Introduction

Injection speed is one of the most critical parameters in injection molding—not only because it directly governs fill time, but because it profoundly impacts molecular orientation, residual stress, surface finish, and overall dimensional stability¹. Properly segmented injection speed profiles enable precise control over melt front velocity, ensuring consistent flow behavior and minimizing defects—even with thermally sensitive or high-viscosity resins.

In this article, we explore the science behind injection speed² segmentation, its relationship with mold geometry and material behavior, and practical strategies to optimize part quality through intelligent speed profiling.

Why Injection Speed Matters

The surface velocity of the melt front should ideally remain constant during cavity filling. Why? Because:

• Constant melt front speed promotes uniform molecular alignment and reduces internal stress.
• Too-slow speeds risk premature freezing (especially in thin sections or cold runners), leading to short shots or flow marks.
• Too-fast speeds cause excessive shear heating, degradation (particularly in thermal-sensitive polymers like PC, POM, or UPVC), jetting, burn marks, and air entrapment.

Since direct measurement of melt velocity is impractical in production, practitioners typically infer it from screw forward speed or cavity pressure curves—provided the check valve is intact and non-leaking.

Core Principles of Multi-Stage Injection Speed Profiling

1. Maintain Constant Melt Front Velocity

A steady melt front ensures homogeneous packing, minimizes orientation-induced warpage, and improves surface replication. This is especially vital for parts with complex geometries or tight tolerances.

2. Prioritize Fast Filling in Critical Flow Paths

• Thin-walled sections require maximum injection speed to avoid premature solidification.
• Long runners demand rapid filling to counteract heat loss—except for high-viscosity materials (e.g., PC), where excessive speed may inject cold slug into the cavity.

3. Decelerate at Gate Entry

This is arguably the most impactful adjustment:

• As the melt passes through the sprue, runner, and approaches the gate, the (melt front) may cool or stall due to sudden constriction.
• High pressure buildup at the gate can cause burn marks, flow lines, gate blush, or even material degradation.
• Solution: Reduce speed just before the gate (typically in the final runner segment), then resume original speed after gate entry. This avoids excessive shear while maintaining momentum.

⚠️ Note: Precise timing is key. Too early deceleration increases cycle time; too late invites jetting or weld-line defects.

4. Final Fill Stage: Controlled Slowdown

• Prevents overpacking, flash, and high residual stress.
• Reduces air trapping by lowering injection velocity near vent areas—critical for parts with poor venting or deep cavities.
• Helps mitigate sink marks by improving pressure transmission and reducing pressure drop across thick sections.

5. Accelerate After Gate Obstruction (for Short Shots)

If localized flow resistance (e.g., sharp corners, inserts, or narrow gates) causes hesitation, a temporary speed boost right after the obstruction can restore flow continuity and eliminate short shots.

Material-Specific Considerations

💡 Key Insight: Higher barrel temperatures reduce viscosity and shear stress—but increase oxidation risk. Optimal speed settings must balance thermal stability and shear-induced degradation.

Mold Geometry Dictates Speed Profile

• Thin walls → High-speed filling throughout.
• Thick sections → “Slow–Fast–Slow” profile: slow start to avoid jetting, fast mid-fill for heat retention, slow end to prevent sink/flash.
• Radial or multi-gate layouts → Ensure balanced flow by synchronizing speed ramps across all gates.
• Intersection zones / ribs → Reduce speed to avoid turbulence, weld lines, or uneven cooling.

✅ Pro Tip: Use mold-filling simulation³ (e.g., Moldflow) to predict optimal speed transition points before trial runs.

Common Defects & How Speed Profiling Fixes Them

Practical Implementation Tips

1. Start with velocity-controlled mode (not pressure-limited) for true speed profiling.
2. Monitor cavity pressure sensors to validate speed-to-pressure correlation.
3. Always verify check valve integrity—leakage invalidates screw-speed-based velocity assumptions.
4. For new molds: Begin with 3–5 speed segments (e.g., runner fill → gate approach → cavity fill → end-of-fill), then refine based on part inspection.
5. Document every profile change: speed values, transition positions (screw position or time), and resulting defect trends.

Conclusion

Mastering injection speed profiling isn’t just about “faster vs. slower”—it’s about orchestrating melt behavior in sync with mold dynamics and polymer physics. When executed correctly, multi-stage speed control eliminates >80% of common cosmetic and structural defects—especially for engineering thermoplastics and reinforced composites.

For manufacturers aiming for zero-defect production, investing time in optimizing injection speed segments yields immediate ROI: reduced scrap, shorter cycles, and higher first-pass yield.