What is screen printing process?
We will define screen printing as a printing technique that consists of preparing a screen or frame with a stretched fabric on which we will have printed a design by applying a varnish that blocks the area of the fabric without an image. We then pass the ink through the screen to print the design on a large number of garments or articles.
It is an ancient technique, which became widely used in the early 20th century thanks to the development of photographic techniques and chemical products that facilitated the process. If you are thinking about screen printing, you probably imagine the final process of marking the T-shirt by spreading the ink on the screen with the help of a squeegee. However, this is one of the last steps in the process, and prior to this, the stencil preparation work is essential to achieve a good result. Stencil preparation is the most expensive step, which is why screen printing is suitable for printing medium to large runs of T-shirts with the same design. This amortises the initial cost of screen preparation.
We will now go through all the steps involved in screen printing T-shirts:
Preparing the design and the photolithograph
First of all we must prepare the design, which can be our own or an image downloaded from the internet. It is important to select an image of sufficient quality and you should bear in mind that in screen printing, each colour of the design represents a screen. For this reason, you must separate the colours of your design and create a screen for each colour. There are programs that facilitate this process of colour separation. In any case, to begin with, we suggest that you choose simple designs, preferably in one or two colours.
Once you have selected the design, you should print it in black on a photolitho. During the subsequent screen exposure phase, the function of the photolith is to prevent light from reaching the screen exactly where it is printed in black. Therefore, it is important to print the photolitho correctly.
When selecting the screen we must take into account which item we are going to screen print, and although it is possible to screen print a large number of items here we are going to focus on textile printing, mainly T-shirts.
The screen consists of a wooden or aluminium frame on which a fabric or textile is stretched. The size of the design will determine the dimensions of the screen we are going to use. And the type of design will determine the mesh or fabric to be used. If the design has very small details we should use a mesh with a higher number of threads per cm. However, for screen printing on textiles it is usually recommended to use a mesh with a weft of T-43 or 43 threads/cm.
This consists of applying a photoreactive emulsion on the screen mesh or fabric. To be able to do this you have to work in a dark room, with red or yellow safety light and avoiding at all times exposure to natural light or light from fluorescent lights or bulbs.
Our emulsion is ready to use and there is no need to apply any sensitiser or reagent. Use a squeegee to spread the emulsion evenly over the screen surface to create a uniform film. The frame should be tilted at a 45° angle and you will first apply 1 coat of emulsion on the outer side of the screen and 2 coats on the inner side (this will be the last one). The frame should then be placed in a horizontal position, with the outer side (print side) facing downwards to ensure that the emulsion forms a thin film. Let the stencil dry in this position for twenty-four hours, at a temperature of 25-35°C and maximum relative humidity of 50%. It is then advisable to run a hairdryer across the stencil to remove the overnight humidity.
Remember that you are still working in a dark room. Now you must place the photolith on the outside of the screen, ensuring that the photolith is in contact with the screen, and you must expose the screen to light to get the development. The emulsion is a light-sensitive product, so the entire screen is exposed to light except for the dark part of the photolith, the design. It is important that the exposure is done for the correct amount of time, as too little time will result in weak designs, while too much time may mask the smallest details or make it impossible to develop. We recommend the use of a heat press that will ensure perfect exposure to light.
We continue to work in a dark room, remove the frame from the exposure machine and proceed to wash it by applying a gentle jet of water on both sides of the screen until the design can be seen and then a jet of water with higher pressure on the outer side of the screen until all the emulsion not exposed to light is removed. Allow the stencil to dry at room temperature, with fans or in a drying cabinet. When the stencil is completely dry, you should apply masking tape around the edges to prevent the ink from getting into the joint between the frame and the fabric.
Screen-printing or screen-printing the T-shirt
Now it’s time to fix the T-shirt on the octopus press and start screen printing. You can use spray adhesive to hold the T-shirt in place. Place the screen over the area of the t-shirt you want to print, and prepare your squeegee. You can use between different hardnesses depending on the type of screen printing job you are going to do. Apply a small amount of ink horizontally across the top of the screen and use the squeegee to spread the ink across the screen, while pressing down. Push the ink in a continuous motion making sure that the ink passes through the uncovered fabric or design.
If you are screen printing a design with more than one colour, you need to include registration marks on the photolithoid so that these marks will also transfer to the screen during the heat transfer process. These registration marks will allow you to correctly align the second stencil on top of the first colour design. But don’t forget to dry the first colour before printing the second colour.
Drying or curing the ink
After screen printing your T-shirt, there is one last important step: you must cure the inks to ensure perfect fixing and maximum durability of the print. It is not enough to dry the screen-printed T-shirt at room temperature, but you must use a drying flash, a transfer iron or a drying tunnel.
The aim is for the print to reach the temperature specified by the manufacturer during the time interval specified by the manufacturer. If you use water-based inks, you should cure the print at 150-160°C for 30 seconds.
These are the basic steps for screen printing a T-shirt. If you are interested in incorporating this marking technique, we recommend that you buy a starter kit and do your own tests until you get good results. It is a very creative process but it requires some learning, which you will only acquire by screen printing. You will discover that having the right tools and knowing how to use them correctly will allow you to obtain a professional result.