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Understanding Injection Speed and Injection Pressure

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Injection molding involves several key factors such as temperature, time, pressure, speed, and position. Temperature, time, and position are relatively straightforward, but injection speed and injection pressure are more complex. Injection speed, in particular, is a challenging aspect of the molding process to control, as it lacks standardized reference data like other process parameters.

Today, we’ll focus on understanding injection speed, injection pressure, and their interrelationship.

Internal movement during injection molding

What is Injection Speed?

Typically, the set injection speed refers to the screw’s forward speed. However, what’s crucial is the flow speed of the melt within the mold cavity, which depends on the cross-sectional area in the flow direction.

The close relationship between injection speed and product quality makes it a critical parameter in injection molding. By setting the filling speeds near the gate, in the main body, and at the flow end, and adjusting the corresponding injection positions, products with good appearance and minimal internal stress can be manufactured.

Concept of Multi-Stage Injection Speed

The control of injection speed involves dividing the screw’s injection stroke into several stages, with each stage using a suitable injection speed.

Steps to Set Multi-Stage Injection Speeds (Example with Three Stages)

First Step: Start by setting V1, V2, and V3 at the same speed, then gradually increase the injection speed by 5% increments from about 5%, observing the appearance. Roughly identify the speeds that produce good appearances near the gate, in the main body, and at the flow end. Existing trial data can also be used to determine appropriate speeds for each stage.

Second Step: Based on an initial estimate of the screw strokes (S1, S2, S3), enter the speed V1 that yields a good appearance around the gate for S1; for S2, input the speed V2 that gives a good appearance in the main body; for S3, input the speed V3 for a good appearance near the flow end, and conduct a trial injection.

Third Step: Move “S1” forward and backward to find the best position for good appearance near the gate and main body; then, adjust “S2” to find the best position for the main body and flow end. Adjustments to the switching position (S3) can also help overcome injection molding defects such as flash and poor appearance at the flow end.

Principles of Injection Speed Setting

1. The surface velocity of the fluid should be constant.

2. Use rapid injection to prevent the melt from freezing during injection.

3. Injection speed settings should consider fast filling in critical areas (like runners) while slowing down at the gate.

4. Ensure the mold cavity is filled and then immediately stop to prevent overfilling, flash, and residual stress.

5. Speed segmentation must consider mold geometry, other flow restrictions, and instability factors.

Proper setting of speed requires a clear understanding of injection molding processes and materials; otherwise, product quality will be difficult to control. Since melt flow speed is hard to measure directly, it can be indirectly calculated by measuring screw advance speed or cavity pressure (ensuring the check valve does not leak).

The Influence of Mold Geometry on Injection Speed Settings

  1. Thin-walled sections require high injection speeds.
  2. Thick-walled parts need a slow-fast-slow speed curve to avoid defects.
  3. To ensure product quality standards, the injection speed setting should keep the melt front flow speed constant. The flow speed of the melt is crucial as it affects the molecular alignment and surface condition of the parts.
  4. When the melt front reaches a cross-sectional area, it should slow down.
  5. For molds with radial diffusion, ensure a balanced increase in melt flow.
  6. Long flow paths must be quickly filled to reduce the cooling of the melt front.
  7. Adjusting the injection speed can help eliminate defects caused by slow flow at the gate. When the melt passes through the nozzle and the runner to reach the gate, the surface of the melt front may have already cooled and solidified, or the melt may stall due to a sudden narrowing of the runner until enough pressure is built to push the melt through the gate, causing a peak in pressure at the gate.
  8. High pressure can damage the material and cause surface defects such as flow lines and scorch at the gate. This issue can be addressed by slowing down just before the gate to prevent excessive shear at the gate, then increasing the injection speed back to the original value. Because precisely controlling the injection speed at the gate is very difficult, slowing down in the final section of the runner is a better strategy.

Improving Product Defects through Injection Speed

Flash

Controlling the speed at the end of injection can avoid or reduce defects such as flash, scorch, and trapped air. Slowing down at the end of filling can prevent overfilling of the cavity, avoiding flash and reducing residual stress. Trapped air caused by poor venting at the end of the mold flow path or filling issues can also be addressed by reducing the venting speed, especially at the end of the injection.

Short shots

Too slow speeds at the gate or localized flow blockage cause Short shots due to solidification of the melt. Increasing the injection speed just after passing the gate or where there is localized flow blockage can solve this problem. Defects such as flow marks, burn marks at the gate, and delamination in heat-sensitive materials are due to excessive shear when passing through the gate.

Splay Mark

The smoothness of the parts depends on the injection speed; materials filled with glass fibers are particularly sensitive, especially nylon. Splay (waviness) is caused by flow instability due to viscosity variations. The type of defect—whether waviness or an uneven mist—depends on the level of flow instability.

Jetting mark

To prevent jetting, the injection speed setting must ensure rapid filling of the runner area and then slow passage through the gate. Identifying this speed transition point is critical. If too early, the fill time will be excessively increased; if too late, excessive flow inertia can lead to jetting. The lower the melt viscosity and the higher the barrel temperature, the more pronounced the tendency for jetting. High-speed, high-pressure injection is needed at small gates, thus being a significant factor causing flow defects.

Sink Mark

The sink mark can be improved through more effective pressure transfer and a smaller pressure drop. Low mold temperature and slow screw advance speed greatly shorten the flow length, which must be compensated by high injection speed. High-speed flow reduces heat loss and, due to high shear heat from friction, causes an increase in melt temperature, slowing down the thickening rate of the outer layers of the part.

Injection System Pressure and Injection Pressure

Injection pressure is provided by the hydraulic system of the injection molding machine. The system pressure acts or transfers to the injection hydraulic cylinder, and from there, through the screw, it is transmitted to the injection melt. The melt then moves from the nozzle into the mold’s main channel and is injected into the mold cavity.

Roles of Injection Machine Pressure and System Pressure

Injection Machine Pressure: During injection, the plastic must be subjected to high injection pressure to overcome flow resistance and fill the mold cavity. The level of injection pressure affects not only the quality and dimensional accuracy of the molded products but also the performance of the plastic melt and the stability of the injection process.

System Pressure: The magnitude of system pressure directly affects the precision, stability, and energy consumption of the injection molding process.

Differences between Injection Machine Pressure and System Pressure

Different Functions

Injection pressure primarily acts on the melt injected into the mold to overcome the viscosity and flow resistance of the plastic. System pressure acts on the injection cylinder, transforming into injection pressure, providing the instantaneous kinetic energy to drive the hydraulic oil.

Different Adjustment Methods:

Injection pressure is adjusted via a PID control system, whereas system pressure is mainly adjusted by the hydraulic system’s control circuit and its boosting unit.

Different Response Times:

Injection pressure adjusts quickly, with response times in milliseconds, allowing the control system to respond promptly to current pressure values. System pressure adjustments are slower, requiring time to pressurize the hydraulic system to achieve the desired high pressure.

Injection Pressure Calculation Formula

  1. The calculation formula for the injection pressure of an injection molding machine is: P = K × Q / S
    • P: Injection pressure, in MPa
    • K: Injection pressure coefficient, varies with different plastics
    • Q: Instantaneous flow rate of injection material, in g/s
    • S: Projected area of the part, in square centimeters.
  2. Determination of the injection pressure coefficient K a. Material properties: Different materials have distinct melt flow characteristics, thus requiring different K values for injection pressure. In production, the appropriate K value should be chosen based on the characteristics of the material. b. Injection process and equipment: The K value also varies with different injection processes and equipment. Therefore, in production, the appropriate K value should be selected according to the performance of the injection machine and the requirements of the injection process.

Calculation of Injection Pressure (Pi) and System Pressure (Pump Pressure)

Injection Pressure Formula Pi (KG/CM2): Pi = P * A / Ao

Pi: Injection pressure

P: Pump pressure

A: Effective area of the injection cylinder

Ao: Cross-sectional area of the screw

A = π * D^2 / 4; D: Diameter; π: Pi = 3.14159

Example 1: Known pump pressure, calculate injection pressure?

Pump pressure = 75 KG/CM2, effective area of the injection cylinder = 150 CM2, cross-sectional area of the screw = 15.9 CM2 (Diameter 45mm).

Formula: 2πR2 = 3.1415 * (45mm / 2)^2 = 1589.5 mm2 Pi = 75 * 150 / 15.9 = 707 KG/CM2

Example 2: Known injection pressure, calculate pump pressure?

Required injection pressure = 900 KG/CM2, effective area of the injection cylinder = 150 CM2, cross-sectional area of the screw = 15.9 CM2 (Diameter 45)

Pump pressure P = Pi * Ao / A = 900 * 15.9 / 150 = 95.4 KG/CM2

Relationship Between Injection Pressure and Speed

The relationship between injection pressure and speed is interactive and directly impacts injection molding. Generally, at the same injection speed, higher injection pressure improves the plastic’s flow capability, enhancing the dimensional precision and surface smoothness of the product. However, excessive injection pressure can cause excessive mold force. This will create gaps and increase the load on the injection machine, destabilizing the injection process. Therefore, in practice, injection pressure and speed must be adjusted based on specific production requirements and material characteristics to achieve optimal molding results.

Conclusion

The insights into injection speed and pressure covered in this article may only scratch the surface. For example, while learning about these factors, injection molding professionals should also understand injection curve graphs.

I am Lee Young. I share insights from the internet and books about injection molding and molds, combined with practical injection molding experience. If you find my content interesting or have any questions, feel free to contact me at firstmold88@gmail.com to discuss further.

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