Reclamation of plate cylinder web offset litho press

Reason for use: Rebuilding of corroded cylinders with a more corrosion resistant surface.
INTRODUCTION 
 
In the offset litho printing process, there is a requirement to use a mixture of water and ink to achieve the correct density of print.
The introduction of water onto any printing process will inevitably cause corrosion problems. One of the main problems on an offset litho press is the plate cylinder, where as moisture seeps under the edge of aluminium print plates it cause the cylinder surface to corrode away thus allowing the print plate to move when in use.
By using the Metallisation Arcspray Process, it is possible to apply a coating of stainless steel onto the corroded area of the plate cylinder building it back to the original size and producing a surface which is more resistant to future corrosion.
The Metallisation Arcspray Deposit possesses a higher degree of bond strength than most other sprayed deposits and the use of compressed air and electricity alone means more economic coatings.
EQUIPMENT 
Metallisation 528E, 340 or 140E Arcspray
MATERIALS 
Bond Coat Arcspray
Metallisation 75E Nickel Aluminium
Specially formulated Arcspray bonding wire which gives an exothermic reaction during spraying producing very high bond strength coatings.
Main Deposit 
Metallisation 84E 316 Stainless Steel
Used to produce a dense corrosion resistance coating.
METHOD 
Cleaning
(A) Remove aluminium print plate
(B) Degrease by solvent vapour process if equipment available
(C) Pay particular attention to holes and plate retaining slot
(D) Inspect for cracking and longitudinal distortion
Pre-Machining 
(A) Rough turn diameter being sprayed to a depth of 1.25mm (0.050″) leaving a witness of original diameter at both ends of 2mm (0.080″) width
(B) Remove sharp edge along both sides of retaining slot forming a small radius
Cleaning
(A) Degrease by solvent vapour process if available
(
B) Check holes and retaining slot are free from contamination and debris
Preparation 
(A) Mask all machined surfaces adjacent to area requiring treatment with heavy duty masking tape
(B) Plug threaded holes with tapered rubber plugs
(C) Thoroughly inspect for contamination prior to blasting
(D) Thoroughly blast with clean no. 30-36 grade aluminium oxide grit
(E) Ensure that area to be treated is thoroughly blasted paying particular attention to both ends of plate retaining slot.
Application of Sprayed Coating 
Masking 
(A) Apply Sprayshield masking fluid, using a small paint brush to all areas adjacent to the area being sprayed (small amounts of masking fluid on areas to be sprayed can be removed with emery cloth)
(B) Check thoroughly area to be sprayed for contamination
(C) IMPORTANT: Area to be sprayed should not come into contact with chains, rope slings, hands or any other form of contamination. Delays between blasting and spraying should not exceed 20 minutes.
Bonding
(A) The Arcspray Equipment should be set up in accordance with the Metallisation Manual for the spraying of 75E Nickel Aluminium Wire
(B) The area to be sprayed should be cleaned with a suitable vacuum cleaner or clean air blast to remove any loose particles of grit
(C) Apply 75E Bond Coat to a depth of 75μm-100μm (0.003” – 0.004”)
(D) The plate cylinder should be rotated to give a minimum surface speed of 18 metres/minute
(E) The Arcspray Pistol should be set so that the spray stream is at 90° to the surface being coated and traversed at an even speed giving a uniform coating.
NOTE: It is recommended that both edges of the plate retaining slot are sprayed with the cylinder stationery 
ensuring both ends are completely covered.
Spraying Parameters Bond Coat
 (i) Range 100mm (4″)
 (ii) Nozzle air pressure 3.7 bar (55 psi)
 (iii) Voltage before spraying 38V
 (iv) Voltage during spraying 34V
 (v) Amperage 200A
NOTE: Parameters may differ in accordance with length and type of powder cables and hoses being used. 
Main Deposit 84E (To be applied immediately after bond coat)
(A) The Arcspray Equipment should be set up in accordance with the Metallisation Manual for spraying 84E (316 Stainless) Wire.
(B) Apply 84E final deposit to the specified thickness including grinding allowance i.e. finished ground dimension plus: 0.375mm – 0.5mm (0.015″ – 0.020″)
(C) The plate cylinder should be rotated to give a minimum surface speed of 18 metres/minute.
(D) The Arcspray Pistol should be set so the spray stream is at 90° to the surface being coated and traversed at an even speed to give a deposit of not more than 0.13mm (0.005”) per pass.
(E) Using pre-set callipers, check final sprayed deposit thickness to ensure there are no areas below finished sprayed diameter
(F) Remove loose particles on surface with wire brush or clean air blast
(G) Allow to cool thoroughly, preferably whilst rotating Spraying Parameters – Main Deposit
 (i) Range 150mm (6″)
 (ii) Nozzle air pressure 4.3 – 4.6 bar (65-70 psi)
 (iii) Volts before spraying 38V
 (iv) Volts during spraying 35V
 (v) Amperage 250A
Sealing 
(A) Apply Sprayseal ‘M’ in accordance with Metallisation Sprayseal ‘M’ instructions. Keep surface wet by re-application for a period of approximately one hour.
(B) Allow to dry thoroughly
(C) Remove all uncured sealer from surface with clean, disposable cloths or paper towels.
Demasking 
(A) Remove all masking tape and rubber plugs
(B) Remove all overspray taking care to prevent coating damage
(C) Remove all traces of Sprayshield with solvent Finish Grind
(A) Grinding wheel type n° 46 grit blue V grade
(B) Wet grind to final diameter taking light cuts using feed and speed in accordance with grinding machine manufacturer’s instructions
(C) Remove sharp edges from plate retaining slot by hand grinding with profiled stone or rotary file.
Inspection 
(A) Check dimensions
(B) Check for cracks, defects in sprayed coating, i.e. large pores or protrusions and loose particles.
Finish Cleaning 
(A) Clean to remove any traces of grinding abrasive and loose particles
(B) Wash with petroleum spirit/paraffin
(C) Dry the surfaces with clean, disposable cloths or paper towel
(D) Final inspection prior to packing
(E) Wrap in clean polythene sheet
(F) Pack and despatch
REFERENCE TECHNICAL BULLETIN N°S :-