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Shopping for a Single-Color Graphics Press, Part 2

(September 2001) posted on Thu Sep 13, 2001

An assessment of standard features and popular options


By Mike Young

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Last month, we began our look at flatbed graphics presses by categorizing the wide selection of press makes and models into three general configurations: clamshell, two post, and four post. We also explored the levels of automation supported by today's equipment, considering manual through fully unattended operation.

On the following pages, we'll continue reviewing press features and optional components, then conclude with some general guidelines to help you match your production needs with the right equipment. Let's pick up where we left off and take a closer look at the take-off and feeding systems that define a press's level of automation.

Take-off systems

As mentioned in Part 1, presses that include take-off units (but not automatic feeders) are identified as 3/4-automatic models. In flatbed screen printing, mechanical grippers are the standard take-off technology used for removing printed sheets from the press bed .

Gripper-based take-off units generally won't require adjustment for varying sheet sizes or thickness. But they don't universally accept material of any thickness--only materials up to the maximum gripper opening size, which in some cases can be adjusted.

One drawback of gripper-based take-off units is that mechanical grippers may have trouble grabbing heavy substrates with a curly or wavy surface. Additionally, the position of the take-off system dictates what edge (back, left, or right) you will use to align sheets for printing. This restricts your ability to align sheets relative to the centerline of the stencil image, which can make it more difficult to compensate for screen distortion in fine-detail printing.

The main alternative to mechanical grippers are take-off devices that utilize suction cups. Suction-based units are also reliable for high-speed production, and they overcome the substrate-thickness limitations of gripper-based systems. However, they lead to longer changeovers between jobs because the suction cups (and sheet-edge guides for registration) must be repositioned for different sheet sizes. They also need larger unprinted areas for material handling, typically a trim area along an unprinted border that is approximately 1 in. wide.

Integrated take-off systems with mechanical grippers operate from the rear of four-post presses and some two-post models and from either side on clamshells and two posts. Suction-based take-off units can operate from either the rear or sides of any of these press configurations.

Both gripper and suction take-off units are available as stand-alone, aftermarket systems. In some cases, these add-ons can effectively turn a semiautomatic press into a 3/4 automatic. But unless the take-off unit was specifically designed for your press, there is no guarantee that it will be able to keep pace with the machine's top cycling speeds.

Gripper-based take-off systems also can be a hindrance with four-post presses when smaller sheets are printed on machines with larger print areas. In these situations, the operator has to reach far back on the pressbed to register sheets for printing.

What are the advantages of removing printed substrates from the rear or sides, and is either side better? The answer really depends on the way your production flow is organized, the space you have available, and the type of press you're using. What can be said with certainty is that rear take off usually means that the press operator cannot see the printed results during press operation.

Take-off direction can also impact drying requirements. For example, on a large-format four-post press, sheets typically take a "landscape" orientation relative to the front of the machine. If the takeoff is in the rear, sheets are removed and placed on the dryer belt in landscape orientation.

On one hand, this means you can utilize more of the belt width and dry more prints in a given amount of time. But it also means you'll need a dryer that is wide enough to handle the maximum sheet size in landscape orientation.

In contrast, a side take-off system will usually operate on either side of the press, and it will remove and feed sheets to the dryer in "portrait" orientation. This means you can use a narrower dryer, but it will have to be longer (or operated at a higher intensity) to achieve the same curing/drying level at the same production rate as the landscape orientation.

Feeders

Feeding systems are typically reserved for presses that also incorporate automatic take-off systems. In other words, they'll be found on fully automatic presses. Historically, presses manufactured to be fully automatic use a single sheet-handling system to serve both feeding and take-off functions. One method uses a series of multiple mechanical grippers or suction cups that reciprocate back and forth. The other uses grippers that rotate beneath the machine to return at the feeding position and repeat the circuit.

On reciprocating systems, each set of grippers or cups moves in unison, transporting sheets from feeding position to registration to printing, and finally, to unloading position, with a different set of suction cups taking over the sheet at each stage. To retrofit existing semi- and 3/4-automatic presses, several third-party automatic feeders are available.

The big question when deciding on a feeding system for a new press or one to retrofit an old press is whether to buy a "single" or "stream" feeder (or one that supports both options). Stream feeding is preferred for most applications because it promotes higher productivity and accuracy. In stream feeding, the rear edge of the sheet is separated from the following sheet in the stack (with the assistance of blowers) and lifted by suction cups in the feeder's pick-up head. The pick-up head (also known as a separator head) then moves the sheet forward a few inches and releases it.

At this point, the sheet's leading edge begins to travel on the feeder's feedboard transport conveyor system and is held down from above with brushes and/or rubber rollers and weighted balls. The pick-up head then returns to separate the next sheet while the previous sheet continues to travel towards the press. The head deposits each sheet several inches behind the previous sheet on the conveyor, producing a stream effect much like playing cards fanned out from the top of a deck.

As the stream of sheets travels up the feedboard, the feeder has to do two things. It has to allow the first sheet's leading edge to ride up against a mechanical fixed stop point (the datum line) for the grippers or suction cups to locate the sheet and register it. The feeder then pushes or pulls the sheet over for proper edge registration (side-to-side). Once in register, the grippers transport the sheet to the print table as the next sheet moves forward for registration. All feeders include detectors that halt the print cycle if they register misfeeds (skewed substrates), as well as "no-sheet" or "two-sheet" feeding errors.

The alternative to stream feeding is the single-sheet feeder. This system separates sheets at the front of the stack and deposits them on the feedboard conveyor one at a time with no overlap. The lack of overlap occurs because the pick-up head cannot separate the next sheet until the previous sheet has completely cleared the stack. As a result, single-sheet feeders must move each sheet a greater distance before the print cycle begins, and the chance for registration errors increases as the distance of movement becomes greater.

Still, single-sheet feeders are the better option in some applications, especially those that involve heavy, thick, limp, wavy/curly, or static-prone materials, which includes many industrial-grade substrates. They may support slower production speeds than their streaming cousins, but the ability to successfully handle troublesome substrates may be worth the tradeoff.

The final thing to note is that a feeder must be both electrically and mechanically interfaced with the press for trouble-free synchronized operation. Unless the press and feeder are automatically synchronized by microprocessor controls on the system, no one other than technicians certified by the press or feeder manufacturer should carry out the job of synchronization.

Other important features and options

Flatbed graphics press manufacturers offer more features and options for their machines than we can possibly cover here. Many functions and features are unique to particular press makes and models, but all machines share certain performance and control attributes, which is where we'll focus. Many manufacturers will also add custom features if a buyer requests them and they are feasible and practical. In the following sections, however, we'll just concentrate on the standard features and options you'll find on presses today.

Automatic frame clamping This feature allows users to slide a screen frame into the press and lock it into position with the press of a button. Systems with this feature use an automatic pneumatic clamping assembly in the printhead. Many automatic clamping systems are designed to work in conjunction with on-press pin-registration systems for frames (discussed later). They're particularly useful with cumbersome large-format frames and short-run jobs, where they can enhance press-setup times.

Digital controls and feedback Although a few single-color graphics presses are still offered with analog controls, the vast majority of manufacturers have switched to digital controls and displays. Some even offer touch-screen user interfaces that are designed to streamline press setup and improve printing consistency.

However, unless your facility already has standardized printing procedures and a comprehensive record-keeping system for job setup parameters, many of the functions and adjustments these control systems allow may be new territory for your press operators. Printers familiar with analog controls may have difficulty adjusting to the menu systems and nomenclature associated with digital controls. At the very least, you need to make sure that the manufacturer will offer training to help your printers take advantage of these systems.

One potential benefit of digitally controlled presses is that many allow you to store setup parameters and recall them later for repeat orders or jobs with similar characteristics. Again, these stored parameters are only useful if all other setup parameters can be recreated as well (e.g., screen tension, emulsion thickness, squeegee durometer, etc.)

Fully adjustable floodbar functions I can never understand why some press manufacturers do not provide the same adjustments for the floodbar as they do for the squeegee. After all, flooding action is just as important as squeegee motion. Some presses not only fail to provide floodbar angle adjustments but also offer no way to adjust floodbar pressure and rely on the floodbar's weight to cause contact with the screen. Because flooding is an important component of accurate printing, make sure the press model(s) you consider offer a full range of adjustments for the floodbar.

Peel control Under ideal circumstances, adequate screen tension and off contact should be sufficient to promote good snap off between screen and substrate during the print stroke. However, if you often produce large-format work or prints with large coverage areas (e.g., reverse printing), then you know that good snap off isn't always possible. A graphics press with an automatic peel feature can help by elevating the screen behind the squeegee during the print stroke to force snap off and maintain print quality.

Pin-registration system Today, it's becoming more and more common for graphics presses to include pin-registration systems for rapid alignment of screens on press. This feature is especially useful in operations that work with a variety of press models and sizes.

Registration begins on a light table marked with datum lines, sheet edges, and possibly centerlines for each of the shop's presses. A film positive is mounted on a clear carrier sheet that is, in turn, affixed to a three-pin registering assembly. This assembly includes two additional pins that correspond to holes in all screen frames. Once a screen frame is placed onto the light table, the image is already exactly positioned relative to the screen and the screen is ready for exposure. Each press has an independent pin assembly attached to it so that the finished screen can be mounted on the press in register.

Squeegee pressure equalizer These systems were originally developed in response to the needs of printers doing close-tolerance work, including those in the electronics field. These devices comprise sensors and pneumatic pistons in the squeegee assembly to equalize pressure across the squeegee blade during the print stroke. Pressure-equalization systems are designed to eliminate image distortion and provide a uniform ink deposit--but they don't address why the deposit was uneven in the first place.

These systems aren't complete cures for image distortion and ink deposit problems. Merely having an image positioned off-center on the screen will lead to image distortion during the print stroke, regardless of how well the equalizing system works. And if pressure on the floodbar was uneven during the flood stroke, it would vary the amount of ink prefilling the mesh, which would, in turn, vary the amount of ink deposited. For this reason, a few systems are offered that equalize both squeegee and floodbar pressure.

Squeegee snowplow Some presses offer a very simple device that allows the squeegee to print at an oblique angle, perhaps a few degrees plus or minus from perpendicular. The angling of the blade helps to eliminate sawtoothing and moiré and promotes excellent coverage when printing fine positive or reverse lines.

Substrate precleaning and static control If you frequently suffer from clogged screens or other defects stemming from dusty or dirty substrates, purchasing a press with a substrate precleaning system may be worth consideration. Typically, these cleaners take the form of adhesive rollers designed to pick up dust and debris from the substrate. More common on presses with reciprocating print beds, the rollers clean the substrate as it is being shuttled toward the print area, thereby adding no time to the production sequence.

Similarly, the press may include an anti-static device along the in-feed area to prevent the substrate from sticking to the press or ink from cobwebbing across the material's surface. The antistatic system may use brushes, an anti-static "bar," or laminar airflow that removes static with ionized air. Note that some cleaning systems include anti-static features.

Troubleshooting via modem Increasingly common are presses that allow connection with a modem so that the press manufacturer can access the machine and troubleshoot performance problems remotely. This can be a useful feature if the problems relate to electronic data, such as improper setup of printing parameters by the operator. But remote troubleshooting won't verify the integrity of all of the machine's mechanical components. If a press you're considering offers remote access via modem, make sure this feature is part of the regular service contract and not a costly add on.

Variable/adjustable table vacuum This doesn't seem to be a difficult thing to ask for, but I am amazed at the number of presses on which the print table's vacuum draw down cannot be adjusted by the operator. To simplify substrate handling on presses where the vacuum is continuous, most operators mask off the print table around the area that will hold the substrate. This means that the vacuum draw down force increases since the motor is trying to pull the same volume of air through fewer holes. The result can include warping of the print surface under the pressure load (which can lead to dot gain and tonal shifts), distortion of prints, damage to substrates, and increased static. But on systems with adjustable vacuum, you can avoid these problems by reducing the pressure.

The right press for the job

Cost not withstanding, you may be able to narrow the field of contenders by looking closely at what and how much you intend to print on the new machine. Certain press types are simply better suited to particular applications.

For comparative purposes, we'll start with manual presses. For the average display-graphics producer, these systems have little use beyond creating prototypes, proofing, and color matching. However, large-format long-stroke manual presses with one-arm squeegee attachments remain popular with printing operations of all sizes for short-run banner and display work.

Clamshells are often considered entry-level machines, a belief inspired by their lower cost as compared to other configurations. But a well-engineered clamshell can hold its own against virtually any two post or four post in terms of print quality, and, in some applications, productivity. The shear number of clamshells already in the marketplace is proof of their popularity. Clamshells are extremely versatile and well suited for both short and long runs, including process-color work.

Two-post machines are ideal for smaller-format, tight-tolerance applications, including graphic overlays, membrane switches, labels and nameplates, instrumentation panels, fascias, dial and gauges faceplates, etc. They're also popular for small-format graphics jobs, ranging from bumper stickers to process-color P-O-P displays. From an operator's standpoint, the clear view of the print area they provide is a major advantage over other press varieties.

For top-of-the-line print accuracy and speed (plus the highest cost), nothing beats a four-post press. This type of machine enjoys the widest range of options, including feeders and takeoff units, sophisticated touch-screen controls, job-storage capabilities, and collections of sensors that continuously relay press performance data back to the operator. These machines are best suited to medium- to high-volume production environments that take advantage of their speed. Except for very large-format models, few single-color four-post systems are ordered today in fully automated versions. However, the four-post configuration serves as the basis for today's high-speed multicolor inline printing systems.

Making a decision

Printers occasionally get stuck with the wrong piece of equipment because they don't spell out precisely what they need to the manufacturer. Occasionally, this occurs because a printing company is afraid of giving away too much about how its products are produced (a problem that could be easily remedied with a non-disclosure agreement). In other cases, the shop simply hasn't thought its purchase through.

When it comes time to shop for a new press, you face the difficult task of matching your production needs--and your budget--with a model that offers the right balance of speed, accuracy, and ease of operation. You have to define the right level of automation to meet your current goals and future demands.

Also, don't overlook space considerations--make sure that the new unit fits without causing your to sacrifice the smooth flow of production on other machines in your operation. Do you have enough dryer capacity to support the new system, or will you also need to invest in new drying equipment? In my experience, shops running two or three semiautomatics can share a dryer, but the average dryer typically won't keep up with two 3/4-automatics.

Additionally, you have to consider the skill of your press operators and the amount of training they'll need to operate the machine effectively. To take full advantage of sophisticated press controls, you may even need to standardize areas of the printing process that you gave little thought to in the past. And above all else, you have to be comfortable that the manufacturer stands ready to provide training and technical support to help you realize the full potential of the equipment.

At the beginning of this series, I compared shopping for a new press to searching for a new home. Like a home, graphics presses are big investments. With price tags starting in the low five-figure range and extending well into six figures, its clear why careful research of press types and evaluation of your needs are important steps in the process. If you short-change yourself with a hasty press purchase, the consequences will stay with you for a long time.


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