How to Expose a Halftone for Screen Printing

Screen printing can be used to print halftone images, such as pencil drawings, ink wash illustrations and black-and-white photos. The key for success is to expose a silk-screen frame using an image that is properly prepared with image editing software such as Photoshop or Corel draw. The image is then printed as a film positive on clear vellum paper and exposed to a high-mesh-count silk screen that has been coated with photo emulsion.

Things You'll Need

• Grayscale image
• Scanner, optional
• Laser printer
• Clear vellum
• Image editing software
• 1/4-inch-thick glass
• Silk screen, 195-230 mesh
• Photo emulsion
• Darkroom
• Foam rubber, 4 inches thick
• Halide lamp
• Newsprint

Instructions

1. Select a photo or illustration to use for halftone printing. Scan the image into a photo editing program such as Photoshop or Corel Draw if it does not already exist as an electronic file. Scan it as a grayscale image at 300 pixels per inch. Set the dot gain for the image at 30 percent. In Photoshop, from the pull down "Edit" menu, select color settings, and set dot gain at 30 percent in the "Color Settings" dialog box.

2. Load your laser printer with clear vellum. Set the "Screen Frequency" to 55 lines per inch, and select "Round" for shape. In Photoshop, these selections are made in the "Print" dialog box. From the this dialog box, select "Screen." This opens the "Halftone Screens" dialog box. Entered the selections above. Click "Ok" when done. Print the image onto the clear vellum.

3. Coat both sides of a silk screen with an even coat of photo emulsion in a darkroom. Use a silk screen with a high mesh count of at least 195 mesh. Place the coated screen in a dark place and let dry overnight.

4. Hang the halide light about 22 inches above the ground in the darkroom. Place the foam rubber beneath it. Put the silk screen frame face-down on the foam rubber. Put the clear vellum with the image centered face-down over the screen. Place the glass over the image and expose the silk screen for five to ten minutes.

5. Remove the glass and clear vellum. In the dark, develop the halftone image on the silk screen with a spray of warm water to both sides of the screen. Once the image area is free of photo emulsion, blot both sides of the silk screen with newsprint.

6. Place the silk screen in sunlight or a well-lit area to dry. Once dry, it may be used for screen printing.

Tips & Warnings

• Increase your exposure time if your design washes out and loses detail. Decrease your exposure time if your halftone dots will not develop.

• Do not use an ink jet printer to print a halftone image for screen printing. Laser printers come designed to print halftone dots, but ink jet printers need plug-in software to execute this task.

References

• Reuels: Silk Screen Printing Instructions