Life Cycle Assessment of micro manufacturing process chains - Application to the microfactory concept.

Transcription

1 Life Cycle Assessment of micro manufacturing process chains - Application to the microfactory concept. A. De Grave a, S.I. Olsen a, H.N. Hansen a a Department of Manufacturing Engineering and Management, Technical University of Denmark DK-2800 Kgs. Lyngby Abstract Historically LCA has mainly been applied to products; however, it can be very useful in assisting the whole product development by identifying more sustainable options in process selection, design and optimisation. It has been shown that it is relevant to apply LCA to micro components and more specifically to the research area of micro manufacturing where both processes and products are stated to be environmentally friendlier because of their small size. It is assumed that environmental impacts can be reduced by reducing the size of the factory using the concept of microfactories. In this paper, first the typology of data used for performing an LCA is investigated in the context of micro technology, then an example of a micro injection moulding insert is given. The concept of microfactories is then addressed with an environmental impact paradigm. Keywords: Life-Cycle Assessment, Environmental impact, Microfactory 1. Introduction The importance of Life Cycle Assessment (LCA) as a tool to assess the environmental sustainability of products and manufacturing is increasing in the European Union. Examples are the use of LCA in the EcoDesign of Energy using Products Directive (2005/32/EC) and the requirements for LCAs in some of the recently published calls for the European Seventh Framework programme for research and technological development. But is it relevant to apply LCA to micro components and more specifically to the research area of micromanufacturing? Micro components are often assumed more environmentally friendly than their macro counterparts although this may be challenged, and in some cases even contradicted as proposed in [1], [2]. Up to now LCA has mainly been applied to products; however, it can be very useful in assisting the whole product development by identifying more sustainable options in process selection, design and optimisation. [3] reviews some of these newly emerging applications of LCA. Likewise it may be assumed that environmental impacts can be reduced by reducing the size of the factory. However, no studies have as yet been established to assess the actual environmental impacts from microfactories and thus sustain this assumption. Presupposing that we aim to develop environmentally sustainable manufacturing methods the need to examine the environmental aspects of micro factories is therefore evident as is the application of LCA as a tool. 2. Data to collect to perform an LCA on a manufacturing process chain LCAs involve the following steps: Choice of product and identification of service (the functional unit) Establishing boundaries for the system Collection of data Environmental assessment Interpretation The initial step in all LCAs is to define the goal and scope of the study [4] what is the aim of the study? What is the object of investigation, the so called functional unit? What are the system boundaries for the investigated system, e.g. should the production of the manufacturing equipment be included? The chain of processes in the system will be defined thus also identifying the processes for which data needs to be collected. LCA is made in an iterative way initially starting with the most easily available data making a screening analysis with focus on materials, energy and chemicals used. The next steps imply more detailed data on inputs and outputs for each process Typology of necessary data One important issue is the type of data needed to be gathered in order to have a pertinent way of evaluating the process chain. This data falls mainly into two categories: process data and material use data. Many problems arise from the difficulty of gathering the necessary data Process data Energy use This is the energy electrical or thermal consumed in the production of a specified amount of product, usually 1 kg of product or e.g pieces. This may be measured directly at the manufacturing equipment or averaged from the energy consumption at the facility. It is important to know the type of energy and whether e.g. it is produced internally as waste energy from other processes. Energy is consumed both at the manufacturing process as well as during handling and assembly

2 Outputs (emissions, waste and products) Outputs from the processes should be inventoried. Of course the product is an important output serving as a reference for all other data but also all types of waste from the processes should be measured. This is e.g. waste material (the difference between what is in the product and what is consumed as raw material). It may also be some emissions from the process e.g. welding fumes. In the initial steps of the LCA, data of the latter type may be hard to acquire. If more than one product is produced by the same process this is important as well, since environmental impacts from the process should be allocated between the products Material use data Data on the use of materials are important both because of the environmental impact associated with the extraction of materials but also to keep track of the losses during manufacturing processes. Raw materials The total amount of raw materials consumed in the process should be registered (i.e. what goes in the factory gate), but also the amount of internal recycling. The type of raw material is important since the different materials cause different impacts during extraction and differ significantly in scarcity. Chemicals Chemicals (for etching, cleaning, metal working fluids etc.) should be registered both in terms of the consumption as well as the losses as emissions or waste. Chemicals may cause toxic impacts on humans and the environment Example of a 4M mould manufacturing project Fig.1. Insert for micro injection moulding The insert (see Fig.1), presented in [5] has been manufactured using an indirect tooling technology involving the making of a master, replicated through electrochemical deposition and then dissolved. In this example the substrate chosen for the master is a silicon wafer (in order to take advantage of the very good planarity and roughness of the surface of the wafer). It is subsequently processed with a micro EDM milling machine and nickel and copper layers were grown with electroforming onto it. A necessary activation stage using PVD was also performed. Data has been collected for all process steps of the mould making and are shown in tables 1 and 2. The data illustrates both what type of data are necessary to gather but also some of the difficulties

3 in acquiring data. There are both data from upstream processes for which it is not possible to measure the data yourself and data for which an estimate is very difficult. An example of the first is the silicon wafer. The production of silicon wafers requires quite a large amount of both energy, raw materials and chemicals of which a significant part is lost during the processing. These data are the property of the suppliers and not easily accessible although it might be possible to search and find some average data for a wide range of commodities. An example where the estimation of data is very difficult is the different solutions that are used during the processing for activation, deposition etc. These solutions are used for many products and may last for a long time, thus how much of the materials use etc. should the investigated product be responsible for? Table 2: Energy consumption in the processing of a micro mould. Process type Energy consumption (in kwh) PVD 14 EDM 122,5 Laser and milling 1,3 Deposition and dissolution Negligible Cleaning and activation Negligible Total 137,8 3. Data analysis As illustrated by the data the amount of materials and energy consumed is rather small, since we are dealing with small products. However, the EDM to a large extent dominates the energy use, reflecting that this is an energy intensive process. Also not included in the tables are of course the materials and energy spent on producing the raw materials or intermediary products like the Si-wafer. It has to be stated that while assessing the complete product development chain the production rate has to be taken into account. Indeed the number of parts produced with one mould/insert/die can justify and give a relative index. Moreover the process chain used is still in the research and development stage. the materials are not used in the same ratio and are not globally comparable the industry is more chemically and thermal processes oriented due to wafer processing the VLSI industry is in the batch processing with a later separation of the final product (chips) the level of purity is higher in white room conditions 4. Application to the concept of the microfactory The concept of microfactory is to create small size production centres for small sized products in a "diverse-types-and-small quantity production". It is based on the idea that the manufacturing and manipulation processes are often much bulkier than the parts they produce and that is even more true for micro components. The manufacturing of micro components is very demanding. Indeed it often requires special working environment for the handling and assembly of the parts but also for their production. The maintenance of this working environment (white or clean rooms with controlled atmosphere and pressure for instance) is very costly. Hence one of the benefits of the microfactory approach are of course that less space occupied hence inducing a lower cost in land occupation and more importantly atmosphere control. It is also assumed that there is a tremendous smaller use of energy. 4.1 Example of micro mechanical systems microfactory and relevant processes The DTU microfactory [6] is targeting at hearing-aid and medical devices. These devices need micro components both in polymer and in metal. Hence the manufacturing processes needed include: micro injection moulding, metal micro forming, handling operations, assembly operations 3.1 Comparison with other production lines The obvious way of evaluating down-scaled process chains is to compare it with a similar conventional process chain. But would it be better to compare with a macro-version of the product, or with the same product? Another often used reference is the clean room, from the micro electronic industry and more recently the micro electro mechanical systems production. It is clear that maintaining the level of purity necessary for VLSI manufacturing is very costly both economically and environmentally. Although again some precaution needs to be taken to use the white room as a reference: Fig.2. the DTU microfactory process chain Fig.2. illustrates a part of the process chain realized in the DTU microfactory. Die making has to be taken into account as an adjacent manufacturing process.

4 Fig. 3. Micro press and obtained components Fig.3 gives the example of manufacturing processes used (here the micro press) and micro products realized with the DTU microfactory. Another example can be seen in [7]: a Japanese microfactory targeting miniature ball bearing. The small factory includes : milling and turning, sheet metal micro forming, handling and assembly. It is stated that each of the processes of the chain are achievable but acknowledged bottlenecks are precision in fixture and time consumption during handling and assembly. The portable version of the microfactory weighs 34Kg and fits into a 625x490x380mm box, it requires a 100V AC power supply although no data about power consumption is given. It can be seen as a suitable example for comparison with the DTU microfactory as it makes use of similarly downscaled manufacturing processes and includes several assembly steps. In the case of sheet metal forming, the making of the die for forming is not studied. Scientists at DuPont have taken the concept of microfactory to the VLSI industry and fabricated a complete chemical plant using just 3 silicon wafers taken from a modern IC process [8]. This microfactory is capable of synthesizing 18,000 kg/yr of the toxic industrial chemical methylisocyanate. Reports of the concept of microfactories can be found in the MEMS industry [9] and in the mechatronics industry as well [10]. Using LCA is as relevant in microfactories as in other types of manufacturing. There are some issues that needs to be taken into account e.g. the flexibility of microfactories making it less comparable with production lines or the potential need for clean room facilities. Nevertheless, there is a clear advantage in the possibilities of identifying environmental improvement potentials in the process chain. 5. Conclusion & perspective As shown by our small example on data gathering even little effort in data collection may reveal insights. It is shown for example that the EDM machining is by far the most energy-intensive process in the chain and that the use of some materials (Si-wafer and gold) may require a large resource consumption. Looking further to the actual use of the mould, the material use and process chains will probably be justified because of better specification of the final plastic product. The example, however, also shows that there are some difficulties in data gathering some of which to a certain degree can be overcome by collecting data continuously: using power meters registering the use of chemicals and raw materials registering the output of useful product and waste The example is not a full fledged LCA i.e. data for raw materials were not yet included and the end of life not estimated. One issue that needs to be better investigated is scenarios of product end of life - waste management. Especially since disassembly and recycling may be troublesome. It is clear that micro components and microfactories will still be seen as the better environmentally friendly solution if profound studies are not performed to assess those claims, in order to know and not guess their friendliness. Acknowledgement: MsC Guido Tosello from IPL, DTU and the 4M polymer division are acknowledged for the use of the micro injection moulding insert as an example. Also, this work was carried out within the framework of the EC Network of Excellence "Multi-Material Micro Manufacture: Technologies and Applications (4M)". References [1] A. De Grave, S.I. Olsen, Challenging the sustainability of micro products development, proceedings of the 2nd International conference on multi-material micro manufacturing (4M 2006), Grenoble, France, September 2006, pp [2] A. De Grave, H.N. Hansen, S.I. Olsen, Sustainability of Products Based on Micro and Nano Technologies, proceedings of the 4th International Symposium on Nanomanufacturing (ISNM 2006), MIT, Cambridge, MA USA, November 2006, pp40-45 [3] A. Azapagic, Life cycle assessment and its application to process selection, design and optimisation, Chemical Engineering Journal 73 (1999), pp 1-21 [4] ISO 14040: htm [5] G. Tosello, G. Bissacco, P.T. Tang, H.N. Hansen, P.C. Nielsen, Micro tools manufacturing for polymer replication with high aspect ratio structures using µedm of silicon, selective etching and electroforming, 7th International Workshop on High-Aspect-Ratio Micro-Structure Technology (HARMST), Besançon, France, 7th- 9th June 2007 (submitted) [6] H.N. Hansen, T.E Eriksson, M. Arentoft, N. Paldan, Design Rules for Microfactory Solutions, proceedings of the 5th International Workshop on Microfactory, Besançon, France, October 2006 [7] N. Mishima, Evaluation of Manufacturing Efficiencies of Microfactories Considering Environmental Impact, proceedings of the 5th International Workshop on Microfactory, Besançon, France, October 2006

5 [8] R. Service, Miniaturization Puts Chemical Plants Where You Want Them, Science 282 (1998), 400. [9] I. Verettas, Microfactory: desktop cleanrooms for the production of microsystems, Proceedings of the IEEE International Symposium on Assembly and Task Planning, 2003, pp [10] Y. Ishikawa, T. Kitahara, Present and future of micromechatronics, proceedings of the 1997 International Symposium on Micro mechatronics and Human Science, 1997, pp