Cobb Sizing Tester for Paper

What is the Cobb Sizing Tester for Paper?

A Cobb sizing tester measures the mass of water absorbed by a paper specimen exposed to water over a defined area and time. The Cobb value indicates sizing quality and resistance to water penetration for printing and packaging grades.

Overview

A ring clamp holds the specimen flat and exposes a defined area, usually 100 square centimetres, to a measured volume of water for a set time, commonly 60 seconds but also 30, 120 or 180 seconds for special grades. The water is then poured off, the surface blotted with a roller under a defined pressure, and the specimen reweighed. The mass gain divided by the exposed area gives the Cobb value in grams per square metre. Results depend strongly on contact time, water temperature, blotting pressure and the number of blotting papers, so these must be controlled exactly, and testing single-sided grades requires the ring to be applied to the treated side. Water temperature is normally controlled to the standard value, since absorption increases markedly with temperature.

How does it work?

Mechanical paper tests apply a controlled force or deformation to a conditioned specimen and record the load or the extent of damage. Tensile testing pulls a strip at constant rate and records breaking force, elongation and tensile energy absorption, from which indices are calculated using basis weight. Tearing resistance is measured by the energy absorbed when a specimen with a pre-cut slit is torn by a pendulum or a falling weight. Folding endurance counts the number of double folds a strip withstands under tension before it breaks. Surface and absorbency tests use different principles: water absorbency is determined by the mass of water taken up by a defined area during a timed contact, smoothness by the air flow between a measuring ring and the sheet surface, and porosity by the air flow through the sheet itself at a defined pressure difference.

What are the key features?

• Ring clamp exposing 100 cm² of specimen
• Controlled contact time from 30 to 180 s
• Water temperature control
• Blotting roller with defined pressure
• Balance resolution for accurate mass gain
• Separate access for single-sided grades

What are the technical specifications?

A typical cobb sizing tester for paper configuration offers test area of 100 cm², water volume of 100 mL, contact time of 60 s standard, 30–180 s options, water temperature of Controlled, typically 23 °C, blotting pressure of Defined roller load and balance resolution of 0.001 g. Values are indicative of the product class; the exact figures for each configuration are confirmed on the quotation and the calibration documents supplied with the instrument.

Which applications is it used for?

The Cobb Sizing Tester for Paper is used in sizing quality of writing and printing papers, water resistance of packaging and board grades, coated and surface-treated paper verification and incoming inspection of liner and carton board. Across the wider Paper Quality Analysis class, laboratories also deploy it for tensile strength and elongation of packaging papers, tearing resistance of sack, tissue and bound papers and water absorbency of sizing and coating grades.

Which standards and certifications apply?

Relevant standards and methods include ISO 1924-2, ISO 1974, ISO 535, ISO 8791-2, ISO 5626, ASTM D828, TAPPI T494, TAPPI T414, TAPPI T441 and IEC 61010-1. Certification candidates for this model are CE and ISO 9001 manufacturing; each is to be confirmed for the specific configuration before shipment, and certificates are supplied only where they have been issued.

Condition specimens to the standard atmosphere, usually 23 °C and 50% relative humidity, before every mechanical test, because strength properties change sharply with moisture. Cut specimens with sharp dies or blades and reject those with damaged edges, since a nick initiates premature failure. Keep grips clean and aligned, and verify load cells with certified masses or a reference load cell at defined intervals. For absorbency and surface tests, control the water temperature, contact time and pressure difference, and clean measuring rings and plates, because deposits change the flow. Record basis weight, test direction, specimen count and conditioning with every result so that indices can be recalculated.

How do I select the right Cobb Sizing Tester for Paper?

Specify a Cobb tester when the specification is expressed as water absorbency in grams per square metre, and confirm the contact time and test area required by your standard. Check the water temperature control and the blotting procedure, since both change the result more than most operators expect, and use a balance of 0.001 g resolution.

How does it compare with other Paper Quality Analysis variants?

Select the test that corresponds to the specification you must meet and to the failure mode you are investigating. Tensile testing is fundamental for packaging, bag and label papers because it defines strength in the principal load direction. Tearing resistance is important for papers that are handled, punched or bound, and for tissue and sack grades. Internal bond, folding endurance and tensile energy absorption matter for papers that are creased or repeatedly flexed. Water absorbency is specified for papers that are glued, coated or used outdoors, while smoothness and porosity are specified where print quality and ink penetration matter. Each instrument should be capable of the exact conditions named in the standard, since test length, strain rate and contact area change results appreciably.