helen@hbxinnuorollforming.com
Closed dovetail re-entrant floor deck cold roll forming production line
36-station progressive forming with specialized locking roller set
Concrete interlocks with dove-tail ribs — full composite without dimples
Twin 15KW motor drive, 450H frame, dual-cylinder shear, PLC control
A re-entrant deck — also called closed deck or dovetail deck — is a structural steel decking panel where the rib profile narrows at the opening, creating a geometric "trap" that mechanically interlocks with the concrete slab after pouring. Unlike open trapezoidal decks that rely primarily on dimple embossments for bond, the re-entrant shape itself provides mechanical interlock: concrete flows into the wide rib base, cures, and becomes physically locked inside the narrowed throat — similar to a dovetail joint in woodworking. This geometric bond is inherently more reliable than surface embossments alone, making re-entrant deck the preferred specification for long-span floors, seismic zones, and projects where composite action cannot be compromised. This machine is engineered specifically for the complex forming geometry that creates the re-entrant dovetail shape — a significantly more demanding roll forming process than standard trapezoidal deck production.
| Performance Factor | Open Trapezoidal Deck | Re-Entrant Closed Deck | Design Advantage |
|---|---|---|---|
| Concrete Bond Mechanism | Primarily dimple embossments on rib surfaces. Bond dependent on embossment quality and consistency. | Geometric interlock from dovetail shape plus embossments. Bond does not rely solely on surface features — the shape itself provides mechanical keying. | Re-entrant bond is inherently redundant — even if embossments degrade, the dovetail geometry maintains composite action. More reliable for critical structures. |
| Longitudinal Shear Capacity | Determined by m+k values from full-scale bending tests per EN 1994-1-1 Annex B. Typical m-values: 150-220 N/mm². | Higher m-values typical: 200-300 N/mm² due to combined geometric + embossment interlock. Requires separate certification testing. | 30-40% higher shear bond capacity for the same slab depth and gauge. Enables longer spans or thinner slabs. |
| Unpropped Span Capability | Typically 2.5-3.5m single span for 0.9mm gauge, 130mm slab | Typically 3.0-4.5m single span for same parameters. The re-entrant profile is stiffer in bending due to the closed rib shape. | 40-50% longer unpropped span. Fewer temporary props = faster floor cycles and lower construction cost. |
| Fire Resistance | Standard performance. Deck acts as permanent soffit but rib voids create paths for heat transmission. | Narrowed rib throat restricts hot gas circulation within the rib cavity. Typically achieves same fire rating with thinner slab or less reinforcement. | 10-15% slab depth reduction for equivalent fire rating. Saves concrete volume and dead weight on supporting structure. |
| Slab Depth Efficiency | Slab depth = structural requirement + fire cover. Rib contributes limited structural depth to composite section. | Slab depth can be optimized — re-entrant shape allows the concrete fill within the rib to contribute more effectively to the composite lever arm. | Thinner overall slab for same structural performance. Reduces building height, cladding area, and foundation loads. |
| Seismic / Cyclic Performance | Adequate for most applications. Bond degradation under cyclic loading is a known concern — dimples can elongate under repeated slip reversal. | Superior under cyclic loading. The dovetail geometry resists slip in both directions — bond degradation is significantly slower because geometric interlock does not "wear out" like surface embossments. | Preferred for high-seismic zones (Zone 3-4) where cyclic bond performance is critical to structural safety. |
| End Slip Resistance | Depends on end anchorage through-deck studs to prevent end slip failure mode. | Re-entrant shape provides inherent end anchorage — concrete wedge in dovetail resists pull-out even without studs at the deck end. | Reduced reliance on end anchorage detailing. Critical where construction errors omit specified end studs. |
| Profile Code | Coverage Width | Rib Configuration | Rib Depth | Coil Width | Gauge Range | Typical Application |
|---|---|---|---|---|---|---|
| CD-510 | 510mm | 2 dovetail ribs | 50mm | 750mm | 0.8-1.0mm | Light commercial floors, residential mezzanine |
| CD-600 | 600mm | 3 dovetail ribs | 50mm | 1000mm | 0.8-1.2mm | Office floors, retail, education buildings |
| CD-688 | 688mm | 2 dovetail ribs | 76mm | 1000mm | 0.8-1.5mm | Medium-span commercial, warehouse mezzanine |
| CD-750 | 750mm | 3 dovetail ribs | 53mm | 1000mm | 0.8-1.2mm | Apartment buildings, hotel construction |
| CD-820 | 820mm | 3 dovetail ribs | 55mm | 1250mm | 0.8-1.5mm | High-rise commercial core, long-span office floors |
| CD-870 | 870mm | 3 dovetail ribs | 60mm | 1250mm | 0.8-1.5mm | Parking structures, transfer slabs, heavy industrial |
| Forming Challenge | Open Deck Solution | Re-Entrant Deck Solution | Machine Design Requirement |
|---|---|---|---|
| Rib Shape Transition | Trapezoidal shape: ribs flare outward. Each forming station simply deepens the V-shape progressively. Straightforward roller geometry. | Dovetail shape: ribs start wide (like trapezoidal) then the rib opening must be folded inward. This requires a reversal of bending direction mid-way through the forming sequence. | Two-phase roller set: Phase 1 (rows 1-18) forms standard trapezoidal shape. Phase 2 (rows 19-36) folds rib walls inward to create the dovetail throat. Phase 2 rollers are more complex — they must support the rib from inside while pressing the walls inward from outside. |
| Material Springback Control | Outward-flared ribs: springback simply widens the rib slightly. Profile tolerances accommodate this. | Inward-folded ribs: springback wants to re-open the dovetail throat. If throat opens beyond specification, the geometric interlock with concrete is compromised. | Over-bending stations at rows 28-34: ribs are folded slightly past the target angle, then allowed to spring back to the correct geometry. Requires precise angle control at these stations — deviation of 1° changes throat width by 1.5-2mm. |
| Internal Roller Access | Open ribs are accessible from above. Rollers apply forming pressure from top and sides with straightforward tooling. | Dovetail throat is narrow — internal rollers that support the rib from inside must be compact enough to fit through the narrowed opening during forming. | Specialized small-diameter internal mandrel rollers on sliding tool posts. As the rib narrows, these rollers retract and withdraw. This is the most technically challenging station set — requires precision mechanical timing. |
| Station Count | 22-28 rows sufficient for trapezoidal geometry | 32-36 rows required. Extra stations handle the rib inversion: 6-8 additional stations just for folding the rib walls from outward-flared to inward-narrowed. | Longer forming bed — 36 rows requires approximately 18-20m of forming unit length. Larger factory floor space, longer drive chains, more mid-plate supports. |
| Material Thinning at Bend | Trapezoidal bends are gradual — material thinning at rib corners is within acceptable limits (typically <5% reduction). | Dovetail bend radius is tighter at the throat. Material thinning at the fold-over point can reach 8-12% if not controlled — weakens the panel at the interlock point. | Gentle bend radius design at dovetail throat — minimum inside radius = 2× material thickness. More stations with smaller incremental bends achieve the final geometry without excessive thinning. |
| Roller Set Component | Function | Design Detail | Failure Mode If Incorrectly Designed |
|---|---|---|---|
| Phase 1 — Trapezoidal Pre-Form Rollers (Rows 1-18) | Form the rib into a standard trapezoidal shape: wide base, outward-angled walls, flat crest. This establishes the internal rib volume that will later be re-shaped. | Standard progressive trapezoidal roller geometry. Rib base width and wall angle precisely calculated to leave correct material volume for Phase 2 folding. | If rib volume is undersized — dovetail walls are too thin, weak interlock. If oversized — excess material bunches during Phase 2 folding, causing surface wrinkles in the finished panel. |
| Phase 2 — Wall Inversion Rollers (Rows 19-26) | Begin folding the rib walls inward. Each station incrementally changes the wall angle from outward-flared (110-120° from horizontal) to vertical (90°) to inward-leaning (70-80°). | Split roller design: upper roller presses wall inward from outside; lower roller supports wall from inside. Gap between upper and lower rollers = material thickness + 0.05mm clearance. | Insufficient internal support: wall buckles inward instead of folding — creates a crease, not a smooth dovetail curve. Buckled wall has zero structural value and cracks under load. |
| Phase 3 — Throat Forming Rollers (Rows 27-32) | Establish the narrowed throat opening. Walls are folded past vertical to create the dovetail undercut. Internal mandrel rollers support the rib cavity to prevent collapse. | Mandrel rollers are spring-loaded — they expand to fill the rib cavity during forming, then compress to withdraw as the roller set opens at the end of each station. | Mandrel roller timing error: if mandrel doesn't compress before roller set opens, it tears the panel edge as the roller separates. This is the #1 cause of scrapped re-entrant panels during commissioning. |
| Phase 4 — Sizing & Lock Rollers (Rows 33-36) | Final stations lock the dovetail geometry. Over-bend slightly past target angle, then release to spring back to exact specification. Verify throat width, rib depth, and wall angle. | Four independent sizing stations, each with micrometer-adjusted roller position. Final station includes a throat width gauge roller — if throat is out of tolerance, the gauge roller flags the operator. | Over-bend too aggressive: throat permanently narrower than specification — concrete cannot flow into the rib cavity during pouring. Over-bend too gentle: springback opens throat — interlock is weak. |
| Component | Specification | Technical Rationale |
|---|---|---|
| Frame | 450H Laisteel I-beam, 12mm wing, full-length stress-relieved weldment | 36 stations on a longer bed = more cumulative forming force. Frame must resist bending and torsion over 18-20m span with zero measurable deflection under load. |
| Mid-Plate | 25mm CNC line-bored, 36 plates at 500mm centers | Increased plate count from additional stations. Line-bored to ±0.02mm position accuracy — essential because dovetail geometry requires tighter roller alignment than trapezoidal. |
| Shafts | Φ115mm from 100mm round bar, chrome plated 0.05mm | Larger diameter for 36-station torque distribution. Chrome thickness increased because internal mandrel rollers experience higher contact pressure. |
| Motor | 2 × 15KW (10#), 8-pole, TEFC, Class F | Dual motor required for balanced drive across long bed. 15KW per side because dovetail forming absorbs more power — folding walls inward requires more energy than flaring them outward. |
| Chain | 2.5-inch heavy series, 4-strand at primary | Heavier chain for higher forming torque. 36 sprocket pairs on longer chain run = more cumulative friction — larger chain compensates. |
| Mandrel Roller Mechanism | Spring-loaded cam-actuated retraction, timed to roller set position | Critical component — mandrel timing is mechanically linked to roller set rotation, not electronically. Mechanical timing cannot drift from PLC fault or encoder error. |
| Forming Speed | 8-12 m/min (slower than open deck) | More forming stations + more complex geometry = slower throughput. Speed is limited by Phase 2 wall inversion stations — these are the bottleneck in re-entrant production. |
| Step | Action | Check Point |
|---|---|---|
| 1 | With machine stopped and locked out, manually rotate the drive to bring Phase 3 station rollers to fully closed position | Mandrel rollers must be fully expanded — filling the rib cavity |
| 2 | Insert test strip (150mm wide × 200mm long × production gauge) into the station with rollers closed | Strip should be firmly gripped between upper/lower roller and mandrel roller — no looseness, no crushing |
| 3 | Continue rotating drive slowly until rollers begin to open | Observe mandrel retraction — it must begin compressing BEFORE the upper roller lifts away from the strip |
| 4 | Verify mandrel is fully compressed (flush with roller surface) before upper roller clears the formed rib | If mandrel is still protruding when roller clears: it will catch the rib edge and tear the panel. Adjust cam timing. |
| 5 | Repeat verification at Phase 3 stations 27, 28, 29, and 30 | Each station's mandrel timing is independently adjustable. All four must be synchronized to within 5° of roller rotation. |
| 6 | Run 10m of test material at slow speed (3-5 m/min) with operator observing each Phase 3 station | Watch for any panel edge lifting, clicking sounds, or surface marking — indicators of mandrel contact during roller opening |
| 7 | Increase to production speed. Run 50m. Inspect every panel for throat width (calibrated gauge), rib depth (depth micrometer), and dovetail wall angle (angle gauge) | All measurements must be within ±1mm / ±1.5° of specification. Adjust individual station roller gaps as needed. |
| Project Type | Why Re-Entrant Deck Is Chosen | Profile Typically Specified | Market Value Proposition |
|---|---|---|---|
| High-Rise Commercial Towers (30+ floors) | Reduced slab depth saves 50-80mm per floor × 30 floors = 1.5-2.4m building height reduction. This saves millions in cladding, vertical services, and foundation cost. | CD-820, 0.9-1.0mm gauge | Premium product — high-rise developers will pay for the depth saving. Your margin is higher than open deck on a per-m² basis. |
| Hospital & Healthcare Facilities | Strict vibration criteria (ISO 10137) for operating theaters and imaging suites. Re-entrant deck's higher composite stiffness reduces perceptible floor vibration. | CD-688 or CD-750, 1.0mm gauge | Healthcare projects have higher budget tolerance and longer design timelines. Engineers specify re-entrant deck early — you're locked into the specification, not competing on price. |
| Seismic Zone 3-4 Buildings | Building codes (IBC, Eurocode 8) require demonstrated cyclic performance for composite diaphragms. Re-entrant deck with dovetail interlock outperforms open deck in cyclic shear testing. | CD-820, 1.0-1.2mm gauge | Regulatory requirement creates mandatory demand. If local code requires cyclic-certified composite deck, your open-deck competitors cannot bid. |
| Multi-Storey Parking Structures | Heavy vehicle loads (5.0 kN/m²), de-icing salt exposure, long spans (5-6m between columns). Re-entrant deck handles all three challenges better than open deck. | CD-820 or CD-870, 1.2-1.5mm gauge | Parking structures are repetitive — once you're approved for one, the same specification repeats for the developer's next 5-10 projects. |
| Data Centers & Server Floors | Heavy equipment loads (server racks 8-12 kN/m²), vibration control for sensitive equipment, raised floor integration. Re-entrant deck provides stiffer platform. | CD-750 or CD-820, 1.0-1.2mm gauge | Data center construction is booming globally. Project sizes are large (10,000-50,000m² per building) — volume opportunity is significant. |
| Transfer / Podium Slabs | Transfer slabs carry concentrated column loads from the tower above. Heavy reinforcement and thick concrete. Deck acts primarily as formwork but must be robust enough for heavy rebar cages. | CD-820 or CD-870, 1.2-1.5mm gauge | Highest gauge = highest margin per panel. Transfer slab projects consume large deck quantities in a single pour — efficient delivery logistics are valued. |
| Selling Point | How to Communicate to Engineer | How to Communicate to Contractor | Supporting Evidence to Provide |
|---|---|---|---|
| Thinner Slab = Lower Building Cost | "Your floor-to-floor height can reduce by 50-80mm per level while maintaining the same structural performance and fire rating. Over 30 floors, that's real money." | "Lower floor height means less concrete, less rebar, shorter columns, less cladding. The deck costs more per m² but the total project saves money." | Comparison calculation: slab depth × number of floors × cost per linear meter of facade + column concrete savings. Show the math. |
| Longer Unpropped Span | "Your beam grid can be wider — fewer secondary beams per floor. Re-entrant deck spans 3.5-4.5m unpropped vs. 2.5-3.0m for open deck. Fewer beams = lower steel tonnage." | "Fewer props needed during construction. Less time setting and removing props. Faster floor cycle — structural steel savings cover the deck premium." | Load table comparison: standard span tables for equivalent gauge open deck vs. re-entrant deck, showing the prop-free span difference. |
| Cyclic Seismic Certification | "If your project is in Seismic Zone 3 or 4, the code effectively requires composite deck with certified cyclic bond performance. We have that testing — open deck typically does not." | "Code compliance is non-negotiable. This deck meets the seismic requirements that standard deck can't certify. No alternative product to compare against." | Full-scale cyclic test report from accredited laboratory. Seismic certification letter from registered structural engineer. |
| Fire Rating Without Added Concrete | "Re-entrant deck achieves R90 with 130mm slab where open deck requires 150mm for the same rating. The dovetail geometry restricts hot gas circulation in the rib cavities." | "Thinner slab at same fire rating means less concrete to pump, less dead load on structure, faster pour per floor. The fire test report is available for submission to the building authority." | Fire resistance test report per ISO 834 or ASTM E119. Fire engineering assessment from accredited fire consultant. |
| Changeover Step | Time Required | Tooling / Consumables | Risk Point |
|---|---|---|---|
| Remove open deck Phase 2 rollers (rows 19-30) | 1.5-2 hours (2 technicians) | Lifting sling for roller sets; roller storage rack; anti-rust spray for stored rollers | Mislabeling rollers during removal — re-installing in wrong station order damages the tooling on first production run |
| Install re-entrant Phase 2 wall inversion rollers (rows 19-26) | 1-1.5 hours | Complete Phase 2 roller set pre-mounted on dummy shafts; installation torque wrench; feeler gauge set | Roller gap setting errors — each station has a specific gap for the target material gauge. Wrong gap = profile distortion or material thinning |
| Install re-entrant Phase 3 throat forming rollers (rows 27-32) | 1-1.5 hours | Phase 3 roller set with integrated mandrel rollers; cam timing alignment tool; spring tension gauge | Mandrel timing misalignment — the critical failure point. Must be verified slowly with test strip before production speed is attempted |
| Install re-entrant Phase 4 sizing rollers (rows 33-36) | 30-45 minutes | Phase 4 sizing roller set; throat width gauge (dedicated to each profile); micrometer depth gauge | Over-bend adjustment at stations 33-34 — too much over-bend permanently narrows the throat. Start conservative, measure, then adjust incrementally |
| Full dry run — no material, full speed | 10 minutes | None — observe chain movement, listen for abnormal noise, check all rollers rotate freely | Any roller binding indicates incorrect installation. Stop immediately, identify, and correct — do not run material through a binding station |
| Material trial — 10m at slow speed (3 m/min) | 15 minutes | 10m of production gauge coil | Operator must observe every station during this slow run. Stop if any panel marking, edge lifting, or abnormal forming behavior is observed |
| Full panel inspection — dimensional check | 20 minutes | Throat width gauge, depth micrometer, angle gauge, straight edge, vernier caliper | All measurements must pass specification before production speed is authorized. Reject panel must be measured and cause of deviation identified before adjustment |
| Production speed ramp-up — 50m at 6 m/min, then 50m at 10 m/min | 30 minutes | 50m of production coil at each speed | Panel quality must remain consistent as speed increases. If quality degrades at 10 m/min, reduce to 8 m/min and investigate — production at reduced speed is better than scrap panels |
| Total Changeover Time | 5-6 hours | Plan for one full shift dedicated to changeover. Schedule at end of week or during planned maintenance window. Do not attempt changeover during active production demand. | |
Hebei Xinnuo is a professional cold roll forming machinery manufacturer with 20+ years of experience. We specialize in C/Z purlin machines, roof panel roll formers, floor deck machines, and custom profile solutions — exported to 50+ countries worldwide.
