Implementation of multi-stage injection molding process

The curve of the multi-stage injection molding process reflects the relationship between the screw feeding stroke and the injection pressure and injection speed provided by the injection molding machine. Therefore, two key factors need to be determined when designing a multi-stage injection molding process: first, the screw feeding stroke and its segmentation, and second, the injection pressure and injection speed.

In actual production, multi-stage injection control programs can be used to rationally set the injection pressure, injection speed, holding pressure, and melt filling method for each stage, based on the structure of the runner system, the type of gate, and the structure of the plastic part. This helps to improve plasticization efficiency, enhance product quality, reduce defect rates, and extend the lifespan of molds and machinery.

Graded settings

When designing multi-stage injection molding processes, the product should first be analyzed to determine the areas for each injection stage. Generally, it is divided into 3 to 5 zones, based on the product's shape characteristics, wall thickness variations, and melt flow characteristics. Areas with consistent or minimal wall thickness differences are designated as one zone; the transition points between zones for multi-stage injection are determined by the points where the material flow changes direction or where the wall thickness changes significantly; the gating system can be set as a separate zone. In the figure above, the product is divided into zones based on its external features, with the point where the material flow changes direction serving as one transition point (between zone 2 and zone 3), and the point where the wall thickness changes serving as another transition point (between zone 3 and zone 4). Therefore, this plastic part is divided into 4 zones for multi-stage injection: 3 zones for the product itself and 1 zone for the gating system.

In practical production, it is generally considered more scientific to set at least three or four injection stages when molding plastic parts. The runner is the first stage, the gate is the second stage, the third stage is when the product is filled to about 90%, and the remaining part is the fourth stage (also called the final stage).

For plastic parts with simple structures and low requirements for surface quality, a three-stage injection process can be used. However, for plastic parts with complex structures, numerous surface defects, and high quality requirements, an injection control program with four or more stages is necessary.

In actual production, the specific number of injection stages required must be determined through scientific analysis and rational setting based on factors such as the runner structure, gate type, location, number and size, part structure, product requirements, and mold venting effectiveness.

• 1) For products with direct gating, both single-stage and multi-stage injection methods can be used. For small plastic parts with simple structures and low precision requirements, a control method with fewer than three injection stages can be used.
• 2) For large plastic products with complex structures and high precision requirements, a multi-stage injection process with four or more stages should be chosen in principle.

Setting up the injection process

For the product shown in the figure, engineers divide it into sections based on its shape characteristics. This division is then reflected in different sections of the injection molding machine's screw. The length of each screw segment can then be estimated based on the product's sectional division. First, the required injection volume (volume) for each section after the product division is estimated. Using a corresponding method, the position of the screw in each segment can be calculated. For example, if the volume of section n is Ω, then the stroke of the nth segment of the screw is:

In multi-stage injection molding production practice, the method for determining the screw injection process is as follows:

• The first stage injection volume (i.e., the end position of the first stage injection) is the end point of the gate in the injection molding system. Except for direct gates, almost all others use medium pressure and medium speed, or medium pressure and low speed. The end position of the second stage injection is from the gate end point to 1/2 to 2/3 of the entire cavity space.
• The second stage injection should use high pressure and high speed, high pressure and medium speed, or medium pressure and medium speed; the specific values ​​depend on the product structure and the plastic material used.
• The third stage injection level should preferably use medium pressure and medium speed or medium pressure and low speed, and the position is exactly where the remaining cavity space is filled. All three stages described above belong to the melt filling process.
• The last stage of injection belongs to the pressurization and holding pressure phase. The pressure holding switching point is between the end positions of this stage of injection. There are two methods for selecting the switching point: time and position.

When injection begins, the injection timer starts, and the termination positions for each injection stage are calculated. If the injection parameters remain unchanged, depending on the fluidity of the material, for materials with better fluidity, the final stage termination position will reach the holding pressure switching point before the timer expires. At this point, the filling and pressurizing processes are completed, and the injection enters the holding pressure phase. If the timer has not yet expired, it stops counting and directly enters the holding pressure phase. Similarly, for materials with poorer fluidity, if the timer completes before the final stage injection termination position reaches the switching point, there is no need to wait for the position to be reached; the process directly enters the holding pressure phase.

In addition, regarding the use of multi-stage holding pressure, it can be determined according to the following methods: For products with few reinforcing ribs and low dimensional accuracy requirements, and products made of high-viscosity materials, use single-stage holding pressure. The holding pressure is higher than the pressure during the boosting process, and the holding time is short; while for products with more reinforcing ribs and low dimensional accuracy requirements, multi-stage holding pressure is generally required.

Setting the injection pressure and injection speed

① Injection pressure and injection speed of the gating system. Generally, gating systems have small runners, so higher injection speeds and pressures (typically 60% to 70% of the maximum) are used to quickly fill the runners and sprues, increasing the melt pressure in the runners and creating a certain mold filling potential. For molds with larger runner cross-sections, lower injection pressure and speed can be set; conversely, for molds with smaller runner cross-sections, higher settings are needed.
② Injection speed and pressure in the second stage. When the melt fills the runners and sprues, overcoming the resistance of the gate (small cross-sectional area) and beginning to fill the mold cavity, a lower injection speed is needed to overcome undesirable flow patterns and improve flow characteristics. In this stage, the injection speed can be reduced, while the pressure reduction is smaller; for larger gate cross-sections, the injection pressure may not need to be reduced.

③ Injection speed and pressure in the third stage. As shown in Figure z, the third stage corresponds to the injection zone 3, which is the main part of the molded part. At this point, the melt has completely filled the mold cavity. To achieve the ideal diffusion state, accelerated mold filling is required, so the injection molding machine needs to provide higher injection pressure and speed in this stage. This section is also a turning point in the melt flow, where the flow resistance increases and pressure loss is significant, requiring compensation. Generally, multi-stage injection uses high speed and high pressure in this section.
④ Injection speed and pressure in the fourth stage. Based on the corresponding relationship in the figure, when the melt reaches zone 4, the part wall thickness may vary or remain constant. The melt has basically filled the mold cavity. Since the melt obtained high pressure and speed in zone 3, buffering can be performed in this stage to achieve approximately uniform linear flow velocity of the melt in all parts of the mold cavity. The general design principle is that when entering zone 4, if the wall thickness increases, the speed and pressure can be reduced; if the wall thickness decreases, the speed can be reduced without reducing the pressure, or the speed can remain unchanged while the pressure is appropriately reduced or not reduced. In short, in the fourth stage, the injection process should exhibit multi-stage control characteristics, and the cavity pressure should increase rapidly.

However, high-speed injection imparts significant kinetic energy to the molten material. When the melt reaches the end of the cavity, it will generate a large inertial impact, potentially leading to energy loss and flashing. Therefore, the melt flow rate should be slowed down to reduce the molding pressure. However, the pressure must still reach the commonly referred to holding pressure (secondary pressure, follow-up pressure) to ensure that the voids caused by melt shrinkage in the mold cavity are filled before the gate solidifies. This requires multiple injection speeds and pressures during the injection molding process. The screw metering stroke shown in the figure is set based on the amount of material used for the product and the buffer amount. The injection screw moves from position "97" to "20" to fill the thin-walled part of the product. In this stage, a high speed of 10 is set to prevent the molten material from cooling and solidifying due to prolonged flow time. When the screw moves from position "20" → "15" → "2", a corresponding low speed of 5 is set to reduce the melt flow rate and its kinetic energy impacting the mold. A higher primary injection pressure is set when the screw is at positions "97", "20", and "5" to overcome the mold filling resistance, and a lower secondary injection pressure is set from "5" to "2" to reduce kinetic energy impact.

Multi-stage injection molding is one of the more advanced injection molding technologies currently available. In the research of multi-stage injection molding processes, the determination of the screw stroke segments during injection is relatively precise, while the selection of injection pressure and injection speed in each segment is largely based on experience. The general empirical method can only determine the corresponding relationship between the injection pressure and injection speed used in each segment. The usual practice is to determine this relationship based on the ratio of the cross-sectional area of ​​each part of the molded product. After designing the multi-stage injection molding process, it requires repeated adjustments through multiple trials to achieve the optimal values ​​for the selected injection pressure and injection speed.