Vented barrel design looks to make short work of volatiles

By: 
October 10, 1998


Figure 1. The Accudose design consists of a vent in the barrel that is sealed by a spool. The pneumatic or hydraulic cylinder is used to open and close the vent to control the release of volatiles from the barrel.

It must be every molder's fantasy: drying nylon no more. As durable, tough, and widely used as nylon is, getting and keeping it dry is a time-consuming chore without which most molders could easily live. And we won't even talk about what it does to the mold. Makoto Yamaguchi, owner of Yama Creative Works, knows this, and it prompted his design for a new vented barrel that would allow molders to purge volatiles from material without any drying.

The key word in the last sentence is "design." As yet, Yamaguchi's brainchild exists in only two dimensions. As a result, it necessarily makes a few assumptions about how material does and might behave in the barrel and what might be done to exorcise volatiles before they hit the mold. Still, the concept is worth exploring and possibly deserves a chance to become a reality. Yamaguchi certainly thinks so and is looking for all the help he can get.

The Concept

Yamaguchi calls his design Accudose Injection Volume Control. A former sales engineer for Meiki, Yamaguchi was looking for a way to allow molders to mold hygroscopic materials without drying. He says he got the idea from a syringe. Nurses, before sticking a patient with a syringe, will hold it upright and push the piston to purge air trapped in the cylinder. Yamaguchi thinks the screw in a barrel can do something similar.

The Accudose design consists of five basic parts: a relief valve in the nozzle end of the barrel, a spool in the valve, a locking clamp, a pneumatic or hydraulic cylinder, and a relatively simple controller tied to the machine's controller (Figure 1). It does not require existing barrel venting.

This theoretical system begins its work at the end of injection and follows these steps:

Figure 2. According to the Accudose design, after the screw builds the shot volume for the next cycle, it backs up even further (possibly as far again as the distance required to build the shot). Yamaguchi believes this "decompression" will create a vacuum to force volatiles out of the melt into the area between the material and the top of the barrel.
Figure 3. After decompression, the screw then moves forward again toward the original metering position. Simultaneously, the cylinder opens the valve, emitting the volatiles in the barrel. The screw stops a few millimeters short of the metering position to prevent material from escaping as the valve closes and injection for the next shot begins.
  1. At the end of injection, as usual, the screw retreats and meters material in front of it in preparation for the next shot. The amount of material that is metered depends, of course, on the size of the part and the diameter of the barrel.
  2. When the screw reaches its starting point, again as usual, it stops and waits for cooling to finish.
  3. This is where the Accudose system deviates. After the screw reaches its starting point, it is then backed up further. But, unlike a typical sag back decompression, Yamaguchi says the Accudose system would take the screw back as far again as the original metering distance. This action, theoretically, creates a vacuum condition in the barrel that forces volatiles out of the molten resin and into a gap between the material and the top of the barrel (Figure 2). "The screw works as a vacuum pump," says Yamaguchi, "creating a vacuum in the metering zone." The question here is how readily volatiles will migrate out from the melt.
  4. Next comes compression. After decompression has forced volatiles to the top of the barrel, the screw is moved back to its original starting point. At the same time, the pneumatic or hydraulic cylinder on top of the barrel actuates the locking clamp, which lifts the spool and opens the valve at the end of the barrel (Figure 3). The volatiles gathered at the top of the barrel would then, in theory, escape through the valve. When the volatiles are gone and the screw has reached its original starting point, the cylinder actuates the locking clamp again, which pushes the spool back down to close the valve before resin itself leaves the opening. Moving the screw forward also locks the check ring against the screw, a technique already practiced by some molders to more precisely meter melt in the barrel.
  5. The next cycle begins. The screw injects the metered, volatile-free material into the mold and another part is made.

Some Details

As interesting as this design is, some questions naturally arise. Some can only be answered by trial and error. Some Yamaguchi has foreseen and has tried to answer with his design.

John Clasen is the manager of process engineering at SeaquistPerfect Dispensing (Cary, IL). This 26-press (Van Dorn, Krauss-Maffei), captive molder is a big user of nylon and would love to get rid of volatiles before they hit the mold. Clasen estimates about 35 percent of his company's business is in nylon, and it shows in the mold. "Basically, the venting of volatiles just cakes up our molds," he says.

Clasen has met with Yamaguchi and has reviewed his design. Without an actual Accudose system to test, Clasen notices two large benefits of the design. "First, it's modular," he says. "It could all be included in the end cap of a machine." Second, by modifying the machine controller logic, a molder could simultaneously vent volatiles and lock the check ring before injection.

His biggest question? "Naturally, our biggest concern would be the venting hole, if it allows any material out and how clean it stays," says Clasen. One also wonders, given the high pressure inside a barrel (up to 40,000 psi in some cases), if the valve would have the strength to remain closed or if it could close once opened. The answer to these concerns will come partially through experimentation. However, Yamaguchi says after decompression, as the screw heads back to its starting point, he would stop the screw a few millimeters (or some fixed percentage) short of the setpoint. Doing this would provide some "wiggle room" necessary to help make sure the melt stays in the barrel. "If the system works and is controlled well, I don't think there will be a problem with material escaping," he says.

Yamaguchi also says, given the sophistication and frequency of screw movement required by the design, only presses with CNC controllers would be adaptable to the system. He also admits some hesitation to run materials such as PET, as excessive hydrolysis and subsequent material degradation can occur in the barrel. But, the only way to find out the real effect is to test it, which will happen as soon as he builds the Accudose. In the meantime, Clasen, and other molders like him, are willing to try. "If we had a working model, we'd definitely try it out," Clasen says. Yamaguchi hopes to have a system built and ready to test before the end of the year.


Contact information

Yama Creative Works

Rolling Meadows, IL

Makoto Yamaguchi

Phone: (847) 925-8017

Fax: (847) 925-8017

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