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11 Steps to Make Ductwork: Duct Manufacturing Process

Key takeaways:

1. Precision in measurement and preparation is critical in the fabrication of air ducts, as even small inaccuracies can lead to significant issues in fit and function, highlighting the need for meticulous attention to detail from the initial field measurement to the final cutting and assembly processes.

2. Material selection and handling techniques are tailored to the specific requirements of air duct construction, with different tools and methods recommended based on the thickness and type of material, ensuring structural integrity and proper fit during installation.

3. The construction of air ducts requires a sequential approach that integrates various specialized processes, such as leveling, sketching, cutting, and folding, with each step dependent on the successful completion of the previous one, emphasizing the importance of workflow efficiency and process mastery.
Making Metal Air Duct

1. Field measurement

Measure the size of the ventilation system at the installation site and turn the results into a sketch as the basis for producing the air duct.

  1. Take note of the following items during the measurement process:
  • Measure the distances between the ventilation system location and the column, partition walls, reserved holes, outer walls, and the height of the floor and ground to the roof.
  • Measure the thickness of the external wall, partition wall, size of the reserved holes, width and height of the doors and windows, cross-sectional size of the column, distance between the bottom of the beam and the flat roof, height of the platform, and other relevant information to the ventilation system.
  • Measure the size, position, height, and relative position of the production equipment, air duct equipment, and the connection ports for the ventilation components connected by the air duct.
  • Measure the size, height, and distance from the wall of the foundation or support for the ventilation equipment.

The specific content of the measurement will depend on the actual situation, and it is important to pay attention to the crossing and spacing of various pipes and electrical lines.

2. Actual sketching

Through the above work, draw processing installation sketches.


3. Sheet correction

(1) Steel coil leveling machines are commonly utilized to straighten coils through a series of repeated bends with multiple rollers.

(2) Usually, the flat plates are corrected for bending deformations using manual hammering correction methods.

If the sheet material is less than 0.8mm thick, a large, soft, flat-head wooden hammer with fast flattening and high efficiency should be used for hammering and smoothing.

If the thickness is ≥ 0.8mm, a steel flat-head hammer is recommended for smoothing.

Based on the unevenness of the sheet, the deformation characteristics such as warpage or unevenness should be identified and then the iron platform should be used for smoothing.

4. Underlining

Determine the thickness of the plate according to the design size of the air duct, select the number of bend pipes, and determine the interface mode. Use calculation and unfolding methods to cut the material, define the cutting line, and make the cutting marks.

5. Unfolding

(1) Select Appropriate Model Material:

When choosing the material for the model, it is important to pick something that is not too thick, with a thickness of 1 to 3mm being ideal. The material used should also be free from curling or deformation, with options such as kraft paper, linoleum paper, soft plastic sheet, and thin iron sheet being preferred.

(2) Calculate Appropriate Length of Sample Plate:

The length of the circular tube sample should equal the length of the pipe’s outer diameter plus the thickness of the sample material, multiplied by π. However, it’s important to note that the actual circumference of the pipe may differ from the calculated length due to seasonal and material influences.

For example, linoleum paper becomes harder in winter and may not fit tightly on the outer wall of the tube, requiring the model length to be increased. In the summer, it may become softer and stretch, requiring a reduction in the unfolded length of the model. It’s important to make these adjustments before drawing the expansion curve, as neither growth nor reduction can be carried out afterwards.

(3) Check Actual Volume of Review:

After creating the model, it’s crucial to check its shape and size by wrapping it on the outer wall of the pipeline and verifying the volume. The model should be close to the pipe wall, with the two ends meeting without any gaps or overlaps. There are three methods for expanding the model: parallel line expansion, radiation expansion, and triangle expansion.

6. Blanking


On the sheet material, mark the unfolding drawing and the clear outline of the blank size. Then, the next step of shearing can be done. Hand shearing is only suitable for steel sheets with a thickness less than 0.8mm, while the thicker ones are usually cut by machinery.

(1) Prior to cutting, it is important to precisely align the scribing line on the plate, and there must be a clear tangent mark on the steel plate to be cut. After cutting, check the size of the blank carefully before proceeding with further processing.

(2) After the cutting is completed, the steel plate should be held vertically and cut along the tangent line. During the cutting process, lifting the cut sheet upward with your hand can reduce the cutting resistance.

(3) When cutting curves, fold lines, and corners, avoid cutting the line marks on the sheet. To do this, the end of the scissors must align with the top of the corner and not be too far away.

(4) When cutting a hole, make a hole first, insert the scissors, and then cut counterclockwise along the line. If cutting a circle, use a curved scissors for smaller diameters and cut counterclockwise. For larger circles with a smaller margin, cutting clockwise is allowed.

(5) After completing the shearing of the sheet, be sure to use scissors or a chamfering machine to bevel the end of the sheet.

7. Closing of the air duct

Closing of the air duct

(1) Select the plate thickness according to the different specifications and sizes of the air duct, then leave a margin for unloading.

(2) The line drawing process must be precise to ensure straight angles, flat lines, and accurate measurements. Geometric sizes should be checked frequently and all necessary lines, such as cutting lines, chamfering lines, folding lines, flanging lines, hole lines, and closing lines, must be drawn.

(3) Cutting and chamfering must be done with accuracy to minimize errors. After cutting, the edges must be chamfered using a chamfering machine or iron scissors before closing the edges. Ensure that there is no overlap or flanging during the operation.

(4) The plate should be placed on the folding machine according to the drawn folding line and folded to the desired angle. During the operation, align the folding line with the upper and lower molds of the square folding machine.

(5) To create a round air duct, use a clapper to shape the edge into an arc, circle the bite, and adjust the arc to make it uniform.

(6) After folding or rounding the steel plate, use a seaming machine or manual seaming. Apply even pressure, avoiding applying too much force to prevent the seams from being uneven or bursting.

(7) The seams of the air duct plates should be staggered, and cross-shaped seams are not allowed.

(8) Common forms of seams:

  • A single seam can be used for splicing and closing circular ducts.
  • Corner seams, joint angle seams, and snap button seams can be used for rectangular air ducts or accessories.
  • Vertical seams can be used for round elbows.

Steel plate duct bite joint:

  • The thickness of the steel plate for making air ducts and accessories δ≤1.2mm can be connected by bite.
  • δ> 1.2mm should be welded.
  • Flange butt welding should adopt gas welding.
  • Galvanized mesh panels for making air ducts and accessories should be bite-joined or riveted.
  • Plastic composite panel air ducts can generally only use bite and riveting methods.To avoid burning the plastic layer by gas welding and electric welding, the bite machine must not have sharp edges to avoid scratches. If the plastic layer is damaged, it should be painted and protected in time.

Stainless steel plate duct bite joint:

  • The bite connection can be used when the wall thickness of the stainless steel plate air duct δ≤1mm.
  • δ> 1mm can use arc welding, argon arc welding, gas welding is not allowed.

The electrode should be of the same type as the base material, and the mechanical strength should not be lower than the minimum value of the base material.

Aluminum plate air duct bite joint:

  • When the wall thickness δ of aluminum duct and accessories is less than or equal to 1.5mm, it can be connected by bite.
  • δ> 1.5mm adopts gas welding or argon arc welding.
  • There should be no scratches on the surface of the aluminum air duct and accessories. When setting out, color pencils or colored pens should be used to set out.The bite or shaping of the air duct should be made of wooden hammer or wooden square ruler to avoid deformation of the bite seam.

(9) Bite Width and Quantity: The width of the bite is determined by the thickness of the air duct. Generally, for single flat bite, single vertical bite, and single angle bite, the width of the bite on the first plate should be equal. On the second plate, it should be twice as wide, so the allowance for the bite is equal to three times the bite width. The quantity of the bite should be left on both sides as required.

(10) Bite Processing: The mechanical bite processing mainly involves operating various bite machines. For curved lines or solid bites, wood square feet and wooden hammers should be used instead of a steel hand hammer to extend the edge of the board, in order to avoid visible marks. The joint of the bite should be tight and there should be no half bites or cracking. The joints of straight pipes should be staggered in the longitudinal bite seam, since the purpose of the air duct often involves elbows, tees, and so on. The round elbow consists of several short inclined tubes and the single bite is formed in one direction when making the elbow, meaning the bite seam of each section is in opposition. This is necessary for the production of elbows and is not limited by this regulation. The width of the bite seam should be uniform to prevent the phenomenon of a wide bite seam at one end and a narrow bite seam at the other, as this affects both the appearance and the firmness and tightness of the bite seam.

8. Welding form of air duct

Welding form of air duct

(1) Butt welding:

It is used for joining plates or for horizontal and vertical closed seams.

(2) Lap welding:

It is used for the longitudinal closed seams of rectangular ducts or pipe fittings, the elbows of rectangular ducts, corner joints of tees, etc.

The general overlap is 10mm, and the overlap area should be marked before welding. Spot welding should be done along the marked line and then the weld should be smoothed with a small hammer before continuous welding.

(3) Flange welding:

It is used for the closing of joints without flanges, round pipes, and elbows. When the sheet is thin, gas welding can be used.

(4) Fillet welding:

It is used for the longitudinal closed seams of rectangular air ducts or pipe fittings, the turning joints of rectangular elbows and tees, and the closed seams of round rectangular air duct heads.

(5) Carbon steel duct welding:

Carbon steel ducts should be welded using a DC welding machine. Before welding, the welding area should be cleaned of dirt, oil marks, and rust. When spot welding or continuous welding is used, oxides must also be removed. The gap should be kept to a minimum, and the nodules at the manual spot welding position should be removed promptly. After welding, the electrode slag and residual welding wire in the welding seam and nearby areas should be removed promptly.

(6) Stainless steel duct welding:

Before welding, the grease and dirt in the welding seam area should be cleaned to prevent air holes and sand holes in the welding seam. Cleaning can be done using gasoline or acetone. When arc welding stainless steel plates, white powder should be applied to both sides of the weld to prevent welding spatter from adhering to the plate surface. After welding, the slag at the welding seam should be removed and the metallic luster should be brushed with a copper wire brush, then pickled with a 10% hydrochloric acid solution, and finally washed with hot water.

(7) Aluminum air duct welding:

Before welding, the welding area must be degreased and the oxide film removed. A stainless steel wire brush can be used for this. Welding must be done within 2 to 3 hours after cleaning and degreasing treatment must also be done after welding. Degreasing can be done using aviation gasoline, industrial alcohol, carbon tetrachloride, or other cleaning agents and wood chips.

(8) Gas welding of thin steel plate ducts:

The direction of gas welding is generally from left to right. The direction of the flame should be mastered to ensure that the heat on both sides of the weld remains balanced. The flame should move forward smoothly and evenly, and the speed of the welding wire into the molten pool should be uniform.

(9) Weld quality requirements:

The surface of the weld should not have any defects such as cracks, burn-through, or missing welding. The longitudinal welds should be staggered. The welding seam should be smooth and spot welding should be symmetrically alternating during welding to prevent deformation. The width of the welding seam should be uniform. After welding, the weld should be cleaned to remove welding slag.

9. Flange production

Flange production

(1) The distance between the bolts and rivet holes on the air duct flange of the low-pressure system of the metal air duct should not exceed 150mm. The high-pressure system’s air duct should not be larger than 100mm. The four corners of the rectangular duct flange must have screw holes.

(2) The distance between the bolts and rivets on the flange for the low-pressure and medium-pressure system air ducts should be less than or equal to 150mm. The high-pressure system air duct should also be less than or equal to 100mm. The four corners of the rectangular flange must be reinforced with bolts or rivets.

(3) Round Flange Production:

The entire angle iron or flat iron is processed by rolling it into a spiral shape on a steel coiling machine. Then, the rolled steel strip is cut and drawn one by one on a platform for leveling and correction. After the adjustment, welding and drilling take place. The holes must be evenly distributed along the circumference to ensure that the flanges can be interchanged.

(4) Rectangular Flange Production:

The rectangular flange is made up of four pieces of angle iron. During marking and blanking, it is crucial to note that the inner edge of the flange after welding must not be smaller than the outer dimension of the air pipe and must be within the acceptable deviation value. Cutting and punching of angle steel must not be done using oxygen and acetylene cutting, and can only be cut using a material cutting machine or hand saw. The angle steel fractures must be smooth, and the burrs at both ends must be removed.

Then, welding takes place on the platform. The angle of the flange must be measured and adjusted after spot welding to ensure that the lengths of the two diagonals are equal. The screw holes’ location must be accurate to ensure a smooth installation of the air duct. The drilling method is the same as for the circular air pipe flange.

(5) Aluminum Plate Flange Production:

The aluminum plate flange is made from flat aluminum or angle aluminum. If angle steel is to be used instead of aluminum plate flange, insulation and anti-corrosion treatment must be done to prevent electrochemical corrosion of the aluminum plate air pipe after contact with the carbon steel flange. Typically, the surface of the angle steel flange is galvanized or sprayed with insulating paint.

(6) Flange and Air Pipe Connection:

When the air pipe and flange are connected using rivets, the riveting must be firm and without any leaks. The flanging must be smooth and close to the flange, with a width of not less than 6mm and without any cracks or holes in the bite seam and the four corners. When the air pipe and flange are connected through welding, the end face of the air pipe must not be higher than the plane of the flange interface.

The air duct of the dust removal system must have full inside welding and intermittent outside welding. The end face of the air duct must not be less than 5mm from the flange interface plane. If the flange of the stainless steel plate or aluminum plate air pipe is made of carbon steel, anti-corrosion treatment must be done as per the design requirements. The rivet must be made of the same material as the air duct or without electrochemical corrosion.

Quality Acceptance of Flange Production:

  • The welding seam of the air duct flange should be well-fused and without any false welding or holes.
  • The permissible deviation for flange flatness is 2mm.
  • The screw hole arrangements for flanges of the same specification processed in a batch must be consistent and interchangeable.

10. The air duct no-flange connected production

(1) Most circular air pipes use either direct socket connection or core tube connection. The core tube connection involves using the core tube as an intermediate connector, where two air pipes are inserted at both ends of the core pipe for connection.

The insertion depth must not be less than 20mm. Then, pull rivets or self-tapping screws must be used to fix the connection between the air pipe and the core pipe and seal the joint tightly with a sealant.

(2) The interface of the rectangular air pipe usually uses inserts, bites, metal spring clips, or mixed connections. The size must be accurate, the shape must be regular, and the interface must be tight.

11. Duct reinforcement

Air Duct reinforcement

(1) Reinforcement Techniques:

Joint height reinforcement technique (using a standing bite). Reinforcing the air duct with an angle steel ring around the circumference. Reinforcing the larger side of the duct with angle steel. Longitudinally reinforcing the inner wall of the air duct with ribs and reinforcing the steel plate of the air duct with rolled grooves or crimped ribs.

Requirements for Air Duct Reinforcement Quality:

The air duct must be firmly reinforced, and for it to be considered excellent, it must be neat.

The spacing between each reinforcement must be appropriate, uniform, and parallel.

(2) Forms and Requirements for Air Duct Reinforcement:

The air duct can be reinforced in the form of corrugated bars, standing bars, angle steel (for both internal and external reinforcement), flat steel (using vertical reinforcement), reinforcing bars, and internal tube support.

Refer to Figure

Reinforcement form of air duct

Figure Reinforcement form of air duct

(3) Reinforcement using corrugated bars or wires should be arranged in a regular pattern with uniform intervals, and there should be no obvious deformations on the surface of the duct.

(4) Angle steel and reinforcement ribs should be arranged neatly and symmetrically, with a height not exceeding the flange width of the air duct. The riveting of angle steel, reinforcement ribs, and air duct should be secure, with even spacing not exceeding 220mm, and the two intersections should be joined as one.

(5) Supports and air ducts should be fixed securely, with uniform spacing between each support point, or the edge or flange of the air duct, not exceeding 950mm.

(6) For medium-pressure and high-pressure system air duct sections with a length greater than 1250mm, reinforcement bars should also be used. The metal air duct of the high-pressure system should have reinforcement or reinforcing measures to prevent bursting at the single bite seam.

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2 thoughts on “11 Steps to Make Ductwork: Duct Manufacturing Process”

  1. We have a project to install duct making assembly line with all relevant options for the hotel industries.
    Appreciate if you send us the list required with your offer for our consideration. It is a turnkey key project including installation, commissioning & training of manpower on site with your personnel to be assigned for 3-6 months.

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