The process of the injection molding process mainly includes four stages: filling-holding pressure-cooling-demolding. These four stages directly determine the quality of the product, which is a complete continuous process.
Ⅰ. Injection parameters
1. Injection pressure
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 gating system, and the cavity of the mold. The shape and thickness of the product, etc.; (3) the process elements of molding.
2. Injection time
Injection time refers to the time required for the plastic melt to fill the cavity, excluding 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.
3. Injection temperature
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 own suitable 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 parts and increase the difficulty of the process; if the temperature is too high, the raw materials are easy to decompose.
Ⅱ. The operation process
1. Filling stage:
Filling is the first step in the entire injection cycle process. The time starts from when the mold is closed and the injection is started until the mold cavity is filled to about 95%. In theory, the shorter the filling time, the higher the molding efficiency, but in practice, the molding time or injection speed is restricted by many conditions.
2. Pressure holding stage:
The function of the holding pressure stage is to continuously apply pressure, compact the melt, and increase the density (densification) of the plastic to compensate for the shrinkage behavior of the plastic. During the pressure holding process, the back pressure is relatively high because the mold cavity has been filled with plastic. In the process of maintaining pressure and compaction, the screw of the injection molding machine can only move forward slowly, and the flow speed of the plastic is relatively slow. The flow at this time is called the pressure maintaining flow.
In the pressure holding stage, the plastic is cooled and solidified by the mold wall faster, and the melt viscosity increases quickly, so the resistance in the mold cavity is very large. In the later stage of pressure holding, the material density continues to increase, and the plastic parts are gradually formed. The pressure holding stage should continue until the gate is cured and sealed. At this time, the cavity pressure in the pressure holding stage reaches the highest value.
3. Cooling stage:
In the injection molding mold, the design of the cooling system is very 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. Since the cooling time accounts for about 70% to 80% of the entire molding cycle, a well-designed cooling system can greatly shorten the molding time, improve injection molding productivity, and reduce costs. An improperly designed cooling system will lengthen the molding time and increase costs; uneven cooling will further cause warpage and deformation of plastic products.
4. Demoulding stage:
The molding cycle of injection molding consists of clamping time, filling time, holding pressure time, cooling time and demolding time. Among them, the cooling time accounts for the largest proportion, about 70% to 80%. Therefore, the cooling time will directly affect the length of the molding cycle of plastic products and the size of the output. The temperature of the plastic product in the demolding stage should be cooled to lower than the thermal deformation temperature of the plastic product to prevent the plastic product from relaxation due to residual stress or warpage and deformation caused by the external force of demolding.
Ⅲ. Matters needing attention
1. Design of plastic products
Mainly the wall thickness of plastic products. 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 1.6th power of the largest runner diameter. That is, the thickness of the plastic product is doubled and the cooling time is increased by 4 times.
2. Mould material and its cooling method
Mold materials, including mold 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 effect of transferring heat from the plastic per unit time, and the shorter the cooling time.
3. Cooling water pipe configuration method
The closer the cooling water pipe is to the mold cavity, the larger the pipe diameter and the larger the number, the better the cooling effect and the shorter the cooling time.
4. Coolant flow
The larger the flow of cooling water, the better the effect of cooling water to take away heat by means of thermal convection.
5. The nature of the coolant
The viscosity and thermal conductivity of the coolant will also affect the thermal conductivity of the mold. The lower the viscosity of the coolant, the higher the thermal conductivity and the lower the temperature, the better the cooling effect.
The plastic raw materials used in the plastic injection molding process are diverse, and the types and forms of mold design are also diverse. In addition, the familiarity of the operators with the specific injection molding machine and the differences in the operating skills and practical experience of the workers are also different. At the same time, the objective environment will also vary with seasons. These objective and subjective conditions together determine the occurrence of defects in injection molded products.
Generally speaking, there are three main aspects in evaluating the performance of plastic products:
First, the appearance quality, including completeness, color and gloss;
Second, the accuracy between size and relative position;
Third, the mechanical properties, chemical properties, electrical properties corresponding to the uses.