This method of application uses a rubber plate, called a flexo plate, which has the desired pattern raised on its surface. The flexo plate is co only made by curing rubber in a metal mold of appropriate design pattern. A newer method of plate manufacture uses a flexible, photopolymer material in which the desired pattern is achieved by photographic etching.
1. Press Types
- Two types of flexographic presses: Central impression and stack-type.
- Central Impression (C.I.) Presses:
- Stack-type Presses
Have a large (4 – 8 ft.), centrally mounted cylinder around which the web travels. Positioned around the central cylinder are up to six flexo application stations. This type of press is more difficult of the two on which to use Cohesives. The wide, some what flattened, impression made by the central cylinder tends to squeeze the Cohesives out to edges of the pattern on the flexo plate. This squeeze-out tends to build coating on the edges of the raised pattern resulting in poor pattern transfer. Recent Cohesives formulation developments have greatly reduced the edge build-up problem.
- Do not have a large central impression cylinder. The individual flexo application stations each has its own, small impression cylinder, similar to a gravure station. The impression cylinder is of steel rather than rubber as in gravure. The small impression roll allows better pattern transfer.
2. Application station types
Two types of application stations are used on either of the two types of presses: Roll Doctored application and Blade Doctored application.
Roll Doctored application:
More standard or traditional type of flexo applicator.
Uses three separate rolls to transfer coating from applicator pan to the moving web – rubber fountain roll (in the pan), overall etched metal anilox roll and steel roll on which are glue mounted the printing plates.
Blade Doctored application:
Newer, more precise tearing type of flexo applicator. Uses two separate rolls and a doctor blade assembly to transfer the coating from applicator pan to moving web – overall etched metal anilox roll (with Doctor Blade) and a steel roll with rubber printing plates.
As with gravure, we recommend that the etched or engraved cells be of Quadrangular shape, to effect the most efficient transfer of the coating. Pyramidal cells are also acceptable but less desireable. In a roll doctored application the anilox roll picks-up the solution (Cohesives, etc.) from the surface of the rubber fountain roll and transfers it to the raised pattern of the flexo printing plate. The quantity transferred is metered or controlled by the etch in the anilox roll.
A 100 line_ per inch (LPI) anilox roll will typically transfer
2.5 – 3.0 pounds per ream (dry) of Cohesive depending on cell depth and shape as well as other machine factors. Normal operating range of anilox rolls with Nip-Welds is 80 to 110 LPI depending on equipment variables.
In a blade doctored application the anilox roll rotates in the fountain or pan, just like a gravure cylinder, picks-up the Cohesives and the excess is metered or doctored off the anilox by a doctor blade. The remaining coating is then transferred to the raised pattern of the flexo printing plate.
The doctor blade assembly for blade doctored flexo has the same considerations as with gravure: stiff, sharp blade, high (90 degree) angle.
3. Transfer and Application rolls
Rubber Fountain or Pan Roll:
Used only with roll doctored flexo application stations. Serves as the initial pick-up roll by rotating directly in the fountain or pan containing the ink, coating, or in our case, the Cohesive. * This roll must not be natural rubber. Nip-Welds are latex based formulations that will react with natural rubber surfaces. Roll should be manufactured from synthetic rubber; Buna N55N, Bunan or Neoprene are satisfactory.
Anilox Transfer Roll:
Used with both. roll doctored and blade doctored flexo application stations. Anilox roll has an etched surface virtually identical to and is manufactured in the same way as a gravure application cylinder.
Printing plate roll:
Steel roll upon which are adhesively mounted rubber printing plates. * The printing plates must not be made of natural rubber. The rubber printing plates, just like the rubber fountain roll, must be made of synthetic rubber. One reliable source of flexo plate material for use with Cohesives is Stero type Equipment Co., Chicago, Illinois. Plates made from photo polymer sheet stock are not recommended (marginally successful with Cohesives). Mounting of the flexo plates to the steel carrier roll must be done with an alkali resistant adhesive.