How Injection Machine Tonnage Affects Plastic Molding Quality

1. What Is Tonnage in Injection Molding?

Tonnage (clamping force) refers to the maximum pressure (in tons) that an injection molding machine can apply to keep the mold closed during plastic injection.

Key Formula:

Required Clamping Force = Projected Area × Injection Pressure

• Projected Area: Total surface area of the part + runner system (cm² or in²).

• Injection Pressure: Typically 300-500 kg/cm² (higher for engineering plastics).

Choosing the wrong tonnage leads to defects like short shots, flash, or mold damage.

2. How Tonnage Affects Plastic Molding

(1) Part Size & Design

• Low Tonnage (<100 tons)

• Best for small, thin-walled parts (e.g., caps, connectors).

• Works well with low-viscosity plastics (PP, PS).

• Medium Tonnage (100-500 tons)

• Suitable for medium-sized parts (e.g., appliance housings, toys).

• Handles glass-filled plastics (e.g., PA66+GF30).

• High Tonnage (>500 tons)

• Needed for large parts (e.g., automotive bumpers, pallets).

• Requires high-flow materials (ABS, PC/ABS) to avoid short shots.

(2) Material Compatibility

• High-Viscosity Plastics (PC, PMMA)

• Require higher injection pressure → Need extra tonnage (20% buffer).

• Low-Viscosity Plastics (LDPE, TPE)

• Excess tonnage causes flash; precise clamping control is crucial.

• Reinforced Plastics (GF/CF composites)

• Abrasive materials demand higher tonnage to prevent mold leakage.

(3) Process Stability

• Injection Pressure: Low-tonnage machines may struggle to fill high-viscosity materials.

• Holding Pressure: Higher tonnage ensures better packing, reducing sink marks.

• Mold Flexing: Insufficient tonnage allows mold separation, causing flash.

3. Problems from Incorrect Tonnage

4. How to Choose the Right Tonnage

1. Calculate Projected Area (Part + Runner).

2. Determine Injection Pressure (350 kg/cm² for standard plastics, 400-500 kg/cm² for engineering plastics).

3. Add 10-20% Safety Margin (for process variations).

4. Example Calculation:

• Part: 500mm × 300mm automotive panel (ABS).

• Projected Area = 50 × 30 = 1500 cm²

• Required Clamping Force = 1500 × 400 = 600 tons

• Recommended Machine: 700-800 tons

5. Special Considerations

• Multi-Cavity Molds: Multiply projected area by cavity count.

• Thin-Wall Molding: High-speed injection may require higher tonnage.

• Soft Materials (TPU, TPE): Avoid excessive tonnage to prevent venting issues.

Conclusion

Selecting the right injection molding machine tonnage ensures:
✔ Proper filling (no short shots)
✔ Reduced defects (flash, sink marks)
✔ Longer mold life (less wear & tear)
✔ Energy efficiency (optimal clamping force)

Pro Tip: Use Moldflow simulation to validate tonnage requirements before production.