Mold Design and Manufacturing Analysis

The moon and stars design of the yoyo is made up of two parts, one injection molded and one thermoformed.  The black injection molded part is meant to frame the white thermoformed part such that white moon and star are visible through the injection molded piece.  Below are the molds for the injection molded moon and star part, core mold on the left and cavity mold on the right.

The cavity mold defines the outer shape and thickness of the part and is complete with sprue and runner. The more complex core mold is shown below in more detail and has four ejector pin holes for  removal of the part from the mold.  This core mold will create the moon and star-shaped stencils in the part, through which the thermoform part will fit.

These molds were scaled in order to account for inherent shrinkage in the injection molding process.  To determine a reasonable scaling factor, a similar injection molded piece found in the lab was measured with digital calipers eight times each for three different features on different iterations of the part.  The same features were measured on the molds and then percent shrinkages were calculated with the averaged eight measurements of the parts.  For example, the average measured disc thickness was 0.147813 mm while the thickness imposed by the mold was 0.149 mm.  This comes out to a percent shrinkage of 0.803382664.  Therefore, since the goal thickness for our injection molded part is 0.1 mm, the molds were designed with a 0.108 mm thickness in mind to compensate for shrinkage.

Manufacturing Process:The cavity and core molds were both manufactured entirely on the mill, with the cavity mold taking approximately two minutes to complete and the more complex core mold taking around one hour to finish.  To begin with, the mold blanks were faced in order to ensure the tops of the islands on the core  mold would be flush with the bottom of the cavity mold.  After the milling process, the ejector pin holes and sprue were reamed.  Below are the process plans for each mold, detailing operations and tool changes. 

Cavity Mold:

Step	Operation	Machine	Tool	Justification
1	Facing	Mill	7	Faces surface of the cavity mold blank.
2	Pocket (Standard)	Mill	7	Removes material in center of piece, depth of .108”
3	Contour (2D)	Mill	9	Creates the runner at a depth of 0.09”

 Core Mold:

Step	Operation	Machine	Tool	Justification
1	Facing	Mill	12	Faces top surface so top of islands will fit perfectly with bottom of cavity mold
2	Drill/Cbore	Mill	13	Center drills for the 4 ejector pin sites.
3	Peck Drill	Mill	17	Creates the through- holes for the four ejector pins.
4	Pocket (Facing)	Mill	5	Faces surface of the core mold blank, leaving a buffer distance around the island.
5	Pocket (Remachining)	Mill	1	Removes closer to the island, creating finer details.

Manufacturing time estimates:

Mold Machining time:

Part	Machining time (min)	Optimization	Final production run
Body Cavity Mold	3	Production runs: 38min Remachining: 5min	110min
Body Core Mold	4
Thermoform M&S insert	35	Production runs: 30min Remachining: 20min	70min
M&S core mold	48	Production runs: 18min Remachining: 20min	50min
M&S cavity mold	27
Thermoform Window mold	2	Production runs: 30min Remachining: 2min	70min
Ring core mold	4	Production runs: 30min Remachining: 3min	85min
Ring cavity mold	2

Total mill/lathe time: 125min

Total Injection mold machine time:  245min

Total Thermoform machine time:  140min

For the process of optimization, it is estimated that 4 production runs of 8 parts will be made. The amount of time to produce one part was estimated based on the part thickness. The components of a cycle of injection molding include mold closure, injection, pack and hold, part cooling, mold opening, and ejection. The driving component of total injection molding time is the cooling time needed.  This was estimated using the formula T_cool = h^2/4α and the material properties of ABS (we were unsure what material will actually be used). For example, the cooling time for the yo-yo body was estimated to be ~45 seconds, and total part production time ~1 minute and 5 seconds (20 seconds for all other components of injection molding process).

For thermoforming it was assumed that the HIPS would take about 30 seconds to heat up and would freeze upon contact. The remachining time for the optimization process was estimated based on the complexity of the mold – e.g. whether it would only be necessary to turn the part to a slightly smaller diameter, or if it might be necessary to redo detailed engraving work.

For the final production run, the time to make one part was multiplied by the 100 of each part needed.

It was decided that the rocket will be no longer be injection molded. Instead it will be laser cut from acrylic, which will take about an hour.

Here is the cavity of the mold for the Space Yo-yo's body. The outer diameter of the upper part of the cavity is expected to be reworked to account for excess shrinkage in the part. This is to make sure that the outer diameter of the space yo-yo matches the expected 2.5".

The dimensions of the body core mold (above) were increased from the original drawings due to the desire for a thinner yo-yo body. The slight burrs left on the top edge were removed with a file to prevent the possibility of that slight undercut affecting the ease with which the finished part can be ejected. The outer diameter of the core is expected to be modified based on shrinkage form.

The ring cavity mold consists of a circular cavity with the outer diameter of the ring and three runners. The top runner leads to the sprue while the remaining runners lead to ejector pin holes located on the core mold. These runners might need rework so as to ensure that there is enough material for the ejector pins to push on. This will ensure that the workpiece is ejected successfully.

The ring core mold consists of a circular island with an inner diameter slightly larger than that of the inner window. The mold has two holes set a distance away from the island, which are for the ejector pins, which will come into contact with the runners on the cavity mold to help remove the ring from the mold. The diameter of the cavity may need to be modified/decreased in order to make sure that the thermoformed window fits inside of the ring, but it is not a critical dimension.



This is a thermoform mold, minus the suction holes and punch guides which have been added since this picture was taken. There may be webbing on the parts this makes between the star and its trail, so more suction holes may have to be added.

This is the completed mold for the thermoformed yo-yo window. The top surface of the mold (which corresponds to the top of the window) was machined at a slower feed rate than the rest of the part for a higher-quality surface finish, which should be reflected in the thermoformed parts. The alignment pins (the two posts on the edges of the material) will act as guides for the punch. Suction holes (very small circle, front center of the part), which were created using a #68 drill bit, should help ensure that the vacuum pulls the plastic sheet into the correct shape. Since this part does not have any critical dimensions, we are not anticipating needing to rework this part.