Airplanes contain rivets that bond layers of metal plates together. These rivets are bonded together using a sealant, which allows the airplane to maintain pressure at high altitudes. This sealant must be applied correctly in order to maintain pressure. Often times the sealant gains air bubbles during the application process. When these air bubbles pop 50,000 feet in the air, it causes air to enter the cabin of the airplane and causes depressurization. In mild cases, this can cause a minor drop in elevation or oxygen levels, but in major cases, it can cause the plane to plummet to the ground. The air bubbles entering the sealant is a major problem in the aircraft industry and this problem is what the Caulkers were tasked with solving.
Figure 1: This picture shows seams where sealant had no air bubbles and shows seams where the air bubbles have popped.
The Caulkers were sponsored by Bergdahl Associates to keep air bubbles from entering the aircraft sealant. When the sealant is dispensed at a fast rate, no air has time to enter the sealant. However, often times a fine application of the sealant must be applied and this is when air enters the sealant. When the caulk gun dispenses slowly with the plunger being compressed in an oscillating motion, it allows air to enter the sealant. The Caulkers were tasked with keeping air out of the sealant but with a few rules: the entire product including manufacturing and shipping must cost under a dollar; the solution cannot modify the caulk gun in any way; the solution must also work in any of the Semco caulk gun versions; must be able to withstand 100 psi; only be used one to five times; and finally have an initial length of one inch and be able to expand between six and twelve inches without putting pressure on the sides of the gun.
Figure 2: This figure shows how the air bypasses the current plunger built into the cartridge of the sealant.
The team decided a disposable insert for the caulk gun would be the best design for the criteria given by Bergdahl Associates. The insert was made out of latex since latex can expand to the six to twelve inch criteria we were given. Also, latex would be disposable and only withstand a couple uses before breaking, which satisfied the one to five time use criteria.
The latex insert works by expanding to the sides of the gun as sealant is being dispensed to keep out air and create another barrier between the air and the sealant. However, it was discovered during testing that applying this latex insert to the caulk gun was incredibly difficult and time consuming. It was decided an applicator was needed in order to cut down the time necessary to apply the latex insert to the caulk gun.
Figure 3: This figure shows the latex insert in its initial state and its expanded state for a six inch caulk gun.
The applicator was made out of polypropylene plastic with a hole down the middle to allow any air to escape. The applicator keeps the latex insert flush to the sides of the caulk gun while also pushing the latex insert into position correctly. This applicator works on every version of the caulk gun so as to be as efficient as possible. The goal of the applicator is to help mitigate time constraints of adding another procedure to the caulk gun process.
Figure 4: This figure shows the plastic applicator applicable to all versions of the caulk gun.
Figure 5: This figure represents the entire assembly of the caulk gun including the cartridge, built in cartridge plunger, the latex insert, and the plastic applicator.
The latex insert is fabricated by dipping a glass mold into a vat of liquid latex several times. This product is then cured in an oven and refined through several quality assurance tests. For this project, there is no need for the team to be fabricating the latex insert when it is readily available on the market. A CNC lathe fabricates the applicator, which is made of polypropylene. This process is done by inputting the design specifications into the machine. The machine’s cutting tool carves the raw material into the desired shape. Meanwhile, the drill and boring bar makes holes in the center of the material as needed. In the future, a company that specializes in lathing or injection molding can manufacture the applicator.
Proof of Concept
For the proof of concept, the team decided to create and test the latex insert as it had the highest chance of failing. In order for the solution to be under a dollar for manufacturing and shipping, the team decided to see if there were any current products on the market that could solve the issue. After extensive research, a condom seemed to have some potential in solving the issue. It was made out of latex; it was durable; and it could expand six to twelve inches. However, there were a few criteria that needed to be tested to determine if the condom would be a viable solution.
- Would the condom withstand 100 psi without breaking and allowing air into the sealant?
- Would the condom have more than one use?
- Would the condom successfully keep air out of the sealant?
After extensive testing, it was determined the condom would be a viable solution! It withstood 100 psi without any tears; it was able to be used multiple times before tears started to appear; and the condom successfully kept air out of the sealant.
In order to simulate the testing procedures done in industry, peanut butter was used to act as a substitute for the sealant. Peanut butter is the same viscosity as the sealant so it is a viable substitute for the testing as the sealant is very expensive. The first step is to place the latex insert over the full length of the applicator rod. The user can then put the rod into the cartridge and pull the insert down and stretch it over the edge of the cartridge. The rest of the caulking gun can now be assembled as usual by installing the tube of sealant filled with peanut butter. The end of gun twists on to enclose the assembly and the user can now tap the trigger to control how they want to dispense the sealant.
Different patterns, such as dots or lines, were tested ranging from slow to fast rates in order to test the effectiveness of the design. It was easy to tell if air had entered the peanut butter as air bubbles would form visually in the peanut butter and a sound would be heard when the air bubbles traveled through the nozzle when being dispensed. Out of the 51 test subjects, only 2 had traces of air bubbles in them. With this result, it was determined the condom effectively kept air bubbles out of the sealant. The 2 bad trials were attributed to the team’s lack of knowledge in using and loading the caulk gun.
Figure 6: This figure shows the different patterns used to test the condom.
Durability was tested next to determine if the condom could last the required five times. 10 tests were conducted with the same condom with the sealant being dispensed quickly, slowly and pausing for periods of time. The condom withstood all tests and it was determined, with a reasonable factor of safety, that the condom would withstand the required one to five trials in the field.
Table 1: This table summarizes the engineering specifications required by Bergdahl Associates against the actual results obtained.
|Specifications||Targeted Results||Actual Results|
|Ease of Use||> 3 minutes||1 minute|
|Withstands pressure||100 psi||–|
|Eliminated air bubbles||>2%||0.05%|
Click the link to watch how the insert and the applicator are tested: Testing Video
While the testing proved the condom would withstand the necessary criteria, the team still wished to calculate the results to be completely confident in their design.
The calculations showed that the latex insert has a 700% elongation rate before it breaks. This means that there is very little chance of the insert breaking except through repeated stress and reuse. Below are the equations used to determine the factor of safety and stress undergoing the latex insert.
Factor of Safety:
These calculations suggest that the latex insert will expand and not break under the parameters of one to five uses.
Also, FEA analysis was used to determine the most likely places of failure on the insert and the applicator. Below are the results of the analysis. The insert is most likely to break at the tip and the applicator is most likely to break at the junction between the rod and the tip.
Figure 6: Maximum applicator stresses
Figure 7: Maximum insert stresses
Design for X
The final part of the project the team decided to analyze was whether there were any parts of the design not accounted for. The team wanted to be sure they accounted for the most important factors of the project and addressed them appropriately.
Cost: The latex insert is already mass manufactured so the design is already optimized for cost. The applicator can be mass produced using the sponsor’s injection molding machine, requiring only the upfront cost of the mold. The applicators can be used more than once so the aircraft industry will be willing to buy them once since they will likely never have to buy them again.
Efficiency: The latex insert itself is not terribly quick to insert but the applicator mitigates this factor. With the use of the applicator, there is no extra time to add the insert to the gun. This means there will be no cut backs to productivity.
Manufacturing/Fabrication: The latex insert is fabricated by dipping a glass mold into a vat of liquid latex several times. This product is then cured in an oven and refined through several quality assurance tests. For this project, there is no need for the team to be fabricating the latex insert when it is readily available on the market.
Overall, the project was a success and followed the criteria and design factors.
Meet the Team
Chris Sietsema is a senior in mechanical engineering, and is close to having a civil engineering and math minor. He currently has an internship at Pacific Drilling and Mining Corporation, doing special projects, design concepts, and filing patents. This will help the team design and construct a their required task. Having taken courses in intermediate dynamics and fluid mechanics will also contribute to the teams design and construct a product that will be durable and efficient.
My name is Chel-Sea Chen, senior Mechanical Engineering student at the University of Nevada, Reno. When I graduate, I would like to pursue a career as a Design Engineer to create innovative aircraft solutions for the Aerospace Industry. On campus, I enjoy being active by being a part of several professional organizations. During my leisure, I am a huge foodie and love trying different cuisines. I also enjoy outdoor activities, dancing, the performing arts, and traveling.
Caitlin Burke is a senior mechanical engineering student at the University of Nevada, Reno. She currently works at CR Engineering as an engineering intern. At this internship, she drafts and designs HVAC, plumbing, and fire protection systems for commercial buildings. This has taught her excellent time management skills and the seeing a project through from start to finish. This will help the team stay on track and finish the project in a timely manner.