Capacitive Ink Level Sensor for Rotary Screen Printing
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Solution Overview
Problem
Cylinder screen printing machines face challenges in precise ink dosing due to limited access and monitoring of the ink fill level, leading to ink wastage and quality issues, as existing sensors are contaminated by the ink and cannot reliably measure the level within the cylindrical mold.
Innovation Solution
A capacitive sensor is integrated directly with the squeegee to measure the ink fill level, regulating the ink supply by adjusting the amount of ink based on measured changes, using a control loop with a microprocessor or PLC for precise control, and optionally employing multiple sensors and individual ink supplies for each squeegee section.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If optical, mechanical or ultrasonic sensors are used to measure ink fill level, then measurement capability is provided, but the sensors are quickly blinded by ink contamination and cannot reliably measure
Solution Approach 1:
The patent replaces optical, mechanical, and ultrasonic sensing systems with a capacitive sensing system. The capacitive sensor measures ink fill level through electrical field interaction without physical contact with the ink, eliminating contamination issues while maintaining measurement capability.
Solution Approach 2:
The capacitive sensor uses an electrical field as an intermediary to detect ink level. The sensor measures changes in capacitance caused by the dielectric properties of ink versus air, allowing indirect measurement without direct sensor-ink contact that would cause contamination.
2Device complexity
If manual monitoring of ink fill level is used, then device complexity is minimized, but ink dosing precision deteriorates leading to wastage
Solution Approach 1:
The patent implements a feedback control system where the capacitive sensor continuously monitors ink fill level and sends signals to the pump control. The system automatically adjusts pump operation based on real-time level data, maintaining precise ink dosing without complex manual intervention.
Solution Approach 2:
The system enables self-regulating ink level control where the capacitive sensor and pump work together autonomously. The sensor detects level changes and the pump automatically replenishes ink, creating a self-service mechanism that eliminates manual monitoring while ensuring precise dosing.
3Reliability
If more ink is fed to prevent screen emptying, then print quality is maintained, but ink wastage increases due to drying and disposal
Solution Approach 1:
The capacitive sensor provides real-time feedback on actual ink availability in the screen, allowing the system to maintain optimal ink levels rather than over-filling. This precise control ensures print quality is maintained while minimizing excess ink that would otherwise dry or be disposed of.
Solution Approach 2:
The system performs preliminary ink replenishment based on predictive capacitive measurements before the screen actually runs empty. This proactive approach maintains consistent print quality by preventing screen emptying while avoiding the need to over-fill, thereby reducing ink wastage.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution enables precise and automatic control of ink dosing, reducing wastage and ensuring consistent print quality by accurately monitoring and adjusting the ink level, suitable for various printing methods beyond banknote printing.
Implementation Method 1
a capacitive sensor measures the fill level of the amount of ink on the squeegee
Implementation Method 2
a time-variable voltage signal is applied to the capacitive sensor between two electrodes and the capacitive coupling of the time-variable voltage signal is measured at a third electrode. If the dielectric constant on the measuring surface of the sensor changes as a result of an external effect
Data Source
Figure 1~2
Figure 3
AI summary
The invention relates to a screen printing machine with a cylindrical rotary screen (1), a squeegee (2) arranged in the rotary screen (1), and a feed and metering device for printing ink, wherein the squeegee (2) rests against the inside of the rotary screen (1) and applies the printing ink to the inside of the rotary screen (1). According to the invention, a capacitive sensor measures the ink level (5) at the squeegee and, depending on the measured ink level (5) at the squeegee, supplies the feed and metering device with less, more, or a constant amount of printing ink to the squeegee (2). Thus, according to the invention, the ink level at the squeegee is measured by means of a capacitive sensor, and when the ink level at the squeegee changes, the ink supply to the squeegee is regulated and adjusted accordingly. The invention further relates to a corresponding screen printing process.