In terms of bagging systems, a bagging scale is a piece of equipment that weighs and dispenses a specific amount of dry bulk material (powder, pellet, granule, etc.) into an open mouth bag. Depending on the application and product characteristics, the bagging scale will utilize one of four main material metering methods, use the bulk and dribble feeding technique and operate based on net-weigh or gross-weigh functionality.
Material Metering Methods for Open Mouth Bagging Scales:
The goal in determining which metering device to use in your bagging system is based on what will transfer the material most consistently. The more consistent the material flow, the more accessible it is for the controller to decide which step to take to achieve accuracy and speed. Knowing the flow characteristics of various products is invaluable when designing a project. Through its Taylor Products and Smoot brands, Magnum Systems has more than 105 years of experience, application knowledge and material handling expertise.
Using cutoff gates, gravity bagging scales use the product's flow characteristics to fill the open mouth bag. These gates close when the bag has reached the desired weight. Free-flowing granular products such as grains, fertilizer, salt and small pellets are packaged using the gravity method.
Vibratory feeding scales utilize side-to-side movement of the vibratory tray to transfer the product into the bag. These scales use varying vibratory speeds to move difficult materials or provide precise, accurate measurements of granular material. This bagging system is best suited for materials that do not flow easily or have irregular shapes, such as ground coffee, light-density grass seed or granola, which usually incorporate vibration with open mouth bag fillers.
Belt-feeding scales are used for packaging material that sticks to itself during filling. The belt feeder uses adjustable product height and speed settings to accurately deliver the product into the open mouth bag. Sweet molasses-based cattle feed, mulch and topsoil are bagged on belt feeder units.
Screw-feeding scales transfer dry materials volumetrically, either horizontally or at an incline, through the scale into a bag. A variable speed drive on the screw feeder and shutting gates on the scale allow for bulk and dribble feeding cycles to ensure accuracy. Various powders and granules, from flour ingredients to asphalt, can be metered with a screw in this bagging system.
Bulk and Dribble Feeding to Increase Accuracy:
Most open mouth bagging scales utilize the bulk and dribble feeding method. Bulk and dribble feeding follows the same principle as gravimetric feeders: filling fast to nearly the target amount and then filling slowly to achieve accuracy of the last 10-20% of the target amount. Each bagging scale utilizes two gates. The first portion of the bulk operation is filling with the gates completely open. Then, the dribble portion is achieved by partially closing the gate opening to hit the desired target fill. When using a screw-to-meter material, like the APO Dual, it uses two different-sized augers – one large and one small – to complete the same method. The large auger fills the bulk of the target fill, and the smaller auger does the target amount's dribble or “trim” filling. Now, a lot of programming goes into the controls to make all of this happen smoothly and efficiently. This is where the T4000 controllers come into play. Learn more about the T4000 here.
Net-Weigh vs. Gross-Weigh Feeding:
The net weight of a dry material during the bagging process is captured by filling a “bucket” inside the scale until the desired weight is met. The scale controls are only concerned with the weight of the material in the “bucket.” Net-weigh feeding allows for faster bagging rates, typically an increase of 1-2 bags per minute since you are doing two actions simultaneously. The next weight (volume of product being weighed) is dispensed into the bucket while a new bag is placed or held in position.
Capturing the gross weight of a dry material during the bagging process is done by zeroing out the weight of the equipment and bag placed on the discharge spout in programming. Gross-weigh feeding is typically slower than net-weigh feeding since you have to wait until the bag is filled and released from the spout before placing another bag and filling.
Within a bagging system, there are four ways dry materials are transferred when using open mouth bagging scales: gravity, vibratory, belt and screw. The bulk and dribble feeding method and programmable controllers increase accuracy. Net-weigh and gross-weigh feeding are the two scale-weighing functions. Last but not least, the product and its flow characteristics determine which method is best, and here at Magnum Systems, we have the material handling experience to guide you in the right direction to achieve your desired results.
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