How Injection Molding Machine And Material Properties Affect Cavity Number

1. Injection Molding Machine Limitations

A. Shot Capacity (Maximum Injection Volume)

• The total material weight (parts + runners) must be ≤ 70–80% of the machine’s maximum shot capacity to ensure consistency.
  Formula:

  n≤0.8×Machine Shot Capacity (e.g., 550g)Part Weight + Runner Weight per Cavityn≤Part Weight + Runner Weight per Cavity0.8×Machine Shot Capacity (e.g., 550g)

  Example: For a 50g part with 10g runner waste on a 550g machine:
  n≤44060≈7 cavitiesn≤60440≈7 cavities.

B. Clamping Force (Tonage)

• The mold’s projected area × cavity pressure (typically 300–500 bar) must not exceed the machine’s clamping force.
  Formula:

  n≤Clamping Force (tons)Projected Area per Cavity (cm²)×40n≤Projected Area per Cavity (cm²)×40Clamping Force (tons)

  Example: 200-ton machine, 50 cm² part area → Max 10 cavities.

C. Mold Size & Platen Space

• Multi-cavity molds are larger; ensure they fit the machine’s tie-bar spacing and min/max mold thickness.

2. Material-Driven Considerations

A. Flowability (Melt Flow Rate – MFR)

• Low MFR (e.g., PC, glass-filled resins):
  Higher injection pressure needed → Fewer cavities or optimized runners.

• High MFR (e.g., PP, HDPE):
  Easier filling → More cavities possible.

B. Shrinkage & Cooling Time

• High-shrinkage materials (e.g., PE) require longer cooling, reducing efficiency gains from multi-cavity molds.

C. Thermal Stability

• High-temperature materials (e.g., PEEK) may need fewer cavities to maintain uniform heat distribution.

3. Balancing Cavity Count: Pros & Cons

4. Practical Examples

Case 1: Thin-Wall PP Container

• Part: 10g, runner 5g, 550g machine (150-ton).

• Calculations:

• Shot capacity: Up to 29 cavities (but limited by clamping force to 16 cavities).

• Final choice: 16-cavity mold (balanced efficiency and machine limits).

Case 2: Thick PC Gear

• Part: 80g, runner 20g, 550g machine (200-ton).

• Calculations:

• Shot capacity limits to 4 cavities (clamping force allows 16).

• Final choice: 4-cavity mold (due to material flow constraints).

5. Optimization Tips

1. Simulate First: Use Moldflow to test runner balance and pressure.

2. Start Small: Prototype with fewer cavities, then scale up.

3. Hot Runners: Eliminate waste, enabling more cavities.