On the Injection Molding Technology for Preform Molds

　　Regarding injection molding technology for eco‑friendly, lightweight PET preform molds, driven by both self‑interest and considerations of environmental protection and sustainable development, an increasing number of bottled‑water manufacturers are now starting at the preform stage to “slim down” their existing PET bottles. By reducing the weight of the preforms themselves, they aim to achieve localized weight reduction.

　　An eco‑friendly lightweight bottle, as its name suggests, is an environmentally sustainable container produced by reducing the amount of PET resin pellets used in the preform. During manufacturing, advanced production techniques are employed to integrate and sequence compatible equipment, streamlining complex process steps and seamlessly coordinating various machines in a systematic, orderly manner. This approach effectively saves floor space, lowers energy consumption, cuts CO2 emissions, and significantly enhances overall efficiency.

　　 1. Preform

　　For PET bottle preforms, the most critical parameter is the acetaldehyde (AA) level. Excessively high AA levels not only compromise taste but also pose health risks. Typically, the AA content in the preform should be kept below 8 ppm. IV, or intrinsic viscosity, is used to characterize the average chain length of the PET polymer and is directly proportional to molecular weight. It is closely linked to the physical properties of injection‑molded preforms and significantly influences their mechanical performance and transparency; a common target range is 70–85 mL/g.

　　 2. Injection Molding Machine

　　For an injection molding machine, the mold‑closing mechanism must exhibit sufficient rigidity, excellent dynamic performance, uniform mold deformation, and high repeatable positioning accuracy. During the no‑load cycle, the cycle time should be short, and the ejection force must be both substantial and rapid to ensure sensitive and effective low‑pressure mold protection. The mold opening position and the ejector pin positions must be precisely aligned to guarantee accurate and reliable handling by the robotic arm during part removal.

　　*Employing a high-performance two-stage plasticizing system, a specially designed plasticizing screw, and highly sensitive backpressure control, this process achieves a large plasticizing capacity at low screw speeds, while minimizing shear stress and reducing the residence time of the molten material. This effectively prevents excessive AA values and IV degradation. The temperature control system must be precise, with barrel and hot-runner mold temperatures freely adjustable as needed. By integrating computer‑based control technology, the entire molding process and all relevant parameters can be monitored online, while networked systems enable remote monitoring and diagnostics. Given the high degree of automation, the complete equipment suite—comprising an injection molding machine, molds, robotic arms, dehumidifiers, and drying systems—should be interconnected via a bus architecture for easy control, thereby facilitating fully unmanned production.

　　Injection molding technology for lightweight preforms:

　　 1. Clamping Unit

　　Optimizing the design of the clamping unit enables faster cycle times: an enhanced five-point inclined‑hinge architecture delivers superior kinematic and mechanical performance; finite‑element analysis is employed to increase mold plate rigidity and reduce deflection, while structural optimization boosts motion speed and enhances stability, making the system particularly well suited for dedicated PET preform injection molding. A specialized mold‑plate design featuring reinforcing ribs ensures more uniform stress distribution and minimizes deformation. A wide tie‑bar spacing accommodates a greater number of cavities. The fully closed‑loop control system utilizes MOOG servo valves to drive servo cylinders in both the clamping and opening/closing mechanisms, achieving finer control and smoother, more stable operation. An auxiliary ejection system further increases ejection force. On a 360‑ton machine, the no‑load cycle time for opening and closing is just 1.8 seconds, with improved low‑pressure protection.

　　 2. Low-pressure mold protection system

　　The low-pressure mold‑protection system can precisely detect changes in the clamping force on the mold and accurately determine whether foreign objects are present inside. It also assesses whether the presence of such objects is unaffected by the mold’s travel speed, ensuring reliable detection even when the mold moves at high velocity. The time from detecting a foreign object to halting the mold’s motion should be extremely short—ideally 10 ms, with actual response times not exceeding 15 ms. Moreover, the system operates at higher speeds than conventional equipment, thereby effectively reducing mold‑closing time and idle cycle durations.

　　PET preform mold

　　 3. Injection Unit

　　In response to the specific molding process conditions of PET preform systems, an advanced two-stage injection method has been adopted. This approach separates and independently controls the plasticization and injection of PET resin, thereby enhancing screw plasticization efficiency and injection accuracy, shortening the molding cycle, and effectively reducing the AA value. The use of a plunger‑type design further improves the precision of injection volume control. A dedicated PET screw, paired with a high‑torque plasticizing motor, ensures thorough resin plasticization, meeting stringent processing requirements and boosting screw plasticization efficiency. Additionally, this unit features temperature monitoring at the feed port, and its pneumatic gate valve also incorporates a discharge function.

　　 4. Robotic arm

　　The cooled‑station take‑out plate is accessed from the side, effectively enhancing preform cooling. The use of linear guide rails ensures precise robotic motion while also providing convenient access for maintenance. A modular, built‑in preform conveyor design further simplifies servicing.

　　The Siemens servo control system employed enables seamless integration with the system’s PLC control, enhancing stability and simplifying operation. Moreover, the 96‑cavity robotic arm features a dual‑drive, dual‑rail design that significantly boosts load capacity and motion speed, helping to shorten the molding cycle.

　　 5. Advanced Control System

　　By collaborating with partners and integrating with the PET preform manufacturing process, this system not only enables closed-loop control of injection speed and holding pressure, but also employs a multi-level management model with varying access permissions, ensuring that all advanced settings can be accessed only by authorized administrators, thereby effectively safeguarding product quality. Furthermore, by consolidating the drying system with the injection molding machine’s control system, management and operation have been streamlined and made more convenient.

　　 6. Unique hydraulic control system

　　The hydraulic system employs advanced variable‑pump control technology and an accumulator‑based approach. The seamless integration of the variable pump and accumulator ensures more stable, intuitive, and rapid control, while delivering exceptional energy efficiency. The entire hydraulic system is equipped with original German‑made components, including high‑performance pumps and valves, enabling precise and reliable operation. Furthermore, in its hydraulic design, Dao Yi Company adheres to the principle of positioning all valves and oil‑distribution plates as close as possible to actuating components such as cylinders, thereby enhancing both speed and accuracy of control.

　　 7. Connection between the preform machine and the blow molding machine: When connecting the preform machine and the blow molding machine, the following points should also be taken into consideration:

　　<1> Before connecting, the preform must be cooled to room temperature, with uniform temperatures on both the inner and outer surfaces.

　　<2>The connecting lines should minimize drop points to prevent defects such as impact damage and scratches on the preforms.

　　<3>Try to avoid corners and collisions.

　　<4>Install cooling devices, such as fans. If possible, *choose a conveyor belt with ventilation features.

　　<5>Sufficient conveyance length is required to ensure adequate cooling of the preform. Typically, lightweight preforms have wall thicknesses close to 2.0 mm and cool more rapidly than standard preforms; therefore, the theoretical conveyance distance should not exceed 30 m.

　　<6>The conveyor speed should be moderate—neither allowing bottle preforms to pile up nor being too fast.

　　<7>The ambient temperature should be appropriate. If the ambient temperature is too high, consider installing a blower to supply cool air.

　　<8>Considering that downstream equipment may experience downtime, an additional bottle‑preform handling unit should be added, enabling direct preform loading and packaging while the preforms are conveyed onto the conveyor belt.