The ideal press locks the screen frame into position, and returns it to the exact same printing position every time. All the other functions found on different presses allow the operator to gain more and more repeatable control over the different actions and variables that come into play during the act of printing. The physical limitations of an individual pulling a squeegee have resulted in some interesting advances in press design.

Different press types have evolved over the years, and can be classified as follows:

MANUAL, meaning the print and flood are pulled by hand.

ONE-ARM, where the squeegee is held in a holder attached to a bearing unit that runs on a rail at the back of a larger manual press. Still hand pulled.

SEMI-AUTOMATIC, meaning the screen lift and squeegee functions are machine driven.

3/4 AUTOMATIC, meaning either the stock feed or stock take-off has been automated as well.

FULL AUTOMATIC, meaning stock feed, print functions, and stock take-off are all automated and integrated.

Presses are further classified by their screen lifting mechanisms:

CLAMSHELL, the most common, involves hinging the screen from the back or side, with the screen lifting at one end like a clamshell to allow removal of the substrate.

VERTICAL LIFT, where the screen travels straight up, usually on 2 or 4 corner posts.

PARALLEL LIFT, where the screen travels up and back.

CYLINDER, where the stock rotates in synch as the screen travels horizontally, with the squeegee staying in one spot.

Due to the wide variety of applications of the screenprinting process, there are a number of hybrid presses that mix and match a number of the classifications listed above.

THE CLAMSHELL PRESS

This is the most common type. It's simple, and the easiest and most versatile press to construct in it's manual form. It can be as basic as 2 door hinges screwed into the frame and a tabletop. Automated versions drive a squeegee and floodbar assembly from the front of the screen to the back, or in long-stroke format, from side to side. The first automated screen presses were built as clamshells.

The advantages to this system are its simplicity and ease of stock handling. The drawbacks are that the ink will run to the back of the screen when it's in the 'up' position. The design also restricts clearance and access at the back of the press near the hinge system.

THE VERTICAL LIFT PRESS

This type of press design with 4 corner poles was first introduced by SVECIA of Sweden in the early 50's, and incorporated a swing-out table to allow stock to be positioned. There are a number of advantages over the clamshell. First, the ink doesn't run, because the screen stays horizontal. Secondly, higher production speeds can be obtained because the screen travel motion is minimized. The system also lends itself to further automation by integrating a simple set of grippers which remove the stock by holding it while the table moves out for reloading. The stock then drops on a conveyor situated below the printing table surface for side or rear delivery.

2-Pole vertical lift presses are very popular in smaller sizes because they provide easy access for placement and removal of stock, with many of the other advantages found on 4 pole presses.

THE PARALLEL LIFT PRESS

This type of press is most commonly used in large format manual printers, where the automation of the up-down movement of the screen speeds up production. Because the screen remains parallel, the ink doesn't run, which becomes a real problem in large format printing, and with the screen travelling up and to the rear, stock is easily registered and removed from the press bed. The press also eliminates exposed counterweight systems commonly found on large format manual clamshell one-arms, which need to be readjusted for different screen sizes and weights.

THE CYLINDER PRESS

These presses achieve the fastest production speeds in automated screenprinting. The cylinder press is used to print flexible flat sheets, and a similar system is used to print mugs, bottles, and round plastic containers. On a cylinder press, the screen moves in synchronization with the stock as it rolls around a drum. Round objects such as jars, bottles, and pails, are held in a gripper that rotates the object in synch with the screen.

OTHER PRESS TYPES

MULTI-COLOURED AUTOMATED PRESSES

With increased demand for automation and speed, manufacturers now produce machines linked together to print multiple colours inline, where the first press prints colour #1, the stock is dried and then fed to the second press, and so on. These systems are expensive, and require long press runs to offset the expense and time required to set the machines up for a print run. They incorporate computer assisted registration systems, computerized processing controls, and UV dryers.

MANUAL & AUTOMATED T-SHIRT PRESSES

T-shirt press manufacturers produce different styles of 4, 6, and 8 colour manual presses known as carousels. The screens lift in a clamshell motion, but each screen and colour rotates into position to print on the shirt, which stays still, atretched on a platen. Automated ovals and circular presses are designed to print thousands of multi-color shirts per hour. The shirts are still loaded and unloaded by hand, but the platens, screens, and squeegee systems are all driven automatically.

WEB SCREENPRINTING

Web screenprinting, where the material is fed from a roll of stock, incorporates a number of presses and dryers linked in line. The material doesn't require specialized grippers for feeding and taking off individual sheets, and allows for high production speeds.

PRINTING PARTS or 3D OBJECTS

Specialized parts printers feature custom jig tables and parts holders, as well as modified frame action to allow 3 dimensional objects to rest under the screen.

PRINTING TIPS & TIDBITS

Registration of the substrate is usually done using 3 tabs, 2 on a corner and one on a side. The tabs, made from self adhesive vinyl or stock with good doublesided tape on the bottom, are stuck to the table top once the stock has been aligned with the stencil image. This is called 3 - point register. In multicolour printing, always register from the same place on your stock. (Corner and side)

Many automatic presses have disappearing register guides. These require you to pre-register your image on the screens when burning stencils, but this technique, once implemented in the screen department, can decrease set-up times on presses dramatically.

Printing flat stock requires the screen to be positioned slightly above the stock. (Off-contact - many textile printers print On-contact, but this method in graphic printing will produce blurred images) To achieve the proper amount of off-contact, use shims placed under the corners of the screen on hand presses. On automatic presses, dial the lift up. Increase the lift if the screen is not releasing from the stock as you print. The correct amount of lift is the minimum off-contact required to produce a sharp print, and achieve an instant peel as the squeegee passes over the screen.

Auto peel is a feature found on many presses, and assists the screen in lifting right after the squeegee. Both 'peel' and 'off-contact' can be kept to a minimum when you use TIGHT SCREENS! (See previous chapters)

Block your airholes on a vacuum bed that aren't under the stock with scrap paper held down with small pieces of tape. You can vary the amount of suction by uncovering more holes. If you cover every hole and leave the vacuum running, you will burn out the motor. Too much vacuum will draw ink down through porous substrates, leaving a faint grid in the print.

Don't tape out airholes, and DON'T CUT TAPE OR STOCK ON THE SURFACE OF YOUR VACUUM TOP. This permanently gouges the surface and will show as print flaws on thin stocks.

Tape out the area around the print image on the screen, and make gutters to control where the ink flows. Don't let it run under the edges of the frame.

Keep adding fresh thinned ink during the press run. Ink gets thicker as you print.

Try and maintain a steady pace as you print. Unnecessary delays in pulling the next stroke will result in drying in with solvent or waterbase inks. This is especially important when starting a run. Many printers make the mistake of stopping and admiring the first print. Ensure you have plenty of print-off paper, and instead of trying to assess all the variables that effect print quality, do them one at a time while maintaining the printing pace. (print with a helper!)

1. Ensure everything is printing, adjust squeegee pressure, ink viscosity, and clean any clogs in the stencil.
2. Check the color and adjust as necessary.
3. Check the register and adjust, locking in tabs and frame before proceeding with the run.
4. Pull a good print when all the above is good, and check at various times during the run against this piece to ensure nothing is changing.

Make sure your squeegee is sharp, flat, and free of nicks in the blade. Squeegees come in different hardness’ (durometer) and material. For most graphic sheet applications, a medium soft squeegee is best (70 durometer)

For further information about workshops and seminars based on SCREENPRINTING TODAY -- contact ANDY MACDOUGALL andy.squeeg@shaw.ca; (250)334-2598. Comments and questions always welcome. Read the Squeegeeville Gazette online at www.squeegeeville.com