Here's a novel approach to the rapidly developing field of micromolding. It's a variation on two-stage injection - but with an in-line screw and plunger combination that delivers shot weights from 0.1 to 4.5 grams. The plasticating cylinder and plunger are joined together into one reciprocating member. The plunger is actually an extension of the injection nozzle, which extends into the mold and hot-runner system. Further, the hot-runner system is an integral part of the molding machine. In what is said to be a cost-saving arrangement, most of the runner-less hot half, the mold base, and the stationary platen are one and the same.

This patented system, called "Direct Injection Molding or D-I-M, comes from Ettlinger Kunststoff-maschinen GmbH in Konigsbrunn, Germany. The model ET 10/16 1 is a compact system with a tiebarless, 11-ton clamp and total footprint of about 6 x 2 ft x 5 ft high.

The latest development in the D-I-M technology is a vertical press with vertical injector, measuring around 5 x 2.7 x 8.8 ft high (with mold open). It allows plenty of room for automation, so it can be integrated into assembly lines, Ettlinger says. The company delivered the first two vertical units, both for coinjection to a German automotive molder as a beta test.

Homogeneous Melt
D-I-M technology is designed for high-precision injection and very consistent melt quality, says Ettlinger. Melt is more consistent because the 16-mm plasticating screw does not reciprocate within the barrel, so the entire shot "sees" the same screw L/D (20:1). Unlike some conventional two-stage systems, the in-line arrangement is said to give true first-in/first-out performance without risk of melt hang-up and inconsistent residence times.

The heart of the D-I-M technology is its plunger, which is, in effect, an extended nozzle with a non-return valve on its end. The entire plasticating unit moves forward to inject melt into the cavity. Injection stroke (up to 2 in.) is measured with a linear transducer.

The plunger tip is a check valve that permits "extremely precise dosing of the smallest shot volumes with very good repeatability and minimum dwell times," according to Ettlinger. The hollow plunger can be of 0.2, 0.3, or 0.4 in. diam. for shot capacities of 1.6, 3.0, and 4.7 cc. Injection-pressure range is 14,510 to 36,275 psi.

The shooting pot, which is located inside the platen, feeds directly into two, four, or eight valve gates of 0.02-0.06 in. diam. The valve pins (whose actuating cylinders are built into the platen) prevent any leakage of melt while the shooting pot is being filled by the screw.