Tuesday, 9 April 2019
Sunday, 31 March 2019
Wednesday, 23 January 2019
Printing Introduction
The
colour designs are developed on fabrics with dyes and pigments in paste form
with specially
designed machines, this process is known as “Textile Printing”. Printing
is used to apply colour only on localized areas. i.e. Localized Dyeing. Printed
fabrics, usually have clear-cut edges in the printed portions on the face of
the fabric.
Printing
allows
§ Flexibility
in creating designs
§ Enables
the creation of relatively inexpensive patterned fabric.
Stages of Textile
Printing
The printing process does involve several stages in order
to prepare the fabric and printing paste, and to fix the impression permanently
on the fabric:
Pre-treatment of fabric:-
Grey cloth as woven on a loom is quite unattractive and contains natural and
added impurities, which impart undesirable properties to the cloth and which
hinder the successful carrying out of the subsequent dyeing, printing, and
finishing processes.
So,
in order to prepare the grey fabric for printing, various pretreatment has to
be done, like – singeing, desizing, scouring, bleaching etc.
Preparation of colours:-
Water soluble dyes are dissolve in low amount of water to add in printing
paste, while disperse dyes are added in dispersion form.
Preparation of printing paste:-
For Printing, the thickened paste is used for sharp boundaries of colour. This
objective is achieved by using suitable thickeners and other ingredients, which
are necessary for fixation.
Impression of paste on fabric using
printing methods:- An impression may be made on a fabric
using the printing paste by any of the printing methods, such as block,
stencil, roller, screen (flat-bed & rotary)
Drying of fabric:-
After the printed cloth leaves the machine, it is passed through hot air or
steam, to dry the cloth. This drying is essential to prevent any spreading of
the colour beyond the boundaries of the design.
Fixing the printing with steam or
hot air (for pigments):- When the cloth is printed and
dried, the dye is not actually transferred into the cloth. In order to bring
about the transfer of the dye into the fabric as much as possible, steaming is
carried out.
After process treatments:- When
the printed cloth leaves the steaming chamber, it contains the exhausted
thickener film, along with certain chemicals dispersed in it. Therefore, to
remove the thickener as well as chemicals after process treatments is done
without affecting dyes.
Fabric Requirements for
Printing
For proper printing the following
are the general requirements of fabrics
ð The
fabric should be stitched with proper face on top and uniform width.
ð Shearing
and cropping treatment is given to remove loose threads.
ð Desizing
is necessary, for proper penetration of the print paste.
ð Singeing
removes hairy fibres, to enable even adhesion of printing paste and sharpness.
ð Scouring
gives better dye absorption.
ð Mercerization
is given for cotton fabric and heat setting for synthetics.
ð Full
bleach is suitable for whiter background.
ð The
fabric should be thoroughly dried.
ð It
should be free from creases, and should be free from weft bowing.
Difference between
Textile Dyeing and Textile Printing
Textile
Dyeing
|
Textile Printing
|
||
1.
|
Only mono colour application can be
done
|
1.
|
Mono or multi-colour application is
possible
|
2.
|
Uniform colouration along length and
width of fabric
|
2.
|
Localized dyeing
|
3.
|
Dyes are applied in liquid form
|
3.
|
Dyes are applied with pastes
|
4.
|
Water plays an important role in dyeing
|
4.
|
Thickening agent plays an important
role in printing
|
5.
|
Percentage shade is calculated on the
weight of the material
|
5.
|
Percentage shade is calculated on the
weight of the paste
|
6.
|
The fabric is handled either in open
width or in rope form
|
6.
|
The fabric is handled only in open
width form
|
7.
|
During dyeing, the fabric may be wet
or dry condition
|
7.
|
During printing, the fabric is to be
kept in dry condition
|
8.
|
Colour is applied uniformly on both
faces of the fabric
|
8.
|
The printed face will be darker, and
the other face will be paler
|
9.
|
Simple machineries are used, such as
tanks, winches, jiggers, soft flow, padding mangles
|
9.
|
Machineries are more complex, by way
of design, screen preparation, printing machines, after process machineries
|
10.
|
Cost of dyeing per meter is lower than
the cost of printing
|
10.
|
Cost of printing per meter is higher
than the cost of dyeing
|
ROTARY SCREEN PRINTING
Fully
automatic screen printing achieved excellent results, but it only associated
with intermittent printing process, printers and designers were still looking
for a method which would offer continuous printing. This
was achieved in mid-1963, when European manufacturers brought out successful
units producing continuous screen prints, by utilizing cylindrical engraved
rotary screens.
Operations involved in Rotary
Screen Printing
Fabric
Feed:-
·
The fabric to be
printed must be introduced in the machine in a crease less, faultless and
tension less manner.
·
In order to have
prolonged printing without stoppages, it should be preferably fed from big
batches.
·
The fabric is to be fed
over a nip roller, according to the speed of the printing machine.
·
For delicate or
warp-knit fabrics, selvedge uncurls are to be provided.
Fixing
the Cloth on endless conveyor Blanket:-
·
Standard two-roller
gluing device and a glue holding trough are used for fixing the cloth.
·
Now a days, coating of
synthetic adhesive is introduced and used widely, which does not require gluing
trough.
·
CMC and PVA are most
widely used adhesives for fixing the cloth on blanket.
Working of Rotary Screen Printing:-
·
The printing effect on
the cloth, in the rotary machines, is achieved by causing a proper and
adequately pressurized contact between the fabric, which is stuck on the
printing conveyor blanket and the cylindrical surface of the electro-deposited
nickel screens.
·
This contact can be
brought about by:
v Bringing
the revolving rotary screens down onto the endless printing conveyor blanket,
or
v By
lifting the printing blanket to meet the rotary screens.
The former
system is usually preferred.
·
The contact of the
rotary screens and the printing blanket is also effected by two alternate
systems:
v Bringing
all the rotary screens down in contact with the printing blanket
simultaneously, or
v Bringing
about this contact in a sequence according to the order of the colours arranged
one screen after the other.
·
It is important to
ensure that, once all the colours in the pattern are properly set to produce the
correct impression of the design.
·
In the system with the
individual screen lowering and lifting, it is possible to register the entire
pattern without any unnecessary loss of printing paste and printing times in
the initial setting of the design will alter.
·
The dyestuff paste is
supplied to the interior of revolving screen, distributed in the entire screen
body and is pressed on the inner side of the screen, pressed through
perforations onto the fabric.
·
It is essential that
the running of rotary unit be very smooth.
·
The assembly of
machine, along with screens and squeegees, are well supported by frames on both
the sides.
·
The height of the
printing assembly from the blanket is adjusted with utmost accuracy, which is
made to suit the thickness of the fabric being printed.
·
Improper height
adjustments may lead to insufficient or excess screen pressure, which results
into inaccurate printing.
·
The lower part of the
printing unit contains the counter pressure system for the printing point and
also the controls for repeat setting.
·
These controls help
adjustments of repeat in the longitudinal and diagonal directions.
·
The top part contains
the screen tensioning system, the screen driving shaft and the traverse repeat
setting system.
Colour
Distribution System:-
·
The pumps used for
supplying the printing paste should work automatically by making use of
accurate sensing instruments.
·
The supply of printing
paste is to be arranged in such a manner, that a specific minimum quantity
should always remain in the squeegee system.
·
This quantity is
maintained by means of the colour probe. The working of probe is based on an
electrical contact system or a pneumatic contact system.
·
Further distribution of
paste in the squeegee body is achieved through smaller distribution tubes
positioning at regular interval to supply equal amount of paste throughout the
screen.
·
The colour pump can be
run both in forward and reverse direction, to ensure pumping back the colour in
case of sudden stoppage of the machine.
Squeegee Unit:-
·
The squeegee units
should to have the necessary sturdy and rigid supports from the sides, where
squeegee blades are located.
·
The pressure applied on
the squeegee blade is to be uniform from side to side and also in middle.
·
The units should be
free of any vibrations and the colour application should be precisely
controlled.
·
Squeegee blade pressure
systems are:
v Pressing
system making use of the squeegee blades themselves as he pressure generating
devices.
v Pressing
system in which independent pressure rollers are used to produce the printing
pressure (electromagnetic rollers)
Drives
of Rotary Screens:-
·
In order to cause and
maintain perfect synchronization between each screen, screen bearing assemblies
are provided, which draw the revolving movement from then main drive of the
printing machine.
·
The rotary screens are
driven by the rotary screen driving motors.
Repeat
adjustment of the Patterns:-
·
The pattern should be
produce exactly according to the printing plan chosen, for that rotary screen
can be revolved in both the forward and reverse direction.
·
These screen adjustment
is possible in three possible directions:
v In
the sidewise, i.e. longitudinal direction
v In
the rotational, i.e. forward or reverse direction
v In
the diagonal direction
Repeat
Size:-
·
The most common repeat
size internationally recognized is 641 mm. i.e. about 25.25”
·
Other repeats are also
favour of some manufacturers like 820 mm (32”) and 1018 mm (40”).
·
An exceptionally larger
repeat size is 1395 mm (55”).
Number
of colours:-
·
Rotary screen printing
machines are suitable for 12, 14 or even 16 coloured designs.
·
Usually, only 5 or 6
coloured patterns are preferred.
Printing
Width:-
·
The most commonly used
machine printing width happens to be 1620 mm (63”), although the width of
designs may vary from 950 to 1620 mm.
·
Some manufacturers also
offered wider width rotary screen printing machines, e.g. 1850 mm (72”), 2400 mm
(96”), 2800 mm (110”) or even 320 0mm (126”).
Rotary
Screen Printing Speed:-
·
The speed of rotary
screen printing machine, available from 4 meters to 100 meters per minute.
·
Printing speed is also
depend on various important factor:-
ð Chemical
nature of the fibre (in the substrate)
ð Twist
on yarn and texture of the fabric
ð Chemical
nature of dye (used for printing)
ð Drying
and post printing facilities
ð Quantity
of material to be printed
·
In the case of normal
cotton, the speed could be high as 60 meters per minute, while for synthetic,
it should be lower as 20-25 meters per minute. For blended material, speed will
be governed by the blend percentage, texture etc.
Cleaning
of Printing Conveyor Blanket:-
·
The fabric is stuck on
blanket using adhesive at entry side.
·
Blanket is running
endless, so the portion of the blanket on which cloth is pasted, returns to the
same position after the return passage.
·
During the return
passage of the blanket from the delivery end back to the entry end below the
printing table, the blanket is thoroughly washed, cleaned and wiped dry.
·
For the purpose of
cleaning, a very efficient cleaning system is provided, which is known as the
conveyor blanket cleaning unit.
·
The unit is provided
with a water-spraying system, along with motorized scrubbing brush rollers.
·
Subsequent wiping
offers a very clean and dry blanket surface.
Colour
Paste Pumps:-
·
The paste pumps perform
the duty of supplying the printing paste to the colour distribution system of
the rotary assembly.
·
The pumps have adequate
capacity to supply the dyestuff paste even under conditions of maximum printing
coverage, say 100 %.
·
The pumps are
reversible and can draw back the printing paste when necessary.
·
The material used for
construction of the pump is such that it is not affected by any of the
ingredients used while preparing the paste.
Screen
Washing Unit:-
·
After completion of
printing, screens should be washed and cleaned as early as possible.
·
Cleaning is to be done
by means of a good water spray.
·
The cleaning unit
should be such a design that water can be sprayed from inside the screen, as
well as from outside.
·
The washing unit should
be located near the printing plant for minimizing the handling of the screen
and to execute the washing operations without any loss of time.
Drying
the Printed Fabric:-
·
Machine is provided
with a suitable jet dryer.
·
The jet dryer should
have enough drying capacity, so that, even at the maximum machine speed, the
printed cloth is well dried during the passage through the dryer.
Reference:- (Book)
1.
Handbook of Textile
Processing Machinery by R.S. Bhagwat
2.
Textile Printing by LWC
Miles
3.
Technology of Printing
by Dr. V.A. Shenai
4.
Technology of Printing
by R.S. Prayag
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