The injection molding process mainly includes plasticizing, mold closing, high-pressure mold clamping, injection, pressure holding, cooling and shaping, and mold opening and product removal, as shown in Figure 1-8. The above process is repeated cyclically, as shown in Figure 1-9, allowing injection molding production to proceed continuously.
The main steps in the above process include the following:
Plasticization of Plastic (Melting)
Plasticization refers to the preparatory process before molding, and its main requirements are: heating the plastic to a stable temperature; the temperature and components must be uniform and able to provide enough melt with sufficient quantity within a specified time; the plastic must be controlled at the lowest temperature.
When the screw is in the pre-plasticizing state, after a back-and-forth rotation, the plastic material is gradually squeezed out from the rear section of the screw, and this accumulation gathers in the space at the front of the screw, forming melt that will establish pressure against the check valve. This pressure is called back pressure. During screw rotation, it is under appropriate working pressure or compression force to rotate backward, and the rotation continues until the measured screw control stroke stops. This process is called metering or quantification.
Due to different plasticization degrees caused by different heat history paths, the materials also have three aggregation states: solid natural aggregation state, viscous flow and semi-solid state, and molten state. Corresponding to the screw, it is divided into solid conveying section, compression section and metering section. The material undergoes absorption and transfer of heat through conduction during the feeding process. At this stage, the screw's main effect is a conveying action. The material's heat mainly comes from the heating of the barrel and the heat generated by the rotation of the screw, which can shorten the molding cycle and increase screw speed to improve quality.
Mold Cavity Filling
After the plastic is heated, pressurized, plasticized and flows in the injection molding machine barrel, the screw will inject the melt into the mold cavity through the injection system. This process is called mold filling.
The filling process is divided into three stages: flow stage, pressure holding stage, and mold cavity opening. When the mold filling starts, the time from when the mold cavity is opened to about 95% filling is reached. After that, upon completion, problems may occur during filling, and production efficiency may be low. However, during actual situations, defects may occur when the molding conditions are not appropriate.
① Too fast speed, premature shear thinning. When plastic is sheared and changes due to rapid usage, it will exist in a tubular form under barrel bottom conditions. Because the liquid flow resistance is low, the local viscosity will increase under the influence of adhesion, which can cause the fusion degree and temperature to change. Therefore, at the flow control stage, the filling is carried out under relatively low pressure in the cavity. In addition, at the flow control stage, due to high-speed filling, the cutting resistance of the melt is significantly reduced at the expected bright area, while the effect of the cooling action is unclear, and the utilization rate of the cooling is unclear. This is the disadvantage of choosing high-speed usage.
② Too slow speed, insufficient heat causes insufficient speed control. When the speed is too slow, the adhesion of the barrel is greater, the flow resistance is greater, and the flow pressure is greater. Due to the high contact time between the molten plastic and the mold, the flow becomes slower, the heat dissipates faster and appears as insufficient, the heat dissipation is severe and appears as a frozen layer, coupled with the adhesion of multiple layers of viscous flow layers, the solidification layer becomes thicker, which further increases the viscous layer resistance and flow resistance.
Pressure Holding
The purpose of the holding pressure stage is to continuously apply pressure to the molten plastic by the screw, compacting the melt and increasing the plastic density to compensate for its shrinkage. During the holding pressure stage, the back pressure is high because the mold cavity is already filled with molten plastic. During this compaction process, the injection molding machine screw can only move forward slowly and slightly, and the flow rate of the plastic is also relatively slow; this flow is called holding pressure flow. Because the molten plastic solidifies rapidly due to cooling by the mold cavity walls during the holding pressure stage, the melt viscosity also increases quickly, resulting in significant resistance within the mold cavity. In the later stages of holding pressure, the density of the molten plastic continues to increase, and the plastic part gradually takes shape. The holding pressure stage continues until the gate solidifies and seals, at which point the mold cavity pressure reaches its highest value.
During the holding pressure stage, due to the relatively high pressure, the plastic exhibits partially compressible characteristics. In areas of higher pressure, the plastic is denser; in areas of lower pressure, the plastic is more porous and less dense. Therefore, the density distribution changes with location and time. During the holding pressure process, the plastic flow rate is extremely low, and flow no longer plays a dominant role; pressure becomes the main factor affecting the holding pressure process. During the holding pressure process, the plastic has filled the mold cavity, and the gradually solidifying melt acts as the medium for transmitting pressure. The pressure in the mold cavity is transmitted to the surface of the mold cavity wall through the plastic, and there is a tendency to open the mold, so sufficient clamping force is required to clamp the mold. Under normal circumstances, the expansion force will slightly open the mold, which helps with the venting of the mold; however, if the expansion force is too large, it is easy to cause burrs, overflow, or even open the mold. Therefore, when selecting an injection molding machine, an injection molding machine with a sufficiently large clamping force should be selected to prevent positive expansion and to effectively hold pressure.
Cooling and Shaping
In injection molding molds, the design of the cooling system is very important. This is because the molded plastic product can only be prevented from deforming due to external force after demolding if it is cooled and solidified to a certain rigidity. Since cooling time accounts for approximately 70%–80% of the entire molding cycle, a well-designed cooling system can significantly shorten molding time, improve injection molding productivity, and reduce costs. A poorly designed cooling system will lengthen molding time and increase costs; uneven cooling can further cause warping and deformation of plastic products.
According to experiments, the heat from the melt entering the mold is dissipated in roughly two parts: 5% of the heat is transferred to the atmosphere through radiation and convection, while the remaining 95% is conducted from the melt to the mold. Within the mold, due to the cooling water pipes, heat is transferred from the plastic in the mold cavity through thermal conduction to the cooling water pipes, and then carried away by the coolant through thermal convection. A small amount of heat that is not carried away by the cooling water continues to be conducted within the mold until it dissipates into the air upon contact with the outside environment.
The injection molding cycle consists of mold closing time, melt filling time, holding pressure time, cooling time, and demolding time. Cooling time accounts for the largest proportion, approximately 70%–80%. Therefore, cooling time directly affects the molding cycle length and production volume of plastic products. During the demolding stage, the temperature of the plastic product should be cooled below its heat distortion temperature to prevent relaxation due to residual stress or warping and deformation caused by demolding forces.
Demolding of Plastic Parts
Demolding of plastic parts is the last step in an injection molding cycle, that is, removing the plastic part from the mold manually or mechanically. Although the plastic part has been cold-cured, demolding still has a significant impact on its quality. Improper demolding methods may lead to uneven stress on the plastic part during demolding, causing warping, deformation, and other defects during ejection. There are two main demolding methods: ejector pin demolding and stripper plate demolding. When designing the mold, a suitable demolding method should be selected based on the structural characteristics of the product to ensure the quality of the plastic part.