Mold closing, filling, holding pressure, cooling, mold opening, and demolding. These six stages not only directly affect the quality of the molded product but also constitute an indispensable continuous workflow.

1. Filling Stage

Filling, as the first link in injection molding, starts from the mold closing and injection, and continues until the mold cavity is filled to approximately 95%. Theoretically, the shorter the filling time, the higher the molding efficiency. However, in actual production, the molding time (or injection speed) is affected by many factors.

During high-speed filling, due to the high shear rate, the viscosity of the plastic decreases due to shear thinning, thereby reducing the overall flow resistance. At the same time, local viscous heating will also reduce the thickness of the solidified layer. Therefore, in the flow control stage, the filling behavior mainly depends on the size of the volume to be filled. Due to the significant shear thinning effect brought by high-speed filling, and the relatively small cooling effect of the thin wall, the rate becomes the dominant factor.

In contrast, during low-speed filling, the shear rate is lower, the local viscosity is higher, and the flow resistance increases accordingly. Because the replenishment of the thermoplastic is slow and the flow is slow, this makes the heat conduction effect more obvious, and the heat is quickly taken away by the cold mold wall. In addition, less viscous heating and a thicker solidified layer further increase the flow resistance at thinner parts of the wall.

In front of the flow front, the polymer chains of the plastic are almost parallel to the flow front. Therefore, when two streams of plastic melt meet, the polymer chains on their contact surface are parallel to each other. Due to the differences in the properties of the two streams of melt (such as residence time, temperature, and pressure in the mold cavity), the microscopic structural strength of the melt confluence area is weaker. By placing the parts at an appropriate angle and observing, the generation of weld lines can be clearly found, which is caused by the difference in properties when the melts converge. Weld lines not only affect the appearance quality of the plastic parts, but also, due to their loose microstructure, they are prone to stress concentration, which in turn reduces the strength of that part and may lead to fracture.

In addition, the strength of the weld line is closely related to the temperature. Usually, the weld line generated in the high-temperature region has better strength, because the activity of polymer chains is enhanced in the high-temperature environment, and they can penetrate and entangle each other. At the same time, the temperatures of the two melts in the high-temperature region are similar, and the thermal properties are similar, thus enhancing the strength of the weld region. Conversely, in low-temperature regions, the weld strength is relatively poor.

Next, we will discuss the holding pressure stage in the injection molding process.
The holding pressure stage plays a vital role in injection molding. Its purpose is to further compact the melt and increase the density of the plastic by continuously applying pressure, thereby effectively compensating for the shrinkage behavior of the plastic. During the holding pressure process, the mold cavity is filled with plastic, causing the back pressure to increase. The injection molding machine screw needs to slowly move forward, and the plastic flow rate slows down accordingly. The flow in this stage is called holding pressure flow. As the mold wall cooling and solidification accelerate and the melt viscosity increases rapidly, the resistance in the mold cavity increases significantly. The holding pressure stage will continue until the gate is solidified and sealed, at which point the mold cavity pressure reaches its peak.

In the holding pressure stage, due to uneven pressure distribution, the plastic density changes with position and time. In high-pressure areas, the plastic density is high and dense; while in low-pressure areas, the plastic density is lower and looser. Therefore, pressure becomes a key factor affecting the holding pressure process. At this time, the plastic flow rate is extremely low, and the flow is no longer the dominant factor, while the gradually solidifying melt acts as a medium for transmitting pressure.

The pressure in the mold cavity is transmitted to the mold wall surface through the plastic, attempting to open the mold. Therefore, appropriate clamping force is needed to keep the mold closed. Appropriate clamping force helps the mold to vent, but excessive clamping force may cause burrs, overflow, or even opening of the mold on the molded product. Therefore, when selecting an injection molding machine, its clamping force should be sufficient to meet the needs of the holding pressure stage. In addition, new injection molding processes such as gas-assisted molding, water-assisted molding, and foam injection molding also bring new challenges and opportunities to injection molding.