As a widely used logistics packaging component, hollow pallets are mainly used for stacking, loading, unloading and transporting goods. They have the advantages of light weight, corrosion resistance, impact resistance, recyclability and long service life. According to the logistics needs, they can be divided into standard size hollow pallets (1200×1000mm, 1100×1100mm) and customized size pallets, with load-bearing requirements ranging from 0.5t to 5t (dynamic load) and 2t to 20t (static load). Hollow pallets need to meet strict core performance requirements such as uniform load-bearing, regular size, impact resistance, wear resistance and no deformation. The production process puts specific requirements on the extrusion stability, molding accuracy, structural adaptability and production efficiency of extrusion blow molding machines. Specifically, it can be divided into the following core dimensions, taking into account both equipment performance and the special load-bearing and structural needs of hollow pallets production.
I. Extrusion System: Ensuring High-Strength Melt and Structural Integrity
The extrusion system is the core of the extrusion blow molding machine for hollow pallets production, which directly determines the plasticizing effect of raw materials, the structural density of pallets, and the final load-bearing capacity and impact resistance. Due to the large volume, thick wall thickness and complex grid structure of hollow pallets, the extrusion system must meet the requirements of stable melt output and sufficient plasticization.
1. Screw Configuration Adapted to HDPE/LDPE Raw Materials
The mainstream raw materials for hollow pallets production are HDPE (High-Density Polyethylene) and LDPE (Low-Density Polyethylene), and HDPE is preferred for its excellent mechanical strength and load-bearing performance. Some high-load-bearing pallets will add reinforcing additives (such as glass fiber, talcum powder) to improve rigidity and wear resistance. Therefore, the extruder screw must be adaptively designed:
- Screw Parameters: The length-diameter ratio (L/D) should be controlled between 30:1 and 33:1, which can ensure sufficient plasticization of HDPE/LDPE raw materials and uniform dispersion of reinforcing additives, avoid melt degradation caused by excessive shearing, and ensure that the melt has excellent mechanical strength and impact resistance; The screw diameter is adjusted according to the size and wall thickness of hollow pallets. A screw of Φ90-120mm can be selected for standard size pallets (1200×1000mm), and a screw of Φ120-150mm should be selected for large-size or high-load-bearing pallets, with corresponding plasticizing capacity (400-600kg/h) to meet the large melt supply needs of thick-walled hollow pallets.
- Screw Structure: A gradual compression ratio screw with mixing section is adopted, and the compression ratio is controlled between 3.8:1 and 4.8:1. The mixing section is added to enhance the dispersion effect of reinforcing additives, avoid uneven distribution of additives leading to reduced load-bearing capacity of pallets; Hollow pallets are mostly single-layer structure, and a single set of high-capacity screw is adopted. For pallets with wear-resistant surface layer, a double-layer co-extrusion screw can be configured to realize synchronous extrusion of base layer (HDPE) and surface wear-resistant layer (modified HDPE), ensuring the wear resistance of the pallet surface while ensuring load-bearing capacity.
- Screw Material: High-wear-resistant and corrosion-resistant alloy material (such as 38CrMoAlA) is selected, and the surface is subjected to deep nitriding treatment (nitriding layer thickness ≥0.6mm) to resist the wear of raw materials and reinforcing additives, avoid screw wear leading to uneven melt extrusion, and ensure the stability of long-term mass production of thick-walled pallets.
2. Barrel and Head Design Meet Thick-Wall Molding Requirements
- Barrel Temperature Control: Adopt high-precision zoned heating (feeding section, compression section, homogenizing section, mixing section) and cooling design, with temperature control accuracy reaching ±1℃. The temperature of each section can be accurately adjusted according to the performance of HDPE/LDPE (when producing HDPE hollow pallets, the barrel temperature is controlled at 165-205℃), to avoid melt degradation caused by excessive temperature and insufficient plasticization caused by too low temperature, ensure stable melt extrusion, and effectively control parison sagging (the sagging amount is ≤3mm/m) —— which is crucial for the wall thickness uniformity of large-size, thick-walled hollow pallets.
- Head Structure: A large-capacity center-fed accumulator head is adopted, and the accumulator capacity is adjusted according to the weight of hollow pallets (for example, the accumulator capacity of 1200×1000mm standard pallet model needs to reach 100-150L), which can stably output thick-walled parison (parison thickness 8-15mm) and avoid parison fluctuation and deflection; The flow channel of the head is designed with a large cross-section, smooth inner wall and no dead corners, to reduce melt retention and pressure loss, ensure uniform parison thickness; The die gap is driven by a servo motor with high precision (adjustment accuracy ±0.02mm), combined with the parison program control system, to realize precise control of the axial and circumferential wall thickness of the parison, compensate for the uneven stretching of the grid structure and support feet during blow molding, and preset corresponding wall thickness parameters for the load-bearing parts (support feet, grid ribs) to improve the load-bearing capacity and structural integrity of the pallet.
II. Blow Molding System: Ensuring Regular Shape and Uniform Load-Bearing
Hollow pallets have regular overall shape, complex internal grid structure and support feet, and extremely strict requirements on size accuracy and load-bearing uniformity. The blow molding system, as the core link of molding, must meet the requirements of precise molding, uniform wall thickness and intact structural details.
1. Parison Control Accuracy Meets Load-Bearing Standards
Hollow pallets have strict requirements on wall thickness uniformity (the deviation should be controlled within ±0.15mm). Uneven wall thickness will lead to uneven load-bearing of the pallet, easy deformation and damage under the action of goods weight, and even potential safety hazards during transportation. Therefore, the parison control must meet:
- Equipped with a high-precision parison wall thickness control system (up to 120 control points), which can accurately adjust the axial and circumferential wall thickness of the parison. For the parison sagging problem that is easy to occur in large-size, thick-walled parisons, compensate by optimizing the extrusion speed curve and increasing the initial thickness of the upper part of the parison, so as to ensure that the wall thickness difference between different parts of the pallet (surface, support feet, grid ribs) is ≤0.1mm.
- The parison weight is accurately controlled (control accuracy ±1%), and the maximum parison weight should match the pallet weight (for example, the parison weight of 1200×1000mm standard pallet can reach 80-120kg), to avoid insufficient parison weight leading to too thin wall thickness and insufficient load-bearing capacity of the pallet, or excessive weight causing raw material waste and affecting the lightweight of the pallet.
2. Stable and High-Pressure Blowing System
- Blowing Pressure and Flow Rate: Adjust according to the size, wall thickness and grid structure of hollow pallets. The blowing pressure is generally controlled at 0.8-1.2MPa, which is higher than that of water tanks and passenger car fuel tanks, to ensure that the thick-walled parison is fully blown up and the internal grid structure and support feet are completely formed; Multi-stage pressure control technology is adopted. A lower pressure is used in the initial stage to avoid local excessive stretching and material damage, and the pressure is increased after the material contacts the mold to ensure that the grid ribs and support feet (key load-bearing parts) are fully formed without shrinkage; The blowing flow rate is stable (flow rate fluctuation ≤5%), to avoid pallet defects such as depression, bulge and incomplete grid caused by pressure fluctuation. The blow-up ratio is controlled between 2:1 and 3:1, which is lower than that of fuel tanks, to ensure the wall thickness and load-bearing capacity of the pallet while realizing molding.
- Blowing Timing and Cooling: The blowing timing is accurately matched with the parison extrusion speed (response time ≤0.15s), too early or too late will affect the material distribution, avoid the parison being too cold to blow up or deformation due to untimely cooling after blow up; The blowing medium adopts dry, clean compressed air (moisture content ≤0.05%), to avoid bubbles and scratches on the inner and outer surfaces of the pallet caused by moisture in the air, which affect the structural density and load-bearing performance. After blowing, air cooling can be assisted to accelerate the cooling speed of the pallet surface and improve production efficiency.
3. Mold Adaptability and Efficient Cooling System
- Mold Adaptability: The mold size is accurately matched with the designed size of the hollow pallet (dimensional tolerance ±0.2mm), which meets the logistics packaging size standards. The mold cavity surface is polished with high precision (surface roughness Ra ≤1.0μm), smooth and free of burrs, to ensure the pallet has a flat surface and no scratches; The mold is designed with a built-in grid and support foot cavity, and the cavity structure is consistent with the pallet’s load-bearing design, to ensure the integrity and uniformity of the pallet’s internal structure; The mold has excellent sealing performance (air leakage rate ≤0.02L/min) to avoid air leakage during blow molding, leading to incomplete pallet molding; The equipment should be compatible with hollow pallet molds of different sizes and specifications, with a mold thickness adjustment range of 1000-2000mm to improve equipment versatility.
- Cooling System: A multi-circuit, high-efficiency cooling system is adopted to independently control the temperature of different areas of the mold (especially the grid ribs and support feet), and the temperature difference on the mold surface is controlled within ±4℃, to avoid pallet shrinkage and deformation caused by uneven cooling; The cooling water circuit is designed according to the pallet structure, with dense cooling points in thick-walled parts (support feet, grid ribs) and reasonable layout in thin-walled parts (pallet surface). The cooling efficiency meets the production cycle requirements. The cooling time of standard size pallets is controlled at 150-300s, and that of large-size, high-load-bearing pallets needs to be extended to 300-500s, to ensure that the pallet is quickly solidified after molding, improve production efficiency, and ensure the dimensional stability and load-bearing capacity of the pallet. The mold is also equipped with a temperature feedback device to realize dynamic adjustment of cooling temperature.
III. Clamping System: Ensuring Molding Stability and Operational Efficiency
Hollow pallet molds are large in size, complex in cavity structure and high in production frequency. The clamping system is responsible for opening, closing and locking the mold, and its clamping force, stroke and precision directly affect the pallet molding quality, production safety and mold service life.
- Clamping Force: Determined according to the pallet size, wall thickness and blowing pressure. The clamping force of standard size hollow pallet models needs to reach 1500-2500KN, and that of large-size, high-load-bearing pallet models should not be less than 2500-4000KN, to ensure that the mold does not loosen or overflow during blow molding, and avoid pallet defects such as flash, burr and dimensional deviation. The clamping force is adjustable (adjustment accuracy ±20KN) to adapt to pallets of different specifications.
- Clamping Stroke and Template: The clamping stroke is designed according to the maximum mold size and pallet shape, generally 1500-3000mm, to meet the needs of mold opening, closing and pallet taking out. The template size is matched with the mold (for example, 1800×2000mm template is suitable for 1200×1000mm standard pallet molds), and the template parallelism error is controlled within 0.1mm, to avoid wall thickness deviation of the pallet or mold damage caused by uneven force on the mold during clamping; The mold thickness adjustment range is 1000-2000mm, which is compatible with hollow pallet molds of different thicknesses and sizes.
- Operational Convenience and Safety: Equipped with automatic clamping, demolding and mold changing mechanisms. The automatic demolding mechanism is designed according to the pallet shape (with special clamping fixtures) to avoid pallet damage caused by manual demolding and reduce labor intensity; The mold changing mechanism adopts quick-change design (mold changing time ≤60min) to improve production efficiency when switching between different size pallets. The clamping mechanism has good stability and durability, and can withstand high-frequency opening and closing actions (≥8000 times/month) for a long time, reducing the impact of equipment failures on production. At the same time, a safety interlock device is equipped to prevent accidental operation during clamping, ensuring production safety.
IV. Control System: Realizing Precise Regulation and Batch Consistency
Hollow pallets production is mass production of logistics packaging products, which has high requirements on the consistency of product size, load-bearing performance and appearance. The control system must realize precise regulation of all parameters, reduce inter-batch errors, and ensure that each pallet meets the standard.
- Control Accuracy: Adopt a high-precision PLC control system (such as Siemens S7-1200/S7-1500 series) with high-speed processing capability, which can accurately control key parameters such as extrusion speed, barrel temperature, head temperature, blowing pressure, clamping force and cooling time. The parameter adjustment accuracy is high (extrusion speed fluctuation ≤1.5%, temperature control accuracy ±1℃), and the response speed is fast (response time ≤0.1s), ensuring the coordinated stability of each process and realizing the consistency of mass production (inter-batch dimensional deviation ≤0.2mm).
- Automation Function: It has full-automatic production functions such as automatic feeding, automatic extrusion, automatic blow molding, automatic cooling, automatic demolding, automatic trimming (trimming of flash and burrs) and automatic inspection, reducing manual intervention (manual intervention rate ≤10%), lowering labor costs, and avoiding product defects caused by manual operation errors; Equipped with a real-time wall thickness monitoring and feedback system (adopting ultrasonic thickness measurement technology, measurement accuracy ±0.02mm), real-time monitor the wall thickness changes of the parison and pallet, feed the data back to the control system for dynamic adjustment, and further improve the wall thickness consistency. At the same time, it is equipped with an online size detection system, which can automatically detect the length, width, height and flatness of the pallet, remove unqualified products, and ensure the qualification rate of products (≥99%).
- Fault Warning and Data Management: It has a complete fault warning and diagnosis function, which can timely detect problems such as extrusion abnormalities, temperature deviations, air leakage, clamping failures and incomplete molding of pallets, and issue audible and visual warning signals, and display fault points and handling suggestions, avoiding the generation of batch unqualified products; At the same time, it can record production parameters (extrusion speed, temperature, pressure, etc.), fault information and product inspection results, store data for ≥1 year, facilitate later problem troubleshooting, production process optimization and quality traceability, which meets the quality management requirements of the logistics packaging industry.
V. Auxiliary System: Adapting to the Special Load-Bearing and Environmental Needs of Hollow Pallets
In addition to the core system, the auxiliary system of the extrusion blow molding machine needs to be adaptively designed to the particularity of hollow pallets production, especially in terms of raw material processing, wear resistance and environmental protection, to ensure the smooth progress of production and the performance of pallets.
- Raw Material Processing System: Equipped with high-precision raw material drying, screening and mixing devices. The HDPE/LDPE raw materials need to be fully dried (moisture content ≤0.1%) to avoid bubbles and pinholes in the pallet caused by moisture, which affect the structural density and load-bearing performance; The screening device (screen mesh size ≤60 mesh) removes impurities in the raw materials to avoid wear of screws and molds and defects of pallets; The mixing device (mixing accuracy ±1.5%) uniformly mixes HDPE/LDPE and reinforcing additives (glass fiber, talcum powder) to ensure the uniform load-bearing performance of the pallet. For pallets with wear-resistant surface layer, independent raw material processing devices are configured for each layer of materials to avoid cross-contamination.
- Exhaust System: A high-efficiency exhaust system is set on the barrel, head and mold to discharge air, volatile substances and harmful gases generated during the plasticization of raw materials, avoid bubbles inside the pallet, ensure uniform wall thickness and dense structure of the pallet, and improve load-bearing capacity and impact resistance. The exhaust gas is treated by a purification device (purification efficiency ≥90%) before emission, meeting environmental protection requirements.
- Safety Protection System: Equipped with a complete safety protection device to meet the safety production requirements. Protective railings, light curtains and emergency stop buttons are set in the clamping area, head area and feeding area to avoid operators contacting high-temperature melt, moving parts and high-pressure gas, ensuring production safety; The high-pressure blowing system and hydraulic system are equipped with pressure protection devices to prevent equipment damage or safety accidents caused by excessive pressure; The electrical system adopts dust-proof and moisture-proof design to adapt to the workshop environment of logistics packaging production.
- Environmental Protection and Energy Saving: The equipment meets national environmental protection standards, reducing waste gas, noise and waste emissions (noise controlled below 85dB); Adopt energy-saving heating (electromagnetic induction heating, thermal efficiency ≥90%), cooling systems and variable frequency motors to reduce energy consumption. For example, the average power consumption of standard size pallet models is controlled at 250-350kw, and that of large-size, high-load-bearing pallet models is controlled at 350-500kw, improving production economy. At the same time, the waste flash and defective products generated during production can be recycled and reused (recycling ratio ≤30%, to avoid affecting the performance of pallets).
VI. Other Special Requirements
- Size and Load-Bearing Adaptability: The equipment should be able to flexibly adjust parameters according to the size and load-bearing requirements of hollow pallets, compatible with the production of pallets of different sizes (1000×800mm to 1500×1200mm) and load-bearing levels (0.5t to 5t dynamic load). For example, the standard size pallet model can be compatible with 1100×1100mm and 1200×1000mm pallets, and the production efficiency is adjusted according to the size and wall thickness (5-8 pieces/hour for standard size pallets, 3-5 pieces/hour for large-size, high-load-bearing pallets), improving equipment utilization. For customized pallets (such as anti-slip surface, special-shaped support feet), the equipment can be adapted to special molds and molding processes.
- Special Performance Adaptability: Hollow pallets need to meet the logistics packaging industry standards (such as GB/T 15234-2021 “Plastic Pallets”, ISO 8611 “Plastic Load-Bearing Pallets for International Logistics”). The extrusion blow molding machine must be compatible with these standards, and the produced pallets can pass tests such as load-bearing, impact resistance, wear resistance, high and low temperature resistance and anti-slip; For outdoor-used pallets, the equipment must be compatible with anti-ultraviolet modified raw materials, ensuring that the pallet does not age, crack or deform under long-term sunlight; For food-grade or pharmaceutical-grade logistics pallets, the equipment and raw material contact parts should be made of food-grade materials to avoid pollution.
In summary, the core requirements of Hollow Pallets production for extrusion blow molding machines are “stability, precision, load-bearing adaptability and high efficiency”. Compared with water tank and automotive fuel tank production, it has higher requirements on melt output, blow molding pressure, mold size and cooling efficiency, focusing on ensuring the load-bearing capacity, size regularity and structural integrity of pallets. It is necessary to ensure the dimensional accuracy, structural strength, load-bearing performance and wear resistance of the pallet through reasonable configuration of screw, head and clamping systems, precise parameter control, and complete auxiliary systems, meet the strict standards of the logistics packaging industry, and realize stable mass production. At the same time, it should be compatible with pallets of different sizes and specifications, improving production efficiency and product qualification rate.
