Additive Manufacturing of Thermoplastic M8 Fasteners Using Photopolymer Jetting Technology for Low-Strength Fastening Applications


 This study investigates the spatial accuracy of additively manufactured M8 fasteners fabricated using photopolymer jetting technology. A number of M8 bolts and their mating M8 nuts are fabricated using Stratasys Object 260 Connex 3 polyjet printer with Vero cyan, Vero magenta, Vero white, and Vero grey thermoplastic resins. Vero series thermo-plastics were used as they offer durability, strength, and flexibility. The additively manufactured nuts and bolts are fabricated with the intention of introducing thermoplastic fasteners that could eventually substitute metal fasteners in certain low strength joining applications such as in fluidics, electronics, biomedical engineering, food packaging, and automobile industries. Manufacturing constraints and critical conditions for fabrication are presented. Vital printing and manufacturing constraints, dimensional stability analysis, and the subsequent dimensional changes for the fabricated fasteners are thoroughly illustrated. Dimensional stability of the printed structures was examined under an ultra-compact 3D laser sensor and the imaging results are also presented. The fabricated thread profile superimposed precisely with the CAD model’s ISO thread profile dimensions. The dimensional performance of the fabricated M8 fasteners was examined in every orientation and along each axis of the fasteners. The high-quality manufacturing of fasteners using photopolymer jetting reached the maximum precision requirements and fitted under IT 06 transition fit grade.


Introduction
3D printing or Additive Manufacturing (AM) [1] is a method of constructing a part by depositing thin layers of the indispensable material one by one. It is amongst the most rapidly rising and high potential manufacturing techniques offering signi cant advantages over conventional manufacturing techniques [2]. AM unlocks the possibility of paths where the weight factor and the design process method is put in effect with the help of data analyzing approach. A wide range of thermo-plastic materials is available, which have the potential to meet the required industry standards. FIGURE 1 compares additive with subtractive manufacturing. As the complexity of the product geometry increases, AM is proven to be more reliable with fewer inventories, less tooling required and minimal material wastage. With the rapid growth in the reliabilities and mass-production capabilities of the 3D printers, AM of traditional mechanical components such as Bolts, Nuts, Gears, turbine blades, etc., is emerging as a cost-e cient [3,4] manufacturing solution. A variety of advancements in accuracy-based 3D printing have been reported in the last ve years [5][6][7][8][9][10]. Few studies have reported the accuracy issues while fabricating the bolt via AM [5,10]. A scale factor is developed for the bolt's fabrication using FDM [5] in which deviations occurred during the printing process that crossed the permissible tolerance limit. The deviations can be evaluated using optimization methods such as the Taguchi method, response surface method (RSM), and Nondominated Sorting Genetic Algorithm (NSGA) [11][12][13]. A similar trend is perceived while fabricating the bolt via metallic 3D printing using stainless steel, aluminium, and steel [10,[14][15][16].
A comparison between different methods of 3D printing primarily focusing on mass customization at both prototyping and industry level, is reported in [17][18][19]. Several manufacturing parameters determine the precision of the fabricated specimens. The printing speed plays a vital role in the 3D printing technique for determining the dimensional deviations. Evaluation of the speed and the accuracy of AM machines was reported in [20]. Parameters such as layer thickness and particle size [21,22], for instance, while fabricating anatomic maxilla and mandible implant guides [21] various processing parameters are examined. Blande Altman analysis and Wilcoxon signed-rank test are performed for checking the mean absolute difference and mean relative difference between SLA and the Polyjet 3D printers. Thermoplastics fasteners such as bolts, screws and gears have become prevalent due to their affordable price, versatility, inability to get corrode, multicolour options, high wear resistance, and re-resistance. Bolts serve as the backbone of the industries and are commonly used as permanent fasteners in mechanical, civil, and hydraulic developments.
Orthopaedic screws were manufactured utilising the Fused Deposition Modelling technique [24] [29] was compared. The study establishes the capacity of various printing technology's to replicate thermoplastics components such as polyjet, SLS, and FDM methods. This paper presents a novel fabrication of M8 thermo-plastic fasteners manufactured using photopolymer jetting technology.
Numerous M8 bolts and M8 nuts were 3D printed utilizing Vero magenta, Vero cyan, Vero white, and Vero grey thermo-plastic resin. A signi cant number of studies have investigated the accuracy and strength of metal 3D printing of bolts; however, only a limited number of studies currently exist on thermos-plastic based fasteners using Polyjet based AM [10]. This fabrication method opens new conventions for AM thermos-plastic fasteners, for low strength joining applications. Vero series photopolymer thermoplastic resins offer resilience, stability, chemical and corrosion resistance. Manufacturing constraints along with the critical prefabricating parameters are highlighted while printing the M8 fasteners with the help of a Polyjet printer. The optimal print conditions were identi ed, and fabrication was carried out in accordance with them. The dimensional inspection of the bolts and nuts is carried out by scanning the fasteners under a high precision 3D scanner. For further analysis the scanned fasteners results are superimposed onto their CAD designed according to ISO 724. The dimensional deviation is examined at three random sections of the design to minimize the recorded data uncertainties. Finally, the behaviours of the 3D printed fasteners are compared to validate the printer's capability to reproduce the M8 fasteners. Hexagonal-headed fasteners are widely used as mechanical fasteners in a wide range of industrial application. Geometric parameters, such as nominal diameter depth (B), pitch, and length of thread engagement (H), listed in Table 1 and thickness of nut (m), width across ats (s), and height of the nut (e), listed in Table 2, serve as the essential link between design and the manufactured product. Using the standard ISO 724 dimensioning data as depicted in FIGURE 2, an M8 bolt and nut specimens are designed on Computer-Aided Design (CAD) software. The CAD model is then converted into the stereolithography le format (.stl) [32,33], which stores the information based on CAD models' triangulations.

Limits and Fits
Limits and ts comprise of 18 classes of fundamental tolerances for both shaft and hole designated as IT 01, IT 0, IT 1, and IT 16. They are termed standard tolerances (IS − 919). The following expression is used to calculate the tolerance on a shaft: Where T is the tolerance (in micrometres), K is the constant, and i is the standard tolerance value (in micrometres) which is obtained by the expression: Where D is de ned as: D 1 and D 2 (in mm) denote the range of maximum and minimum nominal size. The metric threaded bolt is typically fabricated within a narrow tolerance zone. A tolerance zone is a region characterized by the product's upper and lower limit. The amount of deviation and its location with respect to its basic size determine the tolerance zone applied on the thread pro le in the permissible range as stated by the International Organisation for Standards (ISO) [34,35] for manufacturing a fastener. The tolerance zone for a fastener, for instance a bolt and a nut, according to ISO 965, is de ned under IT 06 transition t grade as shown in FIGURE 3, including the standard electroplating tolerances. For instance, a t between the threaded part is designated as M 8*1-7 H / 5 g 6 g speci es that thread has a nominal diameter of 8 mm, pitch 1 mm, 7 H tolerance zone for the nut (female mating part), tolerance zone of 5 g for pitch and 6 g for the major diameter of the thread. The exact thread pro le is crucial for effective thread connections with their mating counterparts. It adversely affects the life and strength of a given threaded joint if the bolt parameters exceed the deviations beyond tolerance limit. As a result, the bolt's geometric parameters' maximum and minimum measurements must be inside the tolerance zone.

Photopolymer Jetting 3d Printing Technology
Photopolymer jetting or polyjet printing is a method of depositing the material in the form of droplets to create thin layers which are then cured to form 3D components. Photopolymer jetting enables the multimaterial and multicolour printing of polymers. It is commonly used to fabricate complicated geometries, such as cavities, overhangs, undercuts, delicate features, and thin-walled sections. Polyjet printing has a good processing accuracy and can print layers as thin as 16 µm. Polyjet printing technique [36-38] uses photopolymer materials for building the actual model and requires another gel-like material as a support structure, followed by subsequent curing of the deposited layers. As the supporting materials are soluble and fusible, removing the supports is a simple and damage-free process. Polyjet printer schematic, and the printer can be seen in FIGURE 4 and FIGURE 5, respectively. The bolts were designed using AutoCAD, and their stl les were uploaded to the Stratasys® Object 260 Connex 3 printer. A Water jet gun is typically used to remove the support material (SUP 705) during post-processing. A number of manufacturing considerations and parametric analysis must be taken into account in the manufacturing process using photopolymer jet printers.

Parametric Analysis
A collection of material properties [36] are available without any recommended printing parameters, so the principle challenge for the designers is to set up the printer and optimize the parameters for the best results as per the design requirements. The impact and interdependencies of the controlling parameters are examined using Design of Experiments (DoE) [38]. A preliminary trial strategy is developed for every fabrication run to determine the optimum printing parameters, minimising the dimensional variation. The expression gives the numbers of trials: (4) Where, N r is the Number of trials, N l is the Number of factor levels, and N f is the Number of factors. A total of sixteen tests are performed as depicted in Table 3 (a) for the material Vero grey to evaluate these deviations' effect on the obtained product dimensions [38]. The manufacturing imperfections were overcome using an optimization method that considered the dimensional variation and design analyses. Hence, the minimum, maximum, and mean values are presented in Table 3 (b). Subsequently, the printer and material's optimum conditions as a prerequisite to obtain the printed part's accuracy are outlined in Table 3 (c). For Vero grey, the optimized process parameters obtained using ANOVA  Table 3 (d) [12,40] are glossy surface nish, 90° orientation of the design, heavy support material and high-speed print condition. These results have served as the optimum match for the print conditions of the M8 bolt using Vero magenta and Vero cyan, respectively, on a photopolymer jetting based 3D printer.

Manufacturing Constraints In Photopolymer Jetting
Accuracy of the thread pro le is an essential parameter for an effective bolt design. Pre-fabrication parameters such as the selection of material, print resolution, turning speed, machine resolution, part size, and machine procurement cost [41][42][43][44][45] are the critical factors in determining the precision of the thread pro le. Along with these critical parameters, numerous manufacturing constraints also had to be overcome while fabricating using photopolymer jetting.
First and the foremost, it is critical to identify the print parameters in photopolymer jetting that affect the critical features of a printed component. Table 4 enlists various printing modes that affect the mechanical properties of the fabricated specimen such as part orientations, support materials, printing speed and surface nish. Therefore, an analysis of the various printer settings is necessary to re ne the results in terms of both strength and accuracy. Secondly, the viscosity of the resin is a critical parameter in polyjet printing which requires a low viscosity to ensure easy deposition. A low viscosity resin with a high degree of uidity reaches the build tray quickly and is suitable for printing high resolution designs [44]. It is essential to have a low-viscosity resin to be injected into the nozzle for a cost-effective solution.
Thirdly, producing support-free 3D printing specimens signi cantly reduces the labour cost. Fourthly, the printing was performed at a 90° orientation of the design and high-speed mode. This printing mode provides 0.002 mm resolution of print. It improved the bolt's surface nish and reduced the need for polishing and post-processing. Finally, the products printed using the photo-curing 3D printing methods are porous and have low energy absorption ability. Therefore, material selection restricts the application of the print mode. Vero thermos-plastics offer high fatigue, tensile and torsional strength [36] that ensembles the requirement of any mechanical fastener. The build cabinet of polyjet printer is 870 mm X 1200 mm X 735 mm in size as demonstrated in FIGURE 6. Parallel printing of bolt samples in various colours was performed to further reduce the printing costs. The material cabinet holding the photopolymer resins measures at 330 mm X 1170 mm X 640 mm. The optimum operational conditions were maintained at 25°C and relative humidity at 60%.

Fabrication
The fabrication of the M8 fasteners was performed using Stratasys® Object 260 Connex 3 [36]. The process parameters during 3D printing consist of laser power and wavelength that were set to 1.5 kilowatts and 1070 nanometre, respectively. The build time taken to fabricate each bolt was approximately 2 hours and nut was 1.5 hours respectively, and the room temperature was at 25°C. FIGURE 7 (a) illustrates M8 bolts manufactured using stainless steel and the 3D-printed thermoplastics bolts using Vero magenta, Vero cyan and the Vero white photopolymer resin. FIGURE 7 (b) demonstrates the ISO dimensions of a 3D printed bolt. The 3D printed M8 Vero grey nut and partially 3D printed nut demonstrating internal threads are depicted in FIGURE 8 (a) and FIGURE 8 (b) respectively. Along with the M8 nut, a partial M8 nut is 3D printed to scan the internal threads while maintaining the same material and manufacturing conditions as that of the M8 nut. Vero thermos-plastics offer high fatigue, tensile and torsional strength [36] that ensembles the requirement of any mechanical fastener. FIGURE 9 illustrates the three randomly selected regions on the thread pro le that are typically located on the bolt's periphery. The specimens were scanned using a Carl Zeiss COMET L3D2 3D scanner [39] as shown in FIGURE 10.
The scanning is performed on randomly three selected sections taken on the periphery of the bolt and nut. The 3D scanner is furnished with a superior 3D sensor that utilizes blue LED lighting and 3D inspection. It can amplify the scanning zone without affecting the precision. Further the data acquisition process generated real-time visualization results by superimposing the printed part with the original CAD model in terms of dimensional deviation as shown in FIGURE 11 (a), (b), and (c) respectively. Stratasys Ltd. gives a collection of material properties [36] without recommending any printing parameterization, so the primary challenge for the client is to set up the printer to arrive at the best results regarding their prerequisites towards the printed part. Object 260 Connex 3 printer offers variation in the material's surface nish, part orientation, support material used, and the printing mode [36]. The overall mechanical performance of a bolt is determined by the accuracy of the thread pro le and its strength. The mechanical properties of the material must be evaluated before deploying it for any fastening application. A bolt is typically subjected to tensile, shear and torsion loads. The tensile strength is the highest tension load that a fastener will withstand until failing whereas exural strength is concerned about the material's susceptibility to deformation. Failure typically occurs due to combination of forces that causes cracking. The mechanical properties of Vero cyan Vero magenta, Vero white and Vero grey are depicted in Table 5. It can be noted that Vero thermoplastics ful l a wide range of design criteriaresilience, stability, chemical and corrosion resistant, as well as being lightweight.

Results And Analysis
The in uence of printing conditions is observed on the average deviation values of the fasteners fabricated with the help of Polyjet Technology. Fasteners were printed at the optimum condition for the printer that were glossy surface nish, heavy support material, 90°-part orientation, high-speed printing, and 25°C ambient temperature. The study compares pitch, depth of thread, thread angle, and thread engagement of the M8 bolt with the standard values.

Thread Pro le Analysis for bolt
Threads are the grooves of a uniform section cut on the internal or external surface of a cylinder in the shape of a helix. The bolt's strength is incredibly reliant on the thread    The positive values indicate a larger dimension than the standard value whereas the negative values indicate a smaller dimension than the standard value. These points function as the reference parameters for evaluating the directional deviation. The directional deviations and total deviation values are calculated, and the dimensional inspection is performed along X, Y, and Z directions. The origin is kept at the bolt's centre for measuring the displacement eld, and horizontal and vertical directions are assumed as the positive X and Y-axis, respectively. FIGURE 18 illustrates the deviation trend for the Vero white bolt.
The green zone of the tolerance chart indicates that there is little variation than the red zone. The deviations are more towards the lower end of the Vero white bolt. FIGURE 19 depicts that the 3D printed Vero cyan bolts pro le slightly deviates from the roots and peaks of the ISO M8 bolt. The major portions of the peaks and roots are missed by Vero magenta. The thread failure of the bolt occurs due to combination of tensile, compressive, shear and torsional stresses. The crack initiates at the thread root and grows until the failure of the bolt. Maximum load is carried by the roots and peaks of the thread. Dimensional deviations, is therefore, the main concern which determines the effective bolt connection. Directional deviations for Vero white, Vero cyan, and Vero magenta M8 bolts are calculated and shown in

3 D Inspection of the nut
The " t" is the dimensional clearance between two mating components, and its magnitude affects the mating components' ability to slide and revolve independently. M8 nuts are fabricated with in the IT 06 transition t grade to determine the tolerance zone for the mating bolt and nut. The mating M8 nut is additively manufactured using Vero grey thermoplastic and fabricated under the similar printing conditions. Internal thread quality is determined using a half3D printed nut, while geometric morphology is determined using a full 3D printed nut. The Carl Zeiss COMET L3D2 3D scanner is used to scan the additively manufactured Vero grey nut [37]. FIGURE 25 illustrates a three-dimensional tolerance chart showing the deviation for the nut. The thread tolerance chart governs the transmission accuracy of the mating parts. The greatest deviation occurs near the nut's top surface, as shown by the red zone on its thread pro le while the smallest deviation occurs towards the nut's threads. The major peaks and crests are missed near the base of the nut. The thread defects among the mating parts initiates failure in the threaded joint. Directional deviations for Vero grey M8 nut are calculated and shown in Fig. 26.
It is perceived that the directional deviations become more prominent as we move towards the lower end of the bolt. They are more along Y-axis for Vero cyan, Vero white bolt and more along the Z-axis for Vero magenta bolt. In case of Vero grey nut the de ection along the Y-axis is more signi cant. The maximum deviation is ± 0.09 mm in case of cyan bolt, ± 0.12 mm in case of magenta bolt and 0.14 mm in case of Vero white bolt. For the Vero grey nut the maximum de ection is ± 0.1 mm along the Y-axis. Standard deviation for Vero white bolt is higher than Vero cyan and Vero magenta bolts, which can be seen from the scanned data set since the values are unevenly located from the actual values. The standard deviation of Vero grey nuts is lower than that of additively manufactured bolts, since the deviations from the ISO standard values are less scattered.
Finally, Table 6  mm. An accuracy of ± 0.12 mm is reported with the fabrication of the presented M8 bolts using Vero magenta colour whereas the accuracy was as low as ± 0.1 mm for Vero cyan. It is observed to be more accurate than FDM [6, 11,20], SLA [13,21].

Conclusion
The AM of thermoplastic M8 fasteners using photopolymer jetting technology was presented with the aim of introducing thermoplastic fasteners which are speci cally designed for low strength joining applications. The dimensional performance of a number of M8 fasteners fabricated using photopolymer jetting technology was examined. The M8 bolts and nuts were fabricated using the Stratasys Object 260 Connex 3 polyjet printer utilizing Vero cyan, Vero magenta, Vero white, and Vero grey thermoplastics resin.
Lightweight Vero thermoplastics exhibit a wide range of essential thermomechanical properties such as hardness, brittleness, and toughness. Various critical fabrication constraints such as the polyjet printer's general printing limits and critical pre-fabrication parameters, were comprehensively addressed. The optimum print conditions were determined using the ANOVA parametric study method to minimise the dimensional deviation of the fabricated parts. The geometric parameters of the 3D printed fasteners were superimposed with the ISO standard parameters. The 3D scanning observations for Vero cyan, Vero magenta, Vero white, and Vero grey scanned M8 bolt and nut samples were analysed using a Carl Zeiss COMET L3D2 3D scanner. After dimensionally inspecting the pitch, depth of thread and the length of the thread engagement for both the mating components, it was established that the effective thread connections fabricated using photopolymer jetting are possible in the IT 06 transition t. It was observed that the minimal error was obtained for the pitch value, and the maximum error was obtained for the depth of thread for Vero magenta bolt. The superimposition of the 3D printed fastener's thread pro le over the ISO-compliant CAD thread pro le demonstrated that Vero thermoplastics fasteners perfectly overlap the majority of the crests and roots. The peaks and valleys of the mating nut's internal threads align perfectly with the Vero thermoplastic bolt. Vero series thermoplastics showed an overall deviation of 0.12 mm while fabricating M8 fasteners. Under the same fabrication circumstances, thread engagement is achievable for all fasteners produced with polyjet technology. The maximum deviation for the M8 bolts and M8 nut lies inside the 6g tolerance band. Therefore, photopolymer jetting as a method could be signi cantly desirable since it did not require any scaling factor in the fabrication of most designs before their fabrication. This study can help professionals and software developers in producing varying models and relationship in the eld of AM.
Plastic fasteners can serve as an excellent alternative for metal fasteners, particularly in various low strength applications. Most commonly used plastic fasteners are screws, washers, nuts, gears, and bearings. This investigation into the manufacturing constraints could play a vital role in developing the guidelines for the fabrication of bolts using techniques such as photopolymer jetting. Measurement results may vary from machine to machine, but the deviation patterns and trends for the photopolymer jetting method, in general, tend to be similar. Products that require de nite dimensions in the tolerance zone can be fabricated using photopolymer jetting technology with Vero series thermoplastic. AM can also be utilized in the production of other bolts printed with their actual dimensions. Further studies can be conducted along the lines in the manufacturing constraints and stability analysis of other fasteners such as M6 and M12 bolts, gears, washers, and nuts.        Thread pro le of M8 bolt measured by 3D scanner (dimensions in mm, angle in degree).

Figure 13
Thread pro le of Vero cyan measured by 3D scanner (dimensions in mm, angle in degree).

Figure 14
Thread pro le of Vero magenta measured by 3D scanner (dimensions in mm, angle in degree).

Figure 15
Thread pro le of Vero white bolt measured by 3D scanner (dimensions in mm, angle in degree).       See image above for gure legend.

Figure 22
Dimensional deviation of a sectional view of Vero White compared with M8 bolt, in mm.

Figure 23
Dimensional deviation of a sectional view of Vero Cyan compared with M8 bolt, in mm.

Figure 24
Dimensional deviation of a sectional view of Vero Magenta compared with M8 bolt, in mm.

Figure 25
3D Tolerance chart of Vero grey nut superimposed with an ISO Standard M8 nut, in mm.

Figure 26
Directional Deviation for Vero grey nut.