Typical structure of injection device
The injection unit is a core component of an injection molding machine. Its primary function is to heat and plasticize the plastic raw material into a molten state and then inject the melt into the mold cavity at a specific pressure and speed. The typical structure of an injection unit generally consists of a feeding system, a plasticizing system, a transmission system, and an injection seat movement system. These components work together to complete the plasticizing, metering, and injection processes of the raw material. The structure of the injection unit may vary between different types of injection molding machines, but the basic components and operating principles are similar.
The feeding system forms the initial stage of the injection molding process and primarily consists of a hopper, a feeding and metering device, and a hopper base. The hopper, typically made of stainless steel, stores the plastic raw material. Its capacity depends on the production capacity of the injection molding machine and typically holds several hours’ worth of raw material. A feeding and metering device is located beneath the hopper to control the amount of raw material added, ensuring an accurate and consistent amount for each injection, which is crucial for ensuring the dimensional stability of the molded part. Common feeding and metering devices include volumetric and gravimetric. Volumetric metering devices measure the raw material by controlling the volume within the screw, offering a simple structure but relatively low accuracy. Gravimetric metering devices, on the other hand, utilize load cells to precisely measure the weight of the raw material, offering high accuracy and making them suitable for applications requiring high-quality parts. The hopper base connects the hopper to the plasticizing system and features an internal feeding port through which the raw material enters the plasticizing system for plasticization.
The plasticizing system is the core component of the injection molding machine, responsible for heating and melting the solid plastic feedstock, plasticizing it into a uniform melt. It primarily consists of a screw, barrel, and heating device. The screw, a key component of the plasticizing system, features spiral grooves on its surface. Driven by a transmission system, it rotates, pushing the feedstock forward from the feed port. During this pushing process, the feedstock is heated by the barrel and sheared and compressed by the screw, gradually transforming from a solid state to a molten state, forming a certain amount of melt at the screw head. The screw’s design varies depending on the characteristics of the plastic feedstock and the requirements of the molded part. Common types of screws include gradual-change, abrupt-change, and universal. Gradual-change screws have a longer compression section and are suitable for plasticizing crystalline plastics; abrupt-change screws have a shorter compression section and are suitable for plasticizing amorphous plastics. Universal screws fall somewhere in between these two types and can be used to plasticize a wide range of plastics. The barrel is a component that works with the screw. A certain gap is maintained between its inner wall and the outer circle of the screw. A heating device is installed on the outside of the barrel, usually a resistance heating ring or an electromagnetic heating device, to heat the barrel to the set temperature and provide heat for the plasticization of the raw materials.
The transmission system provides power for the screw’s rotation and axial movement and primarily consists of a motor, a reduction gearbox, and a transmission mechanism. The motor, typically a servo motor or variable-frequency motor, enables precise speed control to meet the screw’s speed requirements during the different plasticizing and injection stages. The reduction gearbox converts the motor’s high-speed rotation into the low-speed, high-torque motion required by the screw. Common reduction gearboxes include gearboxes and cycloidal pinwheel reduction gearboxes. The transmission mechanism transfers the reduction gearbox’s output power to the screw, driving its rotation. During the injection phase, the transmission system also propels the screw’s axial movement, injecting the melt into the mold cavity. To ensure smooth and precise screw movement, the transmission system typically includes guides and a lubrication system to reduce friction and wear on moving parts.
The injection seat movement system is used to move the injection seat back and forth, ensuring close contact or separation between the injection nozzle and the mold’s sprue bushing. It primarily consists of a movement cylinder, guide rails, and a positioning device. The movement cylinder is typically hydraulically driven, with the piston rod extending and retracting to move the injection seat back and forth along the guide rails. Movement speed and position can be precisely controlled by the hydraulic system. The guide rails provide guidance for the injection seat, ensuring smooth and accurate movement. Rectangular or dovetail guides are typically used, offering high rigidity and wear resistance. The positioning device ensures precise alignment between the injection nozzle and the mold’s sprue bushing, preventing melt leakage during the injection process. Common positioning devices include locating pins and locating sleeves. During the injection process, the injection seat movement system presses the injection nozzle against the mold’s sprue bushing, generating sufficient clamping force to prevent melt leakage. After the injection is complete, the injection seat movement system moves the injection nozzle out of the mold, preparing for the next shot.
