Industrial sustainability is concerned with obtaining a balance in the economic, environment and social aspects during development. This is a laudable aim as the world population keeps increasing in a world with finite natural resources. Although the world population growth rate has eased to 1.13% in 2016, the United Nation has estimated that it will reach 8 billion by 2023. It is obvious that the conservation of natural resources as well as reduction of wastages is of paramount importance.
Within the context of manufacturing, the emphasis in productivity improvements in the Sixties and the more recent drive towards materials and energy efficiency all contribute towards industrial sustainability. The next logical question is how to achieve industrial sustainability. At present, there are no universal laws providing the necessary foundations except a number of frameworks and principles. As an example, the seven principles for eco-efficiency proposed by the World Business Council for Sustainable Development offer sensible guidelines. However, they cover a very wide spectrum of activities and therefore in this article, an effective and direct engineering approach for supporting industrial sustainability, i.e., Lightweighting, is selected for a full discussion.
At a national level in the UK, companies are encouraged to provide an annual corporate sustainability report. The government guideline is to measure and report the performance in five areas: air quality and emissions, water, biodiversity and ecosystem services, natural materials and waste. Significant improvements have been achieved in British industry in the past decade. As an example, the winner of the 2016 Queen’s Award for enterprise (sustainable development), Anglia Print, has used 100% renewable energy and implemented waterless printing process and zero waste to landfill.
The basic idea of lightweighting is simple and straightforward. Essentially, lightweighting means the reduction of weight (actually the mass) of a product without compromising its functionality and structural integrity. This is a direct representation of materials efficiency. If a car is lighter, the material cost and processing cost of the part will be lower; and the fuel cost of the car will also be lower as it is lighter. Further, CO2 emission will be lower as less fuel is consumed. Casadei and Broad reported a 10% reduction if a conventional petrol car could reduce the fuel economy by 4.1%. In fact, lightweighting is particularly beneficial for the entire transportation sector.
For practical purposes, a summary of the methodologies for lightweighting is outlined with illustrative examples in the following sections.
Structural optimization (FEA)
Finite Element Analysis (FEA) is a modern and powerful numerical method for modelling and analysing the physical and mechanical behaviour of an object. In the present context, the object is an engineering product. Nowadays, FEA is seamlessly integrated with 3D modelling of a product within a full computer-aided engineering computer package. Lightweighting through structural optimization is an iterative process. In the following example taken from the Autodesk Sustainability Workshop, a brake pedal for a Formula SAE car was first designed followed by stress and displacement analysis as shown in Figure 1. Through the introduction of I-beam sections and cavities, a new design was created and its corresponding stress and displacement analyses are shown in Figure 2. As a result, the weight was reduced from 309 gm to 141 gm and the safety factor changed from 3.6 to 2.83, still comfortably about the specified safety factor of 2.
The substitution of steel by light alloys such as aluminium, magnesium and titanium is a relatively easy and straightforward option. The density of normal steel is 7.85 g/cm3, while the corresponding densities of Al, Mg and Ti are typically 40%, 25%, 56% compared with steel. As a result, such light alloys are used in modern engineering products for required weight reduction. For instance, Audi A8 and A4 use all aluminium body; magnesium alloys are used by Lenovo, Dell, HP for their laptops; titanium is used in aero-engine components and sports equipment. However, it should be noted that more demanding manufacturing technologies are required to process such materials, especially titanium. Another notable lightweight material is carbon-fibre reinforced composite which has become increasing important for the aerospace industry. For instance, 50% of the airframe of Boeing 787 is made of composite. In deciding materials substitution, numerous factors such as material cost, manufacturability, embedded energy, ease of recycling, have to be considered.
Design for manufacture and assembly
The idea of Design for Manufacture and Assembly (DFMA) was first proposed in 1970s. A central concept of DFMA is reduction of parts through the combination of several parts into a single, albeit more complex part. As claimed by Boothroyd Dewhurst Inc, the propriety owner of DFMA software, the average reduction in labour costs and weight amount to 42% and 22% respectively based on over 100 published case studies. Applying this principle, Porter was able to reduce the part counts of a motor drive from 19 to 7 which represent a significant improvement for both manufacturing and lightweighting as shown in Figure 3.
Additive manufacturing also known as 3D printing, refers to a new class of manufacturing processes by which a part is built layer by layer. The first AM process was stereolithography which was introduced in 1988. Nowadays, AM technology is well established and the global market size has been estimated at US$6.603 billion by Wohlers Associates. Presently, the most popular AM technology is based on extrusion, ink-jet and selective laser melting. As such processes are not constrained by geometry, they are capable of making ‘matrix’ or ‘cellular’ structures for lightweighting with significant weight reduction. An example of a complex matrix structure produced by Renishaw is shown in Figure 4. Due to its lower production rate compared with conventional subtractive machining processes, AM technologies are mainly used in niche high-value applications such as that in the healthcare sector today.
In the advent of micro-engineering and micro-manufacturing technology, there is a continuous trend in product miniaturization. An archetypal example is the development of mobile phone. The first generation mobile phone produced by Motorola in 1973 weighed 1.1 kg and measured 23 cm long, 13 cm wide and 4.45 cm deep with a talk time of only 30 minutes. After four decades of development, a modern mobile phone such as iPhone 7 only weighs 0.138 kg with a volume of less than 5% of the pioneer Motorola mobile phone. But more importantly, the functions and performance of today’s mobile phone hugely surpass the 1973 model. It has become part of everyday necessity and one could hardly function without a mobile phone. Apart from telecommunication and electronic industry, miniaturization is also implemented for mechanical, optical and military products.
The continuous search for higher materials and energy efficiency will act as a powerful driver in achieving industrial sustainability. Lightweighting is demonstrated as a direct and highly effective engineering method, among others, to achieve industrial sustainability. New materials such as graphene, carbon nano-tubes with unique properties will have enormous potential for lightweighting and applications. New business models and concepts, such as circular economy and remanufacturing, will evolve with wide implications to industries. In our strive for ingenuity and innovation to creating a sustainable world, we need to educate the next generation engineers with a greater awareness of the necessity for balancing the economic, social and environmental aspects when developing new products and systems.
About the Author
Prof. Bernard Hon is a world renowned scholar in the area of manufacturing systems and sustainable manufacturing, his has published over a hundred scientific publications. He is actively involved with numerous national and international organizations on research assessment, fundamental and applied research and education. He is also a Fellow of CIRP.