Unit 2 - Printing and Decoration Flashcards
What is white light?
White light is a mixture of all colours (visible wavelengths)
Why do objects appear certain colours
When light shines on an object, the objects absorbs some wavelengths and reflects others - for example, a pink object appears pink because it reflects pink wavelengths
What is colour perception?
Colour perception is a function of the way in which our brain interprets messages received from our eyes
What are retina rods responsible for?
Rods are responsible for vision in low light and give us black and white vision
What are retina cones responsible for?
Cones are active at higher light levels and give us colour vision
What colours do S, M, and L cones detect?
- S-cones detect blue
- M-cones detect green
- L-cones detect red
Name the 4 components of a colour
- Hue
- Saturation
- Brightness / lightness
- Value
Describe hue
Hue - this represents the dominant wavelength of the light emitted by the object, the colour’s position in the spectrum
Describe saturation
Saturation - how strongly coloured the object is, how vivid or dull the colour is
Describe brightness
Brightness / lightness - amount of surface reflectance
Describe value
Value - a colour’s progression to black
Give 5 things to consider when checking colour
- Specify a light course - usually a ‘North Light’ source
- Use similar sizes of samples
- Take account of substrate differences when developing products
- If transparent container, fill with the correct product to be used
- Make sure everyone evaluating colour is tested for colour-blindness
What is used to measure colour density?
Colour density is measured using a Densitometer
Describe the two ways of seeing colour
- Additive synthesis
- Subtractive synthesis
Describe additive synthesis
From a direct light source, we perceive the three primary colours - red, green, and blue
Describe subtractive synthesis
- When looking at an object, we are seeing the reflected light
- With reflected light, the main colours reflected are magenta, cyan, and yellow
Describe line work
- Solid blocks of colour of a single hue
- Each plate lays down one colour with no overlap
Describe continuous tone
One colour is used, so the image in generated by varying the amount of pigment in any one part of the image
Describe process printing
Achieved by combining primary colours in different proportions
Name and describe the 3 methods of process printing
- CMYK - can only match 30% of pantone colours and is not favoured for text and logos, so special colours are often used in an extended colour gamut
- Hexachrome - CMYK plus orange and green to achieve around 90% of pantone colours
- Pantone - international colour specification system
Why are RBG not used for printing?
RBG are not used as they cannot produce light colours, however CMYK can produce RBG colours
How are pictures achieved in CMYK printing?
- Pictures are achieving by printing dots in each of the four process colours and juxtaposition of the dots tricks the brain into seeing a range of shades
- Each colour has its own screen angle which produces characteristic ‘rosettes’
Give the screen angles of CMYK
C - 75/ 15
M - 15 / 75
Y - 0 / 90
K - 45
What determines the quality of illustration in CYK printing?
- Dot size determines the quality of the illustration - smaller dots produce finer details
- Dots are measured in DPI (dots per inch)
Why is dot shape important in CMYK printing?
- Round dots are prone to dot gain
- Elliptical dots are used for mid-tones and vignettes, but are susceptible to moire patterns
Name the 5 steps in preparation for printing
- Graphic design
- Colour separation
- Image assembly
- Plate-making
- Proofing
Name the 6 things to consider in image assembly for printing
- Unwind direction
- Folds and stepping
- Trapping
- Fingerprinting
- Screening
- Dot gain
Describe folds and stepping
- Folds and print areas are shown in a cutter guide
- Stepping is used to fit multiple packs into one reel or sheet of material
Describe what trapping is and why it is used
- Creation of small overlaps in print so no areas are left unprinted
Misregistration of print can be eliminated by a technique called trapping
Name and describe the two types of trapping
- A choke traps a light background to a dark foreground object
- A spread traps a light foreground onto a dark background
Describe fingerprinting
- All presses are different and it it is important to profile to achieve a consistent result
- Gives the printer a benchmark to work from; density reading; contract figure; dot gain; and provides consistency
Name and describe the 2 types of screening
- Half tone / amplitude modulated (AM) screening are where dots are placed in a grid, with darker tones being produced by larger dots
- Stochastic / frequency modulated (FM) screening uses a fixed dot size with random placement, with darker tones being obtained by printing more dots
Describe dot gain
Defined as the increase in the diameter of a halftone dot during the prepress and printing process - it is the difference between the dot size on the film negative and the corresponding printed dot size
Name 3 factors affecting dot gain
- Substrate type
- Printing pressure
- Ink viscosity
Describe how plates are made
Image is superimposed on to a light-sensitive medium which is then treated to create the final plate
Give 7 advantages of CTP (computer to plate)
- No films needed
- Less chance of handling the plate
- Less chance of misregistration
- Less contamination of the plate
- Faster
- Cheaper
- Produced a higher quality image
Name 3 methods of proofing
- Laser print
- Ink jet print
- On-press proof
Name the 4 basic theories of printing
- Relief printing
- Planographic printing
- Intaglio printing
- Stencil printing
Describe relief printing
Print area is raised above the body, so only the raised areas are inked
Describe planographic printing
Print area is level with body, but print area is sensitised to ink
Describe intaglio printing
Print area is recessed which is filled with ink, the impression is then ‘blotted out’ onto paper
Describe stencil printing
Print area is left open and the surround is blinded, the stencil is inked and the impression is squeezed through onto the substrate
Name 4 requirements of printing inks
- Must transfer from roller to plate to substrate
- Must adhere and dry to substrate
- Might be light, product, and rub resistant
- Must be non-toxic
Name the two components of inks and their purpose
- The vehicle - carrier resin / varnish system which transports and dries the pigment
- The pigment for colour
Name the two types of ink and state what they are used for
- Paste inks: high viscosity for litho, silk screen, and dry offset printing
- Liquid inks: lower viscosity for flexo and gravure
Name the 3 methods of drying ink
- Absorption
- Evaporation
- Oxidation
Describe ink curing and name 3 methods of doing it
Cross-linking to harden iks
- UV light
- Electron beam
- Infra-red light
Name 6 risks and hazards of printing inks
- Taint / odour
- Ink spills
- UV / IR varnish
- Poor print adhesion
- Debris in press
- Migration of inks or solvents
Name what 4 things in inks can cause taint or odour
- Solvents
- Pigments
- Resins
- UV photo-initiators such as benzophenones
Name 4 things that can be done in the event of poor print adhesion
- Check Dyne treatment of printing surface
- Check coating system
- Check clay coating for delamination if the substrate is paper-based
- Ensure correct Corona discharge if used
What print theory is flexography based on?
Relief
Describe flexographic printing
Ink is placed in a resevoir and picked up by an anilox roller
Doctor blade scrapes off excess ink from anilox
Anilox transfers ink onto raised surface of printing plate
Plate comes into contact with the substrate and transfers the image
Dried between each colour
What is the function of a flexo anilox roller and describe its structure
- The function of this is to hold a continuous film of ink before transferring it to the printing plates
- Has many small dimples which act as reservoirs for the ink
What 3 materials can anilox rollers be made from
- Steel
- Tungsten
- Carbide
What is the function of a doctor blade?
A doctor blade removes excess ink to improve print quality
What 2 methods can be used to dry flexo inks between stations?
Heat or UV light
What 4 materials is flexo used to print?
- Board
- Films
- Laminates
- Labels
Name the three types of flexo press
- Stack
- In-line
- Central impression (CI)
Describe a flexo stack press
Printing units are stacked vertically with each printing unit having its own impression cylinder
What type of substrates are mainly used on flexo stack presses and why?
Best suited to substrates that don’t stretch to avoid misregistration issues
Describe a flexo in-line press
Similar to a stack press, in a horizontal configuration so tension control must be used
Describe a flexo central impression press
Printing units all grouped around a common impression cylinder
Give an advantage and a disadvantage of a flexo central impression press
- Substrates do not stretch as they move around the cylinder so are good for printing on flexible plastics, and therefore give a good print registration
- Can only print on one side of the substrate
Give 3 ways in which you can identify if something has been printed using flexo
- Halo around the barcode numbers
- Misregistration
- Lack of image sharpness on corrugate
Describe letterpress printing
Image is created in reverse on a hard, raised surface
Ink is applied to raised areas only
Pressed against substrate with pressure to transfer image using an impression roll
Dried on drying stations
What print theory is letterpress based on?
Relief
What 2 things are letterpress plates made from?
- Metal
- Hard polymer
Describe offset letterpress printing
Image transferred to a central blanket, colour by colour (wet on wet) and the total design is transferred to the container but the colours to not overlap
What product is offset letterpress printing used for? Why?
Metal cans due to the excellent quality
What print theory is offset lithography based on?
Planographic
Describe offset lithography
- Planographic process where the printing surface remains flat but areas are sensitized to ink
- Image areas are treated to attract oil and repel water
- Plate cylinder accepts the ink and transfers this to the blanket cylinder, which then transfers the ink to the substrate - hence offset
- Ink application involves a train of rollers to meter an even amount of ink to plate
What drying methods are used during offset lithography printing
UV or IR
Give an advantage and disadvantage of offset lithography
Very accurate but not tolerant of uneven surfaces as the print plate is very smooth
What products is offset lithography used for?
Often used for board, paper, and metal containers
What print theory is gravure based on?
Intaglio
Describe gravure printing
- The image is engraved in reverse on the surface of a hollow metal cylinder and the image is then printed directly onto the substrate
- Recessed cells in the print roll make up the image and the doctor blade ensures that each cell is filled with ink
- An electrostatic charge is applied to ensure that all ink is transferred from the cells onto the substrate
Why is gravure print so accurate?
The depth of the engraved cells determines the ink weight and colour, resulting in very accurate control of ink application - engraved using a diamond stylus, a copper stylus, or a laser
Give 2 advantages of gravure printing
- High production speeds
- Very high quality
Give 4 disadvantages of gravure printing
- Not tolerant of rough or uneven surfaces
- High set-up costs
- Long cylinder lead times
- Volatile organic compound (VOC) emissions must be considered
Give three main uses of gravure print
- Laminates for flexible packaging
- Foil for lidding and blister packs
- Paper for bags and labels
Describe screen / stencil printing
- In the form of a fine polyester or nylon mesh screen
- Medium to high viscosity inks are used and the screen is flooded with the inks
- Ink is squeezed through the mesh using a rubber squeegee to form an image on the substrate beneath
- Different stations for each colour with inter-station drying between each colour
Give an advantage and a disadvantage of screen / stencil printing
- Process is slow
- Good for applying heavy coatings of inks
Give 3 uses of screen / stencil printing
- Reels of labels
- Sheets of paper or board
- Bottles and aerosol cans
What type of product is tampo / pad printing used for?
3D items
What type of inks does tampo / pad printing use?
Uses medium viscosity liquid inks
Inks are usually 2-component and polymerise completely over 24 hours
Describe tampo / pad printing
- An engraved metal printing plate is flooded with ink and a wiper blade removes the excess
- A flexible pad then picks up the inked image from the engraved cells and transfers the image to the substrate
- Inter-station drying
Describe digital printing
Design file sent to press
PIP (photo imaging plate) charged with a laser to create the image using electrostatics
ElectroInk is applied to the charged areas of the PIP
Image transferred from PIP to a blanket cylinder
Blanket cylinder presses the image onto the substrate
Heat and pressure used to dry and fix the inks
Name the two types of digital print press
Electrostatic
InkJet
Give 5 advantages of digital printing
- Fast set-up with no make-ready
- Few, if any, press adjustments required
- No colour variation during the run
- Reduced waste
- Suited to short, customized runs
Give 3 disadvantages of digital print
- Slower press speed
- Not as good quality as offset litho
- Cost of consumables is high
How do the edges of text appear in lithography, flexo & letterpress, gravure, and screen print?
- Lithography - smooth edges
- Flexo and rotary letterpress - ring or halo of ink
- Gravure - serrated edges
- Screen - fluffy edges
Name the 3 methods of finishing after print
- Varnish
- Embossing
- Foil blocking
Give 4 functions of a print varnish
- Improve optics (gloss or matt)
- Rub resistance
- Heat-sealing
- Moisture resistance
Describe wet on wet varnishing
Applied in-line after printing
Describe wet on dry varnishing
Applied off-line once the inks are dry
Where on a pack should you not apply varnish?
Areas that will be sealed, labelled, or date-coded
Why must you check inks are compatible with varnish?
Reduce bleeding and poor adhesion
Describe embossing after print is done
Carried out by a heated metal die during cutting and creasing
What products is embossing normall not used for?
Not usually used for PSL as pressing the label down will flatten the embossing
Describe how foil blocking after print is done
Printed with the design of the image, then foil is applied which adheres to the wet inks
How would you foil block a heat-sensitive substrate?
Cold foil-blocking can be used for heat-sensitive substrates
Give a disadvantage of using foil blocking
It is very expensive
Give 3 examples of variable information printed on a pack
- Barcodes
- Batch numbers
- Date codes
Name 4 methods of applying variable information to a pack
- Blind embossing / debossing
- Thermal transfer
- Inkjet printing
- Laser printing
Describe blind embossing / debossing for variable information
Metal type is pressed into direct contact with the surface with no ink used
Give 2 disadvantages of blind embossing / debossing for variable information
Restricted to simple information and only works on materials which can be easily deformed by pressure (e.g. board and plastic bottles)
Describe thermal transfer of variable information
- A type of digital printing in which a thermal head selectively heats a thermochromic ink to produce the necessary data
- Barcodes are often printed using this method as the data can be easily changed
Describe inkjet printing of variable information
- Non-contact process in which a special ink is dispersed into very fine droplets through a nozzle
- Commonly used for best before dates etc.
- If paper is to be inkjet coded, it is important to leave this area varnish-free on pack in order to achieve good ink adhesion
Describe laser printing for variable information
- Used on high-speed lines where the cost can be justified
- Laser etches the printed surface, leaving the label substrate exposed (such as on beer bottle labels)
What 2 barcodes do retail primary packs use and why?
- Primary packs in the retail environment use EAN-13 or EAN-8 bar codes
- They give fixed information which is not tailored to batches
What 3 barcodes do secondary packs use?
Secondary packs use EAN-13, ITF-14 or EAN-128 bar codes
What barcodes do tertiary packs use and why?
- Tertiary packs use EAN-128 bar codes
- They give variable information such as batch data
Name 7 criteria for printing a good barcode
- Check that number has not been used, ever or for at least the past three years
- Each variant has a unique number
- Secondary packs have different number to the primary packs
- Check barcode scans
- Check it fits dimension standards
- Transparent films must have a printed block colour behind the bars
- Only one visible on the pack (may need to use opaque if secondary packaging, for example)
