Shot Weight Calculation

Professional Injection Molding Weight & Cost Estimator

In the precision-driven world of injection molding, shot weight calculation is a fundamental metric that dictates machine selection, material costs, and cycle efficiency. Whether you are an engineer designing a new mold or a plant manager optimizing production, understanding the exact weight of material required for a single cycle is critical for financial and operational success.

This guide explores the mechanics of shot weight, provides a detailed mathematical breakdown, and offers practical examples to help you utilize our calculator effectively.

What is Shot Weight Calculation?

Shot weight refers to the total amount of plastic material injected into the mold during a single molding cycle. It is not just the weight of the finished product; it encompasses the entire volume of polymer required to fill the mold cavities, the runner system (sprue, runners, gates), and any necessary cushion.

Accurate shot weight calculation is essential for:

• Machine Selection: Ensuring the injection molding machine has sufficient barrel capacity (typically, the shot weight should be between 20% and 80% of the machine's maximum capacity).
• Cost Estimation: Determining the raw material cost per cycle and per part.
• Process Stability: Maintaining a consistent cushion to ensure parts are packed correctly without short shots or flashing.

A common misconception is that shot weight equals part weight. Ignoring the runner system—which can account for 10% to 50% of the total material in cold runner molds—can lead to severe underestimation of material needs and costs.

Shot Weight Formula and Mathematical Explanation

The core formula for calculating shot weight combines the mass of the molded parts with the mass of the feed system. The calculation relies heavily on the relationship between volume and density.

The Formula

Shot Weight (g) = (Part Volume × Material Density × Number of Cavities) + Runner Weight

Where:

• Part Volume: The volume of a single cavity (cm³).
• Material Density: The specific gravity of the plastic resin (g/cm³).
• Number of Cavities: The total number of identical parts produced in one cycle.
• Runner Weight: The weight of the solidified material in the feed system (g).

Variables Table

Variable	Meaning	Unit	Typical Range
$\rho$ (Density)	Material specific gravity	g/cm³	0.90 (PP) – 1.50 (PVC)
$V_{part}$	Volume of one part	cm³	0.1 – 5000+
$N$	Cavity count	Integer	1 – 128+
$W_{runner}$	Feed system weight	grams (g)	5g – 500g+

Practical Examples (Real-World Use Cases)

Example 1: Automotive Interior Clip (ABS)

An engineer is designing a 4-cavity mold for an automotive clip made of ABS plastic.

• Material: ABS (Density: 1.04 g/cm³)
• Part Volume: 12.5 cm³ per clip
• Cavities: 4
• Runner System: Weighed from a prototype at 15 grams.

Calculation:
Part Weight = 12.5 cm³ × 1.04 g/cm³ = 13.0 g
Total Parts Weight = 13.0 g × 4 = 52.0 g
Total Shot Weight = 52.0 g + 15.0 g = 67.0 g

Financial Interpretation: If ABS costs $2.50/kg, the material cost per shot is $0.1675.

Example 2: Medical Vial Cap (Polypropylene)

A high-volume production run for medical caps using a 32-cavity hot runner mold (zero runner waste).

• Material: PP (Density: 0.90 g/cm³)
• Part Volume: 2.0 cm³
• Cavities: 32
• Runner Weight: 0 g (Hot Runner)

Calculation:
Part Weight = 2.0 cm³ × 0.90 g/cm³ = 1.8 g
Total Parts Weight = 1.8 g × 32 = 57.6 g
Total Shot Weight = 57.6 g

Decision: A machine with a 100g barrel capacity would be ideal here, utilizing roughly 57% of capacity.

How to Use This Shot Weight Calculator

1. Select Material: Choose your resin from the dropdown list. The density field will update automatically. If using a specialized compound, select "Custom" and enter the density manually.
2. Enter Part Volume: Input the volume of a single part in cubic centimeters (cm³). This is often found in CAD software properties.
3. Set Cavities: Enter the number of mold cavities.
4. Input Runner Weight: Enter the estimated or measured weight of the runner system. For hot runner molds, enter 0.
5. Optional Cost: Enter the material cost per kg to see financial estimates.
6. Analyze Results: Review the Total Shot Weight to select the right machine size and the Material Efficiency to evaluate runner waste.

Key Factors That Affect Shot Weight Results

Several variables can influence the final shot weight and the financial viability of a molding project:

• Material Density Variations: Additives like glass fiber or talc significantly increase density. A 30% glass-filled nylon is much heavier than unfilled nylon, increasing shot weight and cost per part.
• Runner Design: In cold runner systems, the runner is essentially waste (unless reground). Thick runners increase shot weight without adding value to the part, reducing material efficiency.
• Cushion Size: The machine injects slightly more material than the cavity volume to maintain pressure during the holding phase. This "cushion" (usually 3-6mm of screw travel) adds to the actual material consumed per cycle.
• Shrinkage Rates: While calculations use mold volume, the final part weight is determined by the material density at room temperature. High-shrinkage materials (like PE) require careful volume calculations in the mold design phase.
• Regrind Usage: If you regrind runners, your net material cost decreases, but the gross shot weight (which determines machine size) remains the same.
• Machine Barrel Capacity: If your calculated shot weight is less than 20% of the barrel capacity, the material may degrade due to long residence time. If it exceeds 80%, you may face quality issues like short shots.

Frequently Asked Questions (FAQ)

1. Why is shot weight important for machine selection?

Injection molding machines are rated by clamping force and shot size (max injection weight). You generally want to utilize between 20% and 80% of the barrel's maximum capacity to ensure melt consistency and process control.

2. How do I find the volume of my part?

The most accurate method is using CAD software (SolidWorks, AutoCAD, Fusion 360) which calculates volume based on the 3D model. Alternatively, you can use water displacement for an existing physical prototype.

3. Does shot weight include the sprue?

Yes. In a cold runner system, the shot weight includes the part, the runner channels, the gates, and the sprue. All this material must be melted and injected.

4. How does a hot runner system affect shot weight?

A hot runner system keeps the plastic in the runners molten, so it is not ejected with the part. This reduces the shot weight to just the weight of the parts, significantly improving material efficiency.

5. Can I calculate shot weight using part weight instead of volume?

Yes. If you already know the weight of a single part, you can simply multiply that weight by the number of cavities and add the runner weight. Our calculator uses volume + density for greater flexibility during the design phase.

6. What is a good material efficiency percentage?

For cold runners, an efficiency above 80% (meaning less than 20% runner weight) is good. Efficiencies below 50% suggest the runner system is too large for the parts, which is common with very small parts in large molds.

7. How do I account for the cushion?

The cushion is the material remaining in the barrel after injection. While it is part of the process, it is not ejected with the part. However, for material inventory planning, you calculate based on the ejected shot weight.

8. Does colorant affect shot weight?

Masterbatch or colorants are usually added at 1-3%. While they have different densities, the impact on total shot weight is usually negligible for general estimations unless heavy metal pigments are used.