Estimate Machining & Optimization Times

Last week we machined out first mold. Looking ahead, below are our expected machining times for all our molds. After machining our molds, we will begin injection molding and process optimizations and have included estimates for that below too. 

Machine Time Estimate for Lathe and Mill

Once our molds are made in Solidworks, we import them to Mastercam where we plan out what machines and tools we will use to make them. Mastercam is sync with the 2.008 Mill and CNC Lathe and thus gives us estimates for machining times on both machines. 

Part	Lathe Time	Mill Time
Yoyo Body Cavity	7 min 9 sec	0
Yoyo Body Core	6 min 29 sec	1 min 30 sec
YYB w/Spin Stick Core	6 min 29 sec	6 min 24 sec
Retaining Ring Core	2 min 39 sec	3 min 13 sec
Retaining Ring Cavity	35 sec	0
Thermoform	3 min 12 sec	54 seconds
Letter Mold Core	0	25 min
Letter Mold Cavity	0	1 hr 30 min
Gold Board Mold Core	0	30 min
Gold Board Mold Cavity	0	9 min 21 sec
Entire Wheel of fortune board Mold Core	1 min 1 sec	1 hr 5 min 27 sec**
Entire Wheel of fortune board Mold Cavity	3 min 37 sec	1 hr 16 min 40 sec**

**Has to be machined after letters and board have been injection molded to account for shrinkage before overmolding parts (more on overmolding later this week!)

Projected Timeline

Process Optimization	October 20th - November 10th
Make Final Parts	November 10th -November 24th
Assembly	November 24th - Dec 5th

Previously we planned on having all our molds made last week. However, due to longer machine times than expected and how quick machines get booked up, we have shifted our first injection molding iteration back by a week and will have to shorten our optimization and final production run time. (this is reflected also in our Gantt Chart).

Projected Process Optimization Time

To optimize our process we will run iterations of injection molding parts, measuring parts for accuracy, and re-designing as needed. To get an estimate of how long this process will take we have estimated the time of these iterations. 

Injection Molding time is dominated by cooling time. From class lecture, we learned that cooling time can take upwards of half of the entire injection molding process. Cooling time for our parts can be derived from the equation Cooling Time=(thickness of part)^2/alpha, where alpha is the thermal diffusivity of the material. Our yoyos will be made from polypropylene with a thermal diffusivity of 0.096mm^2/s. Our parts have an average thickness of ~2.54mm, thus the average cooling time is about 67 seconds. To make 10 parts will thus take us about 20 minutes. For all our parts it will take us ~110 minutes (we have already booked 3 hours of IM time so we'll have our first iteration of all parts done this week!). Thermoforming each part also takes us about a minute, adding 20 minutes to our total estimate.

At worse we expect to have to re-machine all our molds and injections mold again. Our total machine time is about 5.5 hours. So our total process optimization time is roughly 16 hours of lab time plus the time to re-CAD any changes. We believe 3 weeks should be sufficient time to get this done between the five of us.

Manufactured Mold: Thermoform

We made our first mold in lab today, using the CNC lathe, mill, drill press, arbor press, and reamer!

The thermoform part only involves one mold, shown below:

The top of our thermoform piece mold, with the dome structure that will form our clear plastic yoyo cover.

The bottom of our thermoform piece mold, with the pin holes at the north and south axes.
The vacuum holes are visible right next to the inner ledge.

We used the lathe to turn the dome shape and ledges, while we used the mill, drill press, and reamer to create the two pin holes. Another drill press was used to create eight tiny vacuum holes, while an arbor press pushed the pins into the mold.

A tiny drill was used to create the vacuum holes.

The arbor press right before we pushed the pins down into the mold.

We also scheduled time to do a couple of test runs -- we made five test covers in all:

The top face of our very first thermoformed part.

The bottom face of the yoyo cover.

As expected, our first run didn't result in perfect yoyo covers off the bat. The texture was speckled (dust!!!), and there was some webbing near the pins.

Thus, for the subsequent runs, we pushed the pins down more to decrease webbing, cleaned the mold and plastic extra well to decrease chances of catching dust, and altered some parameters such as heating time.

Next week, we plan to manufacture the rest of our yoyo part molds, and start injection molding!

Preliminary Mold Design: Retaining Ring

This week, we've been finishing up designing our yoyo part molds in preparation for our first test run! We have one mold for the thermoformed clear plastic cover, and two each (a cavity and core) for the yoyo body, retaining ring, lettering, gold board, and entire wheel of fortune board.

One of the molds ready for its first run is the retaining ring:

Both the core (bottom) and cavity (top) molds.

The core mold features a filleted pocket around the boss extrude.

The cavity mold is stepped for the ring's ledges.

This part sits on top of the first ledge of the yoyo body, keeping the thermoformed cover in place, which will itself sit on the second ledge of the yoyo body.

The mold was created in SolidWorks using its Mold Design tools. The parting line was chosen in the middle of the ring's larger diameter to facilitate easy removal, and a shut-off surface was created at the bottom of the ring's smaller diameter.

A view of the parting line feature in SolidWorks. The body shown is our retaining ring.

The part was scaled by 1.02 based on shrinkage calculations from previous production runs of a similar retaining ring in past classes. The average outer ring diameter size was 1.9415" for this past ring, while the mold diameter itself was 1.98."

The retaining ring cavity mold we based our shrinkage calculations on.

The mold drawing was then transferred to Mastercam to add the runner and ejector pin hole placements:

CNC Lathe Mastercam for the cavity mold.

CNC Lathe Mastercam for the core mold.

ProtoTRAK Mill Mastercam for the core mold.

Since our retaining ring includes fillets which we didn't want the ejector pins to dent when removing our part, we decided to place the ejector pin holes outside of the part and create runners to the ring. (We had to use the largest radius ejector pin holes available since our yoyo body was already at the maximum size of 2.5" in diameter.)

In addition, we decided to place the ejector pin holes on the mold core instead of the cavity (where they typically are) to work around the fillet issue.

To manufacture these molds, we plan to first use the CNC Lathe, then the ProtoTRAK Mill. We chose the tools that would fit and provide the best finishes:

Retaining Ring Core:

Step	Operation	Machine	Tool	Justification
1	Rough Cut	Lathe	T0303	Used Tool 3 because we wanted to cut the outside diameter.
2	Finish Cut	Lathe	T0303	Used Tool 3 to finish to keep it consistent with the rough cut.
3	Groove	Lathe	T0909	Chose Tool 9 as a trepanning tool in order to cut a groove.
4	Center Drill	Mill	3/16” Center Drill (#2)	Chose the only appropriate tool to center drill the ejector pin holes first.
5	Peck Drill	Mill	0.120” Drill (#31)	Chose the only appropriate tool to create the ejector pin holes.
6	Contour	Mill	⅛” Ball End Mill	Chose a ball end mill tool to make one of the runners from the ejector pin holes.
7	Contour	Mill	⅛” Ball End Mill	Chose the same ball end mill tool to make the sprue runner and the other ejector pin runner.

Retaining Ring Cavity:

Step	Operation	Machine	Tool	Justification
1	Rough Cut	Lathe	T1010	Chose Tool 10 because we needed to bore out the cavity.
2	Finish Cut	Lathe	T505	Chose Tool 5 to finish because it has a smaller radius.

Here's to hoping for a successful first test run!