For concentric parts, nozzle allows center gating at the core

The next time you're driving down the highway and pass an 18-wheeler, take a quick peek at the shocks by the wheels. On most trucks you'll see a large rubber-sheathed cylinder, several inches in diameter. Inside that rubber bag is a piston that helps absorb the weight of the truck and trailer above it. That cylinder is expected to support at least 44,000 lb.

Traditionally, that cylinder has been fabricated from metal, since nothing else was thought to be strong enough to hold such weights. But in Mishawaka, IN, one molder has made the thermoplastic leap and now injection molds those cylinders. The molder is Nyloncraft, and a few years ago, it perfected the art of converting the piston cylinders from metal to plastic with such success that, according to air suspensions manager Terry Henderson, the plastic versions are the preference of many truck manufacturers. They're cheaper and lighter than metal, and just as durable - able to withstand up to three times the 44,000-lb minimum load.

Nyloncraft, as the name implies, molds these cylinders from nylon. The Mishawaka plant has 44 presses ranging from a little-used 55-tonner to a 2000-ton press; machine brands include Engel, Cincinnati Milacron, HPM, Nissei, and Toshiba. The chosen nylon for the cylinders is a 33 to 43 percent glass-filled grade, either from DuPont, Solutia (Monsanto), Ticona, or DSM. These bell-shaped cylinders range from 2.5 to 12 inches in length, with diameters of 6.5 to 14 inches. Weights range from 1.5 to 8 lb. The cylinder is hollowed on one side and ribbed on the other for support.

What all these cylinders have in common is a .5- to .75-inch diameter hole that runs right down the center. When Nyloncraft first started converting these cylinders to plastic, it discovered a problem that plagues many molders: it's difficult to center gate a cylindrical part that has a hole cored down the middle.

What Henderson was forced to do was diaphragm gate the part at the center, then clip the sprue and drill the hole in a post-mold process. "That hole always had to be removed in a secondary application," he says. "It left us with a big problem with that hole being inconsistent." Sometimes the drill wandered, the bit got contaminated, or the blades dulled. And the drilling required manual labor - not only an additional cost, but an additional variable.

As a result, many parts were frequently nonconcentric, a vital specification in the cylinder. Also, molding 800,000 to 1.2 million of the cylinders a year, Nyloncraft was losing money in material every time a hole was drilled. "There was scrap involved," Henderson says. "which is one reason we went after a solution. More importantly, the quality concern of ensuring that the hole exists was paramount in obtaining an automatic gating system."

The solution sounds simple. "We wanted a valve gate that can leave a hole in the center," he says. Henderson shopped this idea around the injection molding community, looking for a company willing to give the concept a try. He struck out everywhere until he found Osco, a Rochester Hills, MI-based maker of runnerless molding systems.

Together with Nyloncraft and Granger, IN-moldmaker K Mold & Engineering, Osco developed what is now known as the DSCV-500 pin gate system. It has a valve pin that does two things at once. First, it quickly and cleanly opens and closes to control resin flow through the gate. Second, it acts as a core pin to positively form a cored out hole in the part. With it, Nyloncraft was able to gate the cylinder in the center while simultaneously coring the hole. Parts came out of the mold completely finished and ready to ship. On top of that, part quality was made consistent, and the cost of manual labor was eliminated, freeing up operators for other jobs. "What it leads to is that I don't have to have an operator at every press," Henderson says.

The Osco DSCV-500 valve gate, which can be used under a manifold or as a standalone, consists of a self-contained valve pin and an externally mounted hydraulic cylinder. The valve pin is adjustable from the press side. This setup eliminates the hassle of machining space for the cylinder in the mold and makes access for maintenance simpler. The linear action of the cylinder piston is directly transferred to the valve pin unit through an actuating rod in a 1:1 ratio for fine pin and gate adjustments. The gate opens on the injection forward signal and closes on the injection end signal.

K Mold owner Ken Kasner helped Osco develop the nozzle and says that converting a mold to accommodate the nozzle costs, on average, about $10,000. The biggest challenge, he reports, is creating enough cavity space to fit the valve gate. But with enough depth, he says, the procedure is relatively routine. Bushings on all of the nozzles have been standardized, which makes switchouts simpler.

Henderson says the valve, because it's used to core the part, has to open and close quickly, before solidification occurs. "So timing is critical to this part," he explains. As a result, a gate mark on the part is nearly impossible to find. "If a competitor looked at my parts, he would have a devil of a time finding a gate," says Henderson. Also, because the nozzle helps remove heat from the part faster, cycle times have shortened 10 to 12 percent, he reports. That better cooling is enhanced, says Kasner, by the fact that water is run directly through the carbide pins in the tool.

Henderson also reports that Osco and Nyloncraft significantly changed the gate pins. The original design had three guides, which were changed to full-round pins. The gear system was also beefed up by upgrading to double gearing on the pins to prevent side loading. "Osco's worked well with us on taking our feedback and making a better gate," Henderson says. Nyloncraft is in the process of upgrading several other tools to accept the Osco nozzles.