Offset PrintingXi’an university of Technology

Prof.Dr.Cao Congjun

Feature

Dampening solution Indirect printing (blanket cylinder) Low impression pressure and fast speed Dot gain Thin ink layer Simple technic , low cost

Cp.10,11 Offset press

The basic of offset press Sheet-fed offset machine Web-fed offset machine

Feeding unit + Printing unit (inking unit + dampening unit )+Registration unit + Delivery

unit

1. Feature of offset machine ：

a. Dampening solution ；b. Indirect printing (blanket cylinder)

3. Types ：Flat to round- Flatbed Cylinder Press ： Proof machine

Round to round- Rotary Press ： modern printing machine

2. Structure ：a. Feeding unitb. Registration unitc. Printing unitd. Delivery unit

Inking unitInking unit

Dampening unitDampening unit

structure of printing unitstructure of printing unit

1 Sheet-fed offset machine

Delivery partDelivery part Printing partPrinting part Feeding partFeeding part

Sheet conveyance in a sheet-fed offset press in unit design

1.1 Feeding part1.1 Feeding part

• Feed pile ： auto elevator• Feed pile ： auto elevator

Re-feeding the paper without stopping the machine

• Suction head• Suction head

In the feeder area the sheet is mostly conveyed by suction and friction whereby the top sheet of lifted by suckers.

Sheet separation by sucker tilting

Suction head partSuction head part

• Stream feeder• Stream feeder

•Any double or multiple sheets (i.e., two or more sheetsare picked up together and guided to the feed table) mustbe detected in good time if subsequent damage is to beruled out. As soon as a double sheet is detected in thefeeder, the feeder stops the conveyance of this doublesheet and any following sheets.

•In many cases double sheet detectors still operatemechanically and usually have to be adjusted to thesubstrate thickness. They have certain limits with respectto their sensitivity, as in the case of light-weightpapers. The operation of optical double sheet controlsis very much easier, as they adjust themselves to the materialthickness at the start of the sheet travel.

• Double Sheet Detectors Fig11.19• Double Sheet Detectors Fig11.19

Double sheet detection systems and their applications

The Registration UnitThe Registration Unit

Sheet alignment on the feed table at front and side lays

The accurate position of the print image on the sheetcan only be achieved if each sheet is aligned individuallybefore it enters the press.

During the actual alignment process, the sheet is pushed or pulled with light force against the alignment guides.

Infeed systems to the first printing unit

InfeedInfeed

Between the feeder and the first printing unit a section is needed where the sheet is accelerated. The sheet is stopped on the feed table and the sheet is aligned at acomplete standstill. The impression cylinder moves constantly at printing speed.

The infeed system now has the task of bringing the sheet, guided accurately ingrippers, up to production speed and passing it on to the grippers of the impression cylinder.

Ranger drum, sequence of functions.a Initial position;

b Gripper swings towards the stationary sheet and grasps it;

c Sheet is slowly accelerated up to printing speed;

d Transfer of the sheet to the impression cylinder with synchronousmovement

Sheet Reversal

a MAN Roland system: The sucker system picks up the rear edge of the sheet, transfers it to the perfecting grippers which then transfer it to the straight printing grippers. The front sheet edge is released first.

Sheet turning devices (single-drum system).

b Koenig & Bauer system: The rear sheet edge is gripped by the sucker system of the sheet reversing drum and the front edge is then released.Sucker and gripper system in the sheet reversing drum move towards each other and the sheet is transferred to the grippers

1.2.1. Inking unit1.2.1. Inking unit

1.2. printing part

•Ink fountain with continuous ink duct blade and ink fountain screws.

•A flexible ink blade can be adjusted at varying distances to the ink fountain using ink screws to adjust the amount of ink taken out of the ink fountain.

Conventional offset printing requires a dampening system to supply a very thin film of dampening solution (approx. 2 um) to the nonprinting elements of the printing plate.

1.2.2. Dampening unit1.2.2. Dampening unit

Dampening system design

a Vibrator-type dampening system;b Continuous-type dampening system-Dahlgren;

c Indirect dampening solution application through the ink roller;

d Brush-type dampening system;e Centrifugal( 离心） dampening system

Waterless Printing P240

It uses a special silicone-rubber-coated printing plate and special ink.

A special temperature is maintained for accurate transferring of ink to the substrate.

A waterless plate is an intaglio plate with image areas recessed from the surface of the plate.

•Plate cylinder

•Blanket cylinder

•Impression cylinder

传纸滚筒

1.2.3. Printing unit1.2.3. Printing unit

Cylinder Arrangement

Note ： P ( 印版滚筒 ) B ( 橡皮滚筒 ) I ( 压印滚筒 )

a. Unit ： a unit with three cylinder

b. Combi-satellite ： only one impression cylinder

c. Five cylinder two color printing machine

d. Four cylinder B-B P

I

B

1.3. Delivery1.3. Delivery

• Chain • Chain

• Counter• Counter

• Delivery• Delivery

1.4. Computer control unit1.4. Computer control unit

•Ink adjustment

• Registration

• color control

• pressure adjustment

•Plate changing

•Blanket and press cylinder cleaning

•Feeder adjustments

•Error checking

•Ink adjustment

• Registration

• color control

• pressure adjustment

•Plate changing

•Blanket and press cylinder cleaning

•Feeder adjustments

•Error checking

2 Web-fed offset -Fig.11.37

Sections of the Web Offset Press

※Infeed unit

※Printing unit

※Delivery unit

Types of Web Presses

※Blanket-to-blanket web press

※Common Impression Cylinder web press

※In-line web press

Components of a Web Press

Roll stand Splicer Web tensioner Dryer and chill rolls Folding and cutting Press console

2.1. Infeeding2.1. Infeeding

• Clamping the new reel in the chucks （卡盘） .

• Moving the new reel into working position (the new reel is moved to a distance of 1 cm from the expiringweb, which is protected against fluttering 摆动 by a support roller).

• Acceleration of the new reel to the circumferential speed of the expiring 期满 web.

• On passing the splice marking,

• On passing the splice marking, the expiring web is pressed down to the circumference of the new reelby means of a foam （泡沫塑料 /橡皮） roller (formerly a brush). The adhesive end of the new reel sticks to the expiring reel and at the same time the paste seal is broken.

• Swinging in the separating knife to cut off the remainingpiece of the expiring web.

• Braking and ejection of the expired reel while thenew reel is active.

Splicer

Flying splicer------P255 Zero-speed splicer-------P256

Zero-speed splicer

2.2. Printing 2.2. Printing

• Combi-satellite• Combi-satellite

• B - B• B - B

2.3. Drying2.3. Drying

Drying methods ：Drying methods ：

• microwave

• infrared ray

• ultraviolet radiation

Fig.

Hot-air dryer for web offset printing presses

(Ecoweb Convertible, Heidelberg)

Fig. 2.1-86 Chill roll stand with six chill rolls.

a Chill roll configuration with web travel;

b Chill roll stand (Heidelberg)

Folding

3. Technics Preparation Plate installation Pressure adjustment Printing

平版印刷常见故障平版印刷常见故障

1) 印版引起的故障a. 砂目粗细不当 印刷部分发虚b. 砂目太细 空白易起脏c. 版面氧化或沾有油脂 空白起脏 （用浮石棒摩擦去掉脏点，再涂上阿拉伯树胶）

2) 橡皮布引起的故障a. 高温或橡皮布浸油 橡皮布发粘 （避免高温和浸油，常擦滑石粉）b. 橡皮布未绷紧 图文重影c. 橡皮布表面凹陷或衬垫不匀 图文缺陷 （在橡皮布背面进行衬垫修补，对极小部分可以涂松 节油或煤油使橡皮布膨胀）d. 橡皮布或衬垫滑移 墨色不匀，网点铺展

3) 滚筒引起的故障a. 橡皮滚筒压力过大 版面砂目磨损 , 图文损伤b. 压印滚筒压力过大 纸张伸张、褶皱，套印不准c. 压印滚筒叼纸牙磨损 咬纸不稳，套印不准

4) 墨辊引起的故障a. 墨辊压力过大 造成版面图文部分受损 （用砂纸、浮石磨擦）b. 墨辊直径不一致 版面着墨不匀c. 墨辊有杂质或油墨硬化 输墨不良，印品无光泽

5) 水辊引起的故障a. 水斗辊和传水辊接触不良 供水不匀，使墨色变化b. 水辊绒硬化 磨损印版c. 着水辊压力过大 磨损印版

Homework

P260 review questions 1,4,5,14,15,16,18,20.