halftone screen printing
DESCRIPTION
To be able to screen print photographic or grayscale images, an image must be converted into what is known as “halftones”. A halftone image consists of many small dots that make up the grayscale portion of that image.TRANSCRIPT
Chapter 2 - Halftone Printing
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To be able to screen print photographic orgrayscale images, an image must be convertedinto what is known as “halftones”. A halftoneimage consists of many small dots that make upthe grayscale portion of that image. Screenprinting halftones can present certain challengesand we’ll address them in this chapter.
HOW BIG A “DOT” SHOULD YOU USE?
The size of the halftone dot used will of course bedetermined by the type of artwork that you are
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HALFTONE PRINTING
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going to print or the printing technique that you will use.
For example, screen printing on apparel typically involves printing halftones in a range of 35-65LPI. There may be occasions where you want the halftone to be very noticeable andexaggerated for a special effects look, where you may want to go down to a 25 LPI size halftonewhich will produce a very noticeably large dot.If you are printing four color or simulated process color with a manual screen printing press it isrecommended that you print from 45-55 LPI.
If you have a high end automatic screen printing press along with a point light source exposureunit and are using professionally produced film positives you may want to consider printing 65LPI.
If you are new to halftone printing we suggest you experiment with various LPI sizes so you cansee what the limitations of the equipment are in your particular shop.
DOT SHAPE AND ANGLE
The ideal halftone dot shape for printing on apparel is an elliptical dot. When outputting your filmthrough Adobe Photoshop or similar programs choose an elliptical dot shape wheneverpossible.
If that is not possible, make round halftone dots your second choice.
Regarding the screen angles which refers to the alignment of the dots as they reference to eachindividual color in the print order, a good rule of thumb for simulated process printing is to usean angle of 56 degrees for the white underbase and 26 degrees for all other colors includingblack.
25 L.P.I. HALFTONE 55 L.P.I. HALFTONE
HALFTONE PRINTING
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The dot angle is another function that you can adjust when outputting through AdobePhotoshop.
One of the main purposes of adjusting the dot angles is to minimize or eliminate a moirepattern.
MOIRÉ PATTERNS
Moiré is the visual effect of radiatingcurves created by superimposing tworegular patterns. For example, a moi-ré pattern can result from overlap-ping two halftone screens of differentangles, dot spacing, dot size and theweave of the fabric.
Moiré patterns are the undesirableresult of rescreening an image with adifferent halftone screen or with thesame halftone screen on an angledifferent from the original.
Moiré patterns create a“herringbone” appearance and makethe printed image visibly distracting.
Eliminating moiré can be as simple as choosing a different brand of shirt or as complicated asoutputting the art at different halftone angles and reburning the screens.
Moiré in multi-color prints is covered in more detail in the chapter on simulated process screenprinting.
DRAWBACKS OF VELLUM VS. FILM FOR HALFTONES
The ideal halftone dot on a film positive will be as dense a color black as possible and eachsucceeding color separation will line up in registration as perfectly as possible. Because of thatyou will find that ink jet positives or professional made film positives will produce a film positivethat is vastly superior to using vellum paper.
Vellum paper will shrink as it is going through the laser printing causing a slight registrationerror. And the toner deposited on vellum paper from a laser printer cannot produce a dot asdark as can be obtained from an inkjet printer onto ink jet film.
MOIRÉ CAUSED BY FABRIC WEAVE
HALFTONE PRINTING
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As a result, we recommend that you print halftone and halftone color separations wheneverpossible using an ink jet printer. Be advised that to print halftone dots will require that youhave a Postscript printer and/or a RIP processor for your ink jet output device. There aresoftware utilities that can be used for creating halftone images that can be printed a regularinkjet or laser printer. Wilflexeasyart.com has a free download of one such application. AdobePhotoshop is required for the use of this halftone creation utility.COATING YOUR SCREENS FOR HALFTONE PRINTING
Use the sharp edge of the scoop coater for mesh counts of 162 and up. Most jobs involvingfine detail and/or small halftone dots do not require a heavy ink deposit and you'll want to beable wash out the image on the screen easily. The sharp edge of the scoop coater depositsless emulsion on the screen resulting in a thin ink deposit on the printed product. For meshcounts of 162-305 and up put just one coat of the emulsion on the print side and one coat ofemulsion on the inside using the sharp edge of the scoop coater.
MESH COUNT CHOICES FOR HALFTONES
For apparel printing, we recommend the following mesh counts for halftone printing. 162-200for the white underbase and 305 mesh for all top colors. If you are printing large halftones from25-35 lpi, you can use mesh counts as low as 140.
BURNING HALFTONE SCREENS
High end exposure units that have a lightintegration system are the best possible lightsource for screen making, particularly for exposingscreens that contain halftone dots.
A light integrator is an electronic component in theexposure unit that detects and controls changes inlight intensity. It will automatically adjust the lightoutput to a consistent light level despite changes inelectrical power line levels and for the light intensitychanges due to the aging of the light bulb.
With a light integrator, you will be assured of gettingthe same amount of light each time you expose ascreen.
HALFTONE PRINTING
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When exposing images such as fine halftone dots, having a light integrator will improve thequality of your screens considerably. Without a light integrator, you will experience lightscattering and the halftone dots will not resolve as sharply.
If you are burning screens that contain large halftone dots (25-35 lpi), almost any light sourcewill work provided you have a dense black film positive. When burning halftone screens it isimportant to expose them for the correct amount of time. Over exposure will result in a screenthat is difficult or impossible to wash the dots out.
An under exposed screen will result in the dots being “washed away” during the screen washout process. Also be sure to blot out the screens with newsprint or newspaper after the washout process so no clogging will occur during the screen drying process.
WASHING OUT HALFTONE SCREENS
As was previously noted, care must be taken when washing out any screen that containshalftones. If your screen is exposed properly you should not experience any problems.Washing out a screen containing halftone dots should not be any more difficult than washingout any other type of screen.
PRINTING HALFTONES
This discussion is limited to manual screen printing only. The angle, pressure, and speed atwhich you pull the squeegee will effect how a halftone dot is printed on the substrate.
Our recommendation is to use as little pressure as possible to clear the ink from the screenand to print at a regular and deliberate speed at a normal squeegee angle.
You want to try to print “like you are a machine” with consistent, repeatable strokes of thesame angle, pressure and speed. Automatic screen printing machines always produce a betterresult than a manual printer and your goal is to try to “emulate an automatic press” in yourprinting technique.
HALFTONE PRINTING
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Avoid flood stroking if possible and use asfew strokes as possible with one strokebeing ideal.
Probably the most important element thatwill control manual halftone printing is theangle of the squeegee.
The more the squeegee is angleddownward (assuming equal pressure andspeed in the stroke) the more ink that will bedeposited and the darker the print willbecome.
Printing halftone dots by hand to achieve good looking prints requires a certain amount ofpractice and experimentation.
Further, it is recommended that the squeegees that you use for printing halftones have sharpblades that are free of nicks and other imperfections and are of medium, hard, or multi-durometer.
Again, practice makes perfect. The more experience that you have, the better you will becomeat printing halftones.
DEALING WITH DOT GAIN
As mentioned above, it is important that you limit the number of strokes, be mindful of yoursqueegee angle, and control ink buildup on the bottom of the screen once that starts tobecome a problem.
Dot gain is simply the function of the halftone dot enlarging in size during the printing process.While it can not be eliminated completely, it can be controlled to some extent by using theproper printing techniques described and by periodic “wiping clean” the bottom of the screensduring the print run.
Published by:American Screen Printing Association, Inc.www.aspaUSA.com