What the letters mean
TDF is Transverse Duct Flange. TDC is Transverse Duct Connector. Transverse means the joint runs across the duct, end to end, as opposed to the longitudinal seam that runs along its length. Both names describe the same basic idea: instead of attaching a separate flange to the end of a duct section, the flange is roll-formed out of the duct's own metal, so duct wall and flange are one continuous piece. The trade calls this a parent-metal flange.
SMACNA catalogues the two profiles separately: TDC is T-25a and TDF is T-25b. If a specification names a T-25 profile, that is what it is pointing at.
The misconception worth clearing up
A lot of published material says TDC is an add-on flange fixed with bolts, screws or rivets. That description belongs to a different product: the companion angle flange, where a separate steel angle is attached to each duct end. TDC is not that. TDC is rolled from the duct wall in the same way TDF is. If you are comparing a parent-metal flange against a bolt-on angle, you are having the TDF-versus-angle-flange conversation, not the TDF-versus-TDC one.
The actual difference: hem direction
Both profiles finish the flange with a hem, a folded-back edge. The profiles differ in which way that fold goes.
- TDC (T-25a) turns the hem outward. The raw cut edge of the sheet ends up on the outer face of the flange.
- TDF (T-25b) turns the hem inward. The raw edge is folded back into the profile, so the outside face is a clean fold.
Two consequences follow. The first is handling: an exposed raw edge on the outside of a flange is the edge installers grab when they lift and align a section, which is why the inward-hemmed TDF is generally regarded as the friendlier profile on site. The second is stiffness: folding the metal back into the profile puts a double thickness where the flange takes its bending load.
What it costs you in machine terms
Turning the hem inward is a different forming sequence, so the roll tooling on a flange former is built for one profile or the other. A former tooled for TDC is not switched to TDF without a different roll set, which makes the profile decision partly a machine decision made at purchase. Station counts vary between makers and models and are not a reliable shorthand for the profile, so confirm in writing which SMACNA profile (T-25a or T-25b) the rolls produce.
| TDC | TDF |
|---|
| SMACNA profile | T-25a | T-25b |
| Flange source | Parent metal, rolled from the duct wall | Parent metal, rolled from the duct wall |
| Hem direction | Outward, raw edge exposed | Inward, raw edge enclosed |
| Roll tooling | Rolls set for T-25a | Rolls set for T-25b (inward hem) |
| Edge handling | Raw edge is a site handling consideration | Folded edge, generally preferred to handle |
| Corner pieces | Four corners per joint, bolted | Four corners per joint, bolted |
Everything downstream of the profile is the same in both cases. The joint is closed with four corner pieces and a gasket, and it is clamped rather than screwed through the duct wall, which is what makes the parent-metal family tighter than slip-and-drive on medium pressure work. How tight is a question of sealing rather than profile, and that is covered in duct air leakage classes and SMACNA seal classes.
Which one should a shop build
For most fabricators the honest answer is that the profile follows the market they sell into rather than a technical superiority argument. TDC and TDF are both accepted, both appear in SMACNA, and both make a sound medium-pressure transverse joint. If your specifications name a profile, build that. If they do not, TDF's enclosed edge is the reason it tends to win when a shop is choosing freely, and it is the profile Taokron builds machines for.
Where the decision does bite is second-hand machinery. A used TDC former is a cheaper entry, and unless its rolls are replaced it is limited to TDC. Buying it because the price is right and discovering later that a client base specifies TDF is a common and expensive surprise. This is one of the checks in the duct machine buyer's checklist.
The Taokron TDF machines
Taokron builds the TDF profile on two TDF flange forming machines. Both roll a 35 plus or minus 0.5 mm flange height, which is set by tooling rather than adjusted per job.
| Model | Sheet thickness | Motor | Weight |
|---|
| SBTDF-12 | 0.5 to 1.2 mm | 2.2 kW | 760 kg |
| SBTDF-1.5-16 | 0.8 to 1.5 mm | 4 kW | 910 kg |
The flange former is one station in a TDF workflow. The cleat cutting machine produces the closure cleats, the duct zipper closes the longitudinal seam on heavier gauge, and on a full auto duct line the flange is formed inline as part of the coil-to-duct sequence. Full numbers for every machine in the group are in the TDF flange machine specifications.
Ask which TDF configuration fits your duct schedule →
FAQ
What does TDF stand for?
Transverse Duct Flange. The flange is rolled on the end of the duct from the duct's own sheet, so flange and duct wall are one piece. SMACNA designates it T-25b. TDC is Transverse Duct Connector, T-25a.
What is the difference between TDC and TDF?
Which way the hem is turned. TDC turns it outward and leaves the raw edge exposed on the outer face. TDF turns it inward, enclosing the raw edge and stiffening the flange. Both are rolled from the duct wall, so the difference is not integral versus bolt-on.
Is TDC a bolt-on flange?
No, and this is the most common error in print. TDC is roll-formed from parent metal exactly like TDF. The bolt-on system is a companion angle flange, a separate steel angle attached to the duct end, which both TDC and TDF replace.
Can a TDC flange former make TDF?
Not without a different roll set. The inward hem on TDF is a different forming sequence, so a flange former's rolls are built for one profile. When buying a new or used former, confirm in writing which SMACNA profile (T-25a TDC or T-25b TDF) its rolls produce.
What is the flange height on a TDF machine?
On Taokron's TDF flange forming machines the formed flange height is 35 plus or minus 0.5 mm. Sheet range is 0.5 to 1.2 mm on the SBTDF-12 and 0.8 to 1.5 mm on the SBTDF-1.5-16. Flange height is fixed by the tooling, so it is a machine specification rather than something set per job.
