Modern Machines for Decorating 3-D Products
Read on to find out more about the latest technologies available for this screen-printing specialty.
High-quality, direct-screen-printed images add appeal to glass or plastic containers and bottles, as well as a variety of tubular products. For those who market beverages, cosmetics, and personal-care products, maximum shelf impact is critical, and well-executed decorations are an ideal way to attract attention to their goods. Print buyers in these markets look for images that feature more colors, higher resolutions, and very accurate color-to-color registration (Figure 1). In many cases, they also want to use bottles and containers with sophisticated shapes.
Satisfying these demands poses a continuous challenge for screen printers, especially with the large run lengths that such jobs frequently entail. But manufacturers of screen-printing equipment have answered by developing advanced, new machines for printing on three-dimensional items. These systems enable screen printers to excel in their competitive markets and meet their customers' needs.
In this article, we'll look at the latest generation of screen-printing equipment for containers and other 3-D products. The discussion highlights two families of machinery: mechanically driven presses for printing at very high production speeds and servo-based units for printing onto a wide range of complex-shaped containers. We'll consider the basic functions of these devices and explore the innovative features that can be found in both types of equipment.
Mechanically driven presses
The latest improvements to container and 3-D screen-printing machines were helped by advances in mesh materials, mesh weaving, coating materials, squeegees, and inks—especially UV inks. These developments make it possible to print at high speeds without losing fine details and without significant wear to screens and squeegees that would necessitate frequent machine stops to replace these components.
Mechanical drives allow for the highest production speeds. They are rigid and durable enough to handle the demand. Machines with either one- or two-up operation allow for production speeds up to 100 items/min or 160 items/min, respectively (Figure 2). Mechanical drives resist wear and ensure a long machine life cycle, but machines equipped with these drives are designed for specific types of products and support a limited range of product shapes.
Improved productivity results not only from the faster operating speeds of these machines, but also from enhanced engineering that eliminated process steps, such as re-registering items in each printing station to achieve accurate color-to-color registration. In this situation, rotary indexing tables are used to hold an item in its fixture while the item is moved through the machine. To maintain tight control of the angular position of each fixture on the indexing table, the fixtures remain in positive contact with the main drive gear. As a result, the positions of the fixtures are known at all times, which makes it unnecessary to re-register items in the printing stations. The time saved in re-registering leads to shorter printing cycles and increased production speed. Only the orientation of the item is adjusted for correct image positioning, and this adjustment does not affect throughput, even if it is performed in a separate station (Figure 3).
Servo systems use a set of parameters to simulate gear boxes or make servomotors follow electronic CAM profiles. Servo systems can quickly accept new parameters that change positioning, speeds, CAM profiles, and other operating characteristics. Servo-based multicolor screen-printing machines have a rotary indexing table and individual servomotors for each printhead, each screen, and each fixture. A motion controller synchronizes and controls all print movements.
Servo-based screen-printing machines provide new options for screen printing intricate images onto bottles and containers with sophisticated shapes in one machine pass. During printing, the movements of screens, squeegees, and fixtures in the printing stations depend on the shapes of the items. For cylindrical items, the fixtures rotate and screens move horizontally. For oval-shaped items, the fixtures rotate, screens move horizontally, and, at the same time, the printheads with screen carriages and squeegees move vertically (Figures 4A and 4B). For square items, the fixtures and screens are stationary, and the squeegees move horizontally.
When individual servomotors drive the printing mechanisms, then the print movements in each printing station are independent of the movements in the other stations. This means it's possible to print onto differently shaped surfaces at different printing stations. For example, during a single pass through the machine, a D-shaped bottle can be printed on its round front in one printing station and then on its flat back in the next printing station.
Container and bottle shapes are becoming more complex. In addition to simple shapes such as round, square, and oval, there are combinations of shapes and subtle transitions from one shape to another. Printing onto these types of products is a challenge because the horizontal speed of the screen has to be synchronized with the surface speed of the rotating bottle during printing. Presses for decorating such products feature motion controllers that use geometric data about the product's cross-section to control and synchronize press movements during the print cycle.
If the bottle is slightly conical, the press operator has to select a cross-section within the printing area. The shape of this cross-section will be used to calculate the surface speed, and this speed will be synchronized with the horizontal speed of the screen. The choice is a compromise because only the surface speed of a printing area with the selected cross-section will be equal to the speed of the horizontally moving screen. The surface speed of a printing area with a larger cross-section than the one selected will be faster than the speed of the screen, while the surface speed of a printing area with a smaller cross-section will be slower than the speed of the screen.
When the decoration has two or more images, a printer can optimize print movements for each image by selecting different cross-sections—one for each area—and can still print the images in one pass through the machine. An example of this is the decoration of wine bottles, where the job may require printing onto the cylindrical bodies and the conical shoulders.
Printing on conical items can be made easier on a machine with a rotary indexing table. Here, each screen is driven by its servomotor, and each fixture has its own two servomotors—one to rotate the fixture and one to tilt the fixture. This makes it possible to print at different conical angles, at different printing stations, during one machine cycle. Systems with this capability can save significant time when printing two images at different heights on a conical item where the conical angle varies with height, such as on a coffee pot (Figure 5). While an item is moved to the next printing station, the control system tilts the fixture holding the item to a different angle. On such presses, angular positioning is very precise and repeatable. The motion controller will adjust screen movement and fixture rotation in each printing station to the diameter of the selected cross section.
Higher efficiency reduces costs
Presses for containers and other 3-D parts achieve higher efficiency by reducing the time needed for machine setup and routine maintenance. Many of the improvements made to these machines have resulted from close cooperation between machine manufacturers and screen printers. By observing actual production environments, the machine manufacturers can identify opportunities for streamlining the printing process. As an example, consider the downtime that occurs when a press is stopped so that glass fragments can be removed after a bottle has broken in the machine. By designing equipment that prevents glass fragments from falling into the machine, manufacturers make it possible to quickly remove the glass and continue production.
The use of high-quality components also increases the efficiency of modern container presses. Reliable components reduce the need for maintenance and the likelihood of stoppages caused by component failure. Machines that are designed to quickly return to production speed after a stoppage further increase productivity.
Highly motorized, servo-based screen-printing machines can accommodate automatic format changeovers and provide pre-set parameters to reduce setup times. Innovative solutions are now available to help reduce the time it takes to set up mechanically driven machines as well.
An excenter drive for the screen carriages of a press with rotary indexing table reduces setup times and gives screen printers another feature that improves the press's printing capabilities. Screen carriages of a multicolor high-speed machine are usually driven by gear trains that are linked to the fixtures' rotations. A gear wheel mechanically synchronizes the horizontal speed of its associated screen carriage with the surface speed of the rotating item. When the print job requires a machine setup for a different diameter, then this gear wheel has to be changed at each printing station to be used for the print job.
But now, an excenter drive on a screen-printing machine with a rotary indexing table makes it possible to change the movements of the screen carriages at all printing stations with a single adjustment of the drive (Figure 6). This reduces the time required for changing, saves gear wheels, and removes the limitations of fixed gear ratios. The drive can be adjusted for different item diameters in increments of 0.004 in. If necessary, adjustments can even be made to slightly stretch or compress the printed image.
When the excenter drive is controlled by a servomotor, then the drive can actuate either the screen carriages or the squeegee heads. The screen carriages move when printing onto cylindrical containers; the squeegees move when printing onto square bottles or onto flat sides of oval bottles.
Servomotors significantly shorten changeover times. Each servomotor electronically receives the parameters for the next product, each fixture is moved to the correct angle, and no gear change is required at the printing stations.
The sophistication of servo drives is not visible to operators. A servo-based screen-printing machine is controlled by an industrial programmable controller that is linked to a touchscreen. The screen provides a graphical user interface and intuitive user guidance. During machine setup for a new job, an operator can input item dimensions at the touchscreen or the system can read data from a specifically formatted CAD file. The control system automatically adjusts the print movements for the new item; changing of gear wheels is not required.
When a screen-printing machine needs to be set up for a repeat job, a microprocessor-controlled press with touchscreen and job-storage memory can eliminate the need to again go through a setup process. Selecting a cross-section of an item for optimized print movements and inputting the cross-section data are only required once for a print job. The machine settings can be stored, and the stored settings are then available for recall and automatic machine pre-setting when a repeat job is scheduled.
Downtime caused by machine failure can be reduced when a screen-printing machine has a remote diagnostic link that enables the manufacturer to check equipment status and assist troubleshooting on site. Mechanically driven machines may have a control system with programmable controller and a microprocessor, which can be interrogated via a remote link so that manufacturer can view the error log and assist in troubleshooting when an error has occurred. Servo-based machines make the remote-diagnostics service offered by manufacturers more effective because, in contrast to mechanical drives, servo drives can monitor their own performance. The status of the machine and each drive can be checked.
A servo-driven system also can monitor machine performance. A deterioration in performance caused by a mechanical problem—a gradual seizing of a bearing, slippage in gearing or a coupling, or a loose belt—can be identified by the associated servo drive. The servo drive will signal an error message to the operator, and the operator or maintenance staff can address the problem before it causes extensive and unscheduled downtime. If a servo drive fails, the servo drive system signals an error message that assists the maintenance staff in troubleshooting. Servo drives, in combination with appropriate sensors, also can be self-adjusting, which reduces time spent on setup and maintenance.
Increasing print quality
Today's screen-printing machines produce outstanding print quality with multicolor images. This capability is the result of improvements in machine design, UV inks, and mesh material, as well as enhancements in prepress, such as raster image processing (RIP) and color separation specifically for screen printing.
UV inks and finer meshes enable printing with higher resolution. RIPs that take account of the capabilities of the screen-printing process produce dot patterns that allow the printing of color gradients with virtually invisible tonal transitions.
The maximum number of colors that can be printed in one machine pass is usually six to eight, which gives more options in the design of a decoration. Tight control of the item position in the machine makes it possible to keep color-to-color registration accurate to within ±0.004 in., depending on the manufacturing tolerances of the part.
Manufacturing tolerances for glass containers and bottles are usually such that color-registration accuracy is within 0.008 in. Cylindrical glass bottles and jars can have uneven surfaces, relatively high tolerances in diameter, and a cross-section that can deviate slightly from a perfect circle. Presses equipped with a squeegee-pressure-control system with balanced squeegees can offset the negative effect that these manufacturing tolerances can have on print quality. Such systems maintain constant squeegee pressure during printing around the circumference, even when an item shape or an uneven surface could cause a change in squeegee pressure. These systems also simplify the setting of equal squeegee pressure in all printing stations, and the balanced squeegees extend the operating life of screens.
Modern inks and modern screen-printing machines ensure a uniform ink deposit. Ink formulation allows the ink to flow through the mesh and then even out quickly once the mesh has lifted from the substrate. Vibrations from the press that could affect print quality are prevented through sturdy machine construction and design features such as a sinusoidal screen-carriage acceleration and deceleration, which provides smooth reversal of movement direction. Some systems are even able to print wraparound images with a virtually invisible overprint using UV inks. Such machines use a special UV-curing system that surface cures the leading edge of an image so that it can be overprinted with the trailing edge.
New, high-speed presses for printing with UV inks require curing systems with high UV output power and high UV light intensity focused on the UV-curable ink. At a production speed of 90 cycles per minute, the drying period is reduced to approximately six milliseconds. UV output power of up to 1250 W/in. is required to ensure thorough curing of each layer of UV ink at maximum production speed.
These power requirements would stress a UV lamp and reduce its operating life, but it's possible to provide high output power only during the curing period and reduce the output power of lamps during the part transport period—when the machine is stationary, the UV lamps are switched to stand-by mode to reduce power consumption.
Focusing on quality printing
Modern container-decorating equipment can ensure the competitiveness of the screen-printing process. Advances in the mechanical and electronic capabilities of these systems enable screen printers to directly print images of superior quality onto a wide range of materials, including those designed with challenging surfaces and shapes. These enhancements also allow screen shops to focus their attention where it belongs—on printing.
About the author
Harald Gavin earned a BSc in electronics and mathematics from the University of Hamburg, Germany and an MSc in control engineering from the Cranfield Institute of Technology, England. He has worked in web-offset printing, control-system design, contract engineering, and software. Gavin founded Path 2 Print Ltd. in 2000 as a marketing company for ISIMAT screen-printing machines in the UK and Ireland.