How to do the injection molding process?
1. Injection parameters
The injection pressure is provided by the hydraulic system of the injection system. There are many factors that affect the melt filling pressure, which can be summarized into three categories: (1) Material factors, such as the type and viscosity of plastics, etc.; (2) Structural factors, such as the type, number and location of the gate system and the cavity of the mold . The shape and thickness of the product, etc.; (3) Process elements of molding.
Injection time refers to:
The time required for the plastic melt to fill the cavity, which does not include auxiliary time such as mold opening and closing.
The injection time is much lower than the cooling time, about 1/10 to 1/15 of the cooling time. This rule can be used as a basis for predicting the total molding time of plastic parts.
Injection temperature is an important factor affecting injection pressure. The barrel of the injection molding machine has 5-6 heating sections, and each raw material has its proper processing temperature. The injection temperature must be controlled within a certain range. If the temperature is too low, the melt will be poorly plasticized, which will affect the quality of the molded part and increase the processing difficulty. If the temperature is too high, the raw materials are easily decomposed.
2. Operation process
Filling stage: Filling is a step in the entire injection molding cycle. The time from closing the mold and starting injection until the mold cavity is approximately 95% filled. In theory, the shorter the filling time, the higher the molding efficiency, but in fact, the molding time or injection speed is limited by many conditions.
3. Pressure holding stage:
① The function of the pressure holding stage is to continuously apply pressure to compact the melt and increase the density of the plastic (densification) to compensate for the shrinkage behavior of the plastic. During the pressure maintaining process, since the mold cavity is filled with plastic, the back pressure is relatively high. In the process of maintaining pressure and compacting, the screw of the injection molding machine can only move forward slowly, and the flow speed of the plastic is relatively slow. The flow rate at this time is called the holding pressure flow rate.
②In the pressure holding stage, the plastic cools and solidifies faster through the mold wall, and the melt viscosity increases rapidly, so the resistance in the mold cavity is very large. The pressure holding stage should continue until the gate is cured and sealed.
4. Cooling stage:
In an injection mold, an important point is the design of the cooling system. Why is he so important? This is because the molded plastic product can only be cooled and solidified to a certain rigidity, and then the plastic product can be prevented from being deformed due to external force after being demolded.
Because the cooling time accounts for about 70% to 80% of the entire molding cycle, it is officially because of this, it embodies the importance of a well-designed cooling system, because a well-designed cooling system can greatly shorten the molding time, which can improve injection molding. Molding productivity. It can also reduce costs. However, an improperly designed cooling system will extend the molding time and increase the cost. Uneven cooling will further cause warpage and deformation of plastic products.
5. Demoulding stage:
The molding cycle of injection molding includes clamping time, filling time, holding pressure time, cooling time and demolding time. Therefore, the cooling time will directly affect the length of the plastic product molding cycle and the size of the output.
6. Precautions for injection molding
The design of plastic products mainly depends on the wall thickness of the plastic products. Need to be clear: the greater the thickness of the product, the longer the cooling time. Generally speaking, the cooling time is approximately proportional to the square of the thickness of the plastic product, or proportional to the 1.6 power of the diameter of the large runner. In other words, the thickness of the plastic product has doubled, and the cooling time has increased by 4 times.
Mold material and cooling method. Mold materials, including core, cavity materials and mold base materials, have a great influence on the cooling rate. The higher the thermal conductivity of the mold material, the better the heat transfer effect from the plastic per unit time, and the shorter the cooling time.
The plastic raw materials used in the injection molding process are diverse, and the types and forms of mold design are also diverse. In addition, the operator’s familiarity with a specific injection molding machine and the workers’ operating skills and practical experience are also different. At the same time, the objective environment will also change with the seasons. These objective and subjective conditions jointly determine the occurrence of defects in injection molded products.
Generally speaking, evaluating the performance of plastic products mainly includes three aspects:
①The appearance quality includes completeness, color and gloss;
②The accuracy between size and relative position,
③Mechanical performance, chemical performance, electrical performance, etc. correspond to use