In injection molding¹, mold design directly determines product quality, production efficiency, and manufacturing costs. Below is a comprehensive breakdown of the core design principles, procedures, and key systems that guide professional injection mold development.

Core Design Basis

1. Dimensional Accuracy & Correctness of Related Dimensions

The external quality and specific dimensions of plastic products are defined by their functional requirements:

• Appearance-focused products (e.g., toys): High aesthetic standards but moderate dimensional precision.
• Functional products: Strict dimensional tolerances to ensure performance.
• High-precision products (e.g., cameras): Stringent requirements for both appearance and dimensions.

2. Rationality of Draft Angle²

The draft angle is critical for smooth demolding and product integrity:

• Ensure sufficient draft to prevent sticking during ejection.
• Align the angle with the parting/split mold surface.
• Avoid compromising appearance or wall thickness accuracy.
• Prevent weakening of critical structural areas.

Standard Design Procedure

1. Analysis of Product Drawings & Physical Samples

Thoroughly evaluate the product to establish design constraints:

• Geometric shape and structural complexity.
• Dimensional specifications, tolerances, and design datums.
• Technical requirements (e.g., strength, flexibility).
• Plastic material grade and surface finish demands.

2. Cavity Quantity & Layout

Determine the number of cavities based on:

• Product weight vs. injection machine capacity.
• Projected area vs. clamping force.
• Mold size vs. machine’s tie-bar spacing.
• Product precision, color, and production volume.
• Economic efficiency (output value per mold).

Cavity Layout Considerations:

• Optimize mold size and gating system balance.
• Coordinate with core-pulling mechanisms, inserts, and cooling systems.
• Adjust based on parting surface and gate position selection.

Key Design Elements

1. Parting Surface Selection

The parting surface should:

• Preserve product appearance and precision.
• Simplify mold machining (especially cavity fabrication).
• Facilitate gating, exhaust, and cooling system integration.
• Ensure the product remains on the moving mold during demolding.
• Accommodate metal inserts easily.

2. Gating System Design

The gating system controls plastic flow into the cavity. Key steps:

1. Gate Position Selection (follow these principles):

• Locate on the parting surface for easy cleaning.
• Ensure equal distance to all cavity areas (shortest flow path).
• Direct flow toward thick-walled sections for smooth filling.
• Avoid impinging on cores/inserts (prevents deformation).
• Minimize weld lines or place them in non-critical areas.
• Enable uniform filling and efficient gas evacuation.
• Position for easy post-processing without affecting appearance.

2. System Components:

• Sprue: Main channel connecting the injection machine to the mold.
• Runner: Distributes plastic to multiple cavities (optimize cross-section for flow).
• Gate: Controls flow rate and pressure (e.g., pin-point, edge, or sub-gates).
• Degating Mechanism: For pin-point gates, ensure runner separation during ejection.

3. Exhaust System Design

Effective venting prevents defects like air traps and burn marks:

• Vent Grooves: Position at the last-filled cavity areas. Depth varies by material:
  • ABS: ≤0.04 mm
  • Putty: ≤0.02 mm
  • POM (Delrin): ≤0.02 mm
• Clearance Venting: Use gaps between cores, ejector pins, or special exhaust plugs.
• Air Pins: Prevent vacuum deformation during product ejection.
• Anti-Vacuum Components: Avoid product adhesion to the mold surface.

4. Cooling System Design³

Uniform cooling ensures consistent product quality and cycle time reduction. Design considerations:

• Cooling channel layout (e.g., parallel, series, or spiral).
• Position and size of channels (avoid interfering with other mold components).
• Targeted cooling for high-heat areas (e.g., cores, inserts, sliders).
• Selection of standard cooling components (pipes, baffles, O-rings).
• Sealing structure to prevent water leakage.

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  Note

  Injection mold design is a iterative process—each system interacts with others. Balancing functionality, manufacturability, and cost requires careful adjustment based on specific product requirements.