Plastics applications

Plastic products can be decorated by vacuum metallizing and electroplating. They have replaced metals in a lot of automotive applications,such as mirror housings,control knobs and winder handles as well as decorative metallic trim. It is a field which uses their advantages to the full without relying on properties they lack.

Thin parts must be tough and resistant to the occasional impact. They must be impervious to attack by weather,road salts,extremes of temperatures and all the other hazards that reduce older forms of body embellishments to pitted,rusted,dull,crumbling metal. They do not need high tensile strength or flexural strength as they do not have to carry heavy stresses. They must be cheap and capable of being formed into highly individual and complex shapes. All these requirements are satisfied by thermoplastics and thermosetting resins. They can be pressed, stamped,blow moulded,vacuum formed and injetion moulded into any decorative shape required.
Apart from their decorative properties,the mechanical properties of acrylic resins are among the highest of the thermoplastics. Typical values are a tensile strength of 35–75MN/m2 and a modulus of elasticity of 1550–3250MN/m2. These properties apply to relatively short-term loadings,and when long-term service is envisaged tensile stresses in acrylics must be limited to 10MN/m2 to avoid surface cracking or crazing. Chemical properties are also good,the acrylics being inert to most common chemicals. A particular advantage to the automotive industry is their complete stability against petroleum products and salts.

Acetal resins are mostly used for mechanical parts such as cams,sprockets and small leaf springs,but also find application for housings,cover plates,knobs and levers. They have the highest fatigue endurance limits of any of the commercial thermoplastics,and these properties,coupled with those of reduced friction and noise,admirably qualify the acetal resins for small gearing applications within the vehicle.
Plastics can be self-coloured so that painting costs are eliminated and accidental scratching remains inconspicuous,and they can be given a simulated metal finish. For large-scale assemblies,such as automobile bodies,painting is necessary to obtain uniformity of colour,especially when different types of plastics are used for different components. Plastics can also be chrome plated,either over a special undercoating which helps to protect and fix the finish,or by metal spraying or by vacuum deposition in which the plastic part is made to attract metal particles in a high-vacuum chamber. The use of a plastic instead of a metal base for chrome plating eliminates the possibility of the base corroding and damaging the finish before the chromium plating itself would have deteriorated. The chromecoating can be made much thinner and yet have a longer effective life,with a consequent saving in cost.
Until fairly recently polymer materials were joined only by means of adhesives. Now the thermoplastic types can be welded by using various forms of equipment,in particular by hot gas welding,hot plate machines which include pipe welding plant, ultrasonic and vibration methods,spin or friction welding machines,and induction,resistance and microwave processes. Lasers have been used experimentally for cutting and welding. The joining of metals to both thermoplastic and thermosetting materials is possible by some welding operations and by using adhesives.