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Printing ProcessesIV > Module 3.1, 3.2, 3.3, 3.4, 3.5 > Flashcards

Module 3.1, 3.2, 3.3, 3.4, 3.5 Flashcards

0
Q

Major product categories in web offset printing:

A
  • newspapers
  • direct mail
  • commercial printing/advertising
  • magazines and inserts
  • catalogues and directories
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1
Q

Web offset printing includes:

A
  • indirect (offset)
  • lithographic process
  • on web/ribbon of paper
  • inline finishing
  • up to 3000ft/min
  • up to 96 pg/ revolution
  • up to 60.000 cut offs/ hr
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2
Q

Major WO product categories that use cold-set:

A
  • newsprint
  • inserts
  • magazines
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3
Q

Major WO product categories that use heat-set:

A
  • Magazines
  • books
  • flyers
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4
Q

Major WO product categories that use narrow web:

A
  • forms
  • mailing
  • labels
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5
Q

The comparison between web and sheetfed:

A

- Web: speed, page output, light paper, folding inline, both sides inline printed, rolls are stacks/ logs of folded product, long make-ready, more makeready waste

- Sheetfed: format variability,substrate variability, more spot colours, coating, more precision/better register, multiple pass printing, quick makeready and less makeready waste

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6
Q

What are some current trends in Web offset?

A
  • improved quality
  • improved productivity
  • greater press speed (up to 90,000 cph or 3000+ fpm) - faster make-ready due to automation and process improvement for both coldest and heatset
  • larger presses increasingly common
  • more printing offering increased flexibility
  • hybrid presses which combine heatset with coldset or heatset offset with flexo or digital printing units
  • greater finishing capability
  • plate cylinder gaps decreasing (this increases printable area and reduces waste)
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7
Q

What is the difference between heatset and coldset inks?

A

WO inks are either heatset or coldset and are different in their formulation. A WO press that has a dryer and chill rolls is called a “heatset press”.

- HEATSET INKS: dry by evaporation, it is common for coated stock and sometimes uncoated.

  • COLDSET INKS: dry by absorption, no dryer section required. It is common for newspapers and uncoated stock.
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8
Q

A web offset press is made of multiple press sections or “systems”, what are they??

A
  • roll stand
  • infeed (tension control and web guide)
  • printing units - dryer
  • chill rolls
  • folder and finishing section
  • delivery/stacker
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9
Q

What are the 3 major WO press configurations?

A
  • Inline: sometimes known as a business forms press
  • Blanket-to-blanket perfector: it is very common
  • Satellite (CIC): Rare configuration in web offset , but common in flexography
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10
Q

How does a Web Offset Press Systems look like?

A
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11
Q

Some information on In-line Configuration?

A
  • It has one or more printing units, each with a plate, blanket and impression cylinder
  • Each printing unit prints one colour on one side of the web
  • Perfecting can be accomplished by the use of turning bars that are located betweeen units
  • It is commonly used for business forms
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12
Q

How does an In-line press look like?

A
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13
Q

Some information on Satellite (CIC) Press Configuration?

A
  • It has several printing couples (plate and blanket cylinder) to one impression cylinder
  • Multiple CIC units can be used in one press
  • Perfecting is possible by turning the wbe between CIC or by “double-ending”
  • Less flexible press configuration, but can offer speed advantages
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14
Q

How does a Satellite (CIC) press look like?

A
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15
Q

Some information on Blanket-to-Blanket Configuration?

A
  • Each printing unit contains two “printing couples” capable of printing one colour on both sides of the web.
  • A 4-unit b-to-b can print 4/4
  • A very common and flexible configuration
  • One press can print multiple webs simultaneously
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16
Q

How does a Blanket-to-Blanket press look like?

A
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17
Q

Some information on Blanket-to-Blanket Printing Units?

A
  • It is the basic “building block” of the press
  • Each printing unit consists of two “couples”
  • Each couple contains four basic elements:
  1. Dampening system
  2. Inking system
  3. Plate cylinder
  4. Blanket cylinder
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18
Q

Some information on bearer contact presses?

A
  • Bearers are hardend steel rings at the end of plate and blanket cylinder
  • Most web offset presses have bearers which run in firm contact with each other while on impression
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19
Q

4 Advatanges of bearer contact designs?

A
  1. Smooth rolling contact between cylinders
  2. Eliminates gear backlash
  3. Stabilizes the cylinders
  4. Provides a reference datum for packing
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20
Q

More information on bearer and nonbearer contact presses?

A
  • Bearers compress slightly when on impression, so bearer compression should be checked to ensure proper packing. Bearer compression is greater on older presses.
  • Coldset newspaper presses and all gapless blanket presses do not use bearer contact
  • Run-out is another problem in which bearers or cylinders become slightly out of round and must be repaired or replaced
  • Proper bearer/ cylinder contact can be checked by shining light through cylinder nip
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21
Q

Describe the plate cylinder design:

A
  • The plate cylinder has a small gap to allow for plate lockup (mounting)
  • The plate gap on a web offset press is usually 0.25-0.50 inches, but on sheetfed it can be 6-8” or more
  • The plate gap coincides with the web
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22
Q

Some information on plate bending:

A
  • plates must be prepared for mounting on the plate cylinder. This involves plate bending.
  • plate bending is critical. The bends must be the exact distance from eachother to correctly match the plate cylinder
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23
Q

The plate bending involves several variables, what are these variables?

A
  • The amount of force applied
  • The rate at which the force is applied
  • The radius and angle of the bends
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24
Q

What problems does improper plate bending cause on press?

A
  • misregister
  • plate release or slipping while printing
  • plate cracking due to flexing or slack plate while printing
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25
Q

Plate benders must be adjustable and correctly set for each press. Plate blenders require:

A
  • a pin register system
  • adjustable bending plates
  • a vaccuum system to hold the plate while bending
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26
Q

Plate lockup is commonly either:

A
  • A spring-loaded clamp
  • A reel-rod lockup mechanism
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27
Q

The reel-rod mechanism is more common on modern presses:

A
  • The plate with an acute-angle bend on the lead edge is placed over the cylinder nose
  • The cylinder is turned to wrap the plate and packing around the cylinder and the tail edge is inserted into the off-center reel-rod
  • The reel-rod is turned with a wrench to pull the tail edge of the plate, maintain tension and clamp against the lead edge
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28
Q

Blanket Lockup:

A
  • The blanket is mounted using a reel-rod lockup similar to the plate
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29
Q

Timming the Gaps:

A
  • On web presses (except those using gapless blanket sleeves) the plate gap is smaller than the blanket gap
  • Due to the greater thickness of the blanket
  • The lead edge of the plate tends to accumulate ink dampening solution
  • To prevent excess lead-edge ink and dampening solution from transferring to the blanket, the plate gap should be timed to fall within the blanket gap
  • This is part of press maintenance, not daily operation
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30
Q

Blanket mounting designs come in several forms:

A
  • Premounted bars
  • The blanket is sold premounted onto bars which match the press lockup mechanism
  • Convenient; faster blanket change than with prepunched blankets
  • Prepunched blankets
  • The blankets are prepunched, but not mounted onto bars. The punch holes are used to mount the blanket to bars supplied with the press.
  • Blankets for gapless presses
  • Some presses are designed for gapless blankets. The gapless blanket is a cylinder sleeve which slides over the cylinder.
  • Blanket changes can be very fast - approx 30 seconds
  • The gapless design increases print area and reduces paper waste
  • The smooth gapless blanket reduces bounce and vibration, allowing faster speed printing speed
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31
Q

Press Packing:

A
  • Squeeze for compressible blanket is usually 0.004 - 0.006”
  • Squeeze for conventional (non-compressible) blanket is usually 0.002-0.004”
  • Always pack according to manufacturer’s instructions
  • Allow for paper thickness when calculating packing requirements
  • Excessive blanket tightening decreases blanket thicjness. Use torque wrench when tightening blankets.
  • Squeeze is affected by bearer compression and out-of-round cylinders (run out)
32
Q

Using Packing to Control Image Length:

A
  • The substrate can stretch between printing units due to tension and moisture
  • Progressive packing can be used to make the image “print longer” on any printing unit
  • Shift packing from the plate cylinder to the blanket cylinder to lengthen the print length
33
Q

Infeed systems include:

A
  • rolll stand
  • splicer
  • metering rollers
  • dancer roll
  • lateral positioning (Web) guides
34
Q

Some information on the roll stand:

A
  • Hold the web rolls
  • Contains the splicer
  • Can hold 1-4 rolls of paper
  • It holds the roll currently in use plus a roll which is waiting to be spliced
  • The B-to-B press can be webbed with multipe webs running at once - in this case, there may be as many as four rolls running at any given time
35
Q

The location of the Roll Stand?

A
  • The roll stand is usually inline, ahead of the printing units
  • If the length of the press is a problem, the roll stand can be at the side of the press (sid infeed)
  • Turner bars are required
  • In large installations, the roll stand may be located in the basement, below the press
36
Q

Attaching the roll:

A
  • The roll is mounted on a shaft
  • The shaft expands to tightly hold the inside of the roll core
  • Mechanical expansion
  • Pneumatic expansion
  • A crane is used to lift the shaft and roll onto the roll stand
37
Q

Infeed metering:

A
  • controls the unwind of the roll
  • assists in mainting web tension
38
Q

Dancer Roll and Brake:

A
  • A dancer roll rests in a loop of paper created as the web roll unwinds
39
Q

Web tension Control:

A
  • Tension on the web is required for quality printing
  • Insufficient tension causes poor registe, doubling and slurring
  • excessive tension causes a web break
  • tension must be adjusted according to the tensile strength of the paper being used and the width of the web
  • Tension is controlled after the roll stand and dancer roller and before the first printing unit
40
Q

Metering Rollers:

A
  • Three (sometimes two) infeed metering rollers grip the web in two nips to provide back pressure (pull, or tension) on the web before it enters the first printing unit
  • the lower two metering rollers are steel, driven rollers
  • the top roller is a rubber non-driven roller
  • the nip between metering rollers is adjustable for different paper thicknesses
41
Q

Infeed metering system:

A
  • The drive on the steel infeed metering rollers is variable-speed, controlled by a servo motor
42
Q

Splicers:

A

Join two web rolls

43
Q

Web Splicing:

A
  • To allow non-stop printing, most web presses are equipped with a splicer, which joins an expiring roll to a new roll on the roll stand
44
Q

What are the two different types of splicers?

A
  1. Flying paster
  2. Zero-speed splicer
45
Q

What is a Flying Paster?

A
  • A flying paster is a splicer which makes a splice against an expiring web while it is moving at printing speed
  • Sometimes the press is slowed slightly to enable the splice
  • The new roll is adhere to the expiring roll using glue or double-sided tape
  • This means that in the area of the splice, there is a double thickness or overlap of paper
  • A splice in which the old and new rolls overlap is called a lap (short for overlap) splice
  • In a good splice, only one signature should be wasted at the delivery end - the one containing the splice.
46
Q

The Flying Paster Sequence

A
  1. The new roll is mounted to the roll stand
  2. A reflective tab is attached to the lead edge of the new roll to indicate the nose of the roll
  3. Glue is applied to the splice area of the new roll. The splice area starts at the roll nose and extends back about 12”.
  4. Sensors called “but switches” monitor the diameter of the expiring roll
  5. As the diameter decreases to 8” the reel stand moves the new roll into splice position. Accelerator belts begin to rotate the new roll and the splicing arm moves against the running web.
  6. When the roll diameter is 4” the pressure roller within the splicing arm presses the running web against the pasted area on the new roll, causing a lap splice
  7. At the same time, the knife severs the expiring roll
  8. The acceleration belts and splicing arm rise and the new roll moves into the the running
  9. Web rolls normally have a core diameter of 3 1/2”. With the splice occurring at 4” diameter, this means that 1/2” diameter of paper (1/4” thickness) remains on the old roll at the moment of the splice. The paper is wasted
  10. It is important to minimize waste
47
Q

What is the Zero-speed splicer?

A
  • It makes a splice against an expiring roll which is stopped
  • Because the expiring roll is stopped, the splice can be made more carefully and securely
  • However, the press continues to print at full speed during the zero-speed splice
  • This is possible because the press prints from a supply of paper stored in the festoon
  • A festoon is required on all zero-speed splicers. It is not found on flying pasters
48
Q

What is the festoon?

A
  • A festoon is a vertical (sometimes horizontal) seris of free-spinning steel rollers
  • The unprinted web makes a serpentine (zig-zag) through the festoon
  • When fully expanded, the festoon canstore 80 to 100 feet of paper for several seconds of press run
  • This allows the press to continue to print at full speed while the expiring roll is stopped by the brake and the splice is made against a stopped portion of the expiring roll
49
Q

The Zero-speed splicer sequence:

A
  1. As the running roll begins to run low, the festoon is expanded to store 80-100 ft of paper, enough for several seconds of printing.
  2. The nose of the new roll is prepared for the splice using adheisive tape
  3. The lead edge of the new roll with exposed adheisive tape is held against a splicing roll by vacuum suction. The splicing roll is close to (but not touching) the expiring roll.
  4. As the running roll nears expiry, the roll is stopped using the brake on the roll shaft. The press continues to run from paper stores in the downstream festoon. The festoon collapses to provide paper to the press.
  5. With the paper stopped at the splicing station, the prepared splice is applied to the expired roll, and the expired roll is severed.
  6. The new roll is accelerated to press speed before the festoon runs out of paper.
50
Q

Lap splices and Butt splices:

A
  • A lap splice is when the old and new webs overlap at the splice. A double thickness of paper passes through the press at the splice.
  • A lap slice can be made with glue or tape. Double-sided tape is sometimes used.
  • A butt splice is when the old and new webs butt against each other at the splice and do not overlap. Only a single thickness of paper passes through the press, but the tape adds to the thickness of the splice.
  • A butt splice is made with tape (never glue) on one or both sides of the web
  • A flying paster can make only a lap splice
  • A zero-speed splicer can make a lap splice or a butt splice
51
Q

Web Pre-conditioners:

A
  • Moisture variations within the web cause tension control and register problems
  • Pre-conditioners use heat and chilling to
  • normalize the mositure content
  • burn off slitter dust, paper lint, and debris
  • Consists of a gas-fired heating section and a chilling section
  • gas pre-heater heats the web to 175-200 F
  • chill rollers cool the web prior to printing
  • Pre-conditioners are positioned after the infeed and before the metering rolls
  • Heat applied during preconditioning can make the paper more dry and brittle, aggravating cracking problems in the folder
52
Q

What is tension span?

A
  • A tension span is the area between two tension control points - any area where paper is unsupported in the press
53
Q

Tension is controlled across tension spans by means of tension controls:

A
  • Constant control: constant torque devices apply an unchanging force to the web - eg blanket cylinder
  • Variable control: variable-speed devices control the flow rate of the web - eg. infeed metering roll
54
Q

3 factors affecting tension control:

A
  • Tension: the force the web span is placed under - required to keep the paper flat and tight as it enters printing units
  • Length: the length of the unsupported span - the longer the span, the more force is required to keep the web taut
  • Modulus of Elasticity: the dynamic, ever-chaning, ability or tendency of the paper to stretch. Paper composition, moisture, temperature affect modulus of elasticity.
  • If one of these three factors changes, it will affect the others
55
Q

What is web tension affected by through the press?

A
  1. Surface speed of rollers and cylinders
  2. Circumference of rollers and cylinders
  3. Positive draw on the web
  4. Slip (eg. Chill rolls)
  5. Paper factors: elastic recovery vs plastic flow; reduction in tension due to dampening solution
  6. Dryer: heat shrinks the paper and reduces elasticity
56
Q

Web guiding:

A
  • Side lay controls or web-steering devices which control the lateral position of the web before printing or folding
  • Especially important after a splice, where the edges of the new web may not exactly match the edge position of the old web
  • accuracy of web guides is generally 0.005- 0.010”
57
Q

Webs may be centre-guided or edge-guided:

A
  • Centre-guided: The web is run down the centre of the press, printing units and folder. Best for folded signatures. Requires side-lay sensors on both edges of the web.
  • Edge-guided: The web is run with a side-lay sensor on one edge only, so that print position is fixed relative to that edge of the web. Best for sheeted jobs.
  • Centre-guiding is more common than edge guiding
58
Q

Lateral Web Adjustment:

A
  • Lateral web adjustment is most commonly done by means of a box tilt
  • A box tilt is a unit consisting of four rollers
  • Rollers 2 and 3 are always parallel to each other
  • Rollers 1 and 4 are parallel
  • Rollers 2 and 3 pivot about the central point to change the path of the web
59
Q

Cutoff Compensator:

A
  • Web cutoff is usually made by a rotating knife cutoff cylinder in the finishing section
  • The cutoff must be positioned accurately between impressions
  • A cutoff compensator is a system of rollers which adjusts the length of printed web between the chill rolls and the finishing section, including cutoff knife
60
Q

Web Break Detectors. Web break detectors shut down the press when a web break is detected:

A
  • Mechanical: A roller or finger lightly rides on the web, chnaging position when a web break occurs and shutting down the press
  • Optical: An infrared beam is sent from a source to a detector on the other side of the web, or reflected from the web to a sensor on the same side
  • Pneumatic: An air jet is blown at the web. A sensor on the other side of the web will shut down the press if it detects an air blast
61
Q

How to prevent strip rewind and wrap-up?

A
  • Web break detectors should monitor both edges of the web to prevent strip rewind, when a strip of paper wraps up around a printing cylinder
  • Some web break detectors contain a web severing device to immediately cut off the web in event of a web break to prevent wrap-up
62
Q

Relative Print Size:

A
  • The print can be distorted in length, width, or both directions
  • Relative Print Length (RPL) changes occur along the length of the web
  • Relative Print Width (RPW) changes occur along the width of the web. If the RPW increases from unit to unit, the problem is called fan-out.
  • Print size problems appear as poor image fit or misregister
63
Q

Relative Print Length variations are ususally due to the press and printing units:

A
  • Packing
  • Tension control
  • RPL can vary from unit to unit and even top couple to bottom couple
  • RPL can vary with speed of the press
  • Sometimes paper is a factor
64
Q

Relative Print Width or Fan-out:

A
  • Fan-out usually occurs when the web picks up moisture from the printing units and expands in width (across the grain)
  • Fan-out becomes progressively worse from unit to unit
  • Fan out is commonly controlled usuing bustle wheels small wheels running under the web just before the printing unit
65
Q

Bustle Wheels

A
  • One or two bustle wheels are used to create a “tent” in the web and pull in the web edges
  • Two bustle wheels are more effective than one
  • Air jets may be used in place of bustle wheels, especially if the bustle weehls cause marking of the image
66
Q

What are the types of inks used in web offset

A

- Quickset Inks:

  • Dry by absorption and oxidative polymerization
  • No need for a heat dryer or chill rolls, but drying takes several hours
  • Common in business forms printing
  • News Inks:
  • Dry by absorption only; requires a very absorptive stock
  • Ink does not truly harden; rather absorbs into substrate
  • No need for a heat dryer or chill rolls
  • Newspaper sector, inserts, other printing on newsprint
67
Q

Ultraviolet (UV) and Electron Beam (EB) inks in web offset:

A
  • harden rapidly when exposed to energy source (UV or EB)
  • UV or EB dryers may be interstation or after last printing unit
  • Good ink trap; good control of dot gain, but inks are expensive
  • Suitable for a large variety of substrates
  • No requirement for a heat dryer or chill rolls
68
Q

Heatset Inks in Web Offset:

A
  • Dry primarily by evaporation of ink solvents followed by chilling
  • Suited for a large variety of substrates; dry with a high gloss
  • Ink solvents must have sufficiently high boiling point to not set on warm rollers, but low enough to be able to evaporate in dryer
  • Ink solvents can vary - depends on stock, press speed, type of press dryer. Consult with ink manufacturer.
69
Q

Heatset Drying Process:

A
  • Dryer evaporates volatile solvents from ink
  • Chill rolls set the ink by cooling the heat-softened binding resins
  • Setting on chill rolls must be sufficient to prevent marking of the print by the in-line finisher
  • Final drying occurs off-press by oxidative polymerization
70
Q

Temperatures used in Heatset Drying Process:

A
  • Dryer air tempartures may reach 500F; web may warm to 300F in dryer
  • Ink spoiling solvents have boiling points ranging from 470F - 630F
  • Web spends average of 0.7 sec in dryer oven
  • Chill rolls cool web to 75F
  • Setting of the ink occurs in each phase: impression, dryer, chill rolls and off-press
  • Most setting occurs as web passes “chill rolls”
71
Q

Three types of dryers:

A
  • All modern dryers are “floating dryers” the web makes no contact within the dryer
  • Direct-impingement dryer: exposed gas flames heat the web
  • Hot-air dryer: High velocity hot air is directed at the web bt “air knives” and exhaust ducts between the air knives draw away the solvent-laden air
  • Combination dryer: direct impingement and hot air dryer in same unit
72
Q

Problem of Boundary Layer

A
  • As the web passes through the dryer, a thin layer of air moves along with it due to friction
  • This is called the “boundary layer”
  • The boundary layer quickly becomes saturated with ink solvents
  • If the boundary layer is not removed, further solvent evaporation from the web will not occur
  • Hot-air dryers use air knives which introduce turbulence into the boundary layer to prevent saturation
73
Q

Sine Wave Web Travel:

A
  • Hot air dryer nozzles are staggered on both sides of the web to create a sine wave pattern in the web and improve web control
  • The sine wave better supports the web over a long span and prevents corrugugating wrinkles and web flutter
74
Q

Dryer Pollution Control:

A
  • VOC solvents evaporated from the ink must be removed before exhaust air is released to the atmosphere
  • The most common technology is afterburners which burn the solvents at temperatures of 1400 - 1500F for up to 2 seconds
  • The only resulting by products of complete combustion are harmless carbon dioxide and water
75
Q

Dryer Pollution Control

A
  • The heat from the afterburner can be harnesses for use in the high velocity hot air dryer
  • Afterburner units are normally outside the plant, sometimes on the roof
  • Other systems use catalytic oxidizers and other scrubber technology
  • All sysytems require constant monitoring to measure VOC levels emitted to the atmosphere
76
Q

Chill Rolls

A
  • Several water-cooled chill rolls after the dryer oven cool and harden the ink resins
  • The temperature of the chill rolls decrease from one to the next
  • The faster the press, the more chill rolls required to provide adequate contact time
  • Web contact with the chill rolls ensures web tensio through the dryer oven
  • Slip between the web and chill rolls is normal, as much as 1%
77
Q
A

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Printing ProcessesIV (3 decks)

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