Selecting the right injection molding machine is critical for producing quality parts efficiently. The wrong machine can lead to short shots, flash, excessive cycle times, and poor part quality.
Clamp tonnage is the force that keeps the mold closed during injection. Required tonnage depends on projected area of the part and the material being processed. General rule: 2-5 tons per square inch of projected area. Amorphous materials (ABS, PS) require less tonnage, while high-flow materials (PA, PP) require more due to higher injection pressures.
The shot size must be between 20-80% of the machine's maximum shot capacity. Running below 20% results in excessive residence time and material degradation. Running above 80% risks incomplete melting and inconsistent shot weight. Calculate shot weight including part, runner, and gate volume multiplied by material specific gravity.
General-purpose screws work well for most materials. Dedicated screws with barrier flights, mixing sections, or vented designs are needed for specific applications: PVC requires a low-compression screw to prevent degradation; glass-filled materials need wear-resistant bimetallic barrels; high-temperature materials require special screw steels.
Required clamp force = Projected area (cm²) x Cavity pressure (kg/cm²) / 1000. Typical cavity pressures: 300-500 kg/cm² for easy-flow materials, 500-700 for medium-flow, 700-1000 for difficult-flow. Always add 10-20% safety factor.