For our final project, we decided to cast and mold four Boba cups with a smiley face on it. 

Step 1: 3D model selection and positive creation

Our team was inspired by a file on thingiverse called cute bubble tea key chain. However, when the team tried to use the stl file to create the positive half of the mold there were multiple issues associated with our inability to easily modify the stl. Therefore the team used the model from thingiverse as a model and we created our own bubble tea cup on solidworks and 3D printed it. The ideal size of our model was about 2.5 inches in height. Once the file was created, we followed the instructions given in class to create and 3D print the positive of one side of the mold. The Bambu FDM Printer was used for all 3D prints. 

Figure 1: Inspiration from thngiverse

Figure 2: 3D printed positive and model

Step 2: CNC milling

After creating the 3D printed mold the team used a CNC mill to create the second side of the positive. This was done by creating an stl of the file and converting the 3D object into tool paths for the Shapeoko CNC machine using Vcarve. This machine was used because it is capable of using a ¼ inch end mill which is essential to achieving a CNC cut that doesn’t take multiple hours. In Vcarve we selected two cuts in the toolpath. One cut with the ¼ end mill that was primarily for material removal and one with an ⅛ inch end mill to add more detail. Ensuring that the rotation speed of the endmill is set to an optimal speed is essential because it can cut down on the time associated with cuts and it can decrease the risk of the motor on the CNC machine overheating. To keep the wooden piece in place while it is being milled I cut small divots into the wood that the green CNC securement pieces fit into. This technique is called fixturing.

Figure 3:CNC and 3D printed positives

Figure 4: Example of fixturing

 

Step 3: Silicone Molding

After creating both positives, the team used silicone molding to create both sides of the negative mold. We did this by taping cardboard walls onto the positives and filling the regions with silicone. After about 24 hours, the silicone had set and was ready to be used. 

Figure 5: Silicone mold creation process

Step 4: Polyurethane molding

Using the silicone molds, polyurethane was poured to create multiple replicas of the Boba cut. During this process faring amounts of red dye were added, leading to multiple colors of the replicas. A flaw in the design of the 3-D printed and CNC positives was that they had alignment pegs that did not interlock. In response to this, the team cut off the pegs and tried secure the two sides of the mold with methods such as zip ties which led to some misalignment issues. The misalignment was fixed by propping a popsicle stick underneath the lower side of the mold. The mold was then refilled again using rubber bands in subsequent steps for fast prototyping.

Figure 6: Silicone molds used to mold polyurethane

 

 

Step 5: Post Processing

After the polyurethane mold was completed, 320 grit sandpaper was used to remove excess material and smooth out the shape

Figure 7: Complete pieces

Conclusion

Overall the team was able to produce 4 copies of boba cup of varying colors. If we were to do this project again we would modify a variety of our processes. Firstly we would increase the thickness of the lip of the boba lid as it was too thin and resulted in cracks in that part of the mold and defects in the CNC negative. Additionally we would have added an additional cutting toolpath to make the CNC side of the negative more smooth. 

Figure 8: Clean workspace

Cost Analysis

The majority of costs were associated with labor, especially labor related to creating the positive and negative molds. However, given that the silicone negatives are now created, the average cost of a polyurethane molds would decrease over time as more were created. If we were able to restart the project from scratch this project would likely take half the labor time decreasing costs by over 40%.

 

Cost Type	Cost	Price	Source	Quantity	Total
Materials	PLA	$2.76 /sheet	Amazon.com	1 sheet	$2.76
	Sand Paper	$0.40 / sheet	Amazon.com	1 sheet	$0.40
	Wood	$1.50/ft	Home depot	3 ft	$4.50
	Silicone	$40/ 84oz	Amazon.com	12 oz	$6
	Polyurethane	$37/ 32oz	Amazon.com	8oz	$8
Labor	3D printer operator	$19/ hour	ziprecruiter	4 hours	$76
	CNC operator	$22/ hour	ziprecruiter	3 hours	$66
	Casting	$15.68/hr	careerexplorer.com	4 hours	$62.72
Overhead	CNC machine	30/hour	HLH	3 hours	$90
	3D printer overhead	$1.33/ day	makerbarn	1 days	$1.33
Total					$441.12