Novel injection molding techniques move toward commercialization.

Two relatively new injection molding techniques said to eliminate part defects and increase strength, particularly with fiber-reinforced materials, are now seeing their first commercial applications. Both techniques were developed in Europe and both have in common the use of multiple gates and injection pistons to oscillate the melt back and forth in the mold to achieve their desired orientation effects. One is the Scorim process, developed at Brunel University in England in 1982 and displayed at Interplas '90 in Birmingham under the name Multi-Live-Feed Molding, or MLFM. Equipment for Scorim is offered as a retrofit package by Scortec Inc., Gulph Mills, Pa. The second is the "push-pull" injection molding process of Klockner Ferromatik Desma of Germany (with U.S. offices in Erlanger, Ky.), which was unveiled at the K'89 show in Dusseldorf (PT, Jan. '90, pp. 70, 72; April '90, p. 94).

SHEAR-CONTROLLED ORIENTATION

The Scorim process is based on controlled shearing of the melt in the mold cavity before and during solidification. The retrofit system consists of three separate pieces of equipment: a processing head, a hydraulic package, and an electronic control unit. The processing head is mounted at the end of the injection barrel and adaptor rings. This head has two double-acting hydraulic cylinders, one for each branch of the split molten stream. Pistons attached to these cylinders provide the force that moves the molten plastic inside the mold. The unit requires about 14 in. to fit between the barrel of the injection molding machine and the mold. A separate hydraulic package provides the sequenced driving forces to actuate the hydraulic cylinders and pistons. Although the Scorim package may be operated by the existing machine control, it comes with its own control unit that sets, monitors, and controls heaters, cavity pressure, and sequencing of the hydraulic cylinders.

The processing head splits the melt into multiple feed streams, each of which is gated separately to the mold. Each feed is equipped with its own packing chamber and piston, and is capable of supplying pressure to the cavity independently of the others. The technique provides oscillating packing pressure, which is said to give added control over orientation in the mold.

In its simplest form, the process splits the melt into two identical feeds. During the molding cycle, the molten polymer is injected from the barrel into the mold through one or both piston channels in the processing head, depending on the desired program. Once the mold is filled, the programmed Scorim cycle starts, actuating the pistons in a selected sequence. The piston action first develops fluctuating melt pressure that moves and shears the melt in the cavity and gate areas. Then the pistons are moved to apply compression-decompression forces to the melt. New material is introduced to compensate for shrinkage and voids.

The control software provides extensive flexibility in piston sequencing and pressure modes during packing. Three basic options for packing with the two pistons are available: 180|degrees~ out of phase; in phase; or compressing the melt under equal constant pressure. Major benefits of the technique are said to be enhanced fiber-reinforcement effect and reduction of defects from internal weld lines. It's also said to result in more efficient mold packing and modification of the microstructure of unfilled plastic parts.

According to Dr. Kenneth Hills, CEO of Scortec, a slight increase in cycle time may occur when trying to align fibers in a very thick piece. "But if you're just trying to eliminate knit lines or weld lines, you won't detect a difference in cycle time," he says. In fact, he adds, cycle times in some cases may be shorter than with conventional molding, because better packing allows the part to be demolded quicker.

Theoretically, the only limit on the number of feeds is the complexity of the apparatus. A quadruple live-feed molding device has been developed on a two-color injection molding machine, with each injection unit providing melt to a double live-feed device. A conventional single-barreled injection machine could also accommodate four live-feeds, but requires a more complex assembly, including a crosshead. In either case, the four feeds offer the same benefits of fiber orientation but with greater freedom in possible alignment geometries than is possible with the double live-feed device.

COMMERCIAL INSTALLATIONS

Scortec claims that its process has commercial potential with a wide variety of unreinforced and reinforced thermoplastics (including liquid-crystal polymers) and thermosets. According to Dr. Hills, it also may be used successfully with shear-sensitive materials such as PVC.

Scortec's first U.S. commercial installation is at Toolmaster, a moldmaker and custom molder in Pennsauken, N.J., where a dual live-feed device is fitted to a 475-ton Stokes press. Richard Sabo, owner of Toolmaster, reports success in eliminating weld-line defects and voids, particularly on circular parts and parts with hard-to-fill areas. He adds that the process has allowed parts to be packed out harder than with conventional molding. Toolmaster is now building a test mold for a supplier of office furniture that has experienced problems with desk rollers breaking due to voids. Shortly, a dual live-feed device will be supplied to Stevens Institute of Technology in Hoboken, N.J., for research purposes. Stevens is also considering the addition of a quadruple live-feed device.

Multiple live-feed devices also have been supplied to two licensees in Japan, including a captive molder of pipe fittings and Mitsubishi Gas Chemical, the later using it for research on its resins. Licensees in Europe include a large automotive molder.

Although much of the research has focused on thermoplastics, Scortec claims that the process also has been shown to enhance the physical properties of thermosets, including glass/ polyester BMC. When molding BMC, the pistons are lined with PEEK to prevent jamming due to build-up of filler particles. A major supplier of thermoset resins reportedly is in contact with Brunel University.

Powder injection molding for production of both metal- and ceramic-matrix composites is another area that is said to benefit from the multiple live-feed process because of better control over the alignment of fibers in the solidifying mold. One implication of Scorim's ability to affect weld lines may be the elimination weld-line fractures.

Cost of a Scorim retrofit is $60,000 for the first year, and includes the hardware, controls, hydraulics, and technical assistance. Thereafter the cost is about $3000 per month for commercial use, and around $1500 per month for R&D purposes.

PUSH-PULL FOR LCPs

Klockner's so-called push-pull technique, which also is based on oscillation of the melt in the mold to improve part quality, is said to be particularly suited for liquid-crystal polymer (LCP) molding, although wider applications are foreseen. The technique uses a molding machine with twin injection units, such as are used in multicomponent or two-color work, along with software modifications to control movement of the injection units. After filling a two-gated mold, one injection screw advances while the other retracts and vice versa, creating the oscillation of melt in the cavity.

The first major application of the push-pull process took place beginning in May of this year with the molding of LCP interior window frames for the Airbus A-340 airplane. After the initial filling, orientation of the melt reportedly makes it possible to eliminate weld lines. Klockner also says its process has been shown to increase the tensile strength of LCP by 150% over that indicated by the resin supplier. At the same time, toughness of the material may be increased up to 250% of the supplier's value, due to orientations in the core layer that run parallel to the flow direction.

In addition to LCPs, other possible push-pull applications include high-temperature, high-performance thermoplastics such as PES, PPS, and PEK. Molding with conventional thermoplastics can benefit as well, such as weld-line improvement with ABS or modification of the crystallization in the interior of the mold when using semicrystalline materials. The push-pull process is also said to have potential in powder injection molding of metals and ceramics. One area of particular interest here is complicated molding geometries involving undercuts and wide variations in wall thicknesses.

Machinery cost for the push-pull process, which is performed on a dedicated press, reportedly is comparable to that of a normal two-color press.