Optimization

Injection Molded Disc Optimization: In the last post, we talked about the mold for the injection molded disc with moon and star cut-outs.  The injection molding process for this part of our yoyo was optimized through experimenting with cooling times, feed strokes, and injection boost pressures. Below is the data collected throughout the process optimization.  After changing parameters, a few parts for each trial were  manufactured and diameters and thicknesses at various points were measured.  Additionally, we looked for flash in the windows. MOON AND STAR INJECTION MOLDED PART OPTIMIZATION
Trial #	Cool Time (s)	Feed Stroke (in)	Inj. Boost Pressure (psi)	Diameter 1 (in)	Diameter 2 (in)	Thickness 1 (in)	Thickness 2 (in)	Flash
1	17	1.1	1600	2.016	2.023	0.108	0.108	tiny bit
	"	"	"	2.015	2.019	0.107	0.107	"
	"	"	"	2.015	2.014	0.107	0.106	"
2	17	1	1600	2.017	2.016	0.103	0.107	"
3	15	1	1400	2.013	2.011	0.107	0.106	"
	"	"	"	2.015	2.015	0.107	0.107	"
4	10	1	1400	2.015	2.017	0.107	0.108	none
	"	"	"	2.012	2.018	0.105	0.107	"
5	12	1	1400	2.014	2.016	0.107	0.107	"
	"	"	"	2.011	2.013	0.107	0.107	"

The injection boost pressure was lowered in an attempt to effect the flash we were getting in the disc windows.  Lowering the pressure reduced flash. 
Cooling times were altered to experiment with effects in shrinkage.  Part measurements were taken the day after manufacturing and had time to completely cool.  Parts with less cooling time would shrink more than parts manufactured with longer cooling time.
Having flash is undesirable aesthetically and also in terms of tolerances in fitting the moon and star thermoformed piece through the windows, so all trials in which flash existed were eliminated.   Next, we looked at the average of the two diameter measurements for each part manufactured.  Since the disc diameter is a more critical assembly parameter than the thickness, we eliminated first on the basis of out-of-spec diameter.  While overall the percent shrinkage we accounted for in the mold design was too large (our target diameter is 2 inches), the discs fit into the body well.  We took the lowest average diameters, though, in order to stay in spec as best as possible. 

The conditions of Trial #5 won out in the end, had our first production run, and have 100 beautiful moon and star discs now!

Optimization of Other Yo-yo Parts:
Yo-yo Body:
The cooling time was changed during the optimization runs for the body to experiment with the effects of shrinkage. Our optimized cooling time is 25 seconds. The body mold was edited to remove the sharp edge on the outside diameter. Updated drawing:

Thermoformed Parts - Window, Moon and Star Insert:
The thermoformed parts did not need redesign, but both molds needed remachining so they fit the original  drawing specs. The window mold had nicks around the edge so a completely new mold was made, and lapped on the top surface. The moon and stars mold was machined incorrectly so the mold was remachined to match the drawings. The optimization focused on finish for the window parts and reduction of webbing on the moon and stars part.