How does Screen Printing Work?

Screen printing is the classic, traditional t-shirt printing technique. A screen is essentially a mesh screen with thick material blocking out the portions you don't want printed and clear mesh on the sections you do want it on. It works very much like a stencil in spray painting only with mesh filling the open parts.

The screen is pressed against the substrate. The substrate is whatever you are going to print on, be it a t-shirt, banner, or even wood or other material. The screen is set to be slightly above the substrate and ink is poured onto the screen at the top or bottom. A squeegee is pressed along the length of the screen, forcing ink onto the shirt exactly where you want it. This technique works the best for block printing.

Basic one color shapes such as letters and logo diagrams tend to look really good.

In the case of multiple colors a different screen needs to be prepared for each color. These screens are set up on a rotating carousel and lined up precisely in the correct spot using marks on the screen called registration marks (if you have ever seen a t-shirt where it looks like one color is not lined up correctly that is generally a sign of misaligned registration, which can occur during set up or during the course of printing a larger number of shirts). One color is screened onto the shirt. The shirt is then put under a flash heater to harden the surface of the ink, then the carousel is rotated to the next screen and the process is repeated. Most basic screen printing rigs have up to four screen, although I have seen massive 32 color automatic screen printers. They are pretty cool to watch in action.

On dark shirts black in particular you almost always want a white undercoat in order to show the colors. If your design has white on it already that is not a problem as that will simply be the first color laid down but if you are going with an all one color non-white design odds are you will have to have a white screen made and printed first. When a white undercoat screen is created the good technique is to have that screen be just slightly smaller than the color portion. This helps prevent white ink from showing through on your design. If you look closely at a color print on a black t-shirt you can sometimes see the border has a slightly darker outline where the white undercoat did not cover. If you don't do a white undercoat the colors inevitably look very dark and muted.

Once all the colors are applied the shirt is removed from the platen (the padded plate that that the shirt is placed on) and placed under a heater to permanently bond all the ink. The heater is usually a metal mesh conveyor belt through a big heating element, similar to some pizza restaurant ovens. After the t-shirt passes through the heater it is done and ready to wear.

Screen printing has a lot of advantages. If you are doing more than couple hundred shirts it is by far the most economical with the highest quality. There are cheaper methods but they can sometimes lead to less quality printing. Screen printing is very scalable; in other words the more you are willing to spend the better quality you are going to get. Basic printing will get you a thick plastisol print but if you are willing to spend more you can get water based inks, discharge printing, and any number of other techniques that a good print shop can offer.

One disadvantage is that the error rate is generally higher. If a screen is bumped and knocked out of alignment you can have dozens of shirts ruined before someone notices. It is also very easy to get ink on the wrong part of the screen (or the screeners fingers) and get smudges all over the shirt. Screen printing is a messy process and involves a lot of chemicals and washing. I've never known a screen printer to not have ink all over his or her clothes, face, and hair. The production of the screen is arduous and prone to errors and involves all kinds of toxic chemicals. Most screen shops charge for each screen so as you add colors your cost goes up. Due to the costs screen printing is rarely recommended for smaller print runs and most print shops will not take orders under 48 pieces (and usually 144).

Also getting really good photo realistic or multiple colors blends is extremely difficult. It can be done but only by a really good screen printer with a lot of experience and even then his best work will not match a good DTG or sublmation. If you are doing your company logo in block print with some words screen printing is great but if you want Van Gogh's Starry Night on a t-shirt odds are you should go with Direct to Garment or sublimation.

I'll talk about DTG printing in my next post.