Additive Manufacturing (AM) technologies have progressed in the past few years and many of them are now capable of producing functional parts instead of mere prototypes. AM provides a multitude of benefits, especially in design freedom. However, it still lacks industrial relevance because of the absence of comprehensive design rules for AM. Although AM is usually advertised as being the solution for all traditional manufacturing design limitations, the fact is that AM only replaces these limitations with a different set of restrictions. To fully exploit the advantages of AM, it is necessary to understand these limitations and consider them early during the design process. The establishment of design considerations in AM enables parts and process optimization. This paper discusses the design considerations that will lead to optimize part quality. Specifically, the work discusses the Fused Deposition Modeling (FDM) due to its common use and availability. These considerations are drawn from literature and from experiments done by the authors. The experiments done by the authors include an investigation for the influence of elevated service temperature on the performance of FDM PLA parts, benchmarking the capability of FDM to print overhangs and bridges without supports, studying the influence of processing parameters over dimensional accuracy, and the effect of processing parameters on the final FDM samples modulus of elasticity. The work presents a case study investigating the correct clearances for FDM parts and finally a redesign for AM case study of a support bracket originally manufactured using traditional manufacturing methods taking into consideration the design considerations discussed in this paper.
Fused Deposition Modelling (FDM) is an Additive Manufacturing (AM) technology that builds parts by heating and extruding filaments through a small nozzle. FDM usually deals with thermoplastics or composite materials. The nozzle follows computer controlled paths as in Computer Numerical Control machines (CNCs) while extruding the material to draw layers on top of each other to create the part as shown in
FDM was developed in the 1980s and commercialized in 1990 as a rapid prototyping technology. FDM computer programs perform slicing and patching of the part’s cross section to create a sequence of extruded layers. These extruded layers are formed from contour raster or occasionally called shells. Shells are filled with infill raster as shown in
After printing the outlines, the extruder fills the area inside the contours with infill patterns. Common infill patterns include honey comb, triangular and rectilinear infill patterns as shown in
gap is negative. The generated design can interact with Computer-Aided Design (CAD) tools via an STL file type.
FDM technology is being commercialized as a rapid manufacturing technology as other AM schemes and advertised with “your imagination is the limit”. The fact is, at least currently, most AM technologies only replace the limitations of traditional manufacturing methods with new restrictions and challenges. According to Wittbrodt et al. [
Despite the potential of FDM process in adding flexibility and reducing shape dedicated tools requirements, the current body-of-knowledge is lacking the design considerations necessary for a designer to guide the design of the part to meet the fabrication process requirements. A scheme is known as Design-for Manufacturing (DFM) [
Many researchers investigated the effect of FDM process parameters on the properties of the final part [
However, the body-of knowledge lack comprehensive analysis for the design guidelines for parts fabricated by FDM in terms of the process parameter. This paper research goal is to answer the following questions:
1) What are the FDM process parameters that affect the final parts characteristics?
2) What are the FDM design guidelines necessary to apply DFM principles during initial stages of the part’s design?
3) Finally, a case study is presented to demonstrate the effect of FDM on the final part design.
This paper discusses the major design considerations for FDM parts from manufacturing/fabrication and functionality point of views. The design consideration investigated in this work will focus on the mechanical properties and topology of FDM parts. Other aspects such as thermal or electrical properties are out of the scope of this paper.
FDM parts start with an idea or identification of a need and coming up with a function to aid that need or improve an existing system. Regularly, it is not feasible to request a new design for every updated need. Therefore, experience and use of existing designs are employed to reduce design cost and time. However, available earlier designs are set to be fabricated by other manufacturing methods, which are not necessarily compatible with FDM. In the case of FDM, there are sets of limitations that need to be taken into consideration, such as the part’s orientation, functional optimization and manufacturing, paths optimization [
The following section investigates the FDM processing parameters and their optimized values that can be set during the initial phases of the part’s design.
CAD tools are necessary to successfully utilize AM technologies. CAD tools were optimized to aid the requirements and limitations of traditional manufacturing methods. Most CAD tools are built with the assumption of homogenous part’s interior from both geometrical and materials point of views. An appropriate CAD tool for AM should allow the freedom to tailor the material’s distribution and composition inside the part. In addition, it should allow the part’s fabrication to be free of the constraints of traditional manufacturing methods. Thus, we optimize the topology of the part according to the loads and functionality instead of manufacturability. An example of used CAD tools was discussed in Srinivas Bhashyam et al. 2000 [
Due to the nature of AM that builds the parts layer by layer, the mechanical properties of the part are a function of the material used and the processing parameters. Processing parameters include the building orientation, extrusion temperature, overlap percentage, infill patterns and many others. Therefore, the strength of the part and its stiffness depend on the strength of the fusion and bonding between the extruded infill and the air gaps separating them. This makes simulating FDM parts in FEA a challenging task. Until now, there are no available packages for simulating AM parts and research is still in progress. In literature, there are several approaches taken in simulating AM parts. The first approach is done by testing tensile specimens in all principle directions to collect the stiffness matrix of the material. Then using this stiffness matrix simulations can be performed and verified [
The material library for FDM is constantly increasing to improve the applicability of FDM [
Consequently, when designing for FDM, designers are limited by the small range of materials. Currently, the most common FDM filaments are PLA, ABS and Nylon for printing parts while PVA and HIPS are the most common support filaments due to their dissolvability. New filaments that use TPE or TPU (Thermoplastic polyurethane) print parts with much higher elasticity than ABS and PLA. Composite filaments include carbon fiber, metal powder or wood fibers with PLA, Nylon [
The mechanical properties of the material degrade significantly near the glass transition temperatures as shown in an experiment conducted by the authors. The tensile test specimens of PLA material were created in both orientations; horizontally and vertically and tested at room temperature, 50˚C and 60˚C using a controlled oven chamber attached to the universal tensile testing machine. A total of 18 fabricated specimens were tested in addition to 9 filament (raw material) specimens. The FDM specimens were fused using 0.20 mm layer thickness, three shells, 0.50 mm raster width, an extrusion temperature of 200˚C, and 40 mm/s printing speed with a nozzle of 0.5 mm in diameter on Taz 6® FDM system using Pro series PLA filament of 3.0 mm diameter.
Material | Flexural Modulus [MPa] | Young’s Modulus [MPa] | Flexural Strength [MPa] | Ultimate Strength [MPa] | Ductility [mm/mm] |
---|---|---|---|---|---|
ABS [ | 1250 | 1600 | 40 | 25 | 5% |
PLA [ | - | 3333 | - | 50 | 1.5% |
Nylon [ | 1200 | 1400 | 60 | 50 | 5% |
One of the major current limitations of FDM, and AM in general, is the small build volume that the process permits. Though there are FDM printers with
larger building volumes, most commercial FDM printers have building volumes less than 30 cm × 30 cm × 30 cm. Similarly, most FDM printers have Cartesian coordinate axis. However, many FDM printers have Delta coordinate as shown in
Most FDM printers have its manufacturer nozzle positioning accuracy procedure. Typical values range between 10 - 12.5 microns for the XY plane and 1 - 2.5 microns in the Z direction [
Accordingly, designers should be aware of small details in their designs such as angles, fillets, curves, small holes, and thin walls, as they might not be fabricated as intended in the design. Childs and Juster [
In conclusion, features with smaller dimension than 1 mm should be avoided as they might fail to be rendered. Features between 1 mm and 5 mm will have deviations between 0.15 mm - 0.35 mm. In addition, it is favorable to have thin walls with thicknesses equal to an integer multiplication of the nozzle diameter or raster width [
Overhangs features, shown in
This limitation is important due to the disadvantages of adding supports such as extending the printing time and wasting raw filament.
In some cases, the overhanging parts are unavoidable and require necessary supports. The supports can be located in inaccessible regions and cannot be removed at the end. Such supports are fused using different material filaments that are usually water soluble which works if the solvent has access to the support. However, not all FDM printers have the capability to print using two extruders so that it can build the supports using different materials than the main part. Because of these disadvantages, it is important to know the limits of FDM printer to fabricate overhangs and bridges without supports.
As shown in
In addition to overhangs, islands are another type of features that require a support during deposition. Islands, shown in
isolated from the main body. However, if the part is fabricated in the opposite direction to what is illustrated in
FDM parts need a sufficient base area to ensure adhesion to the building bed. For example, spheres and curved sheets are very hard to fuse due to the small contact area with the building bed, which limits the possible orientations to build a given part with such features. In contrast, having a large base area increases the chances for warping.
Another feature that can be utilized using FDM is the ability to fuse around an insert objects. Although this feature is yet to be investigated thoroughly, in Kataria et al. [
In Klahna et al. [
In conclusion, during the design process the limited possible orientation for such features should be considered to facilitate the printing process. Similarly,
for some shapes one is restricted to specific fusing orientations such as for cylinders, cones, and pyramids. If a part has many features in which no optimized orientation is possible, dividing the part into printable parts might be the only option.
This term refers to how accurate the dimensions of the FDM part are in comparison to the input CAD drawing dimensions. The dimensional accuracy is affected by many parameters, mainly the X, Y, and Z positioning resolution, nozzle diameter and slicing thickness (layer thickness). The processing parameters during the printing process significantly affect the accuracy of the part as it can affect shrinkage, bonding and warping [
In a study done by the authors [
The specimens were measured for their width, thickness and length. It was found that fabricating the part at different orientations affect the deviation in the dimensions. In addition, when the total dimension of the part was equal to an integer multiple of the layer thickness, a lower dimensional error is resulted.
For example, to create a solid cylindrical part with 25.4 mm (1 inch) height using FDM with a common nozzle diameter of 0.4 mm. The optimized cylindrical height to increase the dimensional accuracy will be either 25.5 mm or 25.2 mm instead of 25.4 mm if the slicing thickness used is 0.30 mm.
The dimensional accuracy is influenced by the processing parameters used during fabrication. To investigate the influence of some of these parameters in this work, a total of 8 tensile test specimens fabricated according to ASTM D638 type IV where created using different processing parameters. Four of the specimens, where fabricated using different extrusion temperatures and the rest of the processing parameters, were fixed. The remaining four samples were printed using different layer thicknesses and the rest of the processing parameters were the same for each sample. Each specimen was measured at the locations shown in
Dimension Name | Measured Points | Averaged Value |
---|---|---|
Necking Width | W1, W2, and W3 | W |
Sample Width | OW1, OW2 | OW |
Sample Thickness | OT1, OT2, OT3 | T |
The measurements are averaged based on
This error can be avoided by selecting a layer thickness that is compatible with that dimension. For example, in
During the design phase, clearances should be assigned between any meshing parts. For example, a pin should be fabricated with a slightly smaller diameter, while the hole where the pin is assembled to, should have a slightly larger inner diameter to facilitate the assembly. The Geneva mechanism was taken as a case study to investigate clearances for assembled parts in this work. To find the correct clearance firstly, a fitting calibration test was done as shown in
FDM parts usually have high surface roughness and require surface finishing to get smooth surfaces that are comparable to the surface of injection molding parts. The surface roughness of a part is important in the case of FDM assemblies, especially for moving joints as the poor surface finish will affect the performance of the joint due to high friction. This can cause early part or assembly failure. The processing parameters of FDM directly affect the surface roughness of fabricated parts. FDM processing parameters highly influence the staircase effect such as the extrusion temperature, extrusion width, layer thickness and building orientation [
close to 30˚ have a higher surface roughness. In addition, increasing layer thickness, will increase the part’s surface roughness. A surface at 30˚ angle with layer thickness of 0.178 mm will have a surface roughness around 40 µm. If the surface finishing is an important aspect in the part, then the feasible building orientation will be a multi objective optimization problem. Another way to get a good surface finish is by choosing a chemically treatable filament such as ABS and post process the fused part [
One of the advantages of AM technologies is that they eliminate the lead time needed until the production starts for mold design and production or setting up the production line. However, the fused process itself is very slow. Fusing time is stimulated by many factors such as the speed of the axis controlling the extruder, layer thickness and building orientation. Most commercial FDM printers have extruders that can move at speeds up to 200 mm/s or more. However, during extrusion a speed of 80 mm/s or lower is usually used to enhance the parts’ quality. By orienting the part’s largest surface area as parallel to the layer plane, the printing time can be minimized as shown in
Fusing time is also driven by the layer thickness; the smaller the layer thickness is the longer the fusing time will be. In addition, avoiding the need for supports can reduce the fusing time significantly. That can be done either by eliminating unnecessary features that need support or by choosing an appropriate
building orientation that minimizes the need for support. In Teitelbaum et al. [
Due to the nature of AM, parts suffer from high anisotropy in their mechanical properties. The designer should consider this anisotropy, which is highly affected by the processing parameters during the fusing. Therefore, it is important to consider anisotropy [
Increasing the layer thickness will yield a stronger part as well. It should be noted that FDM parts are usually stronger along the layer direction and weaker along the building direction, which is indicated by having a higher tensile strength and higher young’s modulus [
Production cost is a primary factor that must be considered during the design stage. The cost of the FDM parts depends primarily on the material and the power consumed during fabrication. The material consumed can be divided into the part’s material and the construction material such as the material used for supports, rafts and brims which are for adhesion and warping reduction. The material needed for the part should be optimized by optimizing the part’s topology. The optimization takes into consideration the loading requirements and the part’s intended function. The construction material can be minimized by optimizing the building parameters such as building orientation which could reduce the need for supports. Constructional materials consumption can be reduced by using appropriately heated bed to minimize the need for rafts and brims. As a result, the building orientation of FDM parts is strongly correlated to cost [
Most FDM defects can be eliminated or reduced by using a good combination of processing parameters. However, some types of defects need to be considered during the design process as they might affect the performance of the part. The first defect is oozing or stringing, shown in
Another important defect is under extrusion, shown in
ing the extrusion multiplier or the entered filament diameter. Nevertheless, even after eliminating the under-extrusion defect, there is a gap in the literature about the pressures that FDM parts can handle compared to traditional manufactured parts. Some work presented an experimental methodology to investigate the main factors of profile errors in FDM parts [
To demonstrate the design considerations for FDM, a support bracket shown in
To redesign the support bracket for FDM, the design considerations discussed above are taken into consideration as follows: The CAD tool used for the drawings was Creo 3.0®. For the FEA, ANSYS® Mechanical 17.0 with the free ACT topology optimization extension, which will enable free form design. The material options are limited to ABS, Nylon and PLA due to availability and the li-
mited extrusion temperature up to 300˚C. PLA was chosen over ABS and Nylon due to its higher modulus of elasticity and strength. The envelop dimensions of the original support bracket are (75 mm, 74 mm, 60 mm) which are well within the building volume of the TAZ6® equipment (i.e. 280 x, 280 y, 250 z all in mm). The modified design is not to cross these envelop dimensions to remain functional. The support bracket can be fused while keeping the cylinder rod in its original shape. However, that will limit the building direction to be aligned with the cylinder rod’s axis, which will reduce the ability of the part to bear the bending stress developed normal to the cross section of the cylinder. Therefore, the cylindrical cross section is replaced by a square, which can be fused in many directions. The initial FDM suggested design is shown in
The initial design has no small features that cannot be fused. In addition, the surface roughness of the part has no effect on its function. And because of the elimination of assembly for the part, accurate tolerances are not significant to this design.
The initial design was used to analyze the load of 35 Kg. It can be seen from
bearing very small load. These regions indicate an overdesigned part with unnecessary excess material. Therefore, topology optimization is required to further improve the design and make it more efficient. The topology optimization was done using ANSYS ACT Topology Optimization®. The objective of the optimization was to minimize the compliance. The whole part was subjected to the optimization except the fixed support surface, the area where the load was applied and the bottom of the part to ensure keeping a flat surface for manufacturability. The optimization constraints applied were minimum member size of 5 mm, volume percentage of 50.0%, global stress of 40 MPa and a maximum deformation of 1.5 mm. The minimum member size constraint was used to ensure no small features were created for better manufacturability.
The resulted topologically optimized design is shown in
Another design for the support bracket can be show in
This paper presents the design considerations for FDM from Design for manufacturing aspect. It discusses the limitations of FDM in aspects such as building volume, limited material options, lack of data packages for design and simulation, and discussed unfused features such as very small details, and islands. This work provides the typical dimensional accuracies, surface roughness values and typical clearance values for assemblies. In addition, it delivers benchmarks for fused angles and lengths of overhangs and bridges without supports. This paper summarizes the major guidelines that designers are recommended to follow while creating the parts and fabricating them to optimize the topology, building time and the mechanical properties of the part. Authors presents two experimental studies on the dimensional accuracy of the FDM parts compared to the input CAD model, in addition to the effect of the FDM processing parameters on the modulus of elasticity for PLA samples.
Finally, a case study is presented following the design considerations to redesign a support bracket to be fabricated by FDM. The case study demonstrated the advantages and disadvantages of AM and considered FEA and topological optimization to modify the metal support bracket into a PLA support bracket suitable for FDM. In conclusion, it is important to consider the limitations of FDM and the influence of processing parameters on the quality of FDM parts in early designs stages. This allows better process optimization during production, which might lead to more objectives satisfied compared to optimizing the processing parameters for designs optimized for traditional manufacturing methods.
Currently, FDM is still an immature manufacturing technology for high production volumes and there is still room for improvement and gaps in literature to be filled to improve the DFAM. This work provides a summary of revised literature,
This research was supported by the Hellman Foundation through Hellman Faculty FellowshipGrant-55253 for 2016.
Alafaghani, A., Qattawi, A. and Ablat, M.A. (2017) Design Consideration for Additive Manufacturing: Fused Deposition Modelling. Open Journal of Applied Sciences, 7, 291-318. https://doi.org/10.4236/ojapps.2017.76024