GRC 328: Quiz Flashcards
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-Dampening system -Inking system -Plate cylinder -blanket cylinder -impression cylinder -Transfer drums
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Components of a printing unit
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-Most widely used in sheetfed printing -quick drying time -Double & single sized cylinder -Double is for cardboard , for thin paper -Impression cylinder in 3 cyl system can be Double or Triple sized
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Three cylinder system
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-More compact design -Two blanket cylinders print onto a common impression cylinder.
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Five cylinder system
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-When 2 plates make contact (highest pressure). -Contact line between plate and blanket cyl. -Nip width refers to the contact width between two rollers on the offset printing press, which is determined by roller pressure. -Nip width plays a major role in determining printing quality, so the ability to make period inspection and fine tuning of the nip width, based on factors such as ink characteristics, variations in roller condition (both rubber or metal), and even the printing environment.
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Nip
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the packing gauge is zeroed on the bearer ring
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Bearer ring
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Gutter
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Cylinder body
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distance between cylinder body and bearer ring
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Undercut
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Adjusting pressure between plate and blanket cylinder
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impression pressure is adjusted by setting the thickness of paper.
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Adjusting pressure between blanket and impression cylinder
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Calculating packing height, thickness, undercut, etc.
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-Inadequate packing = too much pressure -Excessive packing = dot gain and worn heat -Hard roller to soft roller creates an ink stripe
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Problems caused by excessive packing & inadequate packing
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-Form rollers and plate cylinder -Plate and blanket cylinder -Blanket and impression cylinder
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Setting up roller pressure
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-Kraft paper: water resistant with different thickness -Polyester: chemical resistant, not for packing blanket -Blanket packing should be cut 1/8 - 1/16 in narrower than the blanket
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Packing materials
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-Affected by the blanket, bearer compression, and print quality -HARD blanket generates more pressure -slightly overpack a blanket when the blanket is new.Raise up the surface a little more than usual.
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Proper packing:
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resistance method: "paper sandwich" -Wider strip of paper: HIGHER pressure -smaller strip of paper: LESS pressure
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How do you set up ink roller pressures?
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prints mostly solids -at the printing nip
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Conventional blanket
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more likely to use -soft blanket -high resolution prints
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Compressible blanket
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-The surface affects dot sharpness & paper release Transfers ink to plate with minimum distortion. -Printing Surface -->Fabric -->Compressible layer -->Fabric
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Blanket surface
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-Tacking ink film break away from blanket surface -Poor release: cause dot distortion, excessive sheet curl or tail end hook
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Ink release
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-Store in dark, dry, cool plate -store in tubes -store flat
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How to store the blanket
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-Used on duplicator and some small presses -Top sheet is separated and lifted at the front -Sheet travels the same speed as press
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Single sheet feeder
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-High-speed presses with large formats -The top sheet is separated and lifted at the back edge -The front edge, floating on a layer of air, is forwarded on the feed board
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Stream Feeder
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Three-point alignment
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-The work-and-turn layout prints the same image on both sides of a sheet with one image carrier or plate. -The first side is printed; then, keeping the gripper edge constant, the side edges are reversed, turning the unprinted side toward the image carrier -The right edge becomes the left edge, the front edge remains the same -switch the side guide to the opposite side
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work & turn
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-one side of the sheet of paper is printed, then the paper is turned upside down to print the other side from the same form, changing the gripper edge -The front edge becomes the rear edge, the side edges stay the same -Must include 2 gripper margins
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Work & tumble
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Pros: -Avoid smearing, offseting, and blocking Neg: -Soiling press -Reduce gloss -Finishing problems -Powder on blankets
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Powder Application
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second image appears alongside the first image
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Doubling
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dots appear elongated or smeared on a single unit
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Slur
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-When ink transfers to the back side of the sheet Remedies: -less ink -more powder -need more drying time -less acid
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Ink Setoff & remedies
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Chemical Ghosting: -can see through the other side -glossier on one side -drying issue -back side printed firstCAUSES: 1) ink coverage on 2 sides are different 2) drying problems-dry fast enough *print heavy coverage first, not light coverage to eliminate this problem Mechanical Ghosting: -faint images -form rollers have hole, less ink -diameter is smaller than the plate CAUSES: 1) ink starvation-4 form rollers are better than 3 2)layout 3)form roller(oscillating=image becomes smaller)
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Chemical & mechanical ghosting remedies:
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Poor fit at tail edge
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Flood coating: coats everything Spot coating: coats on certain area-waterbeads-sometimes spot coating is used to emphasize a picture
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Flood coating & Spot coating
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PROS: .petroleum-based .same formulation as ink formulation but without .pigment, therefore, no color .5 or 6 = for non typical color press .the most economical coating .can apply flood or spot coating .can be applied on any unit .different gloss level-matte, dull, gloss CONS: .can yellow overtime *always put on last unit
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Pro's and Con's of Varnish
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.for flood coating, easy to control and does not yellow overtime. .Requires heavier paper-80 lb or over text stock or will curl -certain pigments may bleed (rhodamine red, reflux blue, purple, violet)
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Pro's and Con's of Aqueous Coating
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-the one after the last printing unit -has anilox roll and blanket
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Tower coater
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-color will change for certain pigments-reflex blue etc. because alkaline will shift color
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Color shift in aqueous coating and UV coating
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-coating on 2 sides: work n turn formula, apply least amount of coating as possible
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Substrates requirements of aqueous coating and UV coating
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aqueous coating, if want on uncoated paper cant apply uv coating first so you have to apply aqueous coating(primer) before UV
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UV primer
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-aqueous: -used to flood coat -used on 80 lb or over text -certain pigments may bleed: rhodamine, reflux blue, purple, violet -varnish: petrolium based -same formulation as ink formulation but w/o pigment, no color -most economical -can apply flood or spot coating -can be applied on any unit -different gloss level- matte, dull, gloss -can yellow ovetime *5/6 is typical for color press) *always put on last unit uv coating: -highest gloss and rub resistance -low vocs -immediately drying -no fda approval for food contact -3-10 times more expensive than aqeuous coating -can be inline or offline
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Coating quality difference (aqueous coating, varnish, and UV coating)
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the "kiss" method applies just enough pressure
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Pressure settings on coater
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special formulation: work and turn formula apply least amount of coating as possible
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Requirements of coating on both sides
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backtrapping-ink on the coating blanket =by excessive pressure, too little coating applied, ink is pulled from sheet, ink is overemlusified, coating does not wet ink properlycolor shift- problem pigments (red, reflex blue etc.), ph (aqueous coating 8-9), ink contains dyes. sheet curl-low weight paper, excessive heat
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Coating problems ( sheet curl, backtrapping, color shift)
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plates: determine the resolution (need to have high resolution), fine screen needs high, not low resolution. blankets: quick-release blankets for sharp dots. sponging= temperapedic=not quick at all paper- 80# or 100# C2S gloss. Grade 1 or 2 inks-acceptable dot gain and overprint traps at reasonable ink film thickness fountain solution-compatible with inks and plates, controlled pH and conductivity.
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Materials used in testing a press
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dry solid: run on each unit and check impression turn off f.s. therefore, ink gets onto platewet solid: run on each unit and check dampening system. burn plate from image
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Dry Solid & Wet Solid
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50 consecutive sheets-determine if ink ductor and oscillator are mechanically operating correctly 50 random sheets-determine if ink and water balance, less than 0.05.
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Check the ink density variation through the press run
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neutral definition: a*0, b* -2 tvi-tone value increase or dot gain- CMY 12-16%, K 14-20% Grey balance %-50-40-40
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Neutral color defined in GRACoL 7
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make a calibration run using a linear CTP plate= first step -dots on plate= dot % in digital file * the info that is out in digital file, software interprets face tone area
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Linear CTP plate
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seen in conventional offset UV, oil based combined use regular rollers to blanket dries by radiation
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Hybrid ink
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Pre-set the ink keys on press for makeready
