Injection speed and injection pressure are the most critical variables in the injection molding process. They together determine how molten plastic fills the mold cavity. Mastering the correct balance between these two factors is essential for manufacturers who want to produce defect-free parts consistently and efficiently.
In this article, we dive deep into the principles of injection speed in injection molding, explore the logic behind multi-stage velocity control, and explain how selecting the right injection molding speed can reduce part defects, enhance surface quality, and improve production output.
What Is Injection Speed?
Injection speed refers to the velocity at which molten plastic is injected into the mold cavity. It is typically measured in cubic centimeters per second (cm³/s) or millimeters per second (mm/s), depending on the machine and material. It directly affects the flow behavior of the material, heat dissipation, and ultimately the appearance and structural integrity of the molded part. A slower speed injection can cause short shots or weld lines, while overly fast speeds may lead to flash, burn marks, or air entrapment.
Modern machines allow processors to program multi-stage injection speeds, dynamically adjusting velocity across different phases of the filling process. This flexibility is essential when dealing with parts that feature sudden changes in wall thickness, intricate geometries, or require tight dimensional tolerances.
The Three Stages of Multi-Stage Injection Speed
In most professional applications, a single constant injection speed is rarely ideal. Instead, the injection process is divided into three or more stages, each customized to a specific flow condition:
- Initial Filling Phase: This stage often uses high injection speed to ensure the molten plastic quickly enters the cavity before significant cooling occurs. It helps reduce premature solidification, especially in thin-walled sections.
- Mid-Filling Stage: The speed is reduced slightly to ensure smooth flow through complex geometries, avoiding turbulent behavior and potential voids.
- Final Filling Stage: As the cavity nears full, the injection molding speed is reduced significantly to prevent overpacking, flashing, or part warpage. This controlled slowdown allows for a seamless transition to the packing stage governed by injection pressure.
Principles for Setting Injection Speed
Setting the right injection speed requires a deep understanding of the mold geometry, material properties, and desired outcomes. Several key principles guide this process:
Maintain Consistent Flow Front Velocity: Maintaining a steady flow front speed ensures uniform filling and reduces the risk of defects like flow lines or hesitation marks. This often requires adjusting injection speed based on the cross-sectional area of the mold cavity.
Prevent Material Freezing: For thin-walled parts, a higher injection molding speed is critical to fill the cavity before the material cools and solidifies. In thin-walled products, rapid flow reduces the risk of premature solidification, minimizing speed injection defects like short shots.
Critical Zones Control: Slowing the injection speed near the gate or sharp corners prevents jetting, flashing, or weld lines surface defects. Controlled speed also reduces shear stress, improving part strength.
Timely Injection Stop: Stopping injection at the right moment prevents overfilling, which can lead to flash or excessive injection pressure buildup. This requires precise coordination between speed and pressure settings.
Geometry-Based Segmentation: Complex molds with thin walls, ribs, or intricate details require segmented speed injection to ensure accurate filling without voids or sink marks.
Avoid Overpacking: As the cavity fills, the machine must detect the correct switchover point to injection pressure to avoid excessive internal stress or deformation.
How to Select Injection Speed Based on Part Zones
A granular approach to velocity control considers the part as a series of zones, each requiring its own speed strategy:
- Main and Sub-Runners: Fast injection here minimizes cycle time and keeps material in a molten state.
- Gate and Adjacent Areas: Speed should be lowered to reduce turbulence and flow marks.
- Main Body of the Part: This zone often benefits from moderate, steady injection speed to maintain consistent packing and prevent flow anomalies.
- Near 85–90% Cavity Fill: Transition to slower speed to reduce internal stress as the cavity nears full and prevent pressure spikes that could lead to flash or mold damage.
- Final Filling: This is the most delicate phase; any sudden velocity can cause severe cosmetic or structural defects. Controlled slowdown is key.
Influence of Mold Geometry on Injection Speed Settings
The complexity of the mold greatly influences how injection molding speed should be calibrated:
Thin-Walled Sections: Require high velocity to prevent freeze-off, particularly in high-speed applications.
Thick Sections: Allow for slower fill speeds and demand more attention during the packing phase to avoid sink marks.
Ribs and Bosses: Need balanced slower injection speeds to prevent voids and achieve full material packing.
Intricate Details: Slower speeds are typically used here to ensure precision and avoid air traps.
Large Cavities: Larger molds may require segmented speed injection to balance filling speed with pressure control, ensuring uniform density and minimal shrinkage.
Gate design: The size and type of gate influence optimal speed—smaller gates may need slower speeds to reduce shear and jetting, while larger gates allow faster injection and smoother cavity filling.
Radial diffusion molds: Molds designed with radial diffusion, where material flows outward from a central point, require segmented injection speed settings. An initial high injection speed ensures smooth flow through the central channels, while a gradual reduction in speed as the material spreads radially prevents overpacking and ensures uniform filling.
High-Speed vs. Low-Speed Injection Molding
The choice between high speed injection molding and low-speed injection depends on the part’s requirements and production goals. Each approach has distinct advantages:
High Speed Injection Molding:
Advantages: High speed injection molding significantly shortens cycle time, boosting productivity for high-volume production in industries like packaging, medical devices, and consumer goods. It is ideal for thin-wall components, ensuring rapid filling to prevent material freezing and achieve complete cavity coverage. High speed injection molding also helps overcome high-flow resistance in complex or narrow mold geometries, enabling smooth material flow. Additionally, it minimizes weld lines by promoting better material fusion at seam points, enhancing part strength and appearance. The rapid injection process can further improve surface finish, producing sharper details and smoother textures, which is critical for aesthetically demanding applications.
Challenges: Risks include burning, jetting, or air entrapment if speeds are not carefully controlled. High injection pressure is often required, which can stress the mold.
Low Speed Injection Molding:
Advantages: Low speed injection enhances control over cavity filling, allowing precise material placement to ensure uniform density and dimensional accuracy, especially in thick-walled or complex parts. It reduces material stress and shear heating, minimizing internal defects and improving the structural integrity of components. This method lowers the risk of air entrapment and flashing, resulting in cleaner parting lines and fewer post-processing needs. Low-speed injection is particularly effective for producing high-quality surface finishes with minimal flow marks, making it ideal for visible components in automotive interiors or consumer electronics. Its controlled approach also reduces warpage, ensuring consistent part geometry in intricate designs.
Challenges: Longer cycle times and potential for incomplete filling in complex molds, requiring careful material selection.
Addressing Defects Through Injection Speed Adjustments
One of the most powerful applications of injection speed in injection molding is its ability to address common defects. By fine-tuning speed injection, manufacturers can resolve issues like:
- Short Shots: Increasing injection speed in critical areas ensures complete cavity filling, especially in thin or complex sections.
- Sink Marks: Adjusting injection speed to balance cooling and filling rates minimizes uneven shrinkage and surface depressions.
- Weld Lines: Optimizing speed injection improves material flow and bonding at seam lines, enhancing part strength.
- Flash: Reducing injection speed during final filling prevents overpacking and material leakage beyond the mold parting line.
- Jetting Marks: Reducing injection speed near the gate prevents jetting, ensuring a smoother finish.
Conclusion
Optimizing injection speed is a strategic tool for improving part quality, ensuring dimensional accuracy, and preventing common molding defects. Whether dealing with thin-walled geometries, complex cavity shapes, or high-precision parts, understanding and controlling injection speed in injection molding is essential for consistent and efficient production. Jiangzhi has extensive experience in injection molding and mold manufacturing. Our team also offers guidance on multi-stage injection settings and mold optimization to reduce shrinkage, warpage, and surface defects. If your project demands high-quality injection-molded parts with optimized flow control, we provide the technical expertise and manufacturing capabilities to turn your design into reliable, production-ready components.
