Learn about Inks, Paper, Printing Methods and print media definitions

It, of course primarily involves putting an image on a medium of paper; but where inkjet and laser printers do that on a small scale, professional printing seeks to do the same at high speed and at a high rate. The whole printing process goes through a series of steps from prepress to finishing.

PRINTING METHODS

There's are many different types of printing which involve different techniques and machinery. Sometimes a combination of printing processes are combined in order to get the desired effect. Older printing equipment has sometimes received new responsibilities in this process.

Letterpress

The first method of commercial printing which Guttenberg and many since then used. The process is similar to an old style typewriter. Ink is applied to an object with a raised form and a recessed form. The raised form picks up the ink and then the object is pressed against a sheet of paper many times to create copies. Although this is the original printing process, it is still used today for specialty work such as numbering, imprinting, diecutting, stamping and embossing.

Offset Lithography

This is by far the most common printing method used today. Printing that uses a stone or metal plates as the template for the reproduction. The word lithos is greek for stone and stones were ususally used in the process but metal plates are used today. Lithography was developed by Alois Senefelder towards the end of the 18th Century. He found that if you draw with a wax crayon on a limestone surface and then soaked the limestone with water, then you could roll greasy ink over the surface and the ink would only get picked up in the parts of the limestone with the crayon. The water would repel the ink and that part of the limestone would not absorb the ink.

Today, metal plates of aluminum replace the stone (which were hard to work with). The problem with this direct process above is that the metal plates can get easily damaged so a method was developed which lets a rubber roller pick up the ink that the plates carry and transfers it to the paper. This is why the process is called offset, the metal plates don't interact directly with the paper.

Offset presses are divided into categories by size into sheetfed (where the paper is cut into sheets) or web (where the paper is fed from a roller). Traditionally sheet fed presses are used for lower quantities (flyers, annual reports, rack cards, etc) and web presses are used for very large quantities (newspapers, high volume magazines, etc.)

Flexography

This process is a variation of the letterpress process and, but where the letterpress used wood blocks or metal to do the pressing, flexography uses a rubbery surface (this process is similar to using a rubber stamp). It is widely used for non-flat surfaces and very popular in the packaging industry (corrugated containers, folding cartons, multiwall sacks, paper sacks, plastic bags, milk and beverage cartons, disposable cups and containers, labels). It can hold pretty good quality and color on even poor paper and cardboard surfaces.

Digital Press

This is the high speed version of your inkjet or laser printing. There are many types, but generally a way to bypass the "middle man" and puts the inks or toner directly on paper. It's generally used for variable data printing where each printout can be customized.

Gravure

Gravure printing is a reverse variation of the letterpress and can compete with Flexography. Gravure printing is an example of intaglio printing (where the depressed part of the surface holds the ink). It is used in food packaging, wallpapers, wrapping paper, greeting cards, etc. It's a way to hold a sharp, fine image with an excellent level of detail. Honeycombed shapped cells in a metal cylinder roll in an assembly as the cylinder rotates in a bath of ink. Excess ink is wiped off by a steel blade. The inks then transfer directly onto the surface.

Screen Printing

Screen printing can print on surfaces of any shape, thickness and size. It is usually used for short runs and widely used in t-shirt designs. The process lays a very thick amount of inks so it's ideal to items in which brightness is important. In this process, a stencil of an image is created in the shape of a screen (metal or fabric) with holes in the screen representing where the ink should go. The screen is then placed over the material to be printed, and ink is applied on top of the screen. Ink is then forced throught the holes in the screen by applying pressure forming an image on the printing surface.

PREPRESS

Pre-press involves getting the files from your computer for output on a press. Presses vary widely on printing techniques so the concepts can be widely different. The process usually involves producing some type of film that will be made into plates to be placed on the press.

Screening

Most of the printing methods are unable to produce continuous tone images (like you see in a photographic method). To approximate this printers convert photographs or continuous tone images into a series of dots,

Halftoning

By far the most popular method of converting photographs, The dots are placed into position by the use of a halftone screen (the screen can also be approximated digitally. The dots are all of the same size, so dots that are close to each other produce darker or more dense colors. The dots can be of different shapes, but are generally round. When the dots are far apart they create lighter parts of the image.

Halftone Angles

When the dots are placed on a grid, they are placed together in an x and y axis because of the shape of the grid. The grid can become visible if ran without tilting the grid at an angle. So most one color jobs should be run at 45 degree angles.

When running multiple colors, it is best to set the screen angle of the colors as far apart from each other as posible. Ideally the best separation of angles works out to be 30 degrees. That means that up to three colors can be used optimally since 90 degrees divides into 30 degrees three times (remember the screen frequency of each color has an angle in the x and y direction so the angles happen to be 90 degrees apart). The four color process uses one more color than would be optimal so printers traditionally arrange their screen angles with Black (the strongest color at the optimal 45 degree angle), Magenta and cyan at 30 degrees from that (75 degrees and 105 degrees respectively, and yellow, the weakest of the colors at 15 degrees in between the magenta and cyan (90 degrees).

Rosettes & Moires

A 30 degree separation between the angles creates a pattern once the colors are placed on the sheet that is pleasant to the eye called a rosette pattern. Any slight deviation from this perfect pattern creates what is called a Moire which appears regularly in four color printing. You can sometimes adjust your screen angles if a color in your image is more predominent to avoid a moire pattern.

Screen ruling

The closeness of the dots is referred to as the screen frequency or screen ruling. This can vary widely, but cheaper papers in which ink tends to spread into the paper, need to have coarser screens. Screen ruling is measured in Lines Per Inch (LPI) So when printing on newsprint for example a screen ruling of 100 lines per inch is common, however when working with coated stock you may go up to 150-175 or higher. Screen ruling also affects the amount of resolution you will scan your pages in. You normally want to scan images at twice the screen ruling. For example, when working with a project that has a 150lpi screen ruling, you would want to scan your photos at 300 dpi. Where for newsprint's 100lpi, 200dpi will be sufficient. Scanning at higher resolutions than the screen ruling is overkill (unless you're scanning images in black and white ONLY mode). Talk to your printer to determine the screen frequency your projects will be running at.

Stochastic Screening

A new kind of screening method in which the dots vary in size an placement. This eliminates many of the problems associated with screen angles, but creates plates that are highly volatile and can be an expensive process. Also presses which use water cannot hold dot sizes that are very small and so this method is best used for a waterless process.

Film output

The next step in the process is to create film for from your digital files. The film is then used to create metal or paper plates to be placed on the presses. Paper plates are used for shorter runs, whereas metal plates are used for longer runs.

Stripping

A lot of film output devices will output film in sizes that are smaller than some presses can accomodate. Presses come in different sizes so you can't make one size of film output device to accomodate all presses. When the two sizes don't match, the film has to be assembled from smaller pieces, this process is known as stripping.

Imposition

When printing on large presses, your originals will often be smaller than the size of the sheets used in the presses. Strippers will try to fit as many pages will fit on a press sheet by assembling different pages together. When printing something like a magazine in which the proper assembly of the job is important, a stripper can arrange the sheets in order to minimize cutting and folding. This process in known as imposition.

Paper

The main material used in printing is paper. It is the canvas upon which all printing is done. because of this, the quality of your job can sometimes hinge on the proper selection of paper. There's a lot that can be accomplished with paper because it can carry it's own color, texture and absorbency properties.

Sheets

Choosing paper is usually done in very large sheets for offset printing. The paper has two sides and can carry coating on one or both sides.

Finish

Refers to the texture or smothness of the paper. Ranging from rough to smooth, it defines how the paper feels to the touch. Finishing can also be emossed (or letterpressed) into the sheet for a ribbed effect.

Coatings

There are many types of coatings that can be applied to the paper. The coating can make the inks better float on top of the sheets. From clay to enamel and other finishes can be applied. When one of the sides is coating, the coating is referred to as C1S (Coated One Side) versus both of the sides being coated (C2S).

Grain

Paper is generally made from wood pulp, cotton or recycled clothing (rags). The material is chemically broken down and then put back together using a screen that holds the material in place. The paper receives a grain or a direction that the material follows based on the screen and is important to keep in mind because folding against this direction can cause cracking. Printers or paper vendors are usually knowledgable about the direction of the grain for the paper they use.

Weights

Referred to as the basis weight and is calculated from the weight of a ream (500 sheets) of the basic size of the ream. Unfortunately, different sizes are used depending on the type of paper yielding different weights. Countries that use the metric standard weigh paper by grammage. The grammage is the weight of one sheet of paper cut to an area of one square meter and is measured by grams per square meter. You won't generally deal with paper in the use in this fashion. (Bond Types of papers

Coated

Best for high quality jobs, the coating on the paper helps the ink prevent from being absorbed and spread into the paper and dry mostly on the top of the paper itself, therefore yielding better colors and brightness. The coating can be glossy or dull.

Uncoated

Uncoated papers do not have any coating and will for the most part absorb the ink somewhat into the paper, causing it to spread. It is much cheaper than coated stock and good quality can be achieved depending on the smoothness of the paper. The smoother the paper, the better the ink will hold on the sheet. This type of paper is also known as text. It is available in many textures and colors (where coated paper is usually white).

Bond

Often used in stationary because it takes in well from inks (it is more difficult to write on coated surfaces). It is also the type of fancier paper you would see in resumes. The quality of the paper is usually associated with how much rag (the amount of recycled cloth or cotton) is in the paper, which is usually 25-50%.

Book

Used primarily for books (duh). The paper come in either antique or smooth finishes and many weights to accomodate different thicknesses of books. It's more opaque than bond paper and characterized by excellent folding qualities and durability.Offset
Similar to coated and uncoated sheets. They are especially suitable for offset printing because of their increased resistance to water and picking. Also more moist to the touch than a paper designe for photocopying.Cover
Generally refers to simply a thicker paper used for covers or thick cards.

Newsprint

Obviously used for newspapers. The sheets are not quite as white as other sheets and very absorbent to ink. Can't generally hold a lot of quality, but is very cheap. Not often used in other types of projects.Copying
Copying paper is designed to sustain more heat than other papers so that toners can attach to them when photocopying or laserprinting. If you use offset papers not designed for laser printing you'll notice they'll start to curl up after printing.

INKS

When drawing on a canvas, our next most important consideration is the types of inks we can use in the process. There is a variety of types of inks and processes we can use to enhance the look of our products.

Process inks

Refer to cyan, magenta, yellow, and black and also referred to as CMYK are used to create a wide gammut of colors.

Spot color

Any color other than process inks is referred to as spot color. When working with spot colors you will genrally used the Pantone Matching System (PMS), which allows a method for different printers to be able to match color on a consistent basis based on standard ink pigments and color formulats.

Hexachrome

A six-color process using CMYK plus Orange and Green inks which yields a greater color gamut.

Fluorescent inks

Can be combined with other inks or used by themselves to create a very bright look.

Metallic inks

Like flourescent inks can by itself or combined with other inks create a special metal look to your projects. The inks are opaque and carry suspended metallic particles in them (giving them their metallic sheen). They have a tendency to scratch or smudge easily, so they often will require you to add a varnish coating to them.

Pearlescent inks

Pearlescent inks have a shimmering effect which changes as the light hits it from different angles. The effect is similar to using overprint varnishes. However, they tend to loose their effect if the ink is too dark and perform best on a high quality gloss paper.

Trapping

Because printing deals with the interaction of physical objects which bring imperfections in terms of the placement of the inks, trapping is a process used to make the size of some areas of inks bigger than then should be so that when different colors are put on top of each other, they overlap ever so slightly.

Dot Gain

Whenever printing on paper, the dots you specify on a piece of film tend to spread once they hit the paper. With black, you'll see that the colors you specify tend to look darker than they should. Different papers, printers, processes will make your dots bigger at different rates so this is important to keep in mind.

FINISHING  

After the paper is printed, the job usually goes through some type of process, wether it's diecutting, coating, embossing, punching, gluing, etc. This process is known as finishing.Trimming
Usually the first item in the finishing process involves some cutting or trimminf of the peice. It can never be completely accurate so allowances have to be made when designing pieces. If you intend printing to go to the very edge of the sheet, it's a good idea to bleed or go past the edge of the sheet in your design by a .25 in so that when it gets imperfectly cut you won't see empty areas in some of your pieces.

Folding

Part of the finishing process involves folding, and in order to do it efficiently and cleanly probably involves high speed (dangerous) machinery. When folding you must learn to leave room for imperfections in the folding process. Some common folds include accordian, gate, roll, double, french, letter and single.

Scoring

When you are going to fold a very heavy or thick piece, scoring is a separate operation on letterpress equipment that makes it easier to fold a piece of thick paper.

Binding

Involves converting the book into some type of unified booklet form in which a number of sheets are attached through some process. Varieties include Saddle and Side stitching, Perfect (gluing and/or stitching to make it look like a book), Spiral and Comb bindings.

Foil Stamping

A foil surface can be applied with letterpress techniques that add character to the printed piece. Foils can also be holographic, tints or pigments.

Embossing

This is also something that can be done with a letterpress and involves raising part of the paper to create a special effect (think of how a notary public and other seals are applied). Embossing can be blind (without an image), foil (application of foil at the same time), or printed (embossing over previously printed work)

Die-cutting

Yet another variation of the letterpress. Instead of printing something onto a sheet, carefully designed knifes (sometimes lasers) are used to cut pieces of the sheet out.

Varnishing

A liquid is applied as coating for protection or special effects. The coating can be glossy or matte and those two are sometimes combined for special effects.

Aqueous Coating

Aqueous coating is a water-based protective coating that adds gloss and rub resistance to your product. Thermography
Thermography is a type of printing that uses a special powder to create a raised feel to the surface. The powder can take on the color of the underlying ink so it is usually used in combination with some other printing process.

blog comments powered by Disqus