Wall Thickness Principles for Injection Moulding Tool Design
Uniform wall thickness design balances polymer flow, prevents shrink defects, cuts cycle times, and reduces landed part cost in injection moulding tools.
Press Tonnage Calculation
Meaning
Mathematical sizing of injection moulding machine clamping force matches required mold hold-closed force against internal hydraulic melt pressure. Performing a press tonnage calculation computes projected part surface area multiplied by average cavity pressure, incorporating safety factors to prevent tool parting line opening during peak packing. The calculation stops being valid if mold platen deflection occurs, if runner projection areas are omitted, or if dynamic peak pressures exceed assumed averages.
Material datasheets provide resin flow length ratios but do not specify required tonnage, which varies according to part geometry and wall thickness. Selecting correct clamp tonnage prevents parting line flash, tool damage, and excessive energy consumption.
Projected Area
Total surface area projected onto the mold parting plane determines the net area subject to opening forces. Summing individual part areas and runner projection surfaces establishes the baseline area for tonnage formulas.
Cavity Pressure
Average internal pressure depends on flow length, wall thickness, resin viscosity and injection velocity. High viscosity engineering resins require higher cavity packing pressures, raising calculated clamping force requirements.
Machine Sizing
Selecting a press with clamp capacity exceeding calculated requirements by ten to twenty percent ensures stable parting line sealing. Operating machines at continuous maximum tonnage accelerates toggle wear and induces platen bending.