately obtain a high-quality cell morphology.

[0030] After pre-plasticization, the gas-containing melt stored at the end of the screw is immediately released by low pressure (X 1 Mpa) injection is compressed (because the nozzle is closed and the check valve at the screw head is also closed), this part of the material is enclosed in this space and compressed T 1 s.

[0031] T 1 The value range of T is affected by many factors. From the perspective of material properties, if it is a polymer material with high gas diffusion rate, T 1 It should be set at about 0.5 to 2 seconds. The material can quickly allow the gas to diffuse in the melt, and the gas-containing melt can reach a better uniform distribution state in a shorter time. For polymer materials with low gas diffusion rate, gas diffusion is relatively difficult, T 1 It needs to be extended to 2 to 5 seconds to ensure complete diffusion of the gas-containing melt.

[0032] X 1 The value range of is also affected by a variety of factors. Generally speaking, a higher low-pressure injection pressure can be set for polymers with low gas solubility, such as 15-30MPa. For polymers with high gas solubility, a lower low-pressure injection pressure can be set, such as 13-15MPa, to ensure that the gas-containing melt is fully integrated and diffused in the melt.

[0033] Self-closing nozzle opening delay (T 2 s) until the injection has started and sufficient pressure has been built up (X 2 Mpa), and then open the nozzle to prevent the self-closing nozzle from opening too early and the injection action not responding in time, causing the gas-containing melt to lose pressure early and cause gas to escape, resulting in a decrease in the quality of the bubble. Therefore, T 2 The value range of is determined by the time it takes for the injection action to go from zero to full speed, which is generally relatively short, about 0.2 to 0.8 seconds.

[0034] X 2Although the value range of X is also affected by a variety of factors, foam molding does not require the mold cavity to be filled, so generally X 2 The value range only needs 50% of that of traditional molding, such as 100-800Mpa.

[0035] The self-closing nozzle closes immediately after the injection is completed and enters the pressure holding action. The pressure value during the pressure holding period must be the same as the pressure value during the screw pre-molding, that is, X 1 Mpa. After conventional injection is completed, the nozzle is still open, and the pressure holding is to continue to compress the material already in the mold. In the embodiment of the present application, the pressure holding is to compress the material remaining between the nozzle and the end of the screw to ensure that the gas in this part of the material will not escape freely and affect the quality of the bubbles in the next mold.

[0036] Holding pressure duration T 3 s and is relatively short, which is a transition measure between the end of injection and screw pre-plasticization. After the short pressure holding period, the screw pre-plasticization of the next cycle will be immediately transferred, and the screw pre-plasticization must be carried out under the set back pressure at this time.

[0037] T 3 The value range of is affected by many factors and can be determined by referring to the back pressure during screw pre-plasticization.

[0038] In S2, the pressure value X of different low pressure injection 1 The effect of Mpa on the pre-molded melt to improve the foaming quality has been confirmed. Figure 4 and Figure 5 , the gas-containing materials at different low-pressure injection pressures were injected into the air and cooled to expand freely at room temperature, and the SEM images of the samples showed huge differences in pores. Figure 4 In the case of 6.9 MPa pressure, the cell diameter is about 1000 μm. Figure 5 In the experiment, a smaller pore size of 250 μm was shown under a holding pressure of 82.8 MPa.

[0039] In the embodiment of the present application, the output Q (kg / h) of the plasticizing screw is simplified to a function of the screw speed N (r / min): (1) Once the screw speed N is selected, the screw output Q is calculated by the above formula, and the gas injection flow rate q (kg / h) of the gas injection system is determined accordingly: (2) Where: q is the gas injection flow rate (kg / h); Q is the plasticization output of the melt (kg / h), which is determined by formula (1); C is the percentage of the gas injection flow rate to the plasticization output. For example, the recommended nitrogen gas injection flow rate and plasticization output for polystyrene material is 0.75% by weight.

[0040] The value of C can be determined experimentally or stored in a material database and can be searched in the database. The data can be entered manually or automatically provided directly from the database.

[0041] Another important intelligent control variable is the gas injection pressure Pg, which must be greater than the melt pressure ΔP at the gas injection inlet but not too large to cause gas surge. The pressure difference ΔP is generally preset, such as 0.5 to 1Mpa. The gas injection system automatically reads the highest melt pressure Pmax in the previous injection molding cycle and automatically sets the gas injection pressure Pg: (3) The implementation principle of a foaming injection molding process in Example 1 of the present application is as follows: the foaming injection molding process provided in this embodiment is aimed at the dependence of the diffusion and integration process of gas in the polymer melt on pressure and time. After the injection screw is completed, a higher back pressure is applied and the cooling time of the workpiece is used to extend the compression time of the pre-molded gas-containing polymer melt, thereby accelerating the diffusion and integration of gas in the pre-molded melt and evenly distributing it. This key process only changes the control program of the machine to replace the traditional back pressure holding action with a low-pressure injection action to complete the high pressure and pressure holding time required for the pre-compression of the pre-molded gas-containing melt without changing the traditional hydraulic system. A series of special processes and corresponding control methods such as a high-pressure resistant self-closing nozzle, a screw head that can maintain high pressure, and a corresponding power device that provides high pressure, as well as the best match between the delayed opening of the self-closing nozzle and the start of injection provided by the control program, short-term pressure holding, and pre-compression of the gas-containing polymer stored in the screw head after the pre-molding is completed, can effectively guarantee the quality of the pores of the foamed parts. This process is a conservation of pressure P and volume V. The volume decreases under high pressure, but the leak-free screw head and nozzle ensure that the quality remains unchanged. Example

[0042] The difference between this embodiment and embodiment 1 is that, referring to Figure 3 In S2, low-pressure injection is used to keep the screw at the position to complete pre-molding by linking with the position sensor, but at the same time maintains an appropriate screw back pressure or at least maintains the minimum pressure to prevent pre-foaming.

[0043] Conventional injection molding processes reduce the back pressure to zero after the screw completes pre-molding. The screw is kept at the pre-molding completion position while maintaining an appropriate back pressure, or at least maintaining the minimum pressure to prevent pre-foaming, thereby effectively avoiding pre-foaming at this stage. This is done by low-pressure injection action to maintain the screw's stop position after pre-molding, because the pressure of the pre-molded gas-containing melt will push the screw back from the pre-molding completion position, and the position sensor links the position change with the pressure generated by low-pressure injection molding to maintain this position unchanged, while also automatically generating back pressure to offset the reverse thrust of the gas-containing melt. Under a stable back pressure environment, the gas can stably dissolve in the polymer melt, and premature bubbles will not be formed due to a sudden drop in pressure, making the bubble structure of the foaming injection molded product of the next mold more consistent, improving the stability and consistency of product quality.

[0044] The above are all preferred embodiments of the present application, and the protection scope of the present application is not limited thereto. Therefore, any equivalent changes made according to the structure, shape, and principle of the present application should be included in the protection scope of the present application.

Claims

1. A foaming injection molding process, characterized in that: The steps include: S1: The self-closing nozzle closes automatically, and the screw completes pre-molding under the preset back pressure; S2: The self-closing nozzle remains self-closing, and the screw switches to low-pressure injection to keep the pre-plastic in a compressed state. The pressure is greater than the preset back pressure during screw pre-plasticization, and the required pressure and time are determined by the diffusion and mixing quality of the gas in the melt; S3: High-pressure injection starts, the self-closing nozzle remains self-closed for a short time (≥0.5s), and the injection changes from low pressure to high pressure, thereby building up high pressure in the pre-plastic before the nozzle opens, and then the self-closing nozzle is fully opened, and high-pressure injection molding begins; S4: After the injection molding is completed, the self-closing nozzle is closed immediately.

2. A foaming injection molding process according to claim 1, characterized in that: In S2, the pre-plastic remains in the compressed state for T1s.

3. A foaming injection molding process according to claim 1, characterized in that: In S2, the pressure value of low-pressure injection is X1Mpa, X1 is greater than the preset back pressure during screw pre-plasticization, and the required pressure and time are determined by the diffusion and mixing quality of the gas in the melt.

4. A foaming injection molding process according to claim 1, characterized in that: In S3, the self-closing nozzle remains open with a short delay, the delay time lasting at least T2s (T2≥0.5), and the injection is switched from low pressure to high pressure until the pressure of high-pressure injection is established.

5. A foaming injection molding process according to claim 4, characterized in that: In S3, the pressure value of high pressure injection is X2Mpa.

6. A foaming injection molding process according to claim 1, characterized in that: In S4, when the self-closing nozzle is closed, the injection is synchronously switched to pressure holding and the start of pre-plasticization is delayed for a short time, and the delay time lasts at least T3s.

7. A foaming injection molding process according to claim 6, characterized in that: In S4, the pressure value during the pressure holding period is X1Mpa.

8. A foaming injection molding process according to claim 1, characterized in that: In S2, the screw stays at the pre-plasticization completion position and is maintained at the preset back pressure and pre-pressing time by low-pressure injection.

9. A foaming injection molding process according to claim 1, characterized in that: Retrieving a preset percentage C from a preset first database according to the injection molding material; The gas injection flow rate q is obtained according to the product of the plasticizing output Q and the preset percentage C. The preset percentage C is used to represent the percentage of the gas injection flow rate q in the plasticizing output. The plasticizing output Q is obtained by the function f(N), where N represents the screw speed.

10. A foaming injection molding process according to claim 1, characterized in that: Read the highest melt pressure Pmax in the last foaming injection molding process; Retrieving a preset pressure difference ΔP from a preset second database according to the injection molding material; The gas injection pressure Pg is obtained according to the sum of the maximum melt pressure Pmax and the preset pressure difference ΔP, and the gas injection pressure Pg is used in this foaming injection molding process.

Images