Essential Injection Molding Formulas Every Engineer Should Know

There are countless injection molding formulas, and every engineer must remember each one.

Injection Molding Formulas Series 1

1. Clamping Force (F in TON):

The clamping force is determined by: F = (Am x Pv) / 1000 Where:

• F is the clamping force in TON.
• Am denotes the projected area of the mold cavity in cm^2.
• Pv stands for the filling pressure, usually between 150-350 kg/cm^2. Materials with good flowability use a lower value, while those with poor flowability require a higher value.

For instance, if the mold cavity projected area is 270cm^2 and the filling pressure is 220kg/cm^2, the clamping force is: F = (270 x 220) / 1000 = 59.4TON

2. Injection Pressure (Pi in kg/cm^2):

The injection pressure is given by: Pi = (P x A) / Ao Where:

• Pi is the injection pressure.
• P represents the pump pressure.
• A is the effective area of the injection cylinder.
• Ao is the cross-sectional area of the screw, calculated as A = (π x D^2) / 4, with D being the diameter.

For a pump pressure of 75kg/cm^2, an effective injection cylinder area of 150cm^2, and a screw cross-sectional area of 15.9cm^2 (for a diameter of 45mm), the injection pressure is: Pi = (75 x 150) / 15.9 = 707 kg/cm^2

3. Injection Volume (V in cm^3):

The injection volume is: V = π x (Do/2)^2 x ST Where:

• V is the injection volume.
• Do denotes the screw diameter.
• ST is the injection stroke.

For a screw diameter of 42mm and an injection stroke of 165mm, the injection volume is: V = π x (4.2/2)^2 x 16.5 = 228.6cm^3

4. Injection Weight (Vw in g):

The injection weight is: Vw = V x η x δ Where:

• Vw is the injection weight.
• V is the injection volume.
• η stands for the specific gravity of the material.
• δ represents the mechanical efficiency.

Given an injection volume of 228.6cm^3, a mechanical efficiency of 0.85, and a specific gravity of 0.92, the injection weight is: Vw = 228.6 x 0.85 x 0.92 = 178.7g

5. Injection Speed (S in cm/sec):

The injection speed is: S = Q / A Where:

• S is the injection speed.
• A denotes the effective area of the injection cylinder.
• Q is the pump discharge volume per revolution.

For a motor speed of 1000RPM, a pump discharge volume of 85 cc/RPM, and an effective injection cylinder area of 140 cm^2, the injection speed is: S = (85 x 1000) / (60 x 140) = 10.1 cm/sec

6. Injection Rate (Sv in g/sec):

The injection rate is: Sv = S x Ao Where:

• Sv is the injection rate.
• S represents the injection speed.
• Ao is the cross-sectional area of the screw.

For an injection speed of 10cm/sec and a screw diameter of 42mm, the injection rate is: Sv = 13.85 x 10 = 138.5g/sec

These formulas offer a foundational grasp of the pivotal parameters in injection molding, ensuring optimal results in the molding process.

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Injection Molding Formulas Series 2

1. Theoretical Injection Volume:

Using the constant π/4 = 0.785:

• Theoretical Injection Volume (cm³) = Screw Diameter² x 0.785 x Injection Stroke.
• Injection Stroke (cm) = Theoretical Injection Volume / (0.785 x Screw Diameter).

2. Injection Weight:

Injection Weight (gr) = Theoretical Injection Volume x Plastic Specific Gravity x Injection Constant (0.95).

3. Injection Pressure:

• Injection Pressure (kg/cm²) = (Injection Cylinder Area² / Screw Area²) x System Maximum Pressure (140kg/cm²)².
• Injection Pressure (kg/cm²) = (Injection Cylinder Diameter² / Screw Diameter²) x System Maximum Pressure (140kg/cm²).
• Injection Pressure (kg/cm²) = Maximum Injection Pressure of the Barrel Assembly x Actual Usage Pressure (kg/cm²) / System Maximum Pressure (140kg/cm²).

4. Injection Rate:

• Injection Rate (cm³/sec) = Screw Area (cm²) x Injection Speed (cm/sec).
• Injection Speed (cm³/sec) = Screw Diameter (cm²) x 0.785 x Injection Speed (cm/sec).

5. Injection Speed:

• Injection Speed (cm/sec) = Injection Rate (cm³/sec) / Screw Area (cm²).
• Injection Speed (cm/sec) = Pump Single Turn Volume (cc/rev) x Motor Speed (rev/sec) / 60 seconds / Injection Area (cm²).

• Motor Speed RPM: 60HZ – 1150, 50HZ – 958.

6. Injection Cylinder Area:

• Injection Cylinder Area (cm²) = Injection Pressure (kg/cm²) / System Maximum Pressure (140kg/cm²) x Barrel Area (cm²).
• For a single cylinder: Injection Cylinder Area (cm²) = (Injection Cylinder Diameter² – Piston Diameter²) x 0.785.
• For a double cylinder: Injection Cylinder Area (cm²) = (Injection Cylinder Diameter² – Piston Diameter²) x 0.785 x 2.

7. Pump Single Turn Volume:

Pump Single Turn Volume (cc/sec) = Injection Cylinder Area (cm²) x Injection Speed (cm/sec) x 60 seconds / Motor Speed.

• Motor Speed RPM: 60HZ – 1150, 50HZ – 958.

8. Screw Speed and Hydraulic Motor Single Turn Volume:

• Screw Speed = Pump Single Turn Volume (cc/rec) x Motor Speed (RPM) / Hydraulic Motor Single Turn Volume.
• Hydraulic Motor Single Turn Volume = Pump Single Turn Volume (cc/rec) x Motor Speed (RPM) / Screw Speed.

9. Total Injection Pressure:

• Total Injection Pressure (kg) = System Maximum Pressure (kg/cm²) x Injection Cylinder Area (cm²).
• Total Injection Pressure (kg) = Injection Pressure (kg/cm²) x Screw Area (cm²).

10. Ounce and Related Unit Conversion:

• 1 ounce (oz) = 28.375 grams (gr).
• 1 pound (lb) = 16 ounces (oz).
• 1 kilogram (kg) = 2.2 pounds (lb); thus, 1 jin = 1.1 pounds.
• 1 pound (lb) = 454 grams (gr) = 0.454 kilograms (kg).

11. Clamping Force:

• For toggle type: Clamping Force (Ton) = Clamping Cylinder Area (cm²) x System Maximum Pressure (140kg/cm²) / 1000 x Toggle Magnification Rate (20-50).
• For single cylinder direct pressure type: Clamping Force (Ton) = Clamping Cylinder Area (cm²) x System Maximum Pressure (140kg/cm²) / 1000.

12. Pillar Diameter and Clamping Force Formula:

Pillar Diameter² (cm²) x 0.785 x Young’s Modulus (scm4 approx. 1000kg/cm²) x 4 = Approximate Clamping Force (Ton).

13. Product Layout Projection Area and Clamping Force Relationship Formula:

Product Layout Projection Area (in²) x Standard Thickness (1.5mm) / Average Product Thickness (mm) x Material Constant / PS Material Constant (1) = Clamping Force (Ton).

14. Ejection Force:

Ejection Force (Ton) = Ejection Cylinder Area (cm²) x System Maximum Pressure (140kg/cm²) / 1000.

15. Power Units:

• 1 Horsepower (HP) = 0.754 Kilowatts (KW).
• 1 Kilowatt (KW) = 1.326 Horsepower (HP) = 1000 Watts (W).
• 1 Kilowatt (KW) = 1 Electricity Measurement Unit (1KW/Hr).

16. Pump Size and Horsepower Relationship:

Appropriate Horsepower:

• HP = P x Q / 540.
• KW = P x Q / 612.

17. Calculating Power Usage:

Actual Hourly Power Consumption (KW/Hr) = (Motor Capacity + Electric Heating Capacity + Dryer Capacity) x Power Usage Constant (approx. 40%).

Injection Molding Formulas Series 3

1. What is the Injection Capacity of an Injection Molding Machine?

Injection Capacity = Injection Pressure (kg/cm²) × Injection Volume (cm³) / 1000

2. What is the Injection Horsepower of an Injection Molding Machine?

Injection Horsepower PW (KW) = Injection Pressure (kg/cm²) × Injection Rate (cm³/sec) × 9.8 × 100%

3. What is the Injection Rate of an Injection Molding Machine?

Injection Rate V (cc/sec) = π/4 × d² × g Where:

• d²: Barrel diameter
• g: Material density

4. What is the Injection Thrust of an Injection Molding Machine?

Injection Thrust F (kgf) = π/4 × (D1² – D2²) × P × 2 Where:

• D1: Cylinder inner diameter
• D2: Piston rod outer diameter
• P: System pressure

5. What is the Injection Pressure of an Injection Molding Machine?

Injection Pressure P (kg/cm²) = [π/4 × (D1² – D2²) × P × 2] / (π/4 × d²)

6. What is the Plasticizing Capacity of an Injection Molding Machine?

Plasticizing Capacity W (g/sec) = 2.5 × (d/2.54)² × (h/2.54) × N × S × 1000 / 3600 / 2 Where:

• h: Depth of the screw’s front teeth (cm)
• S: Material density

7. What is System Pressure? How is it different from Injection Pressure?

System Pressure (kg/cm²) is the highest working pressure set within the hydraulic circuit.

8. What is Injection Speed?

Injection Speed: Speed = Distance / Time