Analysis of the time and humidity-dependent mechanical behaviour of paper webs at offset printing preess conditions

Abstract: This thesis deals with the time, frequency andhumidity-dependent mechanical behaviour of paper webs at offsetprinting press conditions. How the paper web, the set-up andrunning conditions alter press performance is not fullyunderstood, and are difficult to isolate and investigateexperimentally. Several important paper and press performancerelated issues are addressed; these should be relevant toproduct development and customer service in the graphical andpackaging cellulose fibre-based industries.The objective of this thesis is to develop a model, usingthe finite element method, in order to analyse time andhumidity-dependent stresses and strains of moving paper webs atoffset printing press conditions. In the literature, onlylimited multi-axial experimental data is available on therheological behaviour of paper in conditions relevant to offsetprinting. Investigated constitutive models from such data arefew, if any. Multi-axial offset printing press modelsincorporating time and moisture dependency of the paper web arenot generally found.The development includes the design of an experimentalset-up, in-plane rheological characterisation of paper,constitutive modelling and implementation in commercial FEMsoftware. A model with stable performance at arbitrarycombinations of the different boundary conditions, which areactivated as the web passes through the printing press, isrequired. Under this constraint, and with the ability tocapture the experimental observations, a constitutive law basedon hygro-viscoelastic material behaviour is proposed with somespecified limitations. In order to allow analysis of webstructures at stationary conditions in the chosen framework,the development of some approximate boundary conditions of theweb-fed printing press is also performed.Different analysed quantities from the numerical solutionsare linked to web tension, web widening, misregister inmulti-colour printing, sheet size analysis, conditions for webstability and traction conditions at drag rollers to exemplifythe applicability of the suggested model for runnabilityinvestigations.KEYWORDS:CREEP, DEFORMATION, DYNAMIC TEST,HYGROEXPANSION, MEASURING INSTRUMENTS, MECHANICAL PROPERTIES,PAPERBOARD, RELAXATION, RUNNABILITY, TRANSFER FUNCTIONS,VISCOELASTICITY