Oreos Fresh out of the Oven!

After half a semester of hard work, we finally have an assembly of our yo-yo! In this post, we'll share the process that got us here here.

Injection Molded Cookie Part

We will first talk in-depth about the manufacturing process for the injection molded cookie part, and in the next section we will go into the details of the optimization process for each of our parts.

Mold Design and Machining

The molds used for injection molding the part require a lot of detail-oriented design and machining. We started with a SolidWorks model of our cookie part and used SolidWorks' mold-making function to generate models of our molds.

Cavity Mold

Core Mold

Once we had the SolidWorks models of the molds we loaded the models into MasterCam in order to generate the code we needed to machine the molds on the CNC lathe and mill. Before actually machining, we made sure that we had a solid process plan detailing the machine tools we planned on using. Okay, time to machine the molds. After a quick turn on the CNC lathe and a 2-hour long haul on the CNC mill, our molds were Ore-ready! Checkout how great the surface details turned out.

Injection Molding

Now for the big finale--injection molding. There were a lot of parameters to tweak, such as mold temperature, hold pressure, and cooling time. After hours of optimization, we ended up with a set of parameters that we were happy with. We will explain the optimization process for each part in the next section. With optimization in place, we are finally ready for our production run to make 50 yo-yos!

What We Learned

Injection Molded Cookie Part


Through the optimization process of the injection molded cookie part, we learned that subtle errors in mold alignment can be an eyesore on the plastic part.

As soon as we saw our first injection molded piece, we realized that we had a problem. The two halves of the cookie (core and cavity sides) were misaligned rotationally by 1-2 degrees and vertically as well, as seen in the photo to the left. Being perfectionists, we decided that this imperfection was not to be tolerated. Fortunately we were able to fix the rotational misalignment by re-programming and re-machining the core mold. As a result we had to increase the shim thickness from .003” to .032” and decrease the ejector pin length to 5.570”. We fixed the vertical misalignment by switching to 0.212" and 0.2115" dowel pins.

The second problem we encountered was dishing on the edge of the cookie opposite the injection site. We first tried to fix this by increasing the injection speed parameters, but that had no effect on the dishing. We then increased the feed stroke, which increases the shot size, from 1.20” to 1.35”. This reduced but did not eliminate dishing, and we had capsized on shot size. Our next resort was to increased the holding pressure, but not increasing it so much so as to create flash. We were able to find a sweet spot with holding pressure where there was neither dishing nor flash.

We then evaluated the pressfit between the cookie and cream parts to see if we needed to adjust shrinkage. The inner diameter of the cookie averaged at 1.849", while the outer diameter of the cream averaged at 1.860. This was within comfortable margins for a good pressfit (~0.01"), so there was no need to tweak holding and cooling times to adjust shrinkage.

Most importantly, we had to mix the perfect color for our Oreyo-yo. After experimenting like painters for a bit, we decided to go with a 50:50 mixture of mid-brown and black thermoplastic pellets, as seen in the photo to the right.

With all of our parameters set, we ran the injection molding machine on automatic for 5 pieces to find our average cycle time, which turned out to be a reasonable 30 seconds.

Injection Molded Cream Part

Through optimizing the injection molding process of the cream part, we learned that fixing a problem, such as dishing, can take a lot of tries and tweaking of parameters, but in the end it's pretty satisfying to get it right.

Dishing is the ripple effect in the surface finished caused by shrinkage, as seen in the photo to the left. Our first attempt was to increase the holding pressure, which helps the part cure before ejection. We started from the default of 300 Psi, but even after going as high as 1200 Psi we were only able to reduce but not completely eliminate the dishing. The next thing we did was increase cooling time, which also helps the part cure before ejection. We experimented with cooling times ranging from 25 to 45 seconds but saw no significant change after 30 seconds. Next, we raised the holding time from 2 to 8 seconds, but there was no reduction in dishing, which got us suspicious. We started wondering if the sprue was freezing too early, preventing plastic from flowing in through the gate and causing dishing. Dave, one of our wonderful and knowledgeable lab instructors, showed us a way to find out if our suspicion was right--by weighing the part while increasing the holding time until the weight was no longer changing. It turned out that while the part weighted 8.5g with 2 seconds holding time, it remained at 8.7g after 4 seconds. Aha, this means that somewhere between 2 and 4 seconds holding time the sprue was freezing off.

It was easy once we had diagnosed the problem. We remachined our mold to have a larger runner size, which freezes later As expected, the dishing was gone, and the cream part looked a-cream-azing!

Thermoformed Base

The thermoformed base is the seat for the yo-yo. It's supposed to resemble the milk splash associated with Oreo. Pretty sleek, huh?

Through the optimization process of the thermoformed base, we learned that heating time, which determines how stretchable the plastic is, is very important in thermoforming. Just into our trial runs we found that the platform was too low. This was because our part is taller than typical thermoformed parts. Below is a pictured of a base made with the platform too low. Note how the mold did not have enough room to fully form. 

However, after adjusting the platform height our part still wasn’t fully forming. We decided to optimize by adjust the heating time. Adjusting the heating time allowed us to achieve fully formed parts. However, as a compromise, we had to settle for a longer cycle time. Pictured below is our piece formed with a heating time of 30s, 40s, and 50s.