How to Calculate Runner Weight in Injection Molding

Accurately estimate resin consumption, costs, and scrap rates

Injection Runner Weight Calculator

Configure your runner geometry and material properties to calculate total weight.

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Understanding how to calculate runner weight in injection molding is a fundamental skill for mold designers, process engineers, and cost estimators. The runner system—the channel that guides molten plastic from the injection unit to the mold cavities—often represents a significant portion of material usage. Inaccurately estimating this weight can lead to flawed cost quotes, unexpected resin shortages, and inefficient cycle times.

This comprehensive guide explores the mathematics behind runner weight calculations, the factors influencing material density, and the financial impact of runner design on your bottom line.

What is Runner Weight Calculation?

Runner weight calculation is the process of determining the mass of the solidified plastic remaining in the feed system (sprue, runners, and gates) after an injection molding cycle. Unlike the part weight, which adds value to the customer, the runner weight is often considered "necessary waste" in cold runner systems, though it can often be reground and recycled.

Who Should Use This Calculation?

• Mold Designers: To size runners appropriately for pressure drop and cooling time.
• Cost Estimators: To account for gross material needs, not just net part weight.
• Process Engineers: To optimize the runner-to-part ratio and minimize cycle time.

Common Misconceptions

Many novices assume the runner weight is negligible. However, in multi-cavity molds producing small parts, the runner can sometimes weigh more than the parts themselves. Ignoring this leads to substantial underestimation of material costs ($/kg) over a production run.

Formula and Mathematical Explanation

To understand how to calculate runner weight in injection molding, one must first determine the volume of the runner system and then apply the material's specific gravity (density).

Step 1: Calculate Cross-Sectional Area (A)

The area depends on the runner shape geometry:

• Full Round: \( A = \pi \times (\frac{D}{2})^2 \)
• Semicircular: \( A = \frac{\pi \times (\frac{D}{2})^2}{2} \)
• Trapezoidal: \( A = \frac{(W_{top} + W_{bottom})}{2} \times Depth \)

Step 2: Calculate Volume (V)

Multiply the cross-sectional area by the total length of the runner system.

Formula: \( V (mm^3) = Area (mm^2) \times Length (mm) \)

Step 3: Calculate Weight (W)

Multiply volume by density. Note that we must convert units (mm³ to cm³) to match standard density units (g/cm³).

Formula: \( W (g) = \frac{V (mm^3)}{1000} \times Density (g/cm^3) \)

Variable	Meaning	Unit	Typical Range
D	Diameter of runner	mm	3mm – 12mm
L	Total Length of runner	mm	50mm – 1000mm+
Density	Specific Gravity of Resin	g/cm³	0.9 (PP) – 1.4 (PVC)
W	Final Runner Weight	grams	Varies

Practical Examples (Real-World Use Cases)

Example 1: The PP Medical Cap (High Volume)

A mold produces 8 polypropylene caps. The runner system is a Full Round shape.

• Material: Polypropylene (Density 0.91 g/cm³)
• Runner Diameter: 6 mm
• Total Runner Length: 350 mm

Calculation:

1. Area = 3.14159 × (3)² = 28.27 mm²
2. Volume = 28.27 mm² × 350 mm = 9,894.5 mm³ (approx 9.89 cm³)
3. Weight = 9.89 cm³ × 0.91 g/cm³ = 9.00 grams

Financial Impact: If the 8 caps weigh 16g total, the runner is 36% of the shot weight. This is efficient.

Example 2: The ABS Automotive Bezel (Trapezoidal Runner)

A larger mold uses a trapezoidal runner for an ABS part.

• Material: ABS (Density 1.05 g/cm³)
• Dimensions: Top Width 8mm, Bottom Width 6mm, Depth 6mm
• Length: 600 mm

Calculation:

1. Area = ((8 + 6) / 2) × 6 = 42 mm²
2. Volume = 42 mm² × 600 mm = 25,200 mm³ (25.2 cm³)
3. Weight = 25.2 cm³ × 1.05 g/cm³ = 26.46 grams

How to Use This Runner Weight Calculator

1. Select Shape: Choose between Round, Semicircular, or Trapezoidal based on your mold design.
2. Enter Dimensions: Input the diameter, or widths and depth, along with the total length of the runner branches.
3. Select Material: Choose a standard resin density or enter a custom specific gravity from your Material Data Sheet (MDS).
4. Add Financials: Input the resin cost per kg to see the immediate cost per shot.
5. Analyze: Review the "Runner to Shot Ratio". A lower percentage generally indicates higher efficiency, though very low ratios can cause filling issues.

Key Factors That Affect Runner Weight Results

1. Material Density (Specific Gravity)

The heavier the polymer, the heavier the runner for the same geometry. Switching from PP (0.91) to PVC (1.40) increases runner weight by over 50% without changing dimensions.

2. Runner Geometry Efficiency

Full round runners have the best volume-to-surface-area ratio, meaning they hold heat best for the least weight. Trapezoidal runners are often heavier for the same flow capacity.

3. Cavity Layout (Cavitation)

Higher cavitation (e.g., 32 cavities vs. 4 cavities) requires longer and more complex runner branching. While the runner weight increases, the percentage of runner weight relative to total shot weight usually decreases, improving efficiency.

4. Cold Runner vs. Hot Runner

This calculator assumes a cold runner system. Hot runner systems maintain the plastic in a molten state, effectively eliminating the solidified runner weight calculation entirely for the finished part cost model (though manifold volume calculation is still needed for residence time).

5. Regrind Percentage

If you can regrind your runner, the "cost" is reduced to the energy used to plasticize it twice and the degradation of material properties. If you cannot use regrind (e.g., medical parts), the runner weight is 100% financial loss.

6. Scrap and Purge

Large runners increase the volume required to purge the machine during color changes, increasing indirect material waste.

Frequently Asked Questions (FAQ)

What is a good runner-to-part weight ratio?

Ideally, the runner should be less than 20% of the total shot weight. However, for small parts (under 1g), the runner may inevitably exceed 50% or even 100% of the part weight due to minimum flow channel requirements.

Why calculate runner weight before building the mold?

Calculating it early allows you to size the injection unit correctly. If the runner + parts exceed the barrel capacity (shot size) of your machine, you cannot mold the product.

Does runner length include the sprue?

Yes, for a complete weight estimation, you should include the length and equivalent volume of the sprue and the gates, although gates are usually negligible.

How does runner weight affect cooling time?

The runner is often the thickest section of the mold. If the runner is too heavy (thick), it dictates the cooling time of the entire cycle, slowing down production and increasing costs.

Can I reduce runner weight by reducing diameter?

Yes, but be careful. Reducing diameter increases shear heating and injection pressure. If the pressure drop is too high, the mold won't fill. Use mold flow analysis to find the balance.

Is trapezoidal better than round?

Round is hydraulically most efficient but requires machining both mold plates. Trapezoidal is cheaper to machine (only one plate) but slightly less efficient in flow-to-weight ratio.

How accurate is this calculator?

This calculator provides a geometric estimation. Real-world weight may vary slightly due to packing pressure, actual steel dimensions, and specific material shrinkage rates.

What if I use a hot runner system?

If you use a direct-gated hot runner, your runner weight is effectively zero for cost calculations per part. If you use a hot runner to feed a small cold runner branch, calculate only the cold branch.

Related Tools and Internal Resources

Enhance your injection molding cost estimation with these related tools:

• Injection Molding Cycle Time Calculator – Estimate production speed based on wall thickness and material.
• Clamp Tonnage Calculator – Determine the required machine size based on projected area.
• Part Weight Estimator – Calculate the weight of the actual product before runner addition.
• Mold Cooling Calculator – Optimize water flow for faster cycles.
• Resin Drying Time Guide – Best practices for preparing hygroscopic materials.
• Screw Recovery Rate Calculator – Ensure your plasticizing capacity meets cycle demands.