Knife
Coating systems:
With knife
systems, the coating paste is applicated onto the carrier or the substrate with
a knife. Depending on the counter piece to the knife, roller knives and air
knives are distinguished. The knife bar of modern coating plant is equipped
with several coating knives and is turnable according to the so called revolver
principle. This facilitates a quick change of the knives, especially of the
roller and the air knife. All application systems are equipped with either
manually or powered adjustable side scrapers.
In some cases,
the side scrapers are completed to a closed paste basin with a front plate,
which is necessary especially when low-viscous coating pastes are concerned.
Depending on the thickness of the coat to be applicated, on the way of the
knife´s configuration and the viscosity of the paste, knives of different
profiles are used.
With knives, the thickness of the coat is
defined by:
- · Distance between knife and substrate
- · Kind of the substrate to be coated
- · Kind and form of the knives
- · Angle of the knife
- · Position of the knife
- · Viscosity of the paste
- · Speed of the web
For air knives it is especially the web
tension that influences the thickness of the coat.
Roller
Coating:
A 2 roller
system is comparable to an engraved roller system. The difference lies in the
structure of the roller. With the engraved roller, a structured roller is used
to applicate the coating raw material, where as in case of a simple 2 roller
system, a chrome-plated, polished roller is used.
The coordination of the coating raw
material with the application of an unstructured roller is important. This
technology is used when very thin layers have to be applicated.
Depending on the substrate´s surface, the
coating raw material is directly applicated onto the substrate with an engraved
roller, or it is applicated indirectly with a rubber roller. In cases of an
even surface of the substrate, like e.g. when lacquering coated substrates, the
direct application with a raster roller is preferred.
If the substrate´s surface is uneven, the
indirect application is chosen to avoid damages of the substrates and to
achieve a more even appearance of the coat.
In case of the 5 roller system, 5 rollers
are collocated in horizontal direction to each other. By the highly precise
distance of the rollers as well as by the absolute and relative rotating speed
of the individual rollers to each other, the thickness of the coating film that
is applicated through the first roller gap, flattens out increasingly. This is
how coating films with a layer thickness of under 1 µm can be achieved.
Curtain
Coating System
The difference
between a standard die system and the Curtain-Coating System is, that there is
no contact between the die and the web while the coating raw material is
applicated.
After flowing out of the die, the coating film falls like a curtain onto the substrate that moves through below the die. The advantage of the contact-less coating is a more even and, depending on the viscosity of the coating material, much thinner coating film.
Dipping system:
In a foulard,
the coating or impregating of the substrate is realised by dipping it
completely. Depending on the viscosity of the coating raw material and on the
substrate´s structure, the coating compound either attaches superficially to
the substrate or it permeates completely through it. Thus, the texture of the
substrate and the coating viscosity mainly define the application weight.
After the
dipping, the surplus coating amount is squeezed off or scraped off. This can be
realised between two rollers, the so-called squeezing rollers or by cleaning
knives. Exemplary applications are: The impregnation of an open-structured
substrate before the coating process or the complete dipping of glass fibres or
glass non-wovens (for the production of circuit boards). In this case, the
surplus coating raw material is evenly scraped off by bilateral polished
rollers.
Double
Side system
This patented systems allows the doubleside
coating of substrates in one production step. The rear side of the substrate is
coated by a roller system, with the applicating roller being either polished
(mostly chrome-plated) or engraved.
The upside of the substrate is coated with
a knife system which facilitates all standard settings like angle adjustments,
horizontal and vertical knife adjustments. Moreover, roller knives as well as air
knives are possible. Depending on the viscosity of the coating material and the
setting of the application systems, the most different effects can be achieved.
Engraved
Roller System
In case of the engraved roller system, also
called kiss coating system, the carrier material is led through between two
rollers. From a dipping basin, the application roller passes the paste onto the
substrate. The coating paste quantity is determined by the design or the raster
of the applicating roller.
The surplus paste on the applicating roller
is scraped off by a knife. A a lacquering unit can be integrated directly into
the coating machine or as a separate coating system. Coating layers of between
2 and 10 µm can be applicated by the lacquering process.
In most cases, acrylic or PU- varnishes are
applicated. The application of e.g thin varnish layers onto already coated
material is mostly being done with engraved roller systems.
Hot melt Coating System:
Hot-melts are thermoplastic adhesives, based
on polymers, that soften plastically at temperatures between 80°C and 220°C and
harden if they are cooled.
100% dry substance, no dilution
are applicated in a liquid state
easy metering
bonds quickly
short laminating distances and linking times
Hot melt
Slot Die System
With slot dies, products are applicated as
2-dimensional, closed films. They can be used for continuous coatings with a
viscosity range of 50 to 250 000 mPas. In combination with the adequate
"mask" the coating weight and wideness can be determined: If a closed
film is applicated > 10 g/m², in case of partially porous films < 10
g/m².
The coating weight is determined by the
performance of the pump, the web speed and the distance between the die and the
substrate.
Kiss
Coater Coating:
The coating system called Kisscoater for
materials with a low or medium viscosity which has been used as application
system for quite a long time now, has been advanced by COATEMA. Basically, the
system consists of an application roller and two guiding rollers.
The system offers the user a multitude of
adjustment possibilities. The control of the coating unit can be influenced by
the positioning of the guiding rollers and the web tension as well as by the
proportion of the application roller´s speed to the plant speed. The positioning
of the guiding rollers determines the contact angle between the applicating
roller and the substrate. Here, a variation between a large wrap angle and the
application of the coating raw material into the substrate and al small wrap
angle and a linear contact between the substrate and the application roller.
With the linear contact, the coating raw
material is only applicated onto the substrate´s surface. Apart from the
adjustment of the contact angle between the application roller and the substrate,
the quantity of coating raw material is determined by the speed of the
application roller. Here, a higher speed in relation to the substrate´s speed
entails a higher coating weight.
Powder
Scattering System
For certain purposes, powdery materials
need to be applicated. Common examples are: The application of thermoplastic
powder for interlining products, the application of carbon powder for
ABC-protection clothes or the application of glass beads for reflective
products.
A powder scattering unit always consists of
an engraved roller that determines the amount of applicated powder by its
engraving and by its rotating speed during the coating process. To empty the
engraving or rather to strip the powder that may be very sticky in some cases,
off the engraved roller, rotating or changing brushes are used.
Reverse
Roll Coater
This system is called "Reverse Roll
Coater" because the applicating roller moves in the opposite direction to
the web´s moving direction. Principally, this system consists of three roller
that are either arranged vertically align or in an S-shape. The coating raw
material is placed between the two upper rollers and is kept in the roller gap
by double-sided limiting knives.
The coating weight is determined by the
distance between the upper metering roller and the application roller, but also
by varying the roller´s speed. The lower roller that guides the material mostly
is a rubber roller. The coating roller transmits the plastic film on it in
counter direction onto the substrate. Each roller is powered separately. By
default, the speed of each roller can be varied by approximately 50%. The
rotating direction of the upper roller can also be adjusted to the counter
direction.
Because of the various possible settings,
the application can be defined optionally by the gap distance, the rotational
speed of the rollers and the relative roller speed to each other. Also the
thickness of the coating layer can be influenced by changing the relative speed
of the rollers to each other and the relative speed between the applicating
roller and the substrate. The Reverse-Roll-Coater is used for the processing of
pastes with a low viscosity and a good flow behavior. Dilatant pastes, which means
coating raw materials the viscosity of which increases proportionally to high
shearing forces, as well as highly viscous pastes cannot be processed because
of the high shearing forces that appear. However, very thin applications can be
realised with this technology.
Rotary
Screen System
In the field of screen technology, a
general distinction between surface printing and rotary printing can be made.
For the continuous processing of coating compounds, only the rotary screen
technology is used. The coating raw material to be applicated is pumped into a
rotating screen. Then it is applicated onto the substrate by a knife which is
placed inside the screen. The coating weight is determined by the mesh number
of the screen (openings/cm²) and by the thickness of the screening plate. The
rotary screen technology is used for the application of foams and special
designs.
Slot-Die
System
The application of the coating raw material
by a die system is another alternative. A die is a closed application system,
consisting of a die chamber, into which the coating raw material is pumped. For
each coating raw material, the design of the die is evaluated regarding the raw
material´s flow behavior. This guarantees the raw material´s even outflow out
of the die lips.
A special micro pump feeds highly precise metering of the coating material to the die. The coating weight can be
precisely defined by the speed of the pump. Also the die lips and the speed of
the web influence the coating weight.
The advantages of the die system are:
·
a closed system, advantageous
for the processing of solvent-based systems and with regard to the cleanness of
the application
·
depending on the viscosity of
the raw material, very thin layers of under 5 µm can be
realised, but layer thicknesses of up to 300 µm are also possible realisation of intermitting coatings
·
the coating can be realised
either with a contact of the die to the substrate, but also contactless.
The use of the die technology grows
constantly, as the application of coating materials needs to become
increasingly precise
