چاپ آراد >> چاپ افست

واژ‌هٔ چاپ و صورت قدیمی‌تر آن «چهاپ» را برگرفته از واژ‌هٔ مغولی چاو دانسته‌اند که به معنای «فشردن سطحی بر سطح دیگر» است. همچنین "چاپ" در ترکی به معنی کوبیدن و یا حرکت مداوم همراه با کوبش است. چاپ در لغت به‌معنای نقش، اثر، مُهر و نشان آمده است و در متون مختلف کلمات طبع، باسمه، و تافت به‌عنوان مترادف آن به‌کار رفته است. چاپ در اصطلاح، به عمل، فن، و صنعت تکثیر صورت و نقش‌های دوبعدی مانند حروف، ارقام، خط‌ها، تصویرها، و... به‌وسیلهٔ انداختن اثر این نقش‌ها بر کاغذ، پارچه، یا مواد دیگر، به‌ویژه چاپ مواد خواندنی، با تصویر یا بدون تصویر بر روی کاغذ گفته می‌شود. در تعریفی دیگر، چاپ مجموع عملیاتی است که متن خام را به اثر درخور انتشار مانند کتاب، مجله، روزنامه، و جز آن تبدیل کند. چاپ بر روی کاغذ شناخته شده‌ترین کار چاپی است.امروزه با پیشرفت تکنیک‌های چاپ این عمل روی مواد سلولزی، نایلونی، پارچه ای، چرمی، مواد شیمیای، فلزی، شیشه‌ای با اشکال صاف یا ناهموار، مقعر یا محدب و ... صورت می‌گیرد. چاپخانه نیز در لغت به‌معنای محل چاپ کردن و در متون مختلف کلمات مطبعه، دارالطباعه، و باسمه‌خانه به‌عنوان مترادف آن به‌کار رفته است. چاپخانه مؤسسه یا کارگاهی است که از عهد‌هٔ انجام کارهای چاپی بر روی کاغذ و سایر اشیا از طریق انواع چاپ برآید و به تعبیری دیگر، محلی است که چاپ کتاب، نشریات، و مانند آن در آنجا انجام می‌گیرد. امروزه، چاپ به عنوان یک فرآیند انبوه صنعتی در نظر گرفته می شود که بخش اساسی صنعت نشر و بخش مهمی از فعالیت‌های اداری و حکومتی زا شامل می‌شود.

از مهمترین رخدادهای تاریخ، اختراع حروف چاپی مستقل و دستگاه چاپ بوده‌است که به تصور غالب،‌ اولین بار، یوهانس گوتنبرگ ۱۴۶۸-۱۳۹۷ آلمانی در سال ۱۴۵۶ میلادی آن را اختراع کرد؛ اما در حقیقت اختراع فن چاپ به قرن‌ها پیش از گوتنبرگ برمی‌گردد. ادوار تاریخی چاپ و تحولات آن از ابتدا تاکنون به طور کلی به ۶ دسته تقسیم بندی می‌شود:

    پیش از تاریخ و آغاز دوران تاریخی

    دوران باستان

    دوران چینی‌ها

    دوران هنری

    دوران مکانیک

    دوران فتو مکانیک

این دوران از آغاز خط نویسی و پیش از آن تا حدود قرن ۲۰ میلادی را شامل می‌شود.

آسوریان چند هزار سال پیش از میلاد بر خشت‌هایی از گل‌ رس مُهر می‌زدند. انگشترهای خاتم نیز که در زمان باستان استفاده می شد بر همین اساس کار می کرد، چاپ باسمه نیز قرن‌ها قبل از گوتنبرگ در چین شناخته شده بود، در دوران حکومت سلسلهٔ تانگ در چین ۹۰۶-۶۱۸، قدیمی‌ترین نمودهای صنعت چاپ دیده شده است. در این دوره، نقش‌ها بر روی صفحه‌ای چوبی حکاکی، و بعد بر روی پارچه چاپ می‌شد. اولین اشاره به چاپ، در سال ۵۹۳ و یک فرمان حکومتی چینی است که در آن، امپراتور ون‌تی، دستور می‌دهد تصاویر و متون بودایی را چاپ کنند. این متون را اول بر قطعه‌ای کاغذ نازک می‌نوشتند و بعد آن را بر صفحه‌ای چوبی می‌چسباندند و متن را بر روی چوب حکاکی می‌کردند تا یک «زینک» چوبی بسازند و از آن برای چاپ متن استفاده کنند. این شیوه زمان زیادی می‌برد، چرا که هر صفحه از کتاب باید بر یک صفحهٔ چوبی جداگانه حکاکی می‌شد. قدیمی‌ترین کتاب چاپی که تا کنون پیدا شده‌است، یک متن مذهبی بودایی است که در سال ۸۶۸ چاپ شده، این متن در غار دون‌هوانگ در جادهٔ ابریشم کشف شده است. در قرن نهم، کتاب‌های چاپی با تیراژ بالا در شو ایالت چچوان امروز عرضه شد و دلالان خصوصی امکان خرید آن‌ها را داشتند. کمی بعد، فن چاپ به ایالت های دیگر نیز گسترش یافت و در اواخر قرن نهم، در تمام چین رواج یافت. کتاب‌هایی نظیر کتاب‌های کنفوسیوسی، متون بودایی، فرهنگ‌های لغت، کتاب‌های ریاضیات و ... در این دوران چاپ شده است. این فن به سرعت پیشرفت کرد و در سال ۱۰۰۰ میلادی، کتاب‌های صحافی شده به سبک امروز، جانشین تومارها شد. در سال ۱۰۴۱ میلادی، کیمیاگری چینی به نام بی‌شنگ، حروف مستقل چاپی را اختراع کرد این حروف بر روی سفال مرطوب حکاکی می‌شد و بعد از پختن در کوره، دوام زیادی می‌یافت و سرعت حروفچینی و تکثیر متون را بسیار بالا می برد حروف دستی و حروف قلعی که پس از آن ایجاد شد، هیچ یک رواج نیافت، بر عکس حروف چوبی متداول شد. در اوایل قرن ۱۱، اختراع حروف چاپی مستقل باعث رواج کتاب‌های ارزان‌تر چاپی در دوران سلسله‌ی سونگ ۱۲۷۹-۹۶۰ در چین شد.

اعتبار اختراع حروف چاپی فلزی و صنعتی شدن چاپ، از آنِ حکومت کره است که اولین بار در سال ۱۲۴۱ میلادی به آن شاره می‌شود. در سال ۱۳۹۲، ۶۴ سال قبل از دستگاه چاپ گوتنبرگ، دولت کره، وزارت چاپ را تأسیس کرد که وظیفه داشت حروف چاپی فلزی را با فن ریخته‌گری تولید کند. کارخانه‌ی ریخته‌گری دولت کره، در سال ۱۴۰۳، یک قلم مفرغی شامل صدها هزار کاراکتر داشت و تا پایان قرن پانزدهم، ده‌ها قلم کره‌ای دیگر نیز اختراع شد. احتمال دارد که مارکوپولو که در قرن سیزدهم به چین رفت، کتاب‌های چاپی را دیده باشد و او یا مسافران دیگر جاد‌هٔ ابریشم، ‌این دانش را به اروپا آورده باشند که بعدها الهام‌بخش یوهان گوتنبرگ برای اختراع ماشین چاپ شد، چون غربیان نیز قبل از گوتنبرگ با چاپ آشنایی داشتند.

در ۱۴۵۲، گوتنبرگ آلمانی به اید‌هٔ چاپ متحرک تحقق بخشید. وی در کارگاهش فن‌آوری ساخت ورق، جوهر با پایهٔ روغنی و پرس را برای چاپ یک کتاب گرد هم آورد و دستگاه چاپ را اختراع کرد. در واقع او فن‌آوری‌هایی را که سال‌ها قبل برایشان فکر و تلاش شده بود، به ثمر رساند. گوتنبرگ، چاپگر آلمانی، نخستین کسی بود که برای هر حرف، قطعهٔ فلزی جداگانه‌ای درنظر گرفت. وی قطعه‌ها را برای ترکیب کلمات مناسب کنار هم قرار داد، بر آنها مرکب مالید، و بر ورق‌های کاغذ فشرد و به این ترتیب چاپ نوین را ایجاد کرد. وی حروف را ابتدا از جنس چوب، سپس از سرب، و بعدها از آلیاژ سرب، قلع، و آنتیموان ساخت وی نسبت فلزات آلیاژ را به گونه‌ای انتخاب کرد که حروف بیش از حد سخت و نرم نباشند.

گوتنبرگ روزانه بین ۳۰۰ تا ۵۰۰ برگ چاپ می‌کرد. نخستین کتابی که او به این شکل پدید آورد کتاب مقدس ۴۲ سطری بود. کتاب‌هایی که در آن زمان به‌چاپ می‌رسید به اینکونابولا معروف است. ابداع گوتنبرگ ظرف مدتی حدود ۴۰ سال در عمدهٔ کشورهای اروپایی و در شهرهایی مانند ونیز، فلورانس، پاریس و لیونرواج یافت و چاپخانه‌هایی با این روش به‌وجود آمد. در فاصلهٔ سال‌های ۱۴۶۰ - ۱۴۷۰ چاپخانه‌هایی در آلمان و سایر کشورهای اروپایی پدید آمد. در نیم‌قرن نخست پس از کار گوتنبرگ، حدود چهل‌هزار کتاب به چاپ رسید و شمارگان مجموع آنها از ۱۲ میلیون نسخه فراتر رفت. در پایان قرن پانزدهم در اروپا، بیش از ۲۰۰ چاپخانه در ۶۹ شهر فعالیت مستمر داشت. از جمله چاپخانه‌های مهم آن دوره می‌توان به چاپخانه آنتون کابرگر در شهر نورنبرگ اشاره کرد که تعداد ماشین‌های چاپ آن ۲۴ دستگاه بود و صدها نفر در آن چاپخانه کار می‌کردند. از دیگر خدمات گوتنبرگ می توان به موارد زیر اشاره کرد:

    ابداع روش چاپ پیچشی به کمک روغن زیتون که تا سالها در آسیا و اروپا به کار می رفت

    توسعه روش بلوکی چاپ که پس از بازگشت مارکو پولو از آسیا ابداع شده بود

    توسعه روش ساخت کاغذ به تعداد زیاد که از چین وارد اروپا شده بود

    توسعه روشهای ساخت مرکب

    توسعه روش منگنه و قالب بندی کتاب که اجازه چاپ کتابهای قطور را می داد

دستگاه چاپ گوتنبرگ، هزینه‌های بسیار زیادی داشت و درنتیجه بیشتر مورد توجه ثروتمندان آن دوران بود.

۳۰۰ سال پس از اختراع دستگاه چاپ گوتنبرگ، یک نمایشنامه‌نویس آلمانی به نام آلوئیس زنه فلدر، چاپ سنگی یا لیتوگرافی را در سال ۱۷۹۶ میلادی اختراع کرد. هر سنگی که متن یا تصویر با این روش روی آن نقش می‌بست، برای چاپ حدود ۷۵۰ نسخه عملکرد مطلوب داشت و پس از آن نقش روی سنگ قابل چاپ نبود.

دربار‌هٔ شروع چاپ سنگی در ایران روایات متعددی وجود دارد، اما روایت قوی‌تر این است که چاپ سنگی را برای نخستین بار میرزا صالح شیرازی در تبریز راه‌اندازی کرد. میرزا صالح که از سوی دولت ایران برای فراگیری هنرهای جدید به اروپا رفته بود، در بازگشت یک دستگاه چاپ سنگی با خود به تبریز آورد که آن را در سال ۱۲۲۵ هجری قمری راه انداخت. چاپخانهٔ سنگی در مدت کوتاهی در تهران، اصفهان و سپس سایر شهرهای ایران تاسیس شد و بیش از ۵۰ سال تنها روش چاپ در ایران بود و تا اواخر دور‌هٔ قاجار، هر چه در ایران چاپ می‌شد، به روش چاپ سنگی بود. البته هشت سال قبل از ورود چاپ سنگی به ایران چاپ سربی راه‌اندازی شده بود، ولی به علت هزینه و زحمت زیاد آن، پس از ورود چاپ سنگی، کنار گذاشته شد ولی بعدها در اواخر دور‌هٔ قاجار دوباره استفاده از حروف سربی رایج گردید.

حال بعضی از ابداعات چاپ در قرن ۱۹ و ۲۰ م را بررسی نماییم. در دههٔ آخر قرن ۱۸ م تحولاتی در چاپ ایجاد شد؛ از جمله اینکه در همین دوره شخصی به نام آلویز سنفلدر اتریشی چاپ سنگی را برای تکثیر ورقه‌های موسیقی ابداع کرد و به آن به عنوان جایگزینی برای سیاه قلم کاری نگاه کرد. در این روش نگاره به وسیلهٔ یک قلم تیز یا عمل یک اسید بر سطح سنگ ایجاد می‌شود و نگاره با رنگ سیاه بر سطح سنگ خاکستری ترسیم می‌شود. از همین جهت تصور کار نهایی چندان مشکل نبود. در این شیوه روش کار اینگونه است که قسمت‌هایی را که نمی‌خواستند چاپ شود روی سنگ مرطوب می‌کردند و در نتیجه مرکب چاپ که چرب است به آن قسمت‌ها نمی‌چسبید و قسمت‌های خشک مرکب را جذب می‌کردند و بدین صورت سطح آماده چاپ شامل دو قسمت بود:

    بخش مرطوب برای قسمت‌های سفید

    بخش خشک برای قسمت‌های سیاه یا رنگی

همین شیوه چاپ بود که بعدها به اختراع چاپ افست منجر گردید. اختراع عکاسی شاید بیش از هر عامل دیگر بر چاپ و صنعت تاثیر گذار بود و با وجود اینکه تهدیدی برای روش‌های دستی چاپ به حساب می‌آمد اما با اختراع عکاسی روش حساس کردن سطح فلز و برگرداندن تصویر و حروف و نقوش بر روی آن ابداع شد. این روش جدید منشا چاپ با کلیشه‌های خطی گردید و در پی پیشرفت عکاسی شیوه‌های گراوور، فتوگراوور و هلیوگراوور ایجاد شدند. در اوایل قرن ۱۹ م ماشین‌هایی ابداع شد که اعمال مختلف چاپ را به طور خودکار انجام می‌دادند و در نتیجه سرعت چاپ افزایش یافت تا جایی که ماشین‌های روتاتیو که قادر بودند در یک لحظه هر دو روی کاغذ را چاپ کنند به وجود آمد. با توجه به مطالب گفته شده به طور کلی به ۳ روش می‌توان یک نمونه تصویر یا نوشته را به چاپ رساند که عبارتند از:

    چاپ برجسته یا لترپرس یا تایپوگرافی

    چاپ گود یا فرو رفته یا هیلوگراوور

    چاپ مسطح یا پلانوگرافی یا لیتوگرافی

بنابراین اسلوب اصلی چاپ با در نظر گرفتن ۳ مرحله زیر ضروری است:

    تهیهٔ گراوورهای دستی

    تبدیل گراوور به چاپ و انجام چاپ

    استفاده از فتو مکانیک در چاپ

    - چاپ برجسته:در این روش سطح چاپ شونده به صورت برجسته است. در نتیجه قسمت برجسته در مقابل غلتک آغشته به مرکب چاپ رنگ را به خود می‌گیرد و به کاغذ منتقل می‌کند. واضح است که در این شیوه سطح برجسته باید معکوس نقش مورد نظر باشد تا پس از چاپ شدن برروی کاغذ به صورت صحیح نمایان گردد.

    ۲- چاپ گود:در این نوع چاپ نقش‌ها و تصاویر در سیلندر استوانه‌ای که سطح چاپ شونده است، به صورت فرو رفته‌اند. این نوع چاپ از کیفیت بالایی برخوردار است اما به دلیل هزینه زیادی که دارد برای کارهایی با تیراژ بالا مورد استفاده قرار می‌گیرد؛ مثل:تمبر، اسکناس،...

سیلندر چاپ گود دو نوع است:

     تمام سیلندر ریخته گری می‌شود و پس از پایان چاپ دوباره ذوب می‌گردد.

    فقط رویهٔ سیلندر که نقوش در آن فرورفته‌اند عوض می‌شود و پس از هر بار چاپ فقط این رویه تعویض می‌گردد. در این نوع چاپ سطح تصاویر یا حروف چاپ افست است.

    ۳- در این شیوه سطح چاپ شونده نه برجسته و نه فرورفته است و با خاصیت مواد، حروف و تصاویر مرکب را به خود جذب نمی‌کنند. این روش تکامل یافته شدهٔ لیتو گرافی یا چاپ سنگی است. در این شیوه که سطح چاپی صاف است نقش نخست از لوحهٔ فلزی به استوانه لاستیکی منتقل می‌شود و از روی آن به کاغذ انتقال می‌یابد.

صفحهٔ فلزی چاپ یا همان زینک به دور یک استوانه لاستیکی بسته می‌شود. دو غلتک یکی آغشته به مرکب و دیگری آغشته به آب برمبنای تضاد بین آب و چربی باعث می‌شوند نقش روی زینک مرکب بگیرد و قسمت‌های دیگر مرکب را جذب نکنند در این نوع چاپ با ۴ رنگ اصلی و استفاده از ترام، تمام رنگ‌ها ساخته می‌شود.

 Offset printing is a commonly used technique in which the inked image is transferred or “offset” from a plate to a rubber blanket then to the printing surface When used in combination with the lithographic process which is based on the repulsion of oil and water the offset technique employs a flat planographic image carrier on which the image to be printed obtains ink from ink rollers while the non-printing area attracts a water-based film called “fountain solution” keeping the non-printing areas ink-free The modern “web” process feeds a large reel of paper through a large press machine in several parts typically for several metres which then prints continuously as the paper is fed through

Development of the offset press came in two versions in 1875 by Robert Barclay of England for printing on tin and in 1904 by Ira Washington Rubel of the United States for printing on paper

History

Lithography was initially created to be an inexpensive method of reproducing artwork This printing process was limited to use on flat porous surfaces because the printing plates were produced from limestone In fact the word “lithograph” historically means “an image from stone” or “printed from stone” Tin cans were popular packaging materials in the 19th century but transfer technologies were required before the lithographic process could be used to print on the tin

The first rotary offset lithographic printing press was created in England and patented in 1875 by Robert Barclay This development combined mid-19th century transfer printing technologies and Richard March Hoe’s 1843 rotary printing press—a press that used a metal cylinder instead of a flat stone The offset cylinder was covered with specially treated cardboard that transferred the printed image from the stone to the surface of the metal Later the cardboard covering of the offset cylinder was changed to rubber which is still the most commonly used material

As the 19th century closed and photography became popular many lithographic firms went out of business Photoengraving a process that used halftone technology instead of illustration became the primary aesthetic of the era Many printers including Ira Washington Rubel of New Jersey were using the low-cost lithograph process to produce copies of photographs and books Rubel discovered in 1901—by forgetting to load a sheet—that when printing from the rubber roller instead of the metal the printed page was clearer and sharper After further refinement the Potter Press printing Company in New York produced a press in 1903 By 1907 the Rubel offset press was in use in San Francisco

The Harris Automatic Press Company also created a similar press around the same time Charles and Albert Harris modeled their press “on a rotary letter press machine”

Modern offset printing

One of the most important functions in the printing process is prepress production This stage makes sure that all files are correctly processed in preparation for printing This includes converting to the proper CMYK color model finalizing the files and creating plates for each color of the job to be run on the press

Offset lithography is one of the most common ways of creating printed materials A few of its common applications include newspapers magazines brochures stationery and books Compared to other printing methods offset printing is best suited for economically producing large volumes of high quality prints in a manner that requires little maintenance Many modern offset presses use computer-to-plate systems as opposed to the older computer-to-film work flows which further increases their quality

Advantages of offset printing compared to other printing methods include

    consistent high image quality Offset printing produces sharp and clean images and type more easily than for example letterpress printing this is because the rubber blanket conforms to the texture of the printing surface
    quick and easy production of printing plates
    longer printing plate life than on direct litho presses because there is no direct contact between the plate and the printing surface Properly developed plates used with optimized inks and fountain solution may achieve run lengths of more than a million impressions
    cost Offset printing is the cheapest method for producing high quality prints in commercial printing quantities
    a further advantage of offset printing is the possibility of adjusting the amount of ink on the fountain roller with screw keys Most commonly a metal blade controls the amount of ink transferred from the ink trough to the fountain roller By adjusting the screws the gap between the blade and the fountain roller is altered leading to the amount of ink applied to the roller to be increased or decreased in certain areas Consequently the density of the colour in the respective area of the image is modified On older machines the screws are adjusted manually but on modern machines the screw keys are operated electronically by the printer controlling the machine enabling a much more precise result

Disadvantages of offset printing compared to other printing methods include

    slightly inferior image quality compared to rotogravure or photogravure printing
    propensity for anodized aluminum printing plates to become sensitive due to chemical oxidation and print in non-image–background areas when developed plates are not cared for properly
    time and cost associated with producing plates and printing press setup As a result very small quantity printing jobs may now use digital offset machines

Every printing technology has its own identifying marks as does offset printing In text reproduction the type edges are sharp and have clear outlines The paper surrounding the ink dots is usually unprinted The halftone dots are always hexagonal though there are different screening methods

The most common kind of offset printing is derived from the photo offset process which involves using light-sensitive chemicals and photographic techniques to transfer images and type from original materials to printing plates In current use original materials may be an actual photographic print and typeset text However it is more common—with the prevalence of computers and digital images—that the source material exists only as data in a digital publishing system

Offset printing process consists of several parts

    the inking system ink fountain and ink rollers
    the dampening system water fountain and water rollers
    the plate cylinder
    the offset cylinder or blanket cylinder
    the impression cylinder

In this process ink is transferred from the ink fountain to the paper in several steps

    The inking and dampening systems deliver ink and water onto the offset plate covering the plate cylinder
    The plate cylinder transfers the ink onto the blanket covering the offset cylinder
    The paper is then pressed against the offset cylinder by the impression cylinder transferring the ink onto the paper to form the printed image

Inking system

The goal of any inking system is to place a uniform layer of ink across every dimension of the printing plate The lithographic process is unique in that it requires the ink form rollers to pass in contact with the nonimage areas of the plate without transferring ink to them

Inking systems are made up of several elements

    the ink fountain
    the ink fountain roller or ink feed roller
    the ink ductor roller
    the ink distribution rollers
    the ink form rollers

The ink fountain stores a quantity of ink in a reservoir and feeds small quantities of ink to the distribution rollers from the ink fountain roller and the ink ductor roller The ink ductor roller is a movable roller that moves back and forth between the ink fountain roller and an ink distribution roller As the ductor contacts the ink fountain roller both turn and the ductor is inked The ductor then swings forward to contact an ink distribution roller and transfers ink to it There are generally two types of ink distribution rollers the ink rotating rollers or ink transfer rollers which rotate in one direction and the ink oscillating rollers or ink vibrating rollers which rotate and move from side to side The ink distribution rollers receive ink and work it into a semiliquid state that is uniformly delivered to the ink form rollers A thin layer of ink is then transferred to the image portions of the lithographic plate by the ink form rollers

The ink fountain holds a pool of ink and controls the amount of ink that enters the inking system The most common type of fountain consists of a metal blade that is held in place near the fountain roller The gap between the blade and the ink fountain roller can be controlled by adjusting screw keys to vary the amount of ink on the fountain roller The printer adjusts the keys in or out as the ink fountain roller turns to obtain the desired quantity of ink In simple presses the printer must turn these screws by hand The modern state of the art press today the adjusting screws are moved by “servo” motors which are controlled by the printer at a press console Thus the printer can make ink adjustments electronically If the printer needs to increase or decrease ink in an area of the plate print he need only adjust the needed keys to allow more or less ink flow through the blade The ink flow can also be controlled by the rotation velocity of the ink fountain roller

A simple indication of the quality of a printing press is the number of distribution and form rollers The greater the number of distribution rollers the more accurate the control of ink uniformity It is difficult to ink large solid areas on a plate with only one ink form roller With three generally the maximum it is relatively easy to maintain consistent ink coverage of almost any image area on the plate Business forms presses which print very little coverage usually only have one or two ink form rollers Because of this they cannot print large solid or screen images Smaller less sophisticated presses also have the same problem however many of the newer presses today are being equipped with larger better inking systems to meet the growing print demands of the consumer
Dampening system

Most lithographic plates function on the principle of water and ink receptive areas In order for ink to adhere only to the image areas on the plate a layer of moisture must be placed over the nonimage areas The dampening system accomplishes this by moistening the plate consistently throughout the press run

Dampening systems are made up of several elements

    the water fountain
    the water fountain roller or water feed roller
    the water ductor roller in intermittent-flow dampening systems and the water slip roller in continuous-flow dampening systems
    the water distribution rollers
    the water form rollers

Direct dampening systems employ a water fountain roller which picks up the water from the water fountain The water is then passed to a water distributon roller From here the water is transferred to the offset plate via one or two water form rollers

Indirect dampening systems or integrated dampening systems feed the water directly into one of the ink form rollers ink rollers that touch the offset plate via a water form roller in contact with it These systems are known as “indirect” since the water travels to the offset plate passing through the inking system and not directly to the offset plate as direct systems do Some indirect systems will have the ability to feed the water into the inking system as well as to the offset plate A fine emulsion of ink and water is then developed on the ink form roller This is one reason printers need to know about “water pickup” or what percentage of water can be taken up by the ink These systems are also known as “integrated” dampening systems as they are integrated into the inking system One of the benefits of these systems is that they do not use covers thus they react quicker when dampening changes are made One generally finds this type of dampening systems on newer and faster press equipment today

Intermittent-flow dampening systems direct or indirect use a water ductor roller to pick up the water and transfer it to a water distribution roller A drawback of these systems is the slow reaction time in making adjustments due to the back and forth action of the ductor

Continuous-flow dampening systems direct or indirect are used by most newer presses today because they do not have the slow reaction time of intermittent-flow dampening systems They do not employ the water ductor roller but use the water slip roller a roller in contact with both the water fountain roller and a distribution roller contrary to the water ductor roller that moves back and forth between the two for a continuous flow The speed of the water slip roller controls the supply The use of alcohol on these type of dampeners was standard for years Alcohol isopropyl alcohol was used as it increased the water viscosity and made it “more wettable” so that transfer was easier from one roller to the other However alcohol substitutes such as glycol ethers butyl cellusolve etc are being used today to accomplish the same task because alcohol contains volatile organic compounds Roller hardness is also being changed to help accomplish the same job—easy transfer of the water
Variations

Several variations of the printing process exist

    blanket-to-blanket a printing method in which there are two blanket cylinders through which a sheet of paper is passed and printed on both sides Blanket-to-blanket presses are considered a perfecting press because they print on both sides of the sheet at the same time Since the blanket-to-blanket press has two blanket cylinders making it possible to print on both sides of a sheet there is no impression cylinder The opposite blanket cylinders act as an impression cylinder to each other when print production occurs This method is most utilized on offset presses designed for envelope printing There are also two plate cylinders on the press
    blanket-to-steel a printing method similar to a sheet offset press except that the plate and cylinder pressures are quite precise Actual squeeze between plate and blanket cylinder is optimal at 0005″ as is the squeeze or pressure between the blanket cylinder and the substrate Blanket-to-steel presses are considered one-color presses In order to print the reverse side the web is turned over between printing units by means of turning bars The method can be used to print business forms computer letters and direct mail advertising
    variable-size printing a printing process that uses removable printing units inserts or cassettes for one-sided and blanket-to-blanket two-sided printing
    keyless offset a printing process that is based on the concept of using fresh ink for each revolution by removing residual inks on the inking drum after each revolution It is suitable for printing newspapers
    dry offset printing a printing process which uses a metal backed photopolymer relief plate similar to a letterpress plate but unlike letterpress printing where the ink is transferred directly from the plate to the substrate in dry offset printing the ink is transferred to a rubber blanket before being transferred to the substrate This method is used for printing on injection moulded rigid plastic buckets tubs cups and flowerpots

Materials

The plates used in offset printing are thin flexible and usually larger than the paper size to be printed Two main materials are used

    metal plates usually aluminum although sometimes they are made of multimetal paper or plastic
    polyester plates these are much cheaper and can be used in place of aluminum plates for smaller formats or medium quality jobs as their dimensional stability is lower13

Computer-to-plate

Computer-to-plate CTP is a newer technology which replaced computer-to-film CTF technology and that allows the imaging of metal or polyester plates without the use of film By eliminating the stripping compositing and traditional plate making processes CTP altered the printing industry which led to reduced prepress times lower costs of labor and improved print quality

Most CTP systems used thermal CTP or violet technologies Both technologies has the same characteristics in term of quality and plate durability longer runs However the often violet CTP systems are cheaper than thermal ones and thermal CTP systems do not need to be operated under yellow light

Thermal CTP involves the use of thermal lasers to expose and/or remove areas of coating while the plate is being imaged This depends on whether the plate is negative or positive working These lasers are generally at a wavelength of 830 nm but vary in their energy usage depending on whether they are used to expose or ablate material Violet CTP lasers have a much lower wavelength 405 nm–410 nm Violet CTP is “based on emulsion tuned to visible light exposure”

Another process is computer-to-conventional plate CTCP system in which conventional offset plates can be exposed making it an economical option

Sheet-fed refers to individual sheets of paper or rolls being fed into a press via a suction bar that lifts and drops each sheet onto place A lithographic “litho” for short press uses principles of lithography to apply ink to a printing plate as explained previously Sheet-fed litho is commonly used for printing of short-run magazines brochures letter headings and general commercial jobbing printing In sheet-fed offset “the printing is carried out on single sheets of paper as they are fed to the press one at a time” Sheet-fed presses use mechanical registration to relate each sheet to one another to ensure that they are reproduced with the same imagery in the same position on every sheet running through the press
Perfecting press

A perfecting press also known as a duplex press is one that can print on both sides of the paper at the same time Web and sheet-fed offset presses are similar in that many of them can also print on both sides of the paper in one pass making it easier and faster to print duplex
Offset duplicators

Small offset lithographic presses that are used for fast good quality reproduction of one-color and two-color copies in sizes up to 12″ by 18″ Popular models were made by A B Dick Company Multilith and the Chief and Davidson lines made by ATF-Davidson Offset duplicators are made for fast and quick printing jobs printing up to 12000 impressions per hour They are able to print business forms letterheads labels bulletins postcards envelopes folders reports and sales literature
Feeder system

The feeder system is responsible for making sure paper runs through the press correctly This is where the substrate is loaded and then the system is correctly set up to the certain specifications of the substrate to the press
Printing–inking system

The Printing Unit consists of many different systems The dampening system is used to apply dampening solution to the plates with water rollers The inking system uses rollers to deliver ink to the plate and blanket cylinders to be transferred to the substrate The plate cylinder is where the plates containing all of the imaging are mounted Finally the blanket and impression cylinders are used to transfer the image to the substrate running through the press
Delivery system

The delivery system is the final destination in the printing process while the paper runs through the press Once the paper reaches delivery it is stacked for the ink to cure in a proper manner This is the step in which sheets are inspected to make sure they have proper ink density and registration
Slur

Production or impact of double image in printing is known as slur

Web-fed refers to the use of rolls or “webs” of paper supplied to the printing press Offset web printing is generally used for runs in excess of five or ten thousand impressions Typical examples of web printing include newspapers newspaper inserts or ads magazines direct mail catalogs and books Web-fed presses are divided into two general classes coldset or non-heatset and heatset offset web presses the difference being how the inks that are used dry Cold web offset printing dries through absorption into the paper while heatset utilizes drying lamps or heaters to cure or “set” the inks Heatset presses can print on both coated slick and uncoated papers while coldset presses are restricted to uncoated paper stock such as newsprint Some coldset web presses can be fitted with heat dryers or ultraviolet lamps for use with UV-curing inks This can enable a newspaper press to print color pages heatset and black & white pages coldset

Web offset presses are beneficial in long run printing jobs typically press runs that exceed ten or twenty thousand impressions Speed is a determining factor when considering the completion time for press production some web presses print at speeds of 3000 feet per minute or faster In addition to the benefits of speed and quick completion some web presses have the inline ability to cut perforate and fold
Heatset web offset

This subset of web offset printing uses inks which dry by evaporation in a dryer typically positioned just after the printing units This is typically done on coated papers where the ink stays largely on the surface and gives a glossy high contrast print image after the drying As the paper leaves the dryer too hot for the folding and cutting that are typically downstream procedures a set of “chill rolls” positioned after the dryer lowers the paper temperature and sets the ink The speed at which the ink dries is a function of dryer temperature and length of time the paper is exposed to this temperature This type of printing is typically used for magazines catalogs inserts and other medium-to-high volume medium-to-high quality production runs
Coldset web offset

This is also a subset of web offset printing typically used for lower quality print output It is typical of newspaper production In this process the ink dries by absorption into the underlying paper A typical coldset configuration is often a series of vertically arranged print units and peripherals As newspapers seek new markets which often imply higher quality more gloss more contrast they may add a heatset tower with a dryer or use UV ultraviolet based inks which “cure” on the surface by polymerisation rather than by evaporation or absorption
Sheet-fed vs web-fed

Sheet-fed presses offer several advantages Because individual sheets are fed through a large number of sheet sizes and format sizes can be run through the same press In addition waste sheets can be used for make-ready which is the testing process to ensure a quality print run This allows for lower cost preparation so that good paper is not wasted while setting up the press for plates and inks Waste sheets do bring some disadvantages as often there are dust and offset powder particles that transfer on to the blankets and plate cylinders creating imperfections on the printed sheet This method produces the highest quality images

Web-fed presses on the other hand are much faster than sheet-fed presses with speeds in excess of 20000 cut-offs per hour a cut-off is the paper that has been cut off a reel or web on the press  the length of each sheet is equal to the cylinder's circumference The speed of web-fed presses makes them ideal for large runs such as newspapers magazines and comic books However web-fed presses have a fixed cut-off unlike rotogravure or flexographic presses which are variable

Inks

Offset printing uses inks that compared to other printing methods are highly viscous Typical inks have a dynamic viscosity of 40–100 Pa·s

There are many types of paste inks available for utilization in offset lithographic printing and each have their own advantages and disadvantages These include heat-set cold-set and energy-curable or EC such as ultraviolet- or UV- curable and electron beam- or EB- curable Heat-set inks are the most common variety and are “set” by applying heat and then rapid cooling to catalyze the curing process They are used in magazines catalogs and inserts Cold-set inks are set simply by absorption into non-coated stocks and are generally used for newspapers and books but are also found in insert printing and are the most economical option Energy-curable inks are the highest-quality offset litho inks and are set by application of light energy They require specialized equipment such as inter-station curing lamps and are usually the most expensive type of offset litho ink

    Letterset inks are mainly used with offset presses that do not have dampening systems and uses imaging plates that have a raised image
    Waterless inks are heat-resistant and are used to keep silicone-based plates from showing toning in non-image areas These inks are typically used on waterless Direct Imaging presses
    Single Fluid Inks are newer ink that uses a process allowing lithographic plates on a lithographic press without using a dampening system during the process

Ink–water balance

Ink and water balance is an extremely important part of offset printing If ink and water are not properly balanced the press operator may end up with many different problems affecting the quality of the finished product such as emulsification the water overpowering and mixing with the ink This leads to scumming catchup trapping problems ink density issues and in extreme cases the ink not properly drying on the paper resulting in the job being unfit for delivery to the client With the proper balance the job will have the correct ink density and should need little further adjustment except for minor ones An example would be when the press heats up during normal operation thus evaporating water at a faster rate In this case the machinist will gradually increase the water as the press heats up to compensate for the increased evaporation of water Printing machinists generally try to use as little water as possible to avoid these problems
Fountain solution

Fountain solution is the water-based or “aqueous” component in the lithographic process that moistens the non image area of the plate in order to keep ink from depositing and thus printing Historically fountain solutions were acid-based and made with gum arabic chromates and/or phosphates and magnesium nitrate Alcohol is added to the water to lower the surface tension and help cool the press a bit so the ink stays stable so it can set and dry fast While the acid fountain solution has improved in the last several decades neutral and alkaline fountain solutions have also been developed Both of these chemistries rely heavily on surfactants–emulsifiers and phosphates and/or silicates to provide adequate cleaning and desensitizing respectively Since about 2000 alkaline-based fountain solutions have become less common due to the inherent health hazards of high pH and the objectionable odor of the necessary microbiological additives

Acid-based fountain solutions are still the most common variety and yield the best quality results by means of superior protection of the printing plate lower dot gains and longer plate life Acids are also the most versatile capable of running with all types of offset litho inks However because these products require more active ingredients to run well than do neutrals and alkalines they are also the most expensive to produce However neutrals and to a lesser degree alkalines are still an industry staple and will continue to be used for most newspapers and many lower-quality inserts In recent years alternatives have been developed which do not use fountain solutions at all waterless printing
 

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