Optimization of Injection Speed in Injection Molding

Injection speed is one of the most critical parameters in injection molding, directly affecting melt flow, cavity filling, surface quality and mechanical properties. Excessively high speed causes flash, burn marks, gas traps, over-shearing and material degradation; excessively low speed leads to short shots, flow marks, weak weld lines and poor surface finish. Scientific injection speed optimization is essential for stable production, high yield and reduced costs.

Basic Principles of Injection Speed Optimization

Optimization follows the principles of stability first, slow start then fast, multi-stage control and material adaptation. The filling process is divided into sections: slow initial feeding, medium-fast main filling, slow end filling and smooth transfer to holding pressure. Adjustments must be combined with material properties, mold structure, venting and temperature conditions.

Material-Based Speed SettingsHigh-flow materials such as PP and PE require medium-high speed to prevent cold marks. Medium-viscosity materials such as ABS and PC/ABS use medium speed with local fine adjustment. High-viscosity materials such as PC and PMMA are shear-sensitive and require medium-low speed to avoid overheating. Flame-retardant and glass-filled materials need moderate speed with proper back pressure to ensure dispersion and surface quality.

Multi-Stage Injection Speed Control

Modern injection machines support multi-stage speed control. The first stage from nozzle to gate is slow to stabilize flow and prevent jetting. The second stage after the gate is accelerated for efficient filling. The third stage adjusts speed for thin walls, ribs and critical appearance areas. The final stage near the end of filling is slowed down to improve venting and reduce weld lines. The transfer point to holding pressure is set at 95%–98% cavity filling.

Adjustment for Common Defects

Short shots require increased filling speed and improved venting. Flash and burrs indicate excessive speed or pressure, requiring reduction in mid-stage speed. Burn marks and gas traps need reduced end speed and enhanced venting. Visible weld lines can be improved by increasing local speed and optimizing venting. Flow marks and wave patterns require slower initial speed. Silver streaks and bubbles are often caused by over-shearing or moisture, requiring lower speed and improved drying.

Mold Structure Matching

Gate size, runner layout, wall thickness and venting directly influence speed settings. Small gates or narrow runners require higher speed; large gates allow lower speed. Thin-wall areas need higher speed, while thick sections avoid overheating. Poor venting prohibits high-speed filling. Mold temperature must be coordinated: lower mold temperature may require higher speed to compensate heat loss.

On-Site Optimization Procedure

Establish baseline parameters, start with low speed to observe flow pattern, gradually increase mid-stage speed until full filling, then fine-tune each section for appearance and dimensions. Confirm the transfer point and holding parameters. Record all speeds, positions and pressures for standardized process management. Regular checks on nozzle condition, temperature consistency and mold venting maintain long-term stability.

Injection speed optimization aims to achieve smooth, uniform and stable filling. Multi-stage control, material matching and mold coordination ensure high quality, efficiency and consistency in injection production.