Shot Weight Calculator
How do you calculate shot weight in injection molding? Accurate estimation for molders.
Injection Molding Shot Weight Calculator
Formula: Shot Weight = (Part Vol × Density × Cavities) + (Runner Vol × Density)
Weight Distribution
| Component | Volume (cm³) | Weight (g) | % of Shot |
|---|
Table 1: Detailed breakdown of shot components.
How Do You Calculate Shot Weight in Injection Molding?
In the world of plastics manufacturing, precision is profitability. One of the most critical metrics for setting up a process, quoting a job, or selecting a machine is the shot weight. But how do you calculate shot weight in injection molding accurately? This guide covers the definitions, formulas, and practical steps to ensure your calculations are production-ready.
What is Shot Weight in Injection Molding?
Shot weight represents the total amount of polymer required to fill the mold cavities and the delivery system. It is a fundamental parameter used to determine:
- 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).
- Material Cost Estimation: Calculating the exact cost of resin per cycle.
- Cycle Time Optimization: Heavier shots may require longer cooling times.
Many beginners confuse "part weight" with "shot weight." Part weight is simply the mass of the finished product. Shot weight is the gross weight, including the waste or regrind generated by the runner system (in cold runner molds).
The Shot Weight Formula and Mathematical Explanation
To answer "how do you calculate shot weight in injection molding," you need to understand the relationship between volume, density, and quantity. The core formula is derived from basic physics:
Mass (g) = Volume (cm³) × Density (g/cm³)
The expanded formula for injection molding is:
Variable Definitions
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| Part Volume | Volume of a single finished part | cm³ (cc) | 0.1 – 5000+ |
| Density | Specific gravity of the plastic resin | g/cm³ | 0.90 (PP) – 1.50 (PVC) |
| Cavities | Number of parts molded per cycle | Integer | 1 – 128+ |
| Runner Volume | Volume of sprue, runners, and gates | cm³ | 0 (Hot Runner) – 50% of shot |
Practical Examples (Real-World Use Cases)
Example 1: 4-Cavity ABS Housing
A molder is producing electronic housings using ABS plastic. The mold is a 4-cavity cold runner tool.
- Material: ABS (Density ≈ 1.04 g/cm³)
- Single Part Volume: 45 cm³
- Runner System Volume: 12 cm³
- Cavities: 4
Calculation:
Part Weight = 45 cm³ × 1.04 g/cm³ = 46.8 g
Total Parts Weight = 46.8 g × 4 = 187.2 g
Runner Weight = 12 cm³ × 1.04 g/cm³ = 12.48 g
Total Shot Weight = 187.2 g + 12.48 g = 199.68 g
Example 2: Single Cavity Polypropylene Container
A large storage bin is molded in PP using a hot runner system (no runner waste).
- Material: PP (Density ≈ 0.90 g/cm³)
- Single Part Volume: 600 cm³
- Runner Volume: 0 cm³ (Hot Runner)
- Cavities: 1
Calculation:
Total Shot Weight = (600 × 0.90 × 1) + 0 = 540 g
How to Use This Shot Weight Calculator
- Select Material: Choose your resin from the dropdown. This automatically fills the density field. If your material isn't listed, select "Custom" and enter the density from the material data sheet (TDS).
- Enter Part Volume: Input the volume of a single part. This is usually obtained from CAD software (SolidWorks, AutoCAD) properties.
- Set Cavities: Enter the number of mold cavities.
- Enter Runner Volume: If using a cold runner, input the volume of the feed system. For hot runners, enter 0.
- Check Utilization: Optionally, enter your machine's barrel capacity to see if the shot size is appropriate (aim for 20-80%).
Key Factors That Affect Shot Weight Results
When asking "how do you calculate shot weight in injection molding," consider these nuance factors that affect the final number:
1. Material Density Variations
Density is not constant. It changes with temperature and pressure. The "melt density" inside the barrel is lower than the solid density of the finished part. However, for weight estimation, solid density is typically used because the final product is weighed in its solid state.
2. Shrinkage
Plastics shrink as they cool. While mass is conserved, the volume of the mold cavity must be larger than the final part volume. Your CAD volume usually represents the final part, so the weight calculation remains accurate based on solid density.
3. Cushion
The "cushion" is a small amount of plastic left in the barrel after injection to maintain pressure. While not part of the ejected shot, it is part of the required barrel capacity sizing.
4. Additives and Fillers
Glass fibers, talc, or colorants significantly alter density. For example, 30% Glass Filled Nylon is much heavier (approx 1.35 g/cm³) than unfilled Nylon (1.15 g/cm³). Always check the specific datasheet.
5. Runner System Efficiency
In cold runner systems, the runner can account for 20-50% of the total shot weight. Optimizing runner design to minimize diameter while maintaining flow is crucial for cost reduction.
6. Flash and Overpacking
If the mold is worn or clamp tonnage is insufficient, "flash" (excess plastic) occurs, increasing the actual shot weight compared to the theoretical calculation.
Frequently Asked Questions (FAQ)
Shot weight is the actual mass of plastic injected. Shot size usually refers to the maximum volume or weight capacity of the injection molding machine's barrel.
If the shot weight is less than 20% of barrel capacity, the plastic stays in the heated barrel too long and may degrade. If it's over 80%, you may not have enough cushion or melt uniformity.
The most accurate way is using 3D CAD software (like SolidWorks or Fusion 360) which calculates volume based on the geometry. Alternatively, you can use water displacement for existing physical parts.
Yes, but usually negligibly for standard colors (1-3% mix ratio). However, heavy metal-based pigments or high loading rates can slightly increase density.
For hot runner molds, the runner volume is effectively zero regarding the ejected shot. The formula simplifies to: Part Volume × Density × Cavities.
Use solid density to calculate the weight of the final cold part. Use melt density when calculating the volume required inside the hot barrel.
The standard unit is grams (g) or ounces (oz). 1 oz ≈ 28.35 grams.
Regrind doesn't change the volume/density calculation, but it affects cost. If you grind runners and put them back in, your net material usage equals the part weight, though the shot weight remains the same.
Related Tools and Internal Resources
Explore more tools to optimize your molding process:
Injection Molding Cycle Time CalculatorEstimate how long each cycle takes to optimize production rates.
Clamp Tonnage CalculatorDetermine the required clamping force based on projected area.
Plastic Material Density ChartA comprehensive list of specific gravities for common polymers.
Runner System Design GuideLearn how to size runners to minimize waste and cycle time.
Injection Molding Cost EstimatorCalculate the total cost per part including material, machine, and labor.
Mold Cavity Balance GuideEnsure uniform filling across all cavities in multi-cavity molds.