What is the compression ratio for injection molding?

1, The concept and calculation method of compression ratio
The compression ratio in injection molding refers to the ratio of the volume of one screw groove in the feeding section of the injection molding machine screw to the volume of the last screw groove in the homogenization section (metering section). As the core component of injection molding machines, the design of screws directly affects the melting, conveying, and injection processes of plastics. Screw is usually divided into three parts: feeding section, compression section, and homogenization section (metering section). The design of the screw may vary depending on the type of plastic and product requirements.
The calculation method of compression ratio is mainly based on the volume changes of the screw grooves in each section of the screw. Specifically, the screw groove in the feeding section has a larger volume and is used for conveying raw materials; The volume of the compression section spiral groove gradually decreases, used for compacting, melting materials, and establishing pressure; The homogenization section (metering section) has the smallest and constant screw groove volume, which is used to extrude molten plastic at a constant temperature and quantity to the front end of the screw for injection. Therefore, the compression ratio is the ratio of the volume of the feeding section screw groove to the volume of the homogenization section screw groove, usually expressed as ε=h1/h3, where h1 is the depth of the feeding section screw groove and h3 is the depth of the homogenization section screw groove.
2, The influence of compression ratio on injection molding
Compression ratio, as an important parameter in injection molding, has a significant impact on the quality, performance, and production efficiency of plastic products. Specifically, changes in compression ratio can affect the melting, transportation, and injection processes of plastic in the screw, thereby affecting the accuracy, surface smoothness, mechanical properties, and production efficiency of plastic products.
Plastic melting and transportation: The larger the compression ratio, the more complete the melting and transportation process of plastic in the screw. In the compression stage, plastic is subjected to strong shear and friction, which is conducive to the melting and mixing of plastic. In the homogenization stage, the temperature and pressure of the plastic are further homogenized and stabilized, which is beneficial for the quality control of plastic products.
Injection process: Changes in compression ratio can affect the flowability and injection pressure of the plastic during the injection process. The higher the compression ratio, the better the flowability of the plastic, and the injection pressure decreases accordingly. This is beneficial for reducing energy consumption and wear during the injection process, and improving production efficiency. However, excessively high compression ratios may also cause plastic to overheat, burn, or generate excessive frictional heat, thereby affecting the quality of plastic products.
The quality of plastic products: Compression ratio has a significant impact on the accuracy, surface smoothness, and mechanical properties of plastic products. A reasonable compression ratio can ensure uniform flow and filling of plastic in the mold, reducing the occurrence of defects such as bubbles and shrinkage. Meanwhile, changes in compression ratio can also affect the shrinkage rate and internal stress distribution of plastic products, thereby affecting their mechanical properties and dimensional stability.
3, Selection and optimization of compression ratio
In practical applications, the selection of compression ratio needs to be comprehensively considered based on the type of plastic, product requirements, and the performance of the injection molding machine. Different plastics have different melting characteristics and flowability, so it is necessary to choose the appropriate compression ratio based on the characteristics of the plastic. At the same time, the requirements for product accuracy, surface smoothness, and mechanical properties will also affect the selection of compression ratio. In addition, the performance parameters of injection molding machines, such as screw diameter, speed, and injection pressure, can also affect the selection of compression ratio.
To optimize the compression ratio, the following measures can be taken:
Choose the appropriate screw design: Select the appropriate screw design based on the type of plastic and product requirements, including the length, diameter, groove shape, and proportion of each section of the screw.
Adjusting injection molding machine parameters: By adjusting the speed, injection pressure, and temperature of the injection molding machine, the influence of compression ratio on plastic products can be further optimized.
Strengthen quality control: Strengthen quality control in the production process, including pre-treatment of raw materials, cleaning and maintenance of molds, and inspection of finished products, to ensure that the quality of plastic products meets design requirements.