Screenprinting Today: The Basics

PREPRESS

© 2000 Andy MacDougall.

All rights reserved.
Reprinted with permission

Prepress (or make-ready) is the name given to the point in the screen printing process (and other printing processes) where the art and screen are given a final check, the stock or substrate is prepared and arranged for printing, the ink has been mixed and thinned, and the drying system and items needed during the print run are assembled close at hand and ready to use. It is important to carry out these prepress activities before the ink hits the screen, otherwise delays in printing will result, often stopping the print process or requiring cleaning of the screen.

ART & SCREEN CHECK

Using the film as a guide, a final inspection of the stencil should be made. Pay special attention to pinholes, or lines caused by film edges on the stencil, or any other imperfections in the stencil or image. These should be touched up before the screen is put in the press.

Blockout and Touch-up

Touch-up or block-out is applied to the underside of the screen using a brush for spots, or a small flat spreader (old squeegee or sign vinyl applicators work great and clean easily) for larger areas. If touched up on the top-side of a screen, you risk wearing nicks or gouges in the squeegee rubber.

Commercial blockouts are available from most suppliers and are designed for solvent based inks using water soluble stencil systems. When using waterbase inks, direct emulsion can be used as blockout, but it must be dried and re-exposed to harden it for printing. A lacquer blockout works well for waterbase, and does not require curing, but does require more aggressive removal during the reclaiming process.

Ensure that the screen is in good shape, with no serious rips or holes that will let loose during the print run. If there is any doubt that a stencil is suspect, or the image is flawed, re-shoot the screen. It saves time, money, and frustration if the problem is fixed at this point of the process.

Taping off screen gutters along the sides of the frame, or around images, can be carried out at this time, or if it's easier, tape the screen once it is in the press. Use thin packing tape, never paper or masking tape. A good tape job makes production printing easier because it controls where the ink goes, and makes clean-up a snap. On long production runs, especially with waterbase, or when you know a stock edge is going to wear on the screen, a line of tape on the underside can protect the stencil from wearing thin from squeegee pressure along an edge. Make sure the tape goes on flat with no creases or folds.

STOCK PREPSTOCK PREP

Whatever the stock or substrate is, it needs to be prepared for printing. This includes cutting down in size from master sheet to press sheet size, or may mean removing wrapping or packaging, or cleaning dust or other residue from surfaces. The stock should be prearranged or stacked on a convenient table close to the press. Whatever the stock, keep it clean!!! The idea with getting the stock ready before you start printing is to avoid dealing with packaging, cutting, or unnecessary delays once you have inked the screen and started your print run.

Acclimatizing Stock

Some stocks need to breath or acclimatize to the temperature and humidity of the printing area. In the case of limited edition rag paper, it's not uncommon for the paper to expand or shrink up to .125 of an inch after opening a factory sealed package. If paper is printed fresh out of the package, it will then expand and the second colour may not register due to distortion. Other materials, such as plastic or metal, need to reach room temperature for the inks to adhere properly.

Pre-shrinking for Dryers

If you are using a tunnel dryer, the same holds true for many stocks, they will change in size after the first pass through. Multicolour designs will not register if the stock shrinks after the first print. Run the blank sheets through the dryer prior to printing.

Rough Edges

When printing 3 dimensional stocks such as wood, plastic, metal, etc., another consideration is edges. Material that has sharp or burred edges will quickly cut the stencil and mesh under pressure from the squeegee. Either clean the edges, or tape and protect the screen and stencil from direct contact.

Cleaning Dust & Static

When cleaning plastics or metal in preparation for printing, use Isopropyl Alcohol (IPA) available from your local pharmacy. This is a good all purpose cleaner which doesn't attack the surface, evaporates immediately, and reduces the static charge. Buy a tack cloth from an autobody or plastics shop and use this as well for getting rid of static dust on acrylics or other hard surfaces. Dust is your enemy in screenprinting. If you have the money, there are small machines much like old ringer washers which remove dust and dirt from plastics just prior to printing. They have adhesive sleeves which are changed once they pick up too much dust.

The film positive should be taped in exact position on the top piece of stock for registration.

REGISTRATION

There are a number of methods of registration (lining up the screen image with the stock) and many are dependant on the type of press system. Simple hand presses do not allow for adjustment of the screen or printing table, so the stock must be moved to achieve precise register. On more sophisticated press systems, either the screen masterframe can be unlocked and adjusted into position, or the printing table can be adjusted. Both these methods require the image on the screen to be within a working adjustable area. This is called 'pre-registration' and is done when the film positive is exposed on the screen.

3-point register

This is the most common and easiest way to register a print, and refers to your register tab placement. Two tabs will be in one corner, and the third tab will be located along one side. These locations then become your register points on each successive colour pass. Some automated presses have disappearing guides that drop into the table once the stock is located and the screen is lowered. In most cases, printers make their register tabs out of pressure sensitive decal material stuck together to build up some height, and cut into small squares. In the case of larger or 3-dimensional stock, make your tabs with double-sided tape stuck to a piece of coroplast or plastic. In all cases, once located for the run, tape over the guides so they are not accidentally dislodged during the print run. Never use a register guide that sticks up higher than the stock, or you risk ripping the screen when the squeegee passes over.

Placing the Positive

By taping the positive in place on the stock, you now have a visual reference for register by looking through the screen and lining up the stencil with the positive. Keep adjusting until you have both lined up, and then carefully place your register tabs. It's a good idea to keep the vacuum going when carrying out this process because it holds the stock in place while you are adjusting, lifting the screen, etc. Remember, after your stencil and stock are lined up you may encounter some deflection when you start printing, due to screen stretch or play in the press. Always check register once the printing has started and make your final adjustments at that time, then lock everything down for the print run.

PRINTING A CLEAR OVERLAY AND 'PIN REGISTER WITH TABS'

Both these methods are STILL used, mostly in art printing situations. I'll give a couple of reasons why they are not a good idea, although they do have their uses in certain instances….

Overlay Register

Using this method requires the placement of register tabs after the printing has commenced. This causes delays in printing at the most crucial time of the print process, and these delays result in ink drying in on the screen. Pre register as described above is much easier and quicker. Overlay register is good if you have no reference points or need to visually position a print.

Pin Register

The practice of pre-sticking punched tabs on to material, and then locating each piece on two pins stuck to the table top during the print run, has to be one of the most convoluted and time consuming methods of register ever invented. Pin register is used in film making and stripping to register overlays to each other. When used in this context it is accurate and accepted industry wide. I have my suspicions someone saw this done once and borrowed the idea for screenprinting, and now it seems generations of college and university printmaking students have been taught a method that is not very efficient and causes major problems for the printer before, during, and after the printing. All other graphic processes use 3 point x y axis location, where the material itself comes in contact with the register tab or stop. Why? Because 3 point is quick, accurate, and easy to use.

INK PREPINK PREP

Ink for printing should be mixed and thinned to a workable viscosity. Additional thinners should be close at hand for adjustment once printing is started. Always insure that you start with more ink than you need, especially when printing a custom colour. The ink needs to be placed within reach of the press, but in a spot that it won't be accidentally knocked over. More information on inks is contained in chapter 5.

OTHER ITEMS REQUIRED DURING THE PRINT RUN

It's a good idea to ensure that the following items are ready and close at hand before inking the screen and starting the press run.

Drying System

If using racks, they should be empty, flipped up, with enough space available to accommodate the run. If using a dryer, it should be turned on and brought up to operating temperature. If you are laying out printed pieces around the shop, ensure that you have cleared the drying area before starting to print.

Tools

Clear tape, mylar patches, a knife, rags, thinners, sponges and water for waterbase), a pen or pencil, print-off paper, a loupe, and a small ruler are all items that need to be at hand during the run. Depending on the press set-up or material being printed, other specific tools may be required to adjust the press.

Tool Cart

A small wheeled table with a drawer makes an excellent addition to any print area. The tools can be stored in the drawer, and stock and/or ink can be placed on top. It can be easily moved into position during the print run, or moved to facilitate cleaning or adjustments on the press. In our production shop, each printer had his or her own table, and they were responsible for keeping their supplies and tools in order and ready for use.

Helper

It's a good idea to have an assistant during the run. This allows the printing to keep going non-stop. The helper usually racks, which usually speeds up jobs, and provides a second set of eyes for quality control, a runner for more ink, stock, press adjustments or other things that are needed during the run. Helpers should be trained in press operation and control functions, and allowed to print during longer runs.

Artwork, Sample Print, Work Order

Keep these handy for reference at the start and during the run. It's important to check the first prints for positioning, print detail, and colour before proceeding with the entire print run.

Mental Check List

Once all the items involved in prepress and make-ready have been taken care of, it's time to set up the job on the press. In some situations, press set-up can coincide with stock prep and ink mixing. Finally, it's always a good idea to make a quick mental check-off of the above pre-press items before pouring ink in the press.

In Chapter 7, Screenprinting Today gets into the wild world of screenpresses. Where else would you find a carousel and a clamshell, a cylinder or a 4 post, one arms and parallel lifts?

If you have any specific questions on screenprinting problems, drop me a line and I'll try to help.

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