How to Determine the Number of Cavities in an Injection Mold & Its Relationship with Clamping Force

1. Key Factors Affecting Cavity Count

(1) Part Size & Complexity

• Large/Complex Parts (e.g., automotive panels, appliance housings): Typically 1-2 cavities (due to space constraints and filling balance).

• Small/Simple Parts (e.g., bottle caps, buttons): Can use 8-128+ cavities for mass production.

(2) Material Properties

• High-Viscosity Materials (e.g., PC, glass-filled nylon): Fewer cavities (to avoid short shots).

• Low-Viscosity Materials (e.g., PP, PE): More cavities possible.

(3) Production Volume & Lead Time

• High-Volume Orders: Prefer multi-cavity molds.

• Low-Volume, High-Mix: Fewer cavities for flexibility.

(4) Mold Cost & Durability

• More cavities → Higher mold cost but lower per-part cost.

• Precision requirements increase with cavity count.

(5) Process Stability

• Too many cavities may cause:

• Uneven filling (short shots or flash).

• Cooling inconsistencies (warping).

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2. How Clamping Force Limits Cavity Count

The clamping force (tonnage) of an injection machine determines the maximum cavity count. The formula is:

Clamping Force Calculation

Required Clamping Force (T)=Part Projected Area (cm²)×Cavity Pressure (T/cm²)×Number of CavitiesRequired Clamping Force (T)=Part Projected Area (cm²)×Cavity Pressure (T/cm²)×Number of Cavities

• Cavity Pressure: Typically 0.3–0.8 T/cm² (depends on material & flow length).

Example Calculation

For a round part (diameter 10cm, area ≈78.5 cm²) in PP (pressure = 0.5 T/cm²):

• 1 Cavity:

  78.5×0.5×1=39.25 T→Use ≥50T machine.78.5×0.5×1=39.25 T→Use ≥50T machine.

• 4 Cavities:

  78.5×0.5×4=157 T→Use ≥160T machine.78.5×0.5×4=157 T→Use ≥160T machine.

Risks of Insufficient Clamping Force

• Flash (Mold Overflow) – If clamping force < cavity pressure.

• Mold Damage – Excessive deflection over time.

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3. Other Machine Limitations

1. Platen Size & Tie-Bar Spacing – Must fit the mold.

2. Shot Capacity – Enough melt volume for all cavities.

3. Ejection System – Must handle multi-cavity demolding.

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4. How to Optimize Cavity Count?

Design Tips

✅ Prioritize Clamping Force – Avoid flash.
✅ Check Shot Capacity – Prevent short shots.
✅ Ensure Flow Balance – Consistent part quality.

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5. Conclusion

• Cavity count depends on part size, material, and machine tonnage.

• Clamping force is a critical constraint—always verify via calculation.

• Balance efficiency, quality, and cost for optimal production.