101 process choices in screen printing Ed Crutchley
Crutchley Consulting Limited Article no. 101-7 - May 2012 Keywords: Screen printing, tup, ink, squeege.
Sections
Incoming components
Artwork
Screens
Squeegee
Inks
Machine construction
Output and performance Component handling
Pre-treatment
Component holding
Printing stroke
Drying and curing
Camera inspection Introduction
This document lists the most important considerations to be taken into account when specifying and purchasing a screen printing machine and accompanying process for plastic components.
A key consideration in specifying a new machine is the extent to which high levels of repeatability and performance need be assured, i.e. how much technology should be incorporated into the machine conception. The alternative is to have a low cost machine and rely more on operator intervention to tweak and maintain the process.
Incoming components
1.Surface quality
Any shrinkage during moulding may cau decorating issues due to deformation or sinks. Bottle moulds must be carefully maintained so that ams do not
produce unwanted miss in any
banding or peripheral decoration.
Ideally moulding and decorating
should be carried out in protected
areas (pressurisation and filtration)
in order to reduce dust
contamination on component
surfaces.
2.Material
Care should be taken when
lecting the material grade to be
decorated. Internal additives such
as antistats may hinder adhesion.
Artwork
3.Vectorized artwork
Electronic artwork provided for
making dies must be vectorized
instead of pixelated. Adobe
Illustrator is most commonly ud
for artwork preparation.
4.Standardid artwork/screen
positioning
A standard positioning system
should be ud to ensure that any
two screens fall with their images
in exactly the same position once
mounted on the machine.
5.Film positive
The artwork is ud to produce a
film positive the same size as the
print. It should be right-reading
when emulsion side up.
6.Printable area
The screen frame and component
holders, and the flatness of the
component, will determine
printable area. Care must be taken
人的记忆曲线to avoid the screen having to be
placed over sharp edges on parts,
and space should be allowed for
squeegee descent and stabilisation.
7.Bar codes
So as to avoid potential problems
酣畅淋漓
due to slur, bar codes should be
orientated in the print direction.
Fig. 1: Effect of slur on bar code definition
Screens
Fig. 2: Screen
8.In-hou or subcontracted?
The choice has to be made as to
whether screens are manufactured
in-hou or outworked.
Outworking requires good
logistical control, as veral screens
can break in succession.
9.Frame
A screen frame is typically
manufactured in wood or, better,
metal. It is clamped into position on
the printing machine, and micro-
adjustments allow fine
repositioning in order to place the
décor as required on the
component. The frame must be
large enough to allow sufficient
space around the image for
flexibility, squeegee, and flood
return, and rigid enough so as to
ensure the mesh remains taut.
10.Pinning
So that the screen (and printing
image) are always in the required
position, a metal frame can be
pinned to allow it to always land in
the same position in the printing
machine.
11.Mesh
The woven mesh is in nylon or
polyester, usually the latter.
(Stainless steel mesh is ud on
glass.) The material is dyed so as to
avoid gain by light scattering
during exposure of the emulsion.
12.Mesh count
Mesh count determines the
fineness of print. A typical mesh for
UV printing would be 150-180
threads per centimetre,
monofilament plain weave and 34-
micron thread diameter. For solid
areas, a coarr mesh is ud in
order to allow through more ink.
For fine detail, mesh count must be
at least 80 lines per centimetre
when using solvent-bad inks, and
160 lines per centimetre for UV
(zero-solvent) inks.
13.Screen angle
A straight angle of mesh to frame
will produce more jagged edges. Normally the mesh for a single
colour is angled at 22.50.
Fig. 3: Jagged edges and mesh angle 14.Tension
The mesh has to be taut. It is
stretched in a special large frame
prior to fixing the screen frame. Maximum tension is 25-30 Newtons/cm2, minimum 15 Newtons/cm2.
Fixing is usually achieved by adhesive, and sometimes mechanical assistance such as stapling or clamping. In certain industries, lf-tensioning roller frames are ud.
15.Moiré
Moiré is an interference effect
when halftone images (photographs) are super-impod. It is reduced by angling the images to each other. Typically, in 4-colour process printing, yellow is reference, and cyan, black, and magenta are rotated 150, 450, and 750 respectively. A plain weave (one thread over one) is preferred to a twill (one thread over two) in order to limit Moiré effect.
Fig. 4: Moiré
16.Screen preparation
The mesh is abraded and degread.
17.Emulsion The screen emulsion, a light-
nsitive diazo or photopolymer
solution, is prepared well in
advance in order to eliminate
bubbles. It is usually either applied
by hand in what is referred to as a
direct stencil process, or a capillary
film can be alternatively ud for
the printing side. Although it is less
resistant and provides about half
the screen life time, it assures well-
controlled ink film thickness.
Drying at 35-400C is ud. Typical
thickness are 7-10 microns on the
print side and 15 microns on the
squeegee side. Higher thickness
allow heavier ink deposits. The
emulsion is expod under light to
a film positive derived from the
original artwork. Expod areas are
hardened, unexpod areas are
washed away with water in order to
allow ink to pass through during
printing.
18.Exposure
The film positive is vacuum-held in
contact with the printing side of the
screen. Exposure may last veral
minutes under a 0.8 to 8.0 kW
metal halide lamp sufficiently far
away in order to avoid undercutting
(>1.5 x the diagonal of the
positive). The exposure time is
determined with the aid of a light
integrator and exposure calculator.
Fig. 5: Screen/emulsion exposure
19.Spotting out and protection
For protection from squeegee wear
and flexing in order to lengthen the
screen life, non-printing areas on
the screen should be manually
spotted and taped.
20.Split screens
A screen can be manufactured with
a divider glued to the mesh in order
to print two different inks at the
same time. Typically a minimum
distance of 15-20mm is required
between the two images.
Corresponding split squeegees and
flood return blades will be
required.
Fig. 6: split screen
21.Thick deposits
By increasing the emulsion
thickness on the underside of the
screen, deposited film thickness is
incread. In non-flexing metallic
screens, for printing flat substrates,
films of up to 300 microns can be
printed.
22.Fineness of print
Typically the minimum area of
emulsion should be three times the
screen thread diameter. This
implies that the smallest dot size
would be about 100 microns. In
halftone printing, photographic
artwork is divided up into round or
elliptical randomly or regularly
placed dots. This translates into 30-
35 dots per centimetre with the
potential of a tonal range of 10-
90%.
23.Positioning on machine
See Standardid artwork/screen
positioning, above.
24.Cleaning
The underside of screens in u are
kept clean by wiping with a
solvent-laden cloth, preferably non-
linting (Micropure, or other). In the
electronics industry, automatic
cleaning devices have been
developed for machines for screen
printing resists. After solvent
wiping, 1-3 dummy parts are often
run in order to re-stabili the
process and remove all traces of
solvent diluted ink. In manual
operations, a clean sheet of paper
is ud so as not to waste
components.
25.Screen life
This can typically vary from 10,000
to 50,000 prints, depending on the
conditions of u and type of
emulsion.
26.Recycling
Stretched screens can be recycled
by stripping the emulsion and
applying a new coat.
Squeegee
27.Shape
The squeegee is a thin flat strip of rubber material strong enough to push the ink through the screen mesh, and flexible enough to conform to the component. The working edge of the squeegee may be sharp (pointed or square) or rounded. A square edge on its side is the most stable. A ‘V’ shape is best for detail, but is more fragile.
A rounded shape is ud in rougher conditions for less critical work.
Fig. 7: Squeegee shapes
28.Type of material
The squeegee must not be attacked as a result any material with which it comes into contact. For this reason, materials ud include relatively resistant grades such as polyurethane, EPDM, and butyl rubber. Material lection is made after a swelling test.
29.Hardness
Typical squeegee hardness is 850 Shore A. Dual durometer squeegees are also known, whereby the outer coating is harder in order to lend rigidity to a softer edge layer. A higher hardness ensures better ink film thickness control, better definition. A lower hardness is better for less viscous inks, wide meshes, rough and uneven surfaces, lower screen tensions, and longer screen life.
30.Angle
A square-end squeegee must be t at an angle so that the sharp edge faces downwards.
31.Squeegee positioning
When peripheral printing, the squeegee should be positioned centrally.
32.Squeegee pressure
This is provided by springs (more reactive) or adjustable pneumatic cylinders.
33.Squeegee rocking Sometimes squeegees holders are
mounted so that the squeegee can
swivel freely.
34.Curved squeegees
In principle, squeegee edges are
flat along their length. However, it
is possible to decorate surface with
a slight curve along the squeegee
direction, and various techniques
exist for maintaining a curved
squeegee. Screen tension has to be
lowered.
35.Recycling
To avoid squeegees becoming blunt
with u, they must be frequently
re-sharpened with suitable
equipment.
Inks
36.Ink drying/curing type
According to their chemistry, inks
can air-dry with no crosslinking,
with heat (including, in certain
cas, adding catalysts in order to
promote crosslinking), or cure with
ultraviolet (UV) light. Crosslinking
ensures better resistance to
abrasion and chemicals. The u of
UV permits virtually instant curing,
within a few conds (ideal for
multi-colour printing, since a
screen cannot be brought into
contact with a wet ink), and
becau they do not contains
solvents, the inks are stable and
can be left overnight on the screen.
Thermally drying and curing inks
that contain solvents will not be
screen stable if catalyd or
employing fast evaporating
solvents. Very slow solvents
provide screen stability but require
long drying tunnels.
If drying and curing are
delayed, the ink has more time to
flow out, and in certain cas (for
example with large solid areas and
banding), this may be an aesthetic
advantage. In this respect, on-
mandrel curing may not always be
a panacea.
37.Ink constituents
The esntial ingredients of inks
are resin, or mixture of resins
(epoxy, polyurethane, acrylate,
etc), pigment/matting agents,
additives to promote flow-out and
levelling and slip, and in some
cas solvent in order to dissolve a
solid resin and fine-tune viscosity.
38.Overprinting
Certain additives and high degrees
of crosslinking may not be
conducive to good adhesion after
overprinting, or when coating a
screen printed image. For this,
waxes and slip additives may have
to be removed and crosslinking
level reduced. Ink manufactures
should be consulted for
formulations suitable for
overprinting.
39.Rheology - thixotropy
A screen printing ink has to exhibit
some degree of thixotropy
(‘shortness’) in order to prevent it
from flowing through the screen on
its own. This implies that it will
only pass through the screen as a
result of squeegee pressure. A
‘short’ ink, which is said to po sss
non-Newtonian behaviour, will split
if it is allowed to fall from a palette
knife. For high definition work, a
higher yield value is desirable, and
thixotropic additives such as
Aerosil (silica) can be added to help
this.
Fig. 8: Non-Newtonian behaviour (at right)
Fig. 9: ‘Short’ ink (at right)
40.Ink viscosity
Typical viscosity may be 20-100
Pa.s (about the same or more as
toothpaste).
41.Screen printing coatings
It is possible to screen print clear陈皮山楂茶
lacquers. However, due to
thixotropy requirements, the do
not flow out easily, and invisible overlaps are a challenge. The final
result, therefore, does not match a
spray-coated finish, but it may
more easily compete with a roller-coated finish.
42.Colour matching
Many printers will colour match inks on site, as this is more rapid and less costly than using suppliers, especially for small quantities. Colours are obtained from a ba scheme range of up to 15 or more mono-pigment colours. Most ink suppliers will provide colour match formulae, but a final good match will always require the human eye and manual adjustment.
A uful aide to colour matching is the Munll colour circle. When a particular colour is required, tw
o inks which have the clost colour each side to it (the colour circle runs from yellow, through orange, red, purple, violet, blue, green and back to yellow) are mixed together in proportions according to the specific nature of substrate colour, ink, screen, and printing conditions. Black and white are added if required.
Fig. 10: Munll colour system Machine construction
43.Transport system
There are many different screen printing machine configurations from horizontal or vertical dials, to linear horizontal walking-beams, and indexing or continuous chains. Drying and curing may follow a transfer, or may be included in the same transport system (particularly in the ca of UV curing which can be on-mandrel).
One element that remains constant is that the screen is always horizontal.
44.Working height Consideration must be given for
any operator intervention and the
ea with which routine operations,
such as squeegee, ink, or screen
change, can be carried out.
45.Enclosure
It is important that on fully
automated equipment, any
enclosure is hard-wire interlocked
and that enclosures are easy to
dingage for tup and
maintenance.
46.Dust precautions
Falling dust will provoke pinholes
in solid areas. Normal practice is to
enclo the side and roof of the
machine, and to work in an
enclod, pressurized and filtered
area.
47.Rapid change tools
See Component changeover time.
Tooling changes should involve
specific tools rather than tting
adjustments, and single tool
intervention should be provided for
the majority of adjustments or
fixtures.
48.Mobile command console
Depending on the size of the
machine, it may be helpful to have
a hand-held or swinging-arm
console in order to operate or inch
the machine from different sides.
49.Memorized ttings
The scourge of the past has been
variable tups as well as time
wasted as a result of not having
recorded tups. As many
parameters as possible should be
recorded in PLC memory or have
fixed positions.
50.Mechanically or pneumatically
controlled movements
Pneumatic controls are more
variable but cheaper, and they
require more operator intervention.
Mechanical controls can be more
precily t and fixed, provide for
higher output speeds and
reliability. See Squeegee lift-off,
below.
51.Required utilities
Required energy sources such
factory voltage and pha-type
must be defined, as well as needs
for compresd air, gas, and any
other utility.
52.Fault display
Machine faults should be displayed
in clear language.
53.Number of part holders
Output rate can be significantly
improved by loading and unloading
holders in masked time, i.e. by
have at least 3 part holders. Most
automatic screen printing machines
have multiple part holders,
sometimes running into hundreds
where chain conveyors are
involved. The impact of number of
holders on loss of productivity due
查孕酮to changeover time should be
assd.
54.Component shape and size
range
The versatility of the machine to be
able to deal with different shapes
and sizes needs to be defined.
Fig. 11: Different types of decoration
55.Component rotate or flat hit
The esntial screen printing
movements are rotary (or multiple
peripheral ‘flat hit’ whereby a
screen is brought down onto an
indexing component veral times).
In the latter ca, the maximum
number of peripheral flat hits
should be specified (e.g. 4 sides of
a component). Alternatively, the
亡羊补牢文言文
operation may consist of printing a
flat stationary surface.
Output and performance
56.Number of operations
More than one decoration pass may
be required, to the extent that the
machine may require veral heads
with intermediate transfer
mechanisms and drying between
them.
57.Output speed
Screen printing can achieve speeds
in excess of 5,000 per hour. The
main constraint is drying and
curing.
58.Component changeover time
Changeover can involve hundreds
of adjustments. By incorporating
ttings into memory, and applying
basic SMED rules, this can be substantially reduced. However,
when including component feed
and take-off adjustments, this can
still be over 100 ttings. The should be carefully examined when asssing a new machine.
59.Validation procedure
The validation procedure for a new machine needs to be defined at the outt (length of trial, number of component types to run and types of décor, running speed, changeover time, and uptime).
Component handling
60.In-feed system
创业史读后感Components may arrive at random
on a flat conveyor, channelled in
line, pushing each other along towards an escapement, or on
pucks or walking-beam conveyor.
They may also be ranged on trays
or in packaging, or in bulk. A mechanical system which determines preci component position and keeps components apart from each other is always best; if a component can be held in a non-decorated zone, risks of quality and ink transfer problems are further reduced.
Fig. 12: Different types of component feeding
Fig. 13: Walking beam, chain, and dial
transport systems
61.Component reorientation for
printing
A component may have to be
turned on its side to be printed.
Fig. 14: Re-orientating component for
decoration
62.Automatic feed/take-off
Component feed from the in-feed
system may be manual or
automatic, for example with a
ramp, a SCARA or 6-axis robot or
with purpo-built or proprietary
pneumatic, mechanical, or
electromechanical pick and place
devices.
63.Suction cup or finger feed
Robots and other mechanical
manipulators may be equipped
with suction cups or fingers. Cups
have the advantage of flexibility
and creating less physical damage
(although cleanliness has to be
careful monitored) whereas
mechanical fingers provide more
preci positioning, important for
high speed operations.
64.Sensors
Suction cups or fingers should be
equipped with Vaccustats or
captors in order to determine part
prence.
65.Continuous motion or indexing
High-speed production may require
component feeding, decoration,
and removal on the fly.
Pre-treatment
66.Deionising
Deionising is required prior to dust
removal in order to eliminate
electrostatic forces that help retain
dust particles to the component
surface.
67.Compresd air control
Compresd air may be ud with
deionising in order to remove
larger dust particles. This needs to
be carefully regulated with a
preci manometer so as to avoid
dust projections and limit
consumption costs.
68.In-line dust removal
Brushing is usually required after
deionising in order to remove fine
dust. The best system consists of a
rotary brush equipped with an
extraction hood.
69.Compresd air filtration
Any compresd air ud on the
machine must be dried and filtered.
Although this is normally carried
out centrally, individual machines
are often protected with a
desiccator and 3-level oil filtration
unit equipped with automatic
bleeds.
搬家四言八句70.Flame treatment
Flame treatment is the most
commonly ud and versatile pre-
treatment process ud, primarily
to ensure ink wetting and adhesion
to polyolefin materials (PE, PP). In
this, critical parameters are dwell
time (a rotating component should
turn at least 2-3 times in the flame).
Too long a dwell time will result in
surface matting or striping, and so
this should normally be
controllable independently of the machine cycle time. Air to gas ratio
is also critical for optimisation of
flame oxidation and thereby
treatment level, especially at higher speeds. Air and gas flow rates should be controlled with flow meters.
A flame has a reach which can be as much at 50-100mm (although the flame is less stable at longer distances). Rectangular
components are sometimes peripherally treated using CNC-controlled movements.
Burner types may be low or high pressure, ribbon or slot.
Treatment level should pass from 30-32 dynes/cm (untreated) to at least 36-44 dynes/cm. The level is tested using a range of surface tension liquids, or alternatively by using clean water or an adhesion test, for example using an air drying test ink.
71.Corona treatment
Corona treatment, an alternative to flame treatment, is carried out via a high-voltage electrode and earth. Electrode to component distance can only be a few millimetres, and so this process is rerved for concentric or flat components.
72.Plasma treatment
Various forms of plasma treatment exist, and for the purpos of this document can be considered similar to corona treatment.
73.Tunnel treatment Disadvantages related to electrode positioning in electronic treatment can be avoided by using a tunnel. This is more voluminous and expensive, and usually carried out off-line, or before component feeding into the machine.
Component holding during printing
74.Holding device and positioning This may be a nest, mandrel, or cup and cone with inflation for bottles.
Fig. 15: Different component holding
methods (nest, mandrel, cup and cone)
75.Register devices
In peripheral decorating,
particularly of cylindrical
components, a register device may
be required in order to ensure the
décor starts at the appropriate
peripheral position around the
component. This is usually
achieved with a feature and lead-in
moulded in the component that
allows the engagement of a sprung
locating pin, or via electronic eye
四川的乐山大佛aimed at a feature on the
component. Mechanical methods
tend to be more accurate.
76.Component retention
For nests or mandrels, mechanical
holding may be supplemented by
suction, and bottles may have to be
inflated. Inflation, suction, and any
blowing in order to assist ejection
will each need to be timed.
77.Inflation
Bottle may have to be inflated
during printing. Inflation pressure
has to be timed and finely t.
78.Mechanical stabilisation during
printing
A cup-and-cone system is
commonly ud for bottles. A
component held on a mandrel may
require mechanical assist by a
freely-rotating end-stop.
79.Possibility to advance or retard
décor
It may be necessary to shift the
décor sideways in relation to the
component. This adjustment is
usually carried out by adjusting the
screen, but in certain cas the
component position may have to be
altered.
80C or cam-controlled
movement
When peripherally decorating non-
cylindrical shaped components
(e.g. square-round, oval, etc),
screen to component paration is
controlled by two alternative
methods.
In the traditional method, it is
controlled mechanically by, for
example, rack and pinion (link),
where the pinion and an attached
cam are shaped according to the
component, and the cam rides in
contact with a flat plate.
Alternatively it is controlled by
CNC-controlled movement.
Fig. 16: Principle of rack and pinion system
Fig. 17: Principle of CNC control
81.Printing an oval
The diagram below illustrates how
an oval component can be printed
using an off-centre mandrel that
sweeps in an arc the same radius as
the component.
Fig. 18: Printing an oval bottle
82.Possibility to vary differential
peripheral speed
