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Net Weigh Scales: The Heart of Packaging Profitability

October 4, 2016

Net weigh scales packaging .5- to 100-lb bags are a commonly used filler in applications across the powder and bulk solids industry. They are frequently integrated into a FFS automated bagging system. Net weigh scales can also be quite effective in a manual operation where the container may be jugs, boxes, or bags. There are several feeder options available to meet the varying products and product flow demands: servo/gravity and gravity/gravity gate, belt, vibratory, gravity/vibratory and auger feed. Simplex, duplex, triplex, and quad configurations drive dosing and filling speeds.  

Although a net weigh scale is critical for accurate and reliable dosing, it is often under-appreciated and sometimes overlooked in a packaging facility. Producers can lose sight of the importance of fill accuracy to operation profitability. Many companies continue to trust this important component of their business to scale manufacturers using outdated designs based on 1970s or 1980s weighing technology. Low accuracy means packages are intentionally over filled resulting in excessive product give-away. As technology evolves from early electro-mechanical scale designs, a new generation of net weigh scales has emerged. Important innovations in physical design, simple HMI software, and repeatable accuracy now maximize an operation’s packaging performance and profitability.

Key Design Considerations
In terms of physical design, a net weigh scale needs to be robust and built tough with extra steel reinforcement where it is needed most. It is recommended that the housing, frame, weigh bucket, and doors be constructed from heavy gauge steel that can withstand years of dead and shock loading.

Critical load points for pneumatics, door hardware, and door hinges should also be steel reinforced and all bearings sealed. Performance can be enhanced with the weigh bucket directly mounted to the weigh frame. Robust, sound construction includes precision-built components and high-quality pneumatics.

Another important feature of a net weigh scale is its ability to provide accurate and repeatable dosing. Improvement in load cell mounting to isolate the load cell from outside interferences has led to better accuracy in the scale. Fast, accurate, and repeatable weighing is accomplished through the use of platform style, single-point load cells. Weigh indicator technology also plays a key role in ensuring accuracy through the use of advanced digital filtering and very high-speed update rates that help ensure accuracy and control.

A net weigh scale can pace the entire packaging process. Therefore, the speed of the scale is critical for achieving the best overall performance and productivity in the packaging operation. There are several design aspects that contribute to a scale’s speed. These aspects can also affect accuracy, so careful consideration needs to be made with each.

The size of the feed gate will impact how quickly the material can flow through it. New generation designs feature a larger opening than previous designs and tighter part tolerances improve the gate mechanisms as well. The angles and width of the weigh bucket doors directly correlate to how fast the scale can discharge the product. The distance from the gate to the weigh bucket should be minimized to avoid a large amount of product in suspension when the gate closes and time that the product needs to travel from the gate to the weigh bucket.  

A precision-engineered wider weigh bucket with steep sides allows for the product to be processed faster through the scale. Published simplex processing speeds range from 10 to upwards of 30 weights per minute.  Rates will vary based on the type of product and package size. It is important to ask your scale manufacturer at what speeds their scale can run.  

User-friendly interfaces and well-written software are a vital function of net weigh scale longevity and productivity. A well-designed, user-friendly HMI software program should have a simple set up and program, the ability to save dozens of product recipe formats, language customization, fine tuning of weight accuracy while the scale is running, and an operator interface/screen that is easy to use and read. High-tech sensors and over-complicated software can cause issues and downtime for scales. Net weigh scales should be easy to use with software that will help to maximize speed and repeatable accuracy, as well as reduce operator training and errors.

The Core of Packaging Performance
Net weigh scales can pace the production rate, provide the accuracy that can help to control raw material and transportation costs, and if utilized correctly, can be a true profit center. Reevaluating how your net weigh scale measures up in the areas of physical design, accuracy, speed, and ease of use can contribute to measurable profitability in your overall operation.

Dian Mullis is the engineering director at Hamer-Fischbein, a packaging industry leader in large format bagging and robotic palletizing solutions. Mullis is a graduate of General Motors Institute (now Kettering University), with a double major in Mechanical Engineering and Applied Mathematics.

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Hamer-Fischbein