Sometimes an engineer's decision to use a plastic enclosure is clear-cut, for example when the installation will be in a paper mill, where chemical attack will rapidly destroy a metal enclosure. In other instances, the decision to go with plastic is less obvious, and depends upon the severity of the installation environment.
Cost has always been a factor in selecting a plastic enclosure over a traditional metal unit. In the harshest environments, plastic generally is cost effective compared with stainless steel or high solids epoxy coated metal. For less demanding environments, painted metal was often a less costly solution, even though its corrosion resistance was not ideal. However, modern injection and compression molding techniques, coupled with the availability of high performance plastics, have dramatically improved the price-performance level of plastic enclosures, so much so that today, plastic can be considered for packaging in environments that are only moderately hostile.
But that does not mean the designer can be any less particular during the selection process. To acquire the correct plastic enclosure, the user must consider a number of factors that relate to the application. Let's take a look at each of them in some detail.