Comparison Between Economic Aspects of Additive and Conventional Methods of Manufacturing of Plastic Machine Parts

Machine Dynamics Research 2016, Vol. 40, No 2, 101-112 Comparison Between Economic Aspects of Additive and Conventional Methods of Manufacturing of Plastic Machine Parts Tomasz Boratyński *, Paweł Roczniak, and Natalia Żelazo Wroclaw University of Science and Technology Abstract Nowadays the developed Additive Manufacturing (AM) technologies deliver many new opportunities to manufacturing companies. One of their benefits may be the elimination of costly tools in the passage from design to production. This cost is one of crucial factors of decision making process about the use of technology in a defined production case. The managers have to determine at which quantity of produced goods the purchase of tooling is profitable. The objective of the paper is to present literature review about economic aspects of AM and the analysis of production cost of a medium size plastic component in the SLS process compared to the cost of the same part produced with injection moulding (IM). Keywords: additive manufacturing, economic aspect, selective laser sintering, injection moulding. 1 Introduction For years the production was evolving along with the technological progress. Some of the technologies were improved, changed or replaced by new ones. An example of important achievements in the 20th century could be the introduction of CNC technology in the manufacturing industry. It significantly increased the capability of the production (Liu et al., 2014). Another revolutionary point in the history of manufacturing was the application of additive technologies (Berman, 2012) able to overcome restraints imposed on conventional manufacturing technologies, for example injection moulding, subtractive machining or casting (White and Lynskey, 2013). Additive Manufacturing (AM) consists of a successive construction of an entirely functional product from material. The process requires a digitally sliced three-dimensional computer aided design model of the product in order to build it up layer-by-layer on a machine. In previous years, the major task of additive technologies was the creation *tomasz.boratynski@pwr.edu.pl

102 Boratyński T., Roczniak P., Żelazo N. of physical prototypes of products at the level of product development. After some time they developed in such a way that they could be used for manufacturing of final products and tools. Three groups of applications are distinguished with respect to their result: Rapid Prototyping (RP), Rapid Tooling (RT) and Rapid Manufacturing (RM). Compared to the conventional prototyping, RP stands out by significant savings in process planning cost, tooling cost, rework cost, assembly cost, operator cost, machine setup cost, total cycle time and customisation. Some of the above cost factors can be completely eliminated from the process (Sahebrao Ingole et al., 2009). RT provides an opportunity of acceleration of product development process and decreasing the cost of tooling for a limited production (de Beer et al., 2005). Direct part production and fit and assembly totalled 34.7% of AM applications in 2014 (Wohlers, 2014). Naturally there occurred a question whether and when it could compete with the conventional manufacturing. Certainly, one of the estimation criteria of the legitimacy of use of AM methods is the economical aspect. 2 Literature review Researchers have already tried to compare economical aspects of AM with conventional technologies. Gibson et al. in the book "Additive Manufacturing Technologies Rapid Prototyping to Direct Digital Manufacturing" (Gibson et al., 2010), raised an issue of product life cycle costs of AM. The six factors have been enumerated: 1. Equipment cost 2. Material cost 3. Operation cost 4. Tooling cost 5. Service cost 6. Retirement cost As opposed to AM, in conventional manufacturing technologies tools - as moulds, patterns, dies, jigs, gauges and fixtures - are limited for a definite production line because their functional features depend on the products. If the life cycle of the part is long, there is a risk that the tooling used previously is not available after many years. On the other hand, storing tools, especially large ones, can be expensive. AM does not require tools to repair broken parts or to make new ones for replacement. Instead, the digital models of products are stored. The retirement costs are related to taking a product out of service, disassembling and disposing of it. They depend mainly on recyclability of the material used to make it. Plastics are one of types of material used in AM. They have many useful applications in present engineering. They are meant

Comparison Between Economic Aspects... 103 to replace traditional materials like wood and metals (Arena et al., 2003). Thermoplastic polymers used for example in Selective Laser Sintering (SLS) are generally recyclable. Unlike the thermoset photopolymers used in Stereolithography (SLA) which cannot be recycled and must be send to landfills. 3 Production cost During a decision making process on technology selection for a defined production line the enterprise must consider the financial costs (Baumers et al., 2013). The cost estimation allows for answering the question which of available technologies is more cost-effective and thus, which one would bring more profits. An attempt of general description of cost distribution faces challenges because each enterprise has its own individual cost calculating system (Gunasekaran and Singh, 1999). Ruffo et al. (Ruffo et al., 2006) distinguished the following cost estimation techniques: 1. Analogy-based techniques - the cost of a new product is assessed on the basis of the similarity with costs of existing products 2. Parametric models - the cost is determined by a function consisting many variables 3. Engineering approaches - the cost is evaluated in an analytical way after observing process characteristics 4. Artificial neural network models - the digital models simulate real conditions of the process in order to estimate required resources Sectors in which AM is used predominantly to produce components embrace consumer electronics, industry and business machines, automotive and aerospace industries. They constituted 66.1% of total AM applications in 2014 (Wohlers, 2014). Though, these industries still meet barriers to implementation of additive technologies in production. In many cases there is a need of validation and certification of the reliability of AM technologies. Furthermore, a limited choice of materials and excessively high cost of machines and materials can pose a problem to manufacturers. The machine purchase cost impacts the cost of the final products due to the machine depreciation. However, the growing popularity of AM technologies leads to the increase of the market competition, thereupon, the price fall (Hopkinson and Dicknes, 2003). AM also offers the opportunity of lightweighting produced parts, in consequence, savings in raw material and reduction in fuel consumption in case of aircraft and automotive industry (Carpenter et al., 2008).

104 Boratyński T., Roczniak P., Żelazo N. 4 Comparison between IM and AM - previous studies Hopkinson and Dickens (Hopkinson and Dicknes, 2003) performed a cost analysis comparing the injection mounding with three AM technologies: SLA, FDM and LS. The unit cost per part was estimated for various quantities of products. In the calculations they omitted machine power consumption and space rental due to their small contribution to the final cost. Neither overhead costs were considered. The production costs in the calculation model consisted only of machine, labour and material costs. The authors assumed that the machine was producing one type of parts for entire year. The machine cost per part relied on annual machine depreciation and annual machine maintenance cost divided by annual production volume. The labour cost per part depended on machine cost operator per hour multiplied by set-up and post-processing time, divided by the number of parts built during one cycle. The material cost per part in the case of SLA and FDM relied on the mass of the produced part and the material cost per kilogram. It was noticed than the unit cost per part of FDM technology varied depending on the part orientation on the build platform that influenced the building time and the support material volume (Singh, 2013). Since it was assumed that in the LS process the material was not recycled, the cost was calculated on the basis of the sintered and unsintered material mass per build. Two parts were examined: a small lever part and a medium cover part, both characterized by complex geometry. Results showed that LS was the least expensive among the three chosen technologies, the greatest part of the cost was the machine cost. An important remark was that AM technologies could be competitive against injection moulding in terms of cost-effectiveness for thousands of produced parts. Ruffo et al. (Ruffo et al., 2006) expanded on the Hopkinson model. First of all, they rejected the concept of constant unit cost per part for AM. They emphasized than for lower production volume the cost is higher due to machine amortization. The cost of LS was analysed again. The subject of the analysis was the same lever part as in the study performed by Hopkinson and Dickens. The activities involved in the AM process were studied extensively. The activity based costing was chosen as more accurate than the traditional costing system. After identifying the activities, the cost related to each of them was calculated. The cost of the product relied on the consumption of activities (Gunasekaran and Singh, 1999). They included costs of material, software, hardware, capital equipment depreciation, labour, maintenance, production and administration overheads. Only the material cost was considered as a direct cost. The rest of costs were allocated to products proportionally to the required production time. The material recycle was taken into account. According to material suppliers recommendations, it is possible to reuse the unsintered material from the previous SLS process. In order to keep the quality of the process and the parts it is advisable to refresh it with 30-50% of virgin powder (Atzeni et al., 2010). The building time was assessed with the empirical time estimator developed by the authors. The curve representing unit cost per part in function of production volume

Comparison Between Economic Aspects... 105 was characterized by a decline up to few thousands of pieces and a varying tendency. The influence of the packing ratio on the building time and the material usage was accounted for in the costs per part. A significant difference in comparison against Hopkinson and Dickens studies was the contribution of the material cost in the total cost per part. The break-even points with the IM moved towards smaller quantities of products with the new costing method but it kept the range of thousands pieces. In the former approach Ruffo et al. noted that the production of a mix of products on an AM machine is more cost-effective than production of copies of a single part and can minimize the deflection occurring with lower numbers of parts. In fact, in the case of a medium volume production the manufactures prefer to utilize maximum of build space regarding the influence of the packing ratio on the cost-effectiveness of AM processes. Thus, the constant cost model for AM technologies can be accepted (Ruffo and Hague, 2007). Atzeni et al. (Atzeni et al., 2010) comparing the cost-effectiveness of SLS method against IM, focused on the design freedom potential of additive technologies. The analysed part was redesigned from the original form in order to minimize the assembly activities in the production process. The authors took advantage of the ability of AM to fabricate concurrently all components in an assembly. In contrast to previous studies, the authors discussed extensively the cost factors of IM. 5 Case study Until now the analysed parts had small dimensions and complex design. In this paper the question was what quantity of simple machine plastic parts can be manufactured using an additive technology to be competitive for a conventional technology. For a cost estimation based on the engineering approach, a plastic cassette was selected as a case study. In contrast to previous studies, the part was considered as medium and simple (Fig. 1). It was an enclosure of a controller originally produced on an injection moulding machine at SPAMEL company of Twardogóra. The enclosure consisted of two parts: the base and the cover, both parts were produced together in one cycle owing to the mould construction. ABS was used to provide a proper impact resistance of the cassette in its working conditions.

106 Boratyński T., Roczniak P., Żelazo N. Fig. 1. Plastic envelope for a controller, the object of the analysis. The method selected for the comparison was SLS, which was at the moment the most economical from available AM technologies. The material used was PA2200 sold at 270 PLN per kg. 5.1 Assumptions and limitations The calculating model was similar to the ones developed in previous studies (Ruffo et al., 2006; Atzeni et al., 2010) and included the following cost factors: - material cost - machine cost - labour cost - departmental overheads It was assumed that only copies of the same part were fabricated yearly. Thus, the base of allocation of the indirect costs was the process time for both technologies. In both cases the machines worked 5000 hours annually. Departmental overheads consisted of the cost of energy, rent and administration. 5.2 Methodology The direct cost for SLS was presented in the Table 1. In order to calculate the build mass, the formula was used: build mass = density*(build volume+(bed volume-build volume)*waste factor) EOS Formiga P110 machine was used for SLS process analysis. The build space dimensions 200x250x330 [mm] enabled to build 5 entire cassettes with unchanged design (5 bases and 5 covers) in one cycle with the highest packing ratio. Therefore,

Comparison Between Economic Aspects... 107 it could be assumed that when the economical batch volume was a multiple of five the cost per part remained constant. To improve economic efficiency both parts of the cassette housing were redesigned so as to achieve greater density on the working platform construction with unchanged SLS performance requirements. It was assumed that the working space can accommodate 12 sets of housings - see Fig. 2. Fig. 2. Filling of the working space for 5 sets and 12 sets. Table 1. Direct costs of SLS. direct costs 5 sets 12 sets density [kg/cm 3 ] 0.00093 0.00093 build volume [cm 3 ] 1760 1760 bed volume [cm 3 ] 15850 15850 waste factor 0.5 0.5 material cost [PLN/kg] 270 270 number of entire parts per build 5 12 build mass [kg] 8.18865 9.33441 material cost per part [PLN] 442.19 210.02 The direct cost of material in IM was known and equal to 5.91 PLN per cassette while direct costs amounted to 3.01 PLN according to manufacturer. The indirect costs for SLS are presented in the Table 2.

108 Boratyński T., Roczniak P., Żelazo N. Table 2. Indirect costs of IM and SLS. indirect costs SLS 5 sets SLS 12 sets machine and equipment purchase [PLN] 650000 650000 machine depreciation [years] 8 purchase cost/year [PLN] 81250 81250 maintenance cost/year [PLN] 5000 5000 machine cost/h [PLN] 11 11 labour cost/h [PLN] 20 20 production cost/h [PLN] 11.55 11.55 administrative cost/h [PLN] 1.15 1.15 cycle time [h] 4.2 1.75 machine cost/part [PLN] 45.11 18.80 overhead/part [PLN] 53.34 22.23 labour cost/part [PLN] 3.65 1.52 indirect costs [PLN] 102.11 42.54 The machine cost in case of the injection moulding comprehended the cost of additional equipment which was a manipulator used to transport parts. The cost of software was included in the SLS machine purchase. Labour cost per part made in SLS process was calculated by multiplying the machine operator salary by the set-up and post processing time and dividing of the result by number of parts produced in one cycle. The labour cost per part for IM was quoted. The cycle time was evaluated with use of PSW 3.6 software integrated with the SLS machine. The mould purchase cost was 100 000 PLN and was amortised by the number of produced parts. 6 Results and discussion The break-even analysis of IM and SLS was shown in Fig. 3 and 4. The cost per part for SLS was equal to 544.30 PLN for 5 sets and to 252.56 PLN for 12 sets and the cost per part for IM was decreasing towards higher quantities of produced parts. The highest number of produced parts in which SLS was more cost-effective amounted to 184 for 5 sets but according to the assumption the production volume for SLS had to be the most economical possible multiple sets of a break-even point has been established for 12 sets or in the amount of 397 sets.

Comparison Between Economic Aspects... 109 Fig. 3. The break-even analysis of IM and SLS. Fig. 4. The break-even analysis between IM and SLS. In the Fig. 5 the weight of different cost factors in the SLS process was shown. Overhead costs consist 10% for 5 sets and 9% for 12 sets of the total cost per part. It was a result of the long cycle time and the homogeneous production. Fig. 5. Contribution of cost factors in the total cost per part in SLS process. Due to the build space dimensions and the part dimensions not modified only 5 pieces of entire cassettes fitted in the building platform. Significant results were

110 Boratyński T., Roczniak P., Żelazo N. obtained by modifying parts, which up to 12 sets could fit. In consequence, it significantly reduced the cost of parts and improved break-even more than twice. It should be noted that even the cost of 252.56 PLN is not cost-effective compared to the market price that does not exceed 10 PLN for products manufactured with IM. However, such high costs may be acceptable if this type of parts are components of larger products, whose production is relatively low. This type of production is typical for example in the aerospace industry. The use of this type of technology in this industry, allows for improving the efficient production of other important indicators such as: Cost Light weighting Reducing buy-to-fly ratio Performance of materials Utilisation of materials Design freedom "new" or "optimised" parts Increased efficiency of supply chain Energy usage (improved fuel efficiency) Simplified assembly process Production efficiency Life cycle cost Life extension 7 Conclusions The topic of a comparison of cost-effectiveness between AM and conventional technologies becomes increasingly popular. It is a crucial aspect of implementation of additive technologies to manufacturing of machine parts. In case of small and parts with complicated geometry the break-even point can reach thousands of pieces. The case study shown that in case of medium size and simple construction of parts the break-even point was placed at the lower number of parts. The thesis that AM is suitable for low-volume production was confirmed.

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