Is Additive Manufacturing an Environmentally and Economically Preferred Alternative for Mass Production?

The manufacturing sector accounts for a large percentage of global energy use and greenhouse gas emissions, and there is growing interest in the potential of additive manufacturing (AM) to reduce the sector’s environmental impacts. Across multiple industries, AM has been used to reduce material use in final parts by 35–80%, and recent publications have predicted that AM will enable the fabrication of customized products locally and on-demand, reducing shipping and material waste. In many contexts, however, AM is not a viable alternative to traditional manufacturing methods due to its high production costs. And in high-volume mass production, AM can lead to increased energy use and material waste, worsening environmental impacts compared to traditional production methods. Whether AM is an environmentally and economically preferred alternative to traditional manufacturing depends on several hidden aspects of AM that are not readily apparent when comparing final products, including energy-intensive and expensive material feedstocks, excessive material waste during production, high machine costs, and slow rates of production. We systematically review comparative studies of the environmental impacts and costs of AM in contrast with traditional manufacturing methods and identify the conditions under which AM is the environmentally and economically preferred alternative. We find that AM has lower production costs and environmental impacts when production volumes are relatively low (below ∼1,000 per year for environmental impacts and below 42–87,000 per year for costs, depending on the AM process and part geometry) or the parts are small and would have high material waste if traditionally manufactured. In cases when the geometric freedom of AM enables performance improvements that reduce environmental impacts and costs during a product’s use phase, these can counteract the higher production impacts of AM, making it the preferred alternative at larger production volumes. AM’s ability to be environmentally and economically beneficial for mass manufacturing in a wider variety of contexts is dependent on reducing the cost and energy intensity of material feedstock production, eliminating the need for support structures, raising production speeds, and reducing per unit machine costs. These challenges are not primarily caused by economies of scale, and therefore, they are not likely to be addressed by the increasing expansion of the AM sector. Instead, they will require fundamental advances in material science, AM production technologies, and computer-aided design software.


Review of Comparative Environmental LCA Study Methods and Scope
A total of 16 studies analyzed the environmental inventories or impacts of a part when it is produced using AM in comparison to when it is produced using traditional manufacturing methods. All of these studies used a comparative life cycle analysis (LCA) approach. Out of the 16 studies, 7 explicitly report following the ISO 14040 standards for LCA. Table S1 provides details regarding the functional unit, data sources, and methods used for each study.
Several of the studies (6 in total) excluded material extraction and/or end-of-life recycling or disposal in scope. Most of these studies compared identical parts made from identical material that would share the same mining processes and end-of-life. Further, most of them accounted for differences in material scrap across AM and traditional manufacturing processes by using either total material used or the embodied energy and/or CO2 emissions in total material inputs as an indicator. As a result, we would expect the exclusion of material extraction and end-of-life to minimally affect the conclusions of these comparative studies. 10 of the studies excluded materials production and/or post-processing steps from the scope of the analysis. Unlike material extraction and end-of-life, these stages differ across AM and traditional manufacturing processes. AM often requires additional postprocessing steps such as hot isostatic pressing to relieve residual stress. In addition, the material feedstock for AM is generally more energy intensive to produce than that of injection molding or milling. As a result, analyses that omit materials production and/or post-processing steps from the study scope are likely to underestimate the lifecycle environmental impacts of AM in comparison to injection molding or milling.
The studies differ in terms of their goals and the environmental indicators used. Bekker and Verlinden (2018) and Faludi et al. (2015) assess end-point environmental indicators following ReCiPe. Paris et al. (2016) and Raoufi et al. (2020) use mid-point indicators following ReCiPe. The remainder of the studies use life cycle inventories as indicators, primarily assessing energy consumption and/or CO2-eq. emissions. Of the 6 studies that assessed multiple environmental indicators, 5 found consistent results in terms of which manufacturing process had lower environmental inventories or impacts across all indicator categories. The one exception is Tang et al., which found that although BJ had lower energy consumption and CO2-eq. emissions compared to milling, it had a higher effect on human toxicity (in terms of kg DCB-eq.) because BJ uses large amounts of bronze, which has higher toxicity impacts during extraction (Tang et al. 2016). While other studies found that AM had lower human health or toxicity impacts compared to traditional manufacturing methods, it is important to note that they did not account for exposure of production workers to inhalation of ultrafine particles, which may significantly affect health effects, particularly when measures are not taken in the manufacturing facility to mitigate these risks (Kolb et al., 2017).
A consistent finding across the studies is that when production volumes are small (approximately 1,000 parts per year or less) and the part has a geometry with a relatively low solid-to-envelope ratio (<1:7), AM has lower energy consumption, lower greenhouse gas emissions-and for the 4 studies that investigated mid-point or end-point environmental S3 impact indicators-lower impacts on ecosystem and human health indicators. This finding remains even when we exclude studies that omit material production or post-processing steps from the scope of the study, which in our assessment are likely to underestimate the lifecycle environmental impacts of AM. While lifecycle environmental impacts are lower at small production volumes, it is possible that AM processes may result in higher human toxicity, especially for Binder Jetting with bronze infiltration or for other AM processes when mitigation measures are not taken to limit operator exposure to ultra-fine particles during AM production (Katz et al., 2020;Kolb et al., 2017;Tang et al. 2016).
Few of the studies addressed uncertainties in impact categories that may affect the comparison of AM with traditional manufacturing methods. It is important to note that some life cycle impact categories can have large sources of uncertainty and may have differentially larger uncertainty compared to others. This may affect the comparison of the lifecycle environmental impacts of AM compared to traditional manufacturing. For example, uncertainties associated with life cycle human toxicity impacts are high, with large sources of variation in many factors, including intake and the likelihood of developing an adverse health effect. Thus, caution should be taken when comparing the point values of human toxicity impacts across AM and traditional manufacturing methods with point values of other impact categories (e.g., energy use).

Review of Comparative Production Cost Study Methods and Scope
A total of 12 studies analyzed the production costs of a part when it is produced using AM in comparison to when it is produced using traditional manufacturing methods. 11 of these studies used a process-based cost approach that identifies the required inputs (e.g., materials, machine time, labor) for each of the production process steps and determines costs of these inputs as well as indirect costs (e.g., maintenance and overhead) that are needed to run the facility. One of the studies used an activity-based costing approach where both direct and indirect costs of production steps are determined based on the time that it takes to complete the step. Out of the 12 studies, 5 validated their models by comparing the model estimates of predicted inputs or production costs with manufacturing companies and/or an AM fabrication testbed. Table S2 provides details regarding the functional unit, data sources, and methods used for each study.
The studies differ in terms of their goals and scope. All studies include part material, machine, and setup costs in scope. Laureijs et al. (2017) and Liu (2017) are the most comprehensive in terms of additionally accounting for support material, post-processing steps, rejected parts, material waste (scrap), labor, maintenance, and overhead costs. 5 out of the 12 studies exclude energy costs from the analysis, which is a relatively small portion of total production costs. 7 of the 12 studies do not include maintenance or overhead costs, which may underestimate the relative costs of AM compared to traditional manufacturing due to the longer machine time required for AM. 4 of the studies exclude support material and/or post processing steps, which could significantly underestimate the costs of AM S4 because AM often requires significant additional material use for support structures and post-processing steps such as hot isostatic pressing to relieve residual stress.
A consistent finding across the studies is that when production volumes are small (between 42-87,000 per year depending on the AM process and part geometry), AM has lower production costs than traditional manufacturing processes such as casting or injection molding. The upper bound was found from a study that did not include support structures, and therefore the upper bound should be interpreted as an estimate most appropriate for part geometries and manufacturing practices that require little to no support structures. Excluding all studies that omit support structures and/or post-processing steps reduces the range of break-even production volumes between AM and traditional manufacturing methods to 42-14,000 per year. Using AM methods that have faster production speeds and parts with geometries that have a higher buy-to-fly ratio moves the break-even production volume toward the upper end of this range.