Processing Technology for Large-Diameter Industrial Polypropylene Duct

The processing technology for large-diameter PP air ducts (typically DN500 and above, especially DN1000+) differs fundamentally from small-diameter ducts — they cannot be directly formed by extrusion and must follow the PP sheet rolling + welding process route. The detailed processing flow is as follows:

I. Core Processing Workflow

1. Material Selection (Foundation for Large-Diameter Ducts)

Main material: PP sheets conforming to GB/T 12670 standard, with thickness determined by pipe diameter and wind pressure:
- DN500-800: 5-6mm thickness
- DN800-1500: 8-10mm thickness
- DN1500 and above: 10-12mm thickness
Auxiliary materials: PP welding rods of the same material, PP flanges, and reinforcing ribs (must be compatible with base material to avoid stress cracking)
Special requirements: Flame-retardant ducts require B1-grade flame-resistant PP sheets

2. Cutting and Precision Layout

Calculate sheet unfolding dimensions based on duct diameter (circumference = π × diameter + welding allowance)
Use CNC cutting machines for precise cutting with dimensional tolerance ≤ 2mm
Grind cuts into 30° V-grooves, remove burrs to ensure tight welding fit
Special fittings such as elbows, tees, and reducers require separate template layout

3. Rolling and Forming (Core Process for Industrial Polypropylene Duct)

Use three-roll plate rolling machines to gradually roll PP sheets into cylinders
Apply uniform force during rolling, repeatedly adjust roundness to avoid localized stress concentration
Ensure flush butt joints with misalignment ≤ 1mm
Diameters DN1200 and above require segmented rolling followed by assembly into complete pipes

4. Welding Technology (Key to Duct Quality)

Two main welding methods are used for large-diameter PP air ducts:

🔹 Hot Air Welding (Most Commonly Used)

Welding gun temperature controlled at 240-280°C
PP welding rod and base material heated and melted simultaneously, ensuring uniform welds without bubbles
Straight pipe joints use V-groove butt welding with at least 2 weld passes
Additional lap welding at internal seams for enhanced structural strength

🔹 Extrusion Welding (For Extra-Large Diameters)

Ducts DN1500 and above use extrusion welding guns for higher welding strength
Molten PP welding rod continuously extruded to fill weld seams for better integrity

Critical Welding Parameters:

Natural cooling for 24 hours minimum after welding, forced cooling prohibited
Avoid vibration during cooling to prevent stress cracking in welds

5. Reinforcement Treatment (Mandatory for Industrial Polypropylene Duct)

PP ducts ≥ 500mm diameter are highly prone to deformation and must be reinforced:

Flange reinforcement: Weld PP flanges at both ends of each duct section, serving as both connection structure and reinforcement rings
Intermediate reinforcing rings: Add 1-2 PP reinforcing ribs mid-section when pipe length exceeds 2 meters
Flange bolt spacing: ≤ 150mm to ensure connection sealing and structural rigidity

II. Fittings and Connection Methods

Elbow/Tee/Reducer Fabrication

Cut into segments according to angles (e.g., 90° elbows divided into 4-6 segments)
Heat and soften, then weld segment by segment into shape
Grind internal welds smooth to reduce wind resistance

Pipe Section Connection Methods

Hot Melt Butt Welding (Recommended for DN800 and below)
- Heating plate temperature: 210±5°C
- Welding pressure: 0.15-0.25MPa
- Heat absorption time: wall thickness × 10 seconds/mm
- Cooling time: wall thickness × 1.5 minutes/mm
Flange Connection (Mandatory for DN800 and above)
- PP flange butt joint with acid-alkali resistant rubber gasket in between
- Tighten bolts diagonally and evenly to prevent flange deformation
- Detachable design for convenient maintenance and cleaning