Core and Cap STL Files

Please go to this link:  Core and Cap STL Files

Biohazard Thermoform STL

Go to this link! https://www.dropbox.com/s/apr6qazbnp4drxx/BiohazardVacuum.STL?dl=0

Radioactive Thermoform STL

Go to this link!: https://www.dropbox.com/s/9vmhgc3flsbtppi/Radioactive.STL?dl=0

Calculating Shrinkage Allowances

   In order to achieve the desired dimensions in our parts according to the tolerances we specified in a previous post, we had to account for the shrinkage of the injection molded parts after cooling. During injection molding, hot molten plastic fills the molds while under pressure and is held there for a period of time to cool. The pressure at which the mold core and cavity are pressed together as well as the cooling time are two parameters which can be manipulated to affect the degree of shrinkage during the production of the yo-yos.

   To compensate for the inevitable shrinkage of the yoyo parts during production, the yo-yo dimensions were scaled by the shrinkage factor percentage. To determine the shrinkage factor percentage, various yo-yos and the corresponding molds from groups of previous years with body and ring styles similar to our own group were measured. By taking the average critical dimensions of a few yoyo parts and the corresponding dimensions of the mold, the shrinkage factor could be determined. Subtracting the actual yoyo dimension from the mold dimension and dividing by the original mold dimension yields the percent shrinkage.

   The math for the body and cap of the yoyo to calculate the shrinkage factor is shown below. The results show that the body had a shrinkage percentage of 1.47% while the ring shrunk about 1.37%. However, to err on the side of the caution we approximated the shrinkage to be at 2% as the parts we used for comparison were not exact copies of our specific yoyo. Knowing this 2% shrinkage factor, we scaled the design of our yoyo body and cap accordingly in order to be within the correct tolerances and dimensions for our final product. 

By comparing the shrinkage factor of previous yoyo parts similar to our own, we were able to determine an approximate scale factor for our design to compensate for the shrinkage of our part during production. 

Updated Manufacturing Time Estimate

   The machining time estimates were found from MasterCAM by backplotting the functions for creating each piece of the mold for every part. The entire machining process for the injection molded parts is relatively short compared to the thermoformed parts and is accomplished largely on the lathe with the exception of the drilled ejector pin holes. The longer manufacturing time for the thermoformed parts is accounted for by the fact that more intricate milling operations are necessary for the thermoformed designs. However, the actual machining appointment may take longer due to potential edits in the process plan. Thus far, fabrication of the molds has run close to as predicted and no changes in the schedule have yet been made. The process optimization will be performed by testing the thermoforming of the part with the mold to determine if any setting changes need to be made before the high volume production run. This optimization process will take about 1 hour. The final production run will likely take about 2 hours to injection mold 100 parts for the core, 1 hour for the ring, and 2.5 hours for each of the thermoformed parts. Team Batarang will be able to get a more accurate time after optimizing the machine settings for improving the rate.

Presenting the Body Mold for an Injection Molded Part

   The pictures below show the core and cavity of the body of the Batarang Yoyo. The core is relatively simple and reflects the outward facing shape of the yoyo to the user. It is essentially a bored-out hemisphere with a smooth finish to allow a clean look for the finished product and eliminate the chances of snags on the yoyo during use. The cavity encompasses all of the details of the yoyo design which are not outward facing including the hole for the shoulder bolt, the edge for resting the thermoformed face on, and a ridge to include a press-fit for the cap of the yoyo. The ejection pin holes were placed on the flat ridge of the cavity as that proved to be the optimal ejection location and additionally none of the ejection marks will be able to be seen once assembled. The following images show the features described above in the molds. 

Cavity for the body of the yoyo. Notice the bored out hole for the shoulder bolt, the ejection pin holes, and the ridge cutout beyond the ejection pin holes for the press fit.

 Core for the body of the yoyo. A relatively simple outer facing part only with a hole bored through the middle to allow for insertion of the shoulder bolt during injection molding runs.

Both cavity (left) and core (right of the body of the injection molded yoyo.

Brief discussion of  needed rework:

   Before performing the optimization runs, the runners for the body injection mold needs to be added. Furthermore, the ejection pin holes and runners both need to be added to the core injection mold. Team Batarang is working closely with the machine shop to complete these remaining aspects of the mold and expect to finish during lab time this upcoming week. 

Incorporation of Design and Manufacturing Principles:

In our design, we have taken into consideration several design constraints inherent in injection molding and vacuum forming. They center around challenges with warping, extracting the finished part, and making sure our tolerances allow for a functional finished product.

We have decided to use two thermoformed parts as the detailing of our yoyo instead of trying to overlay an injection molded piece with windows and an inner thermoformed piece. This decision is the result of conversations with the lab instructors describing the probable difficulties with the sharp corners and tight tolerancing we want for our yoyo.  With a thermoformed part only, we will be able to achieve the intricate details we want, and we have decided that painting with a stencil will be the best method for adding more color to the design.

Injection Molded Base:

We have attempted to have this part as close to a uniform thickness as possible, in order to decrease non-uniform cooling and, therefore, decrease warping.

In order to have a press fit, we have toleranced the ring acceptor to be slightly smaller than the diameter of the ring itself in order to have a reliable press fit.

Ring:

We have oriented the draft angles such that the slight chamfer helps guide the ring into the receiving pocket of the injection molded base, while maintaining maximum efficiency for removing the part from the mold.

The mold has been adjusted for shrinkage.
Furthermore, we have toleranced it in accordance with the injection molded base such that it will always have a press fit.

Thermoformed parts: For this, we have decided to drape our plastic sheets around an extruded mold rather than pull the plastic sheets into a pocket. This is because we have several sharp corners in our designs, and these corners cannot be machined as a pocket due to the tooling we have access to. If we machined a pocket, every corner would have a fillet matching the tool we used to machine it.  We can negate this by machining material away around the desired shape rather than machining the desired shape (as a pocket) out of a solid piece of aluminum.

Furthermore, we will be ensuring that our parts can come off the mold by applying the appropriate draft angles to the extrusions.

For creating the mold dimensions themselves, we took our original sketch in solidworks and used offset entities to account for the plastic thickness.

Assembly Screenshots

Design Specifications

Summary of Manufacturing & Assembly of Yo-Yo Parts

Core: This is an injection molded part that will house the other two pieces of the yo-yo. It will make up the body and the general shape of the yo-yo. We will be using a mold that is machined to 2% larger in order to account for plastic shrinkage. The inner diameter will be designed to be slightly smaller than the outer diameter of the ring in order to ensure a tight snap fit. This will allow for us to create sturdy, longer-lasting parts. Ring: This is a ring-shaped injection molded part that will be press fit on the inner edge of the core and hold the thermoformed pieces in place. Its core mold will have an extrusion of 0.15” with an inner diameter of outer diameter of 2.05” to account for shrinkage (about 2% of the original 2” dimension and +.01” to allow for a tight press fit with the 2” diameter of the core). With these design considerations in mind, the ring should be able to retain the thermoformed radioactive and biohazard designs in the core. Radioactive & Biohazard Designs: These are thermoformed parts that will sit on opposing faces of the yo-yo. Their molds will have an extrusion of 0.075” at a diameter of 1.5” to fit it into the ring, and a final extrusion composed of the design at another 0.075” to make the symbols visible. The pieces will each be of 2” diameter. Once the plastic has its shape, we will spray paint the extruded sections to give the yo-yo more color contrast. The designed thermoformed parts will be fit into a 2” diameter inset in the core and secured in with the injection molded ring that is press fit into the inner rim of the core piece. Assembly: In each core, we will place the thermoformed piece and center it into a 2” inset, and then press fit a ring around its edge to hold the thermoformed piece tightly into the yo-yo. The ring will be press fit between the cylindrical extrusion of the thermoformed part and the inner diameter of the core. The two cores will each have one nut to be able to screw on an axle to wind the string and hold the two cores together. One side of the complete yo-yo will have the biohazard design, and the other will have the radioactive symbol.

Gantt Chart

Here is a table to detail the schedule for the semester. It specifies the due date for all of the deliverables with an X under the due date in the right of the table. It also lists the team members responsible for each part of the deliverable. Our team has not been able to specify who will be in charge of what in future deliverables, but this will be updated as the semester continues.