Melt-flow oscillation improves part properties.

A relatively new process uses oscillating melt flow to change a resin's microstructure and improve its mechanical properties. Solomat Partners, Stamford, Conn., says its developmental "Rheomolding" process can greatly improve mechanical properties of a wide range of crystalline and amorphous materials in injection molding, extrusion, and blow molding.

Rheomolding is one of a number of apparently related techniques based on oscillation of the melt that have emerged recently. Others include the "push-pull" injection molding technique developed by Klockner Ferromatik Desma of Germany (now Ferromatik Milacron) and the Scorim "multi-live feed" injection molding process developed in England and available for licensing from Scortec Inc., Gulph Mills, Pa. (see PT, Oct. '92, p. 17).

Nonetheless, Solomat's president, J.P. Ibar, claims to hold the seminal patent and several subsequent patents on the use of melt-oscillation technology. The company reportedly has signed development agreements with several resin suppliers, molders, and machinery builders, and Ibar expects to license the technology within the next several months.

HOW IT WORKS

Rheomolding is based on the concept that a material's rheology is a function of vibration frequency and amplitude, as well as temperature and pressure. In general, the rate of cooling a material from liquid to solid state has an important effect on the material's morphology, structure, and properties of the solid, explains Ibar. Plastic materials, because of their large heat capacity and poor conductivity, do not favor conventional heat-transfer techniques. Rheomolding overcomes this difficulty, says Ibar, by introducing another factor--control of internal friction (i.e., shear and hydrostatic pressure) control--to simulate cooling rates that are beyond the values obtainable by conductive cooling, allowing improved orientation effects to take place.

"Internal friction control" is produced by the reciprocating action of one or more melt-accumulator pistons adjacent to the melt-flow path. "There is a tendency for |the effects of~ temperature and vibration to be identical," Ibar explains. "Mechanically we simulate the effects of a high cooling rate so that the material is acting as if we were operating at a lower temperature. This can be used to alter and improve the extent of orientation in amorphous plastics and the morphology (crystal size and number) in the case of semi-crystalline plastics (PE, PP, etc.). These parameters are known to be responsible for stiffness, strength, and impact resistance."

The Rheomolding package consists of both hardware and software components and is adaptable to injection presses, extruders, and even blow molding machines, says Ibar. (Most work so far has been on injection molding.) The hardware consists of one or more piston-type melt accumulators located adjacent to the melt stream after the screw and barrel. These pistons reciprocate, moving the melt back and forth, at a low frequency (from 1 to 60 Hz) and high amplitude. Software, which is in on-going development, controls vibration pressure and frequency during cooling to achieve the desired effects. Each software program is resin-specific and can be supplied in a ROM (read-only memory) cartridge that can be inserted into the controller of the processing equipment.

Rheomolding hardware for injection molding machines is available in two versions. One, supplied as an add-on device that fits between the barrel of the injection press and the mold, is a steel housing containing the pistons, which reciprocate as plastic is injected through the housing into the mold. Alternatively, says Ibar, the mold may be modified to incorporate the pistons directly into the cavity to impart changes exactly where they are needed, such as at knit lines. Both versions require pressure and temperature sensors in the mold, but no other machine modifications are needed.

Versions of Rheomolding hard ware have also been developed for extruders and blow molding machines. In extruders, melt oscillation takes place in several zones located after a crosshead die. Oscillation occurs at different frequencies appropriate to the changing melt temperature of the extrudate as it passes from one zone to another. A typical configuration may include an initial VBS (vibro-barrel system) zone, in which low-amplitude vibration prepares the melt for better flow, followed by an RCU (rheo-cooling unit), which imparts a high degree of orientation in the extruded product.

In blow molding, pistons incorporated into the wall of the mold vibrate the air as the mold closes around the parison, says Ibar.

MANY POTENTIAL BENEFITS

Ibar claims that Rheomolding offers potentially large improvements in appearance, structural strength, and dimensional stability of injection molded parts. For example, by orienting and vibrating material from side to side in the mold cavity, Rheomolding reportedly forces the flow fronts to intermingle at the molecular level, eliminating knit lines. At the same time, the effect of shear vibrational forces on morphology (i.e., the number, size, and orientation of the crystals) is said to increase part strength, which opens the possibility of substituting less expensive materials.

In addition, Rheomolding is said to reduce warpage and birefringence (in transparent parts) by mechanically controlling shrinkage and internal stresses. Ibar, for example, claims success in molding CDs from inexpensive polystyrene without problems of birefringence.

Other potential applications include compatibilizing polymer blends--by affecting the dispersion of one phase in another and the size of the phase domains--with implications both for recycling and masterbatching. In recycling, Rheomolding reportedly can boost the mechanical properties of the recycled material. In masterbatching, Rheomolding may be capable of duplicating the effect of additives such as nucleating agents.

So far, Solomat has concentrated on applying the Rheomolding process to injection molding PS, PP, and PE, though research is being expanded to include polycarbonate and PET. Lab tests of Rheomolded parts molded in a standard ASTM mold have shown vastly improved mechanical properties, says Ibar. For example, Rheomolded PS samples showed improvements in elastic modulus of up to 120% and tensile strength of 90% or better when compared with normally molded, unoriented PS. Rheomolded PP samples have also shown higher stiffness, strength, hardness, and creep resistance by Rheomolding's control of crystallinity and orientation.