I. Filling Stage
Filling is the first step in the entire injection molding cycle, starting from mold closure and injection until the mold cavity is approximately 95% filled. Theoretically, the shorter the filling time, the higher the molding efficiency. However, in practice, the molding time or injection speed is restricted by many conditions.
High-speed filling:High-speed filling involves a high shear rate. The plastic viscosity decreases due to shear thinning, reducing the overall flow resistance. Local viscous heating also thins the solidified layer. Therefore, during the flow-controlled stage, filling behavior often depends on the volume to be filled. That is, in the flow-controlled stage, the shear-thinning effect of the melt is significant under high-speed filling, while the cooling effect of thin walls is not obvious, so the flow rate dominates.
Low-speed filling:Low-speed filling is heat-conduction controlled, with a low shear rate, high local viscosity, and large flow resistance. Since the molten plastic is replenished slowly, the flow is gradual, making the heat conduction effect prominent, with heat quickly dissipated by the cold mold wall. Coupled with less viscous heating, the solidified layer becomes thicker, further increasing the flow resistance in thin-wall sections.
Generally, weld lines formed in high-temperature zones have better strength. At high temperatures, polymer chains have higher mobility and can interpenetrate and entangle. In addition, the temperatures of the two melt streams in high-temperature zones are close, with nearly identical thermal properties, enhancing the strength of the weld area. Conversely, weld strength is poor in low-temperature zones.
II. Holding Pressure Stage
The function of the holding pressure stage is to continuously apply pressure to compact the melt and increase the plastic density (densification), compensating for plastic shrinkage. During holding, the cavity is already full of plastic, resulting in high back pressure. In the compaction process, the injection molding screw only moves forward slowly and slightly, and the plastic flow rate is also low; this flow is called holding flow.
In the holding stage, plastic cools and solidifies rapidly due to the mold wall, and melt viscosity increases quickly, creating high resistance inside the mold cavity. In the late holding period, the material density continuously increases and the plastic part gradually takes shape. The holding stage continues until the gate solidifies and seals, at which point the cavity pressure reaches its maximum.
Due to the high pressure, the plastic exhibits partial compressibility during this stage. Plastic in high-pressure regions is denser, while that in low-pressure regions is looser and less dense, resulting in density variations with position and time. The plastic flow rate is extremely low during holding, with flow no longer dominant; pressure becomes the main factor affecting the process.
III. Cooling Stage
The design of the cooling system is critical in injection molding molds. This is because a molded plastic part must be cooled and solidified to sufficient rigidity to avoid deformation from external forces after demolding. Since cooling time accounts for about 70%–80% of the total molding cycle, a well-designed cooling system can greatly shorten the molding time, improve injection productivity, and reduce costs.
An improperly designed cooling system prolongs the molding cycle and increases costs. Uneven cooling further causes warpage and deformation of the plastic part.
IV. Demolding Stage
Demolding is the final step in an injection molding cycle. Although the part has cooled and solidified, demolding still significantly affects part quality. Improper demolding may lead to uneven stress during ejection, causing defects such as part deformation.
There are two main demolding methods: ejector pin demolding and stripper plate demolding. The appropriate demolding method should be selected according to the structural characteristics of the product during mold design to ensure quality.
