Injection Molding Shot Size Calculator

Estimate shot weight, shot volume, and required machine capacity for injection molding.

Use this shot size calculator to quickly determine required shot weight and machine capacity based on your mold design.

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How to Calculate Shot Size

Shot size in injection molding is determined by part weight, number of cavities, runner system, and safety margin.

This shot size calculator uses the following formula:

• Total shot weight = (Part weight × Cavities) + Runner weight
• Required shot weight = Total shot weight × (1 + Cushion %)
• Shot volume = Required shot weight ÷ Material density

Example:

Part weight = 25g
Cavities = 2
Runner weight = 8g

Total shot weight = 58g
Required shot weight (10% cushion) = 63.8g
Shot volume ≈ 60.76 cm³ (based on PS density)

Key Factors Affect Injection Molding Shot Size

• Part Weight(Higher part weight directly increases total material required per cycle.)
• Number of Cavities(More cavities multiply the total shot weight proportionally.)
• Runner / Sprue System(Cold runner systems add extra material, increasing overall shot demand.)
• Material Density(Different plastics affect the conversion from weight to volume.)
• Cushion / Safety Margin(A safety margin ensures stable injection and prevents short shots.)

Shot Size vs Machine Capacity in Injection Molding

After calculating shot size using this calculator, it is important to compare it with machine capacity.

Injection molding machines typically operate best when shot usage is within 20%–80% of maximum capacity.

• Too low → unstable injection and inconsistent filling
• Too high → risk of overloading and poor process stability

This shot size calculator helps you quickly estimate whether your mold design fits within a suitable machine range.

Shot Size vs Shot Weight in Injection Molding

Shot size and shot weight are often confused, but they are not the same.

• Shot weight refers to the total material injected per cycle (including runners and cavities).
• Shot size refers to the required injection capacity of the machine, usually expressed in PS-equivalent.

This shot size calculator helps convert part weight into actual shot requirements, making it easier to match mold design with machine capacity.

Common Mistakes

• Ignoring runner weight in total shot calculation
• Using incorrect material density
• Not including cushion or safety margin
• Selecting a machine too close to maximum shot capacity

FAQ

How to calculate injection molding shot size?
This shot size calculator helps estimate required shot weight and volume based on part weight, cavity count, runner system, and material density.

What is a typical cushion percentage?
A cushion of 5%–15% is commonly used to ensure stable injection performance.

Is shot size the same as shot volume?
No. Shot size is typically expressed as weight (g), while shot volume depends on material density.

How much machine capacity should be used?
Injection machines generally perform best when operating between 20% and 80% of their maximum shot capacity.

Can this calculator be used for different materials?
Yes, as long as the correct material density is used for calculation.