Discover how the latest developments in part-handling, registration, and programmable-control technology are helping to position screen printing as a leading glass-decorating method.
By Peter Herman
Flat-glass decorating is one of the most diverse application areas for screen printing. But it's also one of the most obscure because of the variety of products that fall under the flat-glass category and the wide range of markets that glass printers serve. Nevertheless, the demand for decorated flat glass continues to grow with the continual introduction of new architectural, industrial, fine-art, and promotional applications. For a great number of these applications, screen printing has emerged as the ideal decoration process.
Screen-printed flat glass is everywhere. You'll find it used in shower doors, architectural partitions and building treatments, automotive windshields, and appliance panels. It's also used in furniture, on gaming equipment (slot machines, pinball machines, etc.), and on countless decorative and artistic products for the home.
Screen printing may be employed in a number of ways to create the final design on glass products. Sometimes it's used to directly print images and colors on glass. In other applications, it is used to apply temporary resistant coatings to non-image areas of a glass surface, masking off specific areas so that they remain unaffected by subsequent acid-etching or sand-blasting processes. Alternately, screen printing may be used to directly apply inks that mimic the appearance of acid-etched or sand-blasted glass. Some applications even turn to screen printing to apply more functional coatings, such as the thin bands of conductive material that make up window defrosters.
Screen printing in the flat-glass market
Besides the variety of decorating options possible with screen printing, another factor that has made the process so popular for flat-glass decorating is its flexibility to handle a wide range of glass-panel sizes and weights. Additionally, screen printing has proven itself as a leading reproduction method for oversized graphics applications, so it naturally appeals to the glass-decorating industry, where requests for large-format flat-glass work have risen sharply in recent years. In fact, the demand for screen printing in large-format flat-glass applications has become so great that printing-equipment manufacturers are adapting their press technology especially for these types of jobs. The result is some of the world's most sophisticated equipment for high-volume rigid-panel printing.
Further fueling the demand for screen printing in the flat-glass decorating market is the introduction of a wide assortment of screen-printing inks designed specifically for glass. Glass inks are available in both opaque and translucent colors that provide a variety of surface finishes after drying or curing. Glass screen-printing inks give additional flexibility to the process by coming in a broad selection of chemistries, including UV inks, solvent-based epoxy and polyurethane inks, and specialty inks, such as electroluminescent and conductive formulations. The coatings may feature pigments and even colorants that incorporate glass frit for products that will be fired in a kiln or lehr after printing. And as mentioned previously, many glass-printing inks are designed to emulate the effects of other decorating options (frosting, etching, etc.) without requiring the extra production steps and costs that go with these more traditional decoration methods.
Flat-glass printing systems
Achieving the level of printing productivity that today's decorators demand with the level of quality that their customers expect creates challenges for screen-printing equipment makers. But the manufacturers have responded with a range of precision printing systems, including equipment designed to support large-format applications (Figure 1).
Today, many high-end glass-printing equipment is designed as part of automated inline production systems, complete with automatic loading, conveyance, registration, printing, drying, and offloading systems. Each component of these decorating lines is engineered to work in concert with other components, ensuring that part positions and print registration are maintained throughout even the most complex printing sequences. For users that don't require this level of automation, but want the same degree of precision, glass-printing equipment can be configured to combine manual loading and handling with automated pre-centering modules and registering stations that ensure accurate and consistent part positions during printing.
Regardless of whether it's a single manually fed printing system or fully automated multicolor glass-printing line, process control is the foundation of any high-volume flat-glass printing machinery. The objective of glass-printing systems today is to move materials smoothly and efficiently through the decorating process, with controls and checks in place at each stage of the process to ensure accurate and predictable results.
Often, screen-printing stations must be adapted to existing glass-processing lines. In such situations, the printing machinery must be designed to mesh with existing part-conveyance systems. In other instances, the glass producer may wish to add screen-printing capabilities offline without sacrificing efficiency. Here again, printing-equipment manufacturers must consider the flow of material and the best handling solutions to ensure that productivity isn't sacrificed.
Whether the screen-printing equipment will be a standalone press or an inline system, the entire production process must be evaluated in order to meet the decorator's needs. Below are some of the primary factors that must be considered before screen-printing technology is added:
* sizes and weights of glass panels to be printed
* substrate pre-processing requirements
* direction and speed of existing inline production systems
* panel-transportation systems required
* printing direction and speed
* registration criteria
* printing controls required (including PLC programming)
* inspection stations required
* drying/curing capacity
As this list of criteria makes clear, moving glass substrates through the system in an efficient and accurate manner is tantamount to creating an effective production process. The explanations that follow clarify how some of the previous factors contribute to the productivity of the final system.
Panel size and weight As demand continues to grow for larger decorated flat-glass products, equipment must be engineered to accommodate both the greater dimensions and the increased weights such products represent. Flat-glass screen-printing equipment is already being used on panels as large as 8 x 18 ft (2.5 x 5.5 m), which can weigh hundreds of pounds. Depending on the application, panels can be conveyed through the system in several different ways, which are discussed under the section "Panel-transport systems."
Pre-processing All processing that occurs prior to printing has a direct effect on the printing line's performance and its overall productivity. For some applications, glass panels must go through a cleaning process or pre-treatment procedure before printing. These production steps can slow down, speed up, or stop the line intermittently, which is why they must be carefully evaluated and accommodated. To maintain an efficient flow of material through the system, accumulators may be required to collect any overflow of panels from preprocessing and then release them for printing at a consistent rate. These devices, which serve as a buffer when production rates between the pre-treatment and printing areas of the system vary, are discussed in the following section.
Panel-transport systems To move glass panels from one stage of production to the next, several panel-conveyance devices may be employed.
Panel conveyors are wide, belt-driven system that are generally used to transport panels between various stages of production. Similar to the conveyors found on screen-printing dryers and curing units, panel conveyors typically appear before the printing stations in a flat-glass production line. Among their benefits, they can move panels very quickly and can direct the flow of panels in various directions, depending on the way they are configured. For example, a conveyor system may be used to feed panels at a high speed from one supply line and alternately direct them left and right to two independent screen-printing lines. The use of use of curved or 90° conveyor sections make such configurations possible.
To smoothly move glass panels from conveyors into position for pre-registration, walking-beam, belt, or roller-transport systems are available. Walking beam transporters are gantry-like devices typically used to move heavy glass, such as the panels used to make automotive windshields. Walking beams usually attach to glass panels via suction cups. Using advanced drive systems to ensure accurate position, the walking beams usually pre-register the glass as they deliver panels to the print station for imaging.
Belt-transport systems operate similar to their larger panel-conveyor cousins. However, these transporters are generally used to move panels short distances, and rather than a single wide belt, they feature multiple thin belts that provide more accurate positioning. Commonly, belt transporters operate by providing continuous movement in one direction, perpendicular to the direction of registration functions and the print stroke. As the part reaches the printing position, register stops and guides in the press bed typically raise to temporarily freeze the part in the printing position. After the print stroke, the register stops withdraw and the belt transporter continues to move the part on to subsequent print stations or the drying unit. Belt transporters may also be employed as part of panel feeding or stacking devices (Figure 2).
For moving extremely bulky or heavy glass panels into registration positions and between print stations on multicolor lines, roller transporters (Figure 3) are usually employed. The wheel-like rollers use direct-drive movement to pull the material to the print station, where it is registered and printed. Then the rollers continue transporting the panels to the drying area. For glass printing, these roller-drive systems feature surfaces designed to prevent scratches, and their rotation is synchronized with the speed of the print stroke and drying system.
As mentioned previously, in order to keep parts from backing up when any portion of the production line is temporarily slowed down, panel accumulators are used. These units temporarily hold excess panel output from earlier production stages and phase the parts back into the line when the source of the slowdown is corrected. Accumulators are capable of sensing problems such as a broken glass panel, a fire in the dryer, or some other line-speed slow down. When such problem are discovered, the units automatically store incoming glass panels, releasing them only when normal production resumes upstream in the process. These systems allow stages of production that precede the bottleneck to continue as normal and help avoid complete shutdowns of the entire line.
Registration systems Registration of glass panels can be achieved by a few different configurations of movable registration pins, lateral stop plates, and manual placement. The shape, thickness, and weight of the glass part are a few of the considerations in choosing the correct registration system.
Both pneumatic and servo-motor-driven devices control the movement of registration mechanisms on today's glass-printing equipment. Some systems rely on manual placement of panels into special nesting receptacles, which deliver the glass to the print station in a precisely registered position. Automatic registration (Figure 4) utilizes side and rear guide stop plates, along with register pins that automatically raise and retract into the press bed. For heavy glass substrates, some decorating lines feature pre-centering stations (Figure 5), which set the general alignment and position of the glass before it moves on to the registration mechanism, which makes the final positional adjustments.
Retractable registration pins are employed to complete the alignment of the glass panel into its final, registered position. The pins are normally set on opposite sides of the panel and automatically engage (raise from the press bed) as the panel is moved onto the press bed by a belt transporter. Pins at the far end of the belt travel area raise to stop the panel; the pins on the opposite side then raise to lock the panel into position along one axis. On one of the sides adjacent to the pins, a moveable stop plate then completes the registration cycle by pushing the glass panel against a fixed stop plate on the side directly opposite. Pin locations and side-stop movement are generally set through the system's controller to support panels of different sizes and those that have curved edges.
Printing controls At the heart of modern high-volume flat-glass decorating equipment is the programmable logic controller (PLC), which is basically an onboard computer that controls all the functions and operating characteristics of the decorating system based on user-defined parameters. The PLC is responsible for controlling a variety of functions, including squeegee and floodbar speed, peel, dwell settings, and the speeds of part-transport systems to which it is connected. Sophisticated PLCs can be interfaced with accumulators and automatic inspection systems, adjusting machine operation based on feedback received from sensors in these components. PLCs also allow users to save specific job settings as independent records that can be quickly recalled for repeat orders. Finally, they typically support a range of extensive diagnostic features to help users pinpoint the source of any performance problems that occur.
Inspection stations Because flat-glass substrates can be quite expensive, most inline systems employ an inspection station of one type or another to help users identify printing problems and avoid lost production time and costly wasted material. These stations can include automatic optical devices to check print registration and warn the operator of any inconsistencies, or they may simply comprise backlit beds that allow for enhanced manual inspection. Most flat-glass printing lines incorporate such stations at the end of the printing station and before the drying module. Multicolor lines may feature additional inspection points between each print station.
Breaking new ground in glass decorating
Advancements in flat-glass printing technology continue to add to the decorating options available for screen printers. Faster material-transport systems and more advanced drive components, coupled with more powerful PLCs, promise even greater productivity and better image quality from future generations of glass-printing machinery. These developments ensure that as the market for decorated flat-glass continues to expand, the opportunities for screen-printing companies will also keep growing.
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