Agora of Additive Manufacturing (2024) paper24002

Agora of Additive Manufacturing
journal homepage: http://additive-manufacturing.or.jp/

Novel snap-lock fastener using 3D printed composites
Masahiro Kubota a, Akira Todoroki *b, and Keisuke Iizuka caGraduate student at Tokyo Institute of Technology, Tokyo Institute of Technology, Tokyo, Japanb

Department of Mechanical Engineering, Tokyo Institute of Technology, Tokyo, Japanc

Department of Mechanical Engineering, Aoyama Gakuin University, Kanagawa, Japan

ABSTRACT
Snap-lock fasteners are very simple for locking two parts. However, because their configuration is complex, snap-lockfasteners are usually made of polymer materials. This implies that the material is usually weak. In this study, we propose anovel snap-lock fastener printed using a 3D printer that prints continuous fiber composites. The novel fastener designed inthis study can be easily attached to and detached from fastened parts. Based on the tensile test results, continuous glass
fibers were placed at the elastic hinges to strengthen the weak elastic hinges. The reinforced snap-lock specimens were
tested under tensile loading. Therefore, the novel snap-lock fastener with continuous glass fiber was able to bear 1.7 times
higher load than that without continuous glass fibers.
Keywords: Composites, Snap-lock, Glass fiber, Carbon fiber

1. Introduction
Recently, metallic materials have been replaced with carbon fiber-reinforced polymers (CFRP) in transportation equipment,such as aircraft and automobiles. CFRP (thermosetting CFRP), which uses thermosetting resin as the base material, has
better specific strength and specific stiffness than conventional metallic materials. However, manufacturing thermosetting
CFRP structures requires skilled technology or large-scale/high-cost equipment. Therefore, the application of thermosetting
CFRP is limited. In 2014, Markforged Inc. in the United States announced Mark One, a new thermoplastic CFRP fabrication
machine that uses a fused filament fabrication (FFF) 3D printer (1). The new 3D printers enable thermoplastic CFRP
fabrication by layering thermoplastics using the FFF method while extruding and cutting continuous carbon filament
composites or glass filament composites extruded from a nozzle different from that of the plastic filament. Mark One andMark Two have already been used worldwide, and the material properties of their composites have been evaluated. Franket al. (2) evaluated the performance of a Mark One 3D printer, obtained the tensile properties, calculated the theoretical
stiffness considering the difference in fiber volume content, and compared the results with experimental values. Todoroki
et al. (3) obtained the mechanical properties such as tensile strength and stiffness in the fiber, transverse, shear, andlamination directions by creating continuous carbon fiber specimens with different printing directions using Mark Two andconducting tensile tests. In addition to studies on tensile properties (2)–(6), various types of mechanical properties havealready been obtained, including compressive properties (7)(8), differences depending on the printing direction in impact
tests (9), fatigue properties (10), and the effect of moisture absorption on the tensile properties of plastic materials (11). This
3D printer can also arrange continuous fibers in a curved shape (12),(13), and a new fracture rule for curved continuous fiber
bundles has been investigated (14). Other composite 3D printers that fabricate products with high degrees of freedom arealso being developed (15),(16).
* Corresponding author
Email: todoroki.a.aa@m.titech.ac.jp
Received 5 June 2024; Revised 24 July 2024; Accepted 26 July 2024
Available online 26 July 2024

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Agora of Additive Manufacturing (2024) paper24002

Composite 3D printers enable the fabrication of parts with complex configurations. A typical structure of complex parts
made of polymer materials is a snap-lock, which enables us to lock two parts easily by inserting a snap-lock into a fastenerhole (17)–(22). Many researchers have attempted to make snap-locks using polymer-based 3D printers (23)–(28). Because
polymers are easy to mold, they are often used for snap-locks. Unfortunately, this limits the strength of the snap-locks
because the strength and stiffness of the polymer are lower than those of conventional metallic materials.
In this study, a novel snap-lock fastener was developed using 3D-printed continuous-fiber-reinforced composite materials.
This fastener enabled us to lock two parts using a simple snap-in process and unlock them from the surface without any
additional complex processes. Using a 3D printer capable of molding complex shapes, we devised a locking mechanismwith a simple locking function that cannot be easily released without a tool. Continuous fibers are used in parts wherestrength is required to make the structure sufficiently strong to withstand practical use. Furthermore, the developed fastener
was fabricated using Mark Two, and the joint strength was obtained using a tensile fracture test.
2. Concept of novel snap-lock fastener
Thermoplastic snap-locks can be easily fastened and removed. Snap-locks are widely used to fasten two parts to which asmall load is applied. In this section, as a first trial, a snap-lock fastener using short-fiber CFRP and continuous-fiber CFRP
is developed to consider its application to parts that are subjected to higher loads than resin. A MarkTwo 3D printermanufactured by Markforged was used for molding. The newly developed snap-lock fastener is shown in Figs. 1 and 2.
The new snap-lock fastener consists of two parts: the snap-lock pin (Fig. 1) and key (Fig. 2). The fastener with the snaplock pin and key integrated is shown in Fig. 3.

Fig. 1. Snap-lock pin.

Fig. 2. Key.

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Agora of Additive Manufacturing (2024) paper24002

Fig. 3. Assembled snap-lock pin and key.
The details of each part are explained below. The snap-lock pin has a body, legs, and feet. At the connecting parts of the
body and legs, elastic hinges were designed, and the elastic hinge parts reacted to the hinges when a key was inserted
between the legs. This process causes the legs to open and the opened legs lock the parts to be fastened. The grooves andbarbs were designed inside the body. When the sub-key of the key is located in the groove of the snap-lock pin, the fastening
process is completed.
The check spring of the key is locked at the barb of the snap-lock pin. This enables integration of the snap-lock pin andkey. The key comprises a U-shaped stopper and key bar parts. When the key is inserted into the snap-lock pin, the pinch
part designed on the U-shaped stopper must be grasped with a plier. The key in the snap-lock pin must be pushed bygrasping the U-shaped stopper. Then, the sub-key of the key fits into the groove of the snap-lock pin, and the key is locked.
Figure 4 illustrates the fastening process. The release process is shown in Fig. 5.

(i) Assembling of snap-lock pin and key(ii) Before fastening#1 part to befastened#2 part to befastened(iii) Inserting key into (iv) Locking the pin
the snap-lick pin(v) Finish of locking

Fig. 4. Fastening process of the novel fastener.

#1 part#2 part(i) Before release(ii) Release the key from the span-lock pin (iii) Pull the key with the snap-lock pin

Fig. 5. Unfastening process of the novel fastener.

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This snap-lock can be locked by inserting a key into the snap-lock pin. The advantages of the novel snap-lock is that it canbe attached and detached only by working from one side, and is designed to prevent detachment. After fastening, a flatworking surface was obtained. Moreover, one side of the fastened section was flat. The key was equipped with a checkspring, and when the key was pulled out from the snap-lock pin, the check spring was caught in the grooved area, as shownin Fig. 3. When the Snap-lock fastener is removed, the Snap-lock pin is removed from the fastener by pulling on the key.
Therefore, the fastener can be easily removed by accessing it from only one side (see appendix movies).
3. Tensile strength measurement of the novel snap-lock fastenerIn the previous section, snap-lock function designs are described. To fasten mechanical parts, the joint strength must be
higher than that of fasteners made of polymer materials. First, a short-fiber CFRP CF/PA-6 filament (Onyx®, fiber volume
content of about 10 %) made by Markforged was used and molded using a MarkTwo 3D printer as a reference. Tensile
tests were conducted on the molded snap-locks to confirm their strengths and weaknesses. Subsequently, we decided toreinforce the weak points with continuous fiber composites.
During the printing process, the axial direction of the cylindrical outline of the snap-lock was aligned with the in-plane
direction of the 3D printer bed, and the wall-print path of the outline was placed in the axial direction. When short-fiber
CFRP filaments are printed on an FFF 3D printer, the fibers are aligned in the direction of the print path. Hence, the print
strength of short-fiber CFRP is the strongest in the print path direction (29). By aligning the length direction of the legs withthe axial direction, the tensile strength of the legs is the strongest for short-fiber CFRP.
To obtain the joint strength, a tensile fracture test was conducted using a SHIMADZU AG-I 100 kN universal testingmachine. Figure 6 shows an outline and drawing of the tensile separation jigs (upper and lower) used in the tensile fracture
test, which simulate the separating force applied to the two fastened parts.

(a) Upper jig(b) Lower jig

Fig. 6. Upper and lower jigs for tensile tests of the novel snap-lock system.
After the snap-lock fastener was fastened, the upper and lower fastened members were gripped using testing machine jigs
and subjected to tensile loading. Figure 7 shows the configuration of the tensile-failure test setup. The printing path of thesnap-lock fastener is shown in Fig. 8. Three tests were conducted.

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Agora of Additive Manufacturing (2024) paper24002

Fig. 7. Tensile fracture test setup. The snap-lock is inside the upper and lower jigs.

Fig. 8. In-fill pattern of the snap-lock pin with Onyx filament.

The load–displacement relationships obtained from the tensile fracture tests are shown in Fig. 9. The abscissa represents
the displacement of the testing machine and the ordinate represents the tensile load. The load–displacement diagrams showthat there was no significant scatter in the three tests. The average value of the load when the load dropped significantly
was 2.63 kN for the three specimens.

Specimen #1

Load [N]Specimen #2
Specimen #3

Average 2.63 kN

Displacement [mm]Fig. 9. Load–displacement diagram of the fastener made from Onyx with fill rate 100 %.

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The specimen after fracture is shown in Fig. 10. Figure 10 shows a snap-lock pin that ruptures at the elastic hinge. This
indicates that the elastic hinge should be reinforced with continuous fibers.

Elastic hinge
Elastic hinge

Leg
Key

Leg

Fig. 10. Configuration of the fractured novel fastener after the tensile fracture test.
4. Strength Improvement using continuous glass fiber composites
To develop a higher-strength snap-lock fastener than that made of short CF/PA-6 fiber (Onyx), the elastic hinge nodes must
be reinforced with continuous fibers. Therefore, we changed the fiber placement in the slice software to strengthen the leg
portion of the snap-lock fastener with continuous glass fibers. The imbedded carbon fibers increased the stiffness and
impeded the action of the hinge. Thus, a continuous glass fiber was adopted in this section.
When modeling with MarkTwo®, it is necessary to slice the modeling parameters with Eiger®, which is a cloud-basedslicing software. The molding parameters that can be set are restricted by Eiger®. Continuous fibers that can be placed in
the same plane are limited to one direction. Therefore, when the snap-lock pin developed in this study was sliced from asingle STL data point, only one side of the elastic hinge could be strengthened, as shown in Fig. 11. This indicates that wemust divide the snap-lock pins into two STL datasets: left and right. Next, we must use the slice software Eiger® for each(left and right) part separately to strengthen both elastic hinge parts. To obtain a perfectly integrated snap-lock pin, we
printed two parts (left and right) simultaneously within the contact distance. We simply placed the left part (Part 1 in Fig.
12) and right part (Part 2 in Fig. 12) side by side on the print bed within the contact distance. To improve the bonding
between the two parts, we added the adhesive layer STL data, which is part 3 in Fig. 12. All parts (Parts 1 to 3) were printed
simultaneously. The distance between Part 1 and Part 2 was 0 mm, and Part 3 overlaps Part 1 and Part 2 by 0.7 mm each.
Figure 13 shows the print paths of the Onyx®-only layer and glass-fiber reinforced layer.

Fig. 11. Print path made with the Eiger® slice software when the snap-lock pin is sliced as an integrated part. The graylines indicate print paths of Onyx®. Blue lines indicate continuous glass fiber paths. The slice software Eiger placescontinuous fiber paths in unidirectional placement or in concentric placement in a sliced plane in a part.

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Fig. 12. Three parts are printed simultaneously. This process enables us to make an integrated snap-lock pin withreinforcement of unidirectional glass fibers in both legs. The part #3 is indispensable to make an integrated snap-lock pin.

Print path forOnyx layer

Print path for continuous
glass fiber and Onyx layer

Fig. 13. 3D printed path of the fiberglass reinforced snap-lock pin.
A continuous glass-fiber-reinforced fastener was fabricated using a Mark Two 3D printer. The printed fasteners were
subjected to tensile fracture tests, as described in the previous section. Three different snap-lock pin specimens wereprepared with different numbers of continuous glass fiber layers: 20 (2.0 mm), 40 (4.0 mm), and 60 (6.0 mm). The load–
displacement diagrams for the tensile fracture test of each type of specimen are shown in Figs. 14(a)–(c). The load–
displacement diagrams show that there is no significant variation in the continuous fiber-reinforced fasteners fabricatedwith MarkTwo®. The MarkTwo® uses the same fabrication parameters that is created by the slice software Eiger and theparameters are not open to users. The average values of the three loads when the load dropped significantly were 3.79 kN
for the 20-layer reinforced specimen, 3.97 kN for the 40-layer reinforced specimen, and 4.49 kN for the 60-layer reinforced
specimen. Failure occurred at the foot of the snap-lock pin in all specimens. As the average maximum load of the specimens
made from the short-fiber CF/PA-6 (Onyx) was only 2.63 kN, the reinforcement of continuous glass of 60 layers gives
approximately 70 % increase of the maximum load without increase of difficulty of pushing the key.
A typical photograph of a specimen after fracture is shown in Fig. 15. As indicated by the arrows in the figure, the fracture
occurred at the foot of the snap-lock pin. This indicates that the elastic hinge area was successfully strengthened using
continuous glass fibers. The fracture load of the 60-layer glass-fiber-reinforced fastener specimen was 1.7 times higher
than that of the short-fiber CF/PA-6 fastener. A higher load indicates that the continuous glass fiber reinforcement was
successful at the elastic hinge.
Further strength improvements can be expected by changing the dimensions of the leg portion and modifying the leg portion
to include continuous fibers. Moreover, shear strength is also important for some snap locks. These will be the focus of our
future work.

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Load [N]M. Kubota et al.

Average 3.78kN

Displacement [mm]Load [N](a) 20 UD glass layers.

Average 3.97kN

Displacement [mm]Load [N](b) 40 UD glass layers

Average 4.49kN

Displacement [mm](c) 60 UD glass layers
Fig. 14. Load–displacement diagram of the snap-lock tensile tests for number of UD glass layers.

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Plastic deformation of the leg

Plastic deformation of the leg

Fig. 15. Failure mode of the snap-lock after the tensile test.
5. Conclusions
In this study, we propose a novel snap-lock fastener that can be printed using a 3D printer (Mark Two). The fastener canbe easily attached to and detached from the fastened parts. Based on the tensile test results, continuous glass fibers wereplaced at the elastic hinge parts. The reinforced snap-lock specimens were tested under tensile loading. The following
results were obtained:(1) By exploiting the characteristics of 3D printed composite materials, we developed a fastener that can be easily joined
and removed from only one side.
(2) The bonding strength of the fasteners was obtained by modeling the developed fasteners with MarkTwo and conducting
a tensile fracture test. We found that an onyx fastener with a 100 % fill rate could withstand loads of up to 2.63 kN.
(3) The notched portion of the snap-lock pin, which functions as an elastic hinge when the fastener is installed or removed,
was reinforced with continuous glass fibers, and actual modeling and tensile fracture tests were conducted. We found thatthe 60-layer reinforcement could withstand loads of up to 4.49 kN. The use of glass fibers for fiber reinforcement enabled
the development of a fastener with high joint strength without losing its function as an elastic hinge.
Acknowledgments
This research was supported in part by the IHI Corporation. Ltd..
Appendix
The STL files and MPF files of the snap-lock pin and key are included. The MPF files are the coded files for printing withMark Two produced by Markforged ©.
<Snap-lock pin STL (https://additive-manufacturing.or.jp/agora-of-additive-manufacturing/)><Snap-lock pin without continuous GF MPF (https://additive-manufacturing.or.jp/agora-of-additive-manufacturing/)><Snap-lock pin with continuous GF MPF (https://additive-manufacturing.or.jp/agora-of-additive-manufacturing/)><Key STL (https://additive-manufacturing.or.jp/agora-of-additive-manufacturing/)><Key MPF (https://additive-manufacturing.or.jp/agora-of-additive-manufacturing/)>
The fastening process video<https://youtu.be/-mncpNK1_YU>
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