Define the parameters of the injection molding machine
Defining injection molding machine parameters is fundamental to ensuring smooth production and part quality. These parameters encompass every aspect of the process, including the injection system, mold clamping system, hydraulic system, and temperature control system. Each parameter has a specific meaning and function and must be precisely set based on the part’s material, structure, and molding process requirements. Properly defining injection molding machine parameters not only improves production efficiency but also reduces material waste and equipment loss, making it a core element of injection molding technology.
Injection system parameters are the core of defining injection molding machine parameters, primarily including shot volume, injection pressure, injection speed, and holding pressure. Shot volume refers to the maximum amount of melt the machine can inject during a single cycle (based on PS). It is determined based on part weight and is typically 1.3-2 times the part weight. For example, for a 200g part, a machine with a shot volume of 260-400g should be selected to avoid material shortages due to insufficient shot volume or material waste due to excessive shot volume. Injection pressure is the force required to propel the melt into the mold cavity. Its magnitude is related to the plastic viscosity, part wall thickness, and runner length. Low-viscosity plastics (such as PE and PP) require an injection pressure of 50-80 MPa, while high-viscosity plastics (such as PC and POM) require 80-150 MPa. Thin-walled parts require 20%-30% higher injection pressure than thick-walled parts. Injection speed affects the melt’s flow and mold filling time, and is generally controlled between 30 and 150 mm/s. Fast injection is suitable for thin-walled parts and materials with poor fluidity, while slow injection is suitable for thick-walled parts and materials prone to flash. Holding pressure, typically 50% to 80% of the injection pressure, replenishes the material needed for melt cooling and shrinkage. The holding time is determined by part thickness, approximately 1 to 2 seconds per millimeter of wall thickness.
The clamping system parameters directly impact mold locking and part molding accuracy, primarily including clamping force, mold opening and closing speed, and platen stroke. Clamping force is the maximum force applied by the injection molding machine to lock the mold. It must be greater than the melt’s expansion force on the mold cavity. The calculation formula is F = P × A, where P is the melt pressure (generally 30-50 MPa) and A is the projected area of the part on the parting surface. The clamping force should be 1.2-1.5 times the calculated value. For example, for a part with a projected area of 1000 cm², the clamping force should reach 3600-9000 kN. Mold opening and closing speeds are divided into three stages: fast, slow, and low-pressure protection. Fast opening and closing speeds shorten production cycles (10-30 m/min), slow speeds are used when the molds are about to make contact (0.5-2 m/min), and the low-pressure protection stage (5-10 bar) prevents mold damage from foreign matter. The template stroke is the maximum moving distance of the movable template, which must be greater than the sum of the plastic part height and the demoulding stroke. It is usually 3-4 times the plastic part height to ensure that there is enough space to remove the plastic part.
The hydraulic and temperature control system parameters guarantee the stable operation of the injection molding machine. Hydraulic system parameters include system pressure and flow rate. System pressure is generally 10-16 MPa, providing power for actions such as mold closing and injection. Flow rate affects the speed of movement and needs to be adjusted according to the needs of different stages. High flow rate is required during the rapid phase, while low flow rate is required during the pressure-holding phase. The oil temperature should be controlled between 35-55°C. Excessively high temperatures will reduce the viscosity of the hydraulic oil and increase leakage, while excessively low temperatures will increase system pressure loss. This can be adjusted using coolers and heaters. Temperature control system parameters mainly refer to the temperature of each section of the barrel and the mold temperature. The barrel temperature is set according to the type of plastic, such as 180-220°C for PP and 260-300°C for PC, with the temperature gradually increasing from the hopper to the nozzle. Mold temperature affects the crystallinity and cooling rate of the plastic part. Crystalline plastics require higher mold temperatures (40-80°C), while amorphous plastics require lower mold temperatures (20-60°C). This can be precisely controlled using a water or oil temperature controller, with the temperature differential kept within ±2°C.
Other important parameters of the injection molding machine include screw speed, back pressure and cycle time. The screw speed affects the plasticization quality and efficiency of the melt, and is generally 50-150r/min. Too high a speed will cause overheating and degradation of the melt, while too low a speed will result in uneven plasticization. It needs to be adjusted according to the thermal sensitivity of the plastic. Back pressure is the pressure on the screw when it retreats. Its function is to compact the melt and discharge gas. It is usually 5-20bar. Too much back pressure will prolong the plasticization time and increase energy consumption, while too little back pressure will result in insufficient melt density. The cycle time is the time to complete an injection molding process, including mold closing, injection, pressure holding, cooling, mold opening and demolding stages. It needs to be shortened as much as possible while ensuring the quality of the plastic part. This can be achieved by optimizing the cooling time (accounting for 50%-70% of the cycle) and the connection of actions. For example, the pre-plasticization function is used to pre-plasticize the screw during the cooling stage to reduce the cycle time.
The process of defining injection molding machine parameters must follow the process of trial mold-adjustment-solidification. First, preliminarily set the parameter values according to the plastic part and mold parameters, and then observe the quality of the plastic part through trial mold. For example, if there is material shortage, the injection pressure or speed needs to be increased, the injection pressure or clamping force needs to be reduced for flash, and the holding time needs to be extended for sink marks. For precision plastic parts, multiple sets of parameter orthogonal tests are required to find the best parameter combination, and record it in the process card for solidification. The stability of the parameters needs to be monitored regularly during the production process. For example, if the injection pressure fluctuates by more than ±5% or the temperature fluctuates by more than ±3°C, the equipment fault needs to be checked in time. With the development of intelligent injection molding machines, parameter definition is gradually developing in the direction of automation and intelligence. The molding data is collected in real time by sensors, and the parameters are automatically optimized by AI algorithms to achieve adaptive production, which greatly improves the efficiency and accuracy of parameter setting.
