The Making of a Throttle Pocket

October 15, 2005
by John Socha-Leialoha, Designer

The new Universal Throttle Pocket is our first injection molding product, and it was both fun and a lot of work. I'd never designed a part for injection molding, so I had a lot to learn. There were terms like draft, straight pull, ejector pins, core, cavity, and many others that were foreign to me when I first start.

Follow along as we take you through the process of creating an injection-molded product.

	I began by designing a 3-D model of the part. This was a little challenging because we need an opening in the front and bottom for the cords on NCE, Lenz, and other DCC throttles that have the cord coming out the bottom. In addition, the thickness of the material needs to be uniform throughout the part to minimize warping and sinking of the plastic. Finally, the part has to avoid any undercuts because they significantly increase the price of a mold. Once I created the initial design for the throttle pocket, it was time for testing. It's very expensive to make a mold, so we wanted to make sure the design worked. Of course, my early designs had problems, which meant design changes.
	Making prototypes of parts is pretty easy these days, thanks to machines that use a technology called Stereo Lithography (SLA). SLA machines build prototype parts one layer at a time from resin. A laser scans across a layer of liquid resin, hardening the resin wherever it touches the surface. Then the part is moved down a little (about .005") and the next layer is hardened. The photograph on the left shows some of our test parts (the black one has been sprayed Grimy Black).
	Once we had the final design, it took about 4 weeks for the mold maker (here in WA state) to cut the molds out of aluminum. The mold is designed to run on this big machine, which is fully automatic. Once we approve the first test parts (called test shots), this machine will crank out a part every minute, completely unattended. There are two sliding doors with windows on either side of the controller, which is in the center of the photograph. The mold is located behind the left door. And the right door has the plastic hopper and plasticizing screw, which melts and mixes the plastic, then injects it into the mold.
	The plastic starts out as pellets that are dried and then fed into the machine. You can see a few black pellets in the bucket, but for the most part the pellets have a natural color. Only a small percentage of black pellets are required to create the final black plastic.
	The machine injects molten plastic into the mold under high pressure. Once the plastic cools a little, the machine separates the mold halves and then pushes the part off the core using eight ejector pins (they're part way extended in this photograph.
	The parts then fall down onto a slide and drop into a box outside the machine. Here my son is holding up a part that just came out of the machine. The sprue is still attached (technically, the sprue, runners, and gates). These will be trimmed flush before the throttle pockets are packed.
	Mark (on the right) inspecting a pocket while Steve, the injection molder, explains the trimming process to Mark.
	Next the mold was sent back to add some texture so the front surface would be matt instead of shiny. This is done by masking the mold and sand blasting it. Here you can see the next test part with the sprue still attached. On further inspection, we noticed a problem, which is very hard to see in the photograph. The plastic enters the mold at the top, on both sides of the opening, and then meets at the bottom at what is known as a "weld line." Unfortunately, the weld line makes the bottom section of the part weak, so it's easy to snap. The mold maker took the mold back and moved the gate (where the plastic is injected into the part) to the bottom of the part. This means there will be no weld lines, and also eliminates the clean-up required around the curves at the top of the Throttle Pocket.
	The first test printing of our logo back from the pad printer. We tried both two "hits" of ink as well as one. Fortunately, a single hit does a very nice job of covering the black plastic. At this point we gave the go ahead for production. After nearly four months of work, we're finally ready to produce parts.