The right choice

Phillip Wallis, technical manager of Harcostar, takes us through the do’s and don’ts of material selection

Poor specification of material accounts for around 30% of product failure – the most common reason – when manufacturing in plastic. Most of these mistakes could be avoided if designers knew a little more about the most commercially important polymer types, their properties and typical applications and, most importantly, the importance of matching a polymer to an appropriate application.

Once a product has been designed, selection of the most appropriate polymer for its manufacture will depend on the properties which will be demanded by its eventual application – mechanical, thermal, chemical, electrical, optical and, recyclability.

Mechanical properties are crucial to products such as rigid packaging, bearings and cog wheels, toys, fuel tanks, aircraft components and automotive parts such as bumpers. Thermal properties such as a polymer’s melting and softening points, distortion, shrinkage and the subsequent effect of this on the polymer’s mechanical and chemical properties will be crucial considerations in applications such as windows, kettles, and automotive parts.

A polymer’s chemical properties include its reaction to substances which may cause ageing, oxidation, embrittlement, permeation or absorption of liquids or gases, and environmental stress cracking. These would all be important considerations in the manufacture of food packaging, air filter casings, fuel pipes and tanks, household articles, wash basins, taps, and drinks bottles.

The manufacture of electrical components and accessories, such as plugs, capacitors, cables and wire covering would require due consideration to a polymer’s insulation and dielectric properties. A polymer’s optical or UV stability will affect its surface gloss and opacity. Obvious applications where these properties are important include plastic windows and frames, film, food containers, and other packaging, traffic light coverings or any component exposed to outdoor conditions.

An additional consideration for food packaging designers is the European Commission Directive (90/128/EEC) relating to plastics and articles intended to come into contact with food stuffs.

A polymer’s mechanical and chemical properties generally determine its recylability. When considering post consumer waste, then one also needs to consider any chemical contamination which may have occurred during the product’s lifetime.

Practical examples of the importance of an appropriate polymer include a sophisticated grade of glass filled polymer with a very high melting point which is to be used to manufacture filler nozzles for beer kegs. As this product will be steam cleaned it is essential to select a polymer which will not distort under heat.

Polypropylene was selected to manufacture traffic light housings. As these are large hollow articles which are blow moulded, polyethylene was an initial consideration. However, it soon became apparent that the polymer would need a higher melting point than polyethylene because of the heat generated by the electrical components fitted inside, and polypropylene was the eventual choice. Additionally, the polymer is always coloured black to protect the product from UV degradation.

A plastic moulding specialist works closely with polymer suppliers to specify not only a polymer type such as polystyrene, ABS, and polycarbonate, but specific grades of each, depending on the application. Some grades are highly sophisticated: for example, for large plastic drums there are very few polymer suppliers with the ability to supply the specific grade of polyethylene which exhibits the required molecular weight distribution to withstand current drop testing regulations.

Once an application has determined that a certain polymer is to be used, this material selection may then go on to determine the method of processing. If the material does not dictate the process, then the final choice between GRP, vacuum forming, rotational, blow or injection moulding will more likely be made by studying the effects of quantity versus unit costs for the different processes. For example, rotational moulding may be a more economic choice when producing a smaller volume of a product which, if many thousands more were required, would be cheaper to injection mould.

The expertise of a moulding specialist will ensure that the most appropriate process is chosen. For example, Harcostar was consulted after the tanks on an industrial floor cleaner began cracking in use. Harcostar discovered these had originally been rotationally moulded – a process which delivers very low resistance to stress cracking. In this case, it is proposed to overcome the problem by blow moulding the tanks in a more resistant material.

When a designer presents a concept or design to a technical moulding specialist, in depth interrogation over volumes, the end product’s application, including under what environmental conditions it will be used and with what substances it will come into contact, must therefore be anticipated. The answers will be crucial in ensuring the most appropriate polymer and manufacturing process is selected and a functional end product is delivered.

Rexam Harcostar Tel 01480 52323