Critical Paper Properties for Digital Printing

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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Critical Paper Properties for Digital Printing