This article explores the critical decision of choosing between sheet metal enclosures and plastic enclosures for your electronic or medical device projects. We’ll delve into the pros and cons of each material, examining factors like durability, cost, customization, and manufacturing processes such as sheet metal fabrication and injection molding. This read is worth your time because it provides a comprehensive comparison of sheet metal and plastic for enclosure design, helping you make informed decisions that align with your project’s specific requirements, whether you’re looking for custom electronic enclosures or other custom enclosures.

1. What is a Sheet Metal Enclosure?

A sheet metal enclosure is a type of enclosure fabricated from thin, flat pieces of sheet metal, typically steel, aluminum, or stainless steel. The manufacturing process involves cutting, bending, and assembling the sheet metal parts to create the desired enclosure shape. Common fabrication techniques used in creating sheet metal enclosures include laser cutting, punching, bending, welding, and finishing processes like powder coating or plating. The sheet metal is used to make the parts of the enclosure.

Sheet metal enclosures are known for their strength and durability, making them suitable for a wide range of applications, including industrial equipment, electronic enclosures, server racks, and custom electronic enclosures. They offer excellent protection for internal components against physical impacts, harsh environments, and electromagnetic interference (EMI). Sheet metal can be used to weld the enclosure. This is a very common manufacturing process. The thickness of sheet metal used in enclosures typically ranges from 032″ to 0.375″ inches, depending on the specific application requirements.

Sheet metal fabrication offers a high degree of customization, allowing for the creation of complex shapes and designs. It is a versatile option for both low-volume prototype development and high-volume production runs. Sheet metal enclosures are a popular choice when robustness, durability, and EMI shielding are primary concerns.

2. What is a Plastic Enclosure?

A plastic enclosure is a type of enclosure made from various types of plastic materials, such as ABS, polycarbonate, or polystyrene. Unlike sheet metal enclosures that are fabricated from flat sheets, plastic enclosures are typically created using molding processes, with injection molding being the most common method for mass production. Plastic can be used in many electronic components as well.

Injection molding involves injecting molten plastic into a tool (mold) cavity, where it cools and solidifies into the desired enclosure shape. This process allows for the creation of complex geometries, intricate details, and integrated features like mounting bosses, ribs, and snap-fit closures, often used to create plastic parts. Plastic enclosures are lightweight, offer good electrical insulation, and are often more aesthetically pleasing than their metal counterparts. They are also resistant to corrosion and can be easily colored or textured during the injection molding process.

Plastic enclosures are commonly used for consumer electronics, medical device housings, and other applications where aesthetics, weight, and cost are important factors. However, they may not offer the same level of durability and impact resistance as sheet metal enclosures, especially in harsh industrial environments.

3. Sheet Metal Enclosures vs. Plastic Enclosures: Key Differences

Choosing between a sheet metal enclosure and a plastic enclosure depends on the specific requirements of your project. Here’s a table summarizing the key differences:

Metal enclosures offer many advantages, including strength and durability. It’s important to weigh these factors carefully to determine which type of enclosure best suits your enclosure needs.

4. When Should You Choose a Sheet Metal Enclosure?

Sheet metal enclosures are the preferred choice in several scenarios:

• Harsh Environments: When your enclosure needs to withstand extreme temperatures, physical impacts, vibrations, or exposure to corrosive substances, the robustness of a sheet metal enclosure is essential. Steel enclosures are particularly good for this. Metal enclosures offer protection from harsh environments.
• EMI/RFI Shielding: If your electronic components are sensitive to electromagnetic or radio frequency interference, or if they emit such interference, a sheet metal enclosure provides excellent shielding, protecting both the internal components and the surrounding environment. However, in the case of RF sensor technology or antennas it is important to remember that metal can sometimes attenuate signals. It is also worth noting that sheet metal can sometimes attenuate signals.
• Security: When security is a concern, the strength and durability of a sheet metal enclosure make it more resistant to tampering and unauthorized access. Fastener can be used to secure the enclosure.
• High Strength and Rigidity: For applications requiring high structural integrity, such as rack-mounted equipment or heavy machine control panels, sheet metal provides the necessary strength and rigidity. It is great for an enclosure or assembly.

Using sheet metal enclosures is a great option when you need an enclosure with specific characteristics. For example, Protocase specializes in creating custom electronic enclosures from sheet metal. Their expertise lies in providing quick-turnaround prototypes and low-volume production runs, catering to customers who need customized sheet metal solutions without the long lead time and high costs associated with traditional sheet metal fabrication.

5. When is a Plastic Enclosure the Better Option?

Plastic enclosures are often the better choice when:

• Weight is a Concern: Plastic is significantly lighter than sheet metal, making plastic enclosures ideal for portable or handheld devices, or any application where weight reduction is crucial.
• Cost-Sensitive Projects: For high-volume production, injection molding of plastic enclosures can be very cost-effective due to lower material costs and faster production cycles compared to sheet metal fabrication. The tool is not expensive. Injection molding can be a good solution for mass production.
• Complex Shapes and Designs: Injection molding allows for the creation of intricate shapes, smooth curves, and integrated features that would be difficult or expensive to achieve with sheet metal. Plastic is better for injection molded parts.
• Electrical Insulation: Plastic is an excellent electrical insulator, making plastic enclosures suitable for applications where electrical isolation is required. Plastic provides electrical insulation.
• Corrosion Resistance: Unlike some metals, plastic is inherently resistant to corrosion, making it a good choice for environments with moisture or chemical exposure. Although, if the enclosure is exposed to water you must be very careful.
• Aesthetics: Plastic enclosures can be easily molded in various colors, textures, and finishes, offering greater aesthetic flexibility than sheet metal enclosures. Plastic can be used for custom plastic enclosures.

If you need an enclosure that will be used for sensor technology or antennas inside you may want to use a plastic enclosure. If you need an enclosure that will have antennas inside the enclosure it may be best to avoid metal enclosures. In this case, plastic would be better.

6. Customization Options: Sheet Metal Fabrication vs. Injection Molding

Both sheet metal fabrication and injection molding offer extensive customization options, but they differ significantly in their capabilities and processes.

Sheet Metal Fabrication:

• Flexibility: Sheet metal fabrication allows for a high degree of design flexibility, especially for low to medium-volume production. Changes to the design can be implemented relatively easily by adjusting the cutting, bending, and welding processes.
• Prototyping: Creating prototypes with sheet metal is often faster and less expensive than with injection molding, as it doesn’t require the creation of a costly mold. Sheet metal is good for time for prototyping.
• Features: Sheet metal fabrication can create features like bends, folds, cutouts, and welded joints. However, achieving very complex shapes or fine details may be challenging or require secondary operations like CNC machining. Sheet metal can be used to create sheet metal parts.
• Finishing: Sheet metal enclosures can be finished with various techniques, including powder coating, painting, plating, and anodizing, to enhance their appearance and durability.

Injection Molding:

• Complex Shapes: Injection molding excels at creating complex geometries, intricate details, and integrated features like snaps, clips, and bosses that would be difficult or impossible to achieve with sheet metal. Injection molding design is great for complex shapes.
• High Precision: Injection molding can produce highly precise and repeatable parts with tight tolerances, making it suitable for applications requiring high accuracy. Injection molding can be used for making injection molded parts.
• Material Options: A wide variety of plastic resins are available for injection molding, each with its own unique properties in terms of strength, flexibility, chemical resistance, and temperature tolerance. You can create custom plastic parts.
• Surface Finishes: Injection molded parts can have various surface finishes, from smooth and glossy to textured or matte, directly from the mold.

Choosing between sheet metal fabrication and injection molding for customization depends on the complexity of the design, the required precision, the desired plastic parts, the production volume, and the budget.

7. Cost Considerations: Sheet Metal vs. Plastic

The cost of sheet metal enclosures versus plastic enclosures is influenced by several factors, including materials, manufacturing processes, tooling, labor, and production volume.

Sheet Metal Enclosures:

• Material Costs: The cost of sheet metal (steel, aluminum, etc.) can fluctuate based on market prices. Specialty metals or finishes can add to the cost.
• Fabrication Costs: Sheet metal fabrication involves labor-intensive processes like cutting, bending, and welding. The complexity of the design and the number of operations required can significantly impact the cost.
• Tooling Costs: While sheet metal fabrication generally doesn’t require expensive molds like injection molding, there may be costs associated with creating custom punches, dies, or fixtures, especially for complex shapes.
• Production Volume: Sheet metal fabrication can be more cost-effective for low to medium-volume production, as the initial setup costs are lower compared to injection molding.
• Prototype will be cheaper with sheet metal than with plastic.

Plastic Enclosures:

• Material Costs: Plastic resins are generally less expensive than sheet metal, although specialty engineering plastics can be costly.
• Injection Molding Costs: The primary cost in injection molding is the creation of the mold (tool), which can be very expensive, especially for complex or multi-cavity molds. However, once the tool is made, the cost per part is relatively low, making it cost-effective for high-volume production. Using a tool can reduce manufacturing costs.
• Production Volume: Injection molding becomes more cost-effective as production volume increases, as the high initial tool cost is amortized over a larger number of parts.
• CNC machining can increase the cost of plastic parts.

In general, plastic enclosures tend to be more cost-effective for high-volume production due to the lower material and per-unit manufacturing costs associated with injection molding. However, for low to medium volumes or custom electronic prototypes, sheet metal enclosures may be more economical due to the lower initial setup costs.

8. How Does Prototyping Differ Between Sheet Metal and Plastic?

Prototyping is a crucial step in the product development process, and the approach differs significantly between sheet metal and plastic.

Sheet Metal Prototyping:

• Faster Turnaround: Sheet metal prototypes can often be produced relatively quickly, sometimes in a matter of days, as they don’t require the creation of complex molds. Companies like Protocase specialize in rapid sheet metal prototyping.
• Lower Initial Cost: The cost of creating a sheet metal prototype is generally lower than that of a plastic injection molded prototype because there are no tooling costs involved.
• Design Flexibility: Design changes can be implemented more easily in sheet metal prototypes by adjusting the cutting, bending, or welding processes.
• Material Properties: Sheet metal prototypes can accurately represent the material properties of the final product if the same sheet metal material is used.

Plastic Prototyping:

• Longer Lead Time: Creating a prototype with injection molding requires the fabrication of a mold, which can take several weeks or even months, depending on its complexity.
• Higher Initial Cost: The cost of the mold makes injection molding prototypes more expensive upfront.
• Limited Design Flexibility: Once the mold is created, making design changes can be costly and time-consuming, often requiring modifications to the mold or even the creation of a new mold.
• Alternative Prototyping Methods: For plastic parts, alternative prototyping methods like CNC machining or 3D printing may be used to create initial prototypes before investing in a costly injection mold. CNC machining can be used for custom electronic enclosures.

Sheet metal is often the preferred choice for rapid prototyping due to its faster turnaround time, lower initial cost, and greater design flexibility. Injection molding prototypes are typically used later in the development process to validate the design for mass production.

9. Case Study: Choosing the Right Enclosure Material

Let’s consider a case study of a company developing a new medical device that requires a custom enclosure. The device contains sensitive electronic components, including a circuit board, and will be used in a hospital environment.

Requirements:

• Durability: The enclosure needs to protect the internal components from accidental drops and impacts.
• Cleanability: The enclosure must be easy to clean and disinfect to meet hospital hygiene standards.
• Aesthetics: The enclosure should have a professional and clean appearance.
• EMI Shielding: The device emits some electromagnetic interference that needs to be contained.
• Production Volume: The initial production run will be 500 units, with potential for higher volumes in the future.

Analysis:

• Sheet Metal: A sheet metal enclosure, such as one made from stainless steel, would offer excellent durability and EMI shielding. It could be designed to be easily cleanable. However, the cost might be higher for the initial low-volume production run.
• Plastic: A plastic enclosure made from ABS or polycarbonate could provide sufficient durability for this application. It would also be easy to clean and could be designed with a more aesthetically pleasing appearance. Injection molding would be more cost-effective for higher volumes but might be expensive for the initial 500 units due to tooling costs.

Decision:

For this case study, the company might choose to start with sheet metal enclosures for the initial production run of 500 units. This would ensure durability and EMI shielding while avoiding the high upfront tooling costs of injection molding. They could use a fastener system. They could partner with a company like Protocase for rapid prototype development and low-volume production.

As production volumes increase, they could then consider transitioning to injection molded plastic enclosures to reduce per-unit costs. They would need to invest in creating a mold but would benefit from the lower material and manufacturing costs of plastic in the long run.

10. What are the limitations of using Sheet Metal and Plastic Enclosures?

While sheet metal enclosures and plastic enclosures offer many advantages, they also have some limitations:

Sheet Metal Enclosures:

• Weight: Sheet metal enclosures, especially those made of steel, can be significantly heavier than plastic enclosures. This can be a drawback for applications where weight is a critical factor, such as portable or handheld devices.
• Cost: While sheet metal fabrication can be cost-effective for low to medium volumes, it can become more expensive than injection molding for very high-volume production runs, especially when complex shapes or features are required.
• Corrosion: Depending on the type of sheet metal used and the operating environment, corrosion can be a concern. While stainless steel offers excellent corrosion resistance, it is more expensive than other options like regular steel. Proper finishing, such as powder coating or plating, can help mitigate corrosion but adds to the overall cost.
• Design Complexity: While sheet metal fabrication offers a good degree of design flexibility, creating very complex shapes or intricate details can be challenging and may require secondary operations like CNC machining, increasing costs and lead time. If you are using sheet metal enclosures you need to remember about limitations.
• It is not ideal for RF sensor technology or antennas. Metal can sometimes attenuate signals, so you must be very careful.

Plastic Enclosures:

• Strength and Durability: While plastic enclosures can be designed to be quite strong, they generally do not offer the same level of impact resistance and durability as metal enclosures, especially in extreme conditions.
• Temperature Sensitivity: Some plastic materials can become brittle at low temperatures or deform at high temperatures. It’s crucial to select a plastic resin that can withstand the expected operating temperature range of the application.
• UV Degradation: Prolonged exposure to ultraviolet (UV) radiation from sunlight can cause some plastic materials to degrade, leading to discoloration, embrittlement, or cracking. UV-resistant additives or coatings can help mitigate this issue but may add to the cost.
• EMI/RFI Shielding: Plastic itself does not provide inherent EMI/RFI shielding. To achieve shielding, conductive coatings or fillers need to be applied or added to the plastic, which can increase the cost and complexity of the enclosure. If you have sensor technology or antennas inside the plastic enclosure it will be easier to transmit the signal.
• Tooling Costs: While injection molding is cost-effective for high-volume production, the initial cost of creating the mold can be very high, making it less suitable for low-volume or prototype runs.

When making custom electronic enclosures it is important to be aware of these limitations.
It is worth mentioning that plastic can be used with threaded fasteners but there are some limitations. It is possible to tap plastic but you need to be careful not to strip the threads. You must be very careful not to strip the threads when installing fasteners. It is better to use a fastener that is designed to be used plastic with bolts rather than threading. 

It is also worth knowing that a CNC router is often used to create plastic enclosure. It is a machine that is often used to create plastic parts and enclosure components. This machine can create a milled feature may be added to the plastic part. A CNC router can create a custom plastic part. A CNC router can machine a part in 2-3 days. The size of the material used in a CNC router can be 4ft x 8ft. The minimum corner radius for a part is 0.022″ for material that is between 0.062″ to 0.750″ thick. For material that is between 0.094″ to 1.000″ thick, the minimum corner radius should be 0.032″. It is important to note that with a CNC router, the tool used means that countersinks cannot be etched during milling.

Designers often use acrylic for an enclosure because it is transparent. Acetal has great dimensional stability which makes it great for close-tolerance machined parts. Designers use acetal for such things as clamps for cables or bus bars, and fiber optic trays.

11. Summary

Here are the 10 most important things to remember about sheet metal enclosures and plastic enclosures:

1. Sheet metal enclosures offer superior strength and durability, making them suitable for harsh environments and applications requiring high impact resistance.
2. Plastic enclosures are lightweight, cost-effective for high-volume production (due to injection molding), and offer excellent design flexibility for complex shapes.
3. Sheet metal fabrication is generally more cost-effective for low to medium-volume production and prototyping, while injection molding becomes more economical for high volumes.
4. Sheet metal provides excellent EMI/RFI shielding, while plastic requires special coatings or fillers for similar shielding capabilities.
5. Injection molding allows for intricate designs, integrated features, and a wide range of colors and finishes, making plastic enclosures often more aesthetically pleasing.
6. Sheet metal prototyping is typically faster and less expensive than injection molding prototyping, which requires creating a costly mold.
7. Plastic is a good electrical insulator, while sheet metal may require additional insulation in some applications.
8. The choice between sheet metal and plastic depends on factors like weight, cost, durability, aesthetics, EMI shielding requirements, and production volume.
9. Both sheet metal fabrication and injection molding offer customization options, but they differ in their processes, capabilities, and limitations.
10. When choosing between sheet metal and plastic, it is essential to consider the specific requirements of your project and weigh the trade-offs of each material and manufacturing process.

Contact us if you need any help in choosing the right enclosure.