The Importance of Injection Speed：A Key Parameter for Quality and Productivity
2025-07-24 10:32:10
Injection speed is often regarded as a secondary setting；however, it plays a critical role in determining filling behavior, product quality, and process efficiency. It directly affects the flow behavior of the melt, cavity pressure distribution, shear heat generation, venting, and mold life. This parameter becomes especially crucial in thin-wall parts, complex mold geometries, or processes requiring multi-stage speed control.
Although closely related to injection pressure, injection speed has its own independent logic and molding impact. From dimensional stability and surface finish to cycle time reduction, the accuracy of injection speed setting directly influences product consistency and overall productivity.
Injection speed refers to the rate at which the screw pushes molten plastic into the mold cavity, typically expressed in mm/s or cm³/s. This parameter directly determines the advancement and flow pattern of the melt, making it a core factor in controlling cavity pressure distribution, shear heat generation, and gas evacuation.
- Excessive shear heat buildup → Causes burn marks, flash, or jetting
- Inadequate gas venting → Leads to flow marks or unbalanced cavity pressure
- Incomplete cavity fill → Results in short shots or cold slugs
- Poor weld lines → Reduces structural integrity and surface quality
Injection speed alters the melt flow pattern (laminar vs. turbulent), affecting how the melt contacts the mold surface. A stable flow regime leads to better surface gloss and detail reproduction.
Speed affects instantaneous filling pressure and heat distribution. Speeds that are too slow may cause insufficient pressure, leading to sink marks and voids. Conversely, excessively fast speeds may result in pressure spikes and localized overheating, leading to dimensional variation or mold damage.
Appropriate high-speed injection can shorten filling time and improve cycle efficiency. However, poor control may increase defect rates, raise mold maintenance frequency, and reduce mold life.
Due to varying geometry within the mold cavity, a single-speed profile cannot fulfill both filling efficiency and part quality. Therefore, segmented speed control has become a mainstream technique in modern injection molding.
| Injection Stage | Suggested Speed | Control Purpose |
| Initial Fill (Sprue to Runner) | High | Rapidly fill runners to prevent cold slug formation |
| Middle Fill (Main Cavity Areas) | Medium | Fill complex geometry smoothly, reduce flow marks |
| Final Fill (85–90% Full to Gate) | Low | Avoid pressure spikes, flash, and burn marks |
This decelerating profile stabilizes flow fronts, improves venting, enhances melt fusion, and ensures a smooth transition into the packing phase.
Further reading：Key Role of Holding Pressure：Enhancing Yield and Stable Mass Production
To ensure optimal filling performance and minimize defects, injection speed should be configured with the following considerations：
- Maintain a Stable Flow Front：Avoid flow imbalance or turbulence that may cause weld lines.
- Prevent Premature Freezing at Flow Fronts：Higher speed is required for thin-wall applications.
- Reduce Speed at Gates and Fine Features：Minimize shear heating and jetting.
- Set Speed Zones Based on Mold Geometry：Use zone-specific speeds to enhance filling completeness and consistency.
Mold structure has a significant influence on speed setting strategies：
| Mold Feature | Recommended Speed Setting |
| Thin-wall sections | High speed to avoid premature freeze-off |
| Thick-wall sections | Medium speed to ensure controlled fill and effective packing |
| Fine structures (ribs, bosses) | Slow speed in final phase to prevent voids or breakage |
| Large cavities | Use decelerating profile to balance pressure and fill uniformity |
| Small gate design | Reduce speed to avoid excessive shear or unstable flow |
Injection speed and pressure are highly interdependent and should be monitored and optimized in tandem：
- Higher speed → Greater flow resistance → Increased pressure demand → Risk of pressure spikes and mold wear
- Lower speed → Inadequate pressure → Poor melt push force → Risk of short shots or poor fusion
It is recommended to integrate in-mold pressure sensors and data acquisition systems for real-time adjustments. Speed and pressure coordination should also be aligned with mold venting and cooling system designs.
Further reading：Injection Pressure in Injection Molding: Complete Guide to Optimizing Quality and Efficiency
Although injection speed appears to be a simple numeric setting, it involves a high degree of complexity. It must be dynamically adjusted based on product requirements, mold design, venting, and cooling systems. By adopting multi-stage speed control strategies, molders can optimize dimensional precision, surface quality, and molding cycle times. Only by combining scientific understanding with hands-on experience, and by leveraging real-time process data and simulation tools, can injection speed become a powerful lever to improve quality, stabilize productivity, and extend mold service life.
- Group Name: Huarong Group
- Brand: Huarong, Yuhdak, Nanrong
- Service Offerings: Injection Molding Machine, Vertical Injection Molding Machine, Injection Molding Automation
- Tel: +886-6-7956777
- Address: No.21-6, Zhongzhou, Chin An Vil., Xigang Dist., Tainan City 72351, Taiwan
- Official Website: https://www.huarong.com.tw/
