1. Our most interesting injected molded part is the base piece.
a. Mold Design
The cavity mold both holds the nut for the pokeball’s shaft and defines the curved back face of the yoyo. This is done by having a curved cavity bored out of the center of the mold, with a center through hole for holding the shaft with the nut.
The core mold is designed to produce the inside face for the yoyo, which includes female snap fits around the outside edge, six press fit pin holes, and two long pins for holding up our thermoformed piece. The snap fits are used to keep our outer ring pieces aligned with the outside edge of the base part and to provide a small amount of support in keeping the final product together. The press fit pin holes are the main source of fastening within the product, with each additional ring part connecting to the base through two of these holes. The holes are made by drilling a through hole in the mold for an ejector pin to rest, and then counterboring a wider hole. This wider hole allows injected plastic to surround the ejector pin tip, creating the pocket that the pins on the ring pieces can press into. In addition, this mold also includes three magnets press fit into holes drilled into the piece. These magnets then hold steel shims around the edge of the base that add some weight to the final product.
b. Manufacturing process (step by step process plan)
PROCESS PLAN FOR BASE MOLD, Cavity
Step
|
Operation
|
Machine
|
Tool
|
Justification
|
1
|
Face front of mold
|
Lathe
|
10
|
Make front of mold flat and square to fit against core side
|
2
|
Cut cavity
|
Lathe
|
10
|
Don’t have any small corners, use large bore tool
|
3
|
Cut gate
|
Mill
|
9
|
Cavity side gets gate
|
PROCESS PLAN FOR BASE MOLD, Core
Step
|
Operation
|
Machine
|
Tool
|
Justification
|
1
|
Face and machine convex curve for profile shape
|
Lathe
|
01
|
Make front of mold flat and square to fit against core side
|
2
|
Cut sharp concave corners from step 1
|
Lathe
|
03
|
Finishes off the general profile of the coreb
|
3
|
Cut snap fit ring on outside
|
Lathe
|
08
|
Small slot, only needs finishing trepan.
|
4
|
Cut bolt insert mold extrusion
|
Lathe
|
12
|
½ inch endmill to cut extrusion for bolt to sit in
|
5
|
Drill holes for pegs
|
Mill
|
1/16 in drill bit
|
Cut holes for pegs which support the button from behind
|
6
|
Drill hole pattern for press fit pegs
|
Mill
|
#8 drill
|
Drill out lower portion of the hole which is smaller diameter first
|
7
|
5 degree taper the hole pattern holes
|
Mill
|
15
|
Gives draft for making ejection easier
|
c. Photographs of mold:
CORE CAVITY
d. Process Parameters:
Injection Pressure: All 400 psi
Injection pressure was high enough to evenly spread plastic throughout the part; 400 psi was approximately selected initially and worked great through production
Injection Hold Time: 8 seconds
This was an initial parameter set, and worked well throughout testing.
Cooling Time: 30 seconds
This was the biggest issue--during optimization (as seen in the photos), the pin heads wanted to flare out and there was significant dimpling in the part; cooling time was almost doubled throughout the optimization process.
Set Screw Feed Stroke (shot size): 1.6 inches
As seen in pictures, initial shot size was not sufficient to fill the part so during optimization this was raised to 1.6 inches
Injection Speed Profile: All 3 in/second
Injection speed was initially low but was raised to completely fill the part.
Injection Boost Pressure: 1600 psi
Injection boost pressure was doubled over the course of optimization; the initial value of about 800 psi was not sufficient and caused problems with issues in the shape.
Intrusion Time/Speed: N/A
Screw Feed Delay Time: 17 seconds
This is a high-pressure process for this particular part and to mitigate these effects, screw feed delay time was raised.
Ejector Counter: 2
Standard; not modified throughout the course of optimization
⅛” Ejector Pins Length: 5.490 in.
Chosen based on geometry of mold.
Total Shim Thickness: 0
Chosen based on geometry of mold.
¼” Ejector Pin Length: 5.327 in.
Chosen based on geometry of mold.
Special Ejector Pin Length/Quantity: N/A
Measurements:
Outer Snap Fit Diameter- This is the most critical dimension on this part once again because this is where it interfaces the other components. This is the base piece that holds the whole yo-yo together, so if this dimension fails in either direction the yo-yo will either be impossible to assemble or will fall apart when used. Shrinkage here ended up being as expected, and was accounted for in this and the other parts so that they still fit together regardless.
Expected Value: 2.14in
Measured Value: 2.138in
2. Summary of what we’ve learned, and issues we’ve come across:
One significant issue that we faced was that our base piece was experiencing extreme short shot, with as much as 1/4th of the part not being filled with plastic. This lack of plastic was exclusively experienced on the side opposite our one gate in the cavity mold. To fix this issue, we initially wanted to increase the shot size for the part, but noticed that with the current size, there was still a significant amount of plastic left in the screw of the machine. This meant that the plastic in the gate was most likely freezing over before the part was completely filled. To remedy this, we were able to file the gate down to a larger size, which resulted in a nearly perfect filling of the mold. We increased the injection pressure by about 10 Pa to get the last amount of needed plastic to finish the part correctly.
The base’s second significant issue was that the press fit pin holes on the part were bending outward unexpectedly. This part required the most plastic out of all of our parts, and therefore experienced a significant amount of shrinkage.
The shrinkage from these bases also caused dimples to appear under each of the pin holes.
We were able to minimize the problems from the press fit pin holes and the dimples by increasing the cooling time.
In general, the biggest challenge our group faced during this process was mold machining; we ran into an unexpected ejector pin placement problem 3 days before the deliverable was due and re-machined the majority of our molds; we got very lucky in that parts fit together on the first try (and definitely learned a lot as a team from the experience), and we look forward to production runs and assembly!
