Shot Weight Calculator
A professional tool for injection moulding process optimization. Calculate total shot weight, plastic utilization, and material throughput instantly.
Calculate Shot Parameters
Weight of a single finished part excluding runner.
Please enter a valid positive number.
Total number of parts produced per cycle.
Must be at least 1 cavity.
Weight of the feed system (waste or regrind) per shot.
Please enter a valid positive number.
Total time to complete one moulding cycle.
Cycle time must be positive.
Total Shot Weight
114.0 g
Material Utilization
89.5%
Throughput per Hour
22.8 kg
Est. Barrel Capacity Required
125.4 g
Formula Used:
Total Shot Weight = (Part Weight × Cavities) + Runner Weight
Throughput (kg/hr) = (Total Shot Weight ÷ 1000) × (3600 ÷ Cycle Time)
Total Shot Weight = (Part Weight × Cavities) + Runner Weight
Throughput (kg/hr) = (Total Shot Weight ÷ 1000) × (3600 ÷ Cycle Time)
Material Distribution Analysis
Visual breakdown of productive material vs. feed system
Production Estimates
| Time Period | Cycles | Parts Produced | Material Consumed (kg) |
|---|
What is Shot Weight Calculation in Injection Moulding?
In the context of plastics manufacturing, shot weight calculation injection moulding pdf resources often define shot weight as the total mass of plastic material injected into the mould during a single cycle. This metric is critical because it determines the machine tonnage required, the barrel capacity needed, and the overall cost efficiency of the production run.
Accurate shot weight calculation ensures that the injection moulding machine has sufficient capacity to fill the cavities and the runner system without reaching its maximum limit, which can cause wear or inconsistent part quality. Engineers and plant managers use this figure to quote jobs, estimate material requirements, and schedule machine time effectively.
Shot Weight Calculation Formula and Mathematical Explanation
While you may search for a shot weight calculation injection moulding pdf to find these formulas, understanding the math dynamically is more valuable. The total shot weight comprises the weight of all finished parts plus the weight of the feed system (sprues, runners, and gates).
The core formula is:
Wtotal = (Wpart × N) + Wrunner
Where:
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| Wtotal | Total Shot Weight | grams (g) | 1g – 5000g+ |
| Wpart | Weight of Single Part | grams (g) | 0.1g – 2000g |
| N | Number of Cavities | Integer | 1 – 128+ |
| Wrunner | Weight of Runner System | grams (g) | 5% – 50% of shot |
Practical Examples (Real-World Use Cases)
Example 1: High Cavity Cap Production
A manufacturer is producing polypropylene bottle caps. The single cap weight is 2.5g. The mould has 32 cavities. The runner system, including the sprue, weighs 35g.
- Part Weight: 2.5g
- Cavities: 32
- Runner Weight: 35g
- Calculation: (2.5 × 32) + 35 = 80 + 35 = 115g Total Shot Weight.
Financial Interpretation: If the cycle time is 10 seconds, the machine consumes 115g every 10 seconds. This high material throughput requires a machine with a high plasticizing rate.
Example 2: Automotive Housing (Single Cavity)
An automotive supplier is moulding an ABS housing. The part weighs 450g. It is a single cavity mould (1 cavity). The cold runner is substantial, weighing 60g.
- Part Weight: 450g
- Cavities: 1
- Runner Weight: 60g
- Calculation: (450 × 1) + 60 = 510g Total Shot Weight.
Financial Interpretation: The runner represents roughly 11.7% of the shot. If this runner cannot be reground and reused immediately, it represents a significant material cost variance that must be factored into the unit price.
How to Use This Shot Weight Calculator
This tool is designed to replace the need for downloading a static shot weight calculation injection moulding pdf. Follow these steps:
- Input Part Weight: Enter the weight of a single finished unit in grams. If you only have volume, multiply volume (cc) by material density (g/cc).
- Set Cavities: Input the number of moulding cavities in your tool.
- Add Runner Weight: Weigh your runner/sprue from a sample shot or estimate it from CAD data and enter it here.
- Define Cycle Time: Enter the total time in seconds from mould close to mould close.
- Analyze Results: The calculator immediately provides the Total Shot Weight, Throughput (kg/hr), and Material Utilization %.
Key Factors That Affect Shot Weight Results
When performing a shot weight calculation, several variables can influence the final financial and technical outcome:
- Material Density: Different polymers have different specific gravities. A part moulded in PP (0.9 g/cc) will weigh significantly less than the same geometry in PC (1.2 g/cc).
- Cushion Value: To ensure consistent pressure transfer, machines maintain a "cushion" of plastic in front of the screw. A barrel capacity should typically be 1.1x to 1.2x the calculated shot weight.
- Regrind Ratio: The financial impact of the runner weight depends on whether you can regrind the material. If you can use 20% regrind, the effective cost of the runner decreases.
- Cycle Time: While cycle time doesn't change the weight of a single shot, it dramatically affects the hourly consumption rate (throughput), which dictates auxiliary equipment size (dryers, loaders).
- Scrap Rate: Real-world calculations should add a safety margin (e.g., 2-5%) for startup scraps and quality rejects.
- Injection Pressure & Packing: High packing pressure compresses more material into the cavity, slightly increasing the actual part weight compared to nominal volume calculations.
Frequently Asked Questions (FAQ)
1. Why is shot weight calculation critical for machine selection?
Injection moulding machines have a maximum injection capacity (in grams of Polystyrene). Your calculated shot weight generally should not exceed 70-80% of the machine's maximum capacity to ensure melt quality and consistency.
2. Can I use this calculator for multi-material moulding?
For 2K or overmoulding, you should calculate the shot weight for each barrel/injection unit independently using this tool, as they are separate processes.
3. How does shot weight relate to clamping force?
Shot weight is volume-based, while clamping force is area-based. However, larger shot weights usually imply larger projected areas, requiring higher tonnage.
4. Is runner weight considered "waste"?
Technically yes, unless it is a hot runner system (where runner weight is effectively zero) or if the material is reground. In cost calculations, runner weight is material paid for but not sold in the final part.
5. Where can I find a shot weight calculation injection moulding pdf?
Many textbooks offer PDF tables, but this dynamic calculator provides more accurate, scenario-specific results than a static document.
6. How accurate is the throughput calculation?
It is mathematically precise based on your inputs. However, real-world throughput varies with machine uptime (OEE) and maintenance pauses.
7. Does temperature affect shot weight?
Yes. Molten plastic is less dense than solid plastic. Shot weight is usually measured in solid state grams, but the barrel volume must account for the melt density expansion.
8. What is the ideal utilization percentage?
Ideally, you want 100% (hot runner). For cold runners, utilization above 80% is good. Below 50% usually indicates an inefficient mould design or a very small part in a large runner system.
Related Tools and Internal Resources
- Cycle Time Estimator – Predict your cooling and injection times.
- Clamping Force Calculator – Determine the required tonnage for your mould.
- Material Cost Analyzer – Calculate part cost including regrind ratios.
- Injection Molding Defects Guide – Troubleshooting common quality issues.
- Machine Hour Rate Calculator – Determine the hourly operational cost of your equipment.
- Cavity Layout Optimizer – Plan your mould layout for optimal flow.