As a common hollow plastic product, Water Tanks can be divided into small household water tanks (tens of liters) and large industrial/civil water storage tanks (500L-5000L) according to their capacity. They need to meet core performance requirements such as leakage prevention, impact resistance, corrosion resistance, and uniform wall thickness for different applications. The production process puts clear requirements on the extrusion stability, molding accuracy, and structural adaptability of extrusion blow molding machines. Specifically, it can be divided into the following core dimensions, taking into account both equipment performance and the special needs of water tank production.
I. Extrusion System: Ensuring Melt Quality and Stable Output
The extrusion system is the core of the extrusion blow molding machine, which directly determines the plasticizing effect of water tank raw materials. The plasticizing quality directly affects the wall thickness uniformity, structural strength, and leakage prevention performance of the water tank, so it is necessary to focus on meeting the following requirements.
1. Screw Configuration Adapted to the Characteristics of Water Tank Raw Materials
The mainstream raw material for water tank production is HDPE (High-Density Polyethylene), and LLDPE, PVC, etc. are used in some scenarios. The raw materials need to have high melt strength and good environmental stress crack resistance. Therefore, the extruder screw needs to be adaptively designed:
- Screw Parameters: The length-diameter ratio (L/D) should be controlled between 30:1 and 32:1, which can not only ensure sufficient plasticization of raw materials but also avoid melt degradation caused by excessive shearing, ensuring uniform melt viscosity and adapting to the melt fluidity requirements of water tank production; The screw diameter should be adjusted according to the water tank capacity. A screw of about Φ80mm can be selected for small water tanks, and a screw of Φ90mm or more should be selected for large 2000L water tanks, with corresponding plasticizing capacity (350-400kg/h) to meet the melt supply needs of water tanks of different capacities.
- Screw Structure: A gradual compression ratio screw is adopted, with the compression ratio controlled between 3.5 and 4.5, to enhance the plasticizing effect and melt strength of raw materials and avoid wall thickness fluctuation of the water tank caused by uneven plasticization; For the production of multi-layer water tanks, multiple sets of screws need to be configured (for example, 3-6 layer models correspond to 3-6 sets of screws), which can realize synchronous extrusion of different functional layers (such as anti-ultraviolet layer and anti-leakage layer) and improve the service life of the water tank.
- Screw Material: Wear-resistant and corrosion-resistant alloy material (such as 38CrMoAlA) is selected, and the surface is nitrided to extend the service life of the screw, avoid the screw being worn by impurities that may be contained in the raw materials, and ensure the extrusion stability of long-term production.
2. Barrel and Head Design Meet Molding Requirements
- Barrel Temperature Control: Adopt zoned heating (feeding section, compression section, homogenizing section) and cooling design, with temperature control accuracy reaching ±1℃. The temperature of each section can be accurately adjusted according to the melting point of different raw materials (when producing HDPE, the barrel temperature is controlled at 160-200℃), to avoid melt degradation caused by excessive temperature and insufficient plasticization caused by too low temperature, ensure stable melt extrusion, and reduce the impact of parison sagging on the wall thickness of the water tank.
- Head Structure: A center-fed accumulator head is adopted, and the accumulator capacity is adjusted according to the water tank capacity (for example, the accumulator capacity of 2000L water tank model needs to reach 80L), which can stably output the parison and avoid parison fluctuation and deflection; The flow channel of the head should be reasonably designed, with smooth inner wall and no dead corners, to reduce melt retention and pressure loss and ensure uniform parison thickness; The die gap can be accurately adjusted, combined with the parison program control system, the die gap is accurately controlled by the servo motor to realize precise control of the axial wall thickness of the parison, compensate for the uneven stretching during the blow molding process, and preset corresponding wall thickness parameters for the corners, reinforcing ribs and other parts of the water tank to improve the structural strength of the water tank.
II. Blow Molding System: Ensuring Water Tank Molding Accuracy and Appearance Quality
The blow molding system is responsible for blow molding the extruded parison into the shape of a water tank. Its performance directly determines the dimensional accuracy, wall thickness uniformity, and appearance integrity of the water tank, so it needs to meet the following requirements.
1. Parison Control Accuracy Meets Standards
The water tank has extremely high requirements for wall thickness uniformity (the deviation should be controlled within ±0.1mm). Uneven wall thickness will lead to weak structure of the water tank, uneven cooling shrinkage, and easy deformation, leakage and other problems. Therefore, the parison control needs to meet:
- Equipped with a high-precision parison wall thickness control system, which can realize more than 100 control points, accurately adjust the axial and circumferential wall thickness of the parison. For the parison sagging problem that is easy to occur in large water tanks, compensate by optimizing the extrusion speed curve and increasing the initial thickness of the upper part of the parison to avoid excessive wall thickness difference between the upper and lower parts of the water tank.
- The parison weight is accurately controlled, and the maximum parison weight should match the water tank weight (for example, the parison weight of 2000L water tank can reach 60kg), to avoid insufficient parison weight leading to too thin wall thickness and insufficient strength of the water tank, or excessive weight causing raw material waste.
2. Stable and Reliable Blowing System
- Blowing Pressure and Flow Rate: Adjust according to the water tank capacity and wall thickness. The blowing pressure of small water tanks is controlled at 0.3-0.5MPa, and that of large water tanks needs to be increased to 0.5-0.8MPa. Multi-stage pressure control technology is adopted. A lower pressure is used in the initial stage to avoid local excessive stretching, and the pressure is increased after the material contacts the mold to ensure detailed molding; The blowing flow rate should be stable to avoid water tank defects such as depression and bulge caused by pressure fluctuation. The blow-up ratio is controlled between 2:1 and 4:1. Too high a ratio is easy to lead to too thin wall thickness, and too low a ratio is difficult to fully stretch the material, affecting the water tank molding quality.
- Blowing Timing and Cooling: The blowing timing should be accurately matched with the parison extrusion speed. Too early or too late will affect the material distribution, avoiding the parison being too cold to blow up or deformation due to untimely cooling after blow up; The blowing medium is preferably dry and clean compressed air, to avoid bubbles and scratches on the inner wall of the water tank caused by moisture and impurities in the air, which affect the leakage prevention performance.
3. Mold Adaptability and Cooling Efficiency
- Mold Adaptability: The mold size should be accurately matched with the designed size of the water tank. The mold cavity surface is smooth and free of burrs to ensure the water tank has a flat appearance and no scratches; The mold should have good sealing performance to avoid air leakage during blow molding, leading to incomplete water tank molding; For water tanks of different capacities (300L-2000L), the equipment should be compatible with corresponding size molds (the maximum mold size can reach 1600x1900mm) to improve equipment versatility.
- Cooling System: A multi-circuit cooling system is adopted to independently control the temperature of different areas of the mold. The temperature difference on the mold surface is controlled within ±5℃ to avoid water tank shrinkage and deformation caused by uneven cooling; The cooling water circuit is reasonably designed, and the cooling efficiency should meet the production cycle requirements. The cooling time of small water tanks is controlled at 30-60s, and that of large water tanks needs to be extended to 120-240s, to ensure that the water tank is quickly solidified after molding, improve production efficiency, and ensure the dimensional stability of the water tank.
III. Clamping System: Ensuring Molding Stability and Operational Convenience
The clamping system is responsible for opening, closing and locking the mold. Its clamping force, stroke and accuracy directly affect the water tank molding quality and production safety. Especially for large water tank production, the requirements for the clamping system are more stringent.
- Clamping Force: Determined according to the water tank capacity and mold size. The clamping force of small water tanks needs to reach 500-800KN, and the clamping force of large 2000L water tanks should not be less than 1800KN, to ensure that the mold does not loosen or overflow during blow molding, and avoid water tank defects such as flash and dimensional deviation.
- Clamping Stroke and Template: The clamping stroke should meet the needs of mold opening, closing and water tank taking out. The template size should match the mold (for example, 1700x1800mm template is suitable for large water tank molds). The parallelism error of the template is controlled within 0.1mm, to avoid wall thickness deviation of the water tank or mold damage caused by uneven force on the mold during clamping; The mold thickness adjustment range should be reasonable (such as 1010-1600mm) to adapt to water tank molds of different thicknesses.
- Operational Convenience: Equipped with automatic clamping and demolding mechanisms. Large water tanks need to be equipped with special automatic demolding devices to avoid water tank damage caused by manual demolding and reduce labor intensity; The clamping mechanism should have good stability and durability, and can withstand high-frequency opening and closing actions for a long time, reducing the impact of equipment failures on production.
IV. Control System: Realizing Precise Regulation and Batch Stability
Water tank production is mostly mass production, which requires precise control systems to ensure the stability of various parameters, reduce inter-batch errors, and improve production efficiency and product qualification rate.
- Control Accuracy: Adopt a high-precision PLC control system (such as Siemens PLC), which can accurately control key parameters such as extrusion speed, barrel temperature, head temperature, blowing pressure, and clamping speed. The parameter adjustment accuracy is high, and the response speed is fast, ensuring the coordinated stability of each process and realizing the consistency of mass production.
- Automation Function: It has automatic functions such as automatic feeding, automatic extrusion, automatic blow molding, automatic cooling, automatic demolding, and automatic flash trimming, reducing manual intervention, lowering labor costs, and avoiding product defects caused by manual operation errors; Equipped with a real-time wall thickness monitoring and feedback system, through ultrasonic thickness measurement, infrared temperature measurement and other technologies, real-time monitor the wall thickness changes of the parison and water tank, feed the data back to the control system for dynamic adjustment, and further improve the wall thickness consistency.
- Fault Warning and Recording: It has a complete fault warning function, which can timely detect problems such as extrusion abnormalities, temperature deviations, air leakage, and clamping failures, and issue warning signals to avoid the generation of batch unqualified products; At the same time, it can record production parameters and fault information, facilitating later problem troubleshooting and production process optimization, and improving production stability.
V. Auxiliary System: Adapting to the Special Needs of Water Tank Production
In addition to the core system, the auxiliary system of the extrusion blow molding machine needs to be adaptively designed to the particularity of water tank production to ensure the smooth progress of production and product quality.
- Raw Material Processing System: Equipped with raw material drying and screening devices to remove moisture and impurities in raw materials (moisture content should be controlled below 0.1%), avoiding bubbles and impurity points after water tank molding, which affect leakage prevention performance and appearance quality; For the use of recycled materials, the proportion of recycled materials should be controlled within 30% to avoid affecting melt strength and water tank performance.
- Exhaust System: Set reasonable exhaust devices on the barrel, head or mold to discharge air and volatile substances in the melt, avoid bubbles inside the water tank, ensure uniform wall thickness and dense structure of the water tank, and improve leakage prevention performance and structural strength.
- Safety Protection System: Equipped with complete safety protection devices. Especially for large extrusion blow molding machines, protective railings and emergency stop buttons need to be set in the clamping area and head area to avoid operators contacting high-temperature melt and moving parts, ensuring production safety; The high-pressure blowing system needs to be equipped with pressure protection devices to prevent equipment damage or safety accidents caused by excessive pressure.
- Environmental Protection and Energy Saving: The equipment should meet environmental protection requirements, reducing waste gas and noise emissions (noise controlled below 85dB); Adopt energy-saving heating, cooling systems and motors to reduce energy consumption. For example, the average power consumption of large water tank models is controlled at 192-280kw, improving production economy.
VI. Other Special Requirements
- Capacity Adaptability: The equipment should be able to flexibly adjust parameters according to the water tank capacity, compatible with the production of water tanks of different capacities (300L-5000L). For example, the 2000L model can be compatible with 500L-2000L water tanks, and the production efficiency is adjusted according to the capacity (16-18 pieces/hour for 500L water tanks, 8-9 pieces/hour for 2000L water tanks), improving equipment utilization.
- Special Performance Adaptability: If producing special-purpose water tanks such as anti-ultraviolet, anti-corrosion, and food-grade, the extrusion blow molding machine needs to have corresponding adaptability. For example, when producing food-grade water tanks, the parts of the equipment in contact with raw materials (screw, barrel, head) should be made of food-grade materials to avoid polluting raw materials; When producing anti-ultraviolet water tanks, a multi-layer extrusion system should be matched to realize synchronous molding of the anti-ultraviolet layer and the base layer.
In summary, the core requirements of Water Tank production for extrusion blow molding machines are “stability, precision, and adaptability”. It is necessary to ensure the molding accuracy, structural strength, leakage prevention performance and mass production needs of the water tank through reasonable configuration of screw, head and clamping systems, precise parameter control, and complete auxiliary systems. At the same time, it should be compatible with the production of water tanks of different capacities and uses, improving production efficiency and product qualification rate.
