Cylindrical surface that receives engraved (or etched) pattern
from which ink is transferred onto the substrate.
Dominant – thin copper layer plated to cylinder base (polished steel).
Cylinder base-steel. Onto steel, thin Ni layer plated, than plated Cu.
Gravure printing- long run jobs- 6-8 million prints from one set of cylinders- Long life achieved by chrome plating the copper engraved cylinder.
Cu surface protected from wear by a chrome surface layer.
* Base cylinder polishing
* Copper plating
* Chromium plating
* Proofing- Corrections in Cr surface
* Dechroming and Rechroming- if run is not finished and Cr is worn out.
PRINCIPLES OF ELECTROPLATING
Cu and Cr are dominant surfaces for gravure- it is important to understand principles of electroplating.
Resurfacing of the gravure cylinder.
FORMULAS USED FOR ELECTROPLATING
First Faraday’s law:
The quantity of material deposited or dissolved at an electrode is directly proportional to the quantity of electricity which flows through the electrolyte.
The mass of the deposit is proportional to the current strength multiplied by duration (time).
Total quantity of electricity in an electrochemical reaction is a product of current and time, the basic unit is COULOMB.
1 COULOMB = quantity of electricity produced by a current strength of 1 A flowing for one second.
Amperehour = 3,600 Coulombs
To calculate actual amount of metal deposited during an electrochemical
reaction, 2nd Faraday’s law must be used:
The weights of different substances deposited from different electrolytes by the same quantity of electricity are in proportion of their equivalent weights.
Equivalent weight = atomic weight/valence
Electro-Chemical Equivalents are different for individual metals (depend on chemical reaction).
1 Ampere = current which will deposit 1.118 mg silver from solution
of silver nitrate in one second.
Ag 107.87 atomic weight valence = 1
Equivalent weight = 107.87g
1 Coulomb (ampere second) deposits 0.001118 g of silver, to deposit 1 gram equivalent will require:
107.87/ 0.001118 = 96,485 Coulombs
1 gram equivalent = 96,485 Coulombs = 1 Faraday
96,485 Coulombs/3,6000 ampere seconds = 26.8 Ampere hours
1 gram equivalent = 26.8 Ampere hours
Example: We will deposit copper from CuSO4 solution. The deposition
time is 20 minutes. The current is 0.9 Ampere. How much copper will be
deposited on the electrode?
(Molecular weight of copper is 63.54 g)
The reaction is:
Cu 2+ + 2e ---> Cuo
It means, gramequivalent of copper is 63.54/2 = 31.77 g because it takes 2 electrons to deposit copper from CuSO4 solution onto the cathode.
Total quantity of electricity = Current x time = 0.9 A x (20 x 60 s) = 1080 Coulombs
We know that to deposit 1 gramequivalent we need total quantity of electricity
96, 485 Coulombs (Faraday’s law).
The question is how much grams we deposit with only 1080 Coulombs?
96, 485 Coulombs will deposit……………….. 31.77 g Cuo
1080 Coulombs will deposit ………………… x g Cuo
x/ 31.77 = 1088/96485 = 0.35 g of Cuo .
DEFINITIONS FOR ELECTROPLATING
Electrons: Subatomic particles that carry negative electric charge. They are the smallest particles responsible for conducting the electric current.
Each atom – made up of a core of protons and neutrons. Protons- positively charged, neutrons- neutral. Electrons- negatively charged. Electrons surround the atom’s core. The number of protons = number of electrons in each atom, characteristic for each element.
Current: Electric current is a flow of electrons. Direct current- electrons flow in the same direction. Alternating current- they reverse in the direction (US 60 times per second = 60 Hz)
Current flows - electrons are moving from atom to atom (copper wire) or they are transported by charged ions- in electrolyte.
Ions: Atoms or parts of molecules that have a positive or negative electric charge. Positive - more protons than electrons occur in the structure
Negative - more electrons than protons occur in structure
Metals - tent to give up electrons- form positive ions Cu2+, Al 3+, Cr 3+ etc.
Electrolyte: Solution that contains ions, giving it ability to conduct the current. Acid copper plating electrolyte: CuSO4 ; in the water environment it ionize, create Cu2+, and SO42- ; and the ions of sulfuric acid SO42- ; and 2H3O+.
Plating: Electroplating is the transport of metal in its ionized form, and deposit of metal in its metal, non-ionized form- which aid to flow the electric current through the electrolyte.
BASIC DESIGN OF A PLATING TANK
Electrolyte through - chemically and electrolytically resistant to the solution used.
Anode- anode holder and anode material (metal that dissolves in the process) the anode- positively charged- anions travel to anode. For Cu plating Cu nuggets are used as anode.
Cathode- cylinder- negative pole- cations travel to cathode.
Both poles connected to rectifier- supplies DC current.
DC current - the negative pole- supplies excess of electrons, positive pole takes electrons away.
IMPORTANT VARIABLES IN PLATING
Plating used for 3 metals: Copper (acid and cyanide bath), chrome and nickel.
Electrolytic degreasing, electrolytically activated dechroming, do not plate, but dissolve old worn out Cr layer, work according the same principle.
CHEMISTRY OF THE ELECTROLYTE
Chemical make-up of the electrolyte- quality and consistency of the result.
Low sulfuric acid content- prolongs the process time.
High chloride content in chrome bath – hazy deposit, abnormal crack patterns. Electrolytes- should be analyzed on regular basis.
CIRCULATION OF THE ELECTROLYTE
Around the anode- electrolyte enriched with metal ions (when plating from anode, like in a copper tank).
Cathode- electrolyte is depleted of metal ions.
Consequently- many cations (positive) around the positively charged anode, and many anions (negative) around cathode.
Uneven distribution of ions in electrolyte- polarization.
Works again even flow of current from rectifier.
High polarization increases the likelihood of imperfections in the deposited layer.
In order to keep efficient process- electrolyte must circulate constantly- pumped between anode and cathode. Large excess of electrolyte- advantage- acts as a buffer.
EFFICIENCY OF THE ELECTROLYTE
Fixed ratio between current and deposition- can be achieved only theoretically. Faraday’s law- starting figure, then multiplied with efficiency factor- (current yield)- for correction of imperfections in the plating process.
Rest of current- side reactions, (water into hydrogen and oxygen-electrolysis), reduction additives
Efficiency of Cu tank- close to 100%, Cr plating- around 23%.
Cathodic efficiency- # ions deposited relative to current flow
Anodic efficiency- # ions dissolved relative to current flow- needs to keep the electrolyte in balance.
What percentage of the cylinder sufrace is immersed in electrolyte at any point of time- active surface.
More current can be conducted through larger surface.
Larger immersion factor- requires a larger anode.
Amount of current flowing, divided by active (immersed area).
Cathodic current density- of interest.
The higher the current density- the faster the plating speed.
Current density is given by electric field at cathode.
Electric field is denser at uneven areas- cylinder ends, closures around cathode surface.
Current density- too high, plating process develops heat- cylinder ends start “burning”.
VOLTAGE LIMITS AND EFFECTS
Increase of current density- polarization increases- and voltage necessary to maintain the plating process.
Increased roughness of deposit.
Voltage limit = f (electrolyte, plating conditions).
Voltage limit is reached; no more Cu ions are available to conduct more current through electrolyte.
Voltage limit decreased beyond limit- hydrogen ions discharged at the cathode (gassing).
Water is split into hydrogen and oxygen.
Voltage limits- maintained by increase of T of electrolyte, adjusting concentrations of electrolyte.
Strong impact on electric resistance of tank. The closer the distance, the lower the resistance.
Lower resistance means: higher current densities can be run to achieve faster plating or, lower voltage is necessary to run at given constant density, lowering electric consumption (kWh).
Most modern plating equipment – small anode/cathode distances 1-2 in.
Anode/cathode ratio- used for comparison of anode surface relative to cylinder to be plated- anode- nuggets- very high surface –differs with size of nuggets.
Ratio- important- to keep the ions in electrolyte in balance.
Important influence on the conductivity of the electrolyte.
Higher temperature- electrolyte more conductive, higher current densities can be achieved for faster plating. T – important for additives behavior in the solution. T- control systems.
Ability of electrolyte to cover the cathode as evenly as possible.
Determined by geometric arrangement in the plating tank.
Shape of electric field in the tank and primary current density (primary current distribution).
Cathode- never spans to complete width of plating tank- cause electric field to bent at the edges- higher current density at the edge of immersed cylinder face.
Higher current density- more current flowing- higher built up at cylinder ends than in the middle.
Another factor- physical placement of anode and cathode towards each other. Ideal – constant distance at all positions across all cylinder-even plating.
Critical- small anode/cathode distances needed to achieve high plating speeds.
CHROMIUM PLATING IN THE GRAVURE PROCESS
Used to lengthen the life of copper engraving.
Early this century- customary to run cylinders without Cr – soft Cu surface easily damaged.
A very low Coefficient of Friction – allows the doctor blade to slide over engraved cylinder with little wear, mirror like surface resistant to oxidation (Cu).
* Cr is very hard, resists the wear of the steel doctor blade.
* Electroplating is easy technology, easy to apply the Cr layer.
* Cr is easy removable, allowing repair of Cu surface and re-chroming
THICKNESS OF CHROME COATING
Gravure cylinders – deposits closely controlled, normally in the range 0.003 -0.006” or 8-15 micron. Must be uniform. Deposit too thin, premature wear, collapse occur.
Too thick deposit- cell volumes reduced.
Regulation of thickness of deposit- important, complicated by the increased area of the cylinder caused by engraving. Solid color- engraving- much higher surface area than in copper plating.
These areas should be calculated and add to the nominal circumference times.
CHROMIUM PLATING BATHS
Metal is deposited out of solution CrO3 - chromic anhydride- chromium trioxide –chromic acid.
* CrO3 - chromic anhydride 250 - 400g/L in de-ionized water
* Catalyst H2SO4 75:1 to 125:1 chromic acid to sulfuric acid
* Deionized water
Chlorides - undesirable-cause gray, hazy deposit.
Cr6+ + 6e ---> Cro Metalic chromium
Chrome deposited from solution, not from anode.
TYPES OF SOLUTIONS
Several general types of solutions: chromic acid, type of catalyst, method of solution control.
Chromium plating solutions- more diluted over years.
Before 375g/L, now 250g/L or even 115g/L.
More concentrated - lower voltages, more tolerant to impurities, improved throwing power, softer deposit.
The more diluted solutions- higher current efficiencies and produce harder deposit. Cr tanks work with 23% efficiency.
TYPES OF CATALYST
SO4 2- is added as sulfuric acid - H2SO4 100:1 chromic acid to sulfuric acid, these small amounts of sulfuric acid are normally present in technical chromium acid- easy to maintain- 100:1 bath – industry standard.
Excess of catalyst (80:1) reduces covering power and current efficiency – eventually- no metal will be deposited.
Lack of catalyst- 125:1 brownish chromium oxide deposits.
Alternate approach- the use of fluoride – catalyzed electrolytes.
F – used in recent years to replace sulfate. Complete replacement- rare- (technical chromic acid contains sulfuric acid). Fluoride added 1.5- 4.0% of chromic acid.
Advantages of fluoride addition:
* Increase current efficiency (50% increase, from 16% to 24%)
* Increased current capacity- within the limits of power source, more current can be applied to bath and still yield a good deposit.
* Increased deposit hardness.
* More “cracks” in the deposit are apparent. Finer crack pattern may increase wear resistance by providing more channels for ink lubricity. Cracks in Cr surface desirable- lubricate the doctor blade.
Disadvantages to fluoride electrolytes:
* Sensitivity to “impurities” particularly iron- form complexes difficult
to remove- poor plating result
* Analytical control of fluorides difficult and costly,
* Tendency for etching cylinder in unprotected low current density areas- almost immediate current application right after immersion of cylinder into bath.
* Environmental safety hazards greater
* Fluorides- volatile- escape from bath- material costs are higher
Self Regulating Electrolytes
Process control- simplified by introducing baths with self-control catalysts level.
Strontium sulfate (SrSO4) and/or K2SiF6 (potassium silicofluoride).
Compounds with very low solubility, portion which does not dissolve is sufficient to maintain the catalyst concentration. Excess of catalyst will precipitate.
Chromium Deposit Crack Pattern
Formation of unstable hydrides during the plating operation.
These responsible for volume shrinkage of 15%, increase of surface tension, and cracking.
Cr- not a very effective corrosion boundary layer.
Crack pattern is useful because it increases the lubricity by retaining some ink.
Cylinder totally immersed and plated in vertical position;
Cylinder partially immersed and plated in horizontal position;
VERTICAL TOTAL IMMERSION SYSTEM
Simplest method in regard to equipment, fixturing and racking.
Plating time minimal.
Hanging the cylinder in the center of an anode ring upon which are hung straight anodes.
Design limits maximum cylinder length (room height and tank depth).
HORIZONTAL PARTIAL IMMERSION TANKS
Most widely used method for larger cylinders.
Cylinders fixtured to rotate on bearings mounted on support columns at either end of tank.1/3 – ¼ of total area- immersed.
Current density calculated on the actual immersed area- plating times 3-4 times longer than when totally immersed cylinder.
High current density – more cracks.
The cylinder rotates constantly during plating process, rotational speed- not very critical – closely controlled ranges- standard operating conditions.
Excessive speed can decrease plating efficiency.
Anode- straight or semi-circular (more effective) –equidistant from cylinder allowing higher current densities to be achieved.
EQUIPMENT FOR CHROMIUM PLATING
Temperature – direct effect on brightness and deposit 60 +/-2 oC.
Not to “burn” the surface. Burn - frosty appearance.
Cylinder must be pre- warmed. Spraying hot water onto cylinder in the degreasing tank before plating.
Automatic heating/cooling system- coils of tantalum, lead, heat exchangers, using waste heat from other plant processes.
Cr solutions- very corrosive- require special tank.
Polyvinylchloride, polypropylene are applied over steel.
Some installations: entire tank made from rigid plastics reinforced with fiberglass.
Cr solutions pH =1.0, highly oxidizing.
CrO3 fumes- extremely toxic, adequate exhaust systems necessary- large amount of H2 being liberated during plating cycle.
Moisture extractors, fume scrubbers necessary in the exhaust system.
Concentration of sulfuric acid- by precipitation with BaCl2 barium chloride.
Chromic acid content calculated from sulfuric acid content and specific gravity.
Base cylinder copper and engraving copper are separate.
Principle: To plate a thin engraving copper layer non- adhesively to the base, then Cr plating so that it can be stripped off, strong enough not to come off in the press. Cu- hard enough for electromechanical engraving.
* Manual stripping of Ballard shell (engraving Cu + Cr layer)
* Preparation for plating: Degreasing, application of new separation layer
* Copper plating (layer 80-100 micron).
* Surface finishing on stone polisher.
Manual stripping: Simple – opening the shell with a knife on cylinder
Pull 2 strips across the whole cylinder face- like zipper
Shell peels off.
Treating the cylinder with abrasive paper.
Problem: Cylinder not degreased completely, or the separation layer not applied completely. Shell does not come off.
Separation layer: Manually poured over the cylinder or automatically sprayed on. (Mercury, silver, or protein based).
Hg- not used any more.
Ag- acceptable as long as the plant waste water treatment plant precipitates metals.
Protein – not environmental problems, must be prepared frequently- shelf life- few hours.
Cu plating- layer 100 micron, Europe, U.S., 80 micron Japan.
Cu not as hard as base Cu- increased hardness- increased brittleness.
Brittle- do not came off easily, pop off in the engraver or on the press.
Hardness 190 – 205 HV (Hardness Vickers).
Ballard shell – simple technology, when correct procedures followed.
Ernest G. Ballard - in 1920s- one of the oldest process technologies in gravure cylinder making.
Ballard shell- eliminates machining time.
Wide –spread popularity publication, packaging printing.
Technology available then - problems – Cu-plating tanks- not able to control process parameters, grinders- needed constant operator attention.
Ballard shell- replaced by base Cu technology.
Polishmaster TM 1970s – eliminating problem of underlying base technology.
Minimum plating technology- high quality plating, fully automatic
Ballard shell- now very popular- Europe, Japan- always used it.
U.S. – many supporters- obvious advantages:
* Process requiring least investment in equipment and space.
* Washing of cylinders is no concern for Ballard shell process (Cr layer
is stripped along with Cu layer).
* Elimination of necessity of dechroming- no need to treat effluents.
* Process is very easy to automate, eliminate labor (except stripping).
* Improved capacity, turnaround times.
* Thin Cu layer – low Cu plating times.
* Manual work cannot be totally eliminated.