Injection Moulding Clamping Force Basics

Injection Moulding Clamping Force

• Clamping force in injection moulding is the force applied to the mould halves to close them and resist the separating force during injection of molten plastic.
• The clamping force must be sufficient to counteract the forces from molten plastic expansion and contraction, as well as to resist the pressure generated by the injection of the molten plastic into the mould cavity.

Factors Affecting Clamping Force

• Insufficient clamping force: Results in poor geometrical symmetry, flashes (excess material), hot spots, loose packing and air bubbles.
• Excessive clamping force: Can lead to insufficient air venting, premature ageing of hydraulic components, and mechanical structural wear, as well as cavities, cracked cores, wall cracks, and parting lines. It also increases machine energy consumption, deflects the platen, and reduces mould life.

Clamping Force Calculation

• A simple formula to calculate clamping force is: Clamping Force (kg) = Cavity Pressure (kg/cm²) × Total Projected Area (cm²) Clamping Force (tons) = Clamping Force (kg) / 1000

• The total projected area is the sum of the projected areas of all cavities and runners, calculated considering their dimensions (diameter, length, etc.) and layout in the mould.

• Hot runner moulds that eliminate runners are considered separately.

• Component geometry (e.g., square) influences the area calculation.

• Cavity pressure is affected by gate size, location, and number, wall thickness of the product, plastic viscosity, and injection speed.

Calculating Cavity Pressure

• Cavity pressure (P) can be determined via the multiplication of the base cavity pressure (P0) and a constant (k).
• The constant (k) is based on the thermoplastic material.
• P = P0 x k

Example Calculations

• Examples are provided for determining total projected area and applying the formula for calculating clamping force for specific injection moulding scenarios involving different materials and dimensions.

Example Application

• Examples illustrate how to determine cavity pressure based on material characteristics (e.g., polycarbonate, polyethylene) and specific component geometries.