Roughness and Smoothness

Smithers Pira can use several test methods to assess the roughness or smoothness of paper to various international standards.

These test methods include lateral air leakage measurement and surface profilometry.

Air leakage methods

The most common method of testing smoothness or roughness in the paper and board sector is by one of three air leakage methods.

Bendtsen roughness is achieved by clamping the test piece between a flat glass plate and a circular metal head and measuring the rate of airflow in ml/minute between the paper and head. The Bendtsen technique is designed to work in the range 30-1500 ml/minute. Applicable test methods are BS 4420, ISO 8791/2, DIN 53108 and SCAN P21. Smithers Pira is UKAS, ISO 17025 accredited to BS 4420.

Bekk smoothness is again measured by the air leak method but, unlike the Bendtsen method, air is drawn across the surface of the test piece under a partial vacuum. Applicable standards are ISO 5627, Tappi T479 and DIN 53107.

Parker print-surf roughness - designed specifically for measuring the surface roughness of printing papers under simulated printing press conditions. The instrument contains an internal gas flow restrictor whose pressure drop versus flow characteristics is closely controlled. The air flow is calculated by comparing the pressure drop across the measuring head and the paper test surface with that across the flow restrictor. Applicable standards are BS 6563, ISO 8791/4, Tappi T555 and SCAN P76.

Profilometry Methods

There are two types of profilometry methods available; contact profilometry and non-contact profilometry. 

Contact Profilometry

Contact or stylus profilometers (such as Smithers Pira's portable Handysurf E-35B) move a loaded diamond probe across the surface and record the vertical movement of the stylus caused by the irregularities.

Several relevant standards have been published, including ISO 3274.

Non-contact profilometry:

Smithers Pira can analyse the three main elements of surface texture - roughness, waviness and form using a Proscan 2000A non-contact profilometer.

The software supports a large number of two dimensional roughness parameters - as detailed in the table below: 

Roughness Parameter Description
(ISO) Ra Roughness average
(DIN) Rz Mean peak to valley height
(ISO) Rz Ten point height
(ISO) Rmax Maximum peak to valley
(ISO) Rm Minimum valley depth
(ISO) Rq Square root of average of the square deviation of the scan
(ISO) Wt Waviness
(ISO) Nr Normalised peak count
(ISO) S Peak profile density
(ISO) D Peak count
(ISO) Sm Mean peak spacing
(ISO) R3z Vertical mean form third highest peak to third lowest valley