Colors from a color separation are output as positives on transparent film for screen printing.Â Film positives can be output onto sheets or rolls of film using inkjet printers. Inkjet printers with the correct film work well for output, are inexpensive, and readily available.
Black and White
Film positives for making a screen printing stencil are printed with solid black shapes on a clear background. Gray tones that are not 100% white or black need to be converted to a shape or pattern. The most common approach is to use aÂ Halftone dot.
Halftones are named by the number of dots used per inch, this it the halftone frequency.Â While the size of the white and black areas in a Halftone pattern changes, the number of dots per inch remain the same. The halftone dots create a pattern of light and dark . Overlapping halftone patterns can be rotated to reduce interference, or moire. The degree of rotation is called the Screen Angle.
Halftone dots and Grayscale
The size of the dot used is determined by the screen printing mesh. For simulated process and process printing it is best to keep the dots as small as possible, so using a fine mesh is best. Mesh counts between 195 and 355 are common, using halftone dot frequencies between 40 and 65 lpi.
To determine the halftone frequency divide the mesh count by 5
Increasing the mesh count can help to reduce moire.
â€˘ Underbase – use a 195 mesh with a 45 to 50 lpi halftone.
â€˘ Colors – Use 230 to 305 mesh with 50 to 55 lpi.
â€˘ Black and Highlight – Use 305 to 355 Mesh with 50 to 65 lpi.
Halftone dots overprinted at various angles
Angle and the Moire Pattern
Moire appears as light and dark areas in an image when two regular patterns are overprinted. When halftone screens are used, their alignment can be changed by a few degrees to reduce this effect. The regular patterns that can cause Moire in screen printing include various halftone dots, the mesh itself, and the garment being printed on.Â
A good set of angles are: 22.5 – 52.5 – 82.5
On light colored garments the white, yellow and blacks can all be at the same angle:Â 22.5
The reds and oranges :Â 52.5
And the blues, greens and purples at the third angle:Â 82.5
On a dark colored garment put the under print at 22.5 and the black at 52.5.
Output resolution determines the line quality of a film
Edge definition and Banding
When considering the quality of your film output it is important to look closely at the edges of the halftone dots, text and graphics. Output resolutions below 600ppi will leave halftone dots poorly defined. Output resolution is determined by the Printers resolution as well as the halftone conversion method.
The ability of a halftone to represent a grayscale is also determined by its output resolution. Lower output resolutions from a printer or a Rip (Raster Image Processor), can limit a halftones range of values, and the result is banding. Banding is an apparent step between different values in a grayscale. To eliminate banding, either increase the output resolution, or decrease the halftone screen count.
Banding can result from low output reslutions
â€˘ Check for banding in the original image first. Banding can be caused by several factors before your image is output.
â€˘ If an image is smooth but the output is banded then it is caused by the output resolution and halftone line count.
â€˘ The number of gray levels that can be created by a halftone line screen at a specific output resolution can be calculated using a formula.
Where R is the output resolution and H is the halftone screen count
(R/ H)sq+ 1 = Gray levels
(The Output resolution (R), Divided by the halftone screen count (H)) squared +1 = the number of gray levels you can print.
For an output resolution of 720 and a line screen of 45 lpi
720 Divided by 45 = 16
16 x 16 + 1 = 257
According to the formula, at an output resolution of 720ppi, we will need to use a 45 line screen to get the 256 levels of gray we see on screen. In practice a 50 line screen works well with an output resolution of 720 ppi. If you do have problems with banding in your output, you can either reduce the halftone line count, or increase the output resolution, until the banding is eliminated.
Printing from Adobe PhotoshopÂ®
In Adobe Photoshop the output settings can be found in the print preview dialog. Under the file menu choose Print, or Print with preview In earlier versions:
( PS7 to CS2 ) File/Print with Preview:
Print using the Print with Preview option.
â€˘ Select More Options to open the options dialog.
â€˘ Set the output drop down to Color Management.
â€˘ Set the print space to Document.
â€˘ Set the Color Handling Option to Separations.
The Color Management should be set to Separations to output film.
â€˘ Change the Color Management Drop down to Output.
â€˘ Select Registration marks and labels.
The Output Settings
(CS3 – CS4) File/ Print
â€˘ Set the drop down to Color Management
â€˘ Set the print space to Document
â€˘ Set the Color Handling Option to Separations
Setting the color management to Separations in Adobe PhotoshopÂ® CS3.
â€˘ Change the Color Management Drop down to Output.
â€˘ Select Registration marks and labels.
Printing from Adobe Photoshop to a third party Rip
A Rip is a software program does the job of controlling a printers output, providing control over a range of functions including color conversion, ink deposit and dot patterns. A Rip can convert grayscale images to a halftone dot screen as part of its function to control a printers output. Some Rips do not produce dots large enough to be screen printed and are designed for color reproduction, while other Rips are developed to produce only film and can not be used for color printing. Consult the Rips instruction manual to ensure it is capable of creating the needed halftones and the proper setup procedure. A Rip needs to be properly configured to create quality film output. A good Rip will allow you to set your Screen angles and frequencies within Adobe Photoshop.
Output Settings – Dot Size, Shape and Angle
â€˘ Choose the Output Settings drop down in the print preview menu, click Screen to set the halftone dots. In the Screen Menu, uncheck “Use Default Screen” and set the Frequency and Screen Angle for each channel. Each spot color and CMYK channel can be selected from the drop down menu. Select Ellipse for the dot shape, and select Use Accurate Screens and Use Same Shape for All Inks.
Setting the screen frequencies and angles
High resolution 1 bit images can be printed straight to film with good results. Separation channels can be converted to halftones using the built in Bitmap function. Converting a document to grayscale or multichannel mode enables the bitmap mode option. Only the single, or top channel will be used in the conversion.
All printing devices produce some dot gain which needs to be compensated for, and a Rip will allow for this gain by applying a linearization curve. A linearization curve can be made by taking a set of readings using a Transmission densitometer, measuring each 10% step along the value range from a printed halftone step wedge. These measurements are used to create a curve that compensates for the changes in dot size that result from printing.
Every printer is different in the amount of gain it produces on a particular film, so ideally we would want to create a Linearization curve for each printer and film combination. For screen printing we just need to get in the ballpark, however, so a generic curve will often work fine. To compensate for dot gain, apply a linearization curve to an image or separation channel prior to conversion to bitmap.
Converting Grayscale images to 1 bit Halftones.
To create a printable 1 bit halftone from an image, convert the document to gray scale using:
Image/Image Mode/ Grayscale
To create a printable halftone from Separation channels, move the channel you want to convert to the top of the channels palette. Convert the image to bitmap using:
Setting the Output Resolution and Method
â€˘ For resolution enter 720ppi for grayscale images and separations that require halftones,
select Pixels/inch in the drop down
â€˘ For conversion Method choose Halftone Screen.
(According to the Gray Level formula above, an output resolution of 720 ppi or higher is needed for a 50 Lpi halftone. For higher line screens, increase this resolution setting to avoid banding)
Setting the Halftone Screen Frequency, size and shape
In the halftone screen dialog choose your dots size angle and shape:
â€˘ Set the Screen Frequency to 50 and set the drop down to Lines /inch
â€˘ Set the angle to 22.5
â€˘ Choose Ellipse for the shape.
Files can be converted back to grayscale and multichannel as needed, being careful not to resize or modify the image. If the image size needs to be changed, it is best to go back and modify the original and reconvert the image to Halftones. To convert the bitmap image back to a channel choose:
Enter 1 in the Size ratio box and click OK,
To create a new spot color channel. Double click the channel to set its color.
Use a size ratio of 1 when converting back to grayscale.
Printing bitmap images onto film using a standard Printer Driver.
Files converted in this way can be printed straight to film using a standard printer driver, avoiding the need for processing with a Rip. Use a quality, transparent polyester film coated for inkjet printing, a printer withÂ resolutions of 1440 or higher, and a quality ink such as Â®Epson Black.
The quality of results from this technique depend entirely on the quality of the ink and film used. Inkjet printers using pigment based inks will require a specially coated waterproof film, while dye based inks generally print on coated, non-waterproof films as well. Most films are coated on a single side, and can be printed on the coated side only.
Printing from Adobe Photoshop
Using the same process as above; Select your printer in the Print with Preview menu, set the color management settings to separations, and the output settings to print labels and registrations marks if needed. Screens should be left to the default setting.
In the Printer Driver select the printer, set the Resolution and media type to print 1440Â by 720 DPI or higher high quality Transparencies, Film or Gloss photo.
Setting the Media type to print 1440 Dpi transparencies.
Increasing print density
In most cases using a resolution of 1440 will provide a high density print when applied to a quality film. In some cases the ink density can be increased by merely increasing the output resolution. In other cases increasing the density may be possible within the driver itself.
In the dialog below the driver has additional output controls which allow for increased ink density. It is important to remember that any increases to the ink density will increase overall dot gain from the output device. The minimum ink deposit required for an opaque print is best.
Setting additional output controls in the printer driver.
Save the setting as a Printer Preset and click Print.
For automatic conversion to bitmap halftones try the Free SimRip droplet for Adobe PhotoshopÂ®.