Screen printing is a printing technique in which a screen is used to transfer ink onto a substrate, except in areas impervious to ink by means of a blocking stencil.
A blade or squeegee moves across the mesh to fill the open apertures in the mesh with ink, and a reverse movement causes the mesh to momentarily touch the substrate along a contact line.
This causes the ink to wet the substrate and be drawn out of the mesh openings as the mesh passes through again after the blade has passed through. One colour is printed at a time, so multiple screens can be used to produce a multi-colour image or design.
There are several terms used for what is essentially the same technique. Traditionally, the process was called silk screen printing or silk screen printing because silk was used in the process. Today, synthetic threads are often used in the screen printing process.
The most popular mesh in general use is polyester mesh. Special-purpose mesh materials, such as nylon and stainless steel, are available to the screen printer. There are also different types of mesh sizes that will determine the outcome and appearance of the finished design on the material.
How is the screen printing process carried out?
A screen consists of a piece of mesh stretched over a frame. The mesh can be made of a synthetic polymer, such as nylon, and for a design that requires a greater and more delicate degree of detail, a finer and smaller mesh aperture will be used. For the mesh to be effective, it must be mounted in a frame and must be under tension.
The frame holding the mesh can be made of various materials, such as wood or aluminium, depending on the sophistication of the machine or the craft process. The tension of the mesh can be checked using a tensiometer; a common unit for measuring mesh tension is the Newton per centimetre (N/cm).
A screen is formed by blocking parts of the mesh in the negative image of the design to be printed, i.e. the open spaces are where the ink will appear on the substrate.
Before printing can take place, the frame and screen must undergo a pre-printing process, in which an emulsion is “removed” from the mesh. Once this emulsion has dried, it is selectively exposed to ultraviolet light, through a film printed with the desired design. This hardens the emulsion in the exposed areas, but leaves the unexposed parts soft. They are then washed with a jet of water, leaving a clean area on the mesh in the same shape as the desired image, which will allow the ink to pass through. This is a positive process.
In textile printing, the surface that supports the fabric to be printed (commonly referred to as the palette) is coated with a wide “palette tape”. This serves to protect the “palette” from any unwanted ink seeping through the screen that may smear the “palette” or transfer the unwanted ink to the next substrate.
The screen and frame are then covered with a tape to prevent the ink from reaching the edge of the screen and frame. The type of tape used for this purpose usually depends on the ink to be printed on the substrate. In general, more aggressive tapes are used for UV and water-based inks due to the lower viscosity of the inks and their greater tendency to slide under the tape.
The last process of “pre-printing” is the blocking of unwanted “holes” in the emulsion. If these holes are left in the emulsion, the ink will continue to pass through and leave unwanted marks. To block these holes, materials such as tapes, special emulsions and “blocking pens” can be used effectively.
The screen is placed on a substrate. Ink is placed on top of the screen and a filler bar is used to push the ink through the holes in the screen. The operator starts with the filler bar at the back of the screen and behind an ink reservoir. The operator lifts the screen to prevent it from coming into contact with the substrate and then, by exerting a slight downward force, pulls the filler bar towards the front of the screen. This fills the mesh openings with ink and moves the ink reservoir to the front of the screen.
Next, the operator uses a squeegee (squeegee blade) to move the mesh towards the substrate and pushes the squeegee towards the back of the screen. The ink in the mesh opening is pumped or squeezed by capillary action onto the substrate in a controlled and prescribed amount, i.e. the wet ink deposit is proportional to the thickness of the mesh and or stencil. As the squeegee moves towards the back of the stencil, the tension in the mesh pulls it away from the substrate (called “snap-off”) leaving the ink on the substrate surface.
There are three common types of screen printing presses: flatbed, cylinder and rotary.
Textile items printed with multi-colour designs often use a wet-on-wet technique, or the colours are dried while on the press, while graphic items are left to dry between colours which are then printed with another screen and often in a different colour after the product is re-aligned on the press.
Most stencils are ready for re-coating at this stage, but sometimes stencils have to undergo a further step in the recovery process called deburring.
This additional step removes the haze or “ghost images” that remain on the screen after the emulsion has been removed. Ghost images tend to faintly outline the open areas of previous stencils, hence the name. They are the result of ink residue trapped in the mesh, often at the knuckles of the mesh (the points where the threads cross).
Although the public thinks of garments in conjunction with screen printing, the technique is used on tens of thousands of items, such as decals, watch faces, balloons and many other products. The technique has even been adapted for more advanced uses, such as placing conductors and resistors in multilayer circuits using thin layers of ceramic as a substrate.
What types of ink are used in screen printing?
Depending on the type of surface on which the screen printing job is to be carried out, one type of ink or another must be used, as not all inks are suitable for printing on all types of surfaces.
The most commonly used inks in screen printing are:
- Plastisol inks : These are the most commonly used because they give a good colour capacity and a clean finish. The finish has a plastic feel to it, hence the name, but it is a very durable option.
- Water-based inks: Ideal for printing darker inks on lighter coloured garments and for large-area printing jobs. They are the second most traditionally chosen ink. The result is a much softer feel than plastisol inks and they are more economically priced than plastisol inks.
- High density inks: These inks give depth and texture, almost like braille. The finish is slightly raised about an eighth of an inch above the fabric and is best applied when working with a lower mesh count.
- PVC- and phthalate-free inks: Eliminate the use of the two main toxic components of plastisol ink without sacrificing any of the benefits and soft touch.
- Nylobond: This is a special ink additive used when we need to print on technical or waterproof fabrics.
What is screen printing used for?
Screen printing is more versatile than traditional printing techniques. The surface does not have to be printed under pressure, unlike engraving or lithography, and it does not have to be flat.
Different inks can be used to work with a variety of materials, such as textiles, ceramics, wood, paper, glass, metal and plastic. As a result, screen printing is used in many different sectors, for example:
- Medical devices
- Printed electronics, including circuit board printing
- Product labels
- Posters and displays
- Snowboard graphics
- Thick film technology
- Pinball machines
What are the advantages and disadvantages of screen printing compared to other printing methods?
Advantages of screen printing
Cost-effective for large quantities
Because this method requires the manufacture of screens for each colour used in a piece of artwork, it is best reserved for large orders. The more garments placed in an order, the cheaper the cost per unit.
Easier to print on special garments
It is difficult to find a printing method as versatile as screen printing. It can be done on almost any surface as long as it is flat, fabric, wood, plastic and even metal, among many others.
Multiple inks can be used
Screen printing allows a greater thickness of ink than other techniques, which translates into greater possibilities when it comes to finishing the piece.
Durable and long-lasting quality
Due to the composition and thickness of the inks used in screen printing, designs placed with this method can withstand much more stress than others without losing the quality of the print.
Disadvantages of screen printing
More money for several colours
Due to the screen printing technique, only one colour can be applied at a time. Therefore, the number of colours on a T-shirt is limited, and each layer requires more time, another screen and stencil and ultimately more money.
It is not practical for small print runs.
Screen printing needs more preparation than other techniques before going into production. This does not make it suitable for “on-demand” printing, which is the creation of a garment as soon as it is ordered.
It may seem counterintuitive, but screen printing can be quite complex depending on the design and the project, as it has more steps than other methods.
Not environmentally friendly
Although efforts have been made to create eco-friendly inks and screens, it remains a reality that screen printing wastes a lot of water. Water is used to mix the inks and clean the screens, which may not seem like much at first, but manufacturers produce hundreds of garments a day and when you start adding it up, it can be scary.
Frequently asked questions about screen printing
On what kind of materials can you print using screen printing
Screen printing is one of the most versatile printing methods available today, allowing us to print on a wide variety of materials such as wood, glass, ceramics, paper, metal or textile fabric.
Can screen printing be used in conjunction with other printing methods?
Yes you can, modern digital processes, while ideal for short runs with multi-colour images, cannot produce some colours or colour opacity. It is possible to combine digital and screen printing to get the best results from both methods.
Want to learn about printing techniques other than screen printing? Visit our printing category where we detail the different types of printing methods that exist and that can be of great help to you when choosing the most suitable option for your printing job.