Quick Answer

Injection shot size is calculated based on the total amount of molten plastic required for one injection cycle, including:

• Product weight
• Runner weight
• Sprue weight
• Cushion material

The selected injection molding machine must provide sufficient shot capacity for stable production while avoiding excessive material residence time and degradation.

What Is Injection Shot Size?

Injection shot size refers to the maximum amount of molten plastic an injection molding machine can inject into the mold during one production cycle.

Shot size is usually measured in:

• grams (g)
• ounces (oz)
• cubic centimeters (cm³)

Shot size is one of the most important factors when selecting an injection molding machine.

Why Shot Size Is Important

Correct shot size selection helps ensure:

• Stable production
• Complete mold filling
• Better product quality
• Reduced material degradation
• Lower production defects
• Better cycle consistency

Incorrect shot size may cause:

• Short shots
• Flash defects
• Burn marks
• Material degradation
• Unstable injection pressure

Basic Injection Shot Size Formula

The total injection shot weight is calculated as:

Wt=Wp+Wr+WsW_t = W_p + W_r + W_sWt​=Wp​+Wr​+Ws​

Where:

• WtW_tWt​ = Total injection weight
• WpW_pWp​ = Product weight
• WrW_rWr​ = Runner weight
• WsW_sWs​ = Sprue weight

Example Shot Size Calculation

Example Product

The machine shot capacity must exceed:

160g

for stable production.

Recommended Shot Size Usage Range

Most manufacturers recommend using:

30% to 80%

of the machine’s maximum shot capacity.

This helps maintain:

• Stable plasticizing
• Better melt quality
• Consistent injection pressure
• Reduced material degradation

Why Oversized Machines Can Be a Problem

Many buyers assume larger machines are always better.

However, oversized machines may cause:

• Longer material residence time
• Plastic degradation
• Black spots
• Burn marks
• Higher energy consumption

Especially for heat-sensitive materials such as PET and PVC, proper shot size matching is critical.

Why Undersized Machines Cause Problems

Machines with insufficient shot capacity may result in:

• Short shots
• Incomplete filling
• Unstable injection pressure
• Poor product consistency

The machine should always maintain a sufficient injection safety margin.

Shot Size vs Product Weight

Product weight alone is not enough.

Buyers must also consider:

• Runner system
• Sprue size
• Cushion volume
• Material shrinkage
• Multi-cavity molds

Actual injection weight is often much higher than the final product weight.

Multi-Cavity Mold Shot Size Calculation

Multi-cavity molds increase total shot weight significantly.

Example

Larger cavity molds require:

• Higher shot capacity
• Faster plasticizing systems
• Higher injection performance

Material Density and Shot Size

Different plastic materials have different densities.

Some machine manufacturers rate shot size using:

PS (Polystyrene)

equivalent capacity.

Actual shot capacity changes depending on material density.

Screw Diameter and Shot Size

Shot size is closely related to screw diameter.

Larger screw diameters provide higher shot capacity.

However, oversized screws may reduce plasticizing quality for small products.

PET Preform Shot Size Considerations

PET preform production requires highly stable shot control.

Important factors include:

• High injection speed
• Stable melt temperature
• Precise shot repeatability
• Specialized PET screw design

Servo injection molding machines are commonly preferred for PET applications.

Thin-Wall Product Considerations

Thin-wall products require:

• High injection speed
• High injection pressure
• Fast response systems

Even lightweight products may require larger injection systems for stable filling performance.

Cushion Volume in Injection Molding

A small amount of molten material called cushion remains in front of the screw after injection.

The cushion helps maintain:

• Stable holding pressure
• Consistent injection control
• Better product quality

Machines should not operate at maximum shot capacity continuously.

Common Shot Size Selection Mistakes

Selecting Machines Based Only on Product Weight

This ignores:

• Runner systems
• Sprue weight
• Cushion volume
• Multi-cavity molds

Choosing Excessively Large Machines

Oversized machines may cause:

• Material degradation
• Higher operating cost
• Increased energy consumption

Ignoring Material Characteristics

Heat-sensitive materials require better shot size matching.

Recommended Machine Selection Strategy

Buyers should evaluate:

• Product weight
• Total shot weight
• Material type
• Mold cavity quantity
• Production volume
• Injection speed requirements
• Future production expansion

Professional machine matching improves long-term production stability.

Common Questions

What is included in shot size calculation?

Shot size includes product weight, runner weight, sprue weight, and cushion material.

What percentage of shot capacity should be used?

Most manufacturers recommend using 30% to 80% of machine shot capacity.

Why is oversized shot capacity bad?

Oversized machines may cause material degradation and unnecessary energy consumption.

How does material type affect shot size?

Different materials have different densities and processing requirements.

Is shot size the same as clamping force?

No. Shot size relates to injection volume, while clamping force relates to mold closing pressure.

How DAYA MACHINERY Supports Machine Selection

DAYA MACHINERY helps industrial customers choose suitable injection molding machines based on:

• Product weight
• Shot size requirements
• Mold structure
• Material characteristics
• Production targets
• Factory conditions

We provide:

• Servo injection molding machines
• Hydraulic injection molding machines
• PET production solutions
• Technical consultation
• Spare parts support
• Industrial sourcing services

Conclusion

Correct injection shot size calculation is essential for stable production, product quality, and efficient machine operation.

Buyers should consider:

• Product weight
• Runner system
• Material density
• Multi-cavity molds
• Cushion requirements

Proper machine selection helps reduce production defects, energy consumption, and long-term operating costs.

Contact DAYA MACHINERY

Need help selecting the right injection molding machine?

DAYA MACHINERY provides professional machine selection support for industrial customers worldwide.

Contact us for:

• Technical consultation
• Machine recommendations
• PET production systems
• Energy-saving solutions
• Spare parts support
• Industrial sourcing services