Plate Mold

Mold Description

We chose to highlight our body mold, which is used to create the plate feature in our waffle-themed yo-yo. The plate mold consists of two parts: the core and the cavity. Since the outside of the plate has a clear curvature, we chose to make the parting line between the core and the cavity molds along the edge of the curvature — to minimize visibility of the parting line on the final produced plastic part. The cavity mold is straightforward — it is a circular cavity with two sections of different depths. This ultimately forms the underside of the finished plate. The core mold was slightly more complicated, with a raised middle that creates the circular rib on the finished plate, where we will snap our finished injection molded waffles.

Shrinkage Calculations

The dimensions that we determined for the mold are based on an estimate of 2% shrinkage, which we determined by measuring similarly sized molds to their finished parts in lab. To obtain a 2.500” diameter in a finished plastic plate, the mold outer diameter was designed to measure 2.550”. To account for shrinkage and achieve a toleranced snap fit, the outside diameter of the plate’s rib feature was designed to measure 2.000”, while the inside diameter of the waffle was designed to measure 1.990”. With shrinkage, the two parts should cool at 1.960” on the plate and 1.950” on the waffle, and create a snap fit. The thickness of the plate was not a major concern in scaling our part up, because due to its current thickness it would be sure not to bend or fail. However the molds measure an inner plate thickness of 0.120”, which would shrink to 0.118”. From the bottom of the plate, the rib according to the mold is designed to be 0.306” tall, though after shrinkage, the final rib should measure 0.300”.

Manufacturing Process

We first manufactured the plate core mold on the lathe, using tools 7, 8, and 9 to create the grooves in the part. Next, we used the 0.125” drill to drill the space for the hex nut. Then, we used the mill to drill the ejector pin holes in the core mold, finishing up with the drill press to ream out the holes. For the plate cavity mold, we first used tool 10 on the lathe to create the necessary features. Then, we used the center drill and the 0.234” drill to bore the sprue hole. Finally, the we used the mill to create the path for the runner.

Step-by-step Process Plan

Plate Core Mold Process Plan

Step Operation Machine Tool Justification
1 Insert stock Lathe N/A Need stock to produce mold
2 Load G-code Lathe N/A To initiate program
3 Zero tool Z axis Lathe 9 So the lathe knows where measurements are relative to
4 Rough groove Lathe 9 Tool 9 has a full radius tip of 0.0385”, which was the right size to bore out the begin the trepan
5 Finish trepan Lathe 7 Tool 7 is good for performing cuts in close quarters, and its point radius of 0.01” was good for finishing the trepan corner.
6 Finish trepan Lathe 8 Tool 8 is good for performing cuts in close quarters, and its complementary geometry to tool 7 made it the right choice to finish the other corner of the trepan.
7 Bore nut hole Lathe 0.125” drill Bore hole in the middle of the piece to create room for the hex nut.
8 Remove piece and clean up Lathe N/A To leave machine ready for the next use

Plate Cavity Mold Process Plan

Step Operation Machine Tool Justification
1 Insert stock Lathe N/A Need stock to produce mold
2 Load G-code Lathe N/A To initiate program
3 Zero tool Z axis Lathe 10 So the lathe knows where measurements are relative to
4 Roughing Lathe 10 Removes material in a facing operation according to the curve we designed. Tool 10 is a boring tool with a small enough radius to produce the curvature and sharp edges that the plate silhouette requires
5 Finishing Lathe 10 Tool 10 is also capable of finishing, when set to a slightly lower feed rate.
6 Drilling Lathe Center Drill Creates an initial hole in the back of the plate cavity
7 Drilling Lathe .234” diam drill Finishes the hole drilled in the previous step, drilling through the entire width of the mold; this creates a channel for the nut to be held in place and the plastic to flow around it
8 Contour Mill ⅛” Ball end mill Creates runner from sprue hole to the back of the part

Manufacturing Time Estimate

Manufacturing Step Estimated Time Justification
Fabrication of Plate Cavity 2m 19.31s Projected machine time from Mastercam
Fabrication of Plate Core 3m 3.43s Projected machine time from Mastercam
Fabrication of Waffle Cavity 12h 30m 25s Projected machine time from Mastercam
Fabrication of Waffle Core 2m 9.18s Projected machine time from Mastercam
Fabrication of Butter Core 1 h 9 min 43.25s Projected machine time from Mastercam
Process Optimization Plate 1 hour 1 hr should be enough time to make minor changes to the mold as well as tweak process parameters on the injection molding machine.
Process Optimization Waffle 1 hr Assuming only minor changes need to be made to the hold, 1 hr should be enough time to fix the mold as well as tweak process parameters on the injection molding machine. If the mold has to be remade, however, then we will require significantly more machine time.
Process Optimization Butter 1 hr Similar to the Waffle, 1 hour should be sufficient for small mold changes as well as process parameter tweaking, but more time will be necessary if the mold needs to be entirely remade.
Final Production for 1 Plate 45 seconds This was a generous estimate of how long we spent for each cycle on the injection molding machine.
Final Production for 1 Waffle 45 seconds We expect the waffle production to take a similar amount of time to the plate.
Final Production for 1 Butter 45 seconds We expect the thermoforming to be a quick process, so 45 sec for each cycle should be reasonable.

The changes made to this schedule are mostly in the final production for each part — we changed the time estimates (to produce one of each part) to 45 seconds. Based on our first testing runs of injection molding with the plate, 45 seconds is a generous estimate of how long each cycle takes on the machine. Since each part will vary (especially for the cooling time), we leave our final estimate at 45 seconds, which should hopefully be reasonable. I don’t think any of these changes will affect our team schedule. The biggest challenge for our schedule will be if we need to remachine the waffle mold.

Photographs of Manufactured Molds

 

Plate Cavity Mold

Screen Shot 2016-04-04 at 2.54.50 PM

Plate Core Mold

Screen Shot 2016-04-04 at 2.54.39 PM

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s