What is Shot Weight Calculation Injection Moulding?
Shot weight calculation injection moulding is a critical process engineering step that determines the total amount of thermoplastic material required to fill the mould cavities and the runner system during a single machine cycle. Accurate calculation is essential for selecting the correct injection moulding machine size (barrel capacity), estimating material costs, and optimizing cycle times.
Engineers, estimators, and plant managers use shot weight calculations to ensure that the injection unit has sufficient plasticizing capacity. If the calculated shot weight exceeds roughly 80% of the machine's barrel capacity, it may lead to quality issues like short shots or inconsistent melt. Conversely, utilizing a machine with a barrel capacity far too large for the shot weight can cause material degradation due to excessive residence time.
A common misconception is that shot weight equals the weight of the finished parts only. In reality, for cold runner systems, the solidified material in the sprue and runners must be included in the total shot weight calculation injection moulding process to ensure accurate machine selection and cost modeling.
Shot Weight Formula and Explanation
The mathematical foundation for calculating the total shot weight is straightforward but must be precise. The formula sums the weight of all productive cavities and the non-productive runner system.
Total Shot Weight ($W_{total}$) = ($W_{part}$ × $N_{cavities}$) + $W_{runner}$
Variable Definitions
Variables used in shot weight calculation injection moulding.
Variable
Meaning
Unit
Typical Range
$W_{total}$
Total Shot Weight
Grams (g)
1g – 5000g+
$W_{part}$
Single Part Weight
Grams (g)
0.1g – 2000g
$N_{cavities}$
Number of Cavities
Integer
1 – 128+
$W_{runner}$
Runner/Sprue Weight
Grams (g)
0 (Hot Runner) – 200g+
After calculating the total weight, engineers often calculate Material Utilization, which is the ratio of useful part weight to total shot weight. A higher percentage indicates greater efficiency and less waste (or regrind) generation.
Practical Examples
Example 1: Multi-Cavity Cap Mould
A manufacturer is producing polypropylene bottle caps.
A large automotive interior panel using a hot runner system (zero waste runner).
Part Weight: 450 grams
Cavities: 1
Runner System: 0 grams
Calculation:
Shot Weight = (450g × 1) + 0g = 450 grams.
In this shot weight calculation injection moulding scenario, the utilization is 100%.
How to Use This Calculator
Enter Part Weight: Input the weight of a single finished unit in grams. If you only know the volume (cm³), multiply it by the material density (g/cm³) first.
Set Cavities: Enter the number of mould cavities (e.g., 1, 2, 4, 8, 16).
Add Runner Weight: Weigh your sprue/runner from a sample or estimate it from CAD. Enter 0 if using a hot runner system.
Material Cost: Enter the price per kilogram of your raw resin to see financial estimates.
Analyze Results: Use the "Total Shot Weight" to select a machine (target 20-80% of barrel capacity) and the "Cost Per Part" for quoting.
Key Factors That Affect Shot Weight Results
Several variables influence the final shot weight calculation injection moulding results and financial outcomes.
Material Density: Higher density materials (like PVC or filled Nylon) will result in a heavier shot for the same mould volume compared to lighter materials like PP.
Runner Type (Hot vs. Cold): Cold runners add significant weight and cycle time. Hot runners require higher upfront capital but reduce material usage and shot weight.
Cushion Maintenance: The machine injects slightly more than the cavity volume to maintain pressure (the "cushion"). While often excluded from the theoretical weight of the plastic removed from the mould, it consumes barrel capacity.
Regrind Usage: If runners are reground and reused, the net material cost decreases, though the gross shot weight remains the same.
Flash and Overpacking: Poor process control can lead to flash (excess material), slightly increasing the actual shot weight versus the theoretical calculation.
Machine Efficiency: Larger shot weights require larger machines with higher hourly rates ($/hr), impacting the total cost of production beyond just material costs.
Frequently Asked Questions (FAQ)
Why is shot weight calculation important for machine selection?
It prevents selecting a machine that is too small (unable to fill the mould) or too large (causing material degradation). Ideally, the shot weight should be between 20% and 80% of the machine's barrel rating.
Does shot weight include the runner?
Yes. In a cold runner system, the runner is part of the shot that the machine must plastify and inject. For hot runner systems, the runner is not part of the ejected shot weight.
How do I calculate part weight from volume?
Multiply the part's volume (in cm³) by the specific gravity or density (g/cm³) of the resin. For example, 10 cm³ of PP (density ~0.9 g/cm³) weighs 9 grams.
What is a good material utilization percentage?
For cold runners, utilization above 80% is good. Below 50% suggests the runner is too large for the parts, wasting energy and material. Hot runners theoretically offer 100% utilization.
How does shot weight affect cycle time?
Larger shot weights require longer cooling times (as there is more mass to cool) and longer plasticizing (dosing) times for the screw to recover.
Should I include the cushion in the shot weight calculation?
For machine sizing, yes, you must account for the required cushion (usually 5-10mm of screw stroke). For material cost estimation per part, no, as the cushion stays in the barrel.
Can I use this for multi-material moulding?
This calculator assumes a single material shot. For 2K or overmoulding, you must calculate the shot weight for each injection unit separately.
Does specific gravity change with temperature?
Yes. Melt density is lower than solid density. However, shot weight is usually measured in solid grams (final part weight), so solid density is the standard reference for cost and weight checks.