Injection molding machines, also known as injection molding machines or injection molding machines, are the main molding equipment used to produce various shapes of plastic products from thermoplastic or thermosetting plastics using plastic molds. They are classified as vertical, horizontal, and all-electric. Injection molding machines heat the plastic, apply high pressure to the molten plastic, and inject it to fill the mold cavity.
Injection molding machines typically consist of an injection system, a mold clamping system, a hydraulic transmission system, an electrical control system, a lubrication system, a heating and cooling system, and a safety monitoring system.
How does an injection system work?
The working principle of an injection molding machine is similar to that of a syringe. It uses the thrust of a screw (or plunger) to inject pre-plasticized molten plastic (i.e., viscous flow state) into a closed mold cavity, and then obtains the product after solidification and shaping.
Injection molding is a cyclical process. Each cycle mainly includes: metered feeding—melting and plasticizing—pressure injection—mold filling and cooling—mold opening and part removal. After the plastic part is removed, the mold is closed again to start the next cycle.
Injection molding machine operation includes three aspects: control keyboard operation, electrical control system operation, and hydraulic system operation. This involves selecting injection process actions, feeding actions, injection pressure, injection speed, and ejection type; monitoring the temperature of each section of the barrel; and adjusting the injection pressure and back pressure.
The general molding process of a screw injection molding machine is as follows: First, granular or powdered plastic is added to the barrel. The plastic is molten through the rotation of the screw and the heating of the barrel's outer wall. Then, the machine closes the mold and moves the injection unit forward, bringing the nozzle close to the mold's sprue. Next, pressurized oil is introduced into the injection cylinder, propelling the screw forward and injecting the molten material into the cooler, closed mold at high pressure and speed. After a certain time and pressure holding period (also known as holding pressure) and cooling, the material solidifies and is then removed from the mold. (The purpose of holding pressure is to prevent backflow of molten material in the mold cavity, replenish material into the mold cavity, and ensure that the product has a certain density and dimensional tolerance.) The basic requirements of injection molding are plasticization, injection, and molding. Plasticization is a prerequisite for achieving and ensuring the quality of the molded product, while injection must ensure sufficient pressure and speed to meet molding requirements. Simultaneously, due to the high injection pressure, a correspondingly high pressure is generated in the mold cavity (the average pressure in the mold cavity is generally between 20 and 45 MPa), therefore, a sufficiently large clamping force is necessary. Therefore, the injection unit and the mold clamping unit are the key components of an injection molding machine.
The evaluation of plastic products mainly involves three aspects: first, appearance quality, including integrity, color, and gloss; second, the accuracy of dimensions and relative positions; and third, physical, chemical, and electrical properties appropriate for the intended use. These quality requirements vary depending on the application of the product. Defects in the products primarily stem from issues with mold design, manufacturing precision, and wear. However, in reality, technicians in plastic processing plants often struggle to effectively compensate for mold defects using technological means.
Adjusting the process during production is essential for improving product quality and yield. Because the injection molding cycle is very short, poor process control can lead to a continuous stream of defective products. When adjusting the process, it's best to change only one condition at a time, observing the changes multiple times. Adjusting pressure, temperature, and time all at once can easily cause confusion and misunderstanding, making it difficult to understand the cause of problems. There are many measures and methods for adjusting the process. For example, there are more than ten possible solutions to the problem of incomplete injection. It's crucial to select one or two key solutions to address the root cause to truly resolve the issue. Furthermore, attention should be paid to the dialectical relationships within the solutions. For instance, if a product exhibits depressions, sometimes the material temperature needs to be increased, sometimes decreased; sometimes the material quantity needs to be increased, sometimes decreased. The feasibility of using reverse engineering methods to solve the problem must be acknowledged.
