Working principle of preform mold
一. Introduction
二. Structural composition of preform molds
(I) Cavity and core
(II) Hot runner system
(III) Cooling system
(IV) Demolding system
三. Working principle of preform mold
(I) Mold closing
(II) Injection
(III) Holding pressure
(IV) Cooling
(V) Mold opening
(VI) Demolding
四. Application of preform molds in the production of different plastic bottles
(I) Mineral water bottles
(II) Beverage bottles
(III) Edible oil bottles
五. Factors affecting the performance of bottle preform molds
(I) Mold materials
(II) Mold design
(III) Processing technology
(IV) Molding process parameters
六. Conclusion
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一. Introduction
In the modern packaging industry, plastic bottles are widely used because of their many advantages such as lightness, durability and low cost. As a key intermediate product in the production of plastic bottles, the quality and production efficiency of preforms directly affect the quality and production cost of the final plastic bottles. Preform molds are the core equipment for realizing efficient and high-precision molding of preforms. In-depth research on the working principle of preform molds is of great significance to improving the quality and efficiency of plastic bottle production and promoting the development of the packaging industry.
二. Structural composition of preform molds
Preform molds are mainly composed of cavities, cores, hot runner systems, cooling systems, demolding systems and other parts.
(I) Cavity and core
The cavity is the external molding part of the preform, which determines the outer contour shape and dimensional accuracy of the preform. The cavity is usually made of high-quality steel, and its inner wall surface finish is extremely high to ensure the smoothness of the outer surface of the preform and reduce subsequent processing steps. The core is responsible for forming the internal shape of the preform, such as the inner diameter of the bottle mouth and the internal cavity of the bottle body. The core needs to withstand the high pressure impact and high temperature of the plastic melt during operation, so its material must not only have good strength and hardness, but also have certain heat resistance and wear resistance. In the design and manufacturing process, the matching accuracy of the cavity and the core is crucial, and the gap between the two needs to be precisely controlled to ensure the uniformity of the wall thickness of the preform.
(II) Hot runner system
The hot runner system is one of the key components of the preform mold. Its main function is to transport the plastic melt evenly and stably into the cavity. The hot runner system usually includes a hot runner plate, a nozzle, a heating element, and a temperature control system. The hot runner plate is the main body of the entire hot runner system. It has a flow channel inside for the transmission of the plastic melt. The nozzle is directly connected to the cavity to inject the melt into the cavity. The heating element is installed around the hot runner plate and the nozzle to keep the plastic melt in a molten state by heating to prevent the melt from solidifying in the flow channel. The temperature control system can accurately control the temperature of each part of the hot runner to ensure the uniformity and stability of the melt temperature. The use of a hot runner system can reduce the generation of plastic waste, improve production efficiency, and improve the quality of the preform, because the melt is always in a molten state in the hot runner, avoiding preform defects caused by cold material injection.
(III) Cooling system
The function of the cooling system is to quickly remove the heat inside the preform after the preform is formed, so that it can be cooled and solidified for demolding. The cooling system is generally composed of cooling water channels, which usually surround the cavity and core and transfer heat by circulating cooling water. The design of the cooling water channel needs to take into account factors such as the shape, wall thickness and cooling efficiency of the preform. A reasonable layout of the cooling water channel can make the preform cool evenly and reduce the internal stress and deformation caused by uneven cooling. In some high-performance preform molds, special cooling technologies are also used, such as conformal cooling technology, that is, the shape of the cooling water channel is adapted to the shape of the cavity or core, further improving the cooling efficiency and cooling uniformity.
(IV) Demolding system
The demolding system is responsible for removing the preform from the mold cavity after it cools and solidifies. The demolding system mainly includes components such as ejector pins, ejector tubes, and demolding plates. When the preform is formed and cooled to a certain degree, the demolding system starts to work. The ejector pin or ejector tube is driven by the power device to eject the preform from the core, and the demolding plate assists in removing the preform from the cavity. The design of the demolding system needs to consider factors such as the shape, size, and demolding force of the preform to ensure that the preform will not be damaged during the demolding process, while ensuring the smoothness and reliability of the demolding action.
三. Working principle of preform mold
The working process of the preform mold mainly includes mold closing, injection, pressure holding, cooling, mold opening, and demolding.
(I) Mold closing
Mold closing is the starting step of the preform mold. During the mold closing process, the movable mold part of the mold moves toward the fixed mold part under the drive of the mold closing mechanism, and the cavity and the core are gradually closed to form a closed molding cavity. The size of the mold clamping force needs to be reasonably set according to factors such as the size of the mold and the injection pressure of the plastic melt to ensure that the mold will not expand due to the melt pressure during the injection process, ensure the close fit between the cavity and the core, and prevent the plastic melt from leaking.
(II) Injection
After the mold is closed, the plastic raw material enters the mold cavity through the hot runner system under the push of the screw. During the injection process, the plastic melt needs to be injected into the cavity at a certain speed and pressure to ensure that the melt can fill the entire cavity. Too fast an injection speed may cause turbulence in the melt in the cavity, entraining air, causing defects such as bubbles in the preform; too slow an injection speed may cause the melt to cool and solidify during the filling process, resulting in insufficient filling of the preform. The size of the injection pressure also needs to be precisely controlled. Too high a pressure may cause the mold cavity to bear too much pressure, affecting the mold life, and may also cause defects such as flash on the preform; too low a pressure cannot ensure that the melt fills the cavity. The hot runner system plays a key role in the injection process. It can evenly distribute the plastic melt to each cavity to ensure the quality consistency of each preform.
(III) Holding pressure
After the melt fills the cavity, the holding pressure operation needs to be performed. The purpose of holding pressure is to supplement the volume reduced by the melt cooling and shrinking, so that the preform can maintain a certain pressure during the cooling process, thereby obtaining better density and dimensional accuracy. The setting of holding pressure and holding time needs to be adjusted according to factors such as the type of plastic, the size and wall thickness of the preform. Too high holding pressure or too long holding time may cause residual stress in the preform, affecting the performance of the preform; too low holding pressure or too short holding time may cause the preform to shrink too much and the dimensional deviation exceeds the standard.
(IV) Cooling
After the holding pressure is completed, the cooling system starts to work, and the heat inside the preform is taken away by circulating cooling water, so that the preform gradually cools and solidifies. The cooling process is a time-consuming link in the work of the preform mold, and its cooling efficiency directly affects the production cycle. During the cooling process, it is necessary to ensure that the preform is cooled evenly to avoid internal stress and deformation due to uneven cooling. The length of the cooling time depends on factors such as the material, size and wall thickness of the preform. Generally, the optimal cooling time needs to be determined through experiments or simulation calculations. When the preform cools to a certain degree and its strength is sufficient to withstand the demolding force, the mold opening operation can be carried out.
(V) Mold opening
Mold opening is the process of separating the movable mold part from the fixed mold part of the mold. The size of the mold opening force needs to be reasonably set according to factors such as the mold structure, the clamping force, and the adhesion between the preform and the mold. The mold opening process needs to be carried out smoothly and slowly to prevent damage to the mold and the preform due to excessive mold opening speed. During the mold opening process, the mold parting surface design needs to be considered to ensure that the parting surface can be separated smoothly and will not affect the appearance and dimensional accuracy of the preform.
(VI) Demolding
After the mold is opened, the demolding system starts to work and removes the preform from the mold cavity. During the demolding process, the demolding components such as the ejector pin and the ejector tube are driven by the power device to eject the preform from the core, and the demolding plate assists in removing the preform from the cavity. The size of the demolding force needs to be precisely controlled. Excessive demolding force may cause deformation or damage to the preform, and too small demolding force may not be able to smoothly remove the preform. After demolding, the preform can enter the subsequent processing steps, such as stretch blow molding, and finally become a plastic bottle product.
四. Application of preform molds in the production of different plastic bottles
Preform molds can be used to produce various types of plastic bottles, such as mineral water bottles, beverage bottles, edible oil bottles, etc. Different types of plastic bottles have different requirements for preform molds.
(I) Mineral water bottles
Mineral water bottles are usually made of polyethylene terephthalate (PET) material. For mineral water bottle preform molds, due to the large output of mineral water bottles, the mold needs to have a high production efficiency. The hot runner system is particularly important in the mineral water bottle preform mold, which can achieve fast and stable melt injection and reduce the production cycle. At the same time, since mineral water bottles have high requirements for appearance transparency and gloss, the surface finish of the cavity and core needs to be strictly controlled to ensure the smoothness of the outer surface of the preform, so that the final formed mineral water bottle has good appearance quality. In addition, the wall thickness of mineral water bottles is relatively thin. During the mold design and manufacturing process, it is necessary to accurately control the clearance between the cavity and the core to ensure the uniformity of the wall thickness of the bottle preform and prevent the strength problem of the bottle body caused by uneven wall thickness.
(II) Beverage bottles
There are many types of beverage bottles. In addition to the common PET beverage bottles, there are also some beverage bottles made of materials such as polypropylene (PP). For PET beverage bottle preform molds, their working principle is similar to that of mineral water bottle preform molds, but the design may be adjusted according to the special shape and functional requirements of the beverage bottle. For example, some functional beverage bottles may have a special hand-held shape or leak-proof structure, which requires the bottle preform mold to make corresponding innovations in the core or cavity design. For PP beverage bottle preform molds, since the characteristics of PP materials are different from PET, there are differences in melt viscosity, crystallization performance, etc., so targeted optimization is required in the temperature control of the hot runner system, injection pressure and speed settings, etc., to ensure the molding quality of the bottle preform.
(III) Edible oil bottles
Edible oil bottles are generally made of materials such as high-density polyethylene (HDPE). Edible oil bottle preform molds need to take into account the high melt strength and slow crystallization rate of HDPE materials. In mold design, the design of the cooling system needs to pay more attention to cooling uniformity, because if the HDPE bottle preform is not cooled uniformly during the cooling process, it is easy to produce large internal stress, which affects the performance of the bottle preform and the quality of subsequent blow molding. At the same time, due to the large capacity of edible oil bottles, the size of the bottle preform is relatively large, which puts higher requirements on the strength and rigidity of the mold. The structural design of the mold needs to be more reasonable to ensure that the mold can work stably and reliably during the long-term production process.
五. Factors affecting the performance of bottle preform molds
(I) Mold materials
The selection of mold materials directly affects the performance and life of bottle preform molds. High-quality mold materials should have the characteristics of high strength, high hardness, good wear resistance, heat resistance and corrosion resistance. For example, some high-performance mold steels can withstand the high temperature and high pressure of plastic melts, and are not easy to deform and wear during long-term work, thereby ensuring the accuracy and stability of bottle preform molds. If poor quality mold materials are used, the mold may deform, crack, and other problems during use, resulting in a decline in the quality of the preform and reduced production efficiency, and will also increase the cost of mold maintenance and replacement.
(II) Mold design
The rationality of mold design is one of the key factors affecting the performance of the preform mold. Reasonable cavity and core design can ensure the shape and dimensional accuracy of the preform, as well as the uniformity of wall thickness. The optimized design of the hot runner system can improve the injection efficiency and quality stability of the melt. The scientific layout of the cooling system can achieve rapid and uniform cooling of the preform, reduce internal stress and deformation. The ingenious design of the demolding system can ensure smooth demolding of the preform and avoid damage. In addition, the overall structural design of the mold, such as the selection of the parting surface and the design of the mold reinforcement ribs, will also affect the strength, rigidity, and smoothness of the mold opening and closing.
(III) Processing technology
The processing technology level of the mold has an important influence on its performance. High-precision processing technology can ensure the dimensional accuracy and surface finish of each component of the mold. For example, the processing accuracy of the cavity and core is directly related to the molding quality of the preform. Advanced processing technology, such as EDM and CNC machining, can produce mold parts with complex shapes to meet the diverse design requirements of preform molds. At the same time, the heat treatment process during the processing can improve the performance of the mold material, increase the hardness and toughness of the mold, and extend the service life of the mold. If the processing technology is rough, the dimensional deviation of the mold parts is large, and the surface finish is poor, it will lead to difficulty in mold assembly, unstable preform molding quality, and even failure to produce normally.
(IV) Molding process parameters
The molding process parameters include injection pressure, injection speed, holding pressure, holding time, cooling time, etc. The reasonable setting of these parameters has a vital impact on the performance of the preform mold and the quality of the preform. As mentioned above, the injection pressure and speed affect the filling process of the melt, the holding pressure and time affect the density and dimensional accuracy of the preform, and the cooling time affects the cooling effect and production cycle of the preform. In the actual production process, it is necessary to determine the optimal molding process parameters through experiments and simulation optimization based on factors such as the type of plastic, the size and shape of the preform, so as to give full play to the performance of the preform mold and produce high-quality preform products.
六. Conclusion
As a key equipment for plastic bottle production, the working principle of preform mold involves multiple complex links and systems. Through in-depth research on its structural composition, working principle, application in the production of different plastic bottles, factors affecting performance and future development trends, it can be seen that preform mold technology is constantly developing and innovating. In the future, with the advancement of development trends such as intelligence, high efficiency and energy saving, high precision and high performance, preform molds will play a more important role in the plastic packaging industry, providing strong technical support to meet the growing market demand and promote the sustainable development of the packaging industry. Relevant technicians and researchers should continue to pay attention to the development of preform mold technology, and actively explore new technologies and methods to promote the further improvement and perfection of preform mold technology.
Foshan Heyan Precision Mold Technology Co., Ltd.'s main business is preform molds, bottle blowing molds, bottle cap molds and other products. It has a strong engineering, R&D, technology, and sales team, advanced processing equipment and rich project experience. If you want to know more or have special needs, please call 13318345050, WeChat ID.pet bottle mold
