Humans have been consuming resources at an alarming pace these days that is certainly not sustainable as the earth that we inhabit cannot regenerate the materials at that rate. For a comparison, between 1950 and 2005, worldwide metals production grew six fold, oil consumption eight fold, and natural gas consumption 14-fold. So, if we continue in the same path, our future generations will not have access to the resources the way that we have now. The harmful effects of our consumption and its final impact on the humankind are well known.
Though manufacturing systems create material wealth for humans, they consume a great amount of resources while generating a lot of waste. The waste generated during the manufacturing processes, during the use of the products and after the end of the life of the products is responsible for the degradation of the environment. Thus, minimising the resource consumption and reducing the environmental impact of manufacturing systems has become increasingly more important. Therefore, it is imperative that manufacturing industries strive for ‘Sustainable Manufacturing’ on their part.
1. Sustainable manufacturing defined
Sustainable manufacturing (SM) or green manufacturing for our purpose can be defined as a method for manufacturing that minimises waste and reduces the environmental impact. These goals are to be obtained mainly by adopting practices that will influence the product design, process design and operational principles. Therefore, sustainable manufacturing may be defined as a system that integrates product and process design issues with issues of manufacturing, planning and control in such a manner as to identify, quantify, assess, and manage the flow of environmental waste with the goal of ultimately reducing the environmental impact to that of the self-recovery capability of the Earth could deal with while also trying to maximise resource efficiency.
A survey of 198 Indian SMEs have identified the following aspects of sustainable manufacturing: “The final quantitative benefits of green manufacturing in order of their decreased ranking are improved morale, improved brand value, lowered regulatory concerns, increased market opportunities, improved product performance and decreased liabilities. The quantitative benefits of green manufacturing are related to either waste (reduced waste handling cost, lowered waste categorisation cost, reduced waste treatment cost, reduced waste disposal cost and reduced waste storage cost) or life cycle of the product (lowered transportation cost, decreased packaging cost, reduced overall cost of the product, lowered cost of production, reduced user operation/use cost, lowered maintenance/service cost and reduced overall cost to the organisation).”
2. Sustainable manufacturing tools
The tool generally used to implement SM is the Life Cycle Assessment (LCA). It is an approach to examine fully the environmental impact of different activities performed by humans including the production of goods and services by corporations. LCA can be applied for any activity that is either at national level or global level in order to identify environmental burdens resulting from the activities of a society, region or industrial sector. In fact, LCA can provide an excellent insight for the engineer to study any given product such that he/she can identify the methods to reduce the environmental impact of a specific product or process. A schematic of the methodology employed for carrying out the LCA is given in Fig 1.
LCA is concerned with identifying the environmental impact of a given product or process at each of these life stages. Full implementation of LCA allows the engineer to make a quantitative comparison of the stages of a product’s life, determine where the greatest environmental benefit is to be gained, and ultimately monitor the long term effect of changes in design and/or manufacturing.
Let us take an example to illustrate the point of how the consideration of LCA makes a very strong contrast to what we normally think about the environmental impact of the products. Consider a car used for personal transportation as an example. The energy consumed during the resource extraction and transportation for making the car is 9.2%, production and selling of a car is 13.9%, during the actual use of car during the lifetime is 76.8%, and the end of life vehicle disposal is 0.1%. As can be seen from this example, it is clear that the biggest impact is to be made at the consumer use life stage. In order to reduce the environmental impact of the car, the approach should be to develop cars with higher mileage and lower emissions.
The ISO standards assume a process-based LCA approach and is organised into four steps: goal and scope definition, inventory analysis, impact assessment, and interpretation as shown in Fig 2. It is important to understand the goals of the LCA and the methods to achieve them. Though this is a framework, the way the goals are formulated, the work involved is going to be enormous. The deeper the level of analysis, better will be the understanding. That will also provide better solutions to achieve the required results. However, this will also call for a lot of effort from the user. Therefore, during the first step, appropriate scope should be defined. This will be followed in the second step to develop a quantitative analysis of the material and energy inputs to the product or process at all levels. In this, it is also important to measure the possible environmental releases. Here, in the problem, though our interest is to find the actual releases over the entire life cycle, the designer/engineer will only have limited knowledge or control on what the user of the product will be doing.
In the third step, the outputs of the system at each stage are related to direct impact on the external world. The trouble with this stage is that data that will be generated is controversial, incomplete or wholly unavailable. Also, many of the impacts will depend on global and will not be necessarily regional in nature. For example, the release of CFC’s into the atmosphere. As a result, data for this step is often qualitative in nature. The last step in the process utilises the findings from the three previous steps and makes recommendations for the environmental improvement of the product or process under consideration. Ideally, this information provides direct input for proactive approaches, such as, design for the environment initiatives.
3. Sustainable manufacturing approaches
Sustainable manufacturing requires that all manufacturing organisations should aim for the following four activities that would help the environment across its entire supply chain with activities that are identified as in Fig 3.
• Energy use reduction
• Water use reduction
• Emissions reduction
• Waste generation reduction
Sustainable manufacturing should integrate sustainable activities at all levels of manufacturing – product, process and system. We are familiar with the 3R as reduce, reuse and recycle that is commonly followed. This needs to be expanded to more R’s, such as, reduce, reuse, recycle, recover, redesign, remanufacturing, repurpose, refurbish, refuse, etc, as shown in Fig 4.
The reduction should always start at the source level to be more effective. Therefore, the first part of the effort should be by incorporating sustainability in the product design to account for environmental impacts over the entire life of the product. Designing products to be environmentally benign can contribute to their successful introduction and maintenance. Designing products with easy disassembly help in the process of repair, reuse, repurpose and remanufacture. Designing products with easy maintainability help in prolonging the use of the product more efficiently. Product flexibility, for example, allows for environmental improvements, like materials substitution, while retaining competitiveness. The expected decrease in product life cycles with increased product customisation is likely to make flexibility increasingly important.
Sustainable manufacturing processes: Manufacturing processes and systems employed should consider sustainability at every level, so that there will be comprehensive adherence to sustainability principles. All the processes used are energy efficient while maintaining requisite quality. All the interconnected systems also share the same philosophy. Reduce energy intensity and emissions in all operations and the supply chain. Zero-emission (i.e. closed-loop) manufacturing views the manufacturing system as an industrial ecosystem, and requires the reuse of wastes or by-products within the manufacturing system. Manufacturing systems employed should have the flexibility for material substitution, and accommodate variations in material flows to assist in enhancing sustainability while maintaining competitiveness. To reduce the environmental impacts of manufacturing processes, it is necessary to optimise the environmental performance of the existing processes as well as develop new green processes.
Optimise the environmental performance: So far, manufacturing processes are generally designed for high performance and low cost with little attention paid to environmental issues. Most of the time, optimisation of a process is done with reference to minimising the machining time or machining cost with no consideration for the environment. The costing models considered rarely included the cost of environmental compliance. However, it is necessary to consider the cost of compliance to the environmental guidance.
For example, when estimating the cutting fluid cost, the following costs are considered as part of the total machining cost.
• Cost of purchasing the cutting fluid including the cost of recharging
• Cost of maintaining the cutting fluid, cost of additives along with the associated labor cost
• Cost of makeup fluid, the cost associated with the volumetric loss of cutting fluid due to evaporation, leakage, etc.
• Cost of pump out of the used cutting fluid
• Cost of system cleaning, i.e. flushing the system after disposing of the spent cutting fluid
It is also possible to improve the efficiency of operating the machine tools by modifying the software. For example, in deep hole drilling when programmed with peck cycle, the tool is withdrawn at programmed intervals to clean the chips. This may not be efficient use of energy. It is possible in deep hole drilling; the power consumption can be reduced with an adaptive pecking cycle, which executes pecking as needed by sensing cutting load. Also, synchronisation of the spindle acceleration/deceleration with the feed system during a rapid traverse stage can reduce the energy consumption up to 10%.
It is possible to develop mathematical models for electrical energy use in machining and validating them along a machine tool path such as in turning and milling. This helps in evaluating tool paths and re-designing machine tools to make them more energy efficient. It is also observed that the total energy requirement is controlled more by the auxiliary equipment than the main process in many situation. So, machine tools need to be redesigned taking this into account.
Develop new green processes
In addition to improving and optimising the existing processes, it will also be important to develop new processes that use less harmful materials and generate fewer emissions which can then be considered as green processes. An example could be processes based on laser. Laser assisted manufacturing processes are likely to bring some environmental advantages by reducing emissions during manufacturing processes while extending the tool life because of its non-contact nature. However the energy consumption of laser processes is more compared to the conventional processes. Direct Metal Deposition (DMD) is an additive manufacturing process that is better for simple molds with a low solid-to-cavity volume ratio that will be less environmentally burdensome compared to CNC milling.
It is known that the selection of process parameters can have a significant influence on the consumed energy and resources. Reducing energy consumption in a machine tool can be done by recovering the energy through the use of a kinetic energy recovery system (KERS) similar to regenerative breaks used in automobiles. It was found that 5 to 25% of energy saving can be achieved by having a KERS device on the machine tool based on simulated conditions. It is possible that the energy consumption for drilling and face/end milling can be reduced by setting the cutting conditions (cutting speed, feed rate and cutting depth) high, thereby shortening the machining time, yet within a value range which does not compromise tool life and surface finish.
4. Sustainable manufacturing examples
Cutting fluids are used extensively in metal machining processes to remove and reduce the heat during the machining operations. The use of cutting fluids greatly enhances the machining quality while reducing the cost of machining by extending tool life. However, the mist and vapour generated during the machining processes is harmful for the operator and stringent regulations exist to control them. Direct exposure of cutting fluids has been responsible for a number of skin cancer cases. Stringent environmental legislations require that the spent cutting fluids be recycled or disposed of in a manner that is not harmful to the environment. This calls for increased expenditure on the recycling and disposal procedures used depending upon the type of cutting fluid.
The machining cost, environmental impact and operators’ health concerns have driven researchers to find alternatives such as Minimum Quantity Lubrication (MQL) or equivalent dry cutting conditions that could satisfy the machining requirements without the use of cutting fluids. In MQL, the fluid used is generally straight oil, but some applications have also utilised water soluble fluids. These fluids are fed to the tool and/or machining point in tiny quantities. When air is combined with the lubricant it becomes atomised in the nozzle to form extremely fine droplets which is termed mist application.
Cutting fluids based on vegetable oils are bio-degradable as well as renewable. The experimental data indicated that the biobased fluids performed better than the mineral oil-based products in drilling in terms of prolonged tool life, better chip breaking, and lower tool wear and lower cutting forces. This will improve its main credentials as green cutting fluid since it does not have any harmful additives that complicate the disposal procedures and hence the disposal cost.
Sustainable manufacturing is the most important aspect to be considered by all production engineers, not because it is a fad but a necessity as an obligation to the world we live in. Product life cycle analysis has become a tool of choice being used to establish the environmental impact of the products that we produce. Though application of PLA is time and data intensive, provides very clear avenues where engineers will be able to reduce the environmental impact. There are a number of areas within manufacturing that can be benefitted greatly by the adoption of green manufacturing practices. The three major principles to be considered are reduce the resource utilisation in the process, use environment-friendly materials, reduce all forms of waste and reuse and recycle as much material as possible to realise the goal of self-recovery capability of the earth.
The article is written by Dr Nageswara Rao Posinasetti, Professor, Dept of Technology, University of Northern Iowa, USA