The best reason for using CMYK separations is are lower set up costs, only 4 colours to produce any number of colours, where index and simulated seps often run to 6+ colours to achieve the best results. This makes true process ideal for promotional prints on white T-shirts if the printer is able to create good separations first time, and able to hold the colours on the printing press.

Bad separations and poorly made screens can amount to a truly terrible result! But with the rapid sharing of information across the internet between screen printing experts many of these problems have been overcome. With the right information, the aid of retensionable screens, and a modern super callibrated printing press very good results can be achieved.

However, on the odd occasion it can produce great results using only 4 colours, as per photo. These index separations can be created in photoshop quite easily without any additional plugins.

and typically consists of well defined seperate areas of colour or half tone. There is sometimes a need to create overlaps where the colours meet when printing in plastisol inks, this is called trapping and is generally created by using an outline to the value of 0.5 in a vector based programme such as corel or illustrator. When dealing with raster artwork (pixels) then a value of 1 or 2 pixels is normal. Advertees favour the use of water based inks, these inks require a different kind of separation where the colours meet and fit exactly with no overlap, this is called butt registration and obviously requires an accurate press. Most T shirt printers have there own spec for separating artwork preferring to be supplied original artwork files. Always ask your screen printer what they require if you want to create the separations yourself.