Molded halves
- Precise mounting features and joint faces
- Even section under close control
- Needs a seam that is joined and sealed
Molded part guide: ducting
Injection Molded Ducting
Ducting carries air from one place to another around everything else in the way. That makes it long, curved, thin, and usually the last part anybody has room for. Molding it well is about filling a long thin shell all the way to the end, keeping it straight, and joining sections that never quite line up.
Long and thin
A long shell is a long way for melt to travel
Air ducts, defrost ducts, blower outlets, vent tubes, and cable ducts.
Air ducts have a large surface and a thin section, which is a hard combination. The melt has to travel a long way through a thin space, cooling as it goes, and if it freezes before reaching the far end the duct comes out short. More than one feed point shortens the distance, at the cost of the flow joints that form where the fronts meet.
Staying straight is the next issue. A long curved shell cools on the outside of the curve and the inside at different rates, and the part relaxes into a slightly different shape than the drawing. On a duct that has to slip into a fixed opening at both ends, a small bow at the middle turns into a part that will not fit at all.
Ducts are also usually split, because a closed tube with bends cannot be formed by cores that pull straight. Two half shells are molded and then joined, which brings the same problem a manifold has: a long seam that has to seal. Air leaks matter less than a fluid leak, but a leaking duct is noisy and sends air where it is not wanted.
Joints between sections are designed to move. Ducts run between parts that shift with temperature, vibration, and assembly tolerance, so a slip joint with a soft lip or a foam seal lets the sections move without opening a gap. Inside, sharp turns and steps make turbulence, which is heard as rush noise, so the inside surface is kept smooth and the turns gentle.
Features
Duct features and what each one is for
Filling, fit, and noise are the three constraints.
| Feature | Purpose | Design note |
|---|---|---|
| Long thin shell | Carries the air | Feed points placed to reach both ends |
| Split halves | Allow bends and turns | Joined by welding or by fasteners |
| Slip joint | Absorbs movement between sections | Soft lip or foam so the joint stays closed |
| Smooth inside | Keeps rush noise down | Gentle turns with no steps at the joints |
| Mounting tabs | Holds the duct in place | Slotted so the duct is not forced straight |
Filling
A thin duct is a long way for melt to travel
The melt cools as it flows, and a long thin path can freeze before the end.
- A large surface with a thin section cools the melt quickly as it travels
- If the melt freezes early the duct comes out short at the far end
- More than one feed point shortens the distance each front has to cover
- Every extra feed point creates a flow joint where two fronts meet
Making the duct
Molded duct halves or blow molded duct
Blow molding makes hollow shapes in one piece. Molding gives precise detail.
Blow molded duct
- Hollow and seamless in one piece
- Handles complex bends easily
- Less control of section and of detail
Drawing
What to send with a duct
The route and the openings at each end set the shape.
Duct CAD with both end interfaces
Airflow and pressure through the duct
Space available along the route
Movement expected between the parts it joins
Noise target and temperature range
Expected annual volume
Keep reading
Related guides
Air parts, thin sections, and the shorts that end a long fill.
Air vents
Vanes and pivots at the outlet.
Air ventsFan blades
What pushes the air along.
Fan bladesManifolds
Split construction and welded seams.
ManifoldsFilter housings
Filters in the air path.
Filter housingsShort shot
When the far end does not fill.
Short shotThin wall design
Filling long thin sections.
Thin wall designPolypropylene
A common duct resin.
PolypropyleneIndustrial and transportation
How we support industrial programs.
Industrial and transportationMolded parts hub
Every part-type guide, grouped by family.
Molded parts hubFAQ
Questions about molded ducting
Why does the far end of our duct come out short?
The melt froze before it arrived. Adding a feed point closer to that end, easing the section, or raising the melt temperature all help, and the first two are usually the durable fix. Checking the fill early, before the tool is cut, shows exactly how far the melt gets and where a second feed point belongs.
Why does the duct not fit between its two openings?
A long curved shell usually relaxes a little as it cools. Balanced cooling and a holding fixture reduce the movement, and slotted mounting tabs absorb what is left. A duct that has to reach two fixed openings is usually checked in a fixture that holds both ends at their real positions.
Why is the duct noisy?
Rush noise comes from turbulence at sharp turns, steps at joints, and edges sticking into the flow. Smoothing the inside and easing the turns is usually enough. Even a small step where two sections meet is enough to be heard once air is moving quickly through it.
How are duct halves joined?
Ultrasonic welding for a permanent joint, or clips and screws where the duct has to open. Air joints are more forgiving than fluid joints, but the seam still needs a continuous fit.
Next step
Quote a duct along its route
Upload the duct and both end interfaces. DFM feedback covers feed points for a long fill, straightness, joint design, and inside smoothness.