PT ZONE: The History of Blending and Dosing
The History of Blending & Dosing
In the early days of plastics processing, most processors either bought pre-colored and/or specially formulated resins for each product they produced, or they measured out color concentrate and additives using a scoop and a scale. Drum tumblers were often used to tumble mix the additive blend with virgin and regrind materials once it was manually created. Inaccuracies and inefficiencies were great, but there was no other alternative. Later, for simple mixing of regrind back into virgin material, proportioning valves were attached to vacuum loaders and receivers to simply time-load layers of regrind and virgin in very rough proportions. As the industry grew and volumes increased, processors demanded equipment that would blend and mix ingredients in larger quantities and with much, much greater accuracy.
This need resulted in the development of the volumetric blender. In this device, the materials being blended, were dispensed from their respective bins based on time and presumed a certain, consistent flow rate for each ingredient. Usually screw augers ran beneath each material bin to move the materials into a mixing chamber where all the ingredients were combined. The augers were either different diameters and/or ran at different speeds and/or ran for different durations to achieve the approximate, desired proportional mix of the materials being blended. Larger quantity materials, like virgin and regrind were often dispensed using electric or pneumatic flappers, gates or plunger valves on their bins, or were proportioned during loading.
The most common customer complaint was inaccuracy of the blend, largely because of the inaccuracy of the main material quantities and the natural “pulsing” action of the augers. The metering accuracy was dependent upon the position of each auger when metering started and stopped and undesirable vibration, common with injection molding exaggerated the errors. Accurate and even regular calibration of metering devices was required, but the calibration of volumetric devices is cumbersome, time consuming and fraught with the risk of human error.
In the higher material consumption world of continuous extrusion, solutions to some of these issues were answered with the use of “loss-in-weight” gravimetric systems. In their original forms, these systems included supply bins for each material ingredient that were suspended on load cells so that their weight could be individually monitored as they metered out each ingredient into the mix. These systems were complex and costly, but were typical for extrusion lines capable of running high volumes for long, uninterrupted periods of time and they did not suffer from the vibrations common to injection molding. This type of device was typically not suitable to injection molding due to cost, complexity, long set-up times and its intolerance for vibration.
Meanwhile, alternate methods of measuring dispensed ingredients utilizing a “gain-in-weight” principle into a single hopper on load cells were being developed along with a blender control that could monitor the load cell status and yet be unaffected by typical vibrations. The result was a range of simplified, cost-effective, easily cleaned and rugged gravimetric blenders, suitable for injection molding and virtually any process.
The affordability and ease of use of this new, simple to use, yet sophisticated “weigh scale” blender caused the use of gravimetric blenders to expand rapidly. Nearly any processor could now afford this device that provided dramatically increased control of blending and lowered operating cost through savings in colorants and other additives and the elimination of the cumbersome calibration process. Processors could now confidently blend expensive additives that were formerly pre-compounded. Processors could now add regrind without the risk of erratic melt behavior due to poor mixing. This provided dramatic cost savings and improved efficiency for plastics processors.
The initial gravimetric blenders introduced could blend up to 4 components using two slide valves and two augers feeders. More models were eventually added providing higher throughput capability and they became practical for use in the extrusion market which had previously depended exclusively on “loss-in-weight” models. Eventually, blenders were developed to dispense up to 12 ingredients including a wide variety of regrinds, virgins, powders, pellets, wood flour, reclaim and flake.
New dispense valves have been introduced as better metering devices are required to enhance dispense accuracy of a wider range of materials.
Since these new blenders initially served the quick-change injection molding marketplace, special emphasis was placed on controls with extremely easy-to-use interfaces for the typical shop worker. Early models, although packed with the computer brains for fool-proof operation, utilized easy setting inputs like thumbwheels for material quantities, allowing those with only modest understanding of the process to set-up and operate them. As blender features were enhanced, new control technology has also been developed. Color touch screen controls and new software have made blenders more versatile – allowing detailed inventory and other record-keeping features and control through network connections like Ethernet, Field Bus and even modems. The ability to read bar codes was added as a way to automatically enter recipe settings. Closed loop extrusion gauge control is also now available. The range of features available on modern loss-in-weight blenders assures their acceptance into the most technically demanding plastics processing facilities.