Insights · Duct Standards
Search this question and you will find two different answers, one of which is wrong. The common claim is that TDF is integral and TDC bolts on. It is not true. TDC and TDF are both parent-metal flanges, rolled from the duct's own sheet. The difference between them is which way the hem is turned, and that single detail decides edge safety, flange stiffness and how many forming stations your machine needs.
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.
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.
Both profiles finish the flange with a hem, a folded-back edge. The profiles differ in which way that fold goes.
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.
Turning the hem inward takes more work, and the machine shows it. Published machine data puts the TDF profile at 16 forming stations, with TDC produced on 12 or 16 stations depending on the machine. A flange former built for TDC at 12 stations cannot simply be switched to TDF, so the profile decision is partly a machine decision made at purchase.
| 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 |
| Forming stations | 12 or 16 | 16 |
| 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.
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 12-station TDC former is a cheaper entry, and it is genuinely 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.
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 →
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.
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.
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.
Published machine data puts TDF at 16 stations, and TDC at 12 or 16 depending on the machine. The extra stations turn the hem inward, so a 12-station TDC former cannot be switched to TDF.
35 plus or minus 0.5 mm on both models. The SBTDF-12 runs 0.5 to 1.2 mm sheet on a 2.2 kW motor; the SBTDF-1.5-16 runs 0.8 to 1.5 mm on 4 kW. Flange height is fixed by tooling.
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