What is offset printing

This basic overview of offset printing discusses sheetfed and web presses, prepress, inks, substrates, finishing, common applications, and trends.

1. What is offset printing?

Thissummer,we’republishingaseriesofpostsexplainingdifferenttypesofprintingprocesses.
In this post, we review the ​basics of offset printing​ (also known as offset lithography).
Offsetlithographyistheprimarymethodusedtoprintlongerrunsofmagazines,books,
manuals,brochures,direct-mailpieces,letterhead,posters,andpaper-basedlabelsand
packaging.
Becauseofthefixedcostsinvolvedwithsettingupeachjobforoffsetprinting,thecostofeach
copy in a 250-copy order will be higher than the cost of each piece in a 10,000-copy job.

How It Works
Offsetlithographyisamulti-stepprocessthatusesetchedmetalplates,water-repellent(e.g.
oil-based) inks, water, blanket cylinders, and impression cylinders.
Theplatesarecreatedsothatoilyinksadhereonlytotheportionsoftheplatethatcontainthe
images and text. A thin film of water over the blank areas of the plate repels the oil-based inks.
Theinkontheplatesisfirsttransferredtoarubber-wrapped“blanket”cylinder.Theink
reaches the paper when the inked blanket cylinder rolls over the paper.
Manypressesusedimpressioncylinderstoapplypressuretothepaperastheinkisbeing
appliedtothepapersurface.Impressioncylindersalsohelpmovethesheetsthroughthe
presses.


Theoffsetprintingprocessrequiresmakingseparateplatesandblanketrollersforeachcolorof
ink that will be used.

2. Differentvariationsofpresses,plates,andinksareusedfordifferentrunlengthsandsubstrate
requirements.Forexample,somepressesmayprintmillionsofcopiesofamagazine.Other
presses may print 250 large sheets that will be folded to make multi-page brochures.

WebPress​:Theseprintersautomaticallyfeedwiderollsofpaperfromoneinkstationto
anotheruntiltheappropriateaboutofinkisappliedtoeachpage.Webpressesaretypically
usedtoprint10,000ormorecopiesofmulti-pagedocumentssuchasmagazines,books,and
catalogs.



In ​coldset web offset​, the ink is fixed by absorption into the paper.

In​heatsetweboffse​t,inkisfixedbyabsorptionintothepaperplusevaporationathigh
temperatures.Heatsetweboffsetpressesarefourtofivetimesfasterthansheetfedpresses
and typically include inline folding equipment.

SheetfedPress​:Thistypeofoffsetprintingequipmentfeedsflatpre-cutsheetsthroughthe
press.Speedsrangefrom5,000to20,000sheetsperhour.Sheetfedpressescanhandleawider
rangeofsubstratetypesandthicknessesthanwebpresses.Smallersheetfedpressescan
handle papers as small as 4 x 6 inches. Larger presses can handle sheets up to 40 in. x 26 in.

3. Largersheetsizesaretypicallyreferredtoas“parents”becauseeachsheetholdsmultiple
pagesofabookormanual.Thepagelayoutsoneachparentsheet(“signatures”)mayhave4,8,
or12bookpagesoneachside.Thepagesmustbelaidoutinaproper“imposition”sothatthe
pages will appear in the proper sequence when the pages are trimmed, folded, and bound.

A​PerfectingPresscanprintonbothsidesofthepaperautomatically.Allwebpressesare
perfecting presses. But not all sheetfed presses are built to print on both sides automatically.

Prepress
Beforeaprintjobcangotoanoffsetpressthepagedesignscreatedonacomputermustbe
separated by the types of inks that will be used to make up each page.
Foryears,aphoto-engravingprocessexposedcolor-separatedfilmsontophotosensitiveplates.
Today,​computer-to-plate(CTP)systemsusedigitallycontrolledlaserengravingsystemstoetch
the imaging data from digital files onto aluminum plates.
CTPsystemshavereducedthetimeandexpenseinvolvedinsettingupoffset-printingjobsand
making last-minute revisions to the plates.
Theprepressprocessforoffsetprintingoftenincludesmoreelaborateproofingmethods,such
asimpositionproofsthatenablebuyerstocheckthatallofthepagesofapublishedmagazine
will appear in the desired sequence.

Offset Inks
A large variety of inks are available exclusively for use with different types of offset presses.

Colors​:Inadditiontothefourprocesscolors(cyan,magenta,yellow,andblack),press
operators can use inks that match a client’s specific Pantone brand colors.

Formulations:Someoffsetinksuseplant-derivedoilssuchassoyaoilorlinseedoiltocarrythe
pigment. Other inks use water-repellant fluids made with synthetic chemical mixtures. .

4. SpecialProperties:Someoffsetinksareformulatedtochangetheperformanceoftheinkon
thepaper.Forinstance,printscanbemadewithsuchascolor-changing(thermochromatic)
inks, metallic inks, magnetic inks, or UV-curing inks. .
Polymer-basedinksthatcurewhenexposedtoUV-LEDlightarepopularbecausetheyenable
ownersofsheetfedpressestomakeinstantlydryprints.Theydon’tneedtouseheator
spray-on powders to accelerate the on-press drying process.
WithcorrectlyformulatedUV-LEDinksandtheproperUV-LEDcuringunits,sheetfedoffset
pressescanprintonpaperboardsforpackaging,non-poroussyntheticsheets,foil-laminated
sheets, plastic sheets, and a greater range of card and label stocks.

Substrates
Thankstoinnovationsinoffsetinksandsubstrates,offsetprintingcanprintonawiderangeof
smoothandtexturedpapers,paperboards,tear-resistantsyntheticpapers,andvariousplastics
usedingiftcards,bags,packaging,andenvelopes.Someofthespecialtymaterialsavailablefor
offsetprintingincludesecuritypapers,carbonlesspapersforbusinessforms,and
metal-laminated papers with silver and gold foils.
Weboffsetpressesprimarilyprintontheinexpensivecoatedanduncoatedpapersneededto
controlthecostsofhighvolumesofbooks,manuals,magazines,andcatalogsButthesepapers
mustbestrongenoughtowithstandroll-to-rollhigh-speedprinting.Mostofthespecialty
materials are designed for use on sheetfed presses.

Finishing Processes
Thetypeoffinishingequipmentuseddependsonthetypeandvolumesofpublications,direct
mail pieces, manuals, stationery, or packaging being produced.
Finishingequipmentincludesdevicesfortrimmingandcuttingthepaper,orfolding,binding,
hole-punching,varnishing,laminating,ordie-cuttingprintedsheets.Someprintedsheetsmay
befoilstamped,perforated,embossed,glued,orinsertedintoenvelopesformailing.Some
finishingequipmentmakesitpossibletoapplyclearUV-curableinksforaddeddurabilityor
decorative effects.

Trends
As​digitalprintingtechnologieshaveimproved,sohastheefficiencyofoffsetprinting.
Manufacturersofoffsetprintingequipmenthavedevelopedprepressprocesses,software,and
consumablesthatenableoffsetpressestoproducemorefast-turnaround,shorter-runprint
jobs.

PressmanufacturerssuchasHeidelbergaredevelopingsystemsthatallowmorestepsinthe
offsetprintingandfinishingprocesstobehandledwithoutanyinterventionbyahumanpress
operator.

Thenextwaveofmajorinnovationsforoffsetprintingislikelytopreviewatthedrupaglobal
print show June 16-26, 2020 in Dusseldorf, Germany.