Estimation and Improvement of the Achievable Tolerance Interval in Material Extrusion Additive Manufacturing through a Multi-State Machine Performance Perspective

Abstract

Dimensional quality is still a major concern in additive manufacturing (AM) processes and its improvement is key to closing the gap between prototype manufacturing and industrialized production. Mass production requires the full working space of the machine to be used, although this arrangement could lead to location-related differences in part quality. The present work proposes the application of a multi-state machine performance perspective to reduce the achievable tolerance intervals of features of linear size in material extrusion (MEX) processes. Considering aspecific dimensional parameter, the dispersion and location of the distribution of measured values between different states are analyzed to determine whether the production should be treated as single-state or multi-state. A design for additive manufacturing strategy then applies global or local size compensations to modify the 3D design file and reduce deviations between manufactured values and theoretical values. The variation in the achievable tolerance range before and after the optimization of design is evaluated by establishing a target machine performance index. This strategy has been applied to an external MEX-manufactured cylindrical surface in a case study. The results show that the multi-state perspective provides a better understanding of the sources of quality variability and allows for a significant reduction in the achievable tolerance interval. The proposed strategy could help to accelerate the industrial adoption of AM process by reducing differences in quality with respect to conventional processes.

A Systematic Method for Assessing the Machine Performance of Material Extrusion Printers

“…For conventional processes (e.g., milling, turning, and moulding [13]), the capability assessment is standardized with ISO 22514 [33]. Two main types of indicators can be computed depending on the statistical stability of the process.…”

“…Out-of-Specification Parts [ppm] U-L Distance Usually, the machine performance assessment is conducted for a process with an already defined TI. However, in the case of additive manufacturing processes and due to their relative youth, there is a lack of standards and knowledge of the proceses [13]. No standard defining the tolerance intervals for AM printers currently exists.…”

“…Table 2 gives a global overview of the existing literature on capability for AM printers. The different gathered papers [13,[42][43][44][45][46][47][48][49][50] were compared depending on:…”

“…This has hampered the widespread use of AM in industry (especially in critical sectors, such as in the biomedical, aeronautical, military, automotive, and energy sectors) [11]. Indeed, even with the creation of two technical committees (ASTM F42 and ISO/TC 261) [3], there is still lack of data in some areas, such as the methods to assess the dimensional and geometrical performance of AM printers [13]. In the design stage, the resulting tolerance intervals (TIs) allow the fitting between parts to be foreseen as well as the machining stock allowance to be determined.…”

“…Despite the large number of studies evaluating the short-term performance of AM printers (by printing a limited number of GBTAs, usually five), only a few studies have focused on their longer-term performance using the concept of capability [13]. However, when producing parts, a more global perspective is needed to foresee the behavior of the machine in the long term as well as its effectiveness to meet the customer needs (TI) [28].…”