Deliverable 2

Description of Mold Design

                The molds for the bowtie parts of our yo-yo were manufactured out of two 6061 (T6) Aluminum blocks (Figure 1). All of the features on the mold were milled on the EZ-TRAK mill. Two identical bowties are molded in a single injection molding run in order to reduce operation time and cost. This was done by creating mold features for two bowtie parts on each mold part. Runners on the cavity mold link the sprue to each of the two sets of parts.

Figure 1. Photographs of machined (a) cavity and (b) core molds for bowtie parts.

                The core mold (Figure 1b) was designed to form the concealed lower ribs that provide structural support for the part. It consists of extrusions that will obstruct the flow of molten plastic during injection molding, and will slide out from underneath the part once it has cooled.
               The cavity mold (Figure 1a) was designed to form the outer surface of the part. It consists of a rounded pocket that will provide the visible upper surface of the part has a smooth, aesthetically pleasing surface finish. It will slide out from above the parts once they have cooled. The parting line of the mold was chosen to lie at the lower plane of the part in order to allow for easy removal of the mold once the part cools. Since the bottom face of the part will be concealed in the yo-yo, any minor defects due to misalignment at the parting line will be concealed.

                The molds include circular holes towards the sides, which ensure that the intended positions and orientations of the core and cavity molds relative to each other and to the injection molding machine are attained. All other features serve to form the parts themselves by directing the flow of molten plastic during injection molding runs.

Quantitative Justification of Dimensions

The bow tie cavity mold was machined to the nominal dimensions.  Since the bow tie is an aesthetic element, shrinkage during injection molding was not considered to be an issue.  The critical dimensions on the core are for the interference fits of the bearing and ballast nuts.

Based on measurements of previous years' parts, we estimate a shrinkage of 2% from the mold dimensions.  The nut has a diameter of .438".  We cut the mating feature in the mold to a diameter of .433"  With an additional 2% of shrinkage, we estimate the interference fit for the nut to be .014"
 
The bearings to be used have an OD of 8mm or .315".  Accounting for shrinkage, we machined the corresponding core mold feature to have an OD of .315".  2% of shrinkage yields an interference of .006"

Description of Manufacturing Process
These molds start from a premade mold blank.  The mold blank is designed with locating holes and a sprue.  The locating holes are used to repeatably fixture the mold to the EZ-TRAK mill bed, and also to the injection molding machine.  A CAD model of the mold is imported to MasterCAM, where toolpaths, depths of cut, and feed rates are specified.  The resulting tested G-code is then taken to the EZ-TRAK, where machining happens.  A human operator monitors the process and changes tools.  After machining, some post processing may be required.  Holes or sharp edges may require deburring.  It is important to avoid deburring or rounding any surfaces that when rounded, might create flash.  If desired, the mold surfaces may be lightly sanded to remove machining marks that may be displayed on the finished injection molded part.

Link to our process plan

Mold Details

The core mold was milled successfully. We even managed not to break the 1/16" end mill when it accidentally cut the full depth of the grooves (all 5/32") in one go. If we have to remachine, we will not be making that mistake again, despite the increase in milling time.
The face of the mold, while appearing patterned, is incredibly smooth, which is important for fitting against the cavity. Any grooves or roughness to the surface would have made flashing more likely. The top of the islands do not need to be finished, as they are not touching the cavity at all. Also, since the sides of the islands remain unseen, they did not have an additional finishing pass applied to them. Since it seems that being slightly rougher will not have an impact on removal from the mold (according to David when I asked him), there is little point in changing that.
If reworking is required, probably decreasing the size of the bearing island (center circles) and increasing the size of the hex nut islands (large trapezoids) to accommodate better press fits will be done.

In spite of its dashingly good looks and polished interior (which was done manually, post-milling to improve surface finish on the injection molded part), the cavity mold was machined to a depth that is 1/16" too shallow.  It would result in a bow-tie with voids where the core unintentionally comes into contact with the cavity.  The operator has made plans to remachine the cavity mold, bringing it to the correct depth.