ASPA
  • Home
  • Online Classes
  • Join ASPA
  • Get Certified
  • Videos
  • Store
  • Equipment & Supplies
Picture

Halftone Screen Printing

12/29/2019

0 Comments

 
Picture
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.

Screen printing halftones can present certain challenges and we’ll address them in this tutorial.


Picture25 LPI halftone screen
HOW BIG A “DOT” SHOULD YOU USE?

The size of the halftone dot used will of course be determined by the type of artwork that you are 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 35-65 LPI. There may be occasions where you want the halftone to be very noticeable and exaggerated for a special effects look, where you may want to go down to a 25 LPI size halftone which will produce a very noticeably large dot.


Picture
WASHING OUT HALFTONE SCREENS

As was previously noted, care must be taken when washing out any screen that contains halftones.

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 washing out any other type of screen.


Picture
COATING YOUR SCREENS FOR HALFTONE PRINTING

Use the sharp edge of the scoop coater for mesh counts of 162 and up. Most jobs involving fine detail and/or small halftone dots do not require a heavy ink deposit and you'll want to be able wash out the image on the screen easily. The sharp edge of the scoop coater deposits less emulsion on the screen resulting in a thin ink deposit on the printed product. For mesh counts of 162-305 and up put just one coat of the emulsion on the print side and one coat of emulsion on the inside using the sharp edge of the scoop coater.


Picture
DOT SHAPE AND ANGLE

The ideal halftone dot shape for screen printing on apparel is an elliptical dot. When outputting your film through Adobe Photoshop or similar programs choose an elliptical dot shape whenever possible.

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 each individual color in the print order, a good rule of thumb for simulated process printing is to use an angle of 56 degrees for the white underbase and 26 degrees for all other colors including black.

The dot angle is another function that you can adjust when outputting through Adobe Photoshop. One of the main purposes of adjusting the dot angles is to minimize or eliminate a moire pattern.


Picture
PRINTING HALFTONES

This discussion is limited to manual screen printing only. The angle, pressure, and speed at which 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 screen and 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 the same angle, pressure and speed. Automatic screen printing machines always produce a better result than a manual printer and your goal is to try to “emulate an automatic press” in your printing technique.

Avoid flood stroking if possible and use as few strokes as possible with one stroke being ideal.

Probably the most important element that will control manual halftone printing is the angle of the squeegee.

The more the squeegee is angled downward (assuming equal pressure and speed in the stroke) the more ink that will be deposited and the darker the print will become. Printing halftone dots by hand to achieve good looking prints requires a certain amount of practice and experimentation.

Further, it is recommended that the squeegees that you use for printing halftones have sharp blades 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 become at printing halftones.


Picture
DEALING WITH DOT GAIN

As mentioned above, it is important that you limit the number of strokes, be mindful of your squeegee angle, and control ink buildup on the bottom of the screen once that starts to become 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 the proper printing techniques described and by periodic “wiping clean” the bottom of the screens during the print run.


Picture
If you are printing four color or simulated process color with a manual screen printing press it is recommended that you print from 45-55 LPI.

If you have a high end automatic screen printing press along with a point light source exposure unit and are using professionally produced film positives you may want to consider printing 65 LPI.

If you are new to halftone screen printing we suggest you experiment with various LPI sizes so you can see what the limitations of the equipment are in your particular shop.


Picture
MOIRÉ PATTERNS

Moiré is the visual effect of radiating curves created by superimposing two regular patterns. For example, a moiré pattern can result from overlapping two halftone screens of different angles, dot spacing, dot size and the weave of the fabric.

Moiré patterns are the undesirable result of rescreening an image with a different halftone screen or with the same halftone screen on an angle different from the original.

Moiré patterns create a “herringbone” appearance and make the printed image visibly distracting. Eliminating moiré can be as simple as choosing a different brand of shirt or as complicated as outputting the art at different halftone angles and reburning the screens.


Picture
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 each succeeding color separation will line up in registration as perfectly as possible. Because of that you will find that ink jet positives or professional made film positives will produce a film positive that is vastly superior to using vellum paper.

Vellum paper will shrink as it is going through the laser printing causing a slight registration error. And the toner deposited on vellum paper from a laser printer cannot produce a dot as dark as can be obtained from an inkjet printer onto ink jet film.

As a result, we recommend that you print halftone and halftone color separations whenever possible using an ink jet printer. Be advised that to print halftone dots will require that you have a Postscript printer and/or a RIP processor for your ink jet output device. There are software utilities that can be used for creating halftone images that can be printed a regular inkjet or laser printer.


Picture
BURNING HALFTONE SCREENS

High end exposure units that have a light integration system are the best possible light source for screen making, particularly for exposing screens that contain halftone dots.

A light integrator is an electronic component in the exposure unit that detects and controls changes in light intensity. It will automatically adjust the light output to a consistent light level despite changes in electrical power line levels and for the light intensity changes due to the aging of the light bulb.

With a light integrator, you will be assured of getting the same amount of light each time you expose a screen.

When exposing images such as fine halftone dots, having a light integrator will improve the quality of your screens considerably. Without a light integrator, you will experience light scattering 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 source will work provided you have a dense black film positive. When burning halftone screens it is important to expose them for the correct amount of time. Over exposure will result in a screen that is difficult or impossible to wash the dots out.

An under exposed screen will result in the dots being “washed away” during the screen wash out process. Also be sure to blot out the screens with newsprint or newspaper after the wash out process so no clogging will occur during the screen drying process.



0 Comments

Your comment will be posted after it is approved.


Leave a Reply.

    Author

    ASPA Staff

    Archives

    January 2020
    December 2019

    Categories

    All
    All Over Screen Printing
    All Over Tshirt Printing
    Athletic Team Shirts
    Athletic Wear Printing
    Black And White Printing
    Black Light Ink
    Build An Exposure Light
    Building An Exposure Unit
    Burning A Screen
    Burn Screens With LED Light
    Buy A Screen Printing Press
    Buy Screen Printing Equipment
    Capillary Film
    Coating A Screen
    Coating Screens
    Cold Peel Transfers
    Color Change Ink
    Creating A Stencil
    Curing A Print
    Curing Plastisol Ink
    Custom Clothing Labels
    Custom Printed Designer Tags
    Custom Printed Tags For Tshirts
    Custom Tshirt Labels
    Custom Tshirt Tags
    DIY Exposure Unit
    DIY Light Box
    Drying A Screen Printed Shirt
    Emulsion
    Exposing A Screen
    Exposure Lamp
    Exposure Light
    Exposure Light Box
    Exposure Unit
    Ez Expose Exposure Unit
    Flash Back Ink
    Flash Cure Unit
    Flash Curing
    Flash Curing A Dark Shirt
    Flashing A Screen Print
    Flood Stroke
    Fluorescent Ink
    Gel Ink
    Getting A Straight Print On A Tshirt
    Glow In The Dark Ink
    Golf Shirt Printing
    Halftone Screen Printing
    Hat Printing
    Heart Print On A T-shirt
    Heat Sealed Transfers
    Heat Transfers
    Homemade Exposure Unit
    Hooded Sweat Printing
    Hooded Sweatshirt Printing
    Hoodie Sweatshirt Printing
    Hot Split Transfers
    How To Build An Exposure Unit
    How To Print Caps
    How To Print Visors
    How To Screen Print Ball Caps
    How To Use A Scoop Coater
    How To Use A Squeegee
    Inkjet Film Positive
    LED Exposure Unit
    LED Light For Exposure Unit
    LED Lights For Exposing Screens
    LED Screen Printing Light
    Light For Burning Screens
    Light Source For Exposure Unit
    Lining Up A Film Positive
    Lining Up A Screen Print
    Lining Up A Screen To Film
    Making A Screen
    Making A Stencil
    Manual Screen Printing
    Mercury Vapor Exposure Light
    Mesh Count
    Metal Halide Exposure Light
    Metal Halide Light Exposure
    Metallic Ink
    Multi Color Printing On Dark Shirts
    Multi-color Screen Printing
    Name And Numbers
    Neon Ink
    Off-contact Screen Printing
    Pellon Squares
    Placing An Image On A Tshirt
    Plastisol Ink
    Polo Shirt Printing
    Printing On Dark Colored Shirts
    Printing Photographic Images
    Printing Transfers
    Puff Ink
    Purchase Printing Equipment
    Reflective Ink
    Screen Coating
    Screen Exposure
    Screen Hold Down
    Screen Mesh
    Screen Printing
    Screen Printing Grayscale Images
    Screen Printing Hoodies
    Screen Printing Inks
    Screen Printing Names And Numbers
    Screen Printing On Black Shirts
    Screen Printing On The Left Chest
    Screen Printing Sweatpants
    Screen Printing Sweatshirts
    Screen Print Shop Clean-up
    Screen Print Shop Maintenance
    Serigraphy
    Shimmer Ink
    Shims For Off-contact
    Silk Screen
    Silk Screener
    Silk Screening
    Silk Screen Printing
    Special Effects Ink
    Special Effects Plastisol Inks
    Squeegee Angle
    Suede Ink
    Team Shirts
    Test Prints
    T-shirt Left Chest
    Tshirt Printing
    Vellum Paper
    Vellum Paper Film Positive
    Wet On Wet Screen Printing

    RSS Feed

Advice Notice & Disclaimers | Privacy Policy | Terms of Use | Sitemap | Contact Us | About ASPA | Advertise with Us | ASPA Members

Copyright © 2004-2023 American Screen Printing Association Inc. (ASPA) | The #1 Ranked Screen Printing Association on the Web | All Rights Reserved.

  • Home
  • Online Classes
  • Join ASPA
  • Get Certified
  • Videos
  • Store
  • Equipment & Supplies