At TOT we cover all forms of garment decoration and processing techniques these include heat transfer, digital printing, embroidery, relabeling and other forms of garment processing.
Our core business is that of screen-printing and this topic will cover a basic introduction into the principles that are used to achieve professional results.
Most people will understand what a STENCIL is from general experience, so the first part of our module is to understand how we get from a simple stencil to more complex shapes that are suitable for commercial screen printing onto garments.
Basically a STENCIL is a template used to create identical letters, symbols, shapes, or patterns on a repeatable basis.
STENCILS are formed by removing sections from a masking material in the form of cutouts. These create what is essentially a physical negative of the required image. The template can then be used to create impressions of the stenciled image, by applying pigment on the surface of the template and through the removed sections, leaving a positive reproduction of the image on the underlying surface.
This
can be done in many ways that include spraying, painting, daubing and
printing. Here at TOT we print using the method of “pulling a squeegee”
across the STENCIL
which
transfers the ink onto the substrate in the selected areas.
There are many you tube videos that explain screen printing. It is worth
watching these to gain a better understanding of the process.
The
link below explains in a simple manner a method of creating a stencil
and how ink can be transferred through the stencil to produce an image.
https://www.youtube.com/watch?v=9OCgFA9RJIo
Note: The method outlined in the video is very basic and unsuitable for a commercial environment.
The next part of this tutorial outlines differing forms of stencils and how they can be used to make up different images.
The simplest form of a stencil is a circle.
This basic stencil has a hole in the middle. If we were to pass ink through the hole, a solid circle would be printed
Stencil Print
Created
A
complex stencil will include islands. The simplest example is that of
the letter “O”
If we wanted to print the letter O using similar stencil method, we would need to create a centre island (2). However, this presents a challenge as the centre is floating so can moved about within the main stencil (1)
It should be obvious that the middle cannot be ‘fixed’ to the outside of the circle and will “fall out” when moved.
One
way to address this is to fix the island stencil to the outside circle
using bridges. However this will cause some issues as the integrity of
the continuous shape will be lost, and the print output will look like
the image on the right
To address the gaps caused by the bridges, we can reduce the width of the bridges.
If we now increase the
number of strands used to hold the ‘island’, more complex stencils can
be catered for.
We have now created a simple example of a mesh
Crude Mesh
The
below images show what a mesh looks like. The photo on the left is a piece
of mesh at normal magnification, whereas the right image shows a representation
of a small area of the mesh highly magnified to demonstrate its composition
and the "holes" between threads that allow liquid through.
Mesh is made up of interwoven threads between which are tiny holes which can allow liquid through. So to represent our shape we would have to block the holes that we don't want ink to pass through, and leave open the ones that ink should pass through. The blocking and unblocking of holes thus represents the stencil. This matrix of fine holes means we can now allow complex shapes.
Now that we have understood the theory of how a mesh can hold complex stencils together, we need to look at the practical method of creating a stabilised mesh suitable for production conditions.
To
stabilise the mesh, it is stretched taut and glued across a rectangular frame
Together these two elements make up what is called a SCREEN
Now we have a blank screen - i.e. it does not contain a stencil - so how do we fix the stencil to the screen to allow us to print our O ?
Now that we understand the components and function of the screen we now need to work out how we can fix the stencil to it. This can be achieved using two methods:
This entails fixing a cut stencil to
the underside of the mesh by use of an adhesive or a capillary action
This is done using a photographic technique
Firstly the mesh has to be masked off
with a photosensitive emulsion, known as coating.
This is then dried in a light safe environment until it’s ready for use.
A coated screen then has a ‘positive” mask attached to it in a suitable position.
It is then exposed to ultraviolet light and the unmasked emulsion changes it’s physical properties while the masked area remains unexposed underneath.
The stencil is then washed with water in the lightsafe environment, which washes the emulsion away from the mesh in the unexposed areas - leaving the mesh permeable - i.e. allow ink to pass. By contrast areas of the mesh that were exposed are blocked - ink cannot pass through the mesh.
The mask can come in 3 main formats:
This
is a solution however it is time consuming to make a mask of this
nature and it also suffers from secondary islands.
Therefore
this method is unsuitable for commercial use.
To
overcome the secondary island problem as noted above, the need for a carrier
to hold the opaque mask is required. The historic method was to use a
clear film with an opaque to ultraviolet mask applied to it.
The clear film is now overcoming the problem that the bridges overcame from the stencil. These can be produced by creating the masks electronically using a suitable artwork package, and printing them onto clear film, which can then be attached to the unexposed screen to be exposed and developed accordingly.
This method is largely redundant now in the industry. TOT does not use this method of mask creation.
With technological developments, it is now possible to print the mask directly onto the screen thus eliminating the need for the clear film carrier noted above.
TOT uses a spyder
printer which loads the electronic separation file
Note the image to be printed is highlighted in the below pic.
and prints each separation image onto a coated frame.
Video of spyder machine printing the image mask
This printed mask will dissolve during the development process - whereby the areas of the mesh with the image applied will be permeable; and the rest of the mesh not.
If
we go back to our original example of the O - once the image has been
applied to the screen and the mesh exposed and washed, we will be left
with the following
Note how ink cannot pass through the exposed (dark) areas - whereas the
lighter areas of mesh will allow ink to pass through.
Once
the stencil has been manufactured ink can be pulled across it at
pressure, leaving the desired image transferred on to the substrate.
It is important to understand at this stage that if an image is made up
of multiple colours, every colour contained within an image required for
screen print will require it’s own separate screen
Screens can be held indefinitely. However if all screens were held indefinitely the storage space becomes a problem, especially in the event where screens are only intended to be used once and the job will not be repeated, e.g. stag do
The materials used to create direct stencils are “reclaimable” thus enabling the “screens to be recycled after printing. This basically entails adding chemicals to the direct stencil that dissolves the emulsion and after high pressure washing the mesh is ready to be recoated and used again.