Optimization of Parts

We have optimized the parameters of the injection molding and thermoforming processes for all parts of our yo-yo. Below, we summarize our optimization process for the orange body mold and discuss a couple of changes we made to the black retaining ring mold to improve injection molding. 

Optimization Process & Outcome
Having begun with parameters suggested by the knowledgeable shop guys Dave & Dave, we made changes to each, one at a time, until we believed that our part was high quality. The main problem we needed to resolve was flash along the extrusions, so we decided to lower the pressure. We adjusted the pressure profile accordingly to make the pressure decrease less drastic. Additionally, we eliminated extra material by decreasing the shot size. Both of these changes greatly reduced flash and resulted in a yo-yo body that both looks and functions the way we designed it to. 

This trial and error method was also used to optimize the black retaining ring and thermoform cover of our yo-yo. The following graphs show the process sheet containing our optimized parameters for the orange yo-yo body. 

Optimized Injection Molding Parameters

Part: Orange yo-yo body

Injection Hold

Injection Hold Pressure Profile: P7 - P16
500	550	600	650	700
650	600	500	400	400
Injection Hold Time			Z2 = 10.0 s
Cooling Time			Z4 = 15 s
Set Screw Feed Stroke			C1 = 1.50 in

Injection Boost

Injection Speed Profile: V12 - V21
1.5	2.0	2.5	2.0	1.5
1.2	1.0	0.7	0.5	0.2
Injection Boost Pressure			P6 = 1399 psi

Intrusion Time:			not needed
Intrusion Speed:			not needed

Screw Feeding

Screw Feed Delay Time

Z3 = 0.0 s

Ejector

Ejector Counter

AZ = 2

Ejector Pin Length:  5.570 in

Total Shim Thickness:  0.0 in

Black Mold Redesign

We observed that the black piece had dishing at each of the flanges, which were especially prominent on those farthest way from the sprue. After speaking with Dave and Dave, we decided that the best way to reduce the dishing would be to create a runner that ran around the periphery of the mold. This would allow for even filling and cooling of the flanges, which would reduce distortion. Creating the runner eliminated dishing completely. The picture below shows the new core mold.

We also observed flash in the middle of the ring. We found that what was causing the flash in the center of was the cavity mold, which had a surface that was not level. The difference in height between one side of the surface and the other was enough to create the flash; therefore, we re-lathed that surface so that it was even. This greatly reduced flash in the center of the ring. The picture below shows the new core and cavity molds for the black retaining ring.