★Implementation of multi-stage injection molding process
The theory of multi-stage injection molding
During the injection of molten plastic into the mold cavity, the melt is subjected to complex thermodynamic and fluid dynamic forces. As shown in the figure describes the melt flow characteristics at four different injection speeds. Figure (a) shows the serpentine flow patterns or "jetting" phenomenon that occurs during high-speed injection molding; Figure (b) shows the flow state at a medium-high injection speed, where the "jetting" phenomenon at the gate is reduced, essentially approaching a "spreading flow" state; Figure (c) shows the flow state at a medium-low injection speed, where the melt generally does not produce a "jetting" phenomenon, and the melt can fill the mold with a low-speed, stable "spreading flow"; Figure (d) shows low-speed injection molding, which may lead to difficulties or even failure in mold filling due to the excessively slow filling speed.
Typically, the extensional flow of polymer melts under the extensional flow model also proceeds in three stages: the initial stage where the melt front exhibits radial flow as it passes through the gate; the intermediate stage where the melt front is arc-shaped under the action of injection pressure; and the final stage of uniform flow with the viscoelastic melt acting as the leading edge.
The flow characteristics of the melt in the initial stage are that the melt flowing out of the gate possesses a certain kinetic energy under the action of injection pressure and injection speed. The magnitude of this kinetic energy (at this point, it has just entered the mold cavity and is not affected by any flow resistance) affects the radial flow characteristics and the volume of diffusion of the melt front. When this force is particularly strong, a "jetting" phenomenon may occur; when the kinetic energy of this force is appropriate, the melt flows evenly in all directions from the source, resulting in a better diffusion state.
As the initial stage progresses, the melt will quickly spread, and two phenomena will occur when it comes into contact with the mold cavity wall: a) the flow direction is altered due to the forces exerted by the mold cavity wall; b) flow resistance is generated due to the cooling and friction effects of the mold cavity wall, resulting in velocity differences in the melt flow at different points. This flow characteristic is manifested as unequal flow velocities at different points of the melt, with the highest flow velocity in the core of the melt, and the flow of the leading edge material exhibiting an arc shape; simultaneously, the flow at each point creates unequal drag and constraint, and the flow resistance tends to increase with the increase in flow distance.
In the third stage, the molten material rapidly flows into the mold cavity, with the viscoelastic melt acting as the flow front. In the second and third stages of injection molding, the kinetic energy generated by the injection pressure and injection speed is the main factor affecting the mold filling characteristics. The figure shows the expansion flow process and velocity distribution. Injection molded parts come in a variety of shapes, and only one model is shown in the figure. Flow characteristics, energy loss during the mold filling process, and the shape of the product are closely related, and different plastics have different flow characteristics.
1. Low-temperature mold; 2. Cold-solidified layer of plastic; 3. Flow direction of the melt; 4. Velocity distribution at the low-temperature mold.
The ideal flow state of the molten material in the mold cavity
As mentioned above, the characteristics of uniform expanding flow and the initial stages of plastic melt flow from the gate should not exhibit phenomena similar to "jetting" or jetting characteristics. This requires that the melt does not possess excessively high kinetic energy in the initial stages of flow to the gate (excessive kinetic energy can lead to jetting and serpentine patterns); in the mid-stage of mold filling, the expanding flow should have sufficient kinetic energy to overcome flow resistance and achieve a uniform expansion state; in the final stage of mold filling, the viscoelastic melt is required to fill the mold rapidly, overcoming the increasing flow resistance with increasing flow distance, and achieving a predetermined uniform and steady flow rate. Based on rheological principles, this ideal flow state can result in injection-molded products with superior physical and mechanical properties, eliminate internal stress and orientation in the product, eliminate sink marks and surface flow lines, and increase the uniformity of the product's surface gloss.
Implementation of a multi-stage injection process
Multi-stage injection molding essentially involves controlling different injection speeds at the moment the plastic melt fills the mold cavity, allowing the plastic melt to reach a near-ideal state during the filling process. This ideal filling process does not introduce quality defects to the plastic product, nor does it generate stress or orientation forces. Generally, the injection molding process is completed within a few seconds to tens of seconds, and the multi-stage injection molding process requires transforming the filling process into a continuous sequence of different filling states controlled by varying injection speeds within this short timeframe.
According to the five-stage requirements of the actual multi-stage injection process, different injection volumes are implemented, and the kinetic energy of the melt must be provided by the injection molding machine. Current injection molding machines can already achieve segmented, or even multi-segment, injection control, as shown in the figure.
As shown in the figure above, five-segment injection control can be achieved, with each segment having a different injection volume. The injection volume controlled by the stroke is:
- Where ΩLn is the injection volume;
- Ln is the injection stroke;
- D is the diameter of the injection molding machine screw;
- p is the density of the plastic.
Therefore, different injection speeds and pressures can be used in each segment to achieve the desired kinetic energy of the molten material during this stage. Each segment corresponds to a specific zone (n-zone) in the mold cavity. Although the kinetic energy of the flow changes due to the influence of the gating system, the variation in volumetric flow rate should be minimal.
In actual production, the injection speed of injection molding machines that achieve multi-stage injection is controlled in multiple stages. Typically, the injection process can be divided into three or four zones, as shown in the diagram, and each zone can be set with its own appropriate injection speed to achieve multi-stage injection molding. Currently, some injection molding machines also have multi-stage pre-plasticizing and multi-stage holding pressure functions.
Multi-stage injection molding process curve
Although multi-stage injection molding describes the state of the molten material during mold filling, its control is implemented by the injection molding machine. From the perspective of the injection molding machine's control principle, the relationship between injection speed (injection pressure) and screw feeding stroke can be utilized. The figure shows a typical curve for a multi-stage injection molding process, where different injection pressures and speeds are applied to different amounts of material during the injection process.
1–5 - 5 different injection speeds
Advantages of multi-stage injection molding
In injection molding, high-speed and low-speed injection each have their advantages and disadvantages. Experience shows that high-speed injection generally has the following advantages: shorter injection time; increased flow distance; improved surface finish of the product; increased strength of weld lines; and prevention of cooling deformation. Low-speed injection, on the other hand, generally has the following advantages: effective prevention of flash; prevention of flow marks; prevention of mold venting issues; prevention of air entrapment; and prevention of molecular orientation deformation.
Multi-stage injection molding combines the advantages of high-speed and low-speed injection to meet the requirements of increasingly complex geometries of plastic products and drastic changes in the cross-sections of mold runners and cavities. It can also effectively eliminate defects such as injection marks, shrinkage, bubbles, weld lines, and burn marks during the molding process.
The multi-stage injection molding process breaks through the traditional injection and holding pressure method, organically combining the advantages of high-speed and low-speed injection processing. By implementing multi-stage control during the injection process, many defects in injection molded parts can be overcome. For example, the figure shows a method that uses low-speed injection at the beginning of the injection process, high-speed injection during mold cavity filling, and then low-speed injection again near the end of filling. Through the control and adjustment of injection speed, various undesirable phenomena such as burrs, jetting marks, silver streaks, or burn marks can be prevented and improved.
a-d: four different injection speeds
Practical experience shows that controlling the oil pressure, injection speed, screw position, and screw speed of the injection molding machine through multi-level program control can largely improve defects in the appearance of injection-mmolded products, such as shrinkage, warping, and flashing.