Main Injection Molding Processes

The production of injection molded parts is primarily completed on injection molding machines, encompassing processes such as plasticization and metering, injection and mold filling, and cooling and solidification. The purpose of studying the injection molding process is to adjust injection molding parameters according to the plastic material and the product requirements, thereby controlling the quality of injection molded products.

Each stage of the injection molding process plays a critical role in determining the final quality, dimensional accuracy, and mechanical properties of the molded part. Understanding and optimizing each phase is essential for achieving consistent results, reducing waste, and improving efficiency in the injection molding process.

Plasticization refers to the entire process by which plastic material in the cylinder is heated to a flowable state with good plasticity. Plastic raw materials are uniformly melted at high temperatures by heat generated from friction with the rotating screw of the injection molding machine or from heaters mounted on the outside of the injection molding machine cylinder, preparing for injection into the mold. Plasticization can be considered the preparation phase of the injection molding process, ensuring that material is properly conditioned before entering the mold cavity.

When molten plastic enters the mold cavity, it should reach the specified molding temperature and be able to provide a sufficient quantity of molten plastic within the specified time. The temperature at each point in the molten plastic should be uniform, with little to no thermal decomposition occurring, ensuring the continuous operation of the injection molding process. Proper plasticization is critical for achieving consistent part quality and preventing defects such as flow lines, burn marks, or incomplete filling.

Plasticization can be divided into plunger-type plasticization and screw-type plasticization. In screw-type plasticization, there is not only rotational movement but also a backward linear movement, with the screw rotating while moving backward. This backward linear movement results from the reaction of the material in the screw channel and the melt at the screw head acting against the screw during rotation. This combined motion is essential for proper mixing, melting, and metering in the injection molding process.

The polymer material in the cylinder undergoes three states from the rear to the front: glassy state, high elastic state, and viscous flow state. Correspondingly, the screw is divided into three sections: the rear solid conveying section (feeding section), the middle melting section (compression section), and the front homogenizing section (metering section). In a general-purpose screw, the depth of the screw channel gradually becomes shallower from the feeding section to the metering section, as illustrated in Figure 1-5. This gradual change in channel depth facilitates the progressive melting and compression of the material as it moves through the screw during the injection molding process.

Functional Details of Screw Sections

The design of the screw directly impacts the efficiency and quality of the plasticization process in the injection molding process. Different materials require specific screw designs to achieve optimal plasticization. For example, crystalline polymers typically require longer compression sections than amorphous polymers. The screw's length-to-diameter ratio, compression ratio, and flight geometry all contribute to its performance in melting and homogenizing the plastic material, making screw design a critical aspect of optimizing the injection molding process.

The injection and mold filling process involves injecting the plasticized melt from the metering chamber into the mold cavity. This stage of the injection molding process is complex yet extremely important, as it involves the flow of high-temperature melt into a relatively cool mold cavity. It is the stage where polymer orientation and crystallization are determined, directly affecting product quality. Proper control of the injection and filling phase is essential for achieving dimensional accuracy, surface finish, and structural integrity in the final part.

Injection and mold filling can be divided into two stages: the injection phase and the packing phase. The injection phase starts when the screw begins to advance the melt and continues until the melt fills the mold cavity. The packing phase begins when the mold cavity is filled with melt and continues until the gate "freezes." The packing phase can be further divided into the packing and compensation flow stage and the packing switchover and backflow stage, each playing a distinct role in the injection molding process.

Packing Phase Details

During the packing phase, under packing pressure, the melt in the mold cavity undergoes cooling compensation and further compression densification. The packing and compensation flow stage occurs when the nozzle pressure (injection pressure) reaches its maximum value, but the mold cavity pressure has not yet reached its maximum. In other words, the peak cavity pressure lags behind the injection pressure by a certain time, requiring a further densification flow process where the melt fills all gaps in the cavity and becomes compressed within a very short time.

The pressure at this point is called the packing pressure, also known as the secondary injection pressure. Both the packing flow and the compaction flow during filling are melt densification flows under high pressure. The flow characteristics at this time are that the melt flow rate is very low and does not play a leading role, while pressure is the main factor affecting the process. This pressure-dominated phase is critical for minimizing shrinkage and ensuring proper part dimensions in the injection molding process.

Switchover to Backflow Stage

When the gate "freezes," packing ends and screw plasticization begins, with the nozzle pressure dropping to zero. Although the gate is "frozen" at this point, the melt inside the mold has not yet completely solidified. Under the reaction of cavity pressure, the melt inside the mold will flow back into the gating system, reducing the pressure inside the mold. The cavity pressure at this point is called the sealing pressure.

The backflow time and sealing pressure depend on factors such as polymer properties, cylinder and mold temperatures, and gate dimensions. Controlling this transition is important in the injection molding process to prevent excessive backflow that could cause part defects, while allowing sufficient pressure release to avoid excessive residual stress in the part.

The injection and mold filling stage is perhaps the most dynamic and critical phase of the injection molding process, as it establishes the initial structure and properties of the molded part. Proper control of this stage requires a deep understanding of material behavior under different processing conditions, as well as the ability to adjust parameters to compensate for variations in raw materials, machine performance, and environmental factors. Advanced injection molding machines incorporate sophisticated control systems that monitor and adjust pressure, temperature, and flow rates in real-time to ensure consistent part quality throughout production runs.

The cooling and solidification process begins when the gate "freezes" and continues until the part is ejected from the mold. This stage of the injection molding process is characterized primarily by temperature-related phenomena. Generally, from gate "freezing" to part ejection, the part must continue cooling in the cavity for a certain period to ensure it has sufficient rigidity to prevent distortion during ejection. During this process, the temperature of the melt in the mold cavity gradually decreases, with corresponding changes in pressure and volume.

Cavity pressure changes are related to packing time. The longer the packing time, the greater the residual stress in the cavity. Ideally, residual stress should be zero when the part is ejected. If residual stress is greater than zero, ejection becomes difficult; if residual stress is less than zero, the part surface is prone to sink marks or internal vacuum bubbles may form. These defects can significantly compromise part performance and aesthetics, making proper cooling control essential in the injection molding process.

The规律 of plastic volume change is actually the same as the规律 of plastic density change. Specifically, the longer the packing time, the higher the plastic temperature when the gate "freezes," and the higher the cavity pressure, resulting in greater part density. When packing time is constant, higher ejection temperatures correspond to higher mold cavity pressures but lower part density, with the part experiencing greater post-shrinkage after ejection, leading to significant internal stress within the part.

Plastic parts can be ejected from the mold when they have cooled sufficiently to possess adequate rigidity. The ejection temperature should not be too high, generally controlled between the heat deflection temperature and the mold temperature. This temperature range ensures that the part can maintain its shape after ejection while minimizing the cooling time required, thus optimizing the overall efficiency of the injection molding process.

Factors Affecting Cooling Efficiency

Cooling Time Calculation

Cooling time in the injection molding process is primarily determined by the maximum wall thickness of the part, as heat must be conducted through the material to the mold surface. For simple geometries, cooling time can be approximated using the square of the maximum wall thickness, multiplied by a material-specific constant. This relationship highlights why part design with uniform, minimal wall thickness is crucial for optimizing cycle time in the injection molding process.

Efficient cooling is essential for optimizing the injection molding process, as it typically accounts for 50-70% of the total cycle time. Improvements in cooling system design can significantly reduce production costs by increasing output while improving part quality. Advanced cooling technologies, such as conformal cooling channels produced through additive manufacturing, allow for more uniform and efficient cooling of complex part geometries, representing an important innovation in the injection molding process. Proper cooling not only improves productivity but also enhances part dimensional stability, reduces residual stresses, and improves overall part quality and performance.

Integration of Injection Molding Process Stages

While the injection molding process is divided into distinct stages for analytical purposes, these stages are highly interconnected and influence each other in practice. Successful injection molding requires a holistic understanding of how parameters set in one stage affect outcomes in subsequent stages. For example, plasticization conditions directly impact melt viscosity and temperature, which in turn affect mold filling behavior, cooling rates, and final part properties.

The successful integration of all stages in the injection molding process requires both technical expertise and practical experience. By understanding how plasticization, injection, and cooling stages interact and influence each other, process engineers can optimize the entire production system for efficiency, quality, and cost-effectiveness. Continuous improvement through systematic experimentation and data analysis is key to mastering the complexities of the injection molding process and achieving consistent, high-quality results.