Injection Moulding Shot Weight Calculation

Professional tool for estimating total shot weight, material usage, and costs

Shot Weight Calculator

What is Injection Moulding Shot Weight Calculation?

Injection moulding shot weight calculation is a critical process in plastics manufacturing that determines the total amount of material required for a single moulding cycle. This calculation includes not just the weight of the finished parts, but also the material consumed by the feed system (runners and sprue) and the necessary safety buffer known as the cushion.

Accurate injection moulding shot weight calculation is essential for machine selection, cost estimation, and material planning. Engineers and production managers use this metric to ensure the injection moulding machine has sufficient barrel capacity (shot size) to fill the mould without degrading the plastic melt. Underestimating the shot weight can lead to short shots (incomplete parts), while overestimating can result in using a machine that is too large, increasing operational costs.

Injection Moulding Shot Weight Calculation Formula

The mathematical foundation for calculating the total shot weight involves summing the mass of three distinct components: the moulded parts, the runner system, and the cushion.

Total Shot Weight (g) = (Part Volume × Cavities × Density) + (Runner Volume × Density) + Cushion Weight

Where Cushion Weight is derived from the screw diameter and cushion length:
Cushion Weight = π × (Screw Radius)² × Cushion Length × Density

Variables Explanation

Variable	Meaning	Unit	Typical Range
Part Volume	Volume of one single part	cm³ (cc)	0.1 – 5000+
Density	Specific gravity of the plastic	g/cm³	0.90 – 1.50
Cavities	Number of parts per cycle	Integer	1 – 128
Runner Volume	Volume of sprue and runners	cm³	10% – 50% of part vol
Cushion	Buffer melt remaining in barrel	mm	3mm – 10mm

Practical Examples of Shot Weight Calculation

Example 1: Automotive Connector (PBT)

A manufacturer is producing a 4-cavity automotive connector using PBT (Density: 1.31 g/cm³).

• Part Volume: 12 cm³ per part
• Cavities: 4
• Runner Volume: 8 cm³
• Screw Diameter: 35 mm
• Cushion: 5 mm

Calculation:
1. Parts Weight = 12 × 4 × 1.31 = 62.88 g
2. Runner Weight = 8 × 1.31 = 10.48 g
3. Cushion Vol = π × (1.75)² × 0.5 = 4.81 cm³ (approx)
4. Cushion Weight = 4.81 × 1.31 = 6.30 g
Total Shot Weight = 62.88 + 10.48 + 6.30 = 79.66 g

Example 2: Medical Cap (PP)

Producing a high-volume medical cap using Polypropylene (Density: 0.90 g/cm³).

• Part Volume: 2.5 cm³
• Cavities: 32
• Runner System: Hot Runner (0 g waste)
• Screw Diameter: 50 mm
• Cushion: 6 mm

Calculation:
1. Parts Weight = 2.5 × 32 × 0.90 = 72.00 g
2. Runner Weight = 0 g
3. Cushion Weight = (Calculated based on 50mm screw) ≈ 10.6 g
Total Shot Weight = 82.6 g

How to Use This Injection Moulding Shot Weight Calculator

1. Select Material: Choose your plastic type from the dropdown. This auto-fills the density. If your material isn't listed, select "Custom" and enter the specific density from the material data sheet (TDS).
2. Enter Part Details: Input the volume of a single part in cubic centimeters (cm³).
3. Set Cavities: Enter the number of cavities in the mould.
4. Runner System: Input the volume of the cold runner. If using a hot runner system, enter 0.
5. Machine Settings: Enter the screw diameter and desired cushion length to account for the material buffer.
6. Review Results: The calculator instantly updates the Total Shot Weight and provides a breakdown of where the material is going.

Key Factors That Affect Shot Weight Results

When performing an injection moulding shot weight calculation, several factors influence the final accuracy and financial outcome:

• Material Density Variations: Density changes with temperature and pressure. The melt density is typically lower than solid density. This calculator uses solid density for weight estimation, which is standard for material purchasing, but melt density is used for barrel volume sizing.
• Runner System Efficiency: Cold runners can account for 20-50% of the total shot weight. Switching to hot runners eliminates this waste, significantly reducing the shot weight and material cost.
• Cushion Consistency: The cushion is vital for transferring holding pressure. If the cushion is too small (near zero), you cannot pack the part, leading to sinks. If too large, material degrades.
• Regrind Usage: If you regrind runners, the net material cost decreases, but the gross shot weight per cycle remains the same.
• Machine Sizing Rule: A general rule of thumb is to utilize between 20% and 80% of the machine's maximum shot capacity. Calculating the exact shot weight ensures you stay within this efficiency window.
• Scrap Rate: Real-world production involves startup scrap and defects. While not part of the theoretical shot weight, financial planning should add a 2-5% buffer to the total calculated weight.

Frequently Asked Questions (FAQ)

1. Why is cushion weight included in the calculation?

The cushion is the material remaining in the barrel in front of the screw after injection. While it doesn't leave the machine with the part, it must be plasticized and available for every shot to maintain pressure. It consumes barrel capacity.

2. Should I use solid density or melt density?

For purchasing material (weight), use solid density. For determining if a shot fits in a specific barrel volume, use melt density (which is roughly 10-20% lower than solid density).

3. How do I calculate runner volume if I don't know it?

If the mould is not built, you can estimate runner volume as a percentage of part weight. For small parts, runners can be 50-100% of part weight. For larger parts, 10-20% is common.

4. What is the ideal barrel utilization percentage?

Ideally, the total shot weight should be between 25% and 75% of the machine's maximum shot capacity (in polystyrene equivalent). Below 20% risks material degradation; above 80% risks inconsistent melting.

5. How does this affect clamping force?

Shot weight does not directly determine clamping force (tonnage). Tonnage is determined by the projected area of the parts and runners multiplied by the cavity pressure.

6. Can I use this for 2-shot moulding?

For 2-shot (2K) moulding, you must perform this injection moulding shot weight calculation separately for each injection unit/material.

7. How accurate is the cost estimation?

The cost estimation is based on raw material weight. It does not include machine hourly rates, labor, energy, or secondary operations.

8. What if my material contains glass fiber?

Glass fiber increases density significantly. Ensure you use the specific density from the datasheet (e.g., PA6 30% GF is ~1.35 g/cm³, while unfilled PA6 is 1.13 g/cm³).

0) ? ((data[i].val / total) * 100).toFixed(1) : 0; row.innerHTML = '' + data[i].name + '' + '' + data[i].val.toFixed(2) + '' + '' + pct + '%'; tbody.appendChild(row); } } function drawChart(parts, runner, cushion) { var canvas = document.getElementById('shotChart'); var ctx = canvas.getContext('2d'); var width = canvas.width; var height = canvas.height; var total = parts + runner + cushion; // Clear canvas ctx.clearRect(0, 0, width, height); if (total === 0) return; var data = [parts, runner, cushion]; var colors = ['#004a99', '#28a745', '#ffc107']; var labels = ['Parts', 'Runner', 'Cushion']; var startAngle = 0; var centerX = width / 2; var centerY = height / 2; var radius = Math.min(centerX, centerY) – 20; // Draw Pie for (var i = 0; i < data.length; i++) { if (data[i] <= 0) continue; var sliceAngle = (data[i] / total) * 2 * Math.PI; ctx.beginPath(); ctx.moveTo(centerX, centerY); ctx.arc(centerX, centerY, radius, startAngle, startAngle + sliceAngle); ctx.closePath(); ctx.fillStyle = colors[i]; ctx.fill(); startAngle += sliceAngle; } // Draw Legend HTML var legend = document.getElementById('chartLegend'); legend.innerHTML = ''; for (var j = 0; j < labels.length; j++) { var item = document.createElement('div'); item.className = 'legend-item'; item.innerHTML = '

' + labels[j]; legend.appendChild(item); } } function resetCalculator() { document.getElementById('materialSelect').value = '0.90'; document.getElementById('density').value = '0.90'; document.getElementById('partVolume').value = '50'; document.getElementById('cavities').value = '4'; document.getElementById('runnerVolume').value = '15'; document.getElementById('screwDiameter').value = '40'; document.getElementById('cushionLength').value = '5'; document.getElementById('materialCost').value = '1.50'; calculateShotWeight(); } function copyResults() { var total = document.getElementById('totalShotWeight').innerText; var parts = document.getElementById('totalPartsWeight').innerText; var runner = document.getElementById('runnerWeight').innerText; var cost = document.getElementById('costPerShot').innerText; var text = "Injection Moulding Shot Weight Calculation Results:\n" + "Total Shot Weight: " + total + "\n" + "Parts Weight: " + parts + "\n" + "Runner Weight: " + runner + "\n" + "Estimated Cost: " + cost; var tempInput = document.createElement("textarea"); tempInput.value = text; document.body.appendChild(tempInput); tempInput.select(); document.execCommand("copy"); document.body.removeChild(tempInput); var btn = document.querySelector('.btn-copy'); var originalText = btn.innerText; btn.innerText = "Copied!"; setTimeout(function(){ btn.innerText = originalText; }, 2000); }