Shear Controlled Orientation in Injection Moulding (SCORIM)
This process was developed in the Wolfson Centre at Brunel University (U.K.) in early 1980's, offers potential benefits to the injection moulding process through controlling the microstructure of moulded materials. This process is the most widely known of the melt manipulation processes and SCORIM technology is now owned and offered for license by Cinpres Gas Injection. The SCORIM process manipulates the polymer melt inside the mould cavity itself. Hydraulic pistons operating in various modes shear the melt during the holding time of the injection moulding process. These piston movements cause shearing at the solid/melt interface producing alignment of fillers, fibres and polymer molecules themselves. The SCORIM process offers improvements in stiffness and strength and controls quality by alignment of filler, fibres and macromolecules. The process offers benefits of moulding to closer tolerances while improving dimensional stability and reproducibility. SCORIM also leads to elimination of microcracks and voids; internal weld marks and sink marks. The SCORIM process takes the "cocktail-shaker" approach of oscillating the melt within the mold cavity. In this case, though, the aim of sloshing the melt back and forth is not mixing but creating stronger, defect-free parts. The process is used to mold products ranging from eyeglass frames to piston rings and VCR remote-control covers.
The process ran short of commercial projects though was widely known. Then, in the late 1990s, Cinpres revamped the auxiliary-equipment package eliminating the need to modify the molding machine which established the process for enhancing strength, and reducing defects including weld lines, sink marks and microcracks. While the former SCORIM equipment housed two auxiliary melt-oscillation cylinders mounted on the end of the injection barrel. This was not very flexible and called for extensive machine modifications. On the other hand, the new SCORIM II equipment fits within a relatively compact hot-half assembly with an easy attachment and removal from the machine's fixed platen. Like the old system, SCORM II also depends on the coordinated action of two hydraulic pistons to oscillate the melt except that these pistons are installed on the fixed platen rather than on the end of the machine barrel. The cylinders can be oriented either vertically or horizontally, depending on the machine design and available space. The improved auxiliary equipment for the SCORIM does not require the molder to commit a modified machine to a single process, thereby eliminating the risk. Within SCORIM II's expanded hot-half assembly, a heated manifold splits the melt flow from the machine nozzle into two streams. The two melt streams then travel through independently balanced hot-runner systems and enter the mold cavity through separate gates, which are typically located at opposite ends of the cavity. By moving out of phase, the two pistons push each melt stream through the cavity as well as the individual gates and runner systems. The start of the oscillation typically coincides with the beginning of the hold cycle. But there are instances where it could begin with the cavity only 50% full.
Besides oscillation, SCORIM has two succeeding operational modes. Post melt oscillation, SCORIM can move the two pistons back and forth in concert, creating compression and decompression effects in the cavity. This movement ensures maximum and consistent packing while the polymer is still in a live state and the material is continuing to freeze in sequential layers toward the center of the part. Finally, the two pistons can be moved to their forward position for some additional compression as the part continues to freeze completely. The system is controlled by a standard hydraulic drive unit and a programmable controller capable of storing up to 16 oscillation/packing profiles. Sloshing the melt back and forth through cavity and gate, together with the subsequent Scorim-aided packing, helps eliminate some common problems affecting both reinforced and unreinforced materials. In reinforced plastics, the process improves the strength through modifying both weld lines and fiber orientation. The shear forces experienced by the unfrozen polymer layers against the previously frozen layers reinforce the fibers in the direction of the flow. Experts say that the process keeps the gates filled with molten polymer and thus keeps them open longer than in conventional molding. The open gate, in turn, allows more material to enter the mold at higher pressures, eliminating defects that arise from underpacking. As a result, SCORIM along with increasing the mechanical properties and structural performance also eradicates the defects due to underpacking.
On the down side, experts have noted that SCORIM process favours materials including polystyrene and ABS over polycarbonate as far as aesthetic consideration is concerned. However, mold temperature or other set-up variables also play an important role in this context. Scaling up is another major issue with the process. On large parts with large wall-surface area, the SCORIM equipment will find it difficult to move a large volume of material before it freezes. The process exhibits potential to expand its materials portfolio with applications in liquid-crystal polymers (LCPs), glass-filled nylons, and reinforced thermosets. |
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