Unlike conventional hot-air and desiccant systems, the Maguire. Low Pressure Dryer™ (LPD™) uses vacuum to draw moisture from pellets, requiring up to one-sixth of the time needed by desiccant dryers to properly dry resin, cutting energy consumption by up to 80%, and decreasing the heat history imposed on the polymer. The short drying time of the machine, combined with its three-station indexing system, makes it possible to run much smaller batches than conventional dryers and still meet required throughput rates. Smaller batches eliminate need to pre-dry resin prior to Monday-morning startup of the molding machine or extruder. By planning ahead, it is possible to change colors without wasting material or stopping production.
 
About twenty thousand Maguire. gravimetric blenders, trade named Weigh Scale Blenders, have been installed around the world since their introduction in 1989. They are designed to be affordable even for small processing operations. Models range from low feed-rate units (60 to 400 lb/hr, or 30 to 180 kg/hr) for injection molding to high-rate machines (up to 5000 lb/hr, or 2270 kg/hr) for extrusion and compounding. The units can be designed to blend up to twelve components.
 
The accuracy of Weigh Scale Blenders has enabled processors to eliminate over coloring and reduce consumption of costly additives. The equipment is self-calibrating. No calculations are required; once the operator sets the desired percentages of color, additive, and regrind, the microprocessor controller automatically adjusts material dispenses so that blend accuracy over time stays within 0.1%.
 
Weigh Scale Blenders use a batch process and weigh each ingredient separately to control use of colors and additives:
 
In blender models with throughputs rated at 450 lb/hr ( 200 kg/hr) or higher, resin and regrind are sequentially dispensed from hopper bins into a weigh chamber by high-speed slide-gate valves with an accuracy per dispense of +/- 4 grams.
Alternatively, in the Micro-Blender™ model (maximum throughput of 140 lb/hr (60 kg/hr) and batch size of only 400 grams) dispensing is accomplished by means of unique vertical pneumatic valves mounted inside the hopper bins (all of which are removable on this model).
Small-percentage (5% or less) ingredients like colors and additives are sequentially metered into the chamber in amounts calculated as percentages of natural to compensate for color already present in regrind. Metering is accomplished either through auger feeding or by means of vertical-valve technology, which allows larger blenders to be outfitted with removable minor-ingredient hopper bins. For applications requiring exceedingly high metering precision, the Micro-Pulse™ pneumatic slide-gate system makes possible dispenses as small as 2 or 3 pellets.
The batch falls into a mixing chamber, then is fed into the processing machine. In subsequent batches, small corrections are made automatically.
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Many competitive gravimetric systems use a continuous loss-in-weight process, in which all ingredients are auger-fed simultaneously into the weigh system. Variable-speed DC motors, each with its own speed control, drive the augers, which continuously dispense ingredients. Weigh Scale Blenders use a combination of timed dispense valves and AC fixed-speed motors. This system is simpler and less costly, and the range of adjustment is much greater. Mix ratios can be varied from 0.01% to 100% with simple thumbwheel settings.
 
Continuous loss-in-weight blenders use a different set of load cells to weigh each ingredient, so that a shift in the calibration of any one cell affects the entire mix ratio. In the Weigh Scale Blender there are only two load cells, and because these are used for all ingredients, target mix ratio is maintained. Since the cells handle smaller loads, they are more sensitive to metering errors, detecting weights as low as 0.025 gram (1 pellet).
 
Important for injection molding is Maguire's system of vibration and shock management, which enables Weigh Scale Blenders to maintain accuracy when mounted on fast-cycling presses. Because these blenders verify weight readings at high speed, they are able to take accurate readings during even the shortest molding cycles