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 four stages, such as filling - holding pressure - cooling - demolding, etc. These four stages directly determine the quality of the products, it is a complete continuous process.

Injection parameters

1. Injection pressure

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 three categories: (1) material factors, such as the type of plastic, viscosity, etc.; (2) structural factors, such as the type, number and location of the pouring system, the shape of the mold cavity and the thickness of the product, etc.; (3) the process elements of molding.

2. Injection time

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, which can be used as the basis for predicting the entire molding time of the plastic part.

3. Injection temperature

The injection temperature is an important factor affecting the injection pressure. There are 5 to 6 heating sections in the barrel of injection molding machine, and each raw material has its own suitable processing temperature (detailed processing temperature can refer to 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.

Operation process

Filling stage:

Filling is the first step in the whole injection molding cycle, and the time is counted from the mold closing to the mold cavity filling 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. At the later stage of holding pressure, the density of the material continues to increase and the molded part is gradually formed. The holding pressure phase should continue until the gate is cured and sealed, at which time the cavity pressure in the holding pressure phase reaches the highest value.

Cooling stage :

In injection molding molds, 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, pressure holding time, cooling time and demolding time. Among them, cooling time accounts for the largest proportion, about 70% to 80%. Therefore, the cooling time will directly affect the length of the molding cycle and the yield of plastic products. The temperature of plastic products in the demolding stage should be cooled down to lower than the heat deformation temperature of plastic products, in order to prevent the relaxation of plastic products caused by residual stress or the warping and deformation caused by external force of demolding.

Caution

Plastic product design. The main thing is the wall thickness of plastic products. The greater the thickness of the product, the longer the cooling time. Generally speaking, the cooling time is proportional to the square of the thickness of the plastic product, or 1.6 times the maximum runner diameter. That is, doubling the thickness of the plastic product increases the cooling time by 4 times.

Mold material and its cooling method. Mold material, including mold core, cavity material and mold frame material, has a great influence on cooling rate. The higher the heat conduction coefficient of mold material, the better the effect of heat transfer from plastic in unit time, and the shorter the cooling time.

The way of cooling water pipe configuration. The closer the cooling water pipe is to the mold cavity, the larger the diameter of the pipe and the more the number, the better the cooling effect and the shorter the cooling time. Coolant flow rate. The larger the flow of cooling water, the better the effect of cooling water to take away heat by thermal convection. The nature of the coolant. The viscosity and heat transfer coefficient of the coolant will also affect the heat transfer effect of the mold. The lower the viscosity of the coolant, the higher the heat transfer coefficient and the lower the temperature, the better the cooling effect.

Defect

The plastic raw materials used in the injection molding process are diverse, and the types and forms of mold design are also varied. In addition, the familiarity of the operator with a specific injection molding machine and the differences in operating skills and practical experience between workers also vary, and the objective environment also varies with the seasons.

Generally speaking, there are three main aspects for the evaluation of the performance of plastic products.

First, the quality of appearance, including integrity, color and luster.

Second, the accuracy between dimensions and relative positions.

Third, the mechanical properties, chemical properties, electrical properties, etc. corresponding to the use.