Screen Printing

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.

How to Sensitise Photo Emulsion

A key part of the screen printing process is the photo emulsion, when purchasing photo emulsion it good to note that it come unsensitized which means if you put it onto a screen it would be useless. Make sure that when you buy photo emulsion to check whether or not it comes with the sensitizer, in this case I have to buy the emulsion and the sensitizer separately.

You'll notice that when you open the emulsion up that it be a bright blue colour (below) this means that it hasn't had the sensitizer added. The sensitizer come in a small bottle (picture above on the right), the bottle will feel light and it will seem that there is nothing in there, DON'T PANIC the sensitizer comes in the form of a paste (normally a dark green colour) that you need to add liquid to. 

For the sensitizer I used I had to fill the bottle 3/4 of the way full and give it a shake before adding it to the emulsion, this may not be the case with all sensitizer (though I think it is). Make sure you read the instructions on the side of the bottle just to make sure.

Once you have added the cold water to the bottle and given it a good shake you then need to add it to the emulsion.

Using a stick give the emulsion a good stir for a few minutes to make sure the sensitizer is fully mixed, you'll notice the emulsion will turn from blue to a green/turquoise colour as you mix it together.

Once it is fully mixed in it should resemble the colour below (colour may vary in shade depending on the brand of emulsion you use). If there is any blue left in the mixture just give it another quick stir until its all gone.

And your done!! it's as simple as that.

Remember that the photo emulsion is light sensitive so make sure the lid is on properly and store it in a cool dry place. Once you have added the sensitizer the emulsion will have a shelf life of about 3 - 3 1/2 months (the emulsion normally has a shelf life of about 1- 3years if left unsensitized).

Applying Photo Emulsion

Applying emulsion to a screen is quite easy but it can be a little messy. I am going to try and explain as best as I can in this post.

First off you need a clean screen, a coating trough and of course photo emulsion.

Coating trough

Second you need to put a generous amount of emulsion into the trough, don't put too much into the trough as when you start to apply it, it will end up flooding over the edges of the trough and it will go everywhere (and it not easy to get off things). I always put a little but more than what I think I will need and then out the excess back into the tub.

Once you have enough emulsion in the trough it's time to apply it to the screen. 

Line the edge of the trough up to one of the bottom corners of your screen, place the trough up right so that the emulsion flows onto the screen (don't let it overflow on the edges!!) once the emulsion is on the screen tilt the trough back, normally to about 45 degree angle and pull the trough up the screen.

You should then be left with a line of emulsion on the screen, if it ends up being a bit patchy then you can go back over it some more emulsion.

Note: you only need to coat the one side of the screen not both.

Once you have coated the screen you need to make sure that it is stored somewhere dark (pitch black) and dry, the emulsion takes about 3 - 5 hours to dry (depending on the temperature of where you store it) unless you have access to a dryer then the emulsion will be dry within about 20 -30 minutes.

How to Clean a Screen for Printing

It's important that you start with a clean screen before you start coating it with emulsion, there are two ways to clean off a screen 

Pregasol - clear liquid that you spray or brush on the screen, this removes the emulsion

Pregan paste - This is an alkaline paste that should only be used if you need to deep clean your clean, by this I mean removing build up of emulsion and removing ghost images (though you can purchase Pregan anti-ghost which is made for removing ghost images). I use the Pregan paste about every 3-4 months, as it is an alkaline and in corrosive I wouldn't recommend using this every time you need to clean a screen (thats what pregasol is for). care should also be taken when using this product as it can cause burns if it comes into contact with your skin.

Green bottle is Pregasol and then Pregan Paste on the right.

Firstly I take my screen outside (I don't have the luxury of a wash room) and I spray the pregasol onto the screen and leave it for a few minutes to sink in.

I then use a pressure washer on full power to clean off the emulsion and screen filler (the red lines on the screen). The pressure washer has to be on full pressure (one jet) so that it can pass through the screen to remove the emulsion, if you have the washer on half pressure you will be standing there for a very long time as there is not enough pressure to pass through the screen (you'll only be washing the surface).

I normally clean the screen in lines, so what I mean is that instead of just moving the jet all over the place I start in the top corner and move down the screen and back up to create lines. This way you should get all the emulsion off and it's quicker.

Note:

Full Pressure (one jet) - to remove emulsion

Half Pressure (two jets) - to wash off ink after printing 

Hold the jet as close to the screen as possible (I normally hold it about 2cm away from the screen) as it will be more effective in removing the emulsion than if you stand 3 ft away.

9 times out of 10 you won't get it all off in one go, especially of you have screen filler on the screen. To clean the stubourn patches off you just simply spray some more pregasol onto the areas and then wash it off.

Afterwards you'll be left with a clean screen and all you need to do is let it dry.

If you are not too sure if your screen is clean or not hold it up the light, if you can see straight through it then all the emulsion is gone, though if you can see some green/blue specks in the screen then there is still some emulsion left on the screen.

In some cases you'll be left with a ghost image on the screen of what ever was on there before, don't worry this shouldn't effect your print in anyway. As you can see on the screen below I have the ghost images of two previous projects, the way to remove these is by using pregan paste.

How to Expose a Screen for Printing

'Burning' your image to a screen is a pretty simple and a fairly quick process. 

First off you will need your freshly coated screen (dried of coarse), your stencils, a piece of glass and some lights (unless you have access to an exposure unit). Place your stencils on the side of the screen with the stencil, so the side you don't put the ink onto, and make sure you put your stencils on the screen the way you want them to print. 

If you are putting text onto a screen make sure you place it on the screen backwards, that way when you turn the screen over to print it will read the right way.

Once you have placed the stencils on the screen make sure to place the glass on top of your stencils and your ready to go. If you are using an exposure unit then the order is reversed a bit, you would put the stencils onto the glass bed and then place your screen (flat side down) on top of the stencils (if you are using an exposure unit and you have text you can put it on the glass the right way round)

I expose my screens using two halogen lights and I normally set the exposure time to about 4 minutes. The exposure time will depend on the wattage of your lights so you may need to experiment a bit to find the right exposure time for you.

Sometimes you will be able to see your image after the exposure is over (picture below) but it may not always be visible.

After you have exposed the screen you then need to wash the unexposed stencil off. You need to wash both sides and using your hand (or a sponge if you don't want to get your hand dirty) rub the stencil away. You will know of the stencil is coming away as your hand/sponge will turn green/yellow.

Keep washing until the stencil has been removed (hold it up to the light and if you can see through the image then its clean or keep washing until the screen stops feeling slimy). Once it's all gone you just need to leave your screen to dry and then your are ready to start printing.