Roller Printing

• Engraved roller printing is a modern continuous printing technique developed in the late 19th and early 20th centuries.

• Until the development of rotary screen printing, it was the only continuous technique.

• The high fixed cost of copper rollers, expense of engraving process, and possible distortion of fabric during printing have led to its reduced use, now being less than 5% of the worldwide textile printing market.

• The fine design detail possible with this technique has always been its main advantage.

Main Parts and their functions

• Color Doctor: It is quite essential and is a thin sharp blade of steel that rests on the engraved roller and serves to scrape off color from its surface, leaving only that which rests in the engraving. On the perfect action of this doctor depends the entire success of printing, and as its sharpness and angle of inclination to the copper roller varies with the styles of work in hand. Any roughness, unevenness or the slightest snip in the edge is sufficient to allow color to escape under it, with the result that the finished parts exhibit serious defects in the form of streak or cloudy patches of color.

• Lint Doctor: It does two functions

• Remove any loose fiber, filaments, neps which are attached from the printing cloth and get stuck up to the wet surface of the roller.
• It cleans other colors of a multicolor design which are already printed on the cloth and being still wet impress themselves on the surface of the roller

• Blankets: This is a tightly woven uniform woolen piece of equal thickness and elasticity. It can be removed from the machine, well washed to remove the dried paste, dried and again put on the machine as before. They are resilient and hard wearing.

• Back Grey: Simple pieces of unbleached cloth that are run between the blanket and the cloth to be printed. This is used to absorb the color forced at the back of the printed piece which would otherwise spoil the blanket.

• Furnishers: Color furnishers are usually wooden roller working partly in the color box and partly above. They supply the color to the design rollers.

• Color Box/Tray: they are long, narrow, shallow, trough of word or copper to carry the printing paste. It is capable of being moved backward or forward.

• Color unit: Consists of color box, two doctors and design roller.

Engraving of copper rollers

In this method, a heavy copper cylinder (roller) is engraved with the print design by carving the design into the copper. Copper is soft, so once the design is engraved, the roller is electroplated with chrome for durability.

Three different methods of engraving

• By hand with a graver which cuts the metal away

• By etching, in which the pattern is dissolved out in nitric acid; and

• By machine, in which the pattern is simply indented.

Pros and Cons

Advantages:

• Suited for large batches

• Very high speed

• Fine line or sharp line can be easily obtained

• Highest production for single color is 18000 yds/hr and 10000/yds for 12 colors

• Can be used for printing any style

• Can be used for all classes of dye on any fabric for all design

• Repeats do not exist as printing is continuous

Disadvantages

• Changing time is high

• Crush effect is produced

• Engraving the printing roller is expensive operation

Defects in Roller Printing

• Scratches: Due to some hard particles of sand or grit in the color paste. These marks being very fine; they are noticeable only after fully development of the color at the stage of soaping and washing

• Removed by burnishing the rollers

• Snappers: These are in the form of ugly lines printed parallel to the selvedges. They are caused by some substances getting under the color doctor which is lifted up at that part and color escapes unscrapped from its either side giving a dirty and smeared printing. Loose thread from the cloth or dried particle of paste or bad mixing of thickener can cause this defect.

• Can be removed by restraining the color paste.

• Lifts: It occurs at regular interval usually equal to the circumference of the design roller. Some hard particle gets stuck into the engraving which lifts the color doctor and allows the color paste to go ahead unscrapped.

• Removed by pulled out of the particle.

• Scrimps: Due to creases or folds in the cloth. They are visible in the form of a double edge with white unprinted part left over in between, below a crease or fold in the cloth.

• Rollers at the back of the feed and drying arrangements must be right.

• Streaks: this defects manifest in the form of two or more fine lines running parallel to the selvedges either straight (due to scratches on the roller) or in a zigzag (cut in the color doctor) manner.

• Removed by polishing the roller and resharpening the doctor edge

• Scumming: due to proper uncleaning of the design roller surfaces. It spoils the whole cloth due to the printing of one or more colors. It may caused by

1. Rough surface of the roller

2. An uneven doctor edge

3. Doctors not properly adjusted

• Removed by correcting the factor responsible for it i.e. by positioning the roller or correcting the uneven edge of the doctor or setting the doctor right.

• Uneven printing: Due to

1. Uneven pressure on the ends of design rollers, show light and deep printing at the selvedges
2. Uneven lapping of the furnishing roller
3. Uneven addition of the color at the one end of the color tray
4. Greater percentage of insoluble substances in the paste
5. Uneven diameter of the design roller due to constant polishing of one end of the roller

Comparison among roller, flat and rotary screen printing

Feature	Roller	Automatic flat screen	Rotary screen
Minimum run (meters)	10,000	500	1000
Design scope	Excellent	Good	Good to excellent
Size of pattern repeat	Limited by roller diameter of 40 to 46 cm	Large designs possible	Limited by screen diameter of about 64 cm
Heaviness of print	Depends on the depth of engravings, generally less than screen prints	Depends on size of holes in screens, percentage of open area and squeegee pressure
Pressure on fabric	More than in screen printing	Low but depends in squeegee pressure
Production rate	Fast	Slower	Fast
Ease of setting up new designs	Costly and length process	Less costly than rotary screens	Technically more difficult than flat screens