About preform Mold Injection Molding Technology

Regarding the injection molding technology of environmentally friendly lightweight PET preform molds, out of self-interest and environmental protection and sustainable development considerations, more and more bottled water manufacturers began to "slim down" existing PET bottles from the preform before the project, hoping to achieve local weight reduction by reducing the weight of the preform itself.

Environmentally friendly lightweight bottle,

as the name implies, is an environmentally friendly bottle made by reducing the amount of PET preform plastic slices on the basis of the original bottle weight. In the production process of this product, due to the use of advanced production technology to integrate and sequence the matching production equipment, so as to optimize the cumbersome production steps, and make the various manufacturing equipment scientifically and orderly integrated. The benefits of this are floor space savings, reduced energy consumption, reduced CO2 emissions, and significantly improved efficiency.

1. Bottle preforms

For PET bottled water preforms, the most important thing is the AA (acetaldehyde) value of the preforms. Too high a AA value not only affects the taste, but is also harmful to the body. Generally, the IV (viscosity) of the preform is required to be less than 8 ppm. IV is used to characterize the average length of PET polymer chains, proportional to molecular weight. It is directly related to the physical properties of injection preforms, and directly affects the mechanical properties and transparency of preforms, and the general value is 70~85ml/g.

2. Embryo injection machine

For the embryo injection machine, the mold clamping mechanism must have sufficient rigidity, excellent motion speed characteristics, mold deformation uniformity and good repeatability accuracy. The empty cycle time period should be short, and the release force should be large and rapid to ensure that the low-pressure mold protector is sensitive and effective. The position of the mold opening and the position of the ejector pin should be accurate and consistent to ensure that the manipulator is accurate during the embryo retrieval process.

* Using high-performance double-stage plasticizing system, special plasticizing screw design and high sensitive back pressure control, so as to achieve large plasticizing amount and low screw speed, and ensure the decrease of shear force, shorten the residence time of molten material, and effectively prevent the possibility of high AA value and IV drop. The temperature control system should be accurate, and the temperature of the barrel and hot runner mold can be freely adjusted as needed. By using computer control technology, the entire molding process and parameters can be detected online, and remote monitoring and diagnosis can be realized using network technology. Due to the high degree of automation of production, a complete set of equipment composed of embryo injection machine, mold, manipulator, dehumidifier and drying system should all be connected by bus, so that it is easy to control, so that the unmanned production mode can be realized.

Injection molding technology for lightweight preforms:

1. Clamping unit

Optimized design of the clamping unit can achieve faster action: the 5-point oblique hinge design structure optimized in cooperation with South China University of Technology is adopted, which has good motion and mechanical properties; Using finite element analysis, the rigidity and deflection of the template are increased, and the structural elements are optimized, which improves the movement speed and increases the stability, making it more suitable for the special injection molding system of PET preforms; It adopts professional template design with stiffener design, so that the force is more uniform and the deformation of the template is smaller; The design of wide tie rod spacing enables the mold to accommodate more cavities; The fully closed-loop control system is adopted, and the clamping and opening and closing system uses MOOG servo valve to control the servo cylinder, which makes the control more precise and the operation more stable; Ejection system with assisted ejection helps to provide greater ejection force. A 360T device has an empty cycle time of only 1.8s for opening and closing dies, and better low-voltage protection.

2. Low-pressure mold protection system

The low-pressure mold protection system can accurately detect the change of resistance on the template, and accurately determine whether there is a foreign object in the mold, and at the same time judge whether the presence of foreign matter is not affected by the moving speed of the template, and can still accurately judge even when the template moves at a high speed. From judging the presence of a foreign object to stopping the movement of the template, this time should be very short, 10ms in theory, but not more than 15ms in practice. The speed of low-pressure mold guarding is also faster than that of other conventional equipment, which can effectively reduce the clamping time and empty cycle time.

PET preform mold

3. Injection unit

In response to the special molding process conditions of PET preform systems, the company adopted an advanced two-stage injection method. This injection method controls the plasticization and injection of PET raw materials separately, which not only improves the plasticizing efficiency of the screw and the accuracy of injection, shortens the molding cycle, but also effectively reduces the AA value. The use of the plunger design allows for more precise control of the injection volume. The plasticization of raw materials is realized by a special PET screw equipped with a high-torque plasticizing motor, which can well meet the plasticization process requirements and improve the plasticizing efficiency of the screw. In addition, the unit monitors the temperature of the discharge port and its pneumatic gate valve also has a discharge function.

4. Manipulator

The station removal plate with cooling enters from the side, effectively increasing the cooling of the preforms. The use of linear guides not only ensures the accuracy of the manipulator's movement, but also provides a convenient maintenance space. The modular built-in preform conveyor belt design makes maintenance easier.

The Siemens servo control system used can be effectively integrated with the PLC control of the whole system, so that the system is more stable and the operation is more convenient. The 96-cavity manipulator adopts a dual-drive, dual-guide design, which significantly improves the load carrying capacity and motion speed, and helps to shorten the molding cycle.

5. Advanced control system

Through cooperation and combined with the production process of PET preforms, the system can not only realize the closed-loop control of injection speed and holding pressure, but also adopt the management mode of different levels and different management rights, ensuring that all advanced settings must be accessed by management permissions, so as to effectively control the quality of products. By integrating the drying system and the control system of the injection molding machine, management and operation are made easier.

6. Unique hydraulic control system

The hydraulic system uses the best variable displacement pump control technology and accumulator method. The perfect combination of variable displacement pump and accumulator makes control more stable, easy and fast, while at the same time providing outstanding energy savings. The whole hydraulic system uses original components from Germany, high-performance pump and valve devices, so that the operation of the whole hydraulic system is more accurate and stable. In addition, in the hydraulic design, the company follows the principle that all valves and oil plates are as close as possible to the operating parts such as the cylinder, so that the control is faster and more accurate.

7. Connection between preform machine and

blowing machine: When connecting preform machine and blowing machine, the following matters should also be paid attention to:

<1> Before connection, the preform must be cooled to room temperature, and the temperature of the inner and outer walls is consistent.

<2> The connection should minimize the drop point to avoid defects such as bumps and scratches in the preform.

<3> Try to avoid curves and collisions.

<4> Add cooling devices, such as fans, etc. If possible, *choose a conveyor belt with a breathable function.

<5> There should be enough conveying length to make the preform get enough cooling. Generally, the wall thickness of lightweight preforms is close to 2.0mm, which cools faster than ordinary preforms, so the theoretical conveying distance should not exceed 30m.

<6> The speed of the conveyor line should be moderate, neither to allow the preforms to accumulate, nor too fast.

<7> The ambient temperature should be suitable. If the ambient temperature is too high, consider providing a blowing device to supply cold air.

<8> Considering that the subsequent equipment may be stopped, it is also necessary to add a set of preform splicing devices, so that the preforms can be directly packaged while being transported to the conveyor belt.