Complete Guide to Injection Pressure: Enhancing Molding Quality and Efficiency
2025-06-13 08:45:19
In injection molding processes, Injection Pressure plays a critical role. It directly impacts product appearance, dimensional accuracy, cavity filling stability, and cycle time. This article provides a comprehensive overview of injection pressure to help manufacturers improve product quality while optimizing production efficiency.
Injection pressure refers to the force exerted by the screw to push molten plastic through the nozzle into the mold cavity. It must be high enough to overcome the resistance of runners, gates, and cavity flow. Insufficient pressure can result in common molding defects such as short shots or weld lines. Conversely, excessive pressure may cause flash, mold damage, or internal stress that compromises product performance.
- Material Properties： melt viscosity, flowability, and filling capability
- Mold Design： cavity shape, part thickness, runner, and gate layout
- Process & Equipment Conditions： injection speed, mold temperature, screw design, and hydraulic system response time
Injection pressure is not a single fixed value but involves multi-stage pressure control during the molding cycle. It is typically categorized into：
This is the peak pressure applied during the initial filling of the cavity. It must be sufficient to rapidly push the melt through the entire mold, particularly in complex geometries or high-viscosity materials. It’s closely related to plastic viscosity, gate design, and flow path resistance.
Once the cavity is nearly filled, the process enters the packing phase. The injection pressure is reduced, and holding pressure is applied to compensate for material shrinkage during cooling. The duration and level of holding pressure should be carefully controlled based on part thickness, mold temperature, and cooling rate to prevent sink marks or voids.
Back pressure is applied by the hydraulic system on the rear side of the screw during plasticizing. Proper back pressure improves melt density and mixing uniformity. However, excessive back pressure can extend plasticizing time and increase melt temperature, while too little may result in inconsistent filling or poor material homogeneity.
| Pressure Type | Function Description | Recommended Setting Range |
| Primary Injection | Rapidly fills the cavity up to 95–98% | Based on flowability and part geometry |
| Holding Pressure | Compensates shrinkage, stabilizes part dimensions | 80–90% of primary injection pressure |
| Back Pressure | Improves melt uniformity and density control | Typically <10–20% of primary injection pressure |
| Clamping Force (not injection pressure) | Prevents mold opening and flash | Calculated based on projected area and mold force |
Further reading： Clamping Force in Injection Molding: Formula & Optimization Tips
Incorrect pressure settings can result in the following common injection molding defects：
- Short Shot： Caused by insufficient pressure or slow injection speed, leading to incomplete cavity filling
- Flash： Caused by excessive pressure or worn parting lines, allowing melt to escape
- Voids & Sink Marks： Result from insufficient holding pressure or short holding time
- Excessive Internal Stress： Occurs when pressure is too high, disturbing polymer chain orientation and causing warpage or poor dimensional stability
When adjusting pressure parameters, change one variable at a time and observe its effect on the molded part to identify optimal conditions.
Further reading： Short Shot in Injection Molding: Causes, Detection & Solutions
Injection pressure settings should be based on part design, mold construction, and material characteristics. Common setup guidelines include：
The flow resistance within the mold determines the required pressure. Long flow paths and thin-wall parts generally require higher injection pressure to ensure complete filling. Mold flow analysis or trial runs can help determine an appropriate starting point.
The switchover to holding pressure should occur when the cavity is about 95–98% filled. Switching too early leads to underfilling, while switching too late risks overpacking and flash. Proper transition timing is essential for molding stability and mold longevity.
Injection speed is closely linked to pressure. High speeds can cause pressure spikes, while slow speeds may lead to premature cooling and incomplete filling. A multi-stage speed and pressure control strategy, fine-tuned based on gate location and cavity geometry, is recommended to improve filling balance and surface finish.
Injection molding is a highly engineered manufacturing process, and injection pressure is one of its core control variables. Proper setting and fine-tuning of injection pressure directly affect product quality and production efficiency. By understanding the roles of each pressure stage and applying practical experience, manufacturers can achieve the ideal balance between quality, yield, and efficiency, paving the way for smart, high-performance production systems.
- 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/
