Cross-Sectional Design of Runner in Injection Mold
The runner in an injection mold is a channel that connects the gate and the main runner. Its cross-sectional design significantly affects the quality of the molded part. A reasonable cross-sectional design can ensure a smooth flow of molten plastic, reduce flow resistance, lower pressure drop, improve injection efficiency, and guarantee uniform filling of the molded part.
Factors Affecting Runner Cross-Sectional Design
Plastic type: Different plastics have different flow characteristics, thus requiring different runner cross-sections. Plastics with good fluidity can have smaller runner cross-sections, while those with poor fluidity need larger ones.
Part shape and size: Complex part shapes, uneven wall thickness, or large part sizes require larger runner cross-sections to ensure filling.
Gate type: Different gate types have different impacts on the runner cross-section.
Injection machine performance: The injection pressure and speed also affect the runner's design.
Selection of Runner Cross-Sectional Shape
Common runner cross-sectional shapes include circular, trapezoidal, U-shaped, and rectangular.
Circular: Offers the least flow resistance but is difficult to process and align on the parting line.
Trapezoidal: Easy to process, moderate flow resistance, and commonly used.
U-shaped: Suitable for high flow rate applications, increasing the cross-sectional area and reducing flow resistance.
Rectangular: Less commonly used, difficult to process, and has higher flow resistance.
Determination of Runner Cross-Sectional Size
The determination of the runner cross-sectional size mainly considers the following factors:
Plastic fluidity: Plastics with poor fluidity should have larger runner cross-sections.
Part filling length: Longer filling length requires larger runner cross-sections.
Number of cavities: More cavities require larger runner cross-sections.
Injection pressure: Higher injection pressure allows for relatively smaller runner cross-sections.
Precautions in Runner Cross-Sectional Design
Avoid dead corners: There should be no dead corners in the runner to prevent the accumulation of molten material and affect product quality.
Consider cooling: The runner design should consider cooling to prevent premature solidification of the molten material.
Consider demolding: The runner design should facilitate demolding and avoid flash formation.
