8/10/2019 Critical Paper Properties for Digital Printing
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Moisture| Curl| Resistivity| Porosity| Smoothness| Stiffness| Formation| Caliper
Basis Weight| Coefficient of Friction (COF)| Brightness| Opacity| Dimensional Stability
You can trust Hammermill Paper to run well and make your work look good. Why? Because our North American Testing
Center continually evaluates critical paper properties for all our Hammermill products. Thats how we can stand behind
the Hammermill 99.99% Jam-Free Guarantee. Here are just some of the many paper properties we continually monitor
and test.
1. Moisture:Amount of water content in sheet.
Too High
Poor toner adhesion
Blotchy look/print mottle
Image deletions
Post-image curl
Too Low
Multi-feeds/misfeeds due to static build-up
Image deletions due to poor toner transfer
Toner scatter
Digi-know?Papers engineered for offset presses typically contain up to 6% moisture. Reprographic papers are engineered
for digital presses and typically contain 4.5% moisture. In general, papers with a high moisture content (over 5%) show a
high propensity for curl in digital equipment so using paper formulated for digital presses means fewer issues.
2. Curl:When a sheet takes on a curved quality, losing its "flatness."
Too Much Manufactured Curl
Feeding problems
Transport problems
Post-image curl
Too Much Post-Image Curl
Transport problems
Delivery (roll-over jams) and post-image finishing
issues
Duplex (second side) jamming
Digi-know?Paper tends to curl toward the heat of the fuser. Paper that's manufactured with a slight built-in curl away from
the heated side reduces the overall post-image curl.
3. Resistivity:Ability to hold a static charge.
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Too High
Image distortion because the charge may pull toner where it
does not belong.
Too Low
Poor or incomplete toner transfer, because the
charge dissipates from the paper too quickly to
complete the toner transfer completely
Digi-know?Paper is manufactured to the ideal moisture content, and should be kept in the ream wrap to protect it from
humidity. Resistivity is greatly influenced by the sheet moisture.
4. Porosity:Characteristic of paper that allows liquids or vapors to seep through it.
More Porous
Misfeeds in vacuum-fed systems
Multi-feeds
Less Porous
Less likely to have misfeeds or multi-feeds.
Digi-know?The terms "porosity" and "density" are often used interchangeably, but they're actually opposites. A moreporous sheet has low density (open formation); a less porous sheet has high density (tight formation).
5. Smoothness:The evenness of the surface of the paper.
Too Slick
Feed and transport problems (sheet slipping in paper path)
Too Rough
Poor toner adhesion; toner incompletely transferring
and fusing to sheet
Toner voids and uneven lay-down (mottle)
Digi-know?High resolution laser technology requires an extra-smooth sheet for even print in large-area screens and solids.
This keeps toner from getting lost in "hills and valleys," thereby reducing print and gloss mottle. Smoothness is generally
measured on the Sheffield scale, with lower numbers (30 to 125) representing smoother surfaces.
6. Stiffness:Force required to bend paper through a specified angle.
Too Stiff
Limits the sheet's ability to negotiate tight bends in the paper
path.
Not Stiff Enough
Sheet buckling/rolling over in the paper path
Inability to accommodate finishing
Limp feel
Digi-know?Stiffness can vary from machine direction (MD) to cross-machine direction (CMD) and is typically stiffer across
the grain due to the resistance of the paper fibers to bend across themselves.
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7. Formation:Uniformity of the sheet.
Poor Formation
Image deletions due to cockling of the sheet
Print mottle
Increased show-through
Uneven toner transfer Limp feel
Good Formation
Even toner lay-down
Reduced print mottle
Visually pleasing
Digi-know?Hold a sheet up to the light and look through it. If you see big fluffy "clouds," chances are solid-coverage areas
will not look uniform. This can also contribute to show-through in areas where there is lower or uneven fiber distribution in
the sheet.
8. Caliper:Thickness of a sheet.
Too High
Poor toner adhesion
Transport jams
Smeared images due to toner disruption before fusing
Too Low
Multiple feeds
Limp feel
Digi-know?Extreme caliper variations in a sheet can cause print quality issues due to varying degrees of toner transfer.
Caliper variations within a single sheet can cause image deletions or a mottled appearance.
9. Basis weight:Weight in pounds of 500 sheets cut to a given standard size for the paper type.
Equipment Setting Is Lighter Than Actual Sheet Weight
Toner ghosting and deposits in unimaged areas, due to toner
flaking off sheet during paper transport and contaminating
internal press parts
Solids may look dull and muted, due to incomplete toner
melting
Poor toner adhesion toner flakes or rubs off the sheet
after imaging
Equipment Setting Is Heavier Than Actual Sheet
Weight
Shortened lifespan of some press parts
Increased toner gloss due to extra heat and/or
pressure applied to the sheet during fusing of the toner
Digi-know?Color and solid images look better on heavier basis weights. Normally 24
28 lb bond is recommended toeliminate sheet cockle, because toner shrinks as it cools. (Typically, toner is a polymer made from plastic.)
10. Coefficient of friction (COF): Force needed to start a sheet feeding, as well as drag a sheet across theone underneath it as it continues to feed.
Too High Too Low
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