Hybrid Sheet Hold-Down Using Vacuum and Electrostatic Tacking
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Solution Overview
Problem
Current sheet hold-down technologies in direct printing processes, such as mechanical grippers, vacuum systems, and electrostatic systems, face challenges in maintaining a consistent and flat print head to media gap, leading to potential contact issues and image quality degradation, especially when dealing with varying sheet lengths and widths.
Innovation Solution
A hybrid system combining a vacuum plenum for flattening the sheet body and electrostatic tacking rollers for securing the edges, which applies a negative pressure and electrostatic charge to maintain the sheet in a flat orientation without excessive complexity or cost, using a nonconductive belt with apertures and tacking devices positioned to align with the sheet edges.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Shape
If vacuum systems are used to hold down sheets, then sheet flatness is improved, but system complexity and cost increase due to high vacuum requirements
Solution Approach 1:
The hold-down function is segmented between the vacuum system (handling sheet body) and electrostatic tacking rollers (handling sheet edges). This segmentation allows the vacuum system to operate at lower pressure levels while electrostatic forces provide additional edge hold-down, reducing overall system complexity and cost.
Solution Approach 2:
Different hold-down mechanisms are applied to different regions of the sheet: vacuum pressure is applied to the sheet body while electrostatic forces are applied specifically to the sheet edges. This local differentiation optimizes the hold-down effect while reducing the vacuum pressure requirements.
2Force
If high vacuum pressure is applied to hold sheet edges, then edge hold-down is improved, but system complexity increases to accommodate varying media widths
Solution Approach 1:
The mechanical vacuum hold-down system is supplemented with an electrostatic hold-down system that uses electrical fields instead of mechanical pressure. The electrostatic tacking rollers can be positioned to accommodate different sheet widths without requiring complex mechanical adjustments to the vacuum system.
3Force
If electrostatic charge is applied across the entire sheet, then sheet hold-down is improved, but ink droplet deflection occurs degrading image quality
Solution Approach 1:
The electrostatic charge is applied locally only to the sheet edges through the tacking rollers, rather than across the entire sheet. This localized application provides sufficient edge hold-down while avoiding charge accumulation in the printing zone that would deflect ink droplets and degrade image quality.
Solution Approach 2:
The hold-down function is segmented spatially: electrostatic forces are applied only at the sheet edges while the sheet body is held down by vacuum. This segmentation prevents electrostatic interference with the printing process while maintaining effective edge hold-down.
4Reliability
If mechanical grippers are used to hold sheet edges, then edge hold-down is reliable, but device complexity and cost increase
Solution Approach 1:
The mechanical gripper system is replaced with an electrostatic hold-down system using charged rollers. This substitution maintains reliable edge hold-down through electrostatic attraction while eliminating the mechanical complexity of gripper mechanisms, making the system more suitable for varying sheet lengths and widths.
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 hybrid system effectively maintains a consistent print head to media gap, preventing contact and ensuring high-quality images by using a modest vacuum level and electrostatic charge to secure the edges, reducing drag and wear on the transport system while accommodating sheets of different widths.
Implementation Method 1
A vacuum plenum has a surface disposed below the belt and is operably connected to a vacuum source. The vacuum plenum is adapted to applying a negative pressure to the media for holding the media to the belt.
Implementation Method 2
The first and second tacking rollers impart an electrostatic charge to the edges of the media for electrostatically securing the inboard and outboard edges of the media to the belt.
Data Source
AI summary
A media sheet transport including a belt for supporting the media thereon. The belt is operably connected to a drive mechanism for moving the belt in a process direction past an image marking unit. The belt has a plurality of openings therein. A vacuum plenum has a surface disposed below the belt and is operably connected to a vacuum source. The vacuum plenum is adapted to applying a negative pressure to the media for holding the media to the belt. An electrostatic hold down apparatus includes a first tacking roller spaced in a cross-process direction from a second tacking roller. The first and second tacking rollers are engagable with the belt. The first tacking roller is disposed to engage the inboard edge of the media, and the second tacking roller is disposed to engage the outboard edge of the media. The first and second tacking rollers impart an electrostatic charge to the edges of the media for electrostatically securing the inboard and outboard edges of the media to the belt.