Design for Improving UV Inkjet Printing Quality (Part ①)(Part ②)(Part ③)(Part ④)(Part ⑤)
Design for Improving UV Inkjet Printing Quality (Part ①)
When using a multi-pass scanning printing method, a UV-curable inkjet printer and its printing method can improve printing strength.
A UV (ultraviolet) curable inkjet printer performs multiple scans over the printing medium using an inkjet printhead to eject UV ink. During the scanning process, the UV ink is irradiated by a UV curing lamp and solidifies simultaneously.
The inkjet unit 7 (KCMY inkjetting) performs multiple scans on the printing medium, ejecting four-color UV inks (KCMY) to form a color image.
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The UV1 curing device is arranged behind the inkjet unit in the X scanning direction and irradiates UV light to partially cure (semi-cure) the UV ink ejected from inkjet unit 7.
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The UV2 curing lamp device is arranged in front of the inkjet unit in the Y direction (perpendicular to the X scanning direction), relative to the movement of the inkjet unit. It fully cures the UV ink that has been semi-cured by the UV1 device.
In this UV-curable inkjet printer design:
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The inkjet unit 7 ejects UV ink in multiple rows.
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It performs multiple passes on the same row, layering UV ink during printing.
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Since the UV2 device is positioned in front of the inkjet unit 7 in the Y direction, during the entire period before the completion of UV ink ejection, UV1 performs semi-curing on the scanned lines, while UV2 does not fully cure the UV ink immediately.
As a result, the layered ink ejected by inkjet unit 7 remains in a semi-cured state.
In other words:
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The lower layer of UV ink is not fully cured and therefore mixes and fuses with the upper layer of UV ink.
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The semi-cured UV ink has a higher viscosity than uncured UV ink, but still maintains sufficient fluidity to blend with subsequently deposited upper-layer ink.
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The surface unevenness of the semi-cured UV ink is reduced and leveled.
Afterward, the UV2 curing lamp irradiates the entire printed image after all scanning by inkjet unit 7 is completed. This allows the layered UV ink to be fully cured in a mixed and integrated state.
Therefore, when using a multi-pass scanning method, the overall printing strength is improved.
The inkjet unit 8 is positioned:
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In front of inkjet unit 7 in the Y direction
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And either in front of the UV2 curing lamp in the X direction or behind the UV2 curing lamp in the Y direction
It is used to eject UV ink.
With this UV inkjet printer:
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Inkjet unit 8 is arranged ahead of inkjet unit 7 in the Y direction.
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After inkjet unit 7 completes its multi-pass scanning, inkjet unit 8 can deposit additional ink layers on top of the printed coating from unit 7.
Thus:
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The print layer formed by inkjet unit 8 can protect the print layer formed by inkjet unit 7.
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When printing on transparent or semi-transparent media, the coating printed by inkjet unit 7 can be visually enhanced.
Since inkjet unit 8 is positioned either before the UV2 curing lamp in the X direction or behind it in the Y direction, the UV2 curing lamp can fully cure both:
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The ink layer printed by inkjet unit 7
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The ink layer printed by inkjet unit 8
simultaneously.
In this system:
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Inkjet unit 7 ejects UV ink (KCMY) to print the image.
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Inkjet unit 8 ejects UV ink (Vr) to print a base layer.
The base layer printed by inkjet unit 8 is layered on top of the image printed by inkjet unit 7. When printing on transparent or semi-transparent media, this structure enhances the vividness of the image.
The intensity of UV light can be adjusted by modifying the irradiation intensity of the UV1 and UV2 curing devices, thereby changing the UV energy.
Since UV energy is proportional to light intensity, adjusting the intensity of UV1 and UV2 changes the corresponding UV energy output.
On the other hand, UV energy can also be adjusted by changing the movement speed of UV1 and UV2:
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Changing speed alters irradiation time
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UV energy is inversely proportional to the UV curing movement speed
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UV energy is directly proportional to UV irradiation time
Therefore, by adjusting the UV curing speed, the curing time of ink on printed products can be controlled, enabling precise management of UV curing energy and printing quality.
Design for Improving UV Inkjet Printing Quality (Part ②)
A UV inkjet printer is used to print color images on a printing medium 1 made of transparent or semi-transparent materials such as polycarbonate that is transported onto a platform. Specifically, the UV inkjet printer prints color images on the medium 1 using UV ink, and further enhances the visual vividness of the color image by overlaying a varnish layer (Vr ink) on top of the image (white ink or other spot colors may also be used as needed).
Moreover, the UV inkjet printer adopts a multi-pass scanning method. The printing medium 1 is transported in the Y direction (transport direction), and the same line is scanned multiple times to complete printing. Specifically, the color image is printed using four passes (4-pass), followed by one additional pass (1-pass) to apply the overlay layer.
Media Transport System
The UV inkjet printer is equipped with a transport roller 2 that conveys the printing medium 1 in the transport direction Y. The transport roller 2 moves the printing medium 1 line by line along the Y direction.
Guide Rail and Inkjet Mechanism
Above the platform, a guide rail mechanism 5 is installed, which is perpendicular to the media transport direction. The inkjet mechanism 6 is mounted on the guide rail 5 and can move along the extension direction of the rail.
The inkjet mechanism 6 is driven to move in both the X scanning direction and the opposite direction. This movement is achieved by a motor-driven belt transmission system. The rear end of the guide rail 5 in the X scanning direction serves as the standby position of the inkjet mechanism 6.
Inkjet Module Configuration
The inkjet mechanism 6 includes:
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Image inkjet unit 7
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Overlay inkjet unit 8
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Semi-curing lamp UV1
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Full-curing lamp UV2 (UV LED)
Image Printing Unit (Unit 7)
The image inkjet unit 7 ejects multiple colors of UV ink to print color images on the printing medium 1.
Specifically, the printheads in unit 7 respectively eject:
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Black ink (K)
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Cyan ink (C)
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Magenta ink (M)
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Yellow ink (Y)
Each printhead is connected to a corresponding ink cartridge storing its respective color, supplying ink to the KCMY printheads.
The image is printed using four passes (4-pass), forming a print length equivalent to four lines in the transport direction Y.
Accordingly, in logical printing, the inkjet unit 7 is divided into four scan regions along the Y direction:
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First pass region: P1
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Second pass region: P2
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Third pass region: P3
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Fourth pass region: P4
Overlay Printing Unit (Unit 8)
The inkjet unit 8 ejects Vr ink to form an overlay layer on top of the color image printed on medium 1.
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It is positioned in front of the image inkjet unit 7 along the transport direction Y
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It is spaced one line apart from unit 7
The inkjet unit 8 is connected to an ink cartridge containing Vr ink, which supplies the overlay ink for printing.
The overlay layer is printed in one pass (1-pass), forming a single line in the Y direction.
Since this pass is performed after the four passes of unit 7, it corresponds to the fifth scan region:
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Fifth pass region: P5
UV Curing System
1. Semi-Curing Unit (UV1)
The semi-curing UV lamp UV1 is positioned behind the image inkjet unit 7 in the X scanning direction.
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When the inkjet mechanism 6 moves in the X scanning direction, UV1 follows behind unit 7
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When moving in the opposite direction, UV1 is positioned ahead of the motion direction
During scanning, UV1 performs semi-curing of the UV ink deposited by unit 7 on the medium 1.
2. Full-Curing Unit (UV2)
The full-curing UV lamp UV2 is positioned:
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In front of the image inkjet unit 7 along the transport direction Y
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Behind the overlay inkjet unit 8 in the X scanning direction
When the inkjet mechanism 6 moves:
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In the scanning direction, UV2 is located in front of unit 7 (Y direction) and behind unit 8 (movement direction)
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In the reverse direction, UV2 is located in front of unit 7 (Y direction) and ahead of unit 8
During operation, UV2 performs full UV curing on:
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The image printed by inkjet unit 7
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The overlay layer printed by inkjet unit 8
This coordinated multi-pass printing and staged curing design ensures improved ink fusion, enhanced layer bonding, and significantly better print quality and durability.
Design for Improving UV Inkjet Printing Quality – Part ③
The control unit sends electrical signals to the transport roller 2, inkjet mechanism 6, inkjet print units 7 and 8, the semi-curing UV1, and the full-curing UV2 for control purposes. Accordingly, the control unit functions as the print media transport control section, the inkjet carriage movement control section, the inkjet timing control section, and the UV curing control section. Additionally, the control unit includes the main computer components such as CPU, ROM, RAM, and PLC.
The print media control section manages the drive of the transport roller 2, moving the print media 1 on the platform along the transport direction Y, advancing by one row spacing per scan.
The inkjet carriage movement control section manages the drive of the inkjet mechanism 6, moving it at a fixed speed along both the scanning direction and the opposite direction.
The inkjet timing control section manages the ink ejection of print units 7 and 8. Inkjet unit 7 ejects black (K), cyan (C), magenta (M), and yellow (Y) UV inks, while inkjet unit 8 ejects Vr clear UV ink. Furthermore, inkjet control for unit 7 is executed separately for each scan region P1–P4.
The UV curing control section regulates the semi-curing UV1 and the full-curing UV2, adjusting the intensity of ultraviolet irradiation so that both UV1 and UV2 reach the required energy levels. UV light intensity can be modified by adjusting the electrical current supplied to UV1 and UV2. When a steady current is applied, the light intensity changes according to the current value; when a pulsed current is applied, the light intensity can be adjusted by modifying the pulse width and pulse count.
Moreover, UV ink (monomer) has the property of curing via polymerization upon UV exposure. Before irradiation, the UV ink is a low-viscosity liquid with a small molecular weight. When exposed to approximately 30 mJ/cm² of UV energy, the molecular weight increases and viscosity rises, resulting in a semi-cured state. The semi-cured state refers to a condition where the liquid exhibits higher viscosity than the uncured state but remains capable of mixing with subsequently deposited uncured UV ink layers. Upon exposure to approximately 200 mJ/cm² of UV energy, the molecular weight increases further, achieving a fully cured state.
Design for Improving UV Inkjet Printing Quality (Part ④)
The control unit issues commands to drive the transport roller 2, conveying the printing medium 1 placed on the platform along the Y transport direction. When the printing medium 1 reaches the initial print position (the starting position for each scan movement), the following printing control is executed.
In addition, during each scan, the inkjet timing control unit controls the inkjet units 7 and 8 to perform printing operations in areas P1–P5.
1. Printing Control of Inkjet Unit 7 (First to Fourth Pass)
First, the control unit drives the inkjet mechanism 6 and issues a command to move it at a constant speed along the X scanning direction. Accordingly, the inkjet mechanism 6 moves at a constant speed along the X direction.
During the printing control of the first to fourth scans, the control unit issues commands to the inkjet unit 7 to eject KCMY inks.
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First pass:
The control unit performs inkjet control in area P1. When the inkjet unit reaches the designated jetting position, a jetting command is sent, and the inkjet unit ejects KCMY inks at that position. -
Second to fourth passes:
Similarly, the control unit performs inkjet control for areas P2–P4. When the inkjet unit reaches the corresponding jetting positions, jetting commands are issued, and KCMY inks are ejected accordingly.
As a result, the inkjet unit 7 ejects KCMY inks at the designated positions, forming a color image on the printing medium.
2. Semi-Curing Control by UV1
At the same time, the control unit issues the following command:
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The semi-curing UV lamp UV1 irradiates at a predefined first light intensity, controlling the semi-curing energy.
The UV energy is defined as the product of light intensity and irradiation time:
UV Energy = Light Intensity × Irradiation Time
The first light intensity is defined such that, when the inkjet mechanism 6 moves at a constant speed, the accumulated UV energy irradiated onto the UV ink is 30 mJ/cm².
Additionally, from the start to the end of UV irradiation, an offset time or offset distance is set according to the position and size of the printed image.
That is:
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After the inkjet unit 7 receives the jetting command, UV1 simultaneously receives the irradiation command
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The offset time or distance is adjusted to ensure that UV light fully covers the printed image area on the medium
During this process:
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When the UV ink monomer is exposed to a cumulative UV energy of 30 mJ/cm²,
→ its molecular weight increases
→ viscosity rises
→ the ink reaches a semi-cured state
3. Printing Control of Inkjet Unit 8 (Fifth Pass)
During the fifth pass printing control, the control unit issues commands to the inkjet unit 8.
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The control unit controls the inkjet unit 8 in area P5
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When the inkjet unit reaches the designated jetting position, it prints on top of the image already formed by the first to fourth passes
Thus:
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The inkjet unit 8 ejects Vr ink at the fifth scan position
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The Vr ink forms a layer on top of the color image printed on the medium
4. Full Curing Control by UV2
Next, the control unit issues a command for the full curing lamp UV2 to irradiate:
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KCMY inks printed by inkjet unit 7 in areas P1–P4
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Vr ink printed by inkjet unit 8 in area P5
UV2 applies a second UV energy level for complete curing.
To achieve full curing, the control unit sets:
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The UV2 irradiation intensity to the second light intensity
The second light intensity is defined such that, when the inkjet mechanism moves at a constant speed, the accumulated UV energy is:
200 mJ/cm²
When the UV ink is exposed to this cumulative energy:
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Molecular weight increases further
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The ink reaches a fully cured state
5. Return and Medium Feeding Process
After completing the above steps:
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The control unit drives the inkjet mechanism 6 to move at a constant speed in the direction opposite to the X scanning direction, returning it to the standby position.
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The control unit drives the transport roller 2 to advance the printing medium 1 by one line in the Y direction.
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The printing medium is positioned for the next scan.
6. Printing Loop Control
The control unit then determines whether the printing process is complete:
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If printing is not complete → the process returns to the initial step and repeats the above operations
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If printing is complete → the control unit terminates the printing process
Design for Improving UV Inkjet Printing Quality – Part ⑤
The control unit drives the inkjet mechanism 6 along the X scanning direction, while inkjet unit 7 scans row 1a of the printing medium 1. Inkjet unit 7’s area P1 deposits UV ink ① to print the first layer of the color image on row 1a (first scan). Then, UV1 irradiates the first light intensity of ultraviolet light, causing UV ink ① to become semi-cured.
The printing medium 1 is conveyed one line along the Y transport direction. As inkjet mechanism 6 moves along the X scanning direction, inkjet unit 7 scans rows 1a and 1b of the printing medium 1. Accordingly, inkjet unit 7’s area P1 deposits UV ink ① to print the first layer of the color image on row 1b (first scan). Meanwhile, inkjet unit 7’s area P2 deposits UV ink ② to print the second layer of the color image on row 1a (second scan). In other words, on row 1a, the second layer of UV ink ② is overlaid on the first semi-cured layer of UV ink ①. Then, UV1 irradiates the first light intensity of ultraviolet light to semi-cure both UV inks ① and ②.
The printing medium 1 is conveyed one line along the Y direction, and as inkjet mechanism 6 moves along the X scanning direction, inkjet unit 7 scans rows 1a, 1b, and 1c of the printing medium 1. Consequently, area P1 of inkjet unit 7 deposits UV ink ① to print the first layer on row 1c (first scan); area P2 deposits UV ink ② to print the second layer on row 1b (second scan); and area P3 deposits UV ink ③ to print the third layer on row 1a (third scan). That is, on row 1b, the second layer of UV ink ② is overlaid on the first semi-cured layer of UV ink ①, and on row 1a, the third layer of UV ink ③ is overlaid on the second semi-cured layer of UV ink ②. UV1 then irradiates the first light intensity of ultraviolet light to semi-cure UV inks ①, ②, and ③.
The printing medium 1 is conveyed one line along the Y direction, and as inkjet mechanism 6 moves along the X scanning direction, inkjet unit 7 scans rows 1a, 1b, 1c, and 1d. Area P1 of inkjet unit 7 deposits UV ink ① to print the first layer on row 1d (first scan); area P2 deposits UV ink ② on row 1c (second scan); area P3 deposits UV ink ③ on row 1b (third scan); and area P4 deposits UV ink ④ on row 1a (fourth scan). Thus, on row 1c, the second layer of UV ink ② overlays the first semi-cured UV ink ①; on row 1b, the third layer of UV ink ③ overlays the second semi-cured UV ink ②; on row 1a, the fourth layer of UV ink ④ overlays the third semi-cured UV ink ③. UV1 irradiates the first light intensity of ultraviolet light, semi-curing UV inks ①, ②, ③, and ④.
The printing medium 1 is conveyed two lines along the Y direction, and as inkjet mechanism 6 moves along the X scanning direction, inkjet unit 7 scans rows 1c, 1d, 1e, and 1f, while inkjet unit 8 scans row 1a. UV inks ①–④ are printed on rows 1c–1f as layers 1–4 and are semi-cured. Meanwhile, row 1a is positioned in front of inkjet unit 7 in the Y transport direction. At this point, inkjet unit 7’s areas P1–P4 have printed UV inks ①–④ to form the color image. Inkjet unit 8’s area P5 deposits UV ink ⑤ to print the fifth base layer on row 1a (fifth scan), thereby overlaying the previously printed color image. Then, UV2 irradiates the second light intensity of ultraviolet light, fully curing all UV inks ①–⑤ on row 1a. That is, UV inks ①–④ deposited by inkjet unit 7 and UV ink ⑤ deposited by inkjet unit 8 are completely cured.