Process Optimization and First Production Run!

Plate Process Optimization

Process sheet for injection molding:

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For the plate, it took a bit of experimentation with the injection molding machine to determine the appropriate process parameters. We started out with a shot size of 20mm, but soon determined that the mold wasn’t filling completely, and there was a short shot every time we injection molded. Bumping the shot size up to 22 mm seemed to produce the best results, with no visible defects. We also experimented with the injection pressure, eventually settling on 500 psi, which produced a plate that had no warping.

Waffle Process Optimization

Process planning sheet for waffle:

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As with the plate, a lot of experimentation was necessary before we arrived at proper parameters for making the waffle. Initially, our shot size was much too large (32mm) and we had difficulties with the packing phase. This, in combination with inadequate holding pressure, led to undesirable flash. To alleviate this problem, we reduced holding pressure, increased packing time, and reduced shot size. After minor adjustments of these parameters and increasing the injection speed from 50% to 70%, our waffles looked very edible.

Summary of Overall Process Optimization

Plate:

Once we had the basic process parameter settings down for the plate, our biggest problem was trying to minimize the sharpness of the edge of the plate. We originally thought that the sharp edge was caused by a bit of flash, so we tweaked a bunch of the process parameters to try to eliminate this feature. Unfortunately, none of the changes in process parameters made a difference in reducing the sharpness of the edge of the plate, and we determined that the issue was primarily caused by the location of the parting line on the piece — it would be extremely difficult to completely eliminate the sharp edge, given the design at that time.

With this in mind, we re-machined the core mold, this time extending the curvature of of the plate so that there would be a smoother edge. We then ran the molds on the injection molding machine, using the our original process parameters. Since the change we made to the plate was not drastic, the original process parameters worked well, and those became our final process parameters to use with the re-machined molds.

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Waffle:

After injection molding a few plates and waffles, we realized that the snap fit was too loose. The waffle component did not shrink as much as we thought it would. We re-machined the core mold of the waffle to get a better snap fit with an overlap of 0.01 inch, whereas before, there was no overlap in between the ID of the waffle and OD of the plate. With the re-machined core mold, the snap fit was tight; we were not able to get the waffle off the plate.

Since our waffle component has uneven thickness, during the process optimization stage, we wanted the part to have the least warping without introduction of defects such as flash or short-shot. When we played around with parameters, we encountered problems balancing pressure and injection speed. The mold was filling very slowly and occasionally had cases of short-shot. When we increased the pressure, flash occurred. To address this, we opened up the runner to allow more area for the plastic to flow in. After doing this, our parameters were easier to optimize. We found an appropriate balance of injection speed and pressure (70%, 500 psi) that addressed both short-shot and flash issues. The shot size of 27mm was optimal for the packing time of 6 seconds without causing flash. Ultimately we were able to achieve consistent results with these parameters.

During our production run, we encountered another problem when trying to achieve the perfect golden-brown waffle color. We discovered that mixing two colors was much more consistent than mixing three colors. When we mixed cream, brown, and yellow, our color ranged from light brown and cream to olive green and dirty yellow. We decided to stick with just the cream and brown plastic pellets.

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Butter:

 

When the only setting we could change was temperature, we found that 625oF worked well with our part. When the display was fixed, we tried to optimize time-wise so production could go just as smoothly but in a shorter amount of time. First, we decreased the heat time from 30 seconds to 22 seconds, and the form and open delay times from 15 seconds to 10 seconds. With those settings, even though production was faster, the base of the butter was not clearly defined enough to snap together with the waffle and plate. Next, we increased the heat time to 25 seconds, but the base was still not well defined enough. We increased the temperature until 640oF, at which point the base of the butter was well defined enough to fit with the rest of our parts, and the top of the butter that’s exposed in the yoyo was still round enough to give it a soft buttery look.

Final thermoforming settings

Not pictured: butters 11-120

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

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Plate Core Mold

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