Round Flange Forming: Complete Guide to Angle Ring Flanges (2026)

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This guide covers everything you need to know about Round Flange Forming: Complete Guide to Angle Ring Flanges (2026). Key takeaways: detailed specifications, cost comparisons, pros and cons, maintenance requirements, and expert recommendations from Preda Machine — a leading HVAC duct machinery manufacturer since 2008, serving 60+ countries.

Types of Round Flanges

Round flanges are used to connect sections of spiral duct, attach duct to equipment (fans, filters, grilles), and provide a rigid, leak-free joint. There are three main types used in HVAC:

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Flange Type	Material	Pressure Rating	Cost	Common Use
Angle Ring Flange	Angle iron (L-profile)	High	Medium	Commercial/industrial spiral duct
Flat Steel Flange	Flat steel strip	Medium	Low	Residential, light commercial
Pressed Flange	Sheet metal (pressed)	Low	Low	Small diameter, low pressure

Angle Ring Flanges (Most Common)

Angle ring flanges are made by rolling angle iron (L-shaped steel profile) into a circle, then welding the joint. The flange is then welded to the end of the spiral duct.

Why Angle Ring Flanges Are Preferred

High strength — The L-profile provides rigidity in two axes
Excellent sealing — Flat mating surface for gasket compression
Easy to bolt — Standard hole patterns match across manufacturers
Wide size range — Available from 80mm to 2000mm+ diameter
Industry standard — Recognized by SMACNA, DW144, and other standards

Angle Iron Sizes

Duct Diameter	Angle Iron Size	Thickness
80-200mm	25×25×3mm	3mm
200-400mm	30×30×3mm	3mm
400-630mm	40×40×4mm	4mm
630-1000mm	50×50×5mm	5mm
1000-1600mm	63×63×6mm	6mm
1600-2000mm	75×75×8mm	8mm

Flat Steel Flanges

Flat steel flanges are made by rolling flat steel strip into a ring. They are simpler and cheaper to produce but less rigid than angle ring flanges.

Flat flanges are suitable for:

Low-pressure systems (under 500 Pa)
Small diameter duct (under 400mm)
Residential and light commercial
Budget-sensitive projects

For medium and high-pressure systems, angle ring flanges are always specified because the L-profile provides much better resistance to bending under pressure.

Hole Patterns and Punching

Flange holes must match between mating flanges. Standard hole patterns are defined by industry standards (SMACNA, DW144, Eurovent).

Duct Diameter	Number of Holes	Bolt Size	Hole Diameter
80-160mm	4-6	M6	7mm
160-315mm	6-8	M8	9mm
315-630mm	8-12	M8	9mm
630-1000mm	12-16	M10	11mm
1000-1600mm	16-24	M10	11mm
1600-2000mm	24-32	M12	13mm

Punching Before Rolling

It is much easier and more accurate to punch holes in the straight angle iron before rolling it into a ring. A flange punching machine automatically spaces holes at the correct pitch. After punching, the angle iron is rolled into a ring and welded.

Welding Flanges to Duct

Proper welding is critical for flange strength and air tightness. The flange is welded to the duct end using one of two methods:

1. Continuous Fillet Weld (Inside Only)

Weld the inside joint between the flange and duct continuously. This is the standard method and provides adequate strength for most applications.

2. Inside and Outside Weld (Heavy-Duty)

For high-pressure or large-diameter duct, weld both inside and outside. This provides maximum strength and prevents flange separation under pressure.

Welding tips:

Use MIG welding with ER70S-6 wire for galvanized duct
Use TIG welding with ER308L wire for stainless steel duct
Ensure full penetration — no gaps between flange and duct
Grind welds smooth on the mating face to ensure flat sealing surface
Use zinc-rich paint to repair galvanized coating damaged by welding

Safety Warning

Welding galvanized steel produces zinc oxide fumes that can cause "metal fume fever." Always weld in a well-ventilated area with fume extraction. Wear a respirator rated for metal fumes. For stainless steel welding, use shielding gas and ensure proper ventilation.

Connection Methods

Bolted Connection (Standard)

The most common method. Two flanges are bolted together with a gasket between them. Use:

Neoprene or EPDM gasket (3-5mm thick)
Galvanized or stainless bolts, nuts, and washers
Even torque sequence (tighten opposite bolts gradually)

Clamp Connection (Quick Assembly)

A ring clamp wraps around both flanges and tightens with a bolt. Faster than bolting but not suitable for high pressure. Common in residential and light commercial.

Van Stone Connection (Spigot and Ring)

The duct end is flared outward (spigot), and a loose flange ring slides over it. The spigot faces seal against each other. This allows the flange to rotate for easy bolt alignment.

Equipment for Round Flange Production

To produce angle ring flanges in-house, you need this equipment:

Round Flange Forming Line — Automatically punches holes and rolls angle iron into rings. This is the core machine for flange production.
Welding machine — MIG welder for welding the ring joint and welding flanges to duct.
Grinder — For smoothing welds and preparing mating surfaces.
Cutting saw — For cutting angle iron to length (if not using coil feed).

A round flange forming line typically produces flanges from 80mm to 1500mm diameter, with automatic punching at standard hole spacing. Production rate is 20-40 flanges per hour depending on size.

Standard Flange Specifications

Standard	Region	Flange Type	Key Feature
SMACNA	North America	Angle ring, flat	Pressure class based (A, B, C, D)
DW144	UK / Commonwealth	Angle ring	Class A, B, C based on pressure
Eurovent 2/2	Europe	Angle ring	Standardized hole patterns
GB 50243	China	Angle ring, flat	Based on duct diameter
AS/NZS 4254	Australia / NZ	Angle ring	Similar to SMACNA

Always check which standard your project specifies. Hole patterns differ between standards, and flanges from different standards may not bolt together.

Frequently Asked Questions

What is the standard thickness for angle ring flanges?

Standard angle iron thickness ranges from 3mm for small ducts (under 200mm) to 8mm for large ducts (over 1600mm). The most common size for commercial HVAC (300-800mm duct) is 40×40×4mm or 50×50×5mm angle iron.

Can I buy pre-made round flanges instead of making them?

Yes, pre-made flanges are available from suppliers, but they cost 2-3x more than in-house production. If you use more than 50 flanges per month, a flange forming line pays for itself within 6-12 months. Preda's round flange forming line starts at around $8,000.

How do I ensure flanges are round after rolling?

Quality flange forming machines roll with consistent pressure and produce round rings within ±2mm tolerance. After rolling and welding, check roundness with a tape measure across multiple diameters. If out of round, you can manually adjust on a mandrel before welding to the duct.

What gasket material should I use between flanges?

For general HVAC, use closed-cell neoprene rubber (3-5mm thick). For high-temperature applications (over 80°C), use EPDM or silicone. For food-grade applications, use FDA-approved silicone. Always use a continuous gasket with a single scarf joint — never multiple pieces.

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Forming Tolerances and How to Hold Them

Angle ring flanges are formed by rolling a length of angle iron to a diameter. The two defects to control are out-of-roundness and the gap at the closing weld.

Diameter	Out-of-round tolerance	Closing gap after cut	Pre-bend allowance
200 – 400 mm	±1.5 mm	2 – 4 mm	Cut long by 8 mm
400 – 800 mm	±2.0 mm	3 – 5 mm	Cut long by 10 mm
800 – 1200 mm	±2.5 mm	4 – 6 mm	Cut long by 12 mm
1200 – 1600 mm	±3.0 mm	5 – 8 mm	Cut long by 15 mm

Cut the blank long, roll the ring, then trim to the measured circumference before closing. Trimming after forming is what removes the cumulative error from the rolling pass — attempting to cut to final length first is the usual cause of an inconsistent gap.

Closing the Ring: Weld Sequence

Clamp the ring in a jig and check the diameter at four points 90° apart.
Tack weld at four points, equally spaced, working opposite sides in sequence.
Re-check the diameter after tacking; correct before final welding.
Weld the joint fully, alternating short runs to limit distortion.
Grind the weld flush on the gasket face.
Re-check flatness on the face — under 1 mm over 300 mm.

Distortion Is the Hidden Cost

Welding one side of a ring continuously pulls the diameter out of round by 2–4 mm on smaller sizes. Alternate short runs and let the ring cool between passes. A ring that is round but not flat will still leak, so check both.

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