Weigh Belt Overview
A weigh-belt feeder primarily consists of a short belt-type conveyor that moves the material from inlet to discharge. As material enters the feeder it passes through an inlet shear gate, forming a shallow bed of consistent cross-sectional area. During transport the material bed is weighed, and belt speed is continuously adjusted to result in a constant gravimetric discharge rate.
Advantages, Limitations and Considerations
Accuracy – Properly maintained and depending on the specific material, a weigh-belt feeder is capable of sustained high gravimetric accuracy, typically controlling feed rate to + ½% of set rate or better. (Related info)
Range – Weigh-belt feeding is typically best suited to medium to higher rate applications where an appropriate belt speed can be accurately maintained. Thus, in plastics processing operations weigh-belt feeding is most frequently found in primary resin production or larger compounding or extrusion application where it controls base resin flow.
Material Handling – Relative to loss-in-weight feeding’s innately high material handling flexibility, the range of material types appropriate to weigh-belt feeding are more limited: free-flowing (not floodable or difficult-to-handle) materials including powders, granules, pellets and the like. However, due to the weigh-belt’s adjustable and relatively unrestricted inlet area, it can handle larger particle-size materials, making it especially appropriate for use in some primary resin production applications. If needed, a weigh-belt feeder’s handling capabilities can often be enhanced through the addition of a pre-feeder, and, particularly in primary resin compounding application, a rotary valve (or rotary delumping) almost always accompanies a weigh-belt.
Material Containment – Both open- and closed-frame designs are available. However, where material containment is an issue and a closed-frame design is appropriate, periodic inspection and cleaning should be performed, as residual dust and particulates may tend to collect within the sealed feeder.
Tare – Because the material is weighed through the moving belt, accurate belt taring is critically important. In weigh-belt feeding, taring errors translate directly to errors in feed rate. Historically an intrusive and disruptive yet necessary burden, the taring operation has now become fully automated through the optional addition of a second weighing system that senses the weight of the (empty) belt just upstream of the feeder’s inlet section. In this way the belt can be automatically and continuously tared along its entire length.
Weighing – Unlike loss-in-weight feeding where a significant portion of the weighing system’s range is employed, the weighing challenge here is characterized by a nearly constant loading applied as the formed material bed passes across the weighing section. Thus, the linearity of the weighing system is not of much concern in weigh-belt feeding, but resolution and dynamic response definitely are. With typical belt speeds ranging from a few ft/min to up to 60 ft/min or so, weigh-belt feeding requires especially nimble, high-resolution weighing if it is to accurately control discharge rate. Compounding the weighing challenge is the fact that weighing is performed through the belt, and, as a result, the belt’s tension and tracking must be mechanically controlled to assure accurate material weight measurement.
Isolation – As a gravimetric device any weigh-belt feeder should be isolated from its environment to prevent ambient vibration from reaching and contaminating the weight measurement. The use of padded installation mounts and flexible inlet and discharge connections are typically sufficient measures to address this concern.
Size – Generally, compared to a complete loss-in-weight feeding system of equal range, weigh-belt feeding offers a relatively more compact installation, especially for moderate to higher rate applications.
While over the years loss-in-weight feeding may have replaced the weigh-belt in many traditional applications, the familiar and time-tested weigh-belt principle continues to serve well as a cost-competitive solution in crucial niche applications that remain outside the reach of their loss-in-weight counterparts.