Focus: CNC Machining and Injection Molding Processes
This project, as part of the IPD 5010 course, was an introduction to injection molding. In teams of two, we designed a part to be molded, then designed and manufactured the tool with which to produce repeatable plastic parts. We started by picking a part that we thought would be a great toy to injection mold: a toy plane, specifically a P51 Mustang.
palmspringsairmuseum.org/p-51-mustang/
First, we made a CAD model of the plane. Then we began redesigning it so that it could be injection molded. This means adding draft to all surfaces, limiting part thickness, making sure there were no undercuts, adding runners and gates to each of the parts. The third image below is one half of the mold, called a tool. It is a negative of the plane and fills with plastic to form the plane in the injection molding process.
In the tool half above, there are a few things to note. In the center, there is a sprue hole, where plastic is injected through. The plastic then passes through a series of passageways called runners and through gates that bring the liquid plastic into the separate parts. These fill with plastic and once the two halves of the mold are taken apart, will need to be snapped off. Some issues we ran into in this design included sharp corners. The tabs to press the wings into the fuselage were very thin and had sharp angles that could not be machined well. Where the main wings connect on the fuselage, the surface along the bottom on the fuselage produced a very thin bit of material that prevented an endmill from removing that material from this mold half.
Above is a finite element analysis (FEA) of the injection molding process. The heat map on the left shows how long it would take to fill the mold with plastic. Nearly 5 seconds is very long for a part of this dimension. The map on the right shows sink. Sink occurs when a part is too thick. The large mass of plastic cools slowly and shrinks as its temperature goes down. Larger volumes of plastic sink more than smaller ones, so any region that is too big, will have indents called sink where the plastic shrank.
These difficulties led my partner and I to scrap this concept upon hitting too many roadblocks in our three week timeline. We still had to machine molds and inject the parts. So, unfortunately, we switched to a less ambitious project.
We started on a quick CAD model of our new project – a shot glass.
The mold for this part had to be much taller than the mold for the plane, but the internal features were much simpler. There was no need for runners or gates since there was only one part. The render below shows the two parts of the mold as well as the finished part that the tool produces, the shot glass.
From the CAD, CAM was programmed and the parts were brought to the mill to machine. Roughing and finishing passes brought the parts to a desired finish. Then the core mold (above, left) was brought to a smooth polish so that the interior of the glass would be smooth.
The final step is to actually inject the part. The University has a desktop injection molding machine, so we could make and test out as many as we needed. This turned out to be a difficult process. We had to adjust the clamping pressure of the machine and the injecting pressure back and forth until we had a consistent shot. This took many tries. Lots of injections were undershot or overshot to produce flash.
The above photos show the many failures involved in tuning the injection molding machine. In the first two, flash is protruding from the edge of the glass, where the two halves of the mold meet. That means that the shot was too long and/or that the clamping pressure pushing the two halves of the mold together was not enough to keep the high pressure liquid plastic from escaping.
The two photos above show the successful injection molded parts we were able to produce once the machine was tuned. The different colors, and the mixed colors, come from the ability to mix the plastic pellets in the hopper that leads into the heated barrel. Additionally, as seen in the bottom right picture, the interior of the glass is perfectly smooth, a result of polishing the core half of the mold.