A closed hem in sheet metal is a 180-degree returned edge pressed close to the parent sheet. It hides the cut edge and can add local stiffness, but its feasibility depends on alloy, temper, thickness, rolling direction and tooling. A sheet metal seam, by contrast, joins two separate sheet edges.
Closed, open, teardrop and rolled hems are edge treatments; Pittsburgh, standing, S-seam and flat-lock patterns are folded joints. The comparison below answers which pattern fits an edge-safety, stiffness, appearance or joining requirement and what a fabricator needs to check before quotation.
Closed-hem quick answer — Use a closed hem when a low-profile returned edge is functional and the selected sheet has enough ductility for the forming sequence. Open or teardrop hems reduce closing strain and are often safer starting points for harder tempers and prefinished sheet. Pittsburgh lock and S-seams are folded joints used in duct and enclosure work. Final geometry still depends on grade, temper, thickness, grain direction and available tooling.
Sheet metal seam and hem types at a glance
| Type | Primary purpose | Common applications | Main design risk |
|---|---|---|---|
| Closed hem | Hide a raw edge and add local stiffness | Doors, covers, cabinet panels | Cracking in hard or low-ductility material |
| Open hem | Create a safer edge with lower forming strain | Handles and exposed panels | Gap variation along the edge |
| Teardrop hem | Fold aluminum or harder material without flattening | Aluminum covers and visible panels | Inconsistent inside opening |
| Rolled hem | Produce a smooth rounded edge | Thin covers and decorative parts | Tooling access and diameter control |
| Pittsburgh lock | Join long sheet sections without continuous welding | Ducts and plenums | Poor lock engagement or leakage |
| Standing seam | Join panels while adding a raised stiffener | Large housings and architectural panels | Height and straightness variation |
| S-seam | Slip two sheet edges together | Duct and light enclosure joints | Pullout under load |
| Flat-lock seam | Create a low-profile folded joint | Covers, trays and light-gauge assemblies | Multiple forming operations |
The search term sheet metal seams covers both folded joints and edge treatments. A hem folds one sheet edge back onto itself; a seam joins two separate sheet edges. That distinction should be clear on the drawing because the tooling, allowance and inspection points are different.
For production examples, folded edges commonly appear on custom sheet metal boxes and trays and drawers and slide-out trays. Those product pages show how the seam or hem decision fits into a complete fabricated assembly rather than an isolated test coupon.
Four sheet metal hem types
1. Closed hem (flat hem)
A closed hem returns the edge through approximately 180 degrees and closes it against the parent sheet. The operation normally starts with an acute pre-hem before the return is flattened in a second forming step. The achievable gap and bend profile are process outputs; “closed” should not be interpreted as a guaranteed zero gap or mathematically sharp inside radius.
Useful starting applications: doors, covers, cabinet panels and other parts that need a low-profile protected edge or local stiffness.
Supplier review is critical when: the alloy or temper has limited ductility, the sheet is thick or prefinished, the bend runs in an unfavorable rolling direction, or the appearance specification restricts tooling marks.
On the drawing, identify the return length, edge function, permitted gap or flushness, cosmetic side and inspection points. A note that says only “closed hem” does not define those acceptance requirements.
2. Open Hem
Same fold as a closed hem, but with a deliberate gap between the hem and the parent surface — typically equal to material thickness. Formed at 145° during the first operation, then closed with a shim or controlled die.
Why bother?: The gap relieves stress at the bend, allowing materials that would crack in a closed hem to form successfully. The gap also creates a finger-safe edge for consumer products.
Best for: Materials that crack in closed hems — harder steels, some aluminum alloys, prefinished material.
Typical applications: Handle edges, consumer appliance panels, parts that contact users.
3. Teardrop Hem
The flange is bent 180° but without flattening against the parent — instead forming a teardrop or D-shaped cross-section. Inside diameter equals material thickness.
Best for: Aluminum (especially 6061-T6, 5052), brittle alloys, prefinished or coated materials that would crack under closed-hem pressure.
Typical applications: Automotive aluminum body panels, aluminum enclosures, premium consumer electronics housings, anything that needs aluminum’s appearance without aluminum’s hemming problems.
Cost effect: A teardrop hem may require different tooling or an extra setup. Ask the supplier to separate tooling, setup and cycle-time effects instead of applying a fixed percentage premium.
4. Rolled Hem (Curled Hem)
A full 360° loop curl that creates a smooth, round edge. The edge wraps completely back to (or past) the original surface.
Best for: Thin sheet (≤ 1.5 mm), decorative edges, smooth-touch applications.
Typical applications: Decorative panels, lighting fixtures, kitchen equipment edges, food-service products. Common in copper and brass where aesthetics matter.
Hem Design Rules
Rule 1: Start the return length review around 4 × thickness
Four times the material thickness is a useful early DFM check, not a universal production minimum. The required return is controlled by the hemming tool, V-opening, material, part access and whether the operation uses a press brake or dedicated hemming die. Short returns can be difficult to locate and restrain during forming.
| Material Thickness | 4T starting value for supplier review |
|---|---|
| 0.5 mm | 2.0 mm |
| 0.8 mm | 3.2 mm |
| 1.0 mm | 4.0 mm |
| 1.5 mm | 6.0 mm |
| 2.0 mm | 8.0 mm |
| 2.5 mm | 10.0 mm |
| 3.0 mm | 12.0 mm |
Values in this table are screening dimensions only. A supplier may accept a shorter return with dedicated tooling or require a longer return for a standard press-brake setup. Put the functional edge requirement on the drawing and confirm the producible return during DFM.
Rule 2: Treat thickness ranges as process-dependent
Closed hems are common in light-gauge sheet, but there is no grade-independent thickness window that guarantees a good result. Very thin sheet can distort or mark; thicker and higher-strength sheet needs more force and may crack during the 180° operation. Tooling and material temper matter as much as nominal thickness.
For a thicker edge treatment, compare an open hem with alternatives such as a deburred edge, welded cap, separate trim or machined feature. The correct choice depends on the safety, appearance, stiffness and sealing requirement.
Rule 3: Teardrop inside diameter = thickness
An inside diameter near the material thickness is a common starting geometry for a teardrop hem. Do not freeze that value before the supplier checks ductility, bend radius and tooling clearance.
Rule 4: Hem allowance calculation
The following relationships are useful for early layout, but the production flat pattern should come from the selected forming method and a validated trial part:
| Hem Type | Material Allowance |
|---|---|
| Closed hem | 2T + R (R = inside radius, typically 0) |
| Open hem | Closed hem + 1.5-2T (for the gap) |
| Teardrop hem | 3-4T (for the loop geometry) |
| Rolled hem | 3.14 × ID + T |
Springback compensation is not a universal percentage. Validate it with the actual material lot, tool radius and forming sequence, then control the production flat pattern through revision management.
Rule 5: Direction of hem relative to grain
For rolled sheet, bend orientation can affect available ductility. Where the design allows it, forming across the rolling direction is often the less demanding orientation. A coupon bend is the practical way to verify a borderline material rather than assigning a fixed cracking multiplier.
For more on bend design rules, see our press brake bending defects guide and 17-point DFM checklist.
Material-Specific Recommendations
This is the most common source of hem failures — specifying a hem type that the material can’t handle:
| Material | Closed Hem | Open Hem | Teardrop Hem | Rolled Hem |
|---|---|---|---|---|
| Mild steel sheet | Common candidate; confirm grade and thickness | Common candidate | Possible | Common in suitable gauges |
| HSLA steel | Higher cracking and force risk | Lower-strain starting option | Lower-strain starting option | Tooling review required |
| Stainless 304 / 316 | Often feasible; finish and work hardening matter | Often feasible | Possible | Tooling and finish review required |
| Galvanized steel | Coating-damage review required | Often safer for the coating | Possible | Coating and tooling review required |
| Aluminum 1100 / 3003 | Temper-dependent candidate | Common candidate | Common candidate | Possible in suitable gauges |
| Aluminum 5052-H32 | Cracking review required | Often safer starting option | Often safer starting option | Tooling review required |
| Aluminum 6061-T6 | High cracking risk; do not assume feasibility | Supplier and coupon review | Supplier and coupon review | Specialist review |
| Aluminum 7075-T6 | High cracking risk | Specialist review | Specialist review | Specialist review |
| Copper C11000 | Temper-dependent candidate | Common candidate | Possible | Possible in suitable gauges |
| Brass C26000 | Temper-dependent candidate | Common candidate | Possible | Possible in suitable gauges |
Aluminum review point: Do not release a closed hem in 6061-T6 or another low-ductility temper without a supplier review and a representative coupon. An open or teardrop profile is usually a safer starting point, but the alloy, temper, thickness and grain orientation still need confirmation. See our 6061 vs 6063 aluminum comparison for more on aluminum formability.
Seams: When Hems Aren’t Enough
When you need to join two sheet metal parts without welding, seams do the work. Common in HVAC, ductwork, large enclosures, and historical metalwork.
5. Pittsburgh Lock Seam
The dominant seam in HVAC rectangular ductwork. One panel has a flange folded into a U-shape (the “pocket”); the mating panel has a single flange that locks into the pocket. Once mated and hammered closed, the joint is mechanically secure and air-tight (with sealant).
Best for: Square/rectangular ducts, large boxes, panels that need to be assembled on-site, weld-free construction.
Typical applications: HVAC ducts, large electrical enclosures, agricultural equipment shrouds, dust collection systems.
Sheet thickness: 0.5 - 1.5 mm typical.
6. Standing Seam
Two adjacent panels each have a vertical flange. The flanges meet and are clipped, crimped, or hemmed together creating a raised “standing” joint.
Best for: Metal roofing, architectural cladding, large panels that need expansion accommodation.
Variants: Single-lock standing seam (1 fold, weather-resistant), double-lock standing seam (2 folds, fully sealed), snap-lock standing seam (mechanical click for tool-free assembly).
7. S-Seam (Slip Joint)
One panel has an S-shaped pocket; the mating panel slides into it. Common in lighter-duty HVAC where Pittsburgh would be overkill, and in field-assembled round/oval ductwork.
Best for: Field assembly of round/oval ducts, removable joints.
8. Flat Lock Seam
Both panels have small interlocking hems that flatten together. Creates a clean flat joint without raised standing seam profile.
Best for: Architectural metal cladding where aesthetics matter, copper roofing, gutters and downspouts.
Common Hemming Defects (and How to Prevent Them)
Defect 1: Edge cracking
Cause: The hem geometry or forming sequence demands more ductility than the specified alloy, temper and grain orientation can provide. Prevention: Review the material condition and bend direction, then compare a closed hem with an open or teardrop section. Validate a high-risk combination with representative material and production-intent tooling.
Defect 2: Springback / hem won’t close
Cause: Material has high yield strength; spring-back exceeds hem allowance. Prevention: Establish compensation from a trial bend using production material and tooling. For high-strength material, review the forming method, tonnage and surface requirements before selecting bottoming or coining.
Defect 3: Visible surface marks on parent panel
Cause: Press brake tooling contacts cosmetic surface during hem closing. Prevention: Specify “Side A cosmetic” on drawing. Use polyurethane-faced tooling or sacrificial film during forming.
Defect 4: Hem dimensions inconsistent along length
Cause: A return that the selected tool cannot locate consistently, tool deflection, part-length variation or an uncontrolled first-bend angle. Prevention: Confirm the minimum return with the chosen tooling, control the pre-hem angle and measure variation along the full formed edge.
Defect 5: Coating damage (galvanized, pre-painted)
Cause: Hem closing forces crack zinc or paint coating at the bend. Prevention: Use open hem instead of closed. Touch-up coating after hemming if closed hem is required.
For more on bending defects, see our press brake bending defects guide.
When to Use Hems vs Alternatives
| Need | Best Solution |
|---|---|
| Eliminate sharp edge for safety | Rolled or open hem |
| Double-up material for stiffness | Closed hem (steel) or teardrop (aluminum) |
| Hide raw cut edge for aesthetics | Closed hem for flush look |
| Join two panels without welding | Pittsburgh lock or S-seam |
| Architectural finish on roofing | Standing seam or flat lock |
| Sealed joint for ducts | Pittsburgh lock + sealant |
| Trim line for car body panels | Teardrop hem (aluminum) |
| Thick plate (> 3 mm) edge finish | Machined edge + edge cap, not hem |
Use this table as a first-pass process choice, not as the complete joint specification. If the connection must be permanent or transfer load, compare the folded seam with the access, coating and inspection limits in the spot-welding sheet metal guide. When several formed panels, fasteners, gaskets or purchased components must fit together, define the joint sequence and finished-interface checks within the sheet metal assembly service scope.
Specification Checklist
Before sending an RFQ that involves hemming:
- ✅ Hem type specified by name (closed, open, teardrop, rolled) — not just “hem this edge”
- ✅ Return length dimensioned and confirmed against the supplier’s tooling
- ✅ Inside opening or radius specified for open and teardrop hems
- ✅ Cosmetic side identified (“Side A visible” notation)
- ✅ Material grade compatibility verified against the recommendations table
- ✅ Tolerance class on overall part dimension (account for hem allowance)
- ✅ Coating sequence specified (hem before coating, or coating before hem)
For complete pre-RFQ rigor, see our 17-point DFM checklist.
What to Send for a Hem or Seam DFM Review
A useful review starts with the actual manufacturing inputs, not only a screenshot of the folded edge:
- 3D CAD and a controlled 2D drawing
- material grade, temper, thickness and finish condition
- cosmetic side and protected-surface requirements
- hem or seam section dimensions, including any required gap
- edge safety, stiffness, sealing or joining function
- production quantity and acceptable inspection method
- coating sequence and any restriction on witness marks
The forming review should confirm tool access, a workable return length, likely surface contact and the inspection datum. For duct seams, it should also identify the sealant and leakage requirement. See our press brake bending services or send the controlled files for review. Feasibility and inspection scope are confirmed against the submitted geometry before quotation.
Closed-hem design bottom line
A closed hem is a useful low-profile edge treatment, not a grade-independent default. Select it only after the edge function, material condition, return length, permitted gap, cosmetic surface and tooling access have been reviewed together. Use an open or teardrop profile when the closing strain or surface risk makes a flat hem unreliable.
For a production decision, validate the flat pattern and acceptance points on the first article. If the hem belongs to a larger formed enclosure or assembly, send the controlled CAD and drawing for a press-brake DFM review so the edge, nearby features and inspection method can be quoted as one process.
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