How to Optimize Injection Speed in Injection Molding
Injection speed is a critical parameter that directly determines the flow behavior of the molten plastic within the mold cavity. It significantly impacts part appearance, mechanical properties, and production efficiency. Excessively high speeds can cause melt fracture, flash, bubbles, and high internal stress, while low speeds often result in visible weld lines, short shots, and sink marks. Optimizing injection speed requires a segmented control strategy, a step-by-step approach, and defect-driven adjustments, tailored to the specific plastic material, part geometry, and mold design.
1. Core Principles of Injection Speed Optimization
1.1 Segmented Control
The filling process should be divided into three distinct phases, each requiring a different speed profile:
Gate Zone (Start): Use a slow speed to ensure the melt enters the cavity smoothly. This prevents jetting (spraying), gate whitening, and air entrapment caused by high-velocity impact.
Mid-Cavity (Fill): Use a fast speed to quickly fill the main body of the cavity. This minimizes the cooling time, reduces the visibility of weld lines, and improves surface gloss.
End of Fill (Pack): Use a slow speed as the melt reaches the final 10-20% of the cavity. This prevents flash at the parting line and reduces internal stress by minimizing the impact on the mold walls.
1.2 Material Compatibility
Speed must match the Melt Flow Rate (MFR) and thermal sensitivity:
High Flow (PE, PP, PS): Tolerate higher speeds.
Low Flow (PC, PMMA, PPS): Require moderate to low speeds to avoid flow marks and melt fracture.
Heat-Sensitive (PVC, POM): Require slow, steady speeds to prevent degradation from excessive shear heat.
1.3 Part Geometry
Thin-Walled Parts: Require high overall speeds to fill before the material freezes (often 30-50% faster than thick parts).
Thick-Walled Parts: Benefit from a "Slow-Fast-Slow" profile to prevent air traps and internal voids.
Complex Parts: Need targeted speed increases in the mid-phase to fill ribs and deep cavities, followed by a sharp deceleration at the end.
2. Step-by-Step Tuning Process
2.1 Define Segments
Divide the total injection stroke into 3-5 segments based on the mold's geometry.
Segment 1 (Gate): 10-20% of total stroke.
Segment 2 (Body): 60-70% of total stroke.
Segment 3 (End): 10-20% of total stroke.
2.2 Set Initial Baseline Speeds
Refer to industry standard speed ranges (mm/s) based on the material:
| Material Type | Gate Zone Speed (mm/s) | Mid-Cavity Speed (mm/s) | End of Fill Speed (mm/s) |
|---|---|---|---|
| PE / PP | 10 - 20 | 50 - 120 | 5 - 15 |
| PC / PMMA | 8 - 15 | 30 - 80 | 5 - 10 |
| ABS | 10 - 25 | 40 - 100 | 5 - 15 |
| PVC | 5 - 10 | 20 - 50 | 3 - 8 |
| POM | 8 - 15 | 30 - 70 | 5 - 10 |
2.3 Defect-Driven Optimization (Single Variable Method)
Adjust only one segment at a time and observe the results:
Gate Issues (Jetting/Whitening): Decrease Segment 1 speed.
Weld Lines/Dull Surface: Increase Segment 2 speed (in 5-10 mm/s increments) until the lines disappear.
Short Shots (Incomplete Fill): Increase the relevant segment speed or increase melt temperature.
Flash (Burrs): Decrease Segment 3 speed and/or reduce injection pressure.
Bubbles/Blisters: Check for trapped air; ensure a slow start and proper venting.
2.4 Link with Other Parameters
Speed cannot be adjusted in isolation:
Temperature: If increasing speed causes flow marks, raise the barrel or mold temperature to improve flow.
Pressure: If speed is too low to fill, increase injection pressure to maintain packing force.
V/P Switchover: High-speed filling usually requires a later switchover point (95-98% full), while low-speed filling may need an earlier switch to prevent overpacking.
3. Key Considerations
Smooth Transitions: Ensure the speed changes between segments are gradual (limit gradient to ≤20 mm/s difference) to avoid shocking the mold or causing pressure spikes.
Documentation: Record all parameter changes and their effects on part quality for future reference and standardization.
Ventilation: Poor mold ventilation can create defects (burn marks, bubbles) that mimic speed-related issues. Always clean vents before assuming speed is the problem.
Validation: After finalizing parameters, run a small production batch (50-100 shots) to confirm stability under thermal equilibrium conditions.
Summary
Optimizing injection speed is about achieving the ideal flow front. By implementing a segmented "Slow-Fast-Slow" strategy, starting with material-specific baseline data, and making incremental adjustments based on observed defects, you can significantly improve part quality and production efficiency. Always remember to balance speed with temperature and pressure for optimal results.
