How to Calculate Shot Capacity of Injection Molding Machine

Shot capacity defines the maximum volume of molten plastic a machine delivers per cycle. Accurate calculation prevents short shots, overpacking, and material waste while ensuring the machine matches mold cavity volume.

Shot Capacity Definition and Calculation Variables

Shot capacity equals the maximum melt volume the screw displaces during one full forward stroke. Two calculation methods exist: volumetric capacity based on screw geometry, and weight capacity based on material density.

The volumetric method calculates cylindrical volume between screw positions. The weight method converts volume to mass using density, yielding the shot weight specification published in machine datasheets.

Volumetric Shot Capacity Formula

Volumetric shot capacity equals screw stroke multiplied by barrel cross-sectional area. Screw stroke represents the linear distance the screw travels from recovery position to full injection position. Barrel cross-sectional area equals pi multiplied by barrel inner radius squared.

Barrel inner diameter must match screw outer diameter with a clearance of 0.05 to 0.15 mm. This clearance allows screw rotation while maintaining pressure seal. The effective area calculation uses barrel inner diameter, not screw diameter, because melt fills the annular clearance space during injection.

Weight Shot Capacity Conversion

Weight shot capacity equals volumetric capacity multiplied by material density. Different polymers have different densities, meaning the same machine delivers different shot weights for different materials. PP at 0.90 g/cm³ yields lighter shots than PC at 1.20 g/cm³ from identical volumetric capacity.

Machine datasheets publish shot weight using PS density as the reference. Users recalculate actual shot weight by multiplying the PS-rated capacity by the ratio of target material density to the PS reference.

Step-by-Step Shot Capacity Calculation

1. Obtain barrel inner diameter from machine specifications. This value typically ranges from 30 mm for small machines to 120 mm for large frames. Record in millimeters.

2. Calculate barrel cross-sectional area using the formula: area (cm²) equals pi multiplied by (diameter/2)² divided by 100. Convert diameter from mm to cm before squaring.

3. Determine maximum screw stroke from machine datasheet. Screw stroke ranges from 60 to 300 mm depending on machine size. Longer strokes provide greater volumetric capacity per cycle.

4. Calculate volumetric shot capacity using: volume (cm³) equals area (cm²) multiplied by stroke (cm). Convert screw stroke from mm to cm before multiplication.

5. Select target material density from material datasheet. Common densities include PP at 0.90, PE at 0.95, PS at 1.05, ABS at 1.07, PA at 1.14, and PC at 1.20 g/cm³.

6. Calculate weight shot capacity using: shot weight (g) equals volumetric capacity (cm³) multiplied by material density (g/cm³). Compare this value against total mold cavity volume plus runner system volume.

7. Verify capacity-to-cavity ratio falls between 40 and 80 percent. Operating below 40 percent wastes machine energy. Operating above 80 percent risks short shots from insufficient recovery time.

Shot Capacity vs Mold Volume Matching

• Under-capacity mismatch occurs when shot capacity falls below total cavity and runner volume. The machine cannot fill the mold completely, producing short shots and dimensional defects.

• Over-capacity mismatch occurs when shot capacity far exceeds mold volume. Excess melt remains in the barrel between cycles, causing material degradation from prolonged heat exposure and increased cooling time per cycle.

• Optimal capacity range targets 40 to 80 percent of rated shot capacity for actual mold fill volume. This range provides adequate recovery buffer without excessive residual melt in the barrel.

• Multi-cavity considerations add cavity count multiplied by single cavity volume plus total runner volume to determine required shot capacity per cycle.

Shot Capacity Calculation Reference

FAQ

How do I convert PS-rated shot capacity to my material?

Multiply the PS-rated shot weight by the ratio of your material density to 1.05 g/cm³. For PP, multiply by 0.90/1.05 equals 0.857. A 200 g PS-rated machine delivers 171 g of PP per shot cycle.

What happens if mold volume exceeds shot capacity?

The machine produces short shots with incomplete cavity fill. Increasing injection pressure cannot compensate for insufficient melt volume. The solution requires either a larger machine frame or reducing total cavity volume by removing cavities from the mold layout.

Does shot capacity change with barrel temperature?

Shot capacity in volumetric terms remains constant regardless of temperature. Weight capacity decreases slightly at higher melt temperatures because polymer density reduces with thermal expansion. The practical difference is less than 2 percent across normal processing temperature ranges.

How does screw design affect effective shot capacity?

Screw flight depth and compression ratio determine the effective melt volume available per stroke. General-purpose screws with moderate compression deliver rated capacity. High-compression screws for engineering materials may deliver 5 to 10 percent less effective volume due to deeper metering zone channels.

Can I use shot capacity at less than 20 percent of rated maximum?

Operating below 20 percent of rated shot capacity creates excessive residence time for melt in the barrel. Material degradation from prolonged heating produces discoloration, reduced mechanical properties, and potential gas generation. Minimum practical utilization targets 40 percent of rated capacity.

Conclusion

Shot capacity calculation requires barrel diameter, screw stroke, and material density inputs applied through a straightforward volumetric formula. Matching calculated capacity to mold volume at 40 to 80 percent utilization ensures production efficiency without short shot risk or material degradation.

NPC Machinery provides precision injection molding machines with clearly specified shot capacity ratings across multiple frame sizes. Request a Rapid Sourcing Quote from NPC Machinery for your required shot weight and cavity volume, or contact the NPC Machinery engineering team for free custom sizing support based on your material and mold specifications.