Photo papers and digital printing

Question 1

What is the difference between thermal and piezo print heads? Name advantages and disadvantages!

Answer 1

The thermal print head uses heat whereas the piezo uses electricity. In the piezo electricity causes charged crystals to expand and the thermal causes air bubble to form.
Thermal:
+ Less expensive
- Limited inks usable
- Limitations on ink properties

Piezo
+ wide range of inks
+ better resolution
+ higher drop rate
- Limited number of nozzles and higher cost per nozzle

Question 2

Describe the difference between swellable and microporous ink jet coatings!

Answer 2

Swellable technology has a gel like coating which absorbs ink by swelling, the ink stays on top of the coating and results in very vivid colour, however it takes a long time to dry. Microporous coating dries much faster as the ink particles are rapidly absorbed via capillary forces into micropores. The colour isn’t as vivid but it has excellent line sharpness.

Question 3

Which materials are used in the coating and what are the advantages and disadvantages of each type? Swellable coating vs microporous technology

Answer 3

Swellable coatings use water soluble polymers such as CMC, PVA and starch.
Microporous coatings contain pigments such as silica. Clay, calcium carbonate and binders such as PVA and latex.
Swellable coatings produce very vivid colour with good dusting but take a long time to dry. Microporous are very quick drying and have great line sharpness but are more prone to dusting.

Question 4

 What are the different phases when printing on an ink jet paper?

Answer 4

Printing
Wetting
Absorption
Fixation
Drying

Question 5

 Which important properties of an ink jet paper do you know? Name at least five properties!

Answer 5

ink absorption and drying time
surface smoothness – allow for even distribution of ink
brightness and whiteness – to allow better colour reproduction, vividity, and contrast
thickness – this will affect the bleed through of inks
coating type – eg whether it is swellable or microporous, this will change the drying times and line sharpness etc
Water/light/humidity resistance
Colour/gloss/curl/waviness

Question 6

 How does a typical matt inkjet coating look like? Name the different components(7), their functions and an example.

Answer 6

Matt inkjet paper does not look as smooth compared to glossy coatings. It appears to be formed of discrete particles which gives it a more rough texture.
Pigment – water absorption – silica
Binder – pigment binding – PVA
Cobinder – Rheology – Latex
Fixing agent – ink fixation – P-DADMAC
Optical brightening agent (OBA) – brightness – stilbene derivative
Dyes – brightness – blue dye
Defoamer – foam reduction – fatty acid

Question 7

 Please describe the principle of a extrusion coater! What are the key elements of an extrusion coater and what polymers can be applied?

Answer 7

Extrusion coating is a process to apply a thin layer of polymer onto paper. The polymeris extruded in molten form, this is coated in an even layer on the moving substrate it bonds to the surface. As it passes over the chill roll the coating sets and hardens.
Polymers used include LDPE/HDPE, these can have various colorants and conductive formulations.
Can also use poly propylene (PP) or EVA.
Key elements are the Extruder
Extruder die – controls speed and thickness
Pressure roll and chill roll