Shenzhen Alu Rapid Prototype Precision Co., Ltd.

In injection molding, the shot size (also called shot volume or injection volume) refers to the total amount of molten plastic material injected into the mold during a single cycle. It includes the volume needed to fill the part cavities, runners, sprue (the entry channel), and a small cushion (extra material to ensure complete filling and compensate for compression). The shot size is crucial for selecting an appropriate injection molding machine, as it must match the machine's barrel capacity—typically aiming for 20-80% utilization to avoid issues like material degradation or uneven melting.

Shot size can be expressed in volume (e.g., cm³) or weight (e.g., grams), with weight being more common for practical calculations since it accounts for material density. There are two main contexts:

Mold shot size: The required amount for a specific part/mold design.

Machine shot capacity: The maximum the machine can deliver, calculated based on screw stroke and barrel dimensions.

Below, I'll focus on calculating the required mold shot size, as this is the most common need for process setup. Machine capacity is often provided by the manufacturer but can be verified using the formulas at the end.

To calculate the required shot size, you'll need details like part geometry (from CAD software), material data (from supplier datasheets), and mold design (e.g., runner type). Use tools like CAD or simulation software (e.g., Moldflow) for precision, but manual calculations work for estimates.

1.Calculate the Part Volume (V_part):

Measure or model the volume of the molded part(s). For simple shapes, use basic geometry (e.g., V = length × width × height). For complex parts, use CAD. If multi-cavity, multiply by the number of cavities.

Units: cm³ (or in³).

Add Volumes for Feed System:  Sprue Volume (V_sprue): The main entry channel; estimate as a cone or cylinder (V = (π × r² × h)/3 for cone).  

2.Runner Volume (V_runner): Channels distributing material; calculate based on design (e.g., trapezoidal cross-section).  

Total Feed Volume = V_sprue + V_runner.

Note: In hot-runner systems, runners are heated and reused, so exclude V_runner from the shot.

3.Account for Shrinkage:

Materials shrink 0.5-3% as they cool (check datasheet). Add this as a factor: Adjusted Volume = Total Volume × (1 + Shrinkage Rate).

Example: For 2% shrinkage, multiply by 1.02.

4.Include Packing Factor and Cushion:

Add 5-15% extra for packing pressure (to fill voids) and cushion (2-5 mm screw cushion). This is often a multiplier of 1.05-1.15.

5.Convert to Weight (if needed):

Multiply total adjusted volume by material density (ρ, e.g., 0.9-1.4 g/cm³ for common plastics like PP or ABS).

6.Verify and Adjust:

Test with trials; over- or under-estimating can cause defects like short shots (underfill) or flash (excess).

Key Considerations

Material Properties: Density and shrinkage vary (e.g., PP shrinks more than PC). Additives like fillers affect flow and density.

Mold Design: Hot vs. cold runners impact waste (hot reduces shot size by 10-30%).

Machine Matching: Required shot should be 20-65% of machine barrel capacity to optimize residence time and avoid degradation. 

 Use 1.3-1.5× buffer for the machine's rated shot size.

Common Errors: Ignoring shrinkage leads to underfill; poor runner design wastes material.

Optimization: Simulate with software to minimize shot size and reduce costs.