What is Screen Printing?

I get a lot of questions about what screen printing entails and when I explain it a lot of people don't understand what I'm talking about or they can't picture what I'm describing. So I thought I would create a blog post mini series about screen printing and how I do it, Now I am by no means an expert and I am still learning about the print process as I go but I'll show what I have learnt to date.

Firstly what is screen printing? 

screen printing is the process of forcing ink through a prepared mesh or fine material screen onto a surface (normally paper or fabric) to create an image or pattern

Below is a photo of one of the screens that I use, This particular one is about A2 in size (the printing area is A2) and it is a 43T screen.

43T may not mean anything to you but the T number lets you know how coarse the mesh is on the screen, so for  example the lower the number the coarser the mesh and the higher the number the finer the mesh. A 43T is usually used for textile printing and a 90T is normally for vinyl, plastic and plastisol inks and for printing onto paper and board. 

The majority of my screens are 43T and I find that they work just as fine as the 90T screen I when it comes to printing onto paper, however when you look closely at the image you will see small dots around the edge when using a 43T so for fine detail I would stick to a 90T or higher screen. The numbers for the mesh can range from 21T through to a 185T.

Next How do you put the image onto the screen? 

This step may not look it but it is actually pretty simple. The method I chose to use is the photo emulsion method, there are other ways of putting the stencil onto the screen using drawing liquid and screen block but I prefer using photo emulsion. Photo emulsion can be a bit messy and you do need to work quick so that the screen doesn't fog or start to expose. 

Firstly you will need to have already mixed up your photo emulsion (this doesn't come pre mixed so you will have to buy the photo emulsion along with the sensitiser) if it's not already mixed the emulsion will be blue, if it has been mixed with the sensitiser it will be a green colour. (I'll do a seperate post on mixing and preparing the photo emulsion). 

Photo emulsion (or photographic emulsion) is light sensitive and when exposed to light it binds itself to the screen to create a stencil.

You need to coat the screen with a thin layer of photo emulsion and store it in a dark place (as in pitch black) to dry for a few hours, (unless you are working in a print studio where they have designated dryers).

This is how your screen should look once coated.

Once dried you then need to place your black stencil (yes it has to be black) onto the screen and cover it with a piece of glass or plexi glass, using an exposure unit or in my case halogen flood lights you can then expose or burn your stencil onto the screen.

Why does the stencil need to be black?

In the burning process the light will bind the photo emulsion to the screen, meaning that it won't wash off unless you use a screen cleaner, Using a black stencil means the light can't pass through that area so when you wash the screen after exposing it under a light the emulsion underneath the stencil will wash off leaving the image behind. 

Depending on the wattage of your lights will depend on the exposure time, for mine the lights are 500w so I find an exposure time of about 3-4 minutes to be fine. Finding the exposure time for you may take a couple of attempts.

After you have exposed the screen you won't see the image on there don't panic the image is there you just need to wash the screen to remove the excess emulsion so your stencil can be used. When washing the screen you'll need to wash both sides of the screen, particularly the printing side, the best way to remove the emulsion is to use your hand a rub the surface. It will feel a bit slimy and you will end up with a green/yellow hand afterwards but it will wash off after a couple of washes. You will know when the screen is ready when the slimy feeling has gone and the screen feels smooth, after washing it you'll need to leave it to dry and then you are ready to print.

Below are example of some of the stencils I have created: 

Once the screen is dry and ready to print, you will need a squeegee, some ink and what ever medium you are printing onto. For my prints I mix up my own ink using acrylic inks and a acrylic printing medium, It's simple to mix it up all you need to do is mix 50% paint with 50% medium and mix well until the you can't see the medium anymore. If you want the ink to be more transparent then all you need to do is add in more base, for example 30% paint 70% medium.

REMEMBER don't print with paint straight from the tube as it will dry to the screen and you will not be able to remove the paint from the screen. it has to be mixed with the medium as the medium slows down the drying process. If you do print with paint straight from the container the only way to remove it is by using Pregan paste (an alkaline based heavy duty screen cleaner) or by replacing the mesh on your screen.

When printing it's best to keep the squeegee at 45 degree angle an don't press to hard as you'll push too much ink through the screen. You may need to pull the ink across 2 or 3 times to get an even coverage. Always pull the squeegee towards you as its easier to print than if you try to move it away form you. To stop the screen moving whilst you are printing you'll need to use one hand to hold the screen in place or ask someone to hold it for you, otherwise you can use a press to hold your screen.

I will do a few more posts in the next few weeks on mixing photo emulsion, creating stencils, mixing inks, how to stop foul printing and cleaning the screen. If there is anything else you would like to see feel free to let me know.