As a group, we built a rotational casting machine. This week, I worked on the central chamber to hold the mold in place. I also reviewed alternatives to 3D printing my final project - creating a silicone mold for the cone top that could wrapped under the base.
The mold would be made of a 5mm thick piece of silicon layed flat. I would most likely use the silicone as in the molding and casting week. I considered assembly of a cone looking at a video by Alexander Dyer, which led me to a cone calculator page.
The sides of the flattened cone could have a sort of flat button system, allowing the material to wrap into shape. The lower section of the top would have a series of bumps that protrude outward. I could push these bumps through gaps in the upper part of the base.
I had used the silicone mold on another pannier – It feel and thickness gave me an idea for the strength of 5mm thick cone top. The material would need to support impact from a football - compared to PLA/ABS it would not (fully) hold its shape but this is a minor issue. It is also heavier and more expensive to use - 1kg currently costs €61.90. It will take a bit of experimenting to get the weight of the base (with sand) right: A balance between it holding itself upright on impact and moving freely. If the top is too heavy, the buoy could be very sturdy and difficult to move – I would like some movement for the players to visualise. It was also useful to consider an earlier prototype done in cardboard.
I estimated the dimensions for each part, including the interior support for a disc housing the electronics. When using the flat buttons, a dip on the opposing size would allow the material to remain flat.
In Fusion 360, I did a straight comparison between ABS and silicone by adjusting the material on the cone top. I returned to my file in progress, removing the grip gap which probably would not work with a flattened piece. Without creating the additional bumps in the lower section, the cone top was already 613grams.
Based on weight and recommendations from Adel, I decided to proceed with a 3D printed piece and find ways to print my design on the Ultimaker 3. The leads appropriately to week 18 where we had to workout material cost.
My initial idea was to 3D print a cylindrical shaped cast, similar to one I'd seen at a rotational molding factory. The idea was to create a clip using a living hinge to hold the two sides in place and close it around two bars running on either side.
The cross-section shows how a tray within the cast could be placed to reuse the different casts.
Based on the time constraints, it was decided that the early concept would take too long to print. An alternative was to create a press-fit box that could function in a similar manner. I set about sketching a chamber with two dividers, similar to a filing cabinet layed flat. To this, I added a top layer which could be placed over the dividers to seal the chamber. On paper, I worked out the dimensions to be used parametrically.
I started working parametrically, entering the amounts in the 'Change Parameters' menu. Despite working out the dimensions, I found it challenging to match up the sides of the press-fit construction and make sure they fit appropriately. I extruded each sketch as a body to check that pieces fit together.
I created a significant amount of construction lines which I needed to remove on transfer to Inkscape. To do this I created a copy of the file in a separate folder named 'production'. Here, I removed all bodies and lines that were not required – Make sure you have the correct copy open before deleting anything!
When the sketches for each side were cleaned up, I right-clicked on each and selected 'Save As DXF'.
In Inkscape, I transferred all DXF to one file with dimensions of 600x300 and saved it as an SVG. I converted all lines/stroke to RGB 255 with a thickness of 1.00px.
Assembly proved challenging because the parts were wafer thin (well, 3mm). The plywood was used for this test and final prototype. I used scotch tape to help me assemble the parts initially, later I used glue. We wanted to ensure that the chamber stayed together on rotation. We also needed to connect it to the axis in some way.
I had to return to Fusion on a couple of occasions to redraw and export sides of the chamber. With so many construction lines, I had confused the height of certain heights and parts did not fit. I also had to adjust the top and bottom layers to include a space for attaching the grips we had in mind. The other issue in Fusion was that I had created a lot of rectangles on top of each other - It would have been better to use lines.
Before proceeding I did a quick test with scotch tape.