For overseas wholesale buyers, the fastest route to predictable quality is not just choosing a capable factory—it is writing a precise RFQ that anticipates how the part will be formed. Hem selection sits at the center of that decision. The correct approach increases yield, passes compliance checks, and shortens ramp‑up; the wrong approach triggers cracking, schedule slips, and rework that quietly erodes margin.
Most purchasing teams already know what is sheet metal, what is sheet metal fabrication, and what their end market expects. What they need at quoting time is a clear, supplier‑agnostic way to connect material, thickness, bend radius, and hem profile with cost and risk. This article does exactly that. It explains what a sheet metal hem is, why hem types matter in fabrication, and translates shop-floor realities into buyer language so you can specify hems that run reliably at scale—whether you source from a sheet metal fabrication company, a sheet metal contract manufacturer, or a multi‑plant network of sheet metal manufacturing companies.
Sheet Metal Hemming Fundamentals for Procurement Teams
Hemming is the act of folding the edge of a sheet back onto itself. Compared with a flange, a hem doubles the edge thickness and eliminates sharp edges. For buyers, the key is not the textbook definition but the effect on risk and landed cost. A hem modifies bend allowance, tool selection, and tonnage, and it influences how coatings build up at the edge. Those variables flow directly into price, takt time, and yield.
When you brief suppliers, align on three ideas: the functional reason for the hem, the acceptable geometry, and the verification method. Function covers safety, stiffness, and fit. Geometry covers length, return gap, and minimum inside radius. Verification covers what will be measured during first article inspection and in‑process quality checks. This shared vocabulary turns an abstract request for custom sheet metal parts into a controlled process in a modern sheet metal works environment.
A quick note on materials. Steel and sheet metal alloys—mild steel, galvanized, 304/316 stainless, and aluminum—behave differently near the bend line. Aluminum, especially in higher strength tempers, is prone to cracking if the radius is too tight; stainless will work‑harden and may spring back more than low‑carbon steel. These behaviors are the reason hem type is not a stylistic choice. It is a forming strategy that must match your alloy, your thickness, and your tolerances across all precision sheet metal works.
The Four Primary Sheet Metal Hem Types—and How Buyers Should Specify Them
Closed Hem
A closed hem folds the edge flat against the parent sheet. It delivers a safe edge and a compact silhouette that resists handling damage. Buyers choose it when the edge will be touched frequently, when aesthetics matter, or when the hem must nest closely with mating parts in an enclosure.
From a sourcing standpoint, confirm that the press brake and tooling can achieve the minimum inside radius for your gauge without micro‑cracks. Closed hems demand more tonnage than open hems because the material is flattened. Vendors should report any pre‑flatten bend and final flattening sequence in their control plan. In high volumes, die hemming can shorten cycle time, but the up‑front tooling cost must be weighed against MOQ and forecast.
Situations where closed hems shine include medical device housings, premium appliance panels, and consumer‑facing covers. If you are buying for automotive sheet metal assemblies, use closed hems where the edge may contact technicians or end users during service. Add measurement points for flatness and reveal in the PPAP or FAI checklist to avoid late surprises.
Teardrop Hem
A teardrop hem intentionally leaves a small cavity at the fold, shaped like a droplet. The form reduces strain at the bend line and protects brittle or work‑hardened materials. Procurement teams often specify this option for aluminum panels, where alum sheet metal gauges above 2.0 mm would otherwise risk edge cracking during flattening.
Ask suppliers to state the target teardrop diameter and the relationship between diameter and material thickness. This is where precision sheet metal forming experience shows: the shop should provide a standard chart or a proven rule of thumb (for example, a diameter near 1.5–2.0× thickness for specific alloys). Teardrop is common in automotive sheet metal parts—hoods, doors, liftgates—because it controls cracking while maintaining a neat edge that assembles cleanly.
When you see production quotes that look unusually aggressive, check whether the vendor quietly swapped a closed hem for a teardrop. The part may pass a quick visual check but fail durability testing. Your RFQ should lock the profile, not just say “hem as needed.” In global sourcing, clarity prevents the “sheet metal sheet metal” problem—vague specs that force a factory to guess.
Open Hem
An open hem leaves a designed gap between the return and the base sheet. It is the fastest and least demanding to form, making it attractive for large runs and for basic sheet metal projects where the edge requires safety but not maximum rigidity. Open hems can also double as functional features: drainage paths, cable routes, or ventilation slots inside an enclosure.
For purchasing, the risk is assuming open hems always cost less. If your drawing demands very tight gap control or long contiguous hems on thin gauges, handling and flattening can slow the cell. Align on the acceptable gap, and ask whether the factory will use a hemming die or a two‑hit brake sequence. In climates with salt spray or chemical exposure, specify post‑process sealing or a protective finish so the open cavity does not trap moisture.
Open hems are common in energy storage cabinets and HVAC components. When you request a sheet metal instant quote, include environmental details and required ingress protection so the supplier can advise the best combination of hem type, sealant, and coating.
Rolled Hem
A rolled hem curls the edge into a rounded bead. It is the most visually refined option and the safest to touch. Buyers use it on display fixtures, retail furniture, and any product where the edge is part of the brand experience. The trade‑off is tooling and cycle time. Rolled hems may require dedicated rolls or special dies not found in every sheet metal fabrication shop.
If your forecast is large enough to justify tooling, rolled hems can be surprisingly economical per piece, especially when the design replaces additional deburring or edge trim. In automotive interiors and appliance trims, rolled hems help designers maintain consistent radii across assemblies. During sourcing, ask vendors to submit sample beads and cross‑sections, and specify the acceptable bead diameter and concentricity. Those data points separate precision sheet metal fabricators from general job shops.
Material, Thickness, Radius—The Trio That Governs Hem Choice
Material Behavior and Buyer Risk
Different alloys change where the risk lives in your order. Aluminum cracks when radii are too tight; stainless fights you with springback; galvanized steel wants surface protection on fresh edges; copper and brass mark easily but form beautifully. If your end market is sensitive to cosmetic issues—consumer electronics, healthcare, or architectural components—choose hem types that minimize post‑polish rework. That safeguard is as important to margin as the headline unit price from a sheet metal factory.
Thickness, Gauges, and Forming Force
As thickness increases, the required bend radius increases and so does forming force. Buyers sometimes forget that press brake tonnage is not an endless resource, especially at the extremes of bed length. Ask for the machine list with force and daylight so you do not design a hem that the cell cannot physically close. In automotive sheet metal, this is routine; make it routine in your RFQs too.
Bend Radius and Return Length
Hems need a practical return length to flatten cleanly. A common starting point is a return length at least 4× the material thickness for closed hems, but experienced shops may advise longer returns for tough alloys. Your acceptance criteria should state inside radius, return length, and flatness after forming. That moves debate off the shop floor and into the drawing, where it belongs.
Reference Table — Typical Starting Points
| Parameter | Mild Steel (1.0–2.0 mm) | Aluminum 5xxx/6xxx (1.0–2.5 mm) | 304/316 Stainless (0.8–2.0 mm) |
|---|---|---|---|
| Min inside radius for closed hem | ≈ 1.0× t | ≈ 1.5–2.0× t | ≈ 1.5–2.0× t |
| Teardrop diameter | n/a | ≈ 1.5–2.0× t | ≈ 1.5× t |
| Typical return length | ≥ 4× t | ≥ 4–6× t | ≥ 5–6× t |
Note: Values are starting guidance. Confirm with forming trials and supplier standards before locking print.
Processes, Throughput, and Cost—Choosing How the Hem Will Be Made
Press Brake Hemming
Press brake hemming uses a two‑stage sequence: a pre‑bend to a target angle, then a flattening hit. It excels in flexible production where order quantities vary and where part families share features. From the buyer’s perspective, it reduces tooling cost and supports engineering changes. The trade‑off is cycle time and operator dependence. For high‑mix programs, it remains the most versatile option in sheet metal shops worldwide.
Die Hemming
Die hemming brings speed and repeatability. A matched die set forms and closes the hem in a controlled motion, delivering consistent results and narrow tolerance bands. Up‑front cost is higher, but the payback is strong above certain volumes. During quoting, request a cost curve that shows piece price against expected volumes so finance can see the crossover where tooling investment becomes sensible for your program.
Roller Hemming
Roller hemming shapes the edge with a rolling head that follows a programmed path. It is common in large contoured panels—think automotive closures—where a straight brake hit cannot follow complex geometry. If your design requires a long curved edge with a consistent profile, ask whether your supplier maintains a cell for roller hemming and how they inspect bead quality along the path. This is a hallmark capability for precision sheet metal fab providers who serve transportation and appliance customers.
Process Comparison — Buyer View
| Method | Tooling Cost | Cycle Time | Best Fit |
| Press brake | Low | Medium | Low‑to‑mid volumes, design agility |
| Die hemming | Medium/High | Fast | Stable designs, mass production |
| Roller hemming | Medium | Medium | Curved edges, automotive closures |
Writing a Hem‑Smart RFQ and Acceptance Plan
Specify Function, Not Just Geometry
State why the hem exists: safety edge, stiffness, clearance, or cosmetic finish. That tells the supplier which failure modes to prioritize. For example, an energy storage enclosure may value drainage and corrosion resistance more than an ultra‑flat edge.
Lock the Profile and the Proof
Include profile type (closed, teardrop, open, rolled), material and temper, inside radius, return length, and the inspection method. For first article approval, list measurable features: gap for open hems, teardrop diameter, bead concentricity for rolled hems, and flatness window. Attach any coating spec, because paint or powder build affects edges.
Align Capacity and Lead Time
Ask for the available bed length, tonnage, and die library. Confirm the plan for jigs or fixtures and how the supplier balances throughput when multiple long hems appear on one panel. Details like these matter more to real‑world delivery than a generic claim of all precision sheet metal works capability.
Template — Hem Requirements Snapshot for RFQ
| Field | Buyer Entry |
| Hem type | Closed / Teardrop / Open / Rolled |
| Material & thickness | e.g., 5052‑H32, 2.0 mm |
| Inside radius | e.g., 2.0 mm |
| Return length | e.g., ≥ 10 mm |
| Critical dimension control | Flatness ≤ 0.3 mm over 300 mm |
| Inspection | Go/No‑Go gauge + caliper points A/B/C |
| Finish | Powder coat RAL 9005, 80 µm total |
| Packaging | Edge protectors + desiccant, export carton |
Failure Modes, Cost of Poor Quality, and How to Avoid Them
Edge Cracking and Micro‑Fractures
Most cracking happens when the radius is too tight for the alloy and gauge, or when a closed hem is forced on a part better suited to teardrop. Preventive actions include specifying minimum radius by material, using proper grain direction on cut blanks, and confirming that press brake tonnage and daylight are sufficient.
Springback, Oil Canning, and Flatness Drift
Stainless and high‑strength steels spring back after forming. Long hems can introduce “oil canning” waves on flat panels. Your acceptance plan should call out flatness zones and allow vendors to recommend stiffening features if cosmetics matter. Precision sheet metal fabricators will share historical data to predict problem zones before you cut tools.
Coating Build and Edge Performance
Powder and paint accumulate at edges. In rolled hems, the bead geometry can amplify this. Coordinate with the finishing line on target film thickness and cure schedule. When quoting, ask suppliers how they mask or support hemmed edges during finishing to avoid rubs or sags. These details are routine for a seasoned sheet metal fabrication shop, but they must be in your print to be enforceable.
Applications and Mini Case Studies with Buyer Takeaways
Automotive Export Components
An OEM sourced aluminum door skins with closed hems to save cost on tooling. Cracks appeared during pilot builds. Switching to a teardrop hem stabilized forming and reduced scrap by double digits. The lesson for buyers in automotive sheet metal: tool savings rarely offset yield loss. Lock the hem type based on alloy, not convenience.
Energy Storage and Industrial Enclosures
Battery cabinets for outdoor use needed safe edges plus ventilation. Open hems created airflow paths and allowed quick forming. The buyer added a sealant spec and a salt‑spray requirement in the RFQ. The result was fast throughput and consistent IP performance without redesign late in the schedule.
Medical Device Casings and Hospital Equipment
For high‑touch surfaces, closed hems avoid edge injuries and support hygienic design. The procurement team required documented capability studies for hem flatness and edge radius, which reduced incoming inspection time and sped up ramp‑to‑volume. This is the practical side of E‑E‑A‑T in manufacturing: experience translated into measurable controls.
Retail Fixtures and Display Systems
Rolled hems lifted the perceived quality of a global retail program and eliminated a separate plastic edge trim. The buyer approved bead samples upfront and locked bead diameter tolerance. The consolidated process saved assembly labor and avoided a second supplier, stabilizing the supply chain for peak season.
Search Behavior, LSI Phrases, and How to Use Them Naturally
Many procurement professionals begin with broad checks—what is a sheet metal, what is sheet metal made of, or how a sheet metal fabrication processes sequence will affect tolerances—before narrowing to a shortlist of sheet metal shops. Others compare a sheet metal fabrication company with a contract manufacturer to decide whether to centralize or split awards. Throughout this article we have used the same phrases you search, from precision sheet metal fab to standard sheet metal works, to ensure that guidance maps to your discovery journey.
If you run vendor surveys, include questions about basic sheet metal projects that reveal a shop’s baseline discipline, then move to complex items like automotive sheet metal parts or curved rolled hems. This progression protects programs from over‑promising. It also helps a buyer distinguish between a general metal works sheet metal provider and a precision sheet metal fabricators group with proven SPC and PPAP practices.
Extended FAQ for Professional Buyers
What is a sheet metal hem?
A sheet metal hem is the process of folding the edge of a sheet back onto itself to create a safe edge, improve strength, or prepare the part for assembly. Common types include closed hem, open hem, teardrop hem, and rolled hem.
How do I translate hem choice into a price‑risk view?
Request two quotes: one with your preferred hem and one with the nearest alternative. Compare piece price, predicted scrap, and cycle time. Often a teardrop hem carries a slightly higher piece price but lowers total cost by improving yield.
Can I switch hem types after pilot builds?
Yes, but only if the geometry and function allow it. For example, moving from closed to teardrop is common on aluminum when cracks appear. Lock changes before hard tooling. Update control plans and acceptance criteria so the supply base can adjust quickly.
What documentation should I expect from a capable supplier?
A clear process flow, a control plan with hem inspection points, machine list with tonnage and bed length, and FAI/PPAP evidence. Reputable vendors—whether a single sheet metal fabrication shop or a multi‑site group—provide these without prompting.
Do I need to worry about grain direction?
Yes. Aligning the hem with or across grain affects cracking risk and cosmetic read‑through. Specify grain direction on flat patterns when you release drawings.
Where does automation make sense?
Roller hemming cells shine on long, curved edges in transportation and appliance panels. Die hemming is the mass‑production workhorse. Press brakes win on flexibility. Choose based on geometry and volume, not fashion.
Conclusion and Next Step
Hem selection is a small line in a big RFQ, but it drives quality, schedule, and cost more than most buyers expect. Specify the profile with purpose, connect it to material and radius, and ask your suppliers to prove capability before you scale. That is how procurement teams turn forming details into durable advantages.
If you want help translating your drawing set into a hem‑ready RFQ, YISHANG can review your target market, material plan, and forecast to recommend a profile and process that run cleanly at volume. Share your part file and we will suggest a path forward—no pressure, just pragmatic engineering support.
