Process principle of multi-stage injection molding
The multi-stage injection molding process achieves high-quality molding of complex plastic parts by dividing the injection process into multiple stages and precisely controlling parameters such as injection speed, pressure, and holding pressure in each stage. This process overcomes the limitations of traditional single-stage injection, which struggles to balance filling speed and pressure. It can dynamically adjust process parameters based on factors such as part wall thickness, runner length, and material properties, effectively addressing common defects such as material shortages, flash, sink marks, and bubbles, offering irreplaceable advantages in the field of precision injection molding.
The core principle of multi-stage injection is the segmented control of the melt’s flow characteristics within the mold cavity. As the plastic melt enters the mold cavity from the nozzle, the flow resistance and cooling rate it faces vary with position. For example, a higher velocity is required at the runner entrance to overcome resistance, while high pressure is required to fill thin-walled areas. As the mold nears fullness, the velocity must be reduced to avoid flash. Multi-stage injection divides the entire filling process into 3-5 stages, each corresponding to different speed and pressure parameters: the first stage (runner filling) adopts medium speed (30-50mm/s) and low pressure (50-80MPa) to ensure that the melt enters the main runner smoothly; the second stage (initial cavity filling) increases the speed (80-120mm/s) to quickly fill most areas of the cavity; the third stage (thin-wall/complex area) reduces the speed (20-40mm/s) and increases the pressure (100-150MPa) to ensure that the melt fills the fine structure; the fourth stage (before the cavity is filled) further decelerates (10-20mm/s) to prevent the melt from impacting the cavity wall and generating flash; the final stage switches to holding pressure to compensate for the melt cooling and shrinkage.
The setting of multi-stage injection parameters requires dynamic optimization in conjunction with the part structure. For parts with uneven wall thickness, such as a shell that gradually transitions from 1mm to 5mm, high speed and high pressure (100mm/s, 120MPa) are required in the thin-walled section, while switching to low speed and low pressure (30mm/s, 80MPa) in the thick-walled section to prevent sink marks in the thick wall due to slow cooling. For parts with long and thin ribs, the rib filling stage requires a separate first-level parameter setting, with the speed controlled at 40-60mm/s and the pressure 20% higher than the main body filling stage to ensure that the melt can overcome flow resistance and fill the ribs. Furthermore, material properties significantly influence the parameters: PE and PP with good fluidity can reduce the number of stages (3 stages are sufficient), while high-viscosity PC and PMMA require 4-5 stages, with the speed gradient within 20-30mm/s at each stage to avoid melt degradation.
The holding phase of multi-stage injection molding also requires segmented control, typically divided into two or three stages. The first stage uses a higher pressure (80% of the injection pressure) for 60% of the total holding time to compensate for shrinkage in the main cavity area. The second stage uses a lower pressure (50%-60% of the injection pressure) to compensate for thick-walled areas of the part. The final stage further reduces the holding pressure to 30%-40% to prevent excessive internal stress caused by over-holding. For example, an 8mm thick ABS part with a total holding time of 15 seconds includes a first stage holding period of 9 seconds (90MPa) and a second stage holding period of 6 seconds (60MPa), effectively reducing surface sink marks. The timing of the holding phase transition is determined by a cavity pressure sensor. When the melt front reaches the end of the cavity, the system immediately switches to the first stage holding period to avoid pressure loss.
The advantages of multi-stage injection molding are particularly prominent in the production of precision plastic parts. Compared with traditional single-stage injection molding, it can improve part dimensional accuracy by 15%-20%. For example, the pitch error of a gear part can be reduced from ±0.05mm to ±0.03mm. Surface roughness Ra can be reduced by 30% to below 0.4μm. Internal stress distribution is more uniform, and impact strength is increased by 10%-15%. These advantages stem from precise parameter control: low-speed filling avoids bubbles caused by melt turbulence, high-pressure holding eliminates sink marks, and gradient pressure reduction reduces internal stress. In practical applications, multi-stage injection requires advanced injection molding machine control systems, such as closed-loop servo-hydraulic systems with pressure and speed control accuracy of ±1%, ensuring stable parameters at each stage. Furthermore, flow simulation using CAE software such as Moldflow can optimize the stage design in advance and reduce the number of mold trials. For example, a simulation analysis revealed pressure loss during the third injection stage of a certain part. By increasing the pressure in this stage by 5MPa, the part passed the mold in a single trial.
The development trend of multi-stage injection molding is toward intelligent and adaptive control. New injection molding machines are equipped with multiple sensors, including those for cavity pressure, melt temperature, and mold deformation, to collect molding data in real time and automatically adjust parameters at each level using AI algorithms. For example, if the melt temperature at a certain stage is detected to be lower than the set value, the system automatically increases the injection speed by 10% to compensate for fluidity loss. If sink marks are detected in a certain area of the plastic part, the holding time in that area is automatically extended. This adaptive multi-stage injection not only improves production stability but also reduces reliance on operator experience, making it particularly suitable for the production of high-variety, small-batch precision plastic parts. In the future, with the application of digital twin technology, multi-stage injection parameters can be optimized in advance through virtual simulation, achieving an efficient production model where “one mold trial leads to mass production.”