Textile colour separation: guide for screen and rotary printing

Colour separation means splitting a design into one layer for each ink. In screen printing each layer is exposed or engraved onto its own screen, so the number of colours in your design decides the number of screens.

The screens are a large part of the cost. If you design for a mill or a print house in Surat, Jetpur or Ahmedabad, the separated file is what actually goes to the engraver.

This guide covers how separation works, what drives the cost and how to prepare a file that does not come back with questions.

What colour separation is

A printed fabric with a red motif, a green leaf, a yellow butta and a navy ground has four inks. For screen printing, the design is broken into four separate images. Each one is black where that ink should print and empty everywhere else.

Each image becomes one screen: a flat screen on a flatbed table, or a nickel cylinder on a rotary machine. The printer then mixes each ink to shade and prints them one after another on the cloth.

In Photoshop, these separate images are kept as spot channels. A file that holds only spot channels (no RGB or CMYK) is a multichannel file.

Textile CAD programs do the same job with their own colour lists, and the idea is identical: one colour, one channel, one screen.

Spot vs process (CMYK)

There are two main ways to separate.

Spot colour separation: every colour is its own ink, mixed to an exact shade (often matched to a Pantone TCX card or a buyer's swatch). The channel for each ink is mostly solid, with sharp edges.

Fabric printers prefer spot colour because each ink prints as a solid layer, so colours come out clean and saturated. The method works on dark grounds as well as light ones.

Process (CMYK) separation: the design is split into CMYK channels, and tiny halftone dots of the four inks mix in the eye to make other colours. This is how magazines are printed. On fabric, the four transparent process inks need a light ground and the result is less bright than spot inks.

Textile mills mostly use spot colour. A typical saree, dress material or bedsheet design has 4 to 12 flat colours, and the buyer wants each one to match a shade card. Spot separation gives you that control.

Note: Photographic designs with soft shading are the exception. Today most of them go to digital printing, which needs no separation at all.

A third method, simulated process or index separation, uses a set of spot inks with dots or dither patterns to imitate photographic shading. It is common on T-shirts and less common in yardage printing.

How separation is done

By hand in Photoshop

Many job-work designers separate in Photoshop. The usual steps are:

1. Reduce the design to the final number of colours. Use Image > Mode > Indexed Color with a custom palette of exactly your ink colours, with dither set to none, so every pixel becomes one of your inks.
2. Clean the result. Zoom in and remove stray single pixels and noise, especially where two colours meet.
3. For each colour, select it (Select > Color Range or the Magic Wand with anti-alias off) and save the selection as a channel.
4. Turn each saved channel into a spot channel and give it the ink colour, so the preview looks like the print. Photoshop's "solidity" setting only changes the on-screen preview; it has no effect on output.
5. Delete the RGB channels, check that every pixel belongs to some channel, and save as PSD or TIFF with the spot channels kept.

This works, but it is slow for detailed florals. The cleaning step is where most hours go.

With textile CAD software

Dedicated textile CAD programs (NedGraphics, Arahne and similar tools used by Indian mills) automate much of this. You choose the number of colours, the program reduces a scan or a painted design to that many colours, and cleaning tools remove stray pixels.

Most of them also handle repeats, colourways and separation output for the engraver in the same program. The trade-off is cost and training, which is why many smaller studios still use Photoshop.

Colours and screen cost

Every extra colour means one more screen to make, store, set up and clean. The main cost drivers are:

Cost driver	Why it grows with colours
Screens	One per colour. A new screen must be made or re-engraved for each design.
Engraving	Each screen is engraved separately from its separation.
Strike-off (sampling)	Every screen is set up and every ink shade is checked on cloth before the run.
Set-up and cleaning time	More screens to fit, register and wash between designs.
Colourways	Screens can be reused, but every colourway needs its own ink mixing and approval.

Indian rupee prices change by city, mill and order size, and few are published. One figure we found: a 2018 comparison by an Indian textile professional said each rotary screen cost about ₹3,500, with engraving charged extra.

The same comparison said a strike-off sample could cost 3 to 4 times the per-metre order price. Treat these as rough, dated references and ask your printer for current rates.

Example: If screens cost a fixed amount each, a 10-colour design carries more than three times the screen cost of a 3-colour design before a single metre is printed. That is why designers are asked to "bring it down to 8 colours", and why separation and colour reduction go together.

Rotary machines also have a hard limit. Industry sources describe rotary machines that run about 12 to 24 screens, and a design cannot have more colours than the machine has screen positions.

Trapping, half-tones and fall-on

Trapping (overlap)

Screens never line up perfectly on moving cloth. If two colours meet edge to edge in the file, a slight shift leaves a thin white line of unprinted fabric.

Trapping fixes this by making one colour slightly larger, so neighbouring colours overlap a little. One separation guide suggests an overlap of about 0.2 to 0.5 mm. The right amount depends on the fabric, the machine and the printer, so ask before you separate.

Tip: Usually the lighter colour is spread under the darker one, so the overlap is hidden.

Half-tones

A half-tone turns a lighter tint of one ink into a pattern of dots. A 40% dot area of red looks like a pink, from the same red screen.

Half-tones need a finer screen mesh. Screen makers list coarse meshes for heavy, solid coverage and finer meshes for fine details and half-tones.

For rotary printing, one guide gives a halftone range of about 30 to 55 lines per inch, much coarser than paper printing, because fabric spreads the ink. Always check with the engraver which half-tone settings their screens will hold.

Fall-on effects

When two transparent inks print on top of each other, they mix on the cloth and give a third colour. Yellow printed over blue gives a green. Textile designers call this a fall-on.

Planned well, a fall-on gives you an extra shade without an extra screen. Unplanned, it shows up as dark outlines wherever your trap overlaps.

If you want a fall-on, draw that area as an overlap of the two channels and tell the printer which inks are meant to overprint.

Repeat size for rotary screens

On a rotary machine, the screen is a cylinder. One turn of the cylinder prints one length of design, so the vertical repeat must fit the screen's circumference exactly, or divide into it exactly. If it does not, you get a visible joint once per turn.

Common screen circumferences listed by screen makers and calculators are 640 mm, 819 mm (often called 820), 914 mm and 1018 mm. Some suppliers also list 530, 688 and 725 mm. Ask your mill which screens their machine takes before you start the repeat.

Here is what each size means in pixels at 300 DPI (mm ÷ 25.4 = inches, then × 300):

Screen circumference	In inches	Pixels at 300 DPI (one full turn)	Repeats that divide it exactly
640 mm	25.20 in	7,559 px	320 mm (2 per turn), 160 mm (4 per turn)
819 mm	32.24 in	9,673 px	409.5 mm (2 per turn), 273 mm (3 per turn)
914 mm	35.98 in	10,795 px	457 mm (2 per turn)
1018 mm	40.08 in	12,024 px	509 mm (2 per turn)

Example: A design with a 30 cm repeat does not fit a 640 mm screen. You would resize it to 320 mm (two repeats per turn) or 213.3 mm (three per turn). Do this in the design stage, because stretching a finished separation also stretches your trapping.

Our fabric DPI calculator gives the pixel size for any length you choose.

Half-drop and other repeats work on rotary too, as long as the full vertical repeat still divides the circumference. Flatbed screens are limited by the size of the flat screen, but they can take longer repeats than a rotary cylinder.

Flatbed, rotary or digital

The three methods differ mainly in set-up cost and speed, so the order quantity (MOQ) decides which one makes sense.

	Flatbed screen	Rotary screen	Digital (inkjet)
Separation needed	Yes, one flat screen per colour	Yes, one cylinder per colour	No
Repeat limit	Size of the flat screen	Screen circumference (640, 819, 914, 1018 mm and others)	No practical repeat limit
Typical speed	15 to 25 yards per minute	27 to 54 metres per minute, faster on some machines	Much slower on scanning machines; single-pass machines are far faster
Colours	One per screen	One per screen, up to the machine's screen count	Any number
Best for	Large repeats, placement prints, borders and pallus	Long runs of allover yardage	Sampling, short runs, many colourways, photographic designs

Where does digital stop being cheaper? Published estimates differ, but they overlap:

• A study by Gherzi, quoted by a rotary screen and printer maker, put the break-even near 1,200 metres per design. Below that digital is cheaper, between about 1,000 and 3,000 metres it depends on the job, and above that rotary wins.
• A textile industry blog uses a rule of thumb of under 2,000 metres for digital and above 5,000 metres for rotary, with the number of colours and colourways deciding the zone in between.
• The 2018 Indian comparison mentioned earlier said digital was cost effective up to about 2,500 metres for a design with 3 to 4 colourways.

For an Indian designer, the practical logic is simple. A boutique order of 200 metres of a placement print kurti goes digital, with no screens and no separation.

A dress-material mill printing several thousand metres of one allover design in a few colourways uses rotary, and the separation pays for itself.

Tip: You can sample on digital, take orders, and move only the designs that sell in volume to rotary. Rotary lead times are longer because screens must be engraved and a strike-off approved.

Preparing the file

Whether you separate yourself or hand the file to a separation artist, a clean file saves days. Before you send it:

• Fix the colour count. Decide how many inks the printer will use and reduce the design to that number. Note it in the file name, for example "rose-border_8col_640.psd".
• Set the repeat to the screen. Match the vertical repeat to the circumference the mill uses, and check the joins with an offset.
• Work at full size. Separate at the final print size and resolution the engraver asks for. A small file enlarged later gives soft, stepped edges on every screen.
• Remove soft edges. Anti-aliased or blurred edges create in-between colours that belong to no screen. Flat colours with hard edges separate cleanly.
• Keep fine lines printable. Very thin lines and tiny dots may not hold on a coarse screen. Ask the minimum line width.
• Name each channel. Name spot channels by ink and order, such as "1 navy ground", "2 red", "3 yellow".

Warning: Save in a format that keeps channels. PSD and TIFF can store spot channels. JPG cannot, and a design sent as a JPG over WhatsApp will have compression noise that must be cleaned first.

If your separated file is too small for the print width, our AI image upscaler accepts multichannel PSDs with spot channels, as well as TIFF and other formats, and enlarges designs 4× or 8×.

Common mistakes

• Too many near-identical colours. Three slightly different reds in a scan become three screens. Merge them in the colour reduction step.
• No trap. Colours that only touch at the edge show white gaps on the fabric.
• Too much trap with dark inks. A wide overlap of two strong colours prints a dark line, an unwanted fall-on.
• Wrong repeat for the screen. A 30 cm repeat on a 640 mm cylinder leaves a joint on every turn.
• Separating from a compressed JPG. JPG noise turns into speckles on every screen. Start from a TIFF, PSD or PNG.
• Forgetting colourways. If the buyer wants four colourways, check that the separation works for all of them. A fall-on that looks good in one palette may look muddy in another.

Our Channel Separation tool, which splits a design into spot colour channels for screen printing, is coming soon. Join the waitlist on the colour separation page to hear when it opens.

Until then, if your design or its separations are too small for the print size, try the AI image upscaler, which already reads multichannel PSDs with spot channels.