Gear Mold Cavity:

The gear mold cavity is composed of all three gears on a single part. The cavity shows the side of the mold that will be exposed on the final yo-yo. There will be a runner going from the injection molding plastic point of entry to each of the gears (not pictured here). 

Gear Mold Core:

The gear mold core is the back face of each of the gears. The mold core has three pegs (two of length 0.15" and one of length 0.1") that correspond to the gear placement on the gear mold cavity. The pegs are uniform in size throughout their length due to the small size of the gears. The gears will shrink around the center pegs and no tapper is needed. Around each peg is two holes for ejector pins. The ejector pin placement was aligned such that there is one pin on either side of the gear.  

Dimensions:

The shrinkage for the gears was derived from measuring a small circular plastic part of similar dimensions as the gears. The circle had a 1.45 in diameter and the mold for it was roughly 2.5 % larger. The shrinkage of the gears was then determined using this formula ( Mold Increase %) = 2.5%* (Gear Diameter) / ( 1.45 in). For the big gear the mold was 1.6% larger, for the middle gear was 1.0 % larger, and the small gear was 0.8% larger. The inner gear holes are sized to have 0.248" diameter to create a press fit on the base which will have pegs of 0.25" diameter.  

Process Plan:

Gear Core Process Plan

Gear Cavity Process Plan

Estimations of total machine time

Part	Machine time on Mill (minutes)	Machine time on Lathe (minutes)	Total Machining Time (min)
Base Core	35	2.68	37.68
Base Cavity	0	1.47	1.47
Gears Core	33.17	0	33.17
Gears Cavity	31.47	0	31.47
Cover Die	1	2.35	3.35
Ring Core	0	4.97	4.97
Ring Cavity	1.27	2.12	3.39
TOTAL	101.91	13.59	115.5

Variable time cost of making one Yoyo:

Part	Process	Rate determining step	RDS time (s)	Total cycle time estimate (s)
Base	Injection molding	Cooling of part	15	30
Gears	Injection molding	Cooling of part	10	25
Cover	Thermoforming	Heating of plastic	5	20
Ring	Injection molding	Cooling of part	5	15
TOTAL				90

As seen from the values above, the greater time cost involves the machining of the molds. Having a total of 7 mold components requires a total machining time of almost 2 hrs. However, the process has been optimized to keep this value to a minimum. The three gears are being made via the same mold, which decreases both machining time and injection molding time by almost a factor of three. Additionally, the thickness of the case and ring have been minimized in order to decrease the time taken for the cooling of these parts, which is the rate limiting step for injection molding. Thus, none of the components have excessive thicknesses. By using both, thermoforming and injection molding, for different components, we will be able to fabricate them simultaneously, improving the total process time of producing 50 Yoyos.

Our estimated cycle times for the different components are based on our knowledge of the rate determining step, the critical dimensions of the parts, and previous time estimates discussed in lectures. Typical cycle times from lecture were taken and scaled down to fit our parts. It is important to note the amount of time it will take to start up machines for both the making of the molds, and the actual injection molding process is relatively significant compared to other parts of the process, since it is not automated. The cycle time estimate considers this component. The addition of all these times sets an estimate of producing a hundred yoyo sides of about 4.5 hours. (This does not consider the need for multiple trial runs, or mold modification.) The assembly time is estimated to be 1 minute per Yoyo. This may seem as an overestimate, but we are accounting for possible fitting problems, coordination of different teams, possibility of errors, and the time it takes to familiarize oneself with the assembly process. (Assembly time can vary depending on the people taking part in the process.) Therefore, total assembly time for all Yoyos is estimated to be 50min.

Manufactured Molds:

The mold has three posts on the core side which the plastic will contract and stick to after cooling. The ejector pins will push out the gears from either side of the post. The cavity side has the gear shapes and includes a runner for the plastic to flow through. These are the second iterations of the gear mold machining. The initial test resulted in parts that did not line up due to an error in mirroring. However, each side of this mold now fits together very well and is ready for an initial injection molding test.