Scholarly article on topic 'Process Planning for the Fuse Deposition Modeling of Ankle-Foot-Othoses'

Process Planning for the Fuse Deposition Modeling of Ankle-Foot-Othoses Academic research paper on "Materials engineering"

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{"Ankle-Foot Orthoses" / "Additive Manufacturing" / "Proess Planning" / "Optimization strategy"}

Abstract of research paper on Materials engineering, author of scientific article — Yuan Jin, Yong He, Albert Shih

Abstract Ankle-foot-orthosis (AFO) is a common assistive device orthoses to provide the support of the foot for users who have the drop foot syndrome. Custom AFOs offer better fit, comfort and functional performance than pre-fabricated ones. The 3D-printing technique is ideal for fabrication of personalized AFOs. Fused deposition modelling (FDM) is a 3D-printing method with the desired strength and material deposition rate for custom AFO applications. The process planning is critical for the cycle time and quality for FDM of AFOs. Four steps in the process planning are: 1) orientation determination, 2) support generation, 3) slicing and 4) tool path generation. In the orientation determination, several factors are taken into accounts to improve the printability and mechanical performance of the fabricated AFO. To reduce the support structure, structural optimizations are applied on the AFO part without compromising of the strength. Adaptive slicing strategy is used to slice the AFO with full consideration of its geometric characteristics. In the tool-path generation, wavy tool-path is developed to improve the manufacturing process and enhance the structural strength by parameters optimization. This study demonstrates the importance of path planning for FDM for custom AFOs.

Academic research paper on topic "Process Planning for the Fuse Deposition Modeling of Ankle-Foot-Othoses"

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Procedia CIRP 42 (2016) 760 - 765

www.elsevier.com/locate/procedii 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM XVIII)

Process Planning for the Fuse Deposition Modeling of Ankle-Foot-Othoses

Yuan Jina,b* , Yong Heb, Albert Shiha,c

a'Department of Mechanical Engineering, University of Michigan, Ann Arbor and 48109, USA bDepartment of Mechanical Engineering ,Zhejiang University, Hangzhou and 310027, China ^Department of Biomedical Engineering, University of Michigan, Ann Arbor and 48109, USA

* Corresponding author. Tel.: +1-734-709-4280 . E-mail address: jinyuan@umich.edu

Abstract

Ankle-foot-orthosis (AFO) is a common assistive device orthoses to provide the support of the foot for users who have the drop foot syndrome. Custom AFOs offer better fit, comfort and functional performance than pre-fabricated ones. The 3D-printing technique is ideal for fabrication of personalized AFOs. Fused deposition modelling (FDM) is a 3D-printing method with the desired strength and material deposition rate for custom AFO applications. The process planning is critical for the cycle time and quality for FDM of AFOs. Four steps in the process planning are: 1) orientation determination, 2) support generation, 3) slicing and 4) tool path generation. In the orientation determination, several factors are taken into accounts to improve the printability and mechanical performance of the fabricated AFO. To reduce the support structure, structural optimizations are applied on the AFO part without compromising of the strength. Adaptive slicing strategy is used to slice the AFO with full consideration of its geometric characteristics. In the tool-path generation, wavy tool-path is developed to improve the manufacturing process and enhance the structural strength by parameters optimization. This study demonstrates the importance of path planning for FDM for custom AFOs.

© 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEMXVIII)

Keywords: Ankle-Foot Orthoses; Additive Manufacturing; Proess Planning; Optimization strategy

1. Introduction

Orthoses, also known as braces, apply force to the body and are designed to meet the biomechanical needs of patients with neuromuscular and musculoskeletal impairments which contribute to functional limitation and disability [1]. Using orthoses can increase users' ability to function and improve their quality-of-life. Each orthosis has specific purposes, such as maintaining or correcting the alignment of a body segment, assisting or resisting joint motion during key phases of the patient's gait, relieving or distributing distal weight-bearing forces, protecting from external stimuli, restoring mobility, and minimizing risk of deformities.

Ankle-foot-orthoses (AFO) are prescribed to patients with musculoskeletal or neuromuscular dysfunction such as stroke, multiple sclerosis, cerebral palsy and others [2]. For patients with unstable ankles, either from injury or muscular imbalance, AFO can be used to support the feet and ankles, maintain optimal functional alignment during activity, and/or to limit motion to protect healing structures. For patients with neuromotor dysfunction, AFO can substitute for inadequate muscle function during key points in the gait cycle, optimize alignment, help to manage abnormal tone, or minimize the risk of deformity (e.g. equinovarus) associated with long term hypertonicity. According to the Healthcare Common Procedure Coding System (HCPCS), AFOs can be categorized depending on whether they are pre- or custom-fabricated [3].

2212-8271 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM XVIII) doi: 10.1016/j.procir.2016.02.315

Pre-fabricated AFOs are usually readily available but provide less comfort and functionality. Custom-fabricated AFOs require an extensive amount of labor time to manufacture. At least two to three patient visits to the clinic are needed. These visits cost patients and caregivers time and travel expenditures, especially for those who live far away from the clinic. Meanwhile, multiple visits for a patient also costs orthotic clinic a tremendous amount of time, thus limiting the number of patients that one orthotist can see.

Current practice for fabrication of custom AFO requires several steps: 1) measurement of the ankle and foot, 2) creating a positive model based on negative plaster impression mold, 3) modifying the positive plaster model to match the anatomy of the patient, and 4) vacuum thermoforming of the AFO and fitting to the patient [4]. This common way to fabricate AFO is beset by four significant challenges: 1) labor intensive and long fabrication time, 2) limited material selection, 3) limited design flexibility, and 4) dependence on the technician's skill to achieve quality consistency.

In order to surmount these drawbacks and realize more effective and efficient fabrication of AFO, new manufacturing methods are required. 3D-printing, also commonly known as additive manufacturing, is a promising technology which could build objects layer-by-layer and directly produce the final parts without any intermediate steps. Meanwhile, novel design geometric features can be readily incorporated using 3D-printing. Although 3D-printing technology has existed for more than two decades, its application areas are often limited to rapid prototyping. Besides great potential in reduction of the product development time and cost, 3D-printing can manufacture fully functional components suitable for loading bearing applications [5].

One of the potential applications of 3D-printing is the fabrication of AFO [6-7]. The concept of using 3D-printing for the fabrication of AFO was published by Milusheva et al.[8] Research has been carried out from the technical and clinical perspectives. From the technical perspective, Faustini et al. [9] fabricated the passive dynamic AFOs using the selected laser sintering (SLS) of Nylon 12, glass-fiber filled Nylon 12 and Nylon 11 and tested in different ways Pallari et al. [4] applied the finite element modeling (FEM) and topology optimization for the design of AFOs fabricated by SLS. Schrank and Stanhope [10] proposed a five-step customization and manufacturing framework on the subject characterization, alignment of the foot and leg segment using landmarks, and the effect of orientation in SLS on the dimensional accuracy. Telfir et al. [2] has developed the AFO with adjustable stiffness in the sagittal plane to change the stiffness on ankle joint and showed the stiffness effect on ankle kinematics in a healthy subject. Schrank et al. [11] integrated the CAD model parameterization and FEM to quantitatively tune and predict and experimentally validate the bending stiffness of the FDM AFO. Clinically, Mavoridis et al. [12] tested two AFOs (flexible and rigid) using stereolithography (SLA) and showed the equivalent walking speed, step length and double support time in the comparison with standard AFO in gait parameter study. Creylman et al. [13] studied the SLS and regular PP AFOs on eight subjects with unilateral drop foot gait. The SLS and PP AFOs showed equivalent performances. Harper et al. [14] conducted a clinical evaluation on 10 subjects with unilateral lower-limb

impairments and measured the gait subjects using regular carbon fiber and stiffness-matched SLS AFOs. Minimal differences in gait performance were observed. SLS AFOs can be applied to study the effects of altering designs on gait performance.

Currently, there are many variations of 3D printing, including SLA, SLS, fused deposition modelling (FDM), binder jetting, and so on. Among these techniques, FDM is easy to implement and can use common thermoplastic material. In the fabrication process, support structure may also need to be designed and built to allow overhanging features to be constructed. FDM has been testified to be able to manufacture parts with detailed and complex geometrical features [15], as well as satisfied strength and stiffness [16], which make it appealing for AFO. Fig. 1 shows the comparison of AFOs made by traditional and FDM methods.

Fig. 1. The AFO fabricated using the traditional and FDM.

This paper presents the process planning for the FDM of AFO by demonstrating several critical steps. Strategies and algorithms for orientation determination, support generation, slicing and tool-path generation are presented respectively by considering the characteristics of FDM technique to illustrate the potential of 3D printing to fabricate AFO.

2. Overview of FDM process planning for AFO

The overview of the process planning for 3D printing of AFO is presented as follows:

(1) Fabrication orientation is determined based on the strength requirement and the characteristic of layer-based manufacturing method, as well as minimal support structures.

(2) An adaptive slicing algorithm is developed to decrease the errors from the staircase effect. The sliced data is further processed to represent the boundary contour more accurately by fitting a large number of short line segments into Non-uniform rational basis spline (NURBS).

(3) Considering the geometrical characteristic of cross sectional shapes, two filling patterns are formulated to apply to different parts of the AFO model. In order to achieve desirable structural strength and acceptable build time, a novel wavy structure filling pattern is developed for the tool-path generation.

Four steps of process planning for 3D printing are shown in Fig. 2, and the strategies are also presented.

Zigzag path Wavy path

Tool-path generation

Fig. 2. Four steps of process planning for 3D printing 3. Methods and characteristics 3.1. Orientation determination

Part orientation is very critical for FDM as it affects build time, support structure, dimensional accuracy, surface finish and cost [17]. Although parts can be deposited in many possible orientations for FDM, the optimal build orientation normally is selected among a few pre-selected orientations, as some other factors will be considered in orientation determination. The selection of candidate plane for part deposition is mainly dependent on four factors: build time, surface finish, support structure, and part strength. Build time is directly influenced by the number of layers, specifically, more layers means more movement along the z axis (perpendicular to the layer) and longer building time consequently. Therefore, minimal number of layers should be one of the goals in selecting part orientation. As the surface quality of parts built by FDM is dominantly affected by the stair stepping effect compared to other 3D printing techniques, surfaces with too large staircase effect are supposed to be avoided. Due to the layer-based manufacturing paradigm, FDM parts have anisotropic property in strength and stiffness, which would affect the mechanical performance of functional parts. For example, the bending strength of the part along the z direction is the weakest as the connection between layers is achieved only by the bonding effect, while not the material internal adhesion. In the orientation determination, surfaces that require high strength should not be along the z direction. Support structure takes time and extra material, as well as deteriorates the surface finish in the support removal process. The part orientation should be selected aiming at minimizing support structures.

Considering aforementioned requirements, three orientations based on the geometric property of AFO, marked as A, B and C, are shown in Fig. 3. Other possible orientations would either affect the surface smoothness that would contact with human body or destroy the integrity of the whole AFO. Orientation A is along the leg axis and it has the most number of layers among all possible orientations, which

is not good for the improvement of fabrication efficiency. Besides, the strength along the direction force commonly applies near the ankle part of the AFO build using FDM is very poor, which would largely affect the lifetime of the AFO. Although minimal support structures are needed in this orientation, it is not suitable in terms of structural strength. Orientation B is implemented by rotating the AFO from the vertical direction by an angle taking the heel as the fulcrum. This approach would enhance the bending strength near the ankle compared to that from direction A, the bending strength at the bottom of AFO, however, becomes poorer gradually as the rotating angle increases. Under this orientation, the support structures are required based on the inclination of the AFO. In order to further improve the bending strength, the third orientation C is provided by lying down the AFO on the base plate. As the extruded lines would be filled along the direction force applies near the back ankle, the bending strength would be sufficient for the daily use. At the same time, the number of layers should be minimal with this orientation.

Candidate orienatation

Support structure Least In between Most

Layer number Most In between Least

Surface quality Best In between Worst

Bending strength

Fig. 3. Different strategies for the orientation determination

The comparison and analysis of three orientations are summarized in Fig. 3. As an orthosis that would be worn by patient every day, AFO should be strong to bear the loads and durable for long-term use. Although orientation C requires more support structure and has poor surface finish on both sides near the ankle due to the stair stepping effect, we select this orientation as the structural strength has the highest priority.

3.2. Support generation

Support generation is a key technology in FDM and plays an important role in guaranteeing the parts process smoothly by constraining the deformation of the parts and enabling the fabrication of overhanging features [18]. After the model orientation has been specified, overhang features of the model are recognized and then some additional structures are designed to support these features and would be removed after the building process. In the support generation, several technical factors should be taken into account, such as material consumption, building time, and surface finish after the removal process. With increasingly complex structures and shapes in FDM, more flexible and robust support generation methodology is required. In recent years, many support generation algorithms have been developed from different perspectives of support structure, support technics and support generation strategies. Among them, sliced data-

In between

based support generation method is desirable and reliable as it can avoid the difficulties in the 3D offsetting and Boolean operation of STL model such as self-intersections and the existence of numerous invalid triangles [19].

As for the FDM of AFO, based on the selected orientation, areas that need support structure can be found after a threshold value for the inclination angle is set. Original AFO without any modification required a large volume of support structure is shown in Fig. 4(a). Through analysis clinically and technically, both side of the AFO near the calf can be removed to significantly reduce the usage of the support structure at the sacrifice of minimal strength loss. As shown in Fig. 4(b), the volume of support structure decrease by about 30% after the structure modification.

\ AFO part^— Support structure

Fig. 4. Support generation on (a) original AFO and (b) modified AFO with reduced calf region

3.3. Adaptive slicing

is defined as the maximum value among the cusp height ct in the whole boundary contour of the layer. The cusp height ct of each point is the distance between the vertex of a layer (O) and the actual surface of part model in the normal direction, and the normal direction is obtained from the average value of normal vectors of the adjacent layers (n1 and n2). With a specified cusp height tolerance cmax, the optimal thickness of each layer (h) can be estimated using its normal vectors. The adaptive slicing is as illustrated in Fig. 5.

(b) Detail of layers Adaptive layers |\ it

(a) Part model

( c) Cusp height

Fig. 5. Scheme of adaptive slicing

: \ 1® ^

The slicing procedure is to get the intersecting boundary contours between sliced layers which are parallel to the building plate and the model based on a designed layer thickness distribution. As the 3D AFO model is reconstructed based on the scanned cloud data from patients' foot and ankle and output as STL file, which represents 3D models using a large number of surface triangles. The achieved boundary contours are consisted of a large number of tiny line segments. In order to alleviate the approximation errors from the triangular models, the sliced contours are fitted into NURBS by taking the intersections as fitting points. When the line segment is shorter than a threshold value and the angles between the current line and its adjacent lines are larger than a specified value, this line can be fitted into NURBS. This procedure can help to maintain the geometrical accuracy of the original AFO model.

Adaptive slicing is an optimization strategy which can relieve the staircase effect by adjustment of layer thickness distribution based on the geometric characteristic of models. In the optimization process, the thickness of each sliced layer is mainly determined by the inclination and curvature of the model surface at a certain height. To avoid large staircase effect, the cusp height c (as shown in Fig. 5 (c)) should be limited to a certain value cmax. The cusp height c on one layer

Fig. 6. Partition of lying AFO based on the inclination of surface

Based on the concept of the adaptive slicing, the lying AFO is divided into three parts, denoted as A, B, and C, according to the inclination of the surface as shown in Fig. 6. Parts A and C are top and bottom parts, respectively, with inclination larger than a give value amax and would lead to the staircase effect. In part B, the sliced layers would have a relatively large layer thickness as the stair stepping effect is very slight. The layer thickness in Parts A and C should be limited to a certain value in case some details would be lost.

Part B

3.4. Tool path generation

Tool-path planning for FDM fundamentally affects fabricating efficiency and part quality. A desirable depositing tool-path can not only improve the part precision, surface quality, and strength of prototypes, but also save the building time and material usage [20]. Commonly, there are two tool-path planning strategies: contour-parallel path and direction-parallel path [21]. Contour-parallel tool-path adopts successive offsets of the boundary contours as tool-path elements, and each successive offset can be obtained using classical Voronoi-diagram approach, while direction-parallel tool-path contains a large number of line segments which are parallel to a specified inclination. These two strategies possess distinct characteristics with respect to depositing quality and efficiency [22]. Although the contour-parallel tool-path has been studied in traditional milling machining, the most common tool-path in 3D printing is the direction-parallel one due to its practical advantages in terms of easy implementation and high-speed depositing [23]. Two keys in the tool-path planning for FDM are filling strategy and tool sequencing strategy [24]. The filling strategy mainly addresses the problems of filling up a patch of internal area continuously without halting the deposition process and has been studied widely in fields of additive manufacturing as well as conventional milling manufacturing. Tool sequencing strategy represents the connection of sub-paths with an appropriate order [25].

In this research, the contour-parallel tool-path is adopted to deposit the boundaries to achieve decent surface smoothness, while the interior part confined by the offsets of the boundaries is filled with direction-parallel tool-path in light of its high efficiency and simple implementation. Generation of contour-parallel tool-path has been studied for years and its complexity and difficulty is mainly determined by the realization of offset algorithm, especially with respect to complex geometries. Few optimizations can be performed to enhance the machining efficiency and increase the depositing quality. By contrast, the strategy of direction-parallel tool-path in the interior filling is still worth more research due to its variable and flexible. Three essential factors that need to be considered comprehensively in direction-parallel tool-path planning are: 1) inclination angle of reference lines, 2) partition of filling areas for dense or sparse filling, and 3) linking of tool-path elements.

Combining aforementioned three factors, tool-paths for layers from the slicing are obtained by Insight software (Stratasys Inc.) as shown in Fig. 7. We can see that tool-paths on one layer have different density distribution depending on their roles: support areas far away from the part are filled with sparse structure while areas closer to the part should be filled with denser structures to hold the part, the part itself have the highest density filling.

Fig. 7. Tool-paths for different layers from bottom to top

The tool-path in some layers with narrow and long cross sectional shapes includes a large number of short line segments, which would impose negative impacts on the deposition time and filling quality. Besides, the anisotropic strength distribution of the parallel tool path strategy would lead to poor mechanical performance of the fabricated AFOs. To solve these issues, we develop a novel filling strategy for layers with narrow and long cross sectional shapes - wavy tool path, which fills the whole internal area with one wave. Fig. 8 shows the difference of wavy path and common zigzag path. It is clear that the wavy path have no sharp turns, which means there is no deceleration and acceleration during the deposition process. The bending tests were applied on specimens fabricated with these two filling strategies, and the results showed that the flexural strength was higher than the paths with zigzag path by 6%, while the build time could be reduced by 23% using the wavy path and the weight was less by 17%.

Fig. 8. Difference of filling strategies: (a) zigzag tool path and (b) wavy tool path

In the AFO fabrication, the cross sectional shape of layers in part B is long and narrow, so the wavy path is adopted. The wavy path generation algorithm is illustrated in Fig. 9. The boundary contours of the sliced layer are preprocessed into two curves (inner one and outer one) firstly. Then the curves are fitted into splines for sampling point generation on both curves. After the sampling points have been picked up, another spline curve can be generated by taking points on both curves alternately as the fitting points.

Fig. 9. Flowchart of wavy path generation algorithm.

Figure 10 shows an example of the wavy tool path on one layer. There are two major parameters for the waves: period length T and the distance between the wave and the boundary S. These two geometric parameters would affect the mechanical strength significantly.

Fig. 10. Wavy tool path on one layer

Because the wavy path on each layer is generated based on its own geometrical shape without consideration of its adjacent layer, wavy paths on adjacent layers would not overlap with each other perfectly. The offset between wavy paths between layers would largely affect the printability and the structural strength. To solve this problem, position modifications on the sampling points of each layers should be applied on one layer considering the wavy paths on its below layers from bottom to top. Wavy paths generated from the adjusted sampling points can be fabricated by FEM smoothly.

4. Conclusions

This study provided an overview of the process planning for FDM of AFO. By illustrating four critical steps in the process planning for FDM, the 3D AFO model from the scanned cloud data were processed step by step for FDM. The future work is to link with FDM machine characteristics on material deposition rate, speed, and acceleration for optimization of AFO FDM time and surface quality as well as to expand the path planning for FDM to other types of custom orthoses, such as foot orthoses and thoracolumbosacral orthosis (TLSO).

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