Halftone for screen printing and DTF: the numbers

Why a screen needs coarse dots#

A screen is a mesh; ink passes through the open holes. A halftone dot smaller than a few mesh openings does not stay on the screen (the emulsion cannot hold it) and a dot bigger than the space around it fills in with ink spread (dot gain). So the dot grid has to be much coarser than the mesh, and the tonal range has to avoid the extremes. Newspapers print at 65 to 85 lpi on paper; shirts print at 35 to 55 lpi because the mesh is coarser and the fabric absorbs.

The formula: dot cell in pixels = dpi ÷ lpi. At 300 dpi, 50 lpi is a 6 px cell. Ask the printer what mesh they will use; if they do not know yet, 45 lpi (a 6.7 px cell at 300 dpi) is the safe middle for most shops.

Getting a Voidcanvas halftone to a number#

The Dot Size slider is grid spacing. In the Editor, a filter layer's cell is about Dot Size ÷ 8 document pixels at a 1× export, so on a 300 dpi document: Dot Size 48 is a 6 px cell (50 lpi), 56 is 7 px (43 lpi), 64 is 8 px (37.5 lpi). In the Halftone tool the setting is scaled with the download so the result matches the preview; measure the exported file rather than trusting the slider.

1. 1Set the document to the print size at 300 dpi (a 12 × 16 in shirt print area is 3600 × 4800 px). Place the photo; convert it to black and white with an adjustment layer and push Curves until the shadows are dark and the highlights nearly white. Screens print ink or no ink, so the photo has to be graphic already.
2. 2Add Filter, Artistic, Halftone. Set Dot Size from the table (48 to 64 for most shirts) and Contrast to about 50, so the darkest dots just fill their cell rather than merging into a solid.
3. 3Zoom to 100 per cent (Ctrl+1) and count: the distance between dot centres is your cell. Adjust Dot Size until it matches the lpi you agreed.
4. 4Check the extremes. Very small dots in the highlights will drop out; very large in the shadows will fill in. Use Curves under the filter to pull the tonal range in, so the smallest printed dot is around 10 per cent and the largest around 90.
5. 5One ink per file. Hide everything except the layers for that ink, export a PNG at 1×. Black is ink; white or transparent is no ink. For a two-colour job, make one halftone per colour on its own layer stack and export each separately, at the same size, so they register.

DTF is not screen printing#

Direct-to-film prints CMYK plus white through an inkjet onto film, and the printer's RIP software converts the whole image into its own fine screen (often 55 lpi or more) before printing. If you send it an image that is already halftoned, the RIP screens your dots again, and two grids at similar angles make moiré. So for DTF:

• Supply full-resolution artwork with a transparent background (PNG at 300 dpi at final size). Let the RIP do the screening; it knows the film and the powder.
• If you want visible halftone dots as part of the design, make them coarse: a cell of 6 px or more at 300 dpi, which the RIP's much finer screen renders cleanly as shapes rather than fighting.
• Keep the smallest solid detail above about 1 mm; very fine dots and hairlines shed powder and peel.
• Transparent means no ink and no white underbase. Anything semi-transparent gets a partial white layer under it, which can look chalky on dark garments. Keep edges hard.

How Voidcanvas handles this#

The Halftone tool and the Halftone filter run the same code; the filter gives you the document-size control and the layer stack for one ink per file. Exports are PNG with transparency preserved, so a halftone on a transparent layer exports as black dots on nothing, which is what a screen positive and a DTF file both want. There is no CMYK separation and no RIP; for process work, send the printer a high-resolution RGB file and let them separate.

Open the Halftone tool