How to choose the right injection molding machine to match the preform mold
一． Understand the basic parameters of the preform mold
1. Number of cavities
2. Mold size
3. Injection volume requirements
4. Required clamping force
二． Consider the characteristics of plastic raw materials
1. The fluidity of plastics
2. Melting point and thermal stability of plastics
3. Plastic shrinkage
三． Matching of injection molding machine performance parameters
1. Injection system parameters
2. Clamping system parameters
3. Temperature control system parameters
四．Requirements for production efficiency and automation
1. Production efficiency considerations
2. Matching the degree of automation
Accurately match the injection molding machine with the preform mold to improve production efficiency and quality. Welcome to consult!
一． Understand the basic parameters of the preform mold
1. Number of cavities
The number of cavities of the preform mold is one of the key factors in choosing an injection molding machine. The number of cavities determines the number of preforms that can be produced per injection molding. If it is a single-cavity mold, the production efficiency is relatively low, but the injection volume and clamping force requirements of the injection molding machine are relatively small. For example, when producing small batches and high-precision special-purpose preforms (such as pharmaceutical preforms), single-cavity molds are more suitable.
Multi-cavity molds (such as 4-cavity, 8-cavity, 16-cavity, etc.) can greatly improve production efficiency and are suitable for large-scale production. However, this requires the injection molding machine to have a large injection volume and sufficient clamping force to ensure that each cavity can be filled and molded normally. For example, for an 8-cavity preform mold, when selecting an injection molding machine, its injection volume must at least meet the volume of plastic melt required for the simultaneous molding of 8 preforms, and the clamping force must be able to resist the expansion force generated by the plastic melt in the cavity.
2. Mold size
The overall dimensions (length, width, and height) of the preform mold determine whether the template size and mold opening stroke of the injection molding machine are appropriate. The installation dimensions of the mold must match the installation template size of the injection molding machine to ensure that the mold can be firmly installed on the injection molding machine. If the mold size is too large, it may not be installed on the selected injection molding machine; if the size is too small, it may shake during the injection molding process due to loose fixation, affecting the molding quality of the preform.
The thickness of the mold (closing height) is also very important. The injection molding machine has a minimum and maximum mold thickness limit, and the thickness of the selected mold should be within this range. Generally speaking, the template spacing of the injection molding machine should be able to accommodate the mold, and there should be a certain adjustment margin when closing the mold to adapt to molds of different thicknesses. For example, if the minimum mold thickness of a certain injection molding machine is 150mm and the maximum mold thickness is 400mm, then the thickness of the selected preform mold should be within this range.
3. Injection volume requirements
Calculate the volume of plastic melt required for each injection of the mold based on the volume of the preform and the number of cavities. The volume of the preform can be calculated by the geometric formula of its shape (usually a cylinder or a near-cylinder). For example, for a preform with a diameter of 20mm and a height of 50mm, its volume is about 15700mm³. If it is an 8-cavity mold, the minimum volume of plastic melt required for each injection is about 125600mm³.
When selecting an injection molding machine, its theoretical injection volume should be greater than the injection volume required by the mold. There are two ways to express the injection volume of a general injection molding machine: one is the injection volume (cm³ or g) based on polystyrene (PS), and the other is the volume corresponding to the diameter and stroke of the screw. According to the density of the plastic raw material and the actual injection volume requirements of the preform, the two should be converted to ensure that the injection molding machine can meet the injection volume requirements of the preform mold.
4. Required clamping force
The clamping force is to overcome the expansion force generated by the plastic melt during the injection process and ensure that the mold can be tightly closed during the injection and holding process. The size of the expansion force is related to the pressure of the plastic melt and the projected area of the cavity. It can be calculated by the formula F = p×A, where F is the clamping force, p is the average pressure of the plastic melt in the cavity, and A is the sum of the projected areas of all cavities on the parting surface.
For preform molds, different plastic materials and preform shapes will result in different expansion forces. For example, the melt pressure of PET plastic is relatively high, and this factor should be considered when calculating the clamping force. Generally speaking, insufficient clamping force will cause flash on the mold parting surface, affecting the quality of the preform; excessive clamping force may damage the mold and injection molding machine.
二． Consider the characteristics of plastic raw materials
1. The fluidity of plastics
Different plastic raw materials have different fluidities. For example, PP (polypropylene) has good fluidity, while PC (polycarbonate) has relatively poor fluidity. If the preform is made of plastic with good fluidity, when choosing an injection molding machine, you can choose a model with a slightly higher screw speed and a slightly faster injection speed to make full use of the fluidity of the plastic and improve production efficiency.
For plastics with poor fluidity, the injection molding machine needs to have sufficient injection pressure and a suitable screw structure to push the plastic melt to fill the cavity smoothly. Some injection molding machines are equipped with special screw heads or plasticizing devices that can better handle plastics with poor fluidity. For example, injection molding machines with barrier screw heads can enhance the plasticizing effect of plastics and improve their fluidity.
2. Melting point and thermal stability of plastics
The melting point of plastics determines whether the heating system of the injection molding machine can heat the plastic raw materials to the appropriate processing temperature. For example, the melting point of PET is about 250-260℃, and the barrel temperature of the injection molding machine should be able to reach and stabilize in this temperature range to ensure that the PET plastic can be fully melted and evenly plasticized.
Plastics with poor thermal stability (such as PVC) are easy to decompose at high temperatures, requiring the injection molding machine to have precise temperature control capabilities. Some high-end injection molding machines are equipped with advanced temperature control systems, which can accurately control the temperature of the barrel, nozzle and other parts to avoid decomposition of plastic due to overheating and ensure the quality of the preform.
3. Plastic shrinkage
The shrinkage of plastic will affect the dimensional accuracy of the preform. Different plastics have different shrinkage rates. For example, the shrinkage rate of PET is generally around 1.2-2.0%, and the shrinkage rate of PP is about 1.0-2.5%. When choosing an injection molding machine, consider whether it can provide appropriate holding pressure and holding time to compensate for the shrinkage of the plastic.
Some injection molding machines have a holding pressure control function, which can accurately adjust the holding pressure parameters according to the shrinkage characteristics of the plastic and the size requirements of the preform to ensure that the size of the preform is stable during the cooling process. For example, by setting the appropriate holding pressure and holding time, the size deviation of the preform can be controlled within a smaller range.
三． Matching of injection molding machine performance parameters
1. Injection system parameters
Injection pressure: The injection pressure should be selected according to the characteristics of the plastic raw material and the requirements of the preform mold. For the preform mold, the injection pressure is generally between 80-150MPa. If the viscosity of the plastic is high or the structure of the preform is complex, a higher injection pressure is required to ensure that the plastic melt can fill the cavity. For example, when producing preforms with fine structures (such as those with complex textures or thin-walled parts), an injection pressure of 120-150MPa may be required.
Injection speed: The injection speed also has a great influence on the molding quality of the preform. The appropriate injection speed can make the plastic melt flow evenly in the cavity, avoiding phenomena such as stagnation and jetting. Generally speaking, the injection speed of the preform mold is between 50-150mm/s. For thin-walled preforms, a higher injection speed can prevent the melt from cooling prematurely and ensure uniform wall thickness of the preform.
Screw parameters: The diameter and aspect ratio (L/D) of the screw should match the injection volume and the characteristics of the plastic raw material. The aspect ratio is generally between 18-25, and a larger aspect ratio is conducive to plasticization and mixing of the plastic. The diameter of the screw is selected according to the injection volume. For example, for a preform mold with a large injection volume, a screw with a diameter of 40-60mm may be required. In addition, the structure of the screw (such as gradual type, sudden type, etc.) will also affect the plasticizing effect of the plastic, and it should be selected according to the fluidity and other characteristics of the plastic raw materials.
2. Clamping system parameters
Clamping force: As mentioned above, the required clamping force should be calculated based on the cavity projection area of the preform mold and the plastic melt pressure, and then an injection molding machine with sufficient clamping force should be selected. The clamping force of a general injection molding machine ranges from tens of tons to thousands of tons. When selecting, it is necessary to ensure that its clamping force is greater than the calculated required clamping force, and there should be a certain margin to deal with possible abnormal situations.
Mold opening stroke and template size: The mold opening stroke must meet the demolding requirements of the preform mold. The mold opening stroke must be at least greater than the total height of the preform and the gate to ensure that the preform can be demolded smoothly. The template size must match the outer dimensions of the mold to ensure that the mold can be stably installed on the injection molding machine.
3. Temperature control system parameters
The temperature control system of the injection molding machine should be able to accurately control the temperature of the barrel, nozzle and other parts. The temperature control accuracy should generally be within ±5℃. For some temperature-sensitive plastics (such as PVC), the accuracy requirement is higher, which may be within ±2℃. The heating power of the temperature control system should be sufficient to quickly heat the plastic raw materials to the required processing temperature and maintain the temperature stability during the production process.
Some injection molding machines also have a cooling function for cooling the barrel and mold. In the production of preforms, the cooling system of the mold and the cooling function of the injection molding machine cooperate with each other to improve the molding efficiency and quality of the preforms. For example, the injection molding machine can control the flow and temperature of the mold cooling medium (such as water) so that the preforms can be cooledickld evenly.
四．Requirements for production efficiency and automation
1. Production efficiency considerations
If the production efficiency of the preforms is required to be high, it is necessary to select an injection molding machine with fast injection speed and fast mold opening and closing speed. For example, the opening and closing speed of some high-speed injection molding machines can reach several hundred millimeters per second, and the injection speed is much faster than that of ordinary injection molding machines, which can greatly shorten the molding cycle of the preforms.
At the same time, the screw speed of the injection molding machine will also affect the production efficiency. Higher screw speed can speed up the plasticization process of plastic and increase the injection frequency. However, the screw speed should also be reasonably adjusted according to the characteristics of the plastic raw material to avoid overheating or uneven plasticization of the plastic due to excessively high speed.
2. Matching the degree of automation
In modern preform production, the degree of automation is getting higher and higher. If you need to realize automated production, such as automatic feeding, automatic demoulding, automatic packaging, etc., you should choose an injection molding machine with corresponding automation interfaces and functions. Some injection molding machines can be integrated with robots or automated production lines to achieve full automation of preform production.
The control system of the injection molding machine is also very important. Injection molding machines with advanced control systems can easily set and adjust process parameters, and can realize automated fault diagnosis and alarm functions to improve production reliability and efficiency. For example, the control system can preset parameters such as the molding cycle, injection pressure, temperature, etc. of the preform, and monitor these parameters in real time during the production process. Once an abnormal situation occurs, an alarm will be issued in time and corresponding measures will be taken.
Foshan Heyan Precision Mould Technology Co., Ltd.'s main business is preform moulds, bottle blowing moulds, bottle cap moulds 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 us at 13318345050, WeChat ID is the same.pet bottle mold
