Abstract
Knife scales are an appropriate candidate for 3D printing. The following paper details the various designs and materials used to additively manufacture an improved set of knife scales. Both physical and finite element methods were used to test and validate design decisions. The final scale design included an increase in thickness to allow for a surface finish and to add strength. Formlabs Tough 1500 was the final chosen material due to its high impact strength.
Introduction
Foldable knives are a useful tool with a purpose in many settings, including both industrial and environmental applications as well as personal use. These knives are designed with two scales that are either riveted or screwed to one another to make up the handle. The complete handle can be made from a variety of materials to target different conditions and uses for various knives [1].
The goal of this paper is to discuss how various 3D printing techniques can be used to produced quality foldable knife scales. The aim is to show how advancements in additive manufacturing technologies have led to the ability to create extremely unique and accurate parts. On top of this, these advancements have allowed for the ability for everyday people to use these technologies to create their own at home designs.
In the Project Proposal, a preliminary SolidWorks model was created, and an initial print was planned for both scales. The initial print was done using FFF with Ultimaker nylon. Subsequent prints were planned using SLA with various Formlabs resins to compare printing methods. Finite element analysis was done for each scale design to ensure that the object is drop-resistant, which is the primary expected mode of failure for this object’s application.
Materials
Three different materials were used during experimentation. The initial print was done using fused filament fabrication (FFF) with Ultimaker nylon
[2]. This proprietary nylon polymer has high strength and resistance to water and other environmental factors that may affect knife users. It is also a highly common material that is easily sourced and inexpensive, making it an appropriate selection for an initial print.
The second print was carried out using stereolithography (SLA) with Formlabs Tough resin [3]. This material was chosen due to its high strength and stiffness and high surface finish quality.
The final print was made using Formlabs Tough 1500, a resin with similar material properties to the original Formlabs Tough but with an improved impact strength [4]. This improved impact strength is especially significant in this application since the main failure mode is expected to be from impact during a drop.
The final, optimal material was chosen based on two main factors: material properties and simulation results. FEA simulations described in a following section will detail the expected maximum stresses experienced during an impact test and compare them with the yield strength of the material. Prints were done with both Tough and Tough 1500 to compare the look and feel of the object as well as each material’s performance.
FFF and SLA Comparison
Overall, the SLA print is more dimensionally accurate and rigid, but has a worse surface finish. This is caused by the support structure used in the SLA process. It consists of many small pillars holding up the handle scale as opposed to FFF which only included a thin layer of material. These imperfections are minor and could easily be cleaned up with sandpaper. Another interesting feature of the SLA scale is that the entire surface has vertical ridges. These ridges were not included in the STL file, but they make the scales feel better and provide more grip than the smooth FFF ones.
Printing Process Initial FFF Print
Once the FFF print was created in UW Makerspace, it was installed onto the knife assembly [5]. The screw holes on the scale did not perfectly align with the metal liner for this print trial. This misalignment is attributed to the imprecision of FFF printing or expansion of the material during or after printing.
Initial SLA Print
A second print was carried out using SLA with Formlabs Tough. The SLA scales fit correctly onto the knife handle and all screws could be installed.
Knife Scale Redesign
The results from the initial prints and experiments discussed above lead to a redesign of the part since there is still room for improvement in terms of ergonomics and strength. The surface was changed to include a pattern which resulted in an increased thickness of the scale. The increased thickness is expected to improve the impact strength of the scale.
Another focus of the project was to show the power of additive manufacturing and how it can be used to create unique shapes and designs. The initial prints were very successful in showing that these scales could be produced with relative accuracy and with a quick turn-around time.
One characteristic of the initial SLA scales were small divots on the backside of the scale caused by the support pillars created during the print. The modified scale contains similar divots, but at different locations. This is likely due to the orientation the scales were in during the print; the modified scales were more angled. Also, the divots a more visible on the new part because the material is opaque.
Similar to the previous scale printed with Formlabs Tough, the modified scale is a perfect fit onto the knife handle. Also, the Formlabs printer was able to perfectly replicate the design of the pattern.
Also, the previous SLA print had vertical ridges that spanned its entire front surface. The modified part has visible lines/stripes, but no deep ridges. This is once again likely due to the orientation at which the two sets of scales were printed.
Finite Element Analysis
Simulations were run using the SolidWorks Drop Test feature for each scale (front and back) individually in 3 different orientations (x, y, and z) to accurately determine the maximum stress experienced by the object when dropped. The final drop test orientation is seen in Figure 10.
Finite element simulations were run using the SolidWorks Drop Test feature to test the redesigned scale with the chosen material. Formlabs Tough 1500 was selected for use as described above; its material properties are described in the following section.
Material Properties and Loading Conditions
Formlabs Tough 1500 has a yield strength of 33 MPa, which is slightly lower than the original Formlabs Tough. The material makes up for this difference though with an increased flexural strength of 33 MPa, and a Notched Izod Impact Strength of 67 J/m. This increase in impact toughness is the desired effect in this knife application, and with a modulus of 1.5 GPa, it has been chosen as the final material to be tested in the following finite element simulation. The loading for these results will be described as followed.
FEA Results
First, a drop test simulation was performed in each of the x, y and z directions to determine which orientation causes the highest stress in the scale. The first test was done with gravity acting in the x- direction. Next, the drop test FEA was carried out with impact occurring in the y-direction. Lastly, the drop test FEA was carried out with gravity acting in the z-direction.
It is evident that dropping in the z-direction results in the highest stress. Moving forward, only this orientation will be considered because it yields the highest risk of failure due to the greatest impact stresses.
Maximum Von Mises Stress (MPa)
20
22.2
25.3
After the orientation with the highest stress was found to be the z-direction, both the front and back scales were then simulated in order to determine which component experienced greater stresses. Since the design of these two scales vary only slightly, they experience a similar amount of stress with the front scale experiencing a slightly higher value than the back scale: 25.3 MPa to 23.6 MPa, respectively. The front scale maximum stress was still below the yield strength of 33 MPa for Formlabs Tough 1500, validating our design and material selection.
Limitations of FEA
These results only account for the dropping of a single scale on the ground. In actuality, the scales will be affixed to a metal lock, liner, and blade. On one hand, the liner will provide more strength. On the other hand, the blade will make the total object heavier and increase mass and thus impact velocity. These considerations reduce the fidelity of the simulations but can only be addressed if a model of the entire knife and blade was made.
Validation of Finite Element Analysis
A rudimentary physical drop test was carried out with each scale to validate the finite element analysis results. Experiment conditions included a 1m drop height with each respective scale to match the simulation parameters. This experiment confirmed the outcome of our simulation results as none of the scales experienced impact failure. The damage is cosmetic and does not affect the structural integrity of the scales.
Discussion
From beginning to end, the improvements in each iteration of additive manufacturing to the knife scales was interesting to see, and it provided the experience to show the improvements in capabilities these printers and their technologies have on making a detailed and effective product over a span of a semester.
It can be duly noted that this design process can be implemented into other design projects as well. As it has been observed that these materials can be effective in a wide variety of performance criteria and geometrical dimensions.
Conclusion
Additive manufacturing is an effective method for creating custom knife handle scales. The SLA print method is capable of printing the scales with high dimensional accuracy which are capable of surviving impact from a drop. Based on physical testing and experimental data, Formlabs Tough 1500 is the recommended material due to its high impact strength. The material can take drops with minor cosmetic damage. Any pattern or design can easily be created and printed within several days. Also, if users do not have access to an SLA machine, FFF still provides a useable scale. However, it is less rigid and less dimensionally accurate.
References
[1] L. Rainey, “Knife Handle Materials Guide,” 13 April 2021. [Online]. Available: https://www.bladehq.com/cat–Knife- Handle-Materials-Guide–3420.
[2] “Ultimaker Nylon,” 15 April 2021. [Online]. Available: https://ultimaker.com/materials/nylon.
[3] “MDS Tough,” 17 April 2021. [Online]. Available: https://formlabs- media.formlabs.com/datasheets/Tough_Technical.pdf.
[4] “MDS Tough 1500,” 17 April 2021. [Online]. Available: https://formlabs- media.formlabs.com/datasheets/Tough_1500_TDS_EN.pdf.
[ 5] “UW Makerspace,” 13 April 2021. [Online]. Available: https://making.engr.wisc.edu/.