Short answer
To upscale spot channel separations, enlarge the art before it is halftoned, never the halftoned channels themselves. Enlarging a halftone makes every dot bigger and halves the line count at 2×. Upscale the color art or a flat preview of the inks to the final print size, re-separate at that size, then halftone last. Our upscaler reads multichannel PSDs with spot channels and returns one flat color PNG.
Key points
- Never enlarge halftoned channels. Resize the art first and halftone last.
- Hard-edged spot art needs more pixels than halftones do; line art guides suggest 600 ppi or more.
- Our upscaler shows each spot channel in its ink color and returns one flat PNG to re-separate.
- Redraw text and simple logos as vector; upscale painted, photo and textured art.
- Keep the old channel order and ink names so the new separation matches the job.
Screen printers get small files all the time: an old job saved at 8 inches that now has to print 12, or a customer logo pulled off a website. When the file is a set of spot channels, the usual "make it bigger" advice needs care, because how you enlarge decides whether the screens still hold.
This guide covers enlarging separations. For how separation itself works (spot vs process, trapping, building channels in Photoshop), see the textile color separation guide.
Resize first, halftone last
A halftoned channel is a 1-bit image: every pixel is ink or no ink, arranged in dots at a set line count (LPI). Enlarging it does not make the dots finer. It makes each dot bigger.
At 2×, a channel halftoned at 45 LPI becomes a 22.5 LPI halftone, with dots twice the size. The resample also blurs the hard dot edges into gray, so the dots are no longer clean ink or no ink.
The fix is to do things in order. UMBC's halftone handout starts from a grayscale file "at least 150 ppi at the size you will print it" and only then converts to a bitmap halftone. ScreenPrinting.com warns that once a design is converted to halftones, "you won't be able to make any changes to the image."
Warning: If all you have is a halftoned separation, do not upscale it. Ask for the art from before the halftone step, or rebuild the tonal channels from a color version of the design.
Solid spot channels with hard edges and no halftones survive a small enlargement better. They still soften at the edges, which is why the next section matters.
Resolution for separations
Halftones need fewer pixels than most people think. Vanderbilt's resolution guide gives the rule of thumb of 2 pixels per halftone line, and says 1.5 to 2 times the LPI is fine. UMBC suggests a starting LPI of mesh ÷ 5, and notes its formulas "are not fixed rules."
| Mesh | Starting LPI (mesh ÷ 5) | Minimum ppi at print size (1.5 to 2 × LPI) |
|---|---|---|
| 156 | 31 | 47 to 62 |
| 230 | 46 | 69 to 92 |
| 305 | 61 | 92 to 122 |
Hard edges are a different story. The same UMBC sheet suggests 600 to 1200 ppi for line art and black-and-white logos, and 300 ppi as the typical file resolution for photographs. A clean spot-color edge needs far more pixels than a halftone does.
Example: A 12 inch wide full-front print. At 300 ppi that is 12 × 300 = 3,600 px wide. For a crisp line-art channel at 600 ppi it is 7,200 px. If the art you received is 900 px wide, you need 4× to reach 300 ppi.
When you enlarge, aim for the final print size at your working resolution, then separate and halftone at that size. Our fabric DPI calculator gives the pixels for any width in inches or cm.
Enlarging a spot-channel PSD
Our Image Upscaler opens multichannel PSDs. Before you rely on it, here is exactly what it does with one:
- It accepts PSD and PSB files: multichannel files made only of spot channels, and RGB, CMYK, grayscale or indexed files with extra spot channels. 8-bit and 16-bit files both open, and the result is 8-bit.
- It reads the flattened image stored in the file, so layers are not edited one by one.
- Each spot channel is laid over the image in its own ink color, using the channel's solidity setting, the way Photoshop previews it.
- Alpha channels and saved selections are skipped, because they do not print. If a multichannel file has no ink colors saved, its channels show as dark gray.
- The result is one flat color PNG tagged 300 DPI, enlarged 4× or 8×. The spot channels do not stay separate.
Note: A TIFF with spot channels should be saved as a PSD first, so the channels show in their ink colors.
So the upscaler gives you a sharp, enlarged picture of how the inks look together. You then separate that picture again at the new size. Because every ink shows in its own flat color, Photoshop's Select > Color Range can pick each one back out, as the separation guide describes.
Steps
- Check the channels in Photoshop. If they are halftoned, stop and find the pre-halftone art.
- Note the channel order, ink names and colors, and the trap you used.
- Save the file as PSD (or PSB), with the spot channels in place.
- Upload it to the Image Upscaler, sign in with your free account and choose 4× or 8×.
- Open the PNG and compare it with the original at 100% zoom, especially fine lines and small text.
- Re-separate at the final size, name the channels in the old order, add trap, then halftone last.
Free results go up to 36 megapixels, so 4× works for files up to about 1,500 × 1,500 px and 8× up to about 750 × 750 px. Pro adds finer detail and 16×, with results up to 50 MP; in the US, credit packs start at $1.49 for 250 credits.
Low-res customer art: upscale or redraw?
Many screen-print shops tell customers to send vector art, and for some art that is still the best answer. Use the art type to decide.
| What the customer sent | Best fix | Why |
|---|---|---|
| Text or a typed logo | Retype or redraw as vector | Exact letter shapes and clean screen edges |
| Simple flat-color logo | Redraw as vector | A few shapes are quick to trace, and vector scales to any size |
| Hand-drawn line art | Redraw, or upscale and test at 100% | Thin lines can break or thicken |
| Painted or textured illustration | AI upscale, then separate | Texture is slow to rebuild by hand |
| Photo or photo-real art | AI upscale, then simulated process | No vector version exists |
| Unreadable, tiny or heavily compressed file | Ask for a better file | The AI would have to guess what was there |
AI upscaling adds new detail where a plain resize only blurs. The AI upscaling vs Photoshop resize guide shows the difference, and how to choose an upscaler for fine detail has a 100% zoom test you can run on your own art.
Tip: Run the separation from a PNG or TIFF, never a re-saved JPG. JPG blocks turn into speckle on every screen. The TIFF vs JPG vs PNG guide explains why.
How many screens?
Once the art is big enough, the screen count decides the method. ScreenPrinting.com describes spot color as best for logos, text and line art with clean edges, and simulated process, which builds images from halftones of spot inks over a white base, as the choice for photos and complex raster art on any garment color.
If the enlarged art comes back with more shades than you have screens, merge them before separating. Reducing colors for screen printing walks through it step by step.
Coming soon: Channel Separation
Our Channel Separation tool will split a design into spot color channels for screen printing. It is not live yet. Join the waitlist to hear when it opens.
What to do next
Check whether your channels are halftoned. If they are solid spot channels, or you have the color art, upload the PSD to the free Image Upscaler, choose 4× or 8×, and re-separate the PNG at the final print size before you halftone.
Questions people ask
Can I resize separations in Photoshop?
You can resize solid, hard-edged spot channels a little, but edges soften with any resample. Halftoned channels should not be resized at all, because the dots grow with the image and the line count drops. Go back to the art before halftoning, resize that, and separate again.
What resolution should screen printing separations be?
At the final print size, a common rule is 1.5 to 2 pixels per halftone line, so a 45 LPI halftone needs at least about 68 to 90 ppi. Hard edges need far more: one university print shop guide suggests 600 to 1200 ppi for line art. The same guide gives 300 ppi as the typical resolution for photographs.
How do I convert a multichannel PSD to PNG?
In Photoshop you can merge the spot channels into the image and save a copy as PNG. Our Image Upscaler also opens multichannel PSDs: it shows each spot channel in its ink color, the way Photoshop previews it, and gives you back an enlarged PNG.
My customer sent a low res logo. Should I upscale it?
For text and simple flat logos, redrawing as vector gives the cleanest screens. For painted, photographic or textured art that cannot be redrawn easily, an AI upscale to the final print size is a good base for separation.