From what we have seen so far the basic principle of screen-printing ink through a stencil, it should be easily understood that a screen is only suitable for single colour application.
Note: There are exceptions such as blends and areas that are not local to each other being “ganged up” on a single screen but these techniques are not really suitable for commercial use so they are not covered in this tutorial.
So the rule of thumb is:
For a multi coloured image, every single colour contained within that print will require its own a separate stencil.
Following on from Understanding Screen Printing Part One - The Stencil, Lets now go through an example of how multicolour images are created using the screen printing technique
Lets take a simple 2 colour design such as the classic RAF roundel
and separate the art into it’s 2 component parts
The blue “separation” and The red “separation”
Separating an image into its constituent colours is done by art workers using specialist separating software, and is beyond the scope of this topic. However, the key concept to grasp is separation.
By using the concepts outlined in Understanding Screen Printing Part One - The Stencil if we use the two separations above to image, expose and develop stencils, we can create two individual screens which will enable printing the RAF roundel
By applying separate coloured inks into each screen (one blue and one red) we are on our way to printing the Roundel
A key challenge with multiple screens / stencils is to ensure that the stencils align consistently so that the relationship between the images is maintained on a repeatable basis. This alignment process is known as registration
Registration
is the term that applies to lining up more than one colour on a single
image. It is achieved by applying registration
marks to each colour separation.
This enables them to be lined up to their original intended relationship.
From
these registration marks it is now possible to line up the stencils when
placing screens into a press.
Once aligned, the screens are then locked
into position ready for use.
Note: This is done using art work technques that are beyond the scope of this tutorial however understanding the principle of separating the artwork at this stage is more important than the method.
If, as covered in screen printing (part 1) we use these separations to image, expose and develop stencils we can create two individual screens to suit
Exhibit 1 the 2 separate screens to suit
We can now place separate colour inks into the screens and apply the different colours to suit.
The challenge is now to LINE UP the two stencils in the correct relationship to each other on a repeatable basis.
REGISTRATION
REGISTRATION is the term that applies to the challenge of lining up more than one colour on a single image.
To overcome this problem, “registration marks” are applied to each colour separation. This enables them to be lined up to their original intended relationship.
From these registration marks it is now possible to line up the STENCILS WHEN PLACING SCREENS into a PRESS The SCREENS are then “locked” into position ready for use.
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FIXING DOWN A GARMENT ONTO A PALLET
There also exists the problem that a garment being a flexible item that it will move about and alter its shape according to how it is handled. To overcome this problem a low tack adhesive is applied to the pallets that the garments are loaded onto, thus holding them in place whilst printing is applied.
FLASH CURING
Under circumstances where it is necessary to overlap or sit colours on top of one another we need to dry the ink in-between colours because as already hi-lighted a garment is of a flexible nature it cannot be removed from the press, dried and realigned.
Drying has to take place in situ, this process is know as “FLASH CURING”.
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The above details can be understood when applied to simple artwork that is made up with a clear number of visible colours, but this does not help to explain photographic style images.
COMPLEX TONAL WORK
These can be broken down into two main areas:
Ø Four Colour Process and
Ø Simulated Process.
These techniques require the separations to be broken down into gradients of small dot patterns of which the basic principles are covered in “TASK 03641 UNDERSTANDING THE DIFFERENCE BETWEEN LINE & TONE WORK”
FOUR COLOUR PROCESS
Is achieved by separating an artwork into the four primary colours in a tonal format.
When these colours are reprinted using transparent colours that go some way to reproducing a convincing representation of a continuous tone real life image.
However the most important aspect of this work is that it is printed on a white background.
The colours used for this type of photographic reproduction are known as process colours and include yellow, magenta, cyan and black. Or CMYK
There are many examples of four-colour process everywhere, but as stated they are always printed on white ground.
PRINTING ONTO DARK GARMENTS
Within the techniques of garment decoration we are faced with an extra problem that rarely occurs in other print formats, that of printing onto a coloured background.
The methods used to overcome this problem for both line and tone work can be explained as follows
Line work
When printing line work onto dark garments there is a problem of the garment colour showing through the ink. To overcome this, “opaque” colours are used and although these go someway to solving the problem they are not sufficient on their own.
The solution is to print a white under base and “flash dry” it in situ, then print the other colours on top of this base. A hi-light white would be included if white was a colour included in the finished image.
Please note; a white underbase requires an additional separate screen
i.e our 2 colour RAF ROUNDEL example would now become a 4 colour
exhibit 2. white base, red, blue & hi-light white stencils
Tonal work
As already covered, the four colour process technique is really only suitable for printing on to white backgrounds.
Common sense suggests that printing a white rectangle (or other suitable shape) onto the garment and then applying the four-colour set on top of the white base is the answer.
Although this will work under certain conditions, the four colour process set tends to have a milky appearance, the colours are difficult to control and the solid block of white leaves an undesirable “hand” to the finished garment.
SIMULATED PROCESS PRINTING
As already covered four colour process
printing is really only suitable for printing onto white ground so to
create vivid and striking multi-coloured images onto coloured garments
has always been a challenge.
This brings us to the unique technique employed by Garment decorators known
as SIMULATED
PROCESS
A separator will assess and image an decide how many spot colours are required to recreate the image in bespoke “opaque” colours and then separate the image into the tonal versions of each of these colour to suit.
These separations will also include a tonal version of the white under base and a hi-light white if required.
The number of heads available on the press dictates the maximum number of colours that can be used in this technique where the print will be processed.