Milk Carton Derby Day

Today was the day of the race for the WOOF 3D printed boat. (There is a story in the Seattle Times.) In the end, stuff happened, and the boat ended up being a simple small boat, similar to the previous year’s boat. The bottom was made out of a pressed board of milk jug HDPE, with more HDPE printed on top This can be seen in figure 1, on top of a piece of plywood, which is on top of the printer bed. You can also see from figure 1 that the boat had a couple leakage points. These had to be manually filled in with an “HDPE gun” made by one of the team members. This was basically a print head not attached to a printer.


Figure 1. The boat in Big Red, WOOF’s large 3D printer.


The Seafair Milk Carton Derby created a special category for us: 3D printed boats. Since we were the only contestants with a 3D printed boat, we raced last year’s boat against this year’s boat (figure 2).

One thing I learned from working on this project is that HDPE is actually pretty tricky to work with. It shrinks by 2% when it cools, which adds up fast on a large object like a boat. For example, the warp in the walls of this years boat that you can see in figure 2 was caused by this shrinkage. (Last year’s boat had the same problem, but to a lesser extent since the walls were thicker.)



Figure 2. Last year’s boat on the left, and this year’s boat on the right.


In the end, the new boat got a crack in the bottom, and kind of sank just before the finish line. We still won our race though, since the old boat made it safely across the finish line, and we were the only people in our category. 🙂

I learned a lot working on the boat team, but probably the most important are: a) how to work with HDPE, and b) never let milk jugs sit around for more than a week before washing them.



Figure 3. Boat team lead Matt, and club president Brandon racing the two boats.


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