The biggest annual project for Duke’s chapter of the American Society of Civil Engineers (ASCE) is designing and constructing a raceable canoe built entirely out of concrete. This year I spearheaded the mold-making process in which I was tasked with building a formwork to shape the canoe.
Identifying the Problem
In years past, we constructed the formwork by sawing cross sections of the canoe out of wood and covering them with a combination of plastic, tape, screws, and foam sheets (too see a detailed description of the old method click here). This year, I sought to completely overhaul the mold making strategy. This process began by looking at the current method and identifying the limitations of the approach. Here are the major weaknesses that I identified:
- The building process was highly labor-intensive, requiring a team and a significant number of man-hours
- The process required a lot of human craft and technical skill which led to the design being imprecise
- The design was difficult to replicate and every year the team had to build from scratch.
- The materials needed to be both flexible and rigid to create the desired form, and the materials were not able to satisfy both criteria
- The materials were both costly and heavy
Designing a Solution
A significant amount of time was spent brainstorming various solutions for the mold. Our primary design criteria were to create a formwork that was dimensionally accurate, sturdy, and low cost. After a lot of discussion between members of ASCE, and outside consultation with experts with design experience, we came up with a solution that excelled in all the categories that the old design struggled.
The proposed solution would seek to create a foam mold that would be shaped using hot wire cutters and laser-cut wood sections. Our team was fortunate to have access to Duke’s state of the art technologies, and I hoped to use these tools to streamline our process.
The design process can be broken down into 3 major phases:
- Cross sections of the canoe were extracted using a 3D model of a canoe, and the sections were cut using a laser cutter.
- The digital file of cross sections was used to cut out segments of the canoe using a FROGWire CNC foam cutter.
- Using the wooden cross sections as a guide-rail, the foam segments were tapered using a hot wire cutter. These sections were taped together to complete the form.
Phase 1: Laser Cutting the Cross Sections
The project began with a 3D model of the canoe. This model was created previously my Duke’s ASCE team and it was determined that the model was an effective design for the purposes of the competition.
The 20-foot canoe model was divided into 20 one-foot long segments, which were imported into a 2D AutoCAD model. The photo below shows a bird’s-eye view of the canoe along with each of the cross-sections.
The cross sections were then transferred into Adobe Illustrator to prepare them for laser cutting. A total of 19 cross sections were cut and labeled using a laser cutter as shown in the photo below.
Phase 2: FROGWire cutting the Foam
Duke’s Foundry building is a large space dedicated to design and projects, which contains 3D printers, CNC machines, and many other design resources. One of these CNC machines, know as the FROGWire cutter, is specifically designed for cutting foam. A 2D design is inputted into the FROGWire cutter and it is able to trace the design into a block of foam.
After tinkering with the layout of the canoe cross sections, it was found that the volume of the canoe would fit into a four-foot cube of foam. The cross sections in AutoCAD were arranged to four 4’X4’ sections a prepared to be read by the FROGWire cutter.
The large foam block was purchased from HIBCO Plastics and was transported into the Foundry to be cut.
The photo above is an 8’X4’X4′ block of foam and ASCE was using half of the block (4’X4’X4′) for this project. Firstly, the cube was cut into 4 evenly cut one foot long sections as shown below
Given that all the foam segments are one foot long, all that was left was to let the FROGWire cutter trace out the sections from the foam sheets
The FROGWire cutting went smoothly and the dimensions of the finished cuts matched perfectly with the digital specifications.
Phase 3: Connecting the Pieces
To achieve the necessary tapering effect of the canoe, the foam segments were attached to the wooden laser cut sections. One side of the foam aligned perfectly with the foam and the other side contained the next smaller section that the canoe was going to narrow down to. These sections were cut using a handheld hot wire cutter. The wire cut easily through the foam but would not penetrate through the wood. The wood was used to act as guide-rails for the wire and each shape was accurately shaped. The photo below shows one of the sections in which the left side has been shaped and the right side is about to be cut.
Given the gradual narrowing of the canoe combined with the large number of cross sections used, the linear tapering between each of the segments resulted in a smooth curved surface across the canoe. These sections were then put into the proper order and then were taped together using gorilla tape. Lastly, packing tape was used to cover the surface of the form so that the finished canoe could be easily removed from the form after the concrete has set.
Overall, the project was vastly successful and a dramatic improvement over last year’s method. Below is a list of the major reasons why the new design is so successful.
- The digital modeling made the mold very dimensionally accurate
- The foam makes the formwork low cost and lightweight
- The AutoCAD files make the design reproducible and modifiable in future years
- The foam is strong and can easily support the weight of the concrete
- The construction is not labor intensive and can be performed by a single person
Moving forward: there are still improvements to be made to the process. The beauty of the digital design is that is it easy to modify and recreate in future years. The performance of the canoe will be analyzed, and changes will be made to the design as the team sees fit. While the large majority of human craft has been removed from the process, the end section still required sculpting. In the future, it is likely that these end sections can be 3D printed as to ensure accuracy and reproducibility. Lastly, it was discovered during the construction process that the FROGWire machine is able to perform independent axis cutting, meaning that the tapering effect can be created entirely using the FROGWire cutter. While performing this 3D cutting is a much more technically demanding, it would remove the need for the laser cutting and any hotwire cutting by hand. Additionally, the end sections could be accurately modeled on a computer and cut directly into a foam block. I look forward to reevaluating our design next year and coming up with an even more technically sound solution.
Acknowledgments: This project could not have been completed successfully without the help of several key people
- Zach McKenzie: Worked side by side with me to develop this new method and carry out the construction.
- Steve Earp: Helped transport the large foam block over 100 miles to get to the Foundry.
- Jeremy Nezaria / Morgan Fuehne: Introduced me to the FROGWire and helped me operate the device.
- Murad Maksumov: Provided me access to a laser cutter and helped cut out the wooden cross-sections.
- Lauren Stulgis: Helped me work with Foundry and provided me logistics on what was possible.
- Chris Northrup: For entrusting me with this project and providing advice along the way.