Electrostatic Coating Backing Roll for High-Speed Web Uniformity
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Solution Overview
Problem
Conventional electrostatic-assisted coating methods face limitations in increasing coating speed due to air layers on both the front-side and rear-side surfaces of the web, leading to non-uniformity and web instability, with existing technologies unable to effectively apply sufficient voltage without causing spark discharge or short-circuit currents.
Innovation Solution
A three-layer structure backing roll with a high-insulating ceramic outermost layer, conductive internal electrode layer, and insulating layer is used, allowing application of up to 6 kV DC voltage safely, ensuring the web adheres to the roll and eliminating air layers through electrostatic forces, preventing spark discharge and short-circuit currents.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If high voltage is applied to the backing roll to eliminate air layers and improve coating uniformity, then coating speed and uniformity are improved, but spark discharge and short-circuit currents occur causing safety issues
Solution Approach 1:
An insulating layer is introduced as an intermediary between the internal electrode and the external environment. This layer has controlled electrical resistance that allows it to block high-voltage spark discharge and short-circuit currents while still permitting the necessary electrostatic field to act on the web for coating uniformity. The insulating layer thus mediates between the conflicting requirements of high voltage application and safety.
Solution Approach 2:
The electrical resistance parameter of the insulating layer is specifically controlled to achieve the desired balance. By adjusting the resistance value, the system allows sufficient electrostatic field penetration for coating uniformity while preventing harmful spark discharge and short-circuit currents, thus changing the electrical parameters to resolve the contradiction.
2Productivity
If web speed is increased to improve productivity, then coating speed increases, but air layers on front-side and rear-side surfaces cause non-uniformity and web instability
Solution Approach 1:
The harmful air layers are actively removed or eliminated from between the web and the coating liquid. By applying electrostatic force through the high-voltage backing roll, the air layers are extracted or pushed away, preventing them from causing non-uniformity even at high web speeds, thus enabling high productivity without sacrificing coating quality.
3Manufacturing precision
If electrostatic force is applied to attract coating liquid to the web, then coating uniformity improves, but the backing roll structure becomes complex requiring multiple insulating layers and internal electrodes
Solution Approach 1:
The backing roll is segmented into distinct functional layers: an internal electrode layer for generating the electrostatic field, an insulating layer for safety and field control, and an outer surface layer for web contact. This segmentation allows each layer to perform its specific function efficiently, achieving coating uniformity through the electrostatic field while managing the structural complexity through clear functional division.
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
The method enables high-speed coating with uniformity and stability, allowing web conveyance speeds of 400 m/minute or more without device damage or web instability, while maintaining safety against spark discharge and short-circuit currents.
Implementation Method 1
electrostatic force is caused to act on a contact line portion (hereinafter, coating point) where the coating liquid comes into contact with the web
Implementation Method 2
causing the web to pass through a coating point while causing the second surface of the web to adhere to a part of a surface of the backing roll to which a DC voltage is applied and which is rotating by means of an electrostatic field
Data Source
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AI summary
There is provided a coating method for a web having a wide width, and the coating method enables high-speed safe coating and is excellent in uniformity of the coating thickness. A method of coating a flexible plastic web with a coating liquid includes conveying the web to a backing roll for coating, causing the web to pass through a coating point while causing a second surface of the web to adhere to a part of a surface of the backing roll to which a DC voltage is applied and which is rotating by means of an electrostatic field of the backing roll; and attracting, at the coating point, the coating liquid by electrostatic force generated due to configuration of charges of the same polarity as that of the DC voltage applied and coating the first surface of the web with the coating liquid. The backing roll includes an outermost layer, an internal electrode layer, and an insulating layer in this order, the backing roll is configured to enable application of a predetermined voltage to the internal electrode layer, and the outermost layer is a ceramic material layer having a volume-specific resistance value of 107 Ωcm to 1013 Ωcm at a temperature of 25°C to 100°C.