Injection molding process for preventing logistics tray from warping

Abstract

The invention discloses an injection molding process for preventing a logistics tray from warping. Comprising the following steps: S1, material pretreatment and injection: adding a nucleating agent into a polypropylene base material, and heating and melting; s2, melt injection and pressure maintaining are conducted, specifically, after the melt is injected into a cavity, multi-stage pressure maintaining is conducted; s3, sequential program control cooling and in-mold annealing stress relaxation are conducted, specifically, asynchronous differential cooling is conducted on different areas of the mold, then cooling is stopped, and heat preservation is conducted to achieve stress relaxation; and S4, constraint release and demolding: after the tray product is cooled and shaped, the constraint of the mold on the tray product is released, and the tray product is ejected out of the mold. The internal stress is systematically guided and relaxed by actively regulating and controlling the cooling program and the in-mold heat history, so that the dimensional stability and flatness of the tray are remarkably improved, and the mechanical property of the tray is enhanced.

Description

Technical Field

[0001] This invention relates to the field of polymer material processing technology, and in particular to an injection molding process that prevents logistics pallets from warping. Background Technology

[0002] Logistics pallets are widely produced using injection molding processes from semi-crystalline thermoplastic plastics such as polypropylene (PP). Typical existing injection molding processes include: material plasticization, melt injection, pressure holding and shrinkage compensation, mold cooling, and demolding. To address pallet warpage, industry-standard improvements primarily focus on optimizing the "pressure holding" and "cooling" stages. Examples include: increasing pressure and duration to enhance shrinkage compensation; optimizing cooling water system design or adjusting cooling water temperature for more uniform cooling; and using shaping fixtures for physical correction after demolding.

[0003] These conventional methods are of little effect on solving the persistent and difficult-to-correct warping deformation that occurs after demolding large pallets. At its root, existing technologies and their improvement ideas are generally based on the superficial understanding that "warping is caused by uneven cooling or shrinkage," and their improvement logic lies in passively pursuing "homogenization" (such as uniform cooling and sufficient shrinkage compensation).

[0004] However, existing processing techniques still have the following limitations: 1. During injection molding cooling of semi-crystalline polymers such as polypropylene, the crystallinity is negatively correlated with the cooling rate. The region near the mold cavity wall forms a "rapid cooling layer" with low crystallinity and high disorder due to rapid cooling, while the core region, which cools slowly, forms a "slow cooling layer" with high crystallinity and high order. The thermal shrinkage rates and mechanical properties of these two structural layers differ significantly. Traditional uniform cooling methods actually exacerbate this crystallinity gradient, generating continuous internal stress after demolding and driving product warping.

[0005] 2. In traditional injection molding processes, there is a sudden change in thickness between the tray reinforcing ribs and the main panel. During cooling and shrinkage, the shrinkage amounts and rates of the two differ, yet they are structurally connected and thus severely restrain each other, generating complex internal shear stresses. Existing technologies employ a single, high-strength pressure-holding strategy, which cannot provide differentiated shrinkage compensation for the ribs and panel to suit their structural characteristics, and therefore cannot alleviate this constraint stress. Summary of the Invention

[0006] The purpose of this invention is to solve the above-mentioned problems by proposing an injection molding process to avoid warping of logistics pallets.

[0007] To achieve the above objectives, the following technical solution was adopted: An injection molding process to prevent logistics pallets from warping includes the following steps: S1: Material pretreatment and injection: The polypropylene-based material is mixed with a nucleating agent, the mixed material is dried, and then it is plasticized by multi-stage gradient heating through an injection molding machine.

[0008] Specifically, the nucleating agent is a β-crystal nucleating agent, and its addition amount is 0.2~0.5% of the mass of the polypropylene-based material.

[0009] Specifically, the multi-stage gradient heating and plasticizing process involves dividing the heating temperature of the injection molding machine barrel into four gradient temperature control stages: the temperature of the nozzle section is controlled at 210~220℃, the temperature of the front section is controlled at 205~215℃, the temperature of the middle section is controlled at 200~210℃, and the temperature of the rear section is controlled at 190~200℃.

[0010] Specifically, during the heating and plasticizing process, the flow index of the mixed melt is monitored by an online viscometer, and a closed-loop feedback control algorithm is used to control the melt viscosity fluctuation within ±5% of the set value. When the melt viscosity exceeds this range, the control algorithm automatically adjusts the heating power of the injection molding machine barrel or the speed of the screw to correct the problem.

[0011] S2: Melt Injection and Holding Pressure: The heated and plasticized mixed melt is injected into the cavity of the tray mold, followed by multi-stage holding pressure.

[0012] Specifically, the multi-stage holding pressure adopts at least two stages of control, and: the first stage holding pressure is 70-80% of the injection pressure and lasts for 5-8 seconds, which is used to compensate for the shrinkage of the tray body panel; the second stage holding pressure is 50-60% of the first stage holding pressure and lasts for 10-15 seconds, which is used to focus on compensating for the shrinkage of the reinforcing rib root of the tray product, while maintaining the melt temperature at the root of the reinforcing rib higher than the material crystallization peak temperature during the second stage holding pressure.

[0013] S3: Sequential controlled cooling and in-mold annealing stress relaxation: Cool the pallet products after they have been pressurized.

[0014] Includes the following sub-steps: S301: Differentiated Cooling Stage: Based on the structural characteristics of the pallet product, the cooling process is divided into multiple cooling periods, and asynchronous cooling control is applied to different areas of the pallet product to cool each part of the pallet product at different rates.

[0015] Specifically, the cooling process includes at least the main panel cooling period, the rib cooling period, the edge reinforcement cooling period, and the rib-plate transition zone buffer cooling period.

[0016] Specifically, the different areas of the pallet product include at least the pallet main panel, reinforcing ribs, pallet edges, and pallet transition areas; the asynchronous cooling control of the differential cooling period is specifically as follows: the temperature of the pallet main panel area is controlled at 60~70℃, the temperature of the reinforcing rib area is controlled at 45~55℃, the temperature of the pallet edge area is controlled at 30~40℃, and the temperature of the pallet transition area is controlled at 30~40℃; wherein, the average cooling rate of the pallet main panel area is controlled at 3~5℃ / min, the average cooling rate of the reinforcing rib area is controlled at 5~8℃ / min, the average cooling rate of the pallet edge area is controlled at 8~12℃ / min, and the average cooling rate of the pallet transition area is between that of the pallet main panel area and the pallet reinforcing rib area, so that the pallet main panel area and the reinforcing rib area form a smooth thermal gradient transition.

[0017] Specifically, different cooling periods are controlled sequentially by intermittently switching the cooling medium on and off, with the on / off cycle being 5 to 15 seconds.

[0018] S302: Thermal relaxation stage: After step S301 is completed, the external cooling of the mold is actively stopped and a controllable heat source is introduced to keep the pallet product warm, so that the pallet product is kept warm in the mold for a period of time.

[0019] Specifically, the controllable heat source maintains the tray product in an isothermal environment of 85~95°C within the mold cavity for 20~40 seconds by controlling the heat exchange state of the mold.

[0020] S4: Constraint Release and Demolding: After the pallet product has cooled and solidified, release the constraint of the mold on the pallet product and eject it from the mold.

[0021] Specifically, the demolding process involves releasing constraints sequentially and then demolding. Specifically, the constraints on the reinforcing rib area of ​​the pallet product are released first, and the reinforcing rib area is ejected from the mold. After a delay of 0.5 to 1 second, the constraints on the pallet body panel area of ​​the pallet product are released, and the pallet body panel area is ejected from the mold. After a further delay of 0.5 to 1.5 seconds, the constraints on the pallet edge area of ​​the pallet product are released, and the pallet edge area is ejected from the mold.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention implements asynchronous, differentiated cooling control of different areas of the mold based on the pallet's structural characteristics, followed by in-mold heat preservation within a specific temperature window. This actively guides and relaxes the internal stress generated by unbalanced crystallization in the thickness and planar directions of the pallet product. This significantly reduces the overall deformation and internal stress of the formed pallet, resulting in excellent flatness. This meets the stringent requirements of high-precision stacking and automated logistics systems for carrier dimensional stability, fundamentally overcoming persistent warping.

[0023] 2. The differentiated multi-stage pressure holding strategy employed in this invention can provide adaptive shrinkage compensation for different structural parts such as the main tray panel and reinforcing ribs, reducing internal defects. Combined with the subsequent sequential demolding method, secondary stress damage during the ejection process is avoided. The synergistic effect of these measures improves the internal quality and stress distribution of the product, thereby enhancing its load-bearing strength, impact resistance, and fatigue durability. Attached Figure Description

[0024] Figure 1 This is a process flow diagram of an injection molding process for preventing logistics pallet warping according to the present invention. Detailed Implementation

[0025] To make the above-mentioned objectives, features and advantages of this application more apparent and understandable, the specific embodiments of this application will be described in detail below with reference to the accompanying drawings.

[0026] This example uses a standard 1200mm × 1000mm mesh plastic pallet. The main panel of the pallet is approximately 4mm thick, and the bottom has reinforcing ribs approximately 15mm high. The raw material is copolymer polypropylene (PP).

[0027] like Figure 1 As shown, an injection molding process to prevent logistics pallets from warping includes the following steps: S1. Material pretreatment and injection; The purpose of this step is to obtain a melt with uniform plasticization and good initial crystallization potential, laying the foundation for the subsequent controlled crystallization process.

[0028] First, the polypropylene-based material is premixed with 0.3% by mass of a β-crystal nucleating agent. The addition of this nucleating agent provides a large number of uniform crystallization initiation points for the mixed melt, which can effectively induce the formation of β-crystals in the polypropylene-based material. The crystallization of this type of material has better toughness and different shrinkage characteristics, which helps to improve the overall stress distribution.

[0029] Subsequently, the mixed materials are dried in an oven at 85℃~95℃ for 3~4 hours to remove moisture and prevent defects in the products caused by moisture vaporization.

[0030] The dried mixture is fed into the injection molding machine barrel for heating and plasticizing.

[0031] Specifically, in this embodiment, the temperature of the injection molding machine barrel is set in a multi-stage gradient: 195°C in the rear section, 205°C in the middle section, 210°C in the front section, and 215°C at the nozzle. The rear section primarily completes the initial melting and conveying of the mixed materials; the middle section further homogenizes and begins to disperse the nucleating agent; and the front section and nozzle ensure optimal fluidity and temperature uniformity of the plasticized melt. This gradually increasing temperature gradient ensures a smooth transition of the material from a solid to a molten state within the barrel, achieving thorough plasticization and uniform mixing, while preventing material degradation due to localized overheating. The residence time of the melt in the barrel should be controlled within 4-6 minutes.

[0032] Furthermore, in order to ensure the stability of the melt state in real time, the melt condition inside the barrel can be monitored in real time by an online viscometer during the plasticizing process. If the melt flow index fluctuates beyond the preset range of ±5%, the injection molding machine will be automatically controlled to fine-tune its heating power or screw speed for correction, so that the injection molding machine can output a stable homogeneous melt.

[0033] S2: Melt injection and holding pressure; The purpose of this step is to achieve smooth filling of the cavity and to selectively compensate for shrinkage in different structural areas in order to minimize stress caused by flow orientation and shrinkage differences.

[0034] During the melt injection stage, this embodiment employs a low-speed filling strategy, injecting the melt into the mold cavity at 35% of the maximum injection speed until the filling volume reaches 80% of the total cavity volume. By filling the mold cavity with the melt at a low speed, the melt can effectively fill areas with high flow resistance, such as tray reinforcing ribs and corners, reducing molecular chain orientation stress caused by high-speed shearing.

[0035] When the melt front reaches the preset switching point, the system immediately switches from the injection stage to the holding stage, which is controlled in real time by a pressure sensor built into the mold cavity. In this embodiment, the switching point is set at a critical location such as the intersection of ribs.

[0036] In this embodiment, a two-stage dynamic pressure holding strategy is adopted during the pressure holding stage: First-stage holding pressure: After the melt filling is complete, a pressure of 75% of the injection pressure is rapidly applied and held for approximately 6 seconds. The purpose of the first-stage holding pressure is to compensate for the macroscopic volume shrinkage caused by the large wall thickness of the tray body panel area.

[0037] The second stage of pressure holding then begins: the applied pressure is reduced to 55% of the first stage pressure, but the duration is extended to 12 seconds. The core purpose of the second stage of pressure holding is to gently replenish the cooling rate of the reinforcing rib roots, which are prone to shrinkage voids, with a lower but sustained pressure, thus maintaining the shrinkage capacity of the melt in that area.

[0038] Furthermore, to achieve this, this embodiment further monitors and controls the temperature at the root of the tray reinforcing rib by using an in-mold thermocouple, ensuring that it remains approximately 10°C above the material's crystallization peak temperature during the second-stage holding pressure. This delays premature solidification of the rib, ensures unobstructed shrinkage channels, effectively reduces the shrinkage difference between the rib and the panel, and lowers the internal stress between them from the source.

[0039] Step S3: Sequential controlled cooling and in-mold stress relaxation; The purpose of this step is to actively guide the product to form a uniform crystalline structure through a non-uniform but coordinated cooling strategy, and to provide it with conditions for stress relaxation.

[0040] It mainly includes the following steps: S301: Differentiated cooling; After the pressure holding period ends, the cooling process is precisely controlled in terms of time and space based on the structural characteristics of the tray.

[0041] Specifically, based on the structure of the pallet product, it is divided into the main panel of the pallet, the reinforcing ribs, the pallet edge, and the transition area between the main panel and the reinforcing ribs. At the same time, the entire cooling process is divided into the main cooling period of the panel, the cooling period of the ribs, the edge strengthening cooling period, and the buffer cooling period of the rib-panel transition area. The pallet product is cooled according to the order of the cooling process.

[0042] Main cooling phase of the panel: After the pressure holding stage, cooling of the main panel area of ​​the tray is initiated immediately. High-temperature cooling water is used to control the temperature of the main panel of the tray at around 65°C, maintaining an average cooling rate of 4°C / min. This allows the thickest part of the main panel to begin to shrink gently first, providing more time for crystallization and forming a more complete crystal structure.

[0043] Rib Cooling Period: After the main panel cooling period begins, cooling of the reinforcing rib area is initiated after a 10-second delay. Meanwhile, panel cooling continues, and both processes enter a parallel cooling phase. Medium-temperature cooling water is used to control the temperature of the tray reinforcing ribs at around 50°C, maintaining an average cooling rate of 6.5°C / min. This ensures that the cooling and shrinkage of the tray reinforcing ribs begins slightly later than the main tray panel, but at a slightly faster rate, thus actively guiding the shrinkage difference between the two towards a controllable direction, rather than allowing them to conflict freely. The edge reinforcement cooling period begins after a 5-second delay.

[0044] Edge Reinforcement Cooling Period: After a 5-second delay following the start of the rib cooling period, reinforced cooling of the tray edge area is initiated. During this time, cooling of the panel and ribs continues. Lower temperature cooling water is used to maintain the tray edge temperature around 35°C, and rapid cooling is performed at an average rate of 10°C / min to facilitate tray edge shaping.

[0045] The buffer cooling period of the rib-plate transition zone occurs between the main cooling period of the panel and the cooling period of the ribs. Five seconds after the main cooling period of the panel begins, the buffer cooling period of the rib-plate transition zone starts, controlling the temperature of the transition zone at around 55°C. This allows for buffer cooling at a rate between the two, of 5°C / min, enabling the transition zone to begin cooling and contracting before the ribs. This ensures that while the rib area is still hot, the transition zone has already begun to cool down and establish a preliminary buffer layer.

[0046] This step employs delayed initiation, systematically regulating the generation, relaxation, and distribution of internal stress by designing different cooling initiation points and intensities for different structural parts, guiding the internal stress state of the tray product towards equilibrium. First, cooling is applied to the main panel area to establish a rigid baseline for the overall structure; then, buffer cooling is initiated in the stiffener transition area to smooth the thermal gradient caused by sudden thickness changes; cooling of the reinforcing rib area is further delayed, allowing its contraction behavior to transform into orderly following on a stable base; simultaneously, early rapid cooling is applied to the edge areas to stabilize the contour, actively forming a stress preparation state with controllable internal distribution and a clear evolution direction within the product. This, together with the subsequent in-mold isothermal annealing step, constitutes a complete process loop from stress guidance to stress relaxation.

[0047] Furthermore, the control of the different cooling periods mentioned above can be achieved through intermittent switching of the cooling medium, with the switching cycle set to 10 seconds. This dynamic control method allows for more precise management of the mold's heat extraction process, preventing localized overcooling.

[0048] S302, Thermal relaxation stage; After approximately 40 seconds of differentiated cooling, the surface of the tray product had solidified, but the core remained at a high temperature of 115°C, marking a critical turning point in the process. First, all external cooling water was actively shut off, and a controllable heat source was introduced, or the residual heat of the mold itself was utilized, to maintain the tray product in an isothermal environment of 85-100°C within the cavity for 30 seconds. This temperature window is above the glass transition temperature of polypropylene, allowing sufficient mobility of the molecular chains; however, it is far below its melting point, preventing product deformation. The internal stress generated by rapid cooling and uneven shrinkage in the early stages is fully relaxed, allowing the macromolecular chains to gain energy for rearrangement, thereby achieving a more stable thermodynamic state in a controlled environment.

[0049] Furthermore, in this embodiment, the stress relaxation degree is monitored by an in-mold ultrasonic probe. When the sound velocity distribution in the thickness direction of the product tends to be uniform or the difference is <5%, this stage ends.

[0050] S4, Constraint Release and Demolding The purpose of this step is to ensure that the product leaves the mold with minimal stress by releasing constraints in an orderly manner during the final stage of demolding, which is prone to deformation.

[0051] After cooling and stress relaxation are completed, the demolding process adopts a sequential release of constraints.

[0052] Specifically, the ejection mechanism specifically designed for the reinforcing rib area is first activated to release the constraints on the rib area and gently eject it from the mold. After a 1-second delay, the constraints on the main panel area of ​​the tray are released and ejected. After a further 1.5-second delay, the constraints on the edge area of ​​the tray are finally released, completing the demolding of the entire product. This sequential demolding avoids the instantaneous stress concentration caused by different demolding resistances in different structural parts, preventing new deformations from occurring at the moment of demolding.

[0053] After demolding, the product can be immediately placed on an online shaping tray that perfectly matches the ideal three-dimensional shape of the tray until it cools to room temperature, thereby further ensuring the stability of its final dimensions.

[0054] The above description is merely a preferred embodiment of this application and is not intended to limit the scope of this application. Those skilled in the art may find other optimizations and additional functions in this application. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. An injection molding process to prevent logistics pallets from warping, characterized in that: Includes the following steps: S1: Material pretreatment and injection: The polypropylene-based material is mixed with a nucleating agent, the mixed material is dried, and after drying, it is plasticized by multi-stage gradient heating through an injection molding machine; S2: Melt Injection and Holding Pressure: The heated and plasticized mixed melt is injected into the cavity of the tray mold, followed by multi-stage holding pressure. S3: Sequential controlled cooling and in-mold annealing stress relaxation: Cool the pallet products after pressure holding; Includes the following sub-steps: S301: Differentiated cooling stage: Based on the structural characteristics of the pallet product, the cooling process is divided into multiple cooling periods, and asynchronous cooling control is applied to different areas of the pallet product to cool different parts of the pallet product at different rates. S302: Thermal relaxation stage: After step S301 is completed, the external cooling of the mold is actively stopped and a controllable heat source is introduced to keep the pallet product warm, so that the pallet product remains stationary in the mold for a period of time. S4: Constraint Release and Demolding: After the pallet product has cooled and solidified, release the constraint of the mold on the pallet product and eject it from the mold.

2. The injection molding process for preventing logistics pallet warping as described in claim 1, characterized in that: In step S1, the nucleating agent is a β-crystal nucleating agent, and its addition amount is 0.2~0.5% of the mass of the polypropylene-based material.

3. The injection molding process for preventing logistics pallet warping as described in claim 1, characterized in that: In step S1, during the heating and plasticizing process, the flow index of the mixed melt is monitored by an online viscometer, and a closed-loop feedback control algorithm is used to control the melt viscosity fluctuation within ±5% of the set value. When the melt viscosity exceeds this range, the control algorithm automatically adjusts the heating power of the barrel or the speed of the screw to correct it.

4. The injection molding process for preventing logistics pallet warping as described in claim 1, characterized in that: In step S1, the multi-stage gradient heating and plasticizing specifically involves dividing the heating temperature of the injection molding machine barrel into four gradient temperature control stages, wherein the temperature of the nozzle section is controlled at 210~220℃, the temperature of the front section is controlled at 205~215℃, the temperature of the middle section is controlled at 200~210℃, and the temperature of the rear section is controlled at 190~200℃.

5. The injection molding process for preventing logistics pallet warping as described in claim 1, characterized in that: In step S2, the multi-stage holding pressure adopts at least two stages of control, and: the first stage holding pressure is 70~80% of the injection pressure and lasts for 5~8 seconds, which is used to compensate for the shrinkage of the tray body panel; the second stage holding pressure is 50~60% of the first stage holding pressure and lasts for 10~15 seconds, which is used to focus on compensating for the shrinkage of the reinforcing rib root of the tray product, while maintaining the melt temperature at the root of the reinforcing rib higher than the material crystallization peak temperature during the second stage holding pressure.

6. The injection molding process for preventing logistics pallet warping as described in claim 1, characterized in that: In step S301, the cooling process includes at least the main panel cooling period, the rib cooling period, the edge strengthening cooling period, and the rib-plate transition zone buffer cooling period.

7. The injection molding process for preventing logistics pallet warping as described in claim 1, characterized in that: In step S301, the different areas of the pallet product include at least the pallet main panel, reinforcing ribs, pallet edges, and pallet transition areas. The asynchronous cooling control of the differential cooling period is specifically as follows: the temperature of the pallet main panel area is controlled at 60~70℃, the temperature of the reinforcing rib area is controlled at 45~55℃, the temperature of the pallet edge area is controlled at 30~40℃, and the temperature of the pallet transition area is controlled at 30~40℃. The average cooling rate of the pallet main panel area is controlled at 3~5℃ / min, the average cooling rate of the reinforcing rib area is controlled at 5~8℃ / min, the average cooling rate of the pallet edge area is controlled at 8~12℃ / min, and the average cooling rate of the pallet transition area is between the pallet main panel area and the pallet edge area, so that the pallet main panel area and the reinforcing rib area form a smooth thermal gradient transition.

8. The injection molding process for preventing logistics pallet warping as described in claim 6, characterized in that: In step S301, the timing of different cooling periods is controlled by intermittently switching the cooling medium on and off, wherein the on / off cycle is 5 to 15 seconds.

9. The injection molding process for preventing logistics pallet warping as described in claim 1, characterized in that: In step S302, the controllable heat source maintains the tray product in an isothermal environment of 85~95°C within the mold cavity for 20~40 seconds by controlling the heat exchange state of the mold.

10. The injection molding process for preventing logistics pallet warping as described in claim 1, characterized in that: In step S4, the demolding step adopts sequential release of constraints and demolding, specifically: firstly, the constraints on the reinforcing rib area of ​​the pallet product are released, and the reinforcing rib area is ejected and demolded; after a delay of 0.5 to 1 second, the constraints on the pallet body panel area of ​​the pallet product are released, and the pallet body panel area is ejected and demolded; after a further delay of 0.5 to 1.5 seconds, finally the constraints on the pallet edge area of ​​the pallet product are released, and the pallet edge area is ejected and demolded.

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