High-speed/low-pressure molding minimizes internal part stresses.
Each version of these so-called "low-pressure/high-speed" injection systems differs from the others somewhat in its details, but all are based on the same premise of combining small and large pairs of hydraulic cylinders to offer molders more flexibility in controlling speed and pressure during the injection process.
Each system offers a choice of four to eight different hydraulic circuits, and some offer the possibility of switching from one circuit to another during a cycle. These circuits allocate different proportions of the machine's available hydraulic capacity to injection speed or pressure. The result is a range of possibilities from high speed/low pressure to low speed/high pressure, with several increments in between. A conventional machine would require different screw sizes to achieve the same results. Note also that different stages of maximum injection pressure can be achieved without changing system hydraulic pressure.
Suppliers say this flexibility is helpful in molding complex parts such as electrical connectors, as well as thin-walled and large, flat parts such as paper trays and syringe barrels. In particular, injecting at low pressure is reportedly less likely to damage pins and other delicate components in a connector mold. It's also said to help prevent flash, sinks, warpage, and burning.
Although the claimed ability to accomplish high filling speeds at low pressures may seem paradoxical, it makes sense when you consider the shear-thinning nature of most plastics. Injection at high speed creates maximum shear, which reduces the melt viscosity and thus the pressure required to inject.
Of course, the ability to fill at high speed and low pressure depends on the mold design and material. One supplier recommends starting by selecting the injection mode with the lowest pressure and highest speed. If the mold does not fill completely under these conditions, step up to the next alternative mode with higher pressure and lower speed until a combination is found that fills the mold.
Multi-mode injection is offered as an option on specific machine models from each supplier. It typically adds 10-20% to the price of the machine.
TOSHIBA BINARY METHOD
Toshiba's "binary" injection unit has a pair of dual-diameter injection cylinders with a common piston shaft extending through the larger and smaller cylinder of each pair. This system offers four "modes" of injection with four levels of speed and pressure over a range of 1:4 and 4:1, respectively. The differing modes are achieved by varying the volume of oil that is available for each function. The first mode is essentially the same as a standard injection unit. Oil is pumped into one side of the unit and displaced oil in front of the piston flows freely back to the tank; maximum injection rate is based on the hydraulic pump capacity.
The remaining three binary modes divert oil from in front of one cylinder to the back side of another cylinder through a regenerative circuit, which creates added oil capacity to develop extra injection speed. Injection-pressure capacity drops proportionately.
Although it's necessary to stay in one mode during the filling stage, it's possible to switch to another mode during the packing phase, says Toshiba v.p. Tim Glassburn. "For example, you can go from 400% filling speed under low pressure to fill the mold, then switch back to a mode where you've got high pressure at low speed for the final packout."
Glassburn says binary injection offers a less expensive alternative to adding an accumulator or extra pump to achieve similar effects. The binary option is available on Toshiba's F and G series machines; the four-stage binary unit is available on machines up to 250 tons, larger presses take a two-stage binary unit. Toshiba also offers a special "Super Repeatable Barrier" (SRB) screw that's said to enhance melt uniformity and aid filling at low pressure.
NIIGATA'S MIRS SYSTEM
Niigata offers its so-called MIRS (Multiple Injection Rate System) as an option on its NN-MI series injection presses from 35 to 500 tons. MIRS uses two sets of different-sized injection cylinders. The smaller-diameter piston uses both the head and rod end for the regenerative (or differential) circuit. Any one of five injection pressure/speed combinations over a 1:5 and 5:1 range can be achieved by selecting different injection modes with different hydraulic circuits.
Niigata's MIRS system differs from those of Toshiba or Toyo in its use of an accumulator. This reportedly allows the press to achieve much higher injection speeds than either low-pressure machines without an accumulator or standard machines with an accumulator. National sales manager Robert Columbus says a single mode setting is normally used throughout the filling and packing cycle, though mode switching is possible if needed.
The MIRS system is said to eliminate the usual cavity-pressure peak during filling. Because tilling pressure is kept to the necessary minimum, flashing and internal stresses are reportedly eliminated.
TOYO'S M8 SYSTEM
Toyo's Plastar G2 series presses (90-550 tons) are available with a new multi-mode injection option. The Multi-Eight (M8) option has a central main injection cylinder with regenerative (differential) pressure circuit and two kinds of outboard pistons. Selection of various configurations provides a total of eight modes of injection. The first mode provides the standard performance of the company's G2 series. Mode 5 provides the same injection-speed capability as a G2 press with accumulator (twice that of a standard press) but with half the injection-pressure capability. Mode 8 provides almost three times the speed capability of an accumulator (5.8 times greater than a standard press) with one-fifth the pressure capability. Injection modes can be switched during a cycle, if needed.
NOT A PANACEA
All three companies market these high-speed/low-pressure machines as niche products for particular applications. Both Columbus of Niigata and Bob Wheeler, Maruka U.S.A. sales manager, caution that these presses are not a panacea for molding problems.
To avoid raising false hopes in customers, Wheeler approaches high-speed/low-pressure molding on a case-by-case basis, analyzing the part and runner system before quoting the machine. One possible limitation, he says, is with certain types of small-diameter gating systems. "We're finding that if molders have small tunnel gates or subgates, they can't get away with using low pressure."
Wheeler also sees the best applications for the M8 system as those using easy-flow materials such as PS, PP, and PE. Some engineering materials, he says, should be analyzed carefully because they may require more pressure to fill a mold. However, Columbus sees no problem with small gates or engineering materials, including ABS. Toshiba's Glassburn says his firm's binary system has been successful with commodity resins, PET, PBT, SAN, nylon, acrylic, and acetal, including glass- and mineral-filled compounds.
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|Author:||De Gaspari, John|
|Date:||Oct 1, 1994|
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