Downgauging Board Caliper through High Bulk Furnish Chemistry Mechanics
High-bulk BCTMP cores maintain boxboard bending stiffness during caliper downgauging while chemical wet-end additives prevent delamination under score creasing.
TAPPI T541
Strength Protocol
Standardized testing methods quantify the internal z-directional tensile strength of paper, paperboard and multi-ply packaging substrates under perpendicular mechanical loads. Formulated by the Technical Association of the Pulp and Paper Industry, tappi t541 establishes the standard laboratory procedure for measuring the internal delamination resistance of cellulosic sheets. The protocol defines the double-sided adhesive tape mounting systems, tensile platen dimensions, compression dwell times and separation speeds required to induce internal sheet failure rather than surface peeling.
This testing method applies to multi-ply paperboard, solid bleached boards, uncoated papers and barrier-coated cartons, ending its applicability when applied to lightweight tissue papers or rigid corrugated structures.
Testing Procedure
Standard testing requires conditioning paperboard specimens at standard laboratory atmospheric conditions before mounting. Square or circular specimens are bonded between two rigid, precision-ground aluminium test blocks using high-adhesion double-sided pressure-sensitive tape. A pneumatic or mechanical clamping unit applies an initial compressive seating pressure of approximately one megapascal for a specified dwell time to establish total adhesion contact between the tape and the substrate without pre-crushing internal fibres.
The mounted assembly is placed in a universal tensile tester that pulls the two aluminium blocks apart in a direction precisely normal to the plane of the sheet at a constant separation velocity. Internal bond strength is calculated by dividing the maximum tensile force recorded at structural rupture by the cross-sectional surface area of the test specimen. Visual examination of the fractured surfaces verifies whether failure occurred internally within the fibrous core or externally through adhesive tape release.
Converting Criterion
Offset lithographic printing inks exert significant tack forces that pull on the sheet surface when exiting the impression nip at high press speeds. If internal z-directional strength fails to exceed operational thresholds, the tacky printing ink delaminates the board, stripping away the top ply and causing immediate press contamination. Multi-ply folding boxboards depend on strong internal bonding to withstand mechanical creasing and high-speed folding without forming delamination blisters along folded box edges.
In blister packaging applications, high internal strength ensures that the paperboard backing ruptures cleanly within the fibre layer during opening rather than suffering premature adhesive interface failure. Testing according to this standard provides the foundational mechanical strength data required to verify the converting reliability of folding cartons and coated packaging boards.
Grammage and Caliper Variation Principles in Multi-Ply Packaging Board Specifications
Grammage and caliper specifications require ISO test method conditioning standards and tight bulk tolerance clauses to prevent converting line failures and yield losses.