A method for hot press forming a polyethylene foam board

Abstract

The present application relates to a kind of foam board production technical field, especially a kind of polyethylene foam board hot press forming method, by the zoned design of foam board production line, and setting up pushing device to link each area, realize the continuous production of foam board hot press forming, main steps include blank preparation-blank preheating-melted material hot press-pressure relief expansion-cooling forming;The polyethylene foam board hot press forming method of the present application, production process does not need to wait for mold cooling, equipment comprehensive utilization rate is greatly improved;And by in melted material hot press step supplementary zoned temperature regulating device and ultrasonic device, make the foaming reaction in polyethylene foam board hot press forming process have controllability, effectively solve the problem of uneven foaming caused by foam board finished product quality defect.

Description

Technical Field

[0001] This invention relates to the field of foam board production technology, and in particular to a method for hot pressing polyethylene foam board. Background Technology

[0002] Polyethylene foam boards are widely used in various industries for structural filling, object protection, and even decoration due to their excellent cushioning and heat insulation properties. The mainstream production process involves mixing polyethylene raw materials with foaming agents and cross-linking agents to form raw materials for further processing. These raw materials are then subjected to a high-temperature, high-pressure environment in a mold to fully foam and cross-link, and finally cooled and molded into usable foam boards.

[0003] The molding of polyethylene foam boards is essentially a dynamic evolution of foaming and cross-linking reactions under thermal influence. However, in traditional polyethylene foam board production processes, the precise control of temperature and pressure during the high-temperature foaming and cross-linking processes of polyethylene is difficult, easily leading to uneven density in the molded foam boards, affecting their performance. Furthermore, traditional foam board molding is intermittent; each foam board must wait for the previous one to finish and the mold to empty before the next can be produced, with a significant cooling period before mold opening during which no output is produced. Overall, this results in low equipment utilization, low production efficiency, and an inability to meet the demands of streamlined, continuous production.

[0004] Therefore, there is an urgent need to develop a foam board production process that enables continuous production of foam boards and allows for proactive and precise control of the polyethylene foaming process to solve the problems in the existing technologies. Summary of the Invention

[0005] The present invention aims to solve the problem in the prior art that polyethylene foam boards cannot be produced continuously, and that it is difficult to ensure that the foaming and cross-linking reactions occur uniformly during the production process, resulting in uneven density of the foam boards after molding.

[0006] To solve the above technical problems, the present invention provides a method for hot-pressing polyethylene foam boards, specifically comprising: dividing the production line for hot-pressing foam boards into a feeding end, a preheating zone, a hot-pressing zone, an expansion zone, and a cooling and shaping zone, which are sequentially connected and equipped with a pushing device; wherein the pushing device is used to push the production material on the production line at a constant speed; and the method for hot-pressing polyethylene foam boards includes the following steps: Step S1, raw material preparation; In the mixing workshop, raw materials containing polyethylene particles, chemical reactants and other materials are hot-melted and mixed, and then calendered, cooled and shaped to prepare raw materials for processing; The chemical reactants include foaming agents and crosslinking agents; Step S2, preheating of the blank: Align the blank with the center line of the production line and put it into the feeding end. The blank is conveyed into the preheating zone at a uniform speed from the feeding end and heated into a molten material with ductility and in which the foaming agent and crosslinking agent inside are not activated. It is then pushed into the hot pressing zone. Step S3, hot pressing of molten material; The hot pressing zone includes an upper pressure plate and a lower pressure plate, and is equipped with a zone temperature control device and several ultrasonic devices. In the hot pressing zone, the molten material undergoes foaming and cross-linking reactions in the high-temperature environment controlled by the zone temperature control device and the high-pressure environment formed by the upper and lower pressure plates. At the same time, the ultrasonic devices are turned on to assist the molten material in uniform foaming. The molten material is initially hot-pressed and formed in this zone. Step S4, pressure release and expansion; the molten material is released from the hot pressing zone outlet and then immediately enters the expansion zone. The expansion zone is designed according to the shape of the finished foam board and is a cavity structure with openings at the front and rear ends. The molten material expands freely in the cavity of the expansion zone to form a preliminary finished foam board, which is then sent to the cooling and molding zone. Step S5, cooling and molding: The initial product is cooled to room temperature in the cooling and molding area by a cooling device, and then trimmed, cut and packaged in this area to form the finished foam board.

[0007] Furthermore, the preheating zone is set at a temperature between the polyethylene melting temperature and the activation temperature of the chemical reactant, and the hot pressing zone and the expansion zone are set at temperatures greater than the activation temperature of the chemical reactant.

[0008] Furthermore, the surfaces of the upper and lower pressure plates in the hot pressing zone and the inner wall of the expansion zone are coated with a Teflon coating.

[0009] Furthermore, the zoned temperature control device includes a temperature sensor connected to a control unit and several zoned heating devices arrayed within the hot pressing zone. The temperature sensor is used to acquire the temperature information of the molten material and transmit it to the control unit. The control unit is used to process the temperature information and drive the zoned heating devices to perform heating or cooling operations on different parts within different hot pressing zones.

[0010] Furthermore, the hot pressing zone consists of at least two hot pressing units connected in series, with the hot pressing unit closest to the entrance of the hot pressing zone being the first hot pressing unit, and the second hot pressing unit being the next; the first hot pressing unit is set to a lower temperature than the other hot pressing units.

[0011] Furthermore, the ultrasonic devices are arranged in an array within the second thermopressing unit and connected to a control system, which can control the ultrasonic energy emitted by each ultrasonic device and the start and stop of each ultrasonic device.

[0012] Furthermore, the pushing device includes a pusher roller disposed at the junction of the preheating zone outlet and the hot pressing zone inlet, and a traction roller disposed at the junction of the expansion zone outlet and the cooling and shaping zone inlet. The traction roller includes steel rollers symmetrically arranged vertically, used to clamp the initial finished product between the upper and lower steel rollers and pull it towards the cooling and shaping zone. The distance between the steel rollers is adjustable, and the surface of the steel rollers is covered with an anti-slip material.

[0013] Furthermore, the expansion zone inlet is connected to a funnel-shaped guide port that gradually widens toward the hot-pressing zone outlet.

[0014] Furthermore, the cooling and shaping zone uses an air-cooling device to cool the initial product, and is divided into a slow cooling zone, a strong cooling zone, and a uniform temperature zone from the inlet to the outlet. The air-cooling device in the slow cooling zone blows hot air at a temperature lower than that in the expansion zone onto the initial product; the air-cooling device in the strong cooling zone blows cold air at a temperature lower than that in the slow cooling zone but higher than that in the room temperature onto the initial product; and the air temperature in the uniform temperature zone is set according to the ambient temperature.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention, through a continuous production line design, eliminates the need for mold cooling during the hot-pressing process of polyethylene foam boards, significantly improving equipment utilization and achieving continuous production from raw material preparation to finished product packaging. Compared to traditional intermittent production processes, this invention, by arranging zoned temperature control devices and ultrasonic devices in the hot-pressing zone, works synergistically to ensure controllable foaming reactions during the hot-pressing process of polyethylene foam boards, effectively solving the problem of quality defects in finished foam boards caused by uneven foaming. The hot-pressing method for polyethylene foam boards of this invention has significant application advantages in the production of polyethylene foam boards with simple shapes and high-volume requirements. Attached Figure Description

[0016] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this application, illustrate exemplary embodiments of the invention and are used to explain the invention, but do not constitute an undue limitation of the invention.

[0017] Figure 1 This is a schematic diagram illustrating the implementation steps of a hot-pressing molding method for polyethylene foam boards according to the present invention; Detailed Implementation

[0018] The technical solution of the present invention will now be described with reference to the accompanying drawings. However, the described embodiments are only some embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0019] It should be noted that in the description of this invention, the terms "upper," "lower," "inner," "outer," etc., which indicate the direction or positional relationship, are based on the direction or positional relationship shown in the drawings. This is only for the convenience of description and is not intended to indicate or imply that the device or element must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, it should not be construed as a limitation of this invention.

[0020] To achieve continuous processing and production of polyethylene foam boards, this embodiment designs the foam board production line into zones. Specifically, the production line is divided into a feeding zone, a preheating zone, a hot pressing zone, an expansion zone, and a cooling and shaping zone, which are connected sequentially and fed by a pushing device at a constant speed. A pusher roller is installed at the junction of the preheating zone outlet and the hot pressing zone inlet, and a traction roller is installed at the junction of the expansion zone outlet and the cooling and shaping zone inlet. In the specific production scenario of this embodiment, the final foam board product to be produced is a low-density, highly uniform foam board with a width of 1.2m, a thickness of approximately 50mm, and a length that can be cut as needed. The specific implementation process is as follows: In a separate mixing workshop, low-density polyethylene granules, azodicarbonamide, dicumyl peroxide, and other raw materials are mixed and heated according to the formula. After hot melting and mixing in an internal mixer, the lumpy material produced by the mixer is sent to a calender. The material is calendered, cooled, and made into a smooth blank with a thickness of 3.0 mm and a width of 1.1 m, which is then rolled up and sent to the foam board production workshop. The selected low-density polyethylene has a melting temperature range of 105℃-115℃, the chemical foaming agent azodicarbonamide has a decomposition temperature range of 150-200℃, and the crosslinking agent dicumyl peroxide has a decomposition temperature range of 120-150℃.

[0021] One end of the billet is centered at the feed end, ensuring the center of the billet aligns with the centerline of the production line. It is then conveyed from the feed end and enters the preheating zone at a uniform speed. Based on the decomposition temperatures of the selected foaming agent and crosslinking agent, the preheating zone temperature is set between 110℃ and 120℃. The billet heats up slowly and uniformly in the preheating zone, preventing the room-temperature billet from directly entering the high-temperature hot-pressing zone and causing localized severe overheating, decomposition, crusting, or scorching. When the billet reaches the preheating zone outlet, the low-density polyethylene has initially melted and gained ductility; the crosslinking agent activity begins to increase significantly, while the foaming agent remains stable but tends towards an activated state.

[0022] Pushed by the feed rollers, the partially molten billet enters the hot pressing zone directly from the preheating zone outlet. Under the continuous thrust of the feed rollers and the spatial constraints of the hot pressing zone, it continues to move towards the end of the production line. The hot pressing zone is equipped with a zoned temperature control device, which includes a temperature sensor connected to a control unit and several zoned heating devices arrayed within the hot pressing zone. The hot pressing zone consists of two series-connected hot pressing units, each including an independent upper and lower pressure plate. The unit closest to the hot pressing zone entrance is defined as the first hot pressing unit, set to a temperature of 120℃-160℃. The lower pressure plate is fixed, while the upper pressure plate, driven by hydraulic pressure, presses down at a constant pressure of 0.8MPa, creating a high-pressure space that confines the billet between the upper and lower pressure plates. After the billet enters this zone, the zoned temperature control device can read the temperature differences in different parts of the billet and make timely temperature adjustments. During continuous production, due to rapid heat dissipation and easy pressure release at the material edges, problems such as excessive edge foaming, low density, and weak strength can easily occur. However, when the temperature sensor detects that the edge temperature is lower than the center temperature, it transmits the temperature signal to the control unit. The control unit adjusts the partition heating devices on both sides of the hot pressing zone to raise the temperature of the edge of the blank to the same as the center temperature, so that the foaming and cross-linking reactions of each part of the molten material are more balanced, ensuring that the final product has uniform performance.

[0023] After passing through the first hot-pressing unit, where the pressure and temperature are set relatively low, the blank can further adapt to the hot-pressing environment in this area, allowing the foaming agent and crosslinking agent to reach their decomposition critical temperatures. This ensures that the foaming and crosslinking reactions across the entire cross-section can be initiated simultaneously and uniformly during the formal hot-pressing process, avoiding excessively violent local reactions caused by sudden high temperatures and pressures. Subsequently, the blank further melts, with both the crosslinking agent and foaming agent entering the second hot-pressing unit in an activated state. The second hot-pressing unit is set at a temperature of 160℃-180℃, with the pressure slightly reduced to 0.5MPa, and is equipped with several ultrasonic generators. These ultrasonic devices simultaneously apply low-frequency, high-amplitude ultrasonic waves to the blank within the second hot-pressing unit, creating numerous additional uniform bubble nuclei within the blank, resulting in finer pores. Furthermore, the mechanical vibration reduces the apparent viscosity of the melt, promoting bubble expansion and uniform gas diffusion. In addition, the ultrasonic waves can break up the agglomeration of fillers or additives in the blank, improving the homogeneity of the melted blank. Within this unit, the crosslinking agent and foaming agent of the raw material react continuously, and with the influence of ultrasound, a homogeneous molten material is finally formed and reaches the outlet.

[0024] The hot-pressing zone outlet connects to the expansion zone inlet. Upon reaching the hot-pressing zone outlet, the molten material is instantly depressurized. The expansion zone inlet is connected to a funnel-shaped guide opening that gradually widens towards the hot-pressing zone outlet, facilitating the reception of the slightly enlarged molten material after depressurization. The expansion zone is designed according to the shape of the finished foam board and is a cavity structure with openings at both ends. In the production line described in this invention, the expansion zone is a detachable structure with adjustable width and height to accommodate expansion according to product dimensions. In this production scenario, the expansion zone is set to a width of 1.22mm and a height of 60mm based on the cross-sectional dimensions of the final product, providing dimensional redundancy for subsequent trimming of poorly shaped edges and surfaces. The expansion zone has a built-in heating device, setting its internal temperature to 120℃-160℃. During the expansion process, the high-pressure dissolved gas inside rapidly expands, driving all microbubbles to grow synchronously, causing the molten material to expand to its final size within the expansion zone. After initial cooling, it forms a preliminary finished product that reaches the outlet.

[0025] The initial product produced in the expansion zone already possesses basic material strength. It can be drawn into the cooling and shaping zone by traction rollers positioned at the expansion zone outlet. These traction rollers consist of symmetrically arranged steel rollers, used to clamp the initial product between them and pull it towards the cooling and shaping zone. The distance between the steel rollers is adjustable, and the surface of the rollers is coated with an anti-slip material, such as highly wear-resistant and highly thermally conductive modified polyurethane, which helps provide a stronger traction force for advancing the initial product.

[0026] The cooling and shaping zone employs a gradient cooling method to cool the initial product, divided into a slow cooling zone where an air-cooling device blows 80°C hot air onto the initial product; a strong cooling zone where an air-cooling device blows 30°C cold air onto the initial product; and a uniform temperature zone where the air blowing temperature is set according to the ambient temperature. After passing through the cooling and shaping zone, the initial product is uniformly cooled to below 35°C, the foam board is formed, and then trimmed, cut, and packaged by the cutting device at the end of the production line to form the final product. This zoned gradient cooling method effectively avoids problems such as stress concentration, deformation, and cracking caused by sudden temperature drops in traditional cooling methods, ensuring that the foam board maintains a stable shape and dimensional accuracy during the cooling process. In actual production, the air cooling temperature and blowing time in each area of ​​the cooling and shaping zone can be flexibly adjusted according to different specifications and performance requirements of the foam board to achieve the best cooling effect.

[0027] In the aforementioned implementation process, the push roller and traction roller provide overall "push-pull" propulsion power on the production line, making the material's movement on the production line smoother. By introducing a zoned temperature control device and an ultrasonic device in the hot pressing zone, the problem of uneven density in foam boards in traditional processes is effectively solved by controlling the temperature of different parts of the blank and by using ultrasonic waves to guide the uniform generation of bubbles. The smooth transition from the feeding end to the preheating zone ensures that the blank reaches a suitable molten state before entering the hot pressing zone, laying a good foundation for subsequent cross-linking and foaming reactions. Within the hot pressing zone, the linkage between temperature sensors and zoned heating devices enables precise control of the temperature at the edge and center of the blank, avoiding density unevenness caused by temperature differences. Simultaneously, the application of the ultrasonic device in the second hot pressing unit further promotes the uniform distribution of bubbles, reduces the viscosity of the blank, and makes the final molten material more homogeneous. The expansion zone cleverly utilizes the expansion force of the high-pressure dissolved gas inside the molten material to drive the synchronous growth of microbubbles, achieving rapid expansion and dimensional stability of the foam board. Finally, the gradient cooling treatment in the cooling and shaping zone not only ensures uniform cooling of the foam board, but also avoids internal stress concentration caused by rapid cooling, thus improving the overall quality of the product.

[0028] Furthermore, the upper and lower pressure plates of the hot pressing zone and the inner wall of the expansion zone are all coated with a Teflon coating, so that the upper and lower pressure plates and the inner wall of the expansion zone slide in contact with the surface of the molten material during production.

[0029] Through the above implementation process, polyethylene foam boards with thickness and width fixed according to design dimensions and length that can be arbitrarily cut are finally formed. By dividing the production line into different zones and setting up conveyor devices to connect each zone, the raw materials are fed into the production line in rolls, ensuring close connection between each step of the hot pressing molding process of polyethylene foam boards. By setting a reasonable production line length and travel speed based on the reaction time of the raw materials, the overall front-end heating and rear-end cooling of the foam board production process can be synchronized. Except for the equipment waiting period at the end of the raw material from entering the feed end to the final product output, subsequent continuous feeding eliminates equipment waiting periods in the intermediate processes. This invention achieves a continuous and efficient operation mode for the hot pressing molding production of polyethylene foam boards.

[0030] Furthermore, the ultrasonic devices are arranged in an array within the hot-pressing zone and connected to a control system. This control system can individually control the ultrasonic energy emitted by each ultrasonic device and the activation / deactivation of each device. Since a more vigorous and uniform foaming reaction typically produces a structure with a slightly higher average density but greater uniformity and strength, while insufficiently foamed areas may develop large pores due to cell merging, resulting in low density and a fragile structure, controlling the ultrasonic waves can ensure that the foam density and uniformity of different areas of the raw material are inconsistent during the foaming reaction, ultimately forming foam boards with varying hardness in different areas. For example, if the ultrasonic devices are activated in the middle of the production line's width direction while being deactivated on both sides, a gradient structure foam board material with high density in the middle and low density on both sides will be produced, eliminating the need for later splicing of finished foam boards of different qualities to meet product requirements and improving the product's adaptability to specific application scenarios.

[0031] This invention has many applications, including but not limited to the following described scenarios: 1. Due to its streamlined production process, this invention offers significant efficiency advantages for the production of foam boards with simple shapes, uniform sizes, and high demand. 2. This invention, by incorporating an ultrasonic device, utilizes ultrasonic-assisted physical foaming, making it suitable for production scenarios involving raw materials with high melt strength where bubble growth is difficult during hot pressing, such as those based on high-density polyethylene or high-filler systems. The ultrasonic device helps reduce the foaming difficulty, ensures finished product quality, and reduces reliance on chemical foaming agents, resulting in a more environmentally friendly product. 3. Because the present invention is equipped with an array-style ultrasonic device, the foaming at corresponding positions can be strengthened or weakened according to a preset pattern, thereby producing foam boards that need to meet special mechanical or functional requirements, such as foam boards with different hardness in different areas used in cushioning packaging.

[0032] It should be noted that the above-disclosed embodiments are merely preferred embodiments of the present invention and should not be construed as limiting the scope of the present invention. Those skilled in the art can understand that implementing all or part of the above-described embodiments and making equivalent changes in accordance with the claims of the present invention are still within the scope of the present invention.

Claims

1. A method for hot-pressing polyethylene foam boards, characterized in that, The production line for hot-pressing foam boards is divided into a feeding end, a preheating zone, a hot-pressing zone, an expansion zone, and a cooling and shaping zone, which are connected sequentially and equipped with a pushing device. The pushing device is used to push the production material on the production line at a constant speed. The method for hot-pressing polyethylene foam boards includes the following steps: Step S1, raw material preparation; In the mixing workshop, raw materials containing polyethylene particles, chemical reactants and other materials are hot-melted and mixed, and then calendered, cooled and shaped to prepare raw materials for processing; The chemical reactants include foaming agents and crosslinking agents; Step S2, preheating of the blank: Align the blank with the center line of the production line and put it into the feeding end. The blank is conveyed into the preheating zone at a uniform speed from the feeding end and heated into a molten material with ductility and in which the foaming agent and crosslinking agent inside are not activated. It is then pushed into the hot pressing zone. Step S3, hot pressing of molten material; The hot pressing zone includes an upper pressure plate and a lower pressure plate, and is equipped with a zone temperature control device and several ultrasonic devices. In the hot pressing zone, the molten material undergoes foaming and cross-linking reactions in the high-temperature environment controlled by the zone temperature control device and the high-pressure environment formed by the upper and lower pressure plates. At the same time, the ultrasonic devices are turned on to assist the molten material in uniform foaming. The molten material is initially hot-pressed and formed in this zone. Step S4, pressure release and expansion; the molten material is released from the hot pressing zone outlet and then immediately enters the expansion zone. The expansion zone is designed according to the shape of the finished foam board and is a cavity structure with openings at the front and rear ends. The molten material expands freely in the cavity of the expansion zone to form a preliminary finished foam board, which is then sent to the cooling and molding zone. Step S5, cooling and molding: The initial product is cooled to room temperature in the cooling and molding area by a cooling device, and then trimmed, cut and packaged to form the finished foam board.

2. The method for hot-pressing polyethylene foam boards according to claim 1, characterized in that, The preheating zone is set at a temperature between the polyethylene melting temperature and the activation temperature of the chemical reactant, while the hot pressing zone and the expansion zone are set at temperatures greater than the activation temperature of the chemical reactant.

3. The method for hot-pressing polyethylene foam boards according to claim 1, characterized in that, The surfaces of the upper and lower pressure plates in the hot pressing zone and the inner wall of the expansion zone are coated with a Teflon coating.

4. The method for hot-pressing polyethylene foam boards according to claim 1, characterized in that, The zoned temperature control device includes a temperature sensor connected to a control unit and several zoned heating devices arrayed in the hot pressing zone. The temperature sensor is used to acquire the temperature information of the molten material and transmit it to the control unit. The control unit is used to process the temperature information and drive the zoned heating devices to perform heating or cooling operations on different parts in different hot pressing zones.

5. The method for hot-pressing polyethylene foam boards according to claim 1, characterized in that, The hot pressing zone consists of at least two hot pressing units connected in series. The hot pressing unit closest to the entrance of the hot pressing zone is the first hot pressing unit, and the next one is the second hot pressing unit. The first hot pressing unit is set to a lower temperature than the other hot pressing units.

6. The method for hot-pressing polyethylene foam boards according to claim 5, characterized in that, The ultrasonic devices are arranged in an array within the second thermo-pressing unit and connected to a control system. The control system can control the ultrasonic energy emitted by each ultrasonic device and the start and stop of each ultrasonic device.

7. The method for hot-pressing polyethylene foam boards according to claim 1, characterized in that, The pushing device includes a pusher roller located at the junction of the preheating zone outlet and the hot pressing zone inlet, and a traction roller located at the junction of the expansion zone outlet and the cooling and shaping zone inlet.

8. The method for hot-pressing polyethylene foam boards according to claim 7, characterized in that, The traction roller includes steel rollers arranged symmetrically on the upper and lower sides, used to clamp the initial finished product between the upper and lower steel rollers and pull it towards the cooling and shaping zone. The distance between the steel rollers is adjustable, and the surface of the steel rollers is covered with anti-slip material.

9. The method for hot-pressing polyethylene foam boards according to claim 1, characterized in that, The expansion zone inlet is connected to a funnel-shaped guide port that gradually widens toward the hot-pressing zone outlet.

10. The method for hot-pressing polyethylene foam boards according to claim 1, characterized in that, The cooling and shaping zone uses an air-cooling device to cool the initial product. From the inlet to the outlet, the cooling and shaping zone is divided into a slow cooling zone, a strong cooling zone, and a uniform temperature zone. The air-cooling device in the slow cooling zone blows hot air at a temperature lower than that in the expansion zone onto the initial product. The air-cooling device in the strong cooling zone blows cold air at a temperature lower than that in the slow cooling zone but higher than that in the room temperature onto the initial product. The uniform temperature zone sets the blowing temperature according to the ambient temperature.

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