A central feeding system is the backbone of any automated injection molding facility. When designed correctly, it delivers consistent material flow, reduces labor costs, and supports uninterrupted production. However, pipeline design errors made during the planning stage — such as incorrect pipe sizing, undersized blowers, or improper airflow calculations — frequently result in blockages, insufficient suction, and frequent equipment failures after commissioning.

This guide covers the 5 critical parameters every engineer must calculate accurately before finalizing a central feeding system pipeline layout.

1. Conveying Capacity (Q) — The Starting Point for All Calculations

Conveying capacity defines how much material (kg/h) the system must transport within a given time frame. It is the foundation upon which all other parameters are based.

How to Calculate:

• Sum the maximum hourly material consumption of all injection molding machines in the facility
• Apply a simultaneous operating rate of 75%–85% to reflect real-world conditions
• Add a minimum design margin of 20% on top of the calculated demand

Design Recommendation: Always base your design on projected capacity 3 to 5 years into the future — not just current production levels. Once pipelines are installed and fixed to walls or ceilings, retrofitting for higher capacity becomes extremely costly.

2. Mixing Ratio (μ) — The Key to System Efficiency

The mixing ratio represents the ratio of material mass to air mass transported per unit of time. It is the primary indicator of how economically the system operates.

Formula: μ = G(material) ÷ G(air)

• A higher mixing ratio means more material per cubic meter of air, reducing energy consumption
• An excessively high mixing ratio causes uneven material-air distribution, significantly increasing the risk of blockages
• Recommended mixing ratio for plastic pellet conveying: 3 to 8

Special Material Note: Hygroscopic materials such as PA (Nylon) and PC (Polycarbonate) require a lower mixing ratio to minimize residence time inside the pipeline and prevent moisture absorption that could compromise product quality.

3. Conveying Air Velocity (v) — The Most Experience-Dependent Parameter

Air velocity is the parameter that most directly impacts system stability and long-term equipment life. Setting it incorrectly in either direction leads to serious operational problems.

Recommended Air Velocity by Material Type:

Engineering Best Practice: All pipeline elbows should use long-radius bends with a curvature radius of R ≥ 7D. Standard 90° short-radius elbows create high-impact zones that accelerate wear and can perforate within months under continuous operation.

4. Air Volume and Pipe Diameter (D) — Translating Numbers Into Physical Design

Once the conveying air velocity is established, the required pipe inner diameter can be calculated using the following formula:

Formula: D = √(4 × Q(air) ÷ π × v)

Key Considerations:

• Air volume calculations do not account for system leakage — blower selection must include an additional 10%–20% margin above the calculated air volume
• For systems with conveying distances exceeding 80 meters, a tapered pipe design is recommended to maintain consistent air velocity throughout the entire line
• In multi-floor installations, vertical and horizontal pipe segments must be calculated separately, as they follow different design logic

5. System Pressure Drop (ΔP) — The Basis for Blower Selection

Total system pressure drop is the cumulative resistance the blower must overcome to maintain stable material flow. It consists of five components:

1. Pipeline friction losses — resistance from air and material moving through pipe sections
2. Acceleration losses — energy required to accelerate material to full conveying velocity
3. Local resistance losses — pressure drops at elbows, tees, reducers, and fittings
4. Auxiliary equipment resistance — contributions from filters and silencers
5. Separation equipment pressure drop — resistance from cyclone separators and dust collectors

Formula: ΔP(total) = ΔP(pipeline) + ΔP(acceleration) + ΔP(fittings) + ΔP(equipment)

Blower Selection Principle: The rated pressure of the selected blower should exceed the calculated total system pressure drop by a safety margin of 15%–20%, ensuring stable operation under full-load conditions and preventing surging.

Quick Reference: 5 Parameters at a Glance

Frequently Asked Questions

Q: What is the most common cause of blockages in central feeding systems?

The most frequent cause is an air velocity that falls below the material’s suspension velocity, allowing pellets to settle and accumulate inside the pipeline. This is often the result of an undersized blower or an incorrectly calculated pipe diameter.

Q: How do I know if my blower is powerful enough?

Calculate your total system pressure drop across all five components listed above, then ensure your blower’s rated pressure exceeds that value by at least 15–20%. If your system frequently surges under full load, the blower is likely undersized.

Q: Can the same pipeline system convey different types of plastic materials?

Yes, but the system must be designed around the material with the highest air velocity requirement. When switching materials with significantly different properties, conveying parameters may need to be adjusted accordingly.

Need Expert Support for Your System Design?

Effective central feeding system design requires a comprehensive evaluation of your facility’s floor plan, material properties, machine layout, and production schedule. Our engineering team provides end-to-end technical support — from initial parameter calculation to on-site commissioning.

Contact Us | Request a Free Consultation |