Abstract
As the flexibility, efficiency, and application space of additive manufacturing continues to grow, many have begun to investigate more sustainable feedstocks as well as options for the end of life of additively manufactured parts. This study examines the effects of mechanical recycling on additively manufactured parts from bio-based feedstock. Articles were printed on the Big Area Additive Manufacturing (BAAM) system at the Oak Ridge National Laboratory using poly (lactic acid)/wood flour (PLA/WF) pellets. These parts were shredded and granulated, and the granulate was fed directly back into the BAAM system for re-printing, skipping the costly and energy-intensive steps of extrusion and pelletization. The chemical, mechanical, thermal, and rheological changes to PLA/WF before and after recycling were investigated. Additionally, the energy savings from directly printing granulate on the BAAM system without extrusion and pelletization is reported. It is shown that PLA/WF is an excellent candidate for recycling of large format additively manufactured parts, and value of these parts can be reclaimed while saving cost and energy through mechanical recycling.
| Original language | English |
|---|---|
| Article number | 110617 |
| Journal | Composites Part B: Engineering |
| Volume | 255 |
| DOIs | |
| State | Published - Apr 15 2023 |
Funding
The authors acknowledge the support from the US Department of Energy (DOE) Advanced Materials and Manufacturing Technologies Office and used resources at the Manufacturing Demonstration Facility at Oak Ridge National Laboratory, a User Facility of DOE's Office of Energy Efficiency and Renewable Energy. This manuscript has been authored by UT-Battelle, LLC under CPS 848 Agreement 35714 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan ( http://energy.gov/downloads/doe-public-access-plan ). Microscopy and spectroscopy studies were completed at the Center for Nanophase Materials Sciences, a DOE Office of Science User Facility. Samples were printed on the Big Area Additive Manufacturing (BAAM) large-scale thermoplastic extruder. The BAAM consists of a single screw extruder mounted on a gantry system above a heated bed. An image of the BAAM extruder and gantry is available in the supporting information (SI, Fig. S1). PLA/WF pellets or recycled granulate (described below) were placed in a forced-air dryer (Dri-Air Industries, Inc.) held at approximately 60 °C for 4 h prior to printing. Dry pellets/granulate were continuously fed from the dryer to the extruder during printing using a vacuum system. A polycarbonate build sheet was placed on the bed heated to 60 °C and held in place with a separate vacuum system. The BAAM extruder consists of five heating zones which were set to a profile of 170-190-210-215-200 °C, with the lowest temperature at the pellet feeding zone (top) and highest temperatures at the nozzle (bottom). The temperature profile was chosen to aid in extrusion of the materials and minimize the load on the extruder motor while avoiding thermal degradation of the feedstocks. The BAAM was fitted with a 7.6-mm diameter nozzle, which is a standard nozzle size produced by Cincinnati, Inc. for the system.All images of the AM samples, both optical and SEM, shown in Fig. 4 appear similar, with no obvious differences between the virgin and recycled or x- and z-directions. However, the fractures surfaces of the IM samples are markedly different than the printed samples. The most distinct feature present on all printed sample fracture surfaces is the presence of voids, some of which are indicated with black arrows in Fig. 4. Such voids were seemingly absent from the IM samples, which supports trends reported in literature comparing IM and AM parts [8,37,43,46]. The voids in the printed samples likely contributed significantly to their reduced mechanical performance in comparison to the IM samples. The void content of all 4 a.m. samples appears similar. The presence of these intra-bead voids is common in AM parts and likely results from air entrapment during feeding and initial melting and potential offgassing of the materials during extrusion. Additionally, gaps between fillers and the polymer matrix of composites due to poor interfacial interactions and variations in the coefficient of thermal expansion of the two phases can increase the overall porosity of both AM and IM samples [45,48]. Another key difference contributing to the higher porosity of AM samples in comparison to IM is that material is subjected to high pressure and shear in AM while moving through the screw barrel and nozzle. When material exits the nozzle, it is released to atmospheric pressure, allowing it to expand before cooling. Conversely during IM, material is injected into a mold at a high pressure, and the material is cooled below its Tg before the pressure is released, resulting in a denser structure with lower porosity. Numerous attempts have been made by researchers to reduce porosity and increase the performance of AM parts through techniques such as venting the extruder during printing and using smaller nozzles or screw designs with higher compression ratios [49,50]. The optimization of printing parameters and extruder design for production of PLA/WF parts with minimized porosity bears further investigation. As the virgin and recycled materials were produced with the same extruder configuration and parameters in this study, it can be assumed from analysis of optical and SEM images coupled with the results of tensile testing previously presented that recycling an additively manufactured PLA/WF part has little to no effect on the overall void content after remanufacturing.The authors acknowledge the support from the US Department of Energy (DOE) Advanced Materials and Manufacturing Technologies Office and used resources at the Manufacturing Demonstration Facility at Oak Ridge National Laboratory, a User Facility of DOE's Office of Energy Efficiency and Renewable Energy. This manuscript has been authored by UT-Battelle, LLC under CPS 848 Agreement 35714 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan). Microscopy and spectroscopy studies were completed at the Center for Nanophase Materials Sciences, a DOE Office of Science User Facility.
| Funders | Funder number |
|---|---|
| Advanced Materials and Manufacturing Technologies Office | |
| Big Area Additive Manufacturing | |
| Center for Nanophase Materials Sciences | |
| DOE Public Access Plan | |
| Dri-Air Industries, Inc. | 45,48, 8,37,43,46 |
| United States Government | |
| U.S. Department of Energy | |
| Office of Science | |
| Office of Energy Efficiency and Renewable Energy | 35714 |
| Oak Ridge National Laboratory |
