Sheet Metal Enclosure Design: Beyond the Box

Introduction

The enclosure is a silent hero in the product design. It is the main defense, support, and face of your product. A properly implemented sheet metal design demonstrates accuracy; however, attempting a design of a box that can be bent is only the ticket to entry.

A box is static, and a high-performance product is living.

The true test, the Beyond the Box philosophy, is the enclosure design that would guarantee that the components flourish. This implies a trade-off between manufacturability, cost, protection, and most importantly, thermal management. Effective sheet metal enclosures are based on the consideration of structural integrity and thermal requirements to achieve long-term success. This guide discusses the DFM principles to have your box right and then moves on to the Beyond factor: to transform that box into a high-reliability system, which shields components against trapped heat.

Material Selection: The Foundation of Your Enclosure

The choice of material affects the cost, weight, durability, and thermal performance. It is a balancing game, and the properties play a big part in the balance; metal selection is used. The right material will make your enclosure, depending on the design requirements, whether it is based on strength or cost-effectiveness, as well as give it durability and long serviceability.

• Carbon Steel (e.g., CRCA)
  • Pro: The workhorse. Inexpensive, strong, durable, and highly formable. Easy to weld and finish (powder coating). It is a good choice where heavy loads are to be discussed, and sheet metal parts are easily made.
  • Con: No natural corrosion resistance. Must be plated or painted to prevent rust.
• Stainless Steel (e.g., 304, 316)
  • Pro: Excellent corrosion resistance, ideal for medical, food-grade, or harsh environments (marine, chemical). 316 is superior for chlorides. Its aesthetic appeal also provides an important trait not only in functionality but also in outward appearance.
  • Con: Much more costly (2-3x carbon steel), more difficult to machine, and is a lower thermal conductor compared to aluminum.
• Aluminum (e.g., 5052, 6061)
  • Pro: The leader in lightweighting. Extraordinary weight-to-strength ratio and is inherently corrosion resistant. It is distinguished by high thermal conductivity, so the enclosure can be a passive heat sink. 5052 is quite good in forming, whereas 6061 is a higher-grade yet will be subject to cracking on small radius bends. The high strength of aluminum is an ideal material to use for enclosures, which will undergo heavy-duty performance requirements.
  • Con: It is even more expensive than carbon steel and may be more difficult to weld.
• Galvanized Steel (G90, etc.)
  • Pro: A low-cost compromise. It is Zinc-coated carbon steel, which is affordable in terms of corrosion resistance
  • Con: Zinc coating may be weakened at the welds. Not comfortable in extremely adverse conditions.

DFM Essentials: Designing for Manufacturability

It is pointless to have a beautiful design in your CAD software and not be able to make it efficiently. DFM (Design for Manufacturability) is the tendency to design parts in such a way that they are as easy and inexpensive to produce as feasible. The most common cause of project overruns and delays is forgetting these rules. Planned manufacturing processes at the time of design will save you needless costs and complexity.

Mastering Bend Radii and Setbacks

When metal bends, the metal on the outside of the bend stretches, and the metal on the inside compresses. This is physics, and you can not struggle against it.

• Bend Radius: The most perpetuated design mistake is that of specifying a sharp or zero-radius internal bend. This causes the material to stretch indefinitely, resulting in cracking and gross stress. To prevent this, one should pay attention to the flange length and provide sufficient space for bends.
  • The Golden Rule: The internal bend radius must not be smaller than 1/X material thickness (e.g., 1.5mm thick steel must not have an internal bend radius less than 1.5mm or more). A fabricator’s standard tooling radii (e.g., 1mm, 1.5mm, 2mm) are even better and do not require any setup cost.
• Setbacks (Bend Relief): You should consider the deformation of the material around the bend. When bending a part, be careful not to put a hole or cutout too near a bend line, or this will be drawn and distorted into an oval or teardrop shape. The type of cut method that you follow will affect this deformation and, therefore, the plan.
  • The Safety Rule: All features (such as holes or slots) must be at least 3 times the thickness of the material plus the bend radius distance away from the bend line. Should you require a feature nearer, you need to make a cutout of a relief in order to avoid the bending and tearing of the material.

Rules for Holes, Vents, and Cutout Spacing

There are also strict rules of features that are cut into the sheet metal so as to ensure structural integrity and to avoid damaging the tooling.

• Hole Diameter: The hole should not be made smaller in diameter than the thickness of the material. Attempting to drill a hole in 2 mm-thick steel, a 1mm drill is a recipe for broken tools.
• Hole Spacing: Intermediate spacing between two holes (edge to edge) must be at least half of the material thickness. Any nearer and the material between them may creep or strangle. Take into account the number of parts you require so that you do not cut or overlap unnecessarily.
• Edge Proximity: The distance between the edge of a hole and the outer edge of the part must be 2 times the material thickness in order to avoid the bulging of the edge.
• Vent Patterns: This is very important for air vents. The above considerations take precedence when formulating a perforation pattern (e.g, of a fan guard). In the event that the thickness of the “webs” between slots is too small, the stress of punching will cause the whole area to bend, which will result in loss of flatness and fan mounting.

Understanding Sheet Metal Gauge (Thickness)

A past number system of sheet metal thickness is called “gauge” (ga). It is counterintuitive: the lower the gauge, the higher the density of a metal and its strength as well as weight. The knowledge will assist in the sheet metal fabrication.

The 12-gauge sheet is thicker and stronger than the 20-gauge sheet. One of the most important trade-offs is the selection of a gauge.

• Too thin (e.g., 20-24 ga): cheaper and lighter; however, the enclosure will feel weak to hold heavy loads and will oil-can too.
• Too Thick (e.g., 10-14 ga): It is very durable and rigid, but heavy, costly, and more difficult to shape (larger bend radii are required). In large enclosures, it can also impact the ease of manufacturing since heavier metals are harder to handle.

The majority of electronic enclosures are in the 16-gauge to 18-gauge range, which provides a reasonable rigidity to cost.

Here are Common Gauge References for better understanding:

Structural Styles: Choosing the Right Enclosure Form

Although there are no limits to the custom design, most of the enclosures are based on variations of a few common topologies.

• U-Shape + Lids: This is probably the easiest and least expensive. It consists of one piece that is the U shape with the base and two sides. The box is then closed with lids (or covers) screwed on. It is good in the case of direct PCB mounting and easy to access, which is why it is a fairly common shape among various electronic products.
• Folded Box (Clamshell): This is a two-part design in which a top and bottom (or front and back) section meet mid-point. This is typical of small electronic goods, and good access is achievable, but the DFM of the mating flanges is required to be flawless, and reliability is assured, particularly in electrical enclosures.
• L-Shape: This resembles the U-shape, only that it may have a base and a single side, and other pieces compose the remaining. This is very tailored and is based on the access requirements of the components, which are commonly applicable in cases where special design deliberations are needed.
• Multi-Piece Assembly: Usual with complex rack-mount units. The front panel is made along with the rear panel, the base, the sides, and the top, and then fastened together using a number of small parts. This makes it easier to manufacture more complicated products and more difficult to assemble, and, in this case, surface finishes are necessary to protect these multi-piece enclosures.

Surface Finishes: Protection, Aesthetics, and Function

Raw metal is a product very seldom used. A finish is also necessary to protect as well as beautify, and even to enhance electrical character, particularly with electrical enclosures in need of greater performance.

• Powder Coating: A Finish that is the most commonly applied to steel and aluminum. Electrostatically applied and then hardened with heat, a dry powder paint creates a hard and durable overcoat that is much more resistant to wear and tear than liquid paint, as well as providing a protective layer to the electronic product as well and a professional appearance.
• Anode (Aluminum Only): This is an electrochemical process where the natural oxide layer of aluminum is thickened. It produces a highly non-conductive, corrosion-resistant, and hard surface. This finish may be dyed in any of several colors (clear, black, red) and is commonly applied to electrical enclosures that require a high level of corrosion resistance.
• Plating (e.g., Zinc, Nickel): This is a metallic overlay on the base metal. This is commonly done as corrosion protection (zinc on steel) or as a protection against good electrical conductivity and EMI shielding (e.g., chromate conversion coating), particularly important to electrical enclosures.
• Silkscreening: It is the method of printing logos, labels, and warning labels directly onto the finished surface. And this is necessary to make it a professional and complete product, and to put a finish on your enclosure that is both functional and pleasing to the eye.

Assembly Methods: Planning How It All Comes Together

Your enclosure is most likely to be constructed of several parts–or sometimes it may be cut out of a single sheet, which is only folded into position. Their joining method is also a major design consideration that influences the strength as well as serviceability.

• Welding (Spot or TIG): Produces a solid, strong, and continuous joint. It is great in terms of structural integrity and in making closed, waterproof seams. The negative side is that it is permanent (no service access) and needs to be processed after so as to make the welds smooth out before the finish of your enclosure is applied.
• Fasteners (Screws, Rivets): The most popular one. This makes it serviceable and disassembled. To make a professional of it, you simply do not drill a hole and insert a nut. Instead, you design for:
• PEM Inserts: (Stands, nuts, studs) This is pressed into the sheet metal to form robust and permanent threads on which PCBs, electrical components, or mating panels can be mounted.
• Tab and Slot: This is an excellent method in which a tab is used on one part to fit into the slots of another part. This is self-aligning, minimizes significantly the use of elaborate jigs in the welding process, and is a very good means of obtaining uniformity and alignment in the assembly process.

Creating your own enclosure or contracting someone to create it, the appropriate joining strategy will provide functionality and reliability without being reliant on the aesthetics or the maintainability.

The IP Rating Challenge: Sealing Your Box vs. Trapped Heat

You’ve done it. You’ve designed a perfect box. You have selected 16-gauge steel, a tough powder coating, and you have your PEM in mind. Now you have to shield the real world from the sensitive electrical components in it.

Here, IP (Ingress Protection) Ratings are involved. The enclosure may be IP65, where it is completely dustproof and may survive water jets of low pressure. Or comparable ratings, such as a NEMA rating, are used to establish the level of protection used in industrial environments to help guarantee that your design is up to its environmental standards. This is provided in gaskets, sealed seams, and waterproof connectors – all these factors influence the finish of your enclosure.

However, when you have solved the external problem (dust, water), you have made yourself a new, internal problem.

You have built a “sealed oven.”

All the internal components, including the power supply, CPU, drivers, and others, produce heat (W). This heat that is not allowed to escape will result in an increase in the internal ambient temperature (Tₐₘᵦᵢₑₙₜ). Each 10 °C (18 °F) increase in operating temperature reduces the lifetime of most electronics by one-half.

Your well-thought-out, IP6 5-rated box is currently throwing the product it was designed to safeguard to the ground. It is the Beyond the Box challenge.

For more knowledge of IP ratings vs NEMA ratings, check out our related blogs here!

The “Beyond” Factor: Active Thermal Management Design

Reliability is not an attribute but a design requirement. And thermal management is equivalent to reliability in a sealed enclosure. The right design tips in the enclosure design can greatly affect the air flow direction, heat release, and the lifetime of the system.

When Passive Cooling (Vents & Heatsinks) Isn’t Enough

Passive cooling depends on natural convection (rise of hot air) and radiation. This may be performed with plain vents or by the enclosure itself being a heat sink (as is common with aluminum). Passive heat dissipation can also be further enhanced by having a durable finish that is efficient in thermal conduction and preserves corrosion protection.

This is applicable to low-power devices (less than 15 W). However, when your power density is high, or when the ambient temperature at the external is large, passive cooling does not work. The heat becomes so saturated in the air that it cannot be moved by natural convection. Also, designers should be sensitive to Electromagnetic interference, which may escalate in case ventilation openings are not well designed

The Solution: Integrating Compact Fans for Active Airflow

Active Cooling is the solution. With the addition of a small AC or DC fan, the physics is altered. You bring about forced convection.

An enclosure cooling fan does two things:

1. It tears the stagnant “boundary layer” of hot air that sticks to components.
2. It establishes a pressure gradient, which causes the hot air to exit and the cool air to enter, establishing a constant exchange of air.

This is the only surest method of ensuring that component temperatures do not exceed their safe operating range. Electromagnetic interference can also be minimized with the help of a durable finish and a clever shielding design, among others, while still leaving the enclosure structure strong in the long run.

However, this brings about a new engineering challenge, which is, say, you are planning to have a 10-year life span of your enclosure, how about the fan?

Why ACDCFAN is the Engineer’s Choice for Enclosure Cooling

A low-cost fan is one of the single failure points that will destroy your whole system. This is the reason why the design of the enclosures and the choice of the fans need to be considered. ACDFAN focuses on high-quality fans that are designed to meet the requirements of contemporary enclosures.

• Enclosure-Grade Durability: Why should an IP-rated enclosure be vulnerable to the fan? We provide fans that have IP68 encapsulation, which makes them dust-proof and waterproof against hostile industrial conditions. They are constructed to withstand the elements as tough as your enclosure.
• Long-Term Reliability: An enclosure is a long-life item. Your fan must match it. Our fans are made with the highest quality ball bearings, and have a 70,000 or more hours Mean Time Between Failure (MTBF) or nearly 8 years of 24/7, non-stop operation.
• The Fan to Fit Any Design: We have the size to suit your design. We offer a small 25mm fan to a large 254mm fan, all fully certified (UL, CE, TUV, EMC) and RoHS 2.0 compliant.

Thermal strategy should not be an additional consideration. We will be able to collaborate on your design to submit a preliminary thermal proposal in 12 hours.

Conclusion: Design a Box That Truly Performs

A masterpiece sheet metal design is a work of balance. It is about the trade-off of cost and quality of a product, the I/O between form and functionality, strength and weight.

The foundation is the box as it is, the DFM, materials, and finishes. It allows you to build your product at a cost-effective level and ensures that the product will endure physical impact. Nevertheless, it takes more than just a little forethought to come up with the final fit to your design, but the key to long-term viability and performance is an insight into critical factors.

However, your product will be able to survive itself, and this is what the Beyond the Box thinking, which is the thermal management strategy, ensures. A good idea is essential when designing something that extends beyond the simple design to have a system that works when stressed, and not only looks nice.

Making these two philosophies go together, you are no longer a box designer but an architect of systems that are reliable and can withstand time. You create a finely dressed enclosure that not only looks pretty, but works, keeps safe, and lasts long enough – you turn your product into an ideal partner in any challenging surroundings.

Ready to build an enclosure that truly performs? Contact our engineers at ACDCFAN today to integrate a durable, smart cooling solution and get a preliminary proposal in 12 hours.