SCREEN PRINTING Tips on coating screens Tricks of the trade with screen printing expert, Kes Cracknell, of Ram Promotions N
o matter how good a printer you are you cant do a good job if the screen has been badly made. So
this month, I went back into the screen department and blasted a few screens for myself. The screen department consists of
washing out (after exposure), reclaiming and coating screens. You will need a dedicated area in your print floor devoted to this process. In this article, I cover everything you need to know about preparing screens for the application of emulsion.
Cleaning the screen
to degrease the screen – this is very important as a well cleaned and desgreased screen helps the emulsion bond to the mesh.
Applying emulsion to a screen Before you begin any screen coating session, you must work under “safe” lighting and yellow bug lights work perfectly. Because emulsion is photo- sensitive, you must not have any regular light present in the room as the emulsion will be exposed by that light and will not work properly. If you are in a room with windows and
you coat your screens during the day, be sure to cover the windows to prevent light from entering and turn off all regular lighting in the room.
Using a ‘trough coater’ It is imperative that you apply emulsion with a trough coater. This specially made, aluminium trough is used to hold emulsion for spreading on the screen during the coating process. Trying to apply emulsion to a screen with
a piece of cardboard or some other makeshift device will only result in a mess. When purchasing a trough coater, make
sure the trough just fits inside your screen. This will help reduce screens breaking down in print production. The last detail about scoop coaters is take care of it. If you get nicks in the coating edge, you will have lines in the emulsion coating every time you coat your screens.
The ideal solution is to have a dedicated washout booth with hot and cold water. We have two, one for the blasting of old screens that need to be reclaimed (usually the ones covered in ink) and this keeps one booth clean for washing out exposed screens. I find large booths easier as you can
place more than one screen in the booth. This speeds up production. So on to the fun part. Put the screen that you are going to
reclaim into the booth, clean all the excess ink off and put the remaining ink back in the ink tub. We use a dip tank – you can fit six screens in each tank, which again, speeds up production. A dip tank cleans ink residue off and at
the same time strips the old emulsion away. After about five minutes in the tank take
out your screen and wash the emulsion away; you will find it comes off easily leaving a clean mesh. We then tidy up the frame with a soft
brush and some screen wash, again washing away with water. Next, we need
www.printwearandpromotion.co.uk
Sharp or rounded edge trough? Having coated thousands of screens, I prefer rounded edge for mesh counts of 43 up to 77 and the sharp edge for mesh counts of 90 and above. The reason is simple – most jobs
involving fine detail and/or small halftone dots do not require a heavy ink deposit and you want to be able to wash out the image on the screen easily. The sharp edge deposits less emulsion on the screen resulting in a thin ink deposit on the printed product.
The coating technique Three-quarters fill the trough coater. If you overfill it, you will most likely spill emulsion, as it has a tendency to run off the sides. Once the emulsion is in your coater, starting on the print side (bottom of the screen), tilt the screen about 30 degrees. We use a fixed location to coat our
screens as we have placed a small piece of metal on the floor that helps hold the screen at the correct angle during coating.
Place your scoop coater at the bottom of
the screen, tilt the scoop coater enough to allow the emulsion to touch the screen and slowly pull the scoop coater until you come within an inch or two of the top of screen. Turn the screen upside down and do the
same coating step again. Next, turn the screen over and repeat this step on the inside of the screen, but this time apply only one coat of emulsion.
How many times should I coat each side? This is a topic where I am sure there will be some debate, but it is this authors experience as to what works best: MESH COUNTS OF 43-61: Two coats on the print side, one coat on the inside of the screen using the rounded edge of the trough coater. MESH COUNTS OF 77-120: One coat on the print side, one coat on the inside using the sharp edge of the trough coater. These coating recommendations are based on printing plastisol ink on T shirts and other apparel, detailed line art, halftones and screens using water-based inks as well. The only controversy here is whether or not you should apply a second coat to the inside of screen. We have found that by eliminating the second inside coating, we are better able to wash out fine details with better results.
Additionally, on the high mesh counts of 90-120 where small halftone dots are involved, you will find that a single coating on each side is all that is needed and that the image and dots will washout
that much easier after exposure. n We have tried many emulsions but as we use plastisol and water based at RAM, we use one emulsion that works very well on both. See
www.trutech.biz CHEMICALS for reclaiming please see
www.inkandsolvents.co.uk MESHES and screens please see
www.pyramid.uk.com
About the author Kes Cracknell has always been involved in the promotional printing industry and started Ram Promotions in 1994. Ram screen prints garments for promotional, school wear and retail companies. In 2010, Ram expanded its print floor and now prints more than 60,000 garments a week. Ram now
decorates garments only for the trade. n For more information or to discuss topics for this column, email Kes Cracknell at info@ram-
promotions.co.uk, call 01277 366164 or visit
www.ram-promotions.co.uk
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