Control of Injection Molding Process and Selection of Injection Speed

The control of the injection molding process has a direct impact on the final part quality and the economic efficiency of production. Process parameters must be thoroughly studied to achieve maximum benefits and optimal product quality. With the continuous upgrading of household appliances, product designs have become increasingly complex, and the requirements for both internal and external quality of injection-molded parts have continued to rise. This places higher demands on mold manufacturing as well as process adjustment and control for injection molding enterprises.

With continuous improvements in mold processing techniques and precision, complex product designs have become achievable. Meanwhile, the ongoing enhancement of electrical control systems in injection molding machines ensures stable production of high-quality parts using complex molds. However, advanced equipment and high-quality molds alone are not sufficient—effective process control is essential to achieve the perfect integration of machine, mold, and product.

The most critical processing conditions in injection molding are temperature, pressure, speed, and the corresponding time factors that influence plasticization, flow, and cooling. These parameters are interrelated and mutually restrictive. For example, increasing melt and mold temperature can reduce injection pressure and speed requirements, and vice versa. Among all process conditions, the key factor is the variation in material viscosity, which plays a crucial role in determining parameter selection and their interactions during injection.

With deeper research into the flow and deformation behavior of polymer melts during injection, it has become increasingly clear that the selection of injection speed is essential for improving product quality.

Characteristics of High and Low Injection Speeds

Advantages of high injection speed:

1. Reduces injection time and shortens the molding cycle
2. Improves flow length, beneficial for thin-wall parts
3. Enhances surface gloss of the product
4. Improves weld line strength and reduces their visibility
5. Prevents cooling deformation

Advantages of low injection speed:

1. Prevents flash formation
2. Reduces jetting and flow marks
3. Avoids burn marks
4. Minimizes air entrapment
5. Prevents molecular orientation deformation

The advantages of high speed correspond to the disadvantages of low speed, and vice versa. Therefore, combining high and low speeds during injection allows the advantages of both to be utilized while avoiding their drawbacks. This is commonly referred to as multi-stage injection technology, which is widely used in modern injection molding machines.

Currently, most medium and large injection molding machines are equipped with 5–6 stages of injection pressure and speed control, as well as 3–4 stages of holding pressure control. (During the holding stage, the cavity is already filled, and the influence of holding speed is minimal.)

Principles for Selecting Injection Speed

Due to the complex geometry of plastic products, the flow and deformation of the melt through the sprue, runner, gate, and cavity are highly complex. Based on rheological studies and CAE analysis, it has been concluded that to achieve high-quality parts with low internal stress, the most important condition is to maintain a uniform and stable melt flow field.

In other words, the velocity of the melt front should remain consistent as it flows through different cross-sections at different times during injection—i.e., linear velocity should remain constant (V = constant).

This ensures:

• High product quality
• Reduced risk of sink marks and short shots
• Uniform flow field in the cavity
• Proper molecular orientation
• Improved surface finish

However, due to varying cross-sectional areas and flow resistance in the mold, the flow rate $Q = V \times S$ (where S is cross-sectional area) becomes a variable. As a result, flow rate and injection pressure both become time-dependent functions.

To balance product quality and economic efficiency (shorter cycle time), multi-stage injection is necessary.

Rational Distribution of Injection Speed

Injection speed is typically divided into five stages:

1. Sprue and runner stage
   High speed is recommended to shorten cycle time, provided surface defects are avoided.
2. Gate and gate area
   Usually low speed, especially for high-viscosity resins such as PC, PMMA, and ABS, to prevent jetting and gate blush.
   For low-viscosity materials (PP, PA, PBT) and less critical surface areas, high speed may be used.
3. Main filling stage (≈70–80%)
   High speed is used to:
   • Reduce cycle time
   • Minimize viscosity variation
   • Improve surface gloss
   • Reduce deformation and improve weld line strength
4. Transition stage (≈85–90%)
   Medium speed is applied to transition smoothly to final filling and prevent flash or uneven thickness.
5. Final filling stage
   Low speed is used to:
   • Prevent flash and burn marks
   • Improve dimensional and weight stability
   • Reduce clamping force requirements

Setting and Adjusting Injection Speed Curves

Setting injection speed:
Two key factors:

1. Injection speed magnitude
2. Injection position

A common method is the “zero injection method”:

• Set the second-stage speed and pressure to zero
• Adjust screw position and observe part shape
• Compare with target position
• Gradually define each stage

Speed is typically increased step by step from low to high, ensuring surface quality is not compromised.

Adjustment of injection speed:
Adjustments are made in reverse order:

• First adjust speed
• Then verify transition positions

Because changing speed affects filling behavior at the same position.

Relationship Between Injection Pressure and Speed

Injection pressure and speed are interrelated:

• Short filling time + long flow path → higher speed → higher pressure required
• Lower speed → longer cooling → thicker frozen layer → higher viscosity → higher pressure required

Therefore, injection pressure is usually set slightly higher, and product defects are controlled by adjusting injection speed across stages.

Defect Causes and Solutions

Flow marks at gate:

• Caused by jetting
• Solution: adjust gate angle, reduce initial speed, then increase second-stage speed

Weld line defects:

• Caused by temperature differences and shear between melt fronts
• Solutions:
  • Modify gate design (e.g., fan gate)
  • Optimize speed profile (slow → fast → slow)
  • Reduce shear and temperature differences

Conclusion

1. Adjusting injection speed can effectively reduce defects and improve product quality and economic efficiency
2. Injection speed curves must be tailored to product design, material type, and gating system
3. The “zero injection method” is effective for setting speed profiles
4. Injection speed is influenced by many factors and must be adjusted based on a thorough understanding of the process