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Dosing devices also known as Feeders or Metering Devices, generally provide for the introduction of a measured amount of material, typically an additive or color, to an existing flow or quantity of material, typically a base resin (virgin) and often, regrind.

Dosing at the machine throat

Dosing can be provided as an add-on to an existing flow of material on the throat of a processing machine where it is triggered or speed-governed by the processing machine itself. In injection molding, as the machine cycles, a dose, calibrated to the shot size of the molding machine is introduced into the gravity flow of the other materials. In extrusion, as the machine operates continuously, constant dosing is provided at a certain rate, in step with the speed of the extruder.

In some cases, multiple dosing devices may be connected to a single machine throat for the introduction of multiple additives, or simply to provide rapid changeover ease with redundant dosing units already installed. In other cases, materials like regrind may be dosed into the flow - in addition to - or instead of, additives. Often times, some form of mixer maybe installed below the dosing device to enhance the blending of the newly introduced additives with the base resins.

The mounting frame for dosing devices of this type is critical to assure:

The ability to support the supply hopper(s) for base resins above it, as well as the dosing unit (typically installed into the side of the support frame) with solid connection to the throat of the processing machine below it.

The ability to readily calibrate the dosing unit to accommodate the additive or material it will be metering. Calibration, is typically accomplished by a catch and weigh procedure, to align the performance of the doser to the material it is metering and the rate at which it will be metering.

The frame device must create a void in the base material flow, so that the dosed additive can readily join the flow of material and not be displaced by its flood feed to the throat.

The frame may be required to accept a mixing unit below it, without sacrificing structural integrity.

Dosing units installed onto processing machine throats may operate either: Volumetrically...by providing doses of additive in measured volumes, by a rotating auger, or a perforated rotating disk, or some other less common means, or Gravimetrically - by weighing the additive as it is metered to assure delivery of additive by weight, not volume.

Since most material recipes depend on materials being combined by weight, gravimetric dosing units are the clear choice for the highest precision and reliable dosing. Plus, they do not require calibration to align metering parameters with the delivery of additive. In many cases, gravimetric units are also the more expensive choice, but in recent years a number of new models have emerged that compete in price with the less precise volumetric units on the market.

Batch Blender

Batch blenders were originally designed to be machine mounted and to serve the needs of injection molders or to work offline, creating a supply of blended material for any type of processing. Their main advantages include low cost, simpler design, easy set-up and cleaning and high accuracy. Originally produced in volumetric configurations, batch blenders have been expanded to operate conventionally now as gravimetric units. They have found their way into many operations because of their simplicity, ruggedness and low cost. This includes very low throughput applications, once considered too minimal for sophisticated automation and even extrusion operations. In fact, some extrusion oriented models are capable of blending up to 8000 lb./hr.

The batch blender provides a metering method for all materials in the blend, but unlike continuous blenders, the batch system uses metering devices better suited to the nature and volume of each ingredient and is not reliant on a similar method for all ingredients. Typical metering devices include timed flapper or plunger-style valves, rotating augers, and even minute metering corner valves that have the ability to trickle in tiny quantities of material at highly controlled and very repeatable rates. Many batch blenders feature a modular design which allows the manufacturer, or even the processor, to freely select and change the metering devices, based on the changing needs of their particular blending task.

The dosing devices are programmed to meter one at a time into a common weighed chamber, which has the only set of load cells on the entire system. Due to advancements in batch blender controls, the traditionally sensitive load cells used on the weigh bin are able to ignore vibrations typical to injection molding by a sophisticated control logic that takes a number of weight readings and then uses its control logic to eliminate readings affected by vibration and then average the remainder for extremely accurate weight monitoring. As each is metered and deposited into the weighing chamber, its specific weight gain is sensed and calculated as a part of the desired blend of materials. Any inaccuracy is noted and the control modifies metering accordingly on the next metering cycle. Several metering cycles are typically included in each batch of blended material, so as time goes on, the blender actually becomes more accurate and self-corrects continuously throughout its operation.

The materials are then discharged from the weigh bin into a mixing chamber, which is integrated into the system. There, the blend is homogenized continuously until use, or to a specification set by the processor.