Member Control Panel
Optimising Plastics Material Selection for Sustainable Performance
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Dr Chris O’Connor, Smithers Rapra
However, despite the advances in manufacturing and design technology, it is a fact that around 70% of plastics fail before their design lifetime, resulting in litigation, expensive recalls, warranty claims, re-tooling and, in an ever competitive market – loss of brand credibility. So where is the issue? From the 5000 plastic product failures which have been the subject of extensive study at Smithers Rapra we conclude that the problems often arise at the design stage. Smithers Rapra has classified an extensive catalogue of plastic product failures on the basis of primary failure caused by material/phenomenological factors, as shown in Figure 1 below: Data compiled clearly demonstrates that all too often, product failure due to poor material selection is the single most common error made by designers or engineers of plastics products. Moreover, our experience indicates that it appears to be caused by a lack of awareness or understanding of the material’s properties. This is perhaps not surprising given that there are over 90 generic classes of plastic, which can be broken down into around 1000 sub-generic modifications and finally over 50,000 commercial grades from a host of materials suppliers. The result is a vast array of trade-names, generic nomenclature, and plethora of often incomplete and inconsistent data with insufficient standardisation which can sometimes confuse and frustrate the best plastic expert. The task to sift through this immense volume of data and make comparative judgements can to the inexperienced seem like an impossible task. The situation is further complicated by the competitive and positive marketing of material suppliers, which makes it sometimes difficult to identify material limitations and disadvantages.
Material selection - Select the right generic plastic for the application From the outset of material selection, the basics of polymer structure and properties should be considered. Polymers are broken down into three main groups – Thermosetting Plastics, Thermoplastic Plastics and Elastomers (rubbers). For the purpose of plastics design, the two basic choices are Thermoset and Thermoplastic. Thermoplastics have two basic types – amorphous and semi-crystalline plastics. Amorphous plastics are preferred for applications where transparency, good appearance, high gloss, high dimensional accuracy and stability are preferred. They should not be utilised for applications involving thermal or mechanical stress cycling, high mechanical abuse or contact with a wide range of chemical environments. Whereas semi-crystalline plastics, as the name suggests, have ordered crystallite structures and are better able to withstand fatigue than amorphous plastics. These plastics are best used where chemical contact, mechanical abuse and resistance to repeated cyclic loading is required. When considering the design and development of a plastic component the designer must understand that:
Consider the effects of time, temperature and stress / strain Visco-plastic materials respond to stress as if they were a combination of elastic solids and viscous fluids. Consequently they exhibit a non-linear stress-strain relationship and their properties depend on the time under load, temperature, environment and the stress or strain level applied. Visco-elastic behaviour can be seen with Silly Putty, a class of silicone polymer marketed as a children’s toy. If this material is pulled apart quickly it breaks in a brittle manner. If, however, pulled slowly apart the material behaves in a ductile manner and can be stretched almost indefinitely. Decreasing the temperature of Silly Putty, decreases the stretching rate at which it becomes brittle. All plastics exhibit creep, consequently design calculations and FEA analysis in order to prove a design are flawed if long term properties such strength and stiffness values are not used. These should be determined through experiments under worst case scenario operating conditions including maximum service temperature, stress levels and the effects of the environment. Unfortunately, many designers too often assume that ‘it is just a plastic’ and readily use the short-term test data provided by the plastic manufacturers datasheets. The result is many product failures due to visco-elastic induced failure mechanisms. Designers and engineers must realise that data sheet information is derived from short term tests which do not take into account time, temperature and environment. Test pieces are also simple shapes, which are moulded under ideal conditions. This rarely applies to moulded products. According to short term data plastics may appear to be able to endure strain levels of 200% or more. However, for long-term performance, the window for design strain is massively smaller. Recommended design strains are as follows: Static stress conditions
Cyclic stress conditions
In order to avoid failure it is imperative that the designer and engineer understand that:
CHRIS O'CONOR |
