The weighing system is the core component of the open-mouth bagging machine, determining the packing accuracy and speed. It consists of the weigh hopper, load cell, and control system. The weigh hopper is connected to the feeding device and is used to load the material to be weighed. The load cell detects the weight of the material, and the control system receives electrical signals from the load cell and controls feeding based on preset target weights.
In this article, we will provide a detailed explanation of the weighing system and weighing methods used in open-mouth bag filling machines, along with their working principles and application scenarios.
Weighing Methods
There are primarily two weighing methods: net weight weighing and gross weight weighing.
1. Net weight weighing involves weighing the material in a weigh hopper before releasing it into the packaging bag. The workflow is as follows: material enters the weigh hopper via a conveying device, the control system receives signals from the load cell, and calculates the weight in the weigh hopper in real time. When the weight approaches the preset target weight, the control system gradually reduces or stops the feeding system to achieve precise measurement.
2. Gross weight weighing involves weighing the packaging bag during the filling process and does not require a weigh hopper. Before filling, first remove the weight of the empty bag. During the filling process, the weight of the bag and material is measured, and the filling stops when the preset weight is reached.
Net weight weighing offers higher accuracy since it is not affected by variations in bag weight, and it supports faster packaging speeds. However, it requires additional weigh hoppers and discharge mechanisms, making it relatively more expensive.
Components of the Weighing System
1. Weigh Hopper: A container used for weighing and batching materials.
2. Load Cell: Provides real-time weight detection and transmits data to the control system.
3. Control System: Receives and processes electrical signals from the load cell and controls feeding and discharging based on preset weight targets.
Single Hopper Weighing vs. Double Hopper Weighing
Depending on packaging requirements, a bagging machine can be equipped with one or multiple weigh hoppers, such as the VPEKS single weigher bagging machine and double weigher bagging machine.
A single weigher bagging machine has only one weigh hopper. When the material inside the hopper reaches the preset weight, the feeding stops. The next weighing and discharging cycle begins only after the material in the hopper has been released into the bag. Each cycle requires waiting for the weighing to complete before discharging, so the filling speed is slower, making it suitable for scenarios with lower output requirements.
A double weigher bagging machine has two independent weigh hoppers that can work alternately. While one weigh hopper completes weighing and prepares for discharge, the other is performing weighing. The double weighing system reduces the waiting time for weighing, offering better continuity and stability. Compared to a single weighing system, it has a higher filling speed and accuracy, but the price is also higher, making it suitable for scenarios with high output requirements.
Weigh Hopper Calibration Methods
Before machine operation, calibration of the weigh hopper is necessary to ensure weighing accuracy. Two common methods are static testing and material testing.
1. Static Testing (Calibration Using Test Weights)
- Zero Calibration: With the hopper empty, zero the real-time weight on the control panel so the display reads zero.
- Place Calibration Weights: Position known calibration weights in the weigh hopper.
- Data Comparison: Compare the calibration weights with the real-time weight readings.
- Verification: Remove the weights and check if the scale returns to zero. Repeat this process to verify accuracy at different weight points.
Static testing verifies the basic accuracy of the weighing system and is used for initial setup and regular performance checks.
2. Dynamic Testing (Calibration Using Actual Production Material)
- Zero Calibration: Zero the real-time weight with the weigh hopper empty.
- System Setup: Set the target bagging weight.
- Material Loading and Weighing: Load actual production material into the weigh hopper and proceed with discharge and bagging.
- Weight Record Comparison: After bagging, compare the recorded data against the preset target weight.
- Adjustment and Optimization: Adjust and optimize control system parameters based on comparison results to achieve the required accuracy under operating conditions.
Dynamic testing helps identify and resolve factors affecting accuracy (e.g., material flow issues) and verifies and fine-tunes precision during actual operation.