Tag Archive for: Mold Gate Design

pin-point gate injection molding

What is a mold gate?

Mold gate is a small opening (or orifice) through which the polymer melt enters the mold cavity. Injection Gate design for a particular application includes a selection of the gate type, dimensions, and location. It is dictated by the part and mold design, the part specifications (e.g., appearance, tolerance, concentricity), the type of plastic raw material being molded, the fillers, the type of mold plates, and economic factors (e.g., tooling cost, cycle time, allowable scrap volume). Gate design is of great importance to part quality and productivity.

Single vs. multiple gates

You’ll usually have better success with a single mold gate unless the length of the melt flow exceeds practical limits. Multiple gates always create weld and meld lines where the flows from the separate gates meet. Except for long, narrow parts, a single gate into the body of the part (as opposed to an edge gate) will assure more uniform distribution of material, temperatures, and packing, and better orientation effects. While a single gate into the body of the part might incur a higher initial tool cost, lower scrap rates and higher part quality will quickly justify this expense.

Gate dimensions

The cross section of the gate is typically smaller than that of the part runner and the part so that the part can easily be “de-gated” (separated from the runner) without leaving a visible scar on the part. The gate thickness is usually two-thirds the part thickness. Since the end of packing can be identified as the time when the material in the gate drops below the freezing temperature, the gate thickness controls the packing time. A larger gate will reduce viscous (frictional) heating, permit lower velocities, and allow the application of higher packing pressure for a longer period of time. Choose a larger gate if you’re aiming for appearance, low residual stress, and better dimensional stability.Types of runner gate

Gate location

Select a gate location that will ensure rapid and uniform mold filling. Position weld lines and air/gas vents so they have the least effect on the appearance and strength of the part. Since gates are locations of high residual stress, position them away from areas that will experience high external stress during use.

Position the gate away from load-bearing areas. The high melt pressure and high velocity of flowing material at a gate cause the area near a gate to be highly stressed.

Position the gate away from the thin section areas, or regions of sudden thickness change.
This will avoid Hesitation or Sink marks and voids.

Mold gate design is a crucial aspect of injection molding, influencing the part quality, cycle time, and overall efficiency of the molding process. The gate serves as the point through which molten plastic enters the mold cavity. The selection of the gate type and its location can significantly impact the final product. Here are key considerations for mold gate design:

  1. Gate Types:
    • Consideration: Different types of gates exist, including:
      • Sprue Gate: Located at the mold’s parting line, connecting the runner to the nozzle.
      • Runner Gate: Connects the runner system to the cavity.
      • Direct or Edge Gate: Located directly at the part’s edge.
      • Submarine Gate: Placed beneath the part surface.
    • Importance: Choose the gate type based on the part geometry, material, and desired aesthetics. Each gate type has its advantages and limitations.
  2. Part Design and Appearance:
    • Consideration: The gate’s location and type impact the part’s appearance and surface finish.
    • Importance: Place gates strategically to minimize visible marks or blemishes on the part. Consider gate vestiges and how they may affect the part’s aesthetics.
  3. Material Flow and Filling Pattern:
    • Consideration: Gate design influences how molten plastic flows into the mold cavity.
    • Importance: Optimize gate design to ensure uniform filling, minimize turbulence, and prevent issues like air entrapment, weld lines, or flow lines.
  4. Gate Size:
    • Consideration: The gate size determines the flow rate of molten plastic into the mold cavity.
    • Importance: Size gates appropriately to balance fast filling times with controlled injection pressure. Avoid oversized gates that can lead to flashing or cosmetic defects.
  5. Gate Location:
    • Consideration: The gate’s position affects the distribution of stress and flow pattern within the part.
    • Importance: Place gates in areas that minimize visible marks on the final part and ensure proper packing of the cavity. Consider part geometry, wall thickness, and material properties.
  6. Number of Gates:
    • Consideration: Some parts may require multiple gates for efficient filling and packing.
    • Importance: Determine the optimal number of gates based on part geometry, size, and material characteristics. Multiple gates may help balance flow and reduce cycle time.
  7. Gate Vestige:
    • Consideration: Gate vestige refers to the small protrusion left on the part after the gate is removed.
    • Importance: Minimize gate vestige through proper gate design to enhance part aesthetics. Consider post-molding operations if gate vestige removal is critical.
  8. Cycle Time Optimization:
    • Consideration: The gate design impacts the overall cycle time of the injection molding process.
    • Importance: Optimize gate design to balance efficient filling and packing with reduced cycle times. Consider the cooling and solidification time of the material near the gate.
  9. Material Selection:
    • Consideration: Different materials may require specific gate designs to address their flow characteristics and cooling rates.
    • Importance: Tailor gate design to the material being processed to prevent issues like premature freezing, gate blush, or degradation.
  10. Gate Wear and Maintenance:
    • Consideration: High-velocity molten plastic flow can cause wear on the gate.
    • Importance: Regularly inspect and maintain gates to address wear issues promptly. Proper maintenance ensures consistent part quality and extends the life of the mold.
  11. Automated Gate Removal:
    • Consideration: Some molds incorporate automated gate removal systems.
    • Importance: Automated systems can improve efficiency and reduce the reliance on manual labor for gate removal, particularly in high-volume production.

Collaboration between part designers, mold designers, and mold makers is essential for effective gate design. Iterative testing and validation during the prototyping phase help refine gate design for optimal performance and part quality.

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