Monday, April 24, 2017

Injection Molding Optimization Process


Optimized Parameter Settings


Presented below is the optimized parameter settings sheet for the main body mold for the cookie. It took us a few iterations on the BOY injection molder to arrive at these settings. We initially had too high of a shot size and still had excess plastic left in the screw. After a few iterations, we got that shot size down to 39.5mm which was ideal for our case and allowed our part to come out with no defects/flash/etc.. We were also able to take the cooling time down from around 30 seconds down to 20 seconds while still maintaining good part quality and minimum shrinkage. After several iterations of 10 or so parts, we arrived at the below sheet which will guide us as we do our final production runs.




Process Optimization Outcome


One major issue resulting from initial injection molding runs concerned the press fit interface between our cookie body and frosting parts.


Here we were able to press the two together right after the body part came out of the machine when it was still flexible, but also really hot. After nearly burning our hands, we decided that this assembly process was not optimal.

The primary cause of this issue was that the inner diameter of the cookie body part (shown above in white) was too small, such that the frosting part (blue) outer diameter could not fit inside. This mishap was potentially caused by differences in shrinkage or misaligned dimensions in the mold. To fix this issue, we needed to re-machine the cookie body core mold to allow for an increased diameter on the cookie body part.

The secondary cause of this issue was that the height of the press fit walls on the frosting part were too short, giving us too little length of engagement at the press fit interface. To fix this, we deepened and widened this wall on the frosting core mold to allow for a longer, stronger press fit connection.

Monday, April 3, 2017

The Mold Manufacturing Process


Body Mold Design


Shown below are our two molds for the yoyo “cookie” part. It is a relatively simple part that is our main body for the yoyo. The core is on the right and the cavity on the left. Both parts were initially put onto the lathe for facing and cutting, and then were milled to add the ejector pin holes to the core. The part is the main body of the yoyo, and hence has a large press fit hole in the center. This hole needed to be vertical, so no draft angle was used on it. The cavity has that ridge built into it in order to reduce the amount of plastic used and the shrinkage defects on the parts.




Body Mold Shrinkage Allowances


To estimate the shrinkage of our part, we looked around for previous yoyo parts that compared similarly to our mold. The variable of interest to us was the inner press fit diameter since this was the most important parameter. The mold we chose was similar in size, geometry, and thickness to our body mold. We then measured that inner diameter parameter on both the mold and the variety of injection molded samples attached to it. From these, we calculated the average shrinkage as 0.0508 inches out of the initial diameter of 2.702 inches. This gave us a shrinkage of 1.89%. We did this for several parts and eventually ended up using a shrinkage value of 2%.


Body Mold Manufacturing Processes


Both parts had the majority of their work done on the lathe first, and then had some final milling done in order to machine the ejector pin holes, etc..  The process plan for both parts is listed below:


Core Mold Process Plan
Step
Operation
Machine
Tool
Justification
1
Lathe Face
Lathe
T0101
Remove surface material to face off part
2
Lathe Rough
Lathe
T0101
Remove material around outer section of mold
3
Lathe Finish
Lathe
T0303
Finish removing material around outer section
4
Lathe Groove
Lathe
T0909
Cut into outer groove (containing press fit feature and outside surface)
5
Lathe Finish
Lathe
T0707
Finish press fit surfaces to tighter radius with turning trepan
6
Lathe Drill
Lathe
T1212
Drill pocket for ⅜” hex nut into mold
7
Mill Center Drill
Mill
T1313
Center drill to facilitate next milling process
8
Mill Ejection Pin Holes
Mill
T1717
Mill holes for the ejection pins






Cavity Mold Process Plan
Cookie Cavity Mold
Step
Operation
Machine
Tool
Justification
1
Lathe Rough
Lathe
T0101
Remove surface material
2
Lathe Finish
Lathe
T0303
Remove remaining material
3
Lathe Rough
Lathe
T0101
Remove surface material on top section
4
Lathe Finish
Lathe
T0303
Remove remaining material on top section
5
Lathe Groove
Lathe
T0909
Cut into the groove
6
Lathe Finish
Lathe
T0707
Finish the groove cut
7
Lathe Rough
Lathe
T1010
Bore out the center pocket
8
Lathe Finish
Lathe
T0505
Finish the boring cut
9
Lathe Drill
Lathe
T0404
Drill roughly 0.1” into the center
10
Lathe Drill
Lathe
T0202
Finish the ¼” diameter center drill hole to house the sleeve that holds the nut in place


Manufacturing Time Estimate
The overall time is dependent on the pure machine running time as well as the time outside the machine setting everything up, aligning all of the parts in the molds, cutting the flash off, and getting the parameters set right.


The machine running time for each mold is given in the spreadsheet below. We estimate that we will need approximately double this amount of time to set up the lathe and mill and align the parts, as well as for process optimization. We will also likely need to re-mill the molds at least once, so we can tack on more time there.

Step
Machine
Time Needed
Cookie Cavity Mold Machining Time
Lathe
3 min, 38 seconds
Cookie Core Mold Machining Time
Lathe

Frosting Cavity Mold Machining Time
Lathe
22 seconds
Frosting Core Mold Machining Time
Lathe, Mill
2 hours, 2 minutes
Sprinkles Die 3D printing Time
3d printer
8 hours



Remill all molds (assuming 1st time is imperfect)

10 hours



Process Optimization

3 hours to optimize parameters (during lab)



Final Production Run (Injection Molding Time (100 yo-yos halves, 200 total injection molded pieces))
Injection Molder
1min*200 parts + setting up time = ~3.5 hours

Manufactured Yoyo Molds




Body Mold


Frosting Mold


Sprinkles Dies (Female and Male)