How to do a good injection molding process
Nowadays, the trend of processing process is developing in the direction of high technology, these technologies include: micro injection molding, high filling compound injection molding, water-assisted injection molding, mixed use of various special injection molding processes, foam injection molding, mold technology, simulation technology, etc.
The process of injection molding process mainly includes 4 stages, such as filling - holding pressure - cooling - demolding, etc. These 4 stages directly determine the quality of the products, it is a complete continuous process.
The injection pressure is provided by the hydraulic system of the injection molding system. There are many factors affecting the melt filling pressure, which are summarized in 3 categories: (1) material factors, such as the type of plastic, viscosity, etc.; (2) structural factors, such as the type, number and position of the pouring system, the shape of the mold cavity and the thickness of the product, etc.; (3) process elements of molding.
The injection time is the time required for the plastic melt to fill the cavity, excluding the auxiliary time for mold opening and closing. The injection time is much lower than the cooling time, about 1/10 to 1/15 of the cooling time, and this rule can be used as a basis for predicting the full molding time of the plastic part.
The injection temperature is an important factor affecting the injection pressure. There are 5 to 6 heating sections in the barrel of the injection molding machine, and each raw material has its own suitable processing temperature (detailed processing temperature can be found in the data provided by the material supplier). The injection molding temperature must be controlled within a certain range. If the temperature is too low, the melt will be poorly plasticized, affecting the quality of the molded parts and making the process more difficult; if the temperature is too high, the raw material will be easily decomposed.
Filling stage: Filling is the beginning of the whole injection cycle, and the time is counted from the time the mold is closed to start injection until the mold cavity is filled to about 95%. Theoretically, the shorter the filling time, the higher the molding efficiency, but in practice, the molding time or injection speed is subject to many conditions.
Holding phase: The role of the holding phase 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 holding pressure process, the back pressure is high because the mold cavity is already filled with plastic. In the process of holding pressure compaction, the injection molding machine screw can only slowly move forward for a small movement, and the flow rate of plastic is also slower, which is called holding pressure flow. As the plastic is cooled and cured by the mold wall, the viscosity of the melt increases quickly, so the resistance in the mold cavity is great. In the later stage of holding pressure, the material density continues to increase, and the molded part is gradually formed.
Cooling stage: In the injection molding mold, the design of the cooling system is very important. This is because only when the molded plastic products are cooled and cured to a certain rigidity can the plastic products be prevented from deformation due to external forces after demolding. Since cooling time accounts for about 70% to 80% of the whole molding cycle, a well-designed cooling system can significantly shorten the molding time, improve injection molding productivity and reduce costs. Improperly designed cooling system will make the molding time longer and increase the cost; uneven cooling will further cause warping and deformation of plastic products.
Demolding stage: The molding cycle of injection molding consists of mold closing time, filling time, holding time, cooling time and demolding time. Among them, cooling time accounts for a large proportion, about 70% to 80%. Therefore, the cooling time will directly affect the length of the molding cycle and the output of plastic products. The temperature of plastic products in the demolding stage should be cooled to a temperature lower than the heat deformation temperature of plastic products to prevent the relaxation of plastic products caused by residual stress or the warping and deformation caused by external force of demolding.