WO2006051558A1

WO2006051558A1 - A servo-synchronized screen printing machine

Info

Publication number: WO2006051558A1
Application number: PCT/IN2004/000351
Authority: WO
Inventors: Kameswaran Blasubramanian
Original assignee: Kameswaran Blasubramanian
Priority date: 2004-11-10
Filing date: 2004-11-10
Publication date: 2006-05-18

Abstract

A screen-printing machine comprising a servo-controlled motion of the frame (9) and cylinder (10) is provided. Independent drives (7, 8) for the frame (9) and the stop cylinder (10), synchronized by sophisticated controls and software are provided. By this arrangement the registration of papers occurs precisely because the screen frame (9) stops at a more accurate position. The stop cylinder (10) is also provided with a double diameter construction with two sets of grippers (18A, 18B), which are arranged at 180 degrees apart from each other. This construction is advantageous since the cylinder (10) rotates only 180 degrees for every print cycle. A dump gate (20) is also provided for separating unprinted stock. When the paper is not aligned properly the sensors (17A, 17B, 17C) present near the two sets of grippers (18A, 18B) in the cylinder (10) and near the side lays of the feedboard send a signal to the dump gate (20) through a solenoid valve and prevents the paper to be printed and thus is passed through the dump gate into a separate bin, thus minimizing paper and ink wastage.

Description

A SERVO-SYNCHRONIZED SCREEN PRINTING MACHINE

The present invention relates to a servo-synchronized screen-printing machine with a double diameter cylinder that enables precision printing with perfect register.

BACKGROUND OF THE INVENTION

The conventional cylinder screen press is a purely mechanical device driven by ^"one motor. AU actions and sequences are derived mechanically through linkages, cams, gears and racks. The more evolved stop-cylinder press, so called because the cylinder would be stationary at every cycle to help register incoming sheets better, is also built along similar lines. While cylinder screen presses are favored for their ease of use and productivity in the small and medium format (up to B2), they have their limitations that require considerable operator skill to overcome such as screen stretch adjustment that needs to be done by adjusting screen tension after imaging, substrate thickness compensation to be built in the imaging, sizes of the cylinder which are small making thick stock and heavy inking difficult. The reciprocating cylinder action calls for extra care in job set up to obtain adequate register.

In addition to these limitations, the job set up requires checking single sheet runs and adjusting settings in the machine to achieve desired result.

Conventionally, in a stop cylinder screen-printing machine, the sets of grippers in the cylinder hold a stock at the right position while the cylinder stops. Then the squeegee moves down and printing starts on the top of the cylinder. The screen frame moves and the vacuum cylinder rotates. After completion of print, the stock is released by the sets of grippers in the cylinder and the printed stock passes on to the delivery board. The cylinder continues to rotate after the stock is delivered until it reaches the initial position ready to receive the next stock for printing. Thus in a stop motion single diameter cylinder screen press the cylinder rotates 360 degrees for every print cycle resulting in less time for register. This in turn leads to frequent register errors and misaligned printing.

SUMMARY OF THE INVENTION

The invention provides a servo-synchronized screen-printing machine with a double diameter cylinder and a means for detecting and separating unprinted stock.

The screen-printing machine according to the invention comprises servo- controlled motion of frame and cylinder. Independent servo drives for frame and cylinder, synchronized by sophisticated software are provided. By this arrangement the screen frame stops at a more accurate position and registration occurs more accurately. The independent drive system allows the forward speed of the frame to be different from the return speed. This means that at a given printing speed, the forward stroke can be slower, yielding better ink transfer through the screen and the lost time can be made up by moving faster on the less critical return stroke. For special requirements printing speed can be made faster and coating speed slower or even equal.

The independent drive system allows the home position of the frame to be changed to the delivery side or the feeder side depending upon the job requirement. For jobs involving special inks, the home position can be moved from the normal delivery side to the feeder side. The independent drive system has another advantage of changing the cylinder to frame ratio to compensate for the substrate thickness, which is not possible on a conventional mechanical system. Frame- to-cylinder ratios can be set dynamically during print stroke using software to realize screen stretch compensation, enabling easy register adjustment and screen tension loss midway through a job.

The independent drive system allows correcting front register errors through software instead of adjusting the screen as done in other machines.

According to . another aspect of the invention, the stop motion cylinder is provided with a double diameter construction with double sets of grippers. In a double diameter cylinder two gripper sets are arranged 180 degrees apart from each other in a large cylinder. When the first set of gripper release a printed stock to the delivery board after the end of the print cycle, the second set of gripper is in position ready to grip the next stock for printing. Therefore, the cylinder rotates only 180 degrees for every print cycle. Thus due to its double diameter construction the cylinder remains stationary during frame return stroke yielding more time for sheet register resulting in perfect register. The diameter of the double diameter cylinder is large, that is, reduced curvature, closer to the ideal situation of flat screen-printing. Superior handling of thick substrates is also possible due to this construction.

According to another aspect of the invention, there is provided a dump gate for separating unprinted stock. A plurality of sensors present near the sets of grippers of the cylinder and in the side lays of the feedboard send a signal to the dump gate and to the squeegee if the paper is not aligned properly. The squeegee does not move down to the printing position, thus preventing paper wastage and faulty printing. The dump gate also lifts up, thus preventing the unprinted stock from moving to the next section in the line for drying. The dump gate then comes back to the normal position ready to accept the next printed stock. The papers rejected by the dump gate are collected in a bin at the bottom and can be re-fed for printing, thus minimizing paper wastage and improving productivity.

BRIEF DESCRIPTION OF THE DRAWINGS

The servo-synchronized screen-printing machine according to the invention is further illustrated with various preferred embodiments in the accompanying drawings.

Figure 1 shows a conventional screen press with a single diameter cylinder that is purely a mechanical device driven by a motor.

Figure 2 shows a servo driven double diameter stop cylinder screen press

Figure 3 shows the independent drive for frame and cylinder movement.

Figure 4 shows the double diameter cylinder construction.

Figure 5a to 5f shows the different positions of the cylinder during a complete cycle of printing.

Figure 6a & 6b shows the dump gate construction in its normal and activated position respectively. DETAILED DESCRIPTION OF THE INVENTION

Figure 1 shows the conventional screen press with a single diameter cylinder with only one set of gripper (2) provided to grip the stock for printing. The cylinder (1) rotates 360 degrees for every print cycle. Moreover the conventional screen press has a mechanically coupled frame-cylinder configuration. In mechanically coupled frame (3) and cylinder (1) linkages, cams, gears, racks etc. are operated for the action and sequencing of the screen press resulting in vibration and excessive wear of the mechanical components.

Figure 2 refers to the servo driven double diameter stop cylinder screen press. In this embodiment, the stop motion cylinder (10) is provided with a double diameter configuration with twin sets of grippers (18A, 18B). In this configuration, the set of grippers (18B) is arranged 180 degrees apart from the set of grippers (18A) in a large cylinder. The screen-printing machine according to this invention is driven by servomotors (7, 8) that independently drive the frame (9) and the cylinder (10). This independent drives for frame (9) and cylinder (10) is synchronized by sophisticated software and controls (30).

Figure 3 clearly shows the independent drive for the motion of the frame and the cylinder.

A first servomotor (7) actuates a first drive mechanism (14), which in turn rotates the cylinder (10) in order to facilitate the grippers of the stock for printing and simultaneously to release the stock after printing. A second servomotor (8) actuates a second drive mechanism (14'), said second drive mechanism (14') rotates the pinion (15) thereby causing the rack and the frame (9) to reciprocate horizontally. The first and second drive mechanisms (14, 14') are preferably a timer belt and pulley, chain drive, gear drive or any other conventional mechanism known to a person skilled in the art. The scope of the drive mechanism (14, 14') that can be employed is not to be limited by only the above-mentioned mechanisms. Thus, this independent drive for the motion of the frame (9) and the cylinder (10) allows the forward speed and return speed to be synchronized by the control according to the printing requirement. The cylinder (10) is provided with two sets of grippers (18 A, 18B) spaced at 180 degrees from each other. Thus, when the first set of grippers (18A) releases a printed stock to the delivery board after the end of the print cycle, the second set of grippers (18B) is in position ready to grip the next stock for printing. Instead of the conventional 360 degrees of rotation for every print cycle, the construction according to this invention requires only 180 degrees of rotation of the cylinder (10) for every print cycle thus considerably increasing the efficiency and productivity of the machine.

Figure 4 shows the constructional features of the sets of grippers (18 A, 18B) that are spaced 180 degrees apart from each other.

Figures 5 a to 5f shows each position of the cylinder (10), frame (9) and the other related components. In the initial position, as shown in figure 5a, the squeegee (16) is in the home position and the first set of grippers (18A) is ready for gripping the paper. In figure 5b, the paper (19) is conveyed from a receiving board and flows into the first set of grippers (18A) to be gripped for printing. The squeegee (16) now moves down to its printing position and normal printing starts. The screen frame (9) now moves horizontally for printing. In figure (5c) and (5d) the paper (19) is gripped by the first set of grippers (18A) and the squeegee is in the printing position thereby starting the printing process. In the process of printing, the frame (9) moves from the feeder side to the delivery side thereby completing the printing. On completion of one cycle of printing, the frame (9) moves back to the delivery side and the squeegee (16) moves back to its home position while the second set of grippers (18B) is now ready to accept the stock from the feeder side as shown in figures 5e. The same cycle now continues till the paper is printed and delivered to the delivery board. Thus, for every 180 degrees movement of the cylinder, one stock gets printed. Figure 5f shows the home position of the cylinder (10) after 360 degrees rotation of the cylinder (10), that is, after two complete printing cycles.

Moreover, if the alignment of the paper (19) is improper, it is detected by sensors (17 A, 17B, 17C) provided adjacent to the sets of grippers in the cylinder (10) and in the side lays of the feedboard. When the sensors (17A, 17B, 17C) detect improper alignment of paper, the squeegee (16) does not move down to the printing position and thereby prevents wastage and faulty printing of the paper The pneumatic cylinder in the dump gate (20) gets the signal from the sensors through the solenoid valve (21) and lifts up the dump gate (20) to allow the paper to fall down into a separate bin.

As shown in figures 6a and 6b, when improper alignment is detected by the plurality of sensors (17A, 17B, 17C), the printing does not occur for that cycle and the faulty paper is prevented form moving to the next section in the line for drying by a dump gate (20). The dump gate (20) gets the signal from the plurality of sensors (17A, 17B, 17C) and lifts up to allow the paper to fall down into a separate bin. The dump gate (20) comes back to its normal position to accept the next printed stock. Thus wastage of both paper and ink are minimized. The papers rejected by the dump gate (20) are collected in the bin and can be re-fed for printing.

Claims

CLAIMS:

1. A screen-printing machine comprising a feeder for precise sheet handling, a feed board for delivering scratch free sheet for printing, a pneumatically clamped squeegee (16), a delivery board for receiving printed stock, a screen printing frame (9), a stop cylinder (10) for registering incoming sheets, characterized in that the said screen printing frame (9) and the said stop cylinder (10) are driven by independently controlled servomotors (7, 8) synchronized by sophisticated controls (30) to precisely register the incoming sheets, said stop cylinder (10) is provided with a double diameter construction with a first and a second sets of grippers (18A, 18B) such that the second set of grippers (18B) is positioned to grip the incoming sheet when the first set of grippers (18A) is positioned to release the printed sheet to the delivery board.

2. The screen-printing machine as claimed in claim 1 wherein the first and the second sets of grippers (18 A, 18B) are located at 180 degrees from each other.

3. The screen-printing machine as claimed in claim 1, wherein a first servo motor (7) is provided to actuate a first drive mechanism (14) said first drive mechanism is provided to rotate the said stop cylinder (10) to facilitate the grippers of the stock for printing and simultaneously to release the stock after printing; a second servomotor (8) is provided to actuate a second drive mechanism (14'), said second drive mechanism (14') is provided to rotate the pinion (15), and wherein the said pinion (15) is adapted to horizontally reciprocate the rack and the frame (9).

4. The screen-printing machine as claimed in claim 3, wherein the said first drive mechanism (14) and second drive mechanism (14') are chosen from a timer belt pulley, a chain drive or a gear drive.

5. The screen-printing machine as claimed in claim 1 wherein a dump gate (20) is provided to prevent misaligned papers from passing to the subsequent section in the printing process.

6. The screen-printing machine as claimed in claim 1, wherein a plurality of sensors (17A, 17B, 17C) is provided adjacent to the sets of grippers and side lays of the said feedboard to detect stock that is misaligned and said sensors (17A, 17B, 17C) are configured to convey the signal to the dump gate (20).

7. The screen-printing machine as claimed in claim 5 wherein a solenoid valve (21) is provided with the dump gate (20) to receive the signal from the plurality of sensors (17A, 17B₅ 17C) and a pneumatic cylinder is activated by the said solenoid valve (21) and wherein the said pneumatic cylinder is provided to lift the said dump gate to allow the sheet to fall into a separate bin.

8. The screen-printing machine as claimed in claim 6 wherein a solenoid valve (21) is provided with the dump gate (20) to receive the signal from the plurality of sensors (17A₅ 17B₅ 17C) and a pneumatic cylinder is activated by the said solenoid valve (21) and wherein the said pneumatic cylinder is provided to lift the said dump gate to allow the sheet to fall into a separate bin.