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.

Yo-yo Molds

We have designed and machined the molds that will create yo-yo. Below, we describe the mold for the orange body of the yo-yo. 

Orange Body

Core Mold

Description: The core mold forms the orange pin-wheel pattern, which snap-fits with the black retaining ring to form a black and orange checkerboard exterior design. (solidworks model below)

Core Mold Manufacturing Process: First, the lathe will create the general shape of the core mold. Next, the shape of the flanges will be created using the mill. A runner will then be milled.

 

Cavity Mold

Description: The cavity mold gives the back side of the body its rounded shape. (soildworks model below)

Cavity Mold Manufacturing Process: First, using the mill, the ejector pins will be drilled into a blank mold. Then, the lathe will carve out the hollow shape of the inner side of the yo-yo. Next, the shafts to hold the nuts in place will be machined on the lathe as well. Finally, the gate will be filed in by hand.

The complete Process Plan for both molds can be found here. 

Justification for dimensions: After measuring molds and their respective parts from completed yo-yos (a chart can be found here), we were able to figure out how to account for shrinkage when deciding the dimensions of our molds. A 0.5 inch shrinkage was estimated for a final outer radius of 2.45 inches, resulting in a cavity mold with an outer radius of 2.50 inches. A drawing of the cavity mold can be found below. It was assumed that circumferential shrinkage would not occur. 

The Manufacturing Time for our yo-yo can be found here and below.

Justification for the described machining times can be found here.

The total machining time for the molds has more than doubled since the initial manufacturing time submission (2 weeks ago) from .5 hours to 1.3 hours. This is due to changes in the machining process of many of our parts including the black cavity and orange cavity. The increased time is also a result of additional time needed for milling injector pins, runners, and other details that were not included in the initial plan. 

The time needed to make revisions to the initial milling/lathing of the molds was unaccounted for and has shifted our team schedule. However, to account for this shift, we have made more lab appointments so that are caught up by the end of this week and to ensure that all deliverables are completed on time.

Here are some pictures of the manufactured molds:

We may have to remachine the connection between the black cavity mold runner and sprue hole. Also, we might want to add a slight draft angle to the thermoform mold for easier removal.

Here's a picture of all of our injection molds in a row. Can't wait to start seeing some plastic parts come out!