Considerations for designing sheet metal enclosures

Custom-designed sheet metal enclosures can offer a low-cost, fast-time-to-market solution for housing electronics. In times where sourcing components has become no trivial matter, local options are available and capable of providing quality solutions.

Here are six things to consider when selecting or designing for a sheet metal enclosure:

1. Why use sheet metal?
2. Which material to use?
3. Which shape best suits the application?
4. Dimensional tolerances
5. Finish options
6. What assembly and testing will be required?

Consideration 1: why choose a sheet metal enclosure?

When is it best to use sheet metal to enclose your equipment? This is fundamentally linked to your application and the outcomes you desire.

Custom-designed enclosure manufacturing with sheet metal is usually reasonably priced because it does not require large volumes to be carried out cost-effectively. Sheet metal can be cut by fast methods such as turret punching or by laser cutting where the metal is melted by a precision laser.

The decision to use punch or laser revolves around whether you require formed cut-outs or irregular shapes. Many manufacturers have options to do both.

Form punched part
Laser-cut sheet metal parts	Sheet metal enclosures

If any of the following statements are true for your application, then a sheet metal enclosure should be considered:

• Custom design is required without high investment in tooling.
• The development process needs to allow for low-cost alterations to the enclosure design.
• Enclosure is required to provide robust protection and good mechanical stability.
• The application is for challenging environments (eg, high chemical and temperature resistance).
• Application requires EMC screening and conductive connections for earthing.

Consideration 2: sheet metal materials

The type of sheet metal material and its thickness defines the best method for designing and manufacturing an enclosure solution. The two most common sheet materials used for enclosures aresteel and aluminium.

Steel

An alloy of iron made by adding small amounts of carbon and other elements to make suitable for forming and fabrication. Typically, the types of steel used for enclosure production are:

• Mild steel: Natural mild steel is generally not suitable for wet or high-corrosion environments. Surface treatments such as paint or heavy treatments like zinc anneal or galvanising can help. It is important to be aware of the environment if this material is to be used.
• Stainless steel: Grade 304 can be considered the most common grade of stainless. Grade 316 offers vastly superior corrosion resistance to chlorides and acids and so is better suited to applications in corrosive environments or medical enclosures where solvents and heavy cleaning is required

Aluminium

Compared to mild steel, aluminium is lighter and corrosion resistant even when unfinished. Although not as strong as stainless steel, it is much lighter and a better thermal and electrical conductor. Aluminium is more reactive than stainless steel so not suitable for some chemicals and food environments.

• 5005 and 5052 alloy is the most used for enclosures in Australia. Both offer good weldability, forming performance and corrosion resistance in marine atmospheres. 5052 is much stronger than 5005 but has slightly better electrical conductivity. Both are good choices for an enclosure.
• Harder, more corrosive resistant 6061 can be used for enclosures but care must be taken with forming as it 6061 will crack if the bend radius is too tight.

Consideration 3: shape of enclosure

It is important to think of the shape of the enclosure at the system design and PCB level process to avoid some common pitfalls. When compared to a moulded plastic enclosure, sheet metal is limited. However, it is possible to design cost-effective, functional and aesthetically appealing enclosures from any of the following common shapes.

Folded box shape Classic four-side box plus a lid Easy access for service Wide range of mounting options
U shape Simple enclosure to produce Solid base Easy access for service
L shape Simple to produce like U shape but more suitable for PCBs with connectors on opposing sides

Consideration 4: dimensional tolerances

Sheet metal enclosures require cut-outs and folds. Things to consider when specifying dimensional tolerances are:

• Keep your tolerance as wide as possible to allow some flexibility in tooling.
  • Unless critical, avoid tight clearance around I/O components and fold to fold.
  • If possible, provide symmetric tolerances for all clearances.
  • Highlight critical dimensions and tolerances in your CAD drawings.
  • Keep in mind that fold allowance factors will vary between manufacturers.
     
• Review how easy it will be to produce the cut-outs required.
  • Are there any unusual or formed shapes or holes?
  • Are small burrs or ‘nibble’ marks acceptable inside the shape?
     
• Avoid distortion.
  • Check cut-out sizes and spacing. Distortion can occur if cut outs are too close.
  • Check that folding allowance will not distort holes close to the fold or edge.

Consideration 5: finishing options

The look and function of a sheet metal enclosure can be greatly enhanced with the right finish.

Plating

• Mild steel
  • Zinc plating provides corrosion protection, but it does not improve the look so usually paint is also required.
• Aluminium
  • Chromate conversion can improve corrosion and electrical conductivity, but is prone to marking
  • Anodising is an oxide film on the surface to make it more scratch resistant and more durable.

Painting

• Wet spray (rarely used) can provide a smooth or textured finish but requires a highly skilled operator to perform.
• Powder coat, which is easier to apply, is very durable with a large array of colours and finishes available as standard.

Labelling

The labelling will largely depend on the design of the enclosure.

• Screen printing — Costly for multicolour as each screen must be separated.
• Digital print — Works well on flat services and has some height restrictions.
• Decals or labels — Wide range of options available.
• Laser marking — Indelible, monochromatic labelling.

Consideration 6: assembly and testing

Consider enhancing the design by adding other custom sheet metal parts or assembly of certain elements during the fabrication process.

A few examples are:

• Brackets
• Fasteners
• Cable glands
   

When it comes to testing, consider:

• IP test/IK test
• Climatic and corrosion test
• Flatness testing
• EMC testing
   

This can all be considered in the design of the enclosure and may even be facilitated by the manufacturer since many can provide a one-stop source for custom, fully finished and partially assembled enclosures.

Erntec designs and manufactures large and small enclosure solutions for a wide range of applications. For more information, visit https://www.erntec.net/enclosure-solutions/enclosure-design/.