INK DRYERS
Gravure- very fluid inks- need to evaporate a lot of solvent. Also, ink coming to next print station must be dry.
Some applications- extremely dry ink- to avoid trapped odor.
Some inks- 100 % solids- hot melts, UV curable, EB curable.
Solvent Ink Film formation
Most inks- solvent integral part of composition. Solvents released by diffusion process- solvent molecules uniformly move through the film
Evaporation rate- constant – free flow throughout the low viscosity fluid.
Film becomes thicker, viscosity increases, solidification process.
Diffusion rate in semi-plastic mass – much lower than in less viscous ink.
At some point- diffusion slow enough to affect the evaporation rate.
Less energy to evaporate the first 80% of solvent in the wet layer, than to remove the last 20%.
The last 5% is extremely difficult to remove. Tendency of ink film to retain solvent.
(Contaminate contents of food package).

WATER BASED INKS
Little or no solvent, therefore little or no explosive hazard.
Water- evaporates more slowly as relative humidity raises.
T of the supply air- well over boiling point of water.
Trace of water remaining in ink film- 0.3%- can make product unacceptable
Tendency- 0.1mg/m2 retainded.
If 1% water retained- does not matter- no odor. WB inks not re-dissolvable (solvent inks are).

SOLVENT AND WATER BASED INKS DRYING COMPARISON
Solvents- much more volatile than the water.
Less energy to change solvent into vapor than water- into vapor.
Amount of heat energy required to raise one gram of liquid one degree C specific heat  SH
SH of solvents…. 0.35
SH of water… ….1.00
3x more energy to raise the T of water than T of solvent.
The amount of E required to transform 1 kg of liquid at vaporization temperature to vapor is LATENT HEAT  LH
LH for solvents ….cca 200 kCal/kg
LH for water…….cca 900 kCal/kg
More than 4x more heat for evaporation of water than solvent- great deal more E to evaporate WB ink.
It does not mean that WB needs 4x more E- because at the the same thickness of ink film much less water is needed to be evaporated at WB than SB ink- there is higher level of solids in WB inks. Water escapes more easily from ink film than solvent.

Skinning- crusting
Creation of an undesirable surface layer-skin over the wet deposit- can be serious problem with both WB and SB.
Skin inhibits the passage of solvent (water) vapors- additional E needed to force them through crust. Aqueous inks tend to crust more easily than SB.
Gradual solvent (water) release- the only way- to prevent blistering, pinholes, and diluent entrapment.
Water based inks can be run at rate equal to or faster than solvent based ones.

DILUENT LOADS ON DRYER
Solvent or water load = total amount of diluent which must be evaporated in one hour-expressed in kg, lb., l, gal per hour.
Best- to use the weight of diluent [g/m2] or [pounds/ream].
Weight= dry weight.  3g/m2-means dry weight.
Solvent weight- relative to water- specific gravity SG. Water SG=1
1 micron thick material with SG=1 will weight 1g/m2.
Coatings measured by dry weight g/m2 or lb/ream

INK LAYDOWN VALUES
How much ink is laydown on material to produce 100% solids in gravure?
Answer depends on type of material.
Tonal quality of ink film depends on pigmentation, percentage of solids in diluted ink.
The tonal quality of solidified ink- density and thickness.
Material being printed- non-absorbent- ink film remains on surface- darkest tonal density.
The same ink film applied onto slightly absorbent substrate- ink absorbs-reduces ink layer.
The same ink applied to very absorbent surface- thinnest film- resulting tone – the lightest.
2 remedies- to increase the pigmentation- too expensive,
to increase % solids- darker tone- this increases the viscosity of the ink- too high visco- interfere with printing system.
To create equally dark tones at all types of substrates- to apply a thicker wet layer on more absorbent substrates.

TABLE: Solvent Ink Mileage (Ink Application Chart)
 
Substrate type  Area for 1 kg of wet ink [m2 Area for 1 lb of wet ink [ft2]
Films with dye inks  200  970
Films with pigment inks  165  800
Coated Papers  150  750
Uncoated Paper, Boxboard 135   670
Solvent Ink Mileage - Ink Application Chart – for obtaining a uniform tone for rotogravure.
Heavier wet ink layers used for more absorbent substrates to create the same tonal density
150 lpi cylinder, 37 micron deep cells for printing 100% tone with 22% solids ink.
Different gravure cell will create different ink mileage.
We must divide the area, which will 1 kg of ink cover in order to find ink laydown values for different materials.
To purchase a new press- (dryer)- estimated ink laydown values under maximum operating conditions must be determined.
 

TABLE: Solvent Ink Laydown Values
Substrate type  [g/m2 [lb/ream]
Films with dye inks 5.0  3.1
Films with pigment inks  6.1  3.8
Coated Papers  6.6  4.0
Uncoated Paper, Boxboard  7.4  4.5
Typical solvent gravure ink would have 20-25 % solids by weight, means it has 80-75 % solvent in it.
Avg = 6.24 g/m2 of solvent in ink.
To find out the solvent load- how many m2 or reams will be printed, and than multiply that with solvent laydown values.
Example: 53” press  (1.35m)
Production speed 300m/min 1000ft/min2
Area of web produced per hour : 1.35 x 300 x 60 = 24,300 m2  [83.4 reams]
Solvent load  6.24 g/m2

6.24 g/m2 x 24,300 m2= 151.6 kg/hour
New printing press must be capable of evaporating 151.6 kg/hr solvent, which is 325.3 lb/hr (to apply 100% ink coverage).
Not every color- 100% coverage first and last station – surface printing background color first, reverse printing- last.
First and last station must be sized to dry full coverage, second- though next to the last- 50% coverage.
Water based inks- 50% solids, WB inks- more pigmentation- much less WB ink is applied than solvent ink.
The average solvent load is about- 4.0g/m2
Shallower cells for WB inks- 20 micron depth (40 micron for solvent based).
The average water load is half- 2.0g/m2
24, 300 m2 or 83,4 reams/hour, the total water load for 100% ink coverage is 48.6 kg (151.6 solvent load).
The water load is about 32% solvent load.

EVAPORATION PRINCIPLES
Energy must be emitted from the drying chamber, transferred to the wet layer, for vaporization to occur.
The released solvent and water must be evaporated to surrounding air, and then extracted from the drying chamber.
Heat transferring action followed by mass transferring action to dry a wet layer.
Heat transfer Q is amount of kilocalories kcal/hr or Btu/hr British thermal units that is implanted in the wet coating under specific operating conditions

Q= h A (Tair -Tsurface)

Where
A= area of dryer
h= coefficient of heat transfer
delta T is the difference in air and surface temperatures
Heat transfer portion of the drying process- ability to raise the temperature of wet layer to its evaporation point- mass transfer can begin.
Heat transfer curve- straight-line function.
After energy is in the liquid, the removal of the diluent begins- the process is referred to as mass transfer.

X= Kx . A (Pair - P surface)

Where
X – evaporation rate in kg/hr , or lb/hr
Kx – mass transfer coefficient
P – vapor pressure
Mass transfer- nonlinear. Factor affecting mass transfer- boundary layer.
Boundary layer of air – present on each moving web.
The ability of drying system to reduce the thickness of boundary layer determines how efficient the dryer is.
Intersection of heat and mass transfer curves- drying rate for specific temperature.
Drying rates are specified in the quantity of water that can be evaporated per m2per hour.

Heat in
Q= h . A . (Ta-Ts)   watt/sec

Mass out

M= k . A. (Pair - Psur)  kg/sec
Vaporization commences at the surface of liquid- works way down when heat applied only on top side.
Ink film enters the dryer- heating up to evaporation T.
Vaporization occurs- T is reduced (cooling process).
Under ideal condition, vaporization from the middle layer begins when the top layer is cool – this prevents crusting.
Crusting- vapor molecules below the skin require greater pressure to force the way through- air dryers use higher air velocities and Temp in the middle sections than at the entrance.
Crust semi-plastic- pinholes are created. Solid crust- pinholes may stay open- inks thermoplastic- softening the crust- pinholes close.
A dryer can only be used to about 33-50 % of its rated efficiency- because high velocity air disturbs the surface of wet layer and excessive evaporation rates will trap solvents in the solidified layers.

BOUNDARY AIR LAYER
The layer immediately above the liquid – boundary air layer:
Laminar zone
Buffer zone
Turbulent air layer located immediately above boundary layer.
The heat is transferred through the boundary layer by conduction and convection.
Released vapors are removed by a diffusion and convection.
Convection heat transfer efficient in turbulent and buffer zones- in these zones uniform air T and released vapor.
Very little convection takes place in laminar sublayer.
Laminar- heat conduction and vapor transmission- means of transferring E and vapors.
Conduction- inhibits the drying action- its resistance proportional to laminar layer. Anything to reduce the laminar layer- will increase the heat, mass and transfer rates.
The use of medium and high velocity air jets to penetrate and disturb the laminar layer- best method to reduce the laminar layer thickness.

AIR FLOW SYSTEMS
Large volume of heated supply air to solidify wet layers, larger volumes of exhaust air to extract the vapor-laden air.
Vertical airflow systems, hole, tube and slot type dryers.
Hot air expelled downwards- vertically- onto wet ink- all of these are classified as impingement dryers.
Medium to high velocity hot air streams to penetrate and disturb the laminar flow- which reduces the thickness of the laminar layer and enlarges the buffer and turbulent layers.
The vapors collect above the wet layer tend to saturate the air.
This inhibits the passage of heat to the liquid as well as the flow of vapors from inside to wet layer.
The impinging air breaks up the boundary layer, scatters the concentration of released vapors, lowering the external vapor pressure.
The low-pressure exhaust system has chance to extract the diluent air from the immediate area around the moving web to the exhaust manifold.

A hole dryer – impingement dryer in which vertical air streams are expelled from holes. The bottom of the air plenum has a series of holes in it, they are spaced to cover the entire web area as it flows through the dryer.
A supply air plenum is a sheet metal enclosure inside the dryer, in which the air is under greater pressure than the air surrounding it.
Air passing through the holes tends to spread.
Major advantage of hole dryers is that 100% area is under the influence of the circular air blast at all times.
The hole dryer is the least costly to print.
Two principal disadvantages: the impinging air velocity is the lowest of any type of the dryer.
The overlapping air pattern causes some areas to receive more energy than others – overdrying in some sections.
Overlapping airflow from hole and tube dryers will cause streaks in the direction of web flow.
Hole dryers- more often used on flexo presses.
Slot dryers- most efficient.

ROTOGRAVURE DRYERS
The length of dryers- function of amount of ink or coating coverage that the unit was specified to.
Single chamber dryers – 50 % ink coverage or slow presses.
Length- 1000-1600 mm 40-60”.
Circulator- supply fan mounted on the side of the unit.
The cover swings from the bottom – easy access for roller cleaning and web threading.
All gravure dryers should have some movable covers, which allow the operator to work inside the dryer.
Dual chamber – total length 2000-3000 mm (6.5-10 ft).
100 % coverage must be designed to allow the web to exit halfway through the dryer.
Circulator mounted on each chamber or one larger supply fan mounted at the side of the unit- single larger supply- less vibration.
EXTENDED DRYERS
Extended or super-extended dryers- about 5-8 meters (15-25 ft).
IR heater – to preheat the wet inks before entering drying chamber.
 

DRYER FUNCTIONING
TIME- most important
TURBULENCE- air impingement
TEMPERATURE

Heat increases drying speed. Excess heat- printed substrate can shrink, curl, become brittle. Ink can skin over- trap solvent in the ink, causing odor, picking on subsequent rollers.
Dryers- airflow can be controlled at specific sections- zones.
Single- zone- the T= const throughout the unit.
Multi-zone- operates independently or with air recirculation from one zone to another within the unit- water based inks, water-borne adhesives, gravure coatings.
Heat increases the process, excessive heat- shrinkage curl, brittleness of substrate.
Dryers- constructed in specific sections “Zones” of the drying unit.
Single zone unit the temperature and air flow are same throughout the unit.
Multi-zone dryers can operate independently or with air recirculation from one zone to another within the unit.
Individual temperature controls for different zones make multi-zone dryers suitable for water-based inks, water-borne adhesives, gravure-applied coatings.

DRYER LIMITATIONS
Cleanable, dimensionally stable over many years- complex system of air nozzles
Drying parameters put into formulas to predict results.
Specification of inks, substrates, coatings. Then calculation of dryer’s potency, from that the operating speed which the dryer will support under those conditions.
Volume of air to solvent must not fall below certain limit.
The air exhaust volume may not be reduced unless a LOWER EXPLOSIVE LIMIT (LEL) detector is used.
LEL- Lower Explosive Limit is concentration of flammable vapors below which they are too lean to ignite.

Supply air can be from 100% fresh air to 100% recirculation air.
Recirculation air economizes on heating energy- it can aggravate the
clogging of supply air nozzles with paper dust. Air recirculation filters must be cleaned weekly.
Supply air must not deflect the web, for tension changes would vary the nip to nip distance.
On presses running foils, films- rollers support the web at each nozzle.

HEAT SOURCES
Steam, gas, electric, hot liquid, gas/ oil, waste heat from incinerators.
Steam- traditional – used in all publication gravure applications.
Steam coils- 100% recirculation loop.
Maximum heat- 250-320oF depending on steam pressure and type of coils
Steam heat is slow and steady.

Electric heat similar characteristics to steam heat- higher energy cost.
Heating coil temperatures can ignite airborne dust or solvents.

Gas- (methane) is cheap, favorite for packaging presses- pipeline infrastructure to deliver gas to any location
Gas- one third to one fifth the cost of electricity.
Gas heaters- can achieve high temperatures with quick warm-ups.
Need fresh air for combustion.

OTHER DRYING METHODS
All mentioned methods use air as the heat transfer medium and for the mass transfer of vapors outside the dryer.
Air – most versatile heat transfer medium and for the mass transfer of vapors outside the dryer.
IR, UV and EB technologies used for certain specialized operations.

ENVIRONMENTAL CONSIDERATIONS
A typical packaging press evaporates ½ to 1 gallon of solvent per minute. It is 450 lb per hour. Press operates 5,000 hours in a year- it is over 1, 000 t of solvent per year.
At present, press producing over 100 tons of solvent per year must have pollution control devices.
Solvent recovery, fume incineration, or use of water based inks and coatings.
 

SOLVENT RECOVERY
Purpose of solvent recovery- to remove evaporated solvents from dryer exhaust air and the pressroom air and collect the solvent for reuse. Fans remove the solvent-laden air.

Air channeled through duct work to one or several adsorbers -beds of activated carbon pellets.
The pellets adsorb the solvent as the air is forced through them.
The cleansed air passes out the adsorber into the atmosphere.

After the bed is reasonably saturated with solvent, steam is forced into the absorber –solvent vapor is forced out of the carbon and into the steam.
The solvent-vapor steam is cooled down and condensed into liquid state.
Mixture is piped into a decant tank, the solvent and water separate into distinct layers (solvent is lighter than water).
Solvent- siphoned out off the top into a collection tank for disposal (Reuse).
Nonpolar solvents- not miscible with water.
Recovered solvent can be reused directly.
Recovered water- traces of solvent.
Packaging and product gravure- use polar solvents or mixtures containing polar solvents. Partially miscible with water. Recovered solvent contains some water and decanted water contains solvent.
Capture efficiency- the ability of solvent recovery unit to get as much exhaust air as possible.

INCINERATION
Incineration- other option. Good pollution control method when solvent recovery is not feasible.
Incineration – burns high- boiling point solvents.
Incinerators- small enough for one small press, where solvent recovery is not economical for single press.
The heat from incinerators can be reused- to preheat successive batches of solvent laden air which is to be incinerated at around 1, 450oF.
The tail gases can be used to heat dryers or winter time fresh air.
Dryers with incinerators can be identical to those with solvent recovery. LEL controls, LEL indicators are essential for keeping the exhaust rates low and safe.
Incinerators can operate at much lower temperatures if a catalyst is used to trigger the oxidation of hydrocarbons.
Catalysts- Pt platinum and palladium- initiate the catalysis of oxidation at 650oF.
Manganese dioxide and cerium oxide blends bring down to 450oF.
Solvent capture limits- 92-98 % recovery crucial for recovery and incineration.
Tracking tonnage of purchased, used, recovered and reused. Control devices, their maintenance. Water based inks more appealing???