This is a valid question and for the sake comme des garcons t shirt of educating the larger masses and for those that are interested I will outline the difference between the printing process and how they may apply to your project. There are generally four printing methods, they are screen printing, heat transfer press, and embroidery “well this is not really printing but I place it there as you still need to place an image on a t shirt”. The last and the latest technology is direct to garment. This process can produce some fantastic high resolution photographic printing but it is at a cost.
Screen printing is most likely the most popular method of printing promotional t shirts, the set up process is similar to offset traditional printing methods where you need to provide your images in to separable colour files, for example if you have a four colour file you need to supply a CMYK file which the print production firm can break each colour down and isolate them from each other creating, four separate print images, one for the cyan colour, one for magenta and one each for the yellow and K being black or grey scale.
Now that the CMYK files have been separated a separate screen is produced for each colour in total four screens a C screen, M screen, and one each for the Y and K screens. So each colour has its own separate screen. After the screens have been produced and have been quality tested and all adjustments have been made they are mounted on to the screen printing press were each colour is aligned to the next using registration marks on the screens. What this means is that each colour is printed separately on the promotional T shirt. Now that the screens have been set and have been tested we can begin the printing the t shirts.
In summarising screen printing we note that there is quite a bit of setup work to be done, you need to allow a hefty lead time for the turnaround of the promotional T shirts, also as the setup process is so intensive there is a substantial cost involved about $70 to $150 per colour so in a four colour print it would be between $280 and $600 depending on the print, and you have yet to print one good promotional T shirt.
The good news is that the set up is the costliest part of the screen printing process and from now on each t shirt is at a minimal price. To sum up, screen printing is the best process for producing large amounts of promotional T shirts, a minimum order of a few hundred and upwards.