Quality management and estimation of quality costs for Additive Manufacturing with SLS

Selective Laser Sintering (SLS) is close to become a production technique for functional parts (Additive Manufacturing). One big difficulty remaining and to be solved in the near future is the establishment of a broadly accepted quality system and standards with acceptable costs. The contribution presents a general approach to develop an adequate Q-system for SLS and provide a cost assessment as well. Every particular phase in the SLS production was analysed in this work and appropriate actions to achieve and maintain a high Q-level are proposed. This analysis enables the assessment of the increase in component cost for different quantities of parts. For fully loaded machines, the single part price increase is acceptable and in the low percentage range. A further considered point is the effort regarding material analysis before and during the processing. Furthermore, the finishing costs for part are also taken into account. This comprehensive approach integrates the most important quality steps of Additive Manufacturing for SLS and SLM and gives a prediction of costs for different Q-actions and the part costs add-on. Introduction Additive manufacturing (AM) is a process of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies, such as traditional machining. Selective Laser Sintering and -Melting (SLS and SLM) of plastic and metal powders are part of these ‘layer by layer’ based additive production techniques, which are considered as the next industrial revolution [1,2]. Solidifying powders with laser radiation by means of digital data opens countless options for production of individualized parts with great freedom in complexity [3]. One drawback for these future dedicated technologies is the lack of a common quality understanding. To date there do not exist neither Standards for AM-process and materials nor a generally accepted ‘Quality Management (QM)’ System. This hurdle for a prospective industrial acceptance of AM was recognized and activities by ASTM and ISO towards worldwide standards started concurrently and recently. Both committees, ISO TC 261 and ASTM F42, are still in basic discussions. We can expect comprehensive AM-Standards only some years ahead. In addition, the German association of engineers VDI prepared a recommendation for AM-processing (VDI 3405), which is in draft status actually. Some approaches described in literature aim to improve processes quality aspects or even set up quality management system for AM in literature [4-8]. The circumstances that standardization activities are in its infancy and only few articles were published so far regarding this topic emphasize the necessity to widen the activities as to achieve a comprehensive Quality Management System for Additive Manufacturing. It is vital for the AM future. Quality Management for Additive Manufacturing The following introduced Quality Management (QM) System for AM, especially SLS, was developed within a research project supported by the European Commission. The project deals with all aspects of a future dedicated model for production of spare parts (project acronym: ‘DirectSpare’ [9]). The development of the QM-System was embedded in the elaboration of different relevant business models [10]. Within the framework of the entire model questions regarding process model, data management, data safety, quality of process, standards, certifications and some others issues are included. In order to have a clear demarcation to other aspects, the QM system described in the following is strongly focused on the manufacturing process. It concerns features of the production itself. Questions of e.g. quality of data files and process or business model are not subject of this report. As a first step of this work, the process chain of AM was analyzed thoroughly regarding the main influence parameter. Along the process chain of SLS/SLM, five aspects are of high impact: ‘Equipment’, ‘Material’, ‘Production’, ‘Batch’ and ‘Part’ connected with ‘Finishing’. Especially in case of material it must be distinguished between polymer and metal powder, as plastic powder will usually not be used as delivered but recycled mixed and sieved by any SLS service bureau under own responsibility. Influence parameter and important aspects linked to any of these main aspects of the process chain are specified as well. All is summarized in an Ishikawa diagram as an outcome of the evaluation (see Figure 1). Figure 1: Parameter of production chain AM processes The complexity of AM processes is indicated in Figure 1 and the scheme was used in the project to set up a questionnaire with Q-relevant questions. This Qquestionnaire was submitted to the project partners. Most of the questions are prepared as multiple-choice options but also comments could be given to any question. The Qquestionnaire turned out to be a valuable instrument for gaining comprehensive input to any of the process regions. The following paragraphs summarize the findings to the different aspects. It is presented along the process chain depicted in Figure 1. Any branch of the Ishikawa diagram is described individually. As a general structure the basic responsibility for any aspect is defined primary. Equipment The equipment fitness and performance and all the connected aspects regarding production system are under the responsibility of the part producer (‘service bureau’). The following Quality activities are recommended in order to fulfill basic obligations: Conduct a logbook for any production equipment (SLS, SLM) and perform the following control and maintenance activities and document them (logbook): • Daily: dust cleaning, remove deposits on laser window and adhere to instruction manual; • For system fitness execute a periodical full service (preferably every three month) and check of laser and optical system, temperature control, inert gas supply, replacement of wear parts (filter, scraper…) It is recommended to outsource the service to specialized and skilled people from equipment supplier or distinct service companies. In order to verify the system performance a specially designed reference part (e.g. recommended by VDI 3404) must be built every month as benchmark. This retain sample is analyzed regarding: weight (density), scaling check, dimensions, tolerances, beam offset, surface roughness (different orientation) and should be stored for the whole production period of the machine. Table 1 summarizes the recommended Q-activities for the Equipment at a glance. Some further comments are given and the frequency of actions is defined as well. Plastic Powder In case of Q-control of plastic powders used in SLS process, two aspects are important as cited already earlier. On the one hand, the quality of the new (virgin) powder must be verified and on the other hand at least as important the quality of the mixed powder from virgin and already used powder as well. Figure 3 summarizes the Qelements for plastic powder. The material supplier or powder configurator must expectedly guarantee the quality of virgin SLS material respectively. However if and how certain intake control of the powder receiver should be performed or not will not be answered currently uniform. A check of thermal properties by DSC measurement (DSC = Differential Scanning Calorimetry) seems to be reasonable according to the authors. A discussion of add-on costs concerning this action is discussed in Q-costs section below. Regarding the new (virgin) SLS powder the following material data should be supplied at least to the customer: Production identification (e.g. Charge), Powder particle size distribution (d10/d50/d90), thermal properties like melting point (Tm) and recrystallization (Tc), bulk density. Further data like BET surface ((Brunauer, Emmett and Teller, 1938). molecular weight distribution (Mw, Mn) and flowability as the quotient of bulk and tap volume (Hausner Ratio) would be preferable.