FACTORS AFFECTING ENCLOSURE MATERIAL SELECTION
The choice of material for your enclosure affects more than just aesthetics. It affects everything from the weight, strength and corrosion resistance of your enclosure to the type of manufacturing methods and surface coatings that can be used. Some of the most common materials for manufacturing enclosures include carbon steel, stainless steel, aluminium, ABS, polycarbonate and PMMA, each of which is available in a large variety of grades or alloys. The purpose of the article is not to compare each material (there are far too many) but rather to outline the factors that you should consider when choosing a material.
1. Enclosure Manufacturing method
The three most common manufacturing methods for product enclosures are sheet metal forming, extruding and moulding. The manufacturing method must be chosen at the beginning of the design process because the choice of manufacturing method affects almost every aspect of the design. Often the manufacturing method is chosen primarily to minimize cost:
sheet metal forming has minimum tooling costs and so is most economical for small quantities.
Extruding has a high tooling cost and so the average cost for low volume manufacturing is very high. However, for medium and high volume manufacturing it is cheaper than sheet metal forming.
Moulding has an even higher tooling cost (than extruding) and so it is only suitable for medium to large volume manufacturing. At high volumes, moulding s generally cheaper than extruding.
In addition to manufacturing cost, there are often additional considerations when choosing a manufacturing method and these are discussed separately. Plastics are not suitable for sheet metal forming but can be extruded or moulded, whereas most metals, including steel and aluminium, can be manufactured by all 3 methods, however:
The specific alloy that is most suitable will differ depending on the chosen manufacturing method.
Tooling costs for extrusion and moulding of metals are significantly higher than for plastics, thereby increasing the minimum manufacturing volume at which metal extruded or moulded enclosures become economical.
Unit cost (excluding tooling cost) for extruded or moulded metal enclosures are higher than for the corresponding plastic enclosures.
2. Mechanical strength
Depending on the application, the enclosure may be required to survive knocks and falls. The ability of the enclosure to withstand mechanical loading is dependent both on mechanical properties of the material and geometry so whilst metals are significantly stronger than plastics, it is possible to manufacture extremely tough thick-walled enclosure out of ABS or fibreglass. The stress at which a material begins to undergo permeant deformation is called the yield stress. A further important material property is its impact strength, which is a measure of the impact energy that can be absorbed (during a knock or fall) before permanent deformation occurs. The impact strength is a factor of yield strength but also heavily reliant on the ductility of the material.
3. Method of fastening
Fastening is required both to hold together individual parts of the enclosure and for mounting components such as circuits boards within the enclosure. The types of fasteners used are largely dependent on the method of manufacturing method and is covered separately. However, there are several differences in fastening methods which are largely a result of material choice:
Plastic moulded parts can be either permanently or non-permanently assembled using clips built into the moulded parts. This is possible but less common in metal casted parts.
When manufacturing by moulding, metal parts can have screw threads cut directly into the part after moulding. Although this can also be done in plastics, it is more advisable to use metal threaded insets
When using self-clinching fasteners in thin-walled metal parts (typically for sheet metal fabrication), there minimum material requirements which vary with the type of metal chosen
4. UV resistance
If a product is likely to be subjected to long periods in direct sunlight, it is important to verify that the materials used for its enclosure are resistant to UV radiation. Whereas metals are largely unaffected by UV radiation, plastics vary widely in their resistance to UV. Over exposure of most plastics to UV radiation will, over time, cause the plastics to become brittle (and therefore fracture easily) and their colour to fade. However, some plastics (e.g. acrylic and PVDF) have excellent resistance to UV and there are anti-UV coatings which can be used to improve the UV resistance of other plastics. Keep in mind when applying surface coats to or printing on enclosures, that some paints, inks and other coating materials can also be susceptible to damage when exposed to UV radiation.
5. Corrosion resistance
The most common form of corrosion is the rusting of unprotected steel, which is why when using steel, you should always protect it by galvanizing or powder coating it (preferably both). Stainless steel, whilst much less susceptible to rusting, is not immune to rusting and particularly if subjected to intense heat (e.g. during welding) well rust due to chromium leaching out of the heated areas. Aluminum and plastics are immune to rusting. If your enclosure is to be placed in a saltwater environment you should also consider the danger of galvanic corrosion, which occurs when two different metals touch in an ionic (i.e. saltwater) environment. Furthermore, if the enclosure is likely to come into contact with acids, alkaline or polar solvents you should research the compatibility of your enclosure material with the respective corrosive substance.
6. Thermal conduction
Thermal conduction is a measure of the ability of a material to conduct heat. The coefficient of thermal conductivity varies greatly with material and is much higher in metals than in plastics. The importance of thermal conductivity typically arises in cases where the enclosure is designed without active cooling and therefore relies on the conduction of heat through the enclosure walls to dissipate the heat generated by the electronics. Despite the difference in thermal conduction between steel and Aluminum, it will make little difference which you choose as metal enclosures are sufficiently thin and both of their coefficients sufficiently high such that any further increase will in most cases result in only a small improvment
However, plastics have much lower thermal conduction coupled with thicker walls due to their lower strength and so are not capable of dissipating as much heat as their metal counterparts. In some cases, this could result in the electronics overheating The lower thermal conductivity can, however, be useful in cases where an internal component, such as a battery, may become hot enough to cause discomfort or harm to the user. In such a case the plastic enclosure can provide thermal insulation to protect the user.