Project Overview
Framing a poster can be expensive and can add a lot of weight. This excess weight can make large nails a necessity to hang up any posters in a dorm or apartment, which are far more difficult to repair due to a larger hole. The use of nails as a hanging method also requires tools such as a hammer. Potential solutions such as thumb tacks and adhesive putty have been developed to combat this, but they can cause damage to the poster itself.
For this reason, a 3D printed poster hanging system that is a cheaper alternative to frames and still preserves the poster was investigated. The initial concept is a device that attaches to the corners of posters via press fit, which has a recessed area in which a small tack can be inserted.
Initial Design
The feasibility of printing a paper-thin gap and tack was observed, so an initial design was made using SolidWorks and printed. This print was done using an Ultimaker S5, and it took two hours and 46 minutes to print. PLA (polylactic acid) filament in combination with PVA (polyvinyl alcohol) filament, a water-soluble support, were used to make the part. Two orientations were tested to see which could print the gap and tack better. Figure 1 displays the finished parts in the two orientations.
Figure 1. Poster clamp initial design, printed in PLA with PVA support.
Unfortunately, both orientations produced less than desirable results since the gaps fused together, and the tacks were too small to the point where they broke when slight pressure was applied. The fused gap can be observed in Figure 2, shown below.
Figure 2. Side profile of poster clamp initial design.
With a fused gap, paper was not able to be slipped into the poster clamp, so this kind of design was deemed infeasible for a 3D printing application. Furthermore, tack redesign was required to properly secure the poster clamp to the wall. A positive take away from the initial design was the appropriate size of the poster clamp itself. The poster clamp length and width were kept the same for subsequent iterations.
Redesign 1
The team went through a few different design iterations to solve the various issues that were observed in the initial design. To solve the issue of the gap fusing together and not leaving enough space for a poster, it was decided to incorporate a living hinge into the design. This hinge can be seen in Figures 3 and 4.
Figure 3. Poster clamp redesigned with live hinge and interlocking tabs (open).
With this design, only one printed piece was necessary to hold the corner of a poster, which makes it faster to manufacture and simpler to install. A living hinge was made possible by simply placing significantly less material at the hinge location.
The hinge was closed by interlocking four tabs shown on the longest side of the poster clamp. These tabs can be seen both in Figures 3 and 4.
Figure 4. Poster clamp redesign with living hinge and interlocking tabs (closed).
These proved more challenging than initially anticipated since they did not align when the hinge was bent closed. The design was reprinted, adjusting the tabs’ location but still had only two of the four tabs properly engaged. This problem primarily stemmed from where the part bends in the live hinge not being the direct center of the thin material.
3D Printed Tacks
A removable tack was added on the back of the design to allow for many different sized tacks to be printed and tested. The hole for these tack inserts can be seen in Figure 3. Tacks of various sizes and angles, found in Figure 5, were printed to see how they would compare in terms of damage they caused and ease of installation.
Figure 5. Three different print sets of tack design.
The leftmost is the SLA tack, and the middle and rightmost sets are FFF tacks printed at different times. Printing the removable tack was originally an idea for easier testing and prototyping, but there are benefits to being able to remove the tack. The primary benefit is that the customer can choose how they want their poster mounted. With a removable tack, they can choose to use adhesive such as Command Strips as the main method to attach the poster clamp to the wall. This would allow for the poster to remain undamaged and allow for use in buildings that do not use drywall, such as cement block or brick walls.
Wall Installment
The 3D printed tracks presented some difficulties when attempting to install them for testing purposes. Holes had to be piloted before installation, as the tacks were not sharp or strong enough to easily puncture the drywall. The tacks also left holes that were larger than desired. A comparison of the holes can be seen in Figure 6. The damage to the wall from the 3D printed tacks makes these tacks a less ideal option for wall installment.
Figure 6: Hole left from 3D printed tack (left), hole left from conventional metal tack (right).
The 3D printed tacks were deemed unusable after testing, as they left large holes in the wall and were difficult to install.
Clamp Material Comparison
It was originally planned to test two materials, ABS (acrylonitrile butadiene styrene) and PLA, to determine which would be better for this application. However, due to ABS unavailability, a comparison between two processes, SLA (stereolithography) and FFF (fused filament fabrication), was done. FFF is far cheaper, takes less time, and is more pliable after post processing. SLA has a better surface finish, but that is really the only positive, and so moving forward, FFF was used. The two prints can be seen side by side in Figure 7. The only aspect of this design that was considered to be made via SLA was the removable tacks as they were strong, and the smoother finish may be less damaging.
Redesign 2
An additional redesign implemented a recessed area to both sides of the live hinge. This allowed the two halves to be easily separated when they snuggly fit together. The lighter gray boxes as seen on the SLA specimens in Figure 7, were the recessed areas.
Figure 7. Further redesigned poster clamp with FFF (left), printed with SLA (right).
It was observed that there was a tendency for the part to snap in half at the live hinge interface upon initially bending it. In order to counteract this, Hubs, a 3D printing prototyping company, recommends two things [1]. One is to orient the part in a way that the central axis of the hinge is parallel to the printer’s z-axis. This ensures that the live hinge is printed layer by layer, leading to less frequent brittle failure by counteracting FFF’s tendency to create anisotropic parts. The other recommendation is to anneal the live hinge surface before initially setting the bend. This can be done by simply raising the temperature at the live hinge interface and immediately bending the part when the material is partially melted, thus eliminating the possibility for a brittle failure. Figure 8 displays the annealing process on one of the FFF poster clamp testing specimens.
Figure 8. Annealing process done to one of the FFF poster clamp test specimens.
Annealing Results
The result of annealing the FFF test specimen was that it did not break in half as others did in the past. Additionally, we were able to manipulate where the live hinge creases, and consequently, we were able to line up the interlocking tabs very easily. The annealed part is shown in Figure 9. The PLA material is a thermoplastic which means reheating it can cause it to be more malleable while retaining its molecular structure. Conversely, the SLA test specimen broke in half just the same as if it were unaffected. This was expected because the resin used in the SLA process is a thermoset that requires UV curing to solidify. Heating is not a part of the process, and as a result, applying heat to the material does not make it more malleable and instead creates a weaker joint.
Figure 9. Annealed FFF poster clamp with functional locking mechanism.
Fatigue Testing
With this design, a fatigue test was conducted. The fatigue experiment consisted of comparing the survivability of an unaffected poster clamp to an annealed one. In order to bring a product like this to market, it was determined that the live hinge should have a lifespan of one hundred cycles in which a cycle consists of one open and one close of the poster clamp. One hundred cycles far exceed the estimated usage in a realistic application. Realistically, the poster clamp will be opened and closed a maximum of ten times in its lifetime. For this reason, we agreed on a fatigue safety factor of ten to account for potential variation in printing quality if this product is manufactured on a larger scale.
The results showed that both the unaffected specimen and the annealed specimen passed with each still possessing the ability to open and close in one piece after one hundred cycles. This suggests that annealing does not affect the live hinge’s structural integrity once it has cooled. On a larger scale, additional consideration should be given to the feasibility of annealing and setting the bend on all parts. For this reason, we tried to eliminate the need for annealing the hinge.
Nylon and Redesign 3
Nylon was identified as another potential material for the clamp, as Hubs states it is the best material for a single-material live hinges [1]. Four nylon clamps were printed on an FFF machine for testing. The geometry on the nylon hinge was the exact same as the previously tested PLA clamp, but with fillets added to the live hinge. It was confirmed that the live hinge was parallel to the printer’s z-axis. A picture of the nylon hinge can be seen in Figure 10.
Figure 10. Nylon poster clamp printed on FFF machine.
Note that the previously square cutout for the 3D printed tack has been replaced with a circular cutout to accommodate store-bought metal tacks. The metal tack sat flush in this cutout and was easily installed into the wall. These metal tacks were also not significantly more expensive than the 3D printed tacks. Figure 11 shows the metal tack in the hinge.
Figure 11. Store-bought metal tack sitting in poster clamp.
Fatigue testing of the nylon clamp live hinge produced promising results. All four samples creased at the correct location on the first bend, and the notches lined up perfectly. The hinges were put through 100 cycles each to test the fatigue life. Each clamp passed this test with no problem, which meant they met our safety factor of 10 when compared to an estimated use of 10 bends per life.
Problems arose when looking at the clamping ability of the nylon clamps. While the hinge did allow for the notches to line up well, the print quality was not great enough for the notches to fit together in all samples. Two samples had overhanging material that blocked one notch from sitting correctly, and two samples sat correctly, but the notches popped out after a short time. While the first problem can be corrected with a higher resolution printer, the second problem is due to material choice.
Nylon was shown to be not stiff enough to provide a clamping force on the paper. When the notches were properly set, the middle of the clamp bowed out and left a large gap in between the two sides of the clamp. This problem can be seen in Figure 12.
Figure 12. Nylon clamp with gap caused from lack of stiffness of material.
This, in combination with the notches not holding firmly, demonstrated that nylon was not the correct material for this application. Moving forward, PLA was the only material used.
More Live Hinge and Holding Testing
Four more samples were printed on the FFF machine using PLA. Once again, fillets were added on the live hinge, and the live hinge axis was oriented along the z-axis. Some success was had with the previous PLA samples when heat was applied to the live hinge, so the repeatability of this was tested. PLA previously passed the fatigue test, so the ability to hold a paper was the focus at this point in the design process.
The first sample was bent without any heat applied, and this resulted in the hinge creasing at the incorrect location. The notches did not fit together, and therefore, this sample could not apply a clamping force. An attempt to force the notches together resulted in hinge failure and a broken part.
Heat was applied to the hinge of the other three samples. Heat made the PLA more malleable so the notches could be lined up during the initial bending of the hinge. The PLA material is a thermoplastic which means reheating it can cause it to be more malleable while retaining its molecular structure. If all three samples were easily bent into the correct position when heated, the annealing process could be feasible on a larger scale.
The first heated sample presented some challenges. The first heating and bending resulted in the notches not lining up correctly. Further cycles of reheating and rebending resulted in the notches sitting well enough to stay clamped, but the cycles of heating caused warpage of the clamp. This warpage took away the clamping ability of the part, which made it unusable for this application. This warpage can be seen in Figure 13. The testing of this sample showed that the notches must be lined up correctly on the first heating and bending cycle.
Figure 13. Warpage of PLA part caused by multiple heating cycles.
The testing of the second heated sample was more promising. The notches were lined up on the first heating and bending attempt, which resulted in a hinge that worked as intended. This sample clamp could hold a piece of paper firmly, with the only problem being slight warpage on one end. This slight warpage did not hinder the clamping ability significantly but was not cosmetically pleasing. A picture of this sample can be seen in Figure 14. The last sample had nearly identical results to this second sample.
Figure 14. Working clamp sample with small warpage in bottom corner.
The results of these three heated samples show that this heating and bending process to align the hinge and notches was not easily repeatable. The heating caused warpage that was not aesthetically pleasing. For this PLA clamp to be mass produced, highly accurate heating and bending machinery would need to be used in the manufacturing process, which would be very expensive. It was decided that this current PLA live hinge design was not feasible, and a final iteration was necessary.
Final Design
The final design was printed in two pieces and eliminated the need for a live hinge. FFF was used with PLA as the material. Notches were used on both sides of the clamp, and this design can be seen in Figure 15. The circular inset for the metal tack was kept, along with the recessed area for easier opening.
Figure 15. Final clamp design.
Overall Testing Synopsis
Two tests were used throughout our design process. A fatigue test, which has been previously discussed, was used to evaluate the strength of our live hinge designs. All live hinge specimens, both annealed and untreated, were put through the fatigue test to verify the hinge would survive the necessary lifespan. Surviving 100 cycles was used as the passing criteria, with a cycle being defined as opening and closing the hinge. It was estimated that a necessary lifespan was 10 cycles for a poster clamp and a safety factor of 10 was desired. All live hinge samples survived the fatigue test, so this design would have been feasible if the live hinge bending interface had been more consistent and repeatable.
The other test used was a hanging test. Only the last design had enough potential to run this final test. This test consisted of clamping a piece of heavy-weight printing paper, and then hanging up the clamped paper using the metal tack. Initial pictures were taken of the testing samples for later comparison, which can be seen in Figure 16. The papers were left hanging for one week and were then inspected.
Figure 16. Hanging test initial picture with final design.
Inspection looked for any movement of the paper, and damage left on the paper from the clamp once removed. The results suggested that the clamps held the paper in place, and they did not damage the paper.
Future Work
Looking ahead, there is plenty of future work to be done on the poster clamp. Research could be done on making the design viable for injection molding. This would be the ideal method of manufacturing if the clamp were mass produced since 3D printing is still very slow. In addition, more research could be done on the design of the interlocking tabs. This research would focus on finding the optimal angle and size of the tabs to increase the reliability and clamping force. The current final design is a great step in the right direction, but it is not perfect yet.
Conclusions
While one 3D-printed piece that includes the tack and poster holder mechanism was the initial concept, it later proved to be not practical. Much work was put into the live hinge design, but there was not enough time to get this design finalized enough for use. This stemmed from variability in print quality. From multiple iterations, a two-piece clamp with interlocking tabs and an insert for a commercially available flat tack is the final design. This design has proven to be the most reliable and consistent in terms of print quality and clamping ability. In addition, it allows the customer to choose their preferred mounting method, whether it be via tack or adhesion. FFF was chosen over an SLA printed clamp since FFF was cheaper and resulted in a rougher surface finish that increased the holding force on the poster. Testing of the final design shows the clamp works as intended and holds paper firmly without causing any damage.
References
1. “How to design living hinges for 3D printing,” Hubs. https://www.hubs.com/knowledge-base/how-design-living-hinges-3d-printing/ (accessed Apr. 02, 2022).