1.What are Half Tones?

Halftones is a reprographic technique that uses of dots of varying size or spacing to make a continuous tone imagery, creating  a gradient-like effect. Halftone is also sometimes refer to the image that is produced by this process. Where continuous tone imagery contains an infinite range of colors or greys, halftones reproduces an image with only one color of ink,which is in dots of differing size or spacing. This reproduction uses basic optical illusion of the human eye by blending halftone dots into smooth tones.This means that at a microscopic level, developed black-and-white photographic film also consists only of two colors, and not an infinite range of continuous tones. Just like color photography with filters and film layers, it is possible to do color printing by repeating the halftone process for each type of color the most common types of color use are called the CMYK color model. The translucent property of ink allows for halftone dots of different colors to create another optical effect full color imagery.

               Halftones                         Continous tones   Halftones with colour    

The resolution of a halftone screen is in lines per inch the suffix is either lpi or hash(#). It is in measured parallel with the screen's angle known as the screen ruling. The higher the pixel resolution the greater the detail. However,increasing also requires a corresponding increase in screen ruling or the output will suffer from posterization which is when a image tone abruptly changes from one tone to another. Thus, file resolution is matched to the output resolution.

These are some of the typical halftone resolutions.

  • screen printing : 45-65 lpi
  • laser printing (300dpi) : 65lpi
  • Laser printer (600dpi) : 85–105 lpi
  • Offset press (newsprint paper) : 85 lpi
  • Offset press (coated paper) : 85–185 lpi

These are some of the typical screen angles(in degrees) for printing CMYK

  • C=15,  M=75, Y=0, K=45
  • C=15,  M=45, Y=0, K=75
  • C=105, M=75, Y=90, K=15
  • C=165, M=45, Y=90, K=105
2.What is CMKY?

The CMYK color model is a subtractive color model which means that it can be mixed to create a wider range of colors, used in color printing. CMYK refers to the four inks used in some color printing: cyan, magenta, yellow, and key (black). However, it varies by print house, press operator, press manufacturer, and press run, ink is typically applied in the order of the abbreviation.

The "K" in CMYK stands for key because the cyan, magenta, and yellow color printing plates are carefully keyed, or aligned, with the key of the black key plate. However there are some who suggest that "K" in CMYK actually comes from the last letter in "black" and was chosen because B already blue.But, some will disagree with this because C for Cyan is classed as the blue when printing in CMYK format. Some sources do claim that K comes only from "Key" because black is often used as outline and printed first.

The CMYK model works by partially or entirely masking colors on a lighter or white background. The light that is normally reflected is reduced by the ink produce. This model is called subtractive because inks "subtract" brightness from white.

The additive color models, such as RGB, white produce through the combination of all primary colored lights, while black is the absence of light/color. In the CMYK model, it's the opposite:  where white is the natural color of the paper or background, while black results from a full combination of colored inks. By saving cost on ink, and producing deeper black tones, unsaturated and dark colors are produced by using black ink instead of the combination of cyan, magenta, and yellow.

The black color generated by mixing commercially practical cyan, magenta, and yellow inks is unsatisfactory, so four-color printing uses black ink in addition to the subtractive primaries. some of the reasons for using black ink are:

  1. In traditional preparation of color separations, a red keyline on the black line art marked the outline of solid or tint color areas. While in other cases a black keyline can be served as both a color indicator and an outline to be printed in black. Because usually the black plate contained the keyline, the K in CMYK represents the keyline or black plate, which is sometimes called the key plate.
  2. Text is typically printed in black and includes fine detail, so to reproduce text or other finely detailed outlines, without slight blurring,while using three inks would require impractical accuracy
  3. Using a combination of 100% of the three inks soaks the paper with ink, making it slower to dry, causing bleeding, or weakening the paper so much that it tears especially on cheap paper like newspaper.
  4. Although in thoery using a combination of 100% cyan, magenta, and yellow inks should completely absorb the entire visible spectrum of light and produce a perfect black, however in practice the inks fall short of their ideal characteristics and the result is actually a dark muddy color that does not quite appear black. Adding black ink absorbs more light and yields much better blacks.
  5. Using black ink is cheaper than using the corresponding amounts of colored inks.

When a very dark area is need, a colored or gray CMY "bedding" is applied first, followed by a full black layer, making a rich, deep black; this is called rich black. A black made with just CMY inks is sometimes called a composite black. The amount of black to use to replace amounts of the other ink is variable, and the choice depends on the technology, paper and ink in use. Processes called under color removal, under color addition, and gray component replacement are used to decide on the final mix; different CMYK recipes will be used depending on the printing task.

3.What is the difference between negative and positive film

A positive image is a normal image. While a negative image is a total inversion, in which light areas appear dark and vice versa and, a negative color image is also color-reversed, with red areas appearing cyan, greens appearing magenta and blues appearing yellow, and vice versa. Negative films usually have less contrast, but a wider dynamic range. The contrast can increases when printed onto photographic paper. A negative film images contrast can be adjusted during the time of scanning or, more usually, during subsequent post-processing. Despite the market's evolution away from film, there is still a desire and market for products which allow fine art photographers to produce negatives from digital images for their use in alternative processes such as cyanotypes, gum bichromate, platinum prints, and many others.

Cross processing is the deliberate processing of photographic film in a chemical solution. Color cross processed photographs often have unnatural colors and high contrast. The results of cross processing differ from case to case, as the results are determined by many factors such as the make and type of the film used, the amount of light exposed onto the film and the chemical used to develop the film. Similar effects can also be achieved with digital filter effects.

Cross processing usually involves one of the two following methods.
Processing positive color reversal film in C-41 chemicals, resulting in a negative image on a colorless base.
Processing negative color print film in E-6 chemicals, resulting in a positive image but with the orange base of a normally processed color negative.
Cross processing can also take other forms, such as negative color print film or positive color reversal film in black and white developer.
Other interesting effects can be obtained by bleaching color films processed in black and white chemistry using a mixture of  solutions. when re-exposed to light and re-processed in their intended color chemistry, subtle, relatively low contrast, pastel effects are obtained.
Cross processing effects can be simulated in digital photography. However, these digital tools lack the unpredictable nature of regular cross processed images.

4.What is silkscreen printing?

Screen printing is a printing method where a mesh is used to transfer ink onto a substrate, except in areas made impermeable to the ink by a blocking stencil.Prior to the invention of polyester mesh silk was used in the process. A blade or squeegee is moved across the screen to fill the open mesh apertures with ink, then a reverse stroke causes the screen to touch the substrate momentarily. This causes the ink to wet the substrate and be pulled out of the mesh apertures as the screen springs back after the blade has passed.

Silkscreen printing technique uses a screen is made of a piece of mesh stretched over a frame. The mesh could be made of a synthetic polymer, such as nylon, for a higher and more delicate degree of detail a finer and smaller aperture for the mesh will be used. The frame which holds the mesh could be made of diverse materials, such as wood or aluminum, depending on the sophistication of the machine. The tension of the mesh may be checked by using a tensiometer which is measured in Newton per centimeter. A stencil is formed by blocking off parts of the screen in the negative image of the design to be printed.

Before printing occurs, an emulsion is 'scooped' across the mesh and the 'exposure unit' burns away the unnecessary emulsion leaving behind a clean area in the mesh with the identical shape as the desired image. The surface to be printed on also known as a pallet is coated with a wide 'pallet tape'. This serves to protect the 'pallet' from any unwanted ink leaking through the screen and potentially staining the 'pallet' or transferring unwanted ink onto the next substrate. After that the screen and frame are lined with a tape. The type of tape used depends upon the ink that is to be printed onto the substrate. These aggressive tapes are generally used for UV and water-based inks due to the inks' lower viscosities. The last process is blocking out any unwanted 'pin-holes' in the emulsion. If these holes are left in the emulsion, the ink will continue through and leave unwanted marks. To block out these holes, materials such as tapes, speciality emulsions and 'block-out pens' may be used effectively.

Ink is placed on top of the screen and the screen is place on top of the substrate. A floodbar is used to push the ink through the holes in the mesh. The operator starts at the rear of the screen and behind a reservoir of ink with the fill bar. The operator lifts the screen to prevent contact with the substrate and then using a slight amount of downward force pulls the fill bar to the front of the screen. This effectively fills the mesh openings with ink and moves the ink reservoir to the front of the screen. The operator then uses a squeegee (rubber blade) to move the mesh down to the substrate and pushes the squeegee to the rear of the screen. The ink that is in the mesh opening is pumped or squeezed by capillary action to the substrate in a controlled and prescribed amount. As the squeegee moves toward the rear of the screen the tension of the mesh pulls the mesh up away from the substrate leaving the ink upon the substrate surface.

There are three common types of screen printing presses. Flat-bed, cylinder and rotary. Textile items printed with multicoloured designs often use a wet on wet technique, or colours dried while on the press, while graphic items are allowed to dry between colours that are then printed with another screen and often in a different colour after the product is re-aligned on the press. Most screens are ready for re-coating at this stage, but sometimes screens will have to undergo a further step in the reclaiming process called dehazing. This step removes haze left behind in the screen once the emulsion has been removed. The haze tend to faintly outline the open areas of previous stencils. They are the result of ink residue trapped in the mesh, often in the points where threads cross. This technique is used on tens of thousands of items, including decals, clock and watch faces, balloons, and many other products. The technique has even been adapted for more advanced uses, such as laying down conductors and resistors in multi-layer circuits using thin ceramic layers as the substrate.

5.what materials are use for indoor and outdoor printing?

Some of the materials use in indoor printing are Polypropylene or PP for short.

while some of the materials use for outdoor printing are Polyvinyl chloride (PVC) and Canvas

Polypropylene (PP) is a thermoplastic polymer used many different types of applications. It is an addition polymer made from the monomer propylene, it can be produced in a variety of structures giving rise to a variety of applications including packaging and labeling, textiles, plastic parts and reusable containers of various types, laboratory equipment, automotive components, and medical devices. It is a white, mechanically rugged, and resistant to many chemical solvents, bases and acids. PP can be made translucent when uncolored but is not as readily made transparent as polystyrene, acrylic, or certain other plastics. It is often opaque or colored using pigments and polypropylene can be resistant to fatigue if made with a specific molecular arrangement. This material is often chosen for its resistance to corrosion and chemical leaching, its resilience against most forms of physical damage, including impact and freezing, its environmental benefits, and its ability to be joined by heat fusion rather than gluing.

Another common application for polypropylene is as biaxially oriented polypropylene (BOPP). These BOPP sheets are used to make a wide variety of materials including clear bags. When polypropylene is biaxially oriented, it becomes crystal clear and serves as an excellent packaging material for artistic and retail products. Polypropylene is also highly colorfast, is widely used in manufacturing carpets, rugs and mats to be used at home. Polypropylene is also light enough to float in water

 
 

Polypropylene when compared to polyvinyl chloride (PVC) emits less smoke and no toxic halogens in low-ventilation environment such as indoors. Polypropylene can also be use in particular roofing membranes as the waterproofing top layer of single-ply systems as opposed to modified-bit systems. Polypropylene is most commonly used for plastic moldings like bottle tops, bottles, and fittings, wherein it is injected into a mold while molten, forming complex shapes at relatively low cost and high volume. It can also be produced in sheet form, widely used for the production of stationery folders, packaging, and storage boxes. The wide color range, durability, low cost, and resistance to dirt make it ideal as a protective cover for papers and other materials. It is used in Rubik's Cube stickers because of these characteristics. Due to the availability of sheet polypropylene, designers can take the opportunity to use the material to create elaborate designs. Polypropylene sheets can also be made into card pockets. Expanded polypropylene (EPP) is a foam form of polypropylene. EPP has low stiffness allowing it to resume its shape after imapct. Polypropylene is also recyclable and has the number "5" as its resin identification code

Polyvinyl chloride also known as PVC, is the world's third-most widely produced synthetic plastic polymer, after polyethylene and polypropylene.

There are two basic forms for PVC rigid or RPVC and a more softer version.  The rigid form of PVC is used in construction for pipe and in profile applications such as doors and windows. It is also used for bottles, other non-food packaging, and cards. There softer and more flexible form of PVC can be used being phthalates, plumbing, electrical cable insulation, imitation leather, signage, phonograph records, inflatable products, and many applications where it replaces rubber.

PVC is formed in flat sheets in a variety of thicknesses and colors. As flat sheets, PVC is often use to create voids in the interior of the material, providing additional thickness without additional weight and minimal extra cost. Plasticized PVC is also used to produce thin, colored, or clear, adhesive-backed films referred to simply as vinyl. Which can be use to produce a wide variety of commercial signage products, including car body stripes and stickers.

Canvas is an extremely durable plain-woven fabric used for making sails, tents, marquees, backpacks, and other items for which sturdiness is required. Modern canvas is usually made of cotton or linen. Canvas comes in two basic types: plain and duck. The threads in duck canvas are more tightly woven. A canvas print is the result of an image printed onto canvas which is stretched, or gallery-wrapped, onto a frame and displayed. Canvas prints are often used in interior design, with stock images, or customised with personal photographs. Canvas prints are intended to reproduce the look of original oil or acrylic paintings on stretched canvas. Canvas prints are commonly used in home decor, either chosen by professional interior designers, or by the homeowner. Canvas prints can be mass produced and available through high street retailers and home improvement stores, such as IKEA, or personalized one-off canvas prints produced from the individual's own photograph, or drawing, usually uploaded via the Internet, or ordered direct from social media websites. Canvas prints are often used as a cheaper alternative to framed artwork as there is no glazing required and the pine frame is not usually visible, so do not need to be varnished, or treated.

6.Difference between dye based and solvent based inks?

Ink is a liquid or paste that contains pigments or dyes and is used to color a surface to produce an image, text, or design. Ink is a complex medium, composed of solvents, pigments, dyes, resins, lubricants, solubilizers, surfactants, particulate matter, fluorescents, and other materials. The components of inks serve a variety of purposes; the ink's carrier, colorants, and other additives affect the flow and thickness of the ink and its dry appearance.

Dye-based inks can produce much more color of a given density per unit of mass when compare to other types of ink for example pigment ink. However, because dyes are dissolved in the liquid phase, they have a tendency to soak into paper, making the ink less efficient and potentially allowing the ink to bleed at the edges of an image. To overcome this weakness, dye-based inks are made with solvents that dry rapidly or are used with quick-drying methods of printing, such as blowing hot air on the fresh print. Other methods include harder paper sizing and more specialized paper coatings. The paper coating is particularly suited to inks used in non-industrial settings, such as inkjet printer inks. Other technique include coating the paper with a charged coating. If the dye has the opposite charge, it is attracted to and retained by this coating, while the solvent soaks into the paper. Cellulose, the wood-derived material most paper is made of, is naturally charged, and so a compound that complexes with both the dye and the paper's surface aids retention at the surface.

An additional advantage of dye-based ink systems is that the dye molecules can interact with other ink ingredients, potentially allowing greater optical brighteners and color-enhancing agents designed to increase the intensity and appearance of dyes.

In recent developments, dye-based inks can react with cellulose to permanently color the paper. Making it unaffected by water, alcohol, and other solvents.  As such, their use is recommended to prevent frauds that involve removing signatures, such as check washing. This kind of ink is most commonly found in gel inks and in certain fountain pen inks

While a solvent dye is a dye soluble in organic solvents. It is usually used as a solution in an organic solvent.

Solvent dyes are used to color organic solvents, hydrocarbon fuels, waxes, lubricants, plastics, and other hydrocarbon-based nonpolar materials. Fuel dyes are one use of solvent dyes. Their molecules are typically nonpolar or little polar, and they do not undergo ionization. They are insoluble in water. They form a colloidal solution in solvents.They have poor to good light fastness.

Solvent dyes are used for gold imitation of metallized polyester films. Also used in marking inks, inkjet inks, glass coloration, and so on.

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