Introduction
Process color screen printing refers to printing full color images using the four primary colors of printing: cyan, magenta, yellow and black (CMYK). It allows printers to print photorealistic images by combining layers of the four process colors.
Process color printing is the industry standard for printing high quality, full color images onto T-shirts, posters, brochures and other items. It works by using screens to apply each process color as a separate layer. The colors overlay one another to create the full spectrum of hues in the finished print.
Compared to spot color printing which uses premixed solid ink colors, process color provides far greater color accuracy and range. With thousands of possible color combinations, process printing can reproduce virtually any photograph or digital design in full color. It offers superior image quality and color consistency across large print runs.
In this 4000 word guide, we provide a comprehensive overview of process color screen printing, explaining what it is, why it’s used, how the colors are printed and what’s involved in setting up process color screen printing.
What are the four process colors?
Process color printing uses four standard process color inks:
– Cyan – The cyan ink produces a light blue color and is used to print blues and greens. Cyan absorbs red light.
– Magenta – The magenta ink makes a pinkish hue and is used to print reds and purples. Magenta absorbs green light.
– Yellow – The yellow ink produces a light yellow color and prints yellows. Yellow absorbs blue light.
– Black (K) – The black ink is used to print black elements and also darkens other colors when mixed.
These four process color inks are transparent and overlay each other to produce the full spectrum of colors. Different combinations create different hues. For example, cyan + magenta = blue, cyan + yellow = green, magenta + yellow = red, and all three combined = various shades of gray.
The black ink is used to print black elements, shadows and outlines. It also controls contrast, darkness and color balance. The combination of all four inks (CMYK) can reproduce almost all visible colors.
Process color printing is sometimes called full color printing or 4-color printing based on the use of these four process colors. The CMYK model aligns with the way our eyes perceive color through combinations of primary colors of light.
How does process color printing work?
Process color printing works by separating a full color image into four colors (CMYK) and applying each as a halftone layer on press. Here is an overview of the process:
1. Color separation: The full color artwork is digitally separated into CMYK channels using software like Adobe Photoshop. This breaks the image down into the four process printing inks.
2. Halftones: Each CMYK channel is converted into a halftone image which uses various sized dots to produce different densities of ink. This is necessary because the presses can only print solid ink, not gradients.
3. Film positives: The halftone separations are printed onto transparent film to make the film positives. These contain the dot patterns for each color layer.
4. Screens: Screens are made by adhering the film positives to pre-stretched mesh. A finer mesh is used for detail, coarser for large areas of color.
5. Registration: Screens are precisely aligned on press using registration marks so the color layers print in perfect register.
6. Printing: Ink is squeegeed through the screens to apply each process color layer in sequence, gradually building up the full image.
7. Curing: Ink is cured and bonded onto the material between color layers, often using a heat press or conveyor oven.
The combination of the four printed halftone layers combines to create the full color image when viewed at normal distance. The principle is similar to how color TV and computer monitors mix RGB colors to display images.
Why is process color printing used?
There are several key reasons why process color printing is the preferred method for printing full color artwork onto apparel, signage, packaging and other products:
– Photographic image quality – Process printing can reproduce subtle gradients and seemingly unlimited colors, allowing photographic images to be printed.
– Consistent and accurate color – With process colors mixed on press, consistent and reliable color is achieved across the entire print run. Pantone colors can also be simulated using CMYK mixes.
– Large color gamut – Thousands of color combinations are possible by mixing the four process inks. Much greater than spot colors.
– Cost effective long runs – For long print runs, process color is far more affordable than spot colors as only four inks are required no matter the design.
– Standardization – Process colors are industry standards, used for reproducing all types of full color print projects. The files are device independent.
– Visual sharpness – Layering transparent inks on press creates sharp details compared to pre-mixing inks. The registration can be finely adjusted.
– Special effects – Visual effects like gradients, vignettes and drop shadows can only be produced using process color overprints.
For these reasons, process color is used for the vast majority of full color printing jobs, especially apparel decoration, signage, packaging, marketing materials, publications and photobooks. It provides the most accurate and cost effective way to print high quality, photographic images.
What is the alternative to process colors?
The alternative to printing with process colors is to use solid spot color inks. Common examples include:
– Pantone matching system (PMS) – A popular proprietary spot color standard used in graphic design. Involves premixed solid inks.
– Custom spot colors – Spot colors can be custom created by ink mixing companies to produce signature brand colors like Coca-Cola red.
– Metallic and fluorescent colors – Special effect pigments produce colors like silver, gold or neon hues.
– RGB and latex inks – Used on wide format inkjet printers. Limited for apparel printing.
Compared to process colors, spot colors provide more intense, vibrant hues but with substantially reduced color gamut. gradations aren’t possible. They involve higher setup costs and ink mixing.
Spot colors are best for simple graphics, flat areas of color, branding applications and specialty effects like metallics. For photorealistic images, process colors are superior.
Many print projects will use both spot colors and process colors together. The spot colors are normally printed first as a solid background, followed by the process color image layers.
How many process colors are possible?
With process color printing, thousands of distinct colors can theoretically be reproduced. Here’s an overview:
– 4-color process – Using just CMYK, around 4,000 colors can be mixed although color gamut is reduced.
– 6-color process – Adding light cyan and light magenta extends the color range to around 6,000 colors.
– 8-color process – Further adding orange and green increases the gamut to over 8,000 colors for optimal quality.
– Digital printers – The latest wide format printers can reproduce over 16 million colors using CMYK plus light inks.
– Hexachrome printing – A 6-ink system of CMYK plus orange and green for over 1 million colors.
– Custom color profiles – Using ICC color management profiles, targeted colors can be tuned.
While thousands of colors are possible, in reality visible limitation reduce the number of discernible colors. The human eye can only see around 10 million different surface colors.
For most purposes, 4-color process printing delivers sufficient range and quality at low cost. Six colors may be used for fine art reproduction requiring maximized color gamut. Beyond six or eight colors has diminishing returns.
What is a halftone in screen printing?
Halftones are a key part of reproducing photographic images using process color screen printing.
During the prepress stage, each process color separation is converted from continuous color to a halftone image. This breaks the image into a pattern of variably sized dots ranging from small to large.
These halftone dots are spaced evenly at a certain lines per inch (LPI) frequency. Common examples are 45 LPI for t-shirts or 150 LPI for higher resolution printing like posters.
The larger the halftone dot, the more ink coverage. Small dots deposit less ink. Varying the sizes produces lighter and darker areas. Grouping the dots together makes different colors.
The smooth blends of tiny halftone dots combined with transparent process inks overlaying creates the impression of continuous color or tone to our eyes. The dots are only visible under magnification.
Halftones allow the solid inks used in screen printing (and other printing processes) to print what appears to be variable color tones. This essential technique makes process color printing possible.
What does lines per inch (LPI) mean in screen printing?
Lines per inch (LPI) refers to the frequency or ruling of the halftone dot pattern in process color printing. It determines how fine or coarse the halftone pattern is.
Common LPI values used in screen printing include:
– 20 to 34 LPI – Very coarse halftones best for printing on rough or uneven surfaces where detail is not essential.
– 35 to 54 LPI – Suitable for printing simpler images onto t-shirts, tote bags and other promotional items.
– 55 to 85 LPI – Mid-range detail good for posters, murals, signage and gaining photographic effect.
– 86 to 120 LPI – High resolution halftones for decals, packaging, displays and publishing requiring precision.
– 121 to 150 LPI – Ultra fine details for logo printing, point of sale materials, overlays and decals demanding crispness.
The coarser the halftone (lower LPI), the larger dots blend over greater distance so images look smooth from further away. Finer LPI values provide precision but must be viewed up close.
LPI must be matched both to the image being printed and the printing application. A photograph requires higher LPI than a cartoon graphic. Fine details need higher LPI than large solid color areas.
How is image registration achieved?
Registration refers to aligning the printed color layers so they overlay each other precisely. Here are the main techniques used:
– Screen registration – Screens have registration marks. Micro-registration allows adjusting the screens minutely between print strokes.
– Press registration – Precision engineered presses hold tight tolerances. Some have micro-registration adjustments.
– Jig system – Print platens are aligned to jigs ensuring each print is positioned accurately.
– Laser registration – Laser crosshairs align the garment and print pallet for automated screen printing.
– Print flash print – Flash curing between each print stroke helps consecutive colors register correctly.
– Vacuum pallets – Holding the print material fixed in place aids registration between colors.
– Dimensional stability – Using stable and rigid screens and pallets prevents stretching or distortion when printing multiple layers.
– Operator skill – Experienced press operators learn to adjust pressure and angle to gain good registration across color passes.
Maintaining correct registration is especially important with process color printing to gain sharpness and depth as the color overlays build up. Even small misalignment leads to blurring.
What mesh counts are used for process printing?
In process color printing, finer mesh counts are used for the lighter cyan, magenta and yellow colors to control ink deposit. Coarser mesh counts are used for black and for base platens. Typical mesh counts include:
– 305 to 355 mesh – Used for base platens when printing on fabric. Provides good ink laydown.
– 355 to 420 mesh – Suitable for the cyan, magenta and yellow colors. Controls dot gain.
– 420 to 490 mesh – Often used for black ink layers. Lays down smooth coverage.
– Higher mesh counts – Can be used where maximum detail and resolution is essential.
Finer mesh counts result in sharper edge definition and registration. But excess ink deposit can cause the dots to gain in size. So mesh counts around 300 to 420 offer the best balance.
Coarse mesh counts may be used for underbases, discharge inks that bleed easily, and composite black layers requiring high ink deposit. Specific mesh selection depends on factors like ink viscosity and density.
Do waterbase and plastisol inks differ for process printing?
Waterbase and plastisol ink types can both be used for process color printing, each with pros and cons:
– Waterbase pros – brighter more transparent colors, very soft hand, reduced dot gain, wide color gamut
– Waterbase cons – slow drying, risk of washout on dark fabrics, pretreatment often required
– Plastisol pros – very opaque coverage, fast drying, prints well on dark fabrics, durable prints
– Plastisol cons – slightly narrower color range, harder hand, increased dot gain
Waterbase excels where high vibrancy and softness is desired such as printing on lightweight fashion garments and soft signage. Washout is managed using underbases.
Plastisol is preferred for printing onto darker fabrics thanks to its dense ink film opacity. It also performs well for harder, industrial applications like banners where abrasion resistance is needed.
Many printers use plastisol for the base and black layers then waterbase for the CMY colors to gain benefits of both ink types. This works very effectively.
How is vignetting achieved with process colors?
Vignetting refers to an image effect where the color and tone gradually fades out towards the edges. It focuses attention on the central subject. Here are two ways to achieve vignettes with process printing:
1. Halftone vignettes – The percent dot values in the image channels decrease evenly towards the edges. For example from 100% to 0%. This lightens the colors fading into the substrate.
2. Color blending – Light base coats are printed then overprinted with the process colors. The central colors mix over the base producing richer hues that graduate outwards.
Vignette effects require precise halftone alignment between separations. They are best printed with a high LPI halftone ruling to gain smoothness in the gradient.
Using transparent waterbase inks makes it easier to produce color blending vignettes compared to opaque plastisols. Discharge bases can also mimic this graduated fading.
How are halftones created for screen printing?
There are several alternative workflows for producing halftones ready for screen printing:
1. Film positives – Halftones are output to film then the films are used to make screens. This analog process has largely been replaced by digital methods.
2. Direct emulsion screens – Using a digital halftone output device like an Epson printer or CTS imaging system, the separations are printed or imaged directly to pre-coated screen mesh.
3. Vintage film processing – Some screen printers process halftone positives themselves using analog darkroom techniques for artistic effect.
4. Direct inkjet film – Special inkjet printers using opaque ink can print film positives ready for screen making. Offers full digital workflow.
5. Hybrid workflows – Many printers combine digital and analog steps. For example, scanning hand-drawn prints to generate the films digitally.
Dedicated halftone RIPs (raster image processors) optimize the separations for the screen printing process. Most now output direct for photopolymer emulsion exposure or printing.
What is the benefit of CMYK printing versus RGB?
Process color printing uses CMYK (cyan, magenta, yellow, black) inks but designers normally work in RGB (red, green, blue) colorspace. Here’s why CMYK is used for printing instead:
– CMYK aligns with real process printing inks. RGB is just light on screens. CMYK gives accurate printed color.
– Cyan, magenta and yellow are transparent printing primaries. RGB red, green and blue cannot be printed.
– Black ink is needed for shadows, outlines and gray mixing. RGB has no separate black channel.
– CMYK has a smaller gamut than RGB but matches the printable spectrum. RGB has colors that cannot print.
– CMYK allows for press color profiles. RGB varies on every monitor so is not color managed for print.
– Converting RGB files to CMYK separates the channels ready for printing plates or films.
– Only CMYK provides WYSIWYG (what you see is what you get) printed output matching the design file.
For these reasons, using CMYK source files is vital for achieving predictable, accurate color matching in any printing process including process color screen printing.
How are spot colors simulated using process colors?
Special brand colors and Pantone colors are often specified in designs to be screen printed. By mixing process inks, these spot colors can be simulated on press.
1. Color lookup tables – Reference charts list the CMYK mixes needed to match Pantone and other spot colors as closely as possible.
2. Color profiling software – Applications like Pantone Color Manager calculate the closest CMYK formula for spot colors.
3. Visually match on press – The printer mixes and tests process ink combinations during setup until achieving a match.
4. Print a color swatch book – Save swatch samples of each spot color match in a book for easy reference.
No simulated color will ever exactly match a solid spot ink. But with care, process colors can reproduce spot colors within an acceptable delta-E color difference. Modern color management improves results.
For best effect, spot colors should be specified consistently across designs rather than converting different formulas on the fly during printing.
Can special effects be created with process colors?
Many decorative effects can be achieved by manipulating the process colors during printing:
– Metallic look – Add silver or gold metallic powder to the base ink. Then print process colors on top.
– Pearlescent effect – Mixing mica powder pigments into the process inks create a soft shimmering pearlized look.
– Day-Flu
