Yo-yo Base: An Injection Molded Part

A. The base mold has a parting line at surface where the base would meet the ring. The core mold is made using the lathe and the mill (the center shaft of the base mold, where the spinner is placed, needed to be milled into the core mold). Here is what the process of machining the cavity would look like. Here is what the process of machining the core on the lathe would look like. After the core is lathed, it is then put into the mill to drill the ejector pin holes and to hollow out the center shaft of the base mold. The cavity mold then uses the mill to create the runner from the sprue to the part. 

B. In class, there were example yo-yo base molds and a few of their injection molded parts. From these examples, I measured the injection molded base outer diameters and compared them to the outer diameter on their respective molds. From this I saw that the base molds shrunk around 2% in size from their mold sizes. Thus, we decided to have a scale factor of 1.02 for the base mold.

C. Process Plan for yo-yo base mold

Step	Operation	Machine	Tool	Justification
Cavity Mold
1	Bore cavity mold Rough Pass	PUMA Lathe	T1010 0.03125 Large Bore	Use bore tool to make a rough pass and cut out the design for the base of the yoyo cavity
2	Bore cavity mold finish pass	PUMA Lathe	T1010 0.03125 Large Bore	Use bore tool again on final pass to smooth mold
				1m 16.94s
3	Make runner from sprue to part	Protrak Mill	#9 sphere endmill	Use the sphere endmill to create the runner from the sprue to the part
Core Mold
1	Lathe rough pass core mold	PUMA Lathe	T0101 Turning Face tool	Turn down the face of the mold to match the core of the base part of the yoyo
2	Lathe finish pass core mold	PUMA Lathe	T0101 Turning Face tool	Finish pass over previous cut to smooth and clean
3	Lathe Groove rough pass	PUMA Lathe	T0909 Trepan Tool	Turn the center of the part down to match the central axis and mounting point for the earth part onto the base mold.
4	Lathe groove finish pass	PUMA Lathe	T0808 Trepan tool	Smooth out previous step using a slightly smaller radius trepan
5	Mill Peck Drill	Protrak Mill	3/16” flat endmill	Drill out the center shaft of the core mold
6	Circle Mill for center shaft	Protrak Mill	Circle mill	Fully mills out center shaft
7	Center drill for ejector pin holes	Protrak Mill	#13 drill: #2 center drill	Preps ejector pin holes to be drilled
8	Drill ejector pin holes	Protrak Mill	#17 Drill	Drills out holes for the ejector pins

Manufacturing Time Estimate

Mold Fabrication:	Time
Base	Lathe Core: Mill Core: Lathe Cavity: Total Tool Change Machine Setup	7m 3m 2m 1m 10m
Dome	Lathe: Mill: Lathe Tool Change: Machine Setup:	1m 12.90s 2.04s 30s 10m
Spinner	Core: Cavity: Total Tool Change: Machine Setup:	30m 8m 30s 10m
Earth	Core: Cavity: Total Tool Change: Machine Setup	3m 6.44s 3m 15.9s 1m 10m
Ring	Lathe Cavity: Lathe Core: Core Mill: Tool Changes: Machine Setup:	3m 6.44s 5m 37.68s 7m 58.51s 1m 10m
Total		133min
Part Fabrication	Time/1 yoyo	Time/100 yoyos
Base	131s	3.6hrs
Dome	10s (not including cutout)	20m (Including loading material)
Spinner	105s	2.9 hrs
Earth	90s	2.5hrs
Ring	80s	2.2 hrs
Assembly	215s	8.3hrs
Total	631s	20 hrs

We used the injection molding notes from lecture to evaluate process time for making each part. Thicker take longer to inject, pack, and cool so we accounted for additional time in our thicker parts. We used Mastercam to automatically calculate time to cut the molds, and included setup and tool change time for each mold in order to keep our time estimates conservative. We estimated assembly time thinking about each part being added in assembly line fashion. The assembly time accounts for stamping paint on the Earth piece, putting together all the pieces for both sides of the yoyo, screwing them together, and tying on the string. In conclusion the time from making the molds to full assemble will be 20 hours and the time to make the molds will be 2 hours and 13 minutes.  This has extended our time allotted to manufacture the molds by 2 days, everything else remains on the same schedule.  

Here is the link to our updated schedule.   

Thermoform Mold: Yoyo Dome

The thermoform mold for the clear outer dome is shown below. Areas that may need alteration are the diameter of the central circular bevel and also the shape of the dome extrusion. The diameter may need to change based on any unexpected shrinkage in the ring or base mold, and the shape of the dome may change for purely aesthetic or functional reasons after experimentation. 

Key features on this part are the exterior cut in the side of the mold and drill holes in the top, which will all be used to line the mold up in the thermoform machine. Vent holes will also be added to the to ensure a close fit on the mold.

Gantt Chart

Here is a link to our Gannt Chart with a timeline of our process.

For the division of labor in our group we decided to split up the work by parts of the yo-yo. John and Al will be working on the spinner, which includes the CAD of the part, CAD of the mold, making of the mold, injection molding of the part, and painting of the spaceship/moon during assembly. Christine will be working on the ring, which includes the CAD of the part, CAD of the mold, making of the mold, and injection molding of the part. Meaghan will be working on the thermoform dome, which includes the CAD of the part, CAD of the mold, making of the mold, and thermoforming of the part. Morgan will be in charge of the earth cap piece, which includes the CAD of the part, CAD of the mold, making of the mold, injection molding of the part, and painting of the earth during the assembly process. Lastly, Sam will be responsible for the base of the yo-yo, which includes the CAD of the part, CAD of the mold, making of the mold, and injection molding of the part. We will all, contribute to the assembly of the yo-yos.

In short, each team member's responsibility is listed below:

Sam - Yo-Yo base

Meaghan - Thermoform dome

Morgan - Earth cap

John and Al - Spinner piece

Christine - Ring (outer piece)

Table of Specifications

Below are the specifications and critical dimensions for our Yo-Yo Design!

Specification	Target Numerical Value	Expected Tolerance	How to Measure
Max Diameter	2.42 in.	+/- 0.01 in.	Use calipers
String Gap	0.1 in.	+/- 0.02 in.	Use calipers
Total Mass			Use mass scale
Max Rotational Speed	4000 rpm	+/- 1500 rpm	Use strobe light tachometer and/or high speed camera
Thickness (through axel)	1.63 in.	+/- 0.04 in.	Use calipers
Diameter of Spinner (Washer inside)	0.34 in.	+/- 0.01 in.	Use calipers
Diameter of Spinner Length (end to end)	1.47 in.	+/- 0.01 in.	Use calipers
Diameter of Earth Cap Piece	0.80 in.	+/- 0.01 in.	Use calipers
Diameter of Post (attachment between Earth and Base)	0.290 in.	+/- 0.01 in.	Use calipers
Depth of Detailing on Earth Cap Piece	0.039 in.	+/- 0.01 in.	Use depth gauge
Distance between Spinner Washer and Base Shaft	0.02 in.	+/- 0.02 in.	Use calipers

*300 micron = 0.0118 in. tolerance estimated using assumptions of tolerances due to injection molding and mold fabrication

Initial Yo-Yo Design

Our yo-yo design consists of a base, a spinner with a moon and a rocket, an earth, a ring, and a thermoformed clear cover.

Here are some views of our yo-yo solidworks assembly:



The spinner fits loosely around a shaft in the center of the base. The earth snaps onto the shaft above the spinner, and thus keeps the spinner in place. The thermoformed cover fits onto the base, and the ring snaps into the base keeping the cover in place. The earth, spinner, ring, and base are made via injection molding; the molds are machined using mastercam, based on the solidworks parts, and g-code is created for the lathe and mill CNC machines. The thermoformed part is made by heating up a clear sheet of plastic and pushing the mold for the cover into the warmed plastic.

The base, ring, and spinner is made of black plastic. The earth is made of blue plastic. The spinner's moon and rocket will be spray painted different colors and the earth is "stamped" into a sponge of green paint to make the land green.

Design for manufacturing principles were considered in every facet of the design process. Draft angles and overhangs had to be correct for machining and molding, serial laser cutting processes were eliminated due to rate concerns, and parts had to be designed with correct thickness and geometry to allow for good moldflow without significant shrinkage or warping. Tolerance analysis was also conducted to ensure parts that are press fit or loose fit would perform across multiple iterations.