Robert Hewitt, senior screen printer and developer at Red Oak Roller, breaks down how he manually created a simulated process colour separation of his original artwork
To create great prints, you must start at the beginning in Photoshop and learn how to separate the artwork into channels (which store colour information) ready for film output.
For this artwork, my first step was to consider dot gain, which is essentially the spreading of ink on the substrate. There are a few variables on press that will cause this so it’s a good idea to compensate for any gain within your separations. To accommodate for this, go to Edit, then Preferences then Colour Settings – in that last window you’ll find some options that can be adjusted.
To tell Photoshop to compensate for dot gain and display colours correctly, in the dropdown RGB window (under Colour Settings) I set it to Apple RGB. In the CMYK window, I selected Custom CMYK and set the ink colours as ‘SWOP (newsprint)’ – this gives a 30% dot gain in CMYK mode and mimics an absorbent substrate such as a T-shirt. For ink generation, selecting Medium for Black Generation, with the Black Ink Limit at 85% and Total Ink Limit at 250%, will back othe amount of ink you lay down with your CMYK process prints, giving a less harsh colour saturation. I then changed the Grey and Spot to 30% dot gain (see step 1).
For this step-by-step, as I was separating for a simulated process print instead of a separating a CMYK process print, I used more opaque inks and specific colour selections which give a much punchier print as well as more control over my seps, instead of four transparent inks blending together to create a full-colour image.
Look at the artwork (which must be 300dpi at its final print size) – how should it be printed? Are there any adjustments to be made? Has it been flattened onto a background that needs removing? What type of garment and what colour? How many colours can you print on your press? Four? Eight? 12? Will it be printed manually or on an automatic? Plastisol? Water- based? Are you printing discharge? As this example uses tonal artwork, I opted for a Gildan Softstyle Ringspun T-shirt (GD01) as it has a tight weave which ensures the halftone dots don’t get lost in the depths of the fabric – the garment choice could very well make or break your print.
I was using a six-colour press so I needed to make this design work with six colours or less. As the tee is white that acted as my opacity so there was no need for an underbase here, but I have used a highlight white. A highlight really ties your inks together so try to use one if you can. Even with CMYK use a highlight as well as any spot colours you feel the print may benefit from: maybe two screens for your black, one for the shades and one for the solid spot blacks, and use an opaque ink to really get that contrast.
There are different methods to separate your art: working in RGB mode, I got my highlight from a black and white adjustment (see step 2, over page). I selected Maximum Black, which pushes all the white backwards leaving only the hottest areas of white – perfect for a highlight. This was inverted then copy and pasted into a channel. I’ll also use this method for an underbase or a black screen: I use Maximum White, which does the opposite and brings more white forward and pushes back all the black in the image. Invert this for a base or leave it as it is for a black screen then again copy and paste into a channel.
Next, I named the channel, then set it to Spot and assigned it a colour before setting the solidity. The solidity does not change the colour information stored in a channel; instead, you want to imitate the opacity of the ink on the computer screen. Can you get a 100% opaque white in one hit? No? Okay, set your underbase opacity to around 75-85%. Maybe print some white on a shirt and compare it to what is displayed on your monitor to judge a more accurate opacity.
For the highlight, I brought it up to 90-95% because I was printing over a white base (in this case, the white shirt). Most colours are around 5% solidity; reds, blues and greens in general are not strong enough by themselves on a dark shirt so it’s a good idea to boost your base under those colours to bring back the opacity, unless that’s what you’re going for. Up the solidity for inks with more white in them like greys or flesh tones, etc. Discharge inks should be set to a solidity of 100%. As with all separations, each channel will need individual attention with minor adjustments. The applied dot gain made the white highlight look way too strong so it needed bringing back a bit – you can use the Levels or Curves here, or use the Apply Image option.
Pulling colours is a little different and again there are various ways of doing things: hue saturation and brightness (HSB) sliders, black and white adjustment layers, or manually using the Colour Range tool. If you’ve opted for the Colour Range tool, use it to sample the required colour in your image then use the Fuzziness tool to select how much you want to pull (see step 6). You can select more than you need and knock it back a little using the adjustment tools.
The presets within the Colour Range tool are another great way of grabbing colours, especially if you have limited print heads. I used presets together with custom pulls to create this set of seps because I had a six-colour press and the art could easily have been separated into eight or nine colours. As I was using a white shirt I knew I needed a black screen and I wanted that highlight, which left me with four colours to work with. The presets allowed me to merge some information in the channels together and throw away channels I didn’t need.
For instance, the background colour is made up from green and cyan. I could have thrown both of these away and pulled the green manually, but under the blue in the art there was some information I wanted to keep as it blended in nicely underneath, so I merged these two channels together by CTRL clicking the separation, which loads that channel as a selection. With that selected, I clicked on the channel I wanted to merge it with and filled the selection with black – in this case I filled 100%. Then I assigned this channel with the colour I wanted by sampling the light green in the art – and then I could throw away a channel. I did the same with the magenta and red channels, but needed to boost the saturation in areas so I manually pulled more of what I wanted and added it to the channel, and assigned a brighter pink/reddish colour.
Check density then output
In the Info Panel read the K value on your channels as this tells you how much ink you’ll be putting down in that area. In this step-by-step, the black screen had a lot of 2-4% ink coverage across the whole image and those 2% dots, although extra small, would have caused the colours to become a touch darker and make the print look quite dull overall. An adjustment curve and a little nudge to the left in the highlight removed the unwelcome guests.
Once I had pulled all the colours from my art, assigned Pantones and everything was looking cool on the screen, I determined the print order. Generally, it’s light to dark, or smallest to largest area, or even putting colours of importance that you don’t want to get stepped on a bunch of times later in the sequence. If, for example, yellow is dominant in your design, print it later to keep it bright.
Finally, it’s time to output the films, check them carefully, then expose your screens.
As I wanted smooth colours and it was going on an auto (an M&R Diamondback), I went for 65lpi with an angle of 22°. If I’d been printing manually I would have dropped it down to 45-55lpi. I put these 65 line halftones on 120T statics with a yellow mesh – you get better halftones on a yellow mesh because it absorbs light and stops it spreading. On a white mesh, there’s greater risk of losing the fine halftone dots through undercutting when exposing. I exposed the screens for one minute.
On press I used 62-90-62 durometer squeegees all at a 10° angle – it helps to lower your angle for each screen when printing wet-on-wet as you will lay down more ink with less pressure, which will help to reduce pick up. The ink order I used is shown below. The colour inks are Union Mixopake, the black is Wilflex Epic Rio Mixing Black, and the highlight is Total Ink Solutions Bright White – this is the best white ink I’ve ever used!
Remember: ink viscosity, pressure, flood and print speed are all variables on press and each of them will have an impact your print.
(4) Pantone 285C