Conversion of an old single colour sheetfed offset machine into an UV coating machine is the best possible option for a printer to adopt the new technology. There are several advantages of such conversions over traditional coater conversion, as mentioned below :

Offset machines are designed and fabricated to deposit or transfer a very thin ink film on to the substrates, It is in the range of two to three micron film thickness,which is nearly equal to a deposition of 2.5 GSM. So, the chances of unwanted higher deposition of costly UV coating are reduced, hence financial loss is negligible.
Control over film thickness or GSM deposition is more compared to coating machines.
In order to obtain a higher deposition, dampening system of the machine may be renovated to be used as a coating unit.
Automatic feeding unit ensures higher productivity.
Possibility to apply a sealing base coating (generally low cost aqueous formulation) with dampening unit prior to application of UV coating through inking unit of the press reduces coating cost in achieving higher gloss. (Before going for a long run job with this process, printers are recommended to test the success of it with their combination of paper/board, ink, varnish, machine and work environment.
Photopolymer flexographic plate can be used in place of lithographic plates for spot varnishing application.
It is easy to keep the glue flap unvarnished using photopolymer plates or pill of blanket

In converting a sheet-fed machine following points need to be considered:

Since the production speed is higher, the radiation source need to be of higher wattage to ensure proper curing of the coating.
Molleton cloth covers for dampening rollers should be removed.
Proper blanket (EPDM blankets) should be chosen for the UV material.
The soft rollers of the inking unit should be resistant to UV varnishes. Generally EPDM rollers serve the purpose.
Number of rollers in inking unit should be reduced to mini'mise the splitting of varnish film into thinner films. It helps in getting thicker deposition and reduces the maintenance cost of the machine as well.
To obtain thicker coating film deposition through dampening unit intermittent type dampen.

Conclusion:

The UV varnishing system is the next generation solution to the ever-demanding speed of the printing and finishing process. With proper handling of materials, logical analysis of specific requirement and implementation, the technology has the capability to yield most wanted extra profit for the printers On the other hand, it can be a powerful tool for the designers in giving shape to their imagination with wide range of finishes, better colour gamut (for UV inks) etc.

Coating Equiptment
Coater / Dampeners

The EPIC Coater / Dampener or and the DAHLGREN Coater / Dampener are the most widely accepted and popular units representing about 95% of the market. Press manufacturers are only now beginning to design their own units. These units can be changed from Dampener to Coater mode in about 15 to 30 minutes. The EPIC was originally engineered as a plate - type Coating Applicator. Later, this successful design was modified to become a Dampener with the inherited advantages of a plate type unit. Namely the press retains the number one ink form roller. Therefore, the EPIC Dampener and Coater Dampener are similar in appearance, adjustment and operating ease. It’s rugged construction dependable and proven design combined with accurate but forgiving characteristics have firmly established both the EPIC Dampener and Coater Dampener as a favorite of management and press operators alike. DAHLGREN however, has changed from its normal Ink-type configuration to a plate-type also for the Coater/ Dampener purpose. Both units are a continuous flow design. Therefore, while the units are different in many respects we can discuss both units and their functions in general terms. Manufacturer recommendation for setting and operation should be followed carefully. This information is offered as an aid for set-up and application of water based coatings in general term only.

Both the EPIC and DAHLGREN Coater / Dampeners are a three roller configuration. Both units have a clutch equipped gear driven from roller to the plate cylinder, which allows for light settings without slipping when handling high viscosity coatings. The roller 'configuration may differ, but the basic function of each of the three rollers remains the same. The metering Roller Pan roller and transfer Roller are coupled by gears and driven by a variable speed DC Motor.

Coater Settings

METERING ROLLER TO TRANSFER ROLLER - This adjustment controls the amount of coating allowed to reach the Form Roller regardless of the roller configuration.
TRANSFER ROLLER TO FORM ROLLER - This roller transfers the coating to the Form Roller from the Metering Roller (Pan roller). This is also the slip nip position and should be adjusted to manufacturer's specifications.
FORM ROLLER TO PLATE - The adjustment should be light just a kiss.
PLATE TO BLANKET - Adjust the amount of packing under the blanket to avoid squeezing the coating off the blanket.
BLANKET TO IMPRESSION CYLINDER - Back off the impression cylinder adjustment to avoid squeezing the coating off the sheets.
SPEED CONTROL - All units have a "Percent of power" or "percent of speed" indicator and control at the control box. This will increase or decrease the speed of the metering roller and transfer roller to control the rate of flow of coating to form roller. Since the coating has a higher viscosity than fountain solution it may be necessary to increase the power settings.

Application Technique

The most favorable results will be obtained when the coating area on the blanket is slightly smaller than the sheet size. By restricting the coating to within 1/4" of the four sides of the sheet excess coating can not build on the sheet edge to cause blocking or sling the coating. To obtain this desired area coating on the blanket, the press has the option of undercutting the packing from the plate or from the blanket. It will not be necessary to undercut the packing from both, except when dealing with extremely stiff or heavy stock.

My preference is to undercut the pecking from the plate not the blanket. The blanket is not damaged and can be used for many future jobs. The following procedure is suggested for a fast and effective set up of a Coater / Dampner.

Pack the blanket to normal height. Do not under cut the packing.
Select a used one-sided plate. The unused side should be smooth in good conditions from wrinkless, dents and chemicals.
Measure the width of the sheet to be coated.
Cut the plate 1/2" inch smaller in width only, than the press sheet.
Caliber the stock to be coated and select packing sheets which will bring the plate to normal height. Some experimentation may be necessary to determine the exact packing best for coating purposes on your press.
Cut the packing sheets 1/2 inch smaller than the paper sheet size to be coated (25 x 38 sheet - 24 1/2 x 37 1/2 packing) a) It is recommended the packing sheets be tapered 1/1 6 inch four sides per packing sheet from top to bottom to avoid coating build on the outside edges.

a) It is recommended the packing sheets be tapered 1/1 6 inch four sides per packing sheet from top to bottom to avoid coating build on the outside edges.
The minimum print distance from the edge of the plate for each press is known. For example purposes it will be assumed this distance is three (3) inches. Measuring from the leading edge of the plate, when inserted in the plate clamp make a 3 inch mark on the plate cylinder. This will always be the starting point for mounting packing sheets or to measure from if less than the full sheet is to be coated.
Mount the packing sheets under the plate and jog the press forward. Insert and tighten the tail of the plate in the rear plate clamp.
The plate surface. is now raised to receive the coating. and will transfer the coating only to the desired area of the blanket.

Applying the Coating

Be sure the coater / dampener unit is clean and free from chemical residue.
Under normal operating conditions, recirculation of the coating material is not necessary. Since ORGACRYL coatings are formulated for excellent pan life.
Set the Coater up prior to the set up of the printing units. After you have register and color you do not want to wait and leave the press dry up.
Prior to starting the press apply a light mist from a spray bottle of 90% water and 10% alcohol to the plate and blanket. This will prevent the first sheet from sticking to the blanket.
Start the press but do not drop the coater form roller until the sheet is in the next forward printing unit. This will prevent excess coating build up on the roller and blanket.
We recommend the printed stock be aired immediately after printing to speed drying and to check on the amount of coating that is being applied. This will also prevent any blocking problems which might result from excess coating build up at the edges of the sheet. a) It is recommended the packing sheets be tapered 1/1 6 inch four sides per packing sheet from top to bottom to avoid coating build on the outside edges.
ORGACRYL coatings are designed for fast set up. If retardants are added, the drying process will also be slowed down. Keep a wash up bucket of 90% water, 10% alcohol and a cap full of liquid cleaner (AM 8491) at the unit. After an impression trip off or press shut down wipe down the rollers and blanket to avoid the coating drying up and causing mottle on the sheet.
If the sheet is to be coated on the second side it is important to air the stock a second time while loading the feeder and again after coating the second side.
Spray powder - it can be reduced but unless your press is equipped -with a good I.R. dryingunit use spray powder

A-FORM ROLLER
B-DRIVE GEARS
C-METERING ROLLER
D-TRANSFER ROLLER

How To Cut the Plate

Plate and Blanket Cylinder Relationship

Coating Procedure

The following procedures are to prevent beading of coating on the gripper edge of the sheet., squeezing coating off onto the tail end of blanket and heavy buildup of coating on the sides of the sheet. To accomplish this we must reduce the effective impression size of the blanket relative to the sheet by 1/8" around the sheet. We must also reduce the effective impression size by 1/8" around the plate relative to the blanket by 1/8" around the blanket.