Sunil Bhandari

Environmental Durability of Bio-Based and Synthetic Thermoplastic Composites in Large-Format Additive Manufacturing

Published article

Abstract

This research investigates the durability of large-format 3D-printed thermoplastic composite material systems under environmental exposure conditions of moisture and freeze–thaw. Durability was evaluated for two bio-based composite material systems, namely wood-fiber-reinforced semi-crystalline polylactic acid (WF/PLA) and wood-fiber-reinforced amorphous polylactic acid (WF/aPLA), and one conventionally used synthetic material system, namely short-carbon-fiber-reinforced acrylonitrile butadiene styrene (CF/ABS). The moisture absorption, coefficient of moisture expansion, and reduction of relevant mechanical properties—flexural strength and flexural modulus—after accelerated exposure were experimentally characterized. The results showed that the large-format 3D-printed parts made from bio-based thermoplastic polymer composites, compared to conventional polymer composites, were more susceptible to moisture and freeze–thaw exposure, with higher moisture absorption and greater reductions in mechanical properties.

Efficient Residual Stress Modeling for Large-Format Polymer Composite Extrusion-Based Additive Manufacturing

Published article

Abstract

Residual stresses are generated in thermoplastic composite parts produced using extrusion-based additive manufacturing (AM). The layer-by-layer deposition of molten thermoplastic composite material and the subsequent cooldown results in the differential thermal contraction of the deposited layers, giving rise to residual stress in the additively manufactured parts. These residual stresses depend on the material properties of the polymer composite and the processing parameters used during the AM process. The residual stresses affect the mechanical performance and the final printed shape of the manufactured part. Efficient numerical modeling methods can predict residual stresses for AM parts and hence help design geometrically accurate and mechanically reliable parts. However, large-scale AM introduces computational challenges for evaluating residual stresses due to the large number of degrees of freedom in the numerical models. This research work explores the use of an explicit thermal model and a mesh merging technique to expedite the numerical analysis of thermal history and residual stresses in large-scale additively manufactured thermoplastic composite parts. The goal of the research is to develop an efficient modeling method for aiding the design of AM polymer composite parts.

Recycling Large-Scale 3D Printed Polymer Composite Precast Concrete Forms

Published article

Abstract

Large-scale thermoplastic extrusion-based 3D printing, also referred to as additive manufacturing (AM), has shown promise for a multitude of applications in civil infrastructure. Advancements in this technology have led to increased usage and a subsequent increase in generated waste. Recently, the opportunity to recycle this material has contributed to lessening this waste. Presented in this work are established baseline thermo-mechanical and physical properties of carbon fiber–acrylonitrile butadiene styrene (CF-ABS) and wood flour–amorphous poly lactic acid (WF-aPLA). Additionally, this work is to be used in comparison with subsequent recycling cycles in the evaluation of the mechanical recycling process for large-scale 3D printed CF-ABS and WF-aPLA with intended applications as formwork for precast concrete.

Design and Manufacture of Precast Concrete Formworks Using Polymer Extrusion-Based Large-Scale Additive Manufacturing and Postprocessing

Published article

Abstract

Large-scale thermoplastic polymer extrusion-based additive manufacturing (AM) has been used to fabricate precast concrete formworks. There are some limitations inherent to the large-scale AM process that need to be overcome to design complex, multipart additively manufactured formworks to be used for precast concrete. This research work uses a large-scale polymer composite AM process to manufacture two-part formworks. Postprocessing was used to repair imperfections, create smooth casting surfaces, achieve precise dimensional tolerance, and incorporate assembly mechanisms for multipart formwork. Two biodegradable polymer composites (wood-fiber polylactic acid and wood-fiber amorphous polylactic acid) and a conventional polymer composite (carbon fiber acrylonitrile butadiene styrene) were selected to manufacture four sets of two-part formwork. Design details, including the cellular infill pattern, continuous toolpath, and layer time selection, are presented. Postprocessing and repairs performed on the manufactured formworks to get the required dimensional tolerance and surface smoothness are discussed.

Coupled thermo-mechanical numerical model to minimize risk in large-format additive manufacturing of thermoplastic composite designs

Published article

Abstract

The collapse of deposited thermoplastic composite material under self-weight presents a risk in large-format extrusion-based additive manufacturing. Two critical processing parameters, extrusion temperature and deposition rate, govern whether a deposited layer is stable and bonds properly with the previously deposited layer. Currently, the critical parameters are determined via a trial-and-error approach. This research work uses a simplified physics-based numerical simulation to determine a suitable combination of the parameters that will avoid the collapse of the deposited layer under self-weight. The suitability of the processing parameters is determined based on the maximum plastic viscous strains computed using a sequentially coupled thermo-mechanical numerical model. This computational tool can efficiently check if a combination of temperature and extrusion rate causes layer collapse due to self-weight, and hence minimize the manufacturing risk of large-format 3D-printed parts.

A graph-based algorithm for slicing unstructured mesh files

Published Article

Abstract

Process planning is an important step in additive manufacturing processes that involves the creation of a deposition toolpath from input geometry. The most commonly used input geometry file formats are in the form of unstructured triangular meshes. Previous research works have presented different algorithms for slicing an unstructured triangular mesh by a series of uniformly spaced parallel planes. Recent research works show that the use of hashtable data structures results in an optimal algorithm that improves the efficiency of the slicing process. The presented algorithm minimizes the use of hashtable in the slicing process by using graph-based data structures. It was found that the slicing can be sped up by about 10 times by using the graph-based algorithm compared to the previous algorithm. The use of parallel processing on multi-core processors was explored to further speed up the slicing process. The proposed efficient algorithm can be useful in process planning optimization.

Large-scale extrusion-based 3D printing for highway culvert rehabilitation (Best paper award at SPE-ANTEC 2021 Additive Manufacturing / 3D printing track)

Published Conference Paper

Abstract

A significant problem associated with repairing deteriorating highway culverts is the resultant lowered flow capacity. This can be mitigated by the use of culvert diffusers. Current culvert diffusers are made using fiberglass reinforced thermosetting epoxy polymers, which require custom-made molds. This research work explores the use of large-scale 3D printed thermoplastic polymer composite to manufacture culvert diffusers. The research work shows that 3D printing technology reduces the manufacturing time as well as the cost of culvert diffusers. Large-scale 3D printing technology is well-suited for the manufacture of individualized culvert diffusers with unique geometrical designs without the need for molds. 3D printing technology is also capable of using different materials according to environmental requirements. The use of segmental manufacturing in conjunction with large-scale 3D printing enables the manufacturing of culvert diffusers larger than the build envelope of the 3D printer. Different post-processing techniques used for cutting, finishing, and joining the 3D printed segments are discussed.

Discrete-event simulation thermal model for extrusion-based additive manufacturing of PLA and ABS 

Published Article

Abstract

The material properties of thermoplastic polymer parts manufactured by the extrusion-based additive manufacturing process are highly dependent on the thermal history. Different numerical models have been proposed to simulate the thermal history of a 3D-printed part. However, they are limited due to limited geometric applicability; low accuracy; or high computational demand. Can the time–temperature history of a 3D-printed part be simulated by a computationally less demanding, fast numerical model without losing accuracy? This paper describes the numerical implementation of a simplified discrete-event simulation model that offers accuracy comparable to a finite element model but is faster by two orders of magnitude. Two polymer systems with distinct thermal properties were selected to highlight differences in the simulation of the orthotropic response and the temperature-dependent material properties. The time–temperature histories from the numerical model were compared to the time–temperature histories from a conventional finite element model and were found to match closely. The proposed highly parallel numerical model was approximately 300–500 times faster in simulating thermal history compared to the conventional finite element model. The model would enable designers to compare the effects of several printing parameters for specific 3D-printed parts and select the most suitable parameters for the part. 

Large Scale 3D Printed Thermoplastic Composite Forms for Precast Concrete Structures 

Published Conference Paper

Abstract

Recent advances in large-scale 3D printing and thermoplastic composite materials with bio-based fillers and reinforcements have great potential for expanding the possibilities of making forms for precast concrete structures. The 3D printing technology for making molds, forms and tooling for precast concrete is expected to reduce labor cost and minimize waste. 3D printed forms allow design optimization of precast concrete parts since the additive manufacturing cost is only function of thermoplastic material weight and is independent of part complexity. Additionally, 3D printed forms can become and asset, since thermoplastic composite materials can be reprocessed. The design and manufacturing of 3D printed forms for casting concrete encompass three steps, as follows: 1) Prediction and reduction of shrinkage effects; 2) Structural design for strength and stiffness requirements; and 3) Model slicing, part printing, and surface finishing. The type of surface finishing and the dimensional stability and durability of the thermoplastic material selected for concrete forms are examined. The lessons learned from projects using large-scale 3D printing technology for formwork are discussed. 

Elasto-plastic finite element modeling of short carbon fiber reinforced 3D printed acrylonitrile butadiene styrene  composites 

Accepted Manuscript

Published Article

Abstract

 This research extends the existing classical lamination theory (CLT) based finite element (FE) models to predict elasto-plastic and bimodular behavior of 3D printed composites with orthotropic material properties. Short carbon fiber (CF)-reinforced acrylonitrile butadiene styrene (ABS) was selected as the 3D printing material. Material characterization of a 3D printed unidirectional laminate was carried out using mechanical tests. A bimodular material model was implemented using explicit FE analysis to predict the tension and bending behavior of a 3D printed laminate. The results of the FE model predictions were experimentally validated.  Hill’s yield function was effective at predicting the elasto-plastic stress-strain behavior of the laminate in tension. In bending, bimodular material behavior along with Hill’s yield function worked reasonably well in predicting the elasto-plastic bending of the laminate. The material model proposed can be used to predict the mechanical behavior of 3D printed parts with complex geometry under complex loading and boundary conditions.

Enhancing the interlayer tensile strength of 3D printed short carbon fiber reinforced PETG and PLA composites via annealing 

Accepted Manuscript

Published Article

Abstract

Previous studies have shown that 3D printed composites exhibit an orthotropic nature with inherently lower interlayer mechanical properties. This research work is an attempt to improve the interlayer tensile strength of extrusion-based 3D printed composites. Annealing was identified as a suitable post-processing method and was the focus of this study. Two distinct thermoplastic polymers, which are common in 3D printing, were selected to study the enhancement of interlayer tensile strength of composites by additive manufacturing: a) an amorphous polyethylene terephthalate-glycol (PETG), and b) a semi-crystalline poly (lactic acid) (PLA). It was determined that short carbon fiber reinforced composites have lower interlayer tensile strength than the corresponding neat polymers in 3D printed parts. This reduction in mechanical performance was attributable to an increase in melt viscosity and the consequential slower interlayer diffusion bonding. However, the reduction in interlayer tensile strength could be recovered by post-processing when the annealing temperature was higher than the glass transition temperature of the amorphous polymer. In the case of the semi-crystalline polymer, the recovery of the interlayer tensile strength was only observed when the annealing temperature was higher than the glass transition temperature but lower than the cold-crystallization temperature. This study utilized rheological and thermal analysis of 3D printed composites to provide a better understanding of the interlayer strength response and, therefore, overcome a mechanical performance limitation of these materials.

 Finite element modeling of 3D-printed part with cellular internal structure using homogenized properties 

 Accepted Manuscript

Published Article 

Abstract 

The purpose of this research is to create a homogenized linearly elastic continuum finite element model of a 3D-printed cellular structure. This article attempts to answer the following research question: can the homogenization technique based on using virtual experiments, commonly employed in micromechanics solid modeling, be used for homogenization of 3D-printed cellular structure to generate orthotropic material properties? Virtual experiments were carried out for homogenization of cellular structure. These virtual experiments generated homogenized material properties for the continuum finite element model. Physical experimentation was carried out to validate the accuracy of results obtained from the continuum finite element model. Results show that the outlined procedure can be used to generate a fast, yet reasonably accurate, continuum finite element model for predicting the linearly elastic structural response of 3D-printed cellular structure. This study extends the micromechanics homogenization approach to homogenize the 3D-printed partial infill cellular structure to create input material properties for a continuum finite element model. The outlined procedure would enable faster iterative design of 3D-printed cellular parts. The continuum model generated is valid only for a linearly elastic structural response. This framework, however, has potential for extending the analysis to the inelastic range.