Technical Information dispelling traditions - Formica

formica_technical.qxd:formica_technical.qxd 28/11/08 16:01 Page 10Pre-conditioningCounter Veneering and Board Flatness 1 of 2The most important factor in achievingstability in bonded panels is thepre-conditioning of core materials,surfacing and backing laminatesprior to bonding.Pre-conditioning ensures that the effectsof differential movement, caused by thematerials’ reaction to changes in relativehumidity, are minimised.The following procedure will allow thelaminates to reach equilibrium; anysubsequent movement, caused bychanges in humidity, will then be equalon each side of the bonded panel andthe risk of bowing will be greatly reduced.Decorative laminates and core materialsshould be conditioned before veneeringso that all materials reach equilibriumand are neither too dry nor too damp,the latter being most important at the timeof pressing. Optimum conditions are bestachieved in a dry storage area (about20°C and 50-60% Relative Air Humidity).The sheets that will form the oppositefaces of the same composite board arebest conditioned as a pair, with theirsanded backs together. Sheets paired inthis manner should be stacked, covered,and left for a minimum period of threedays in order to reach moistureequilibrium. This will ensure that theyachieve near identical moisture contentsprior to bonding, and any subsequentdimensional movements will thereforebe similar in both magnitude and directionon each side of the composite panel.Wood-based core materials should havea moisture content of around 9%. Themoisture content of laminates cannot bemeasured with a normal moisture meter,but it is essential that the face laminatehas the same moisture content as thecorresponding backing board.If the composite boards are to be exposedto constant low relative humidity in theirsubsequent application (e.g. radiatorcasings), the laminates and core materialsshould be pre-conditioned in warm dryconditions for a suitable period in order topre-shrink the materials and so avoid anysubsequent shrinkage stresses.Panels and boards faced with decorativelaminate will nearly always be required tohave the reverse side faced with a similarmaterial to counter-balance the effects ofdimensional changes that may take place.Optimum flatnessBest results are obtained by the use ofidentical laminates simultaneously bondedto both face and back, a practice whichmight appear unnecessarily expensive,but is entirely rational when the extra costis related to the degree of flatness requiredand the final ‘installed price’ of the panel.This method should always be consideredfor large free standing panels such asscreens, balustrades, cubicles, slidingdoors, thin ceiling panels, shelving, etc.BS 4965 flatness requirementsThe majority of installations have somewhatless demanding flatness requirements andare usually satisfied by the criteria specifiedin BS 4965, viz. 1mm maximum distortionover 600mm length.Two important clauses are contained inBS 4965 relating to flatness limits:1. Because of the difficulty of achievingflatness in thin panels, no tolerance isspecified for boards or panels usingsubstrates less than 12mm thick.2. Numerous factors, including changesin temperature and relative humidity (ascan be encountered at building sites)can cause boards and panels to bowand twist irreversibly.Therefore flatness to BS 4965requirements can be guaranteedonly at the time of delivery from thefabricator. This is particularly applicableif opposite sides of a panel are likelyto be exposed to marked temperatureand/or humidity differences.Other requirementsLinings to kitchen cabinets and inside facesto cupboard doors are required to have anattractive easily cleanable surface. Whilstthe flatness of these components mustbe considered, their overall dimensionsare relatively small and experience hasshown that the Cabinet Liner productsoffered by Formica Limited in both Whiteand Beige will satisfy both aesthetic andflatness requirements.For certain installations, where the reverseside has to be backed to add stiffnessor to seal the substrate but where flatnessand appearance are not critical, FormicaLimited offer a variety of backing boards.These are produced for maximum economy,and whilst when properly fabricated theygenerally provide an acceptable level ofcounter-balance, no flatness guaranteescan be given. They are suitable for usein applications such as curved panels,postformed worktops, window sill boards,and panels which will be fixed back toa stout framework.9Formica Limited, Coast Road, North Shields, Tyne & Wear NE29 8RE. Tel: 0191 259 3000. Fax: 0191 258 2719.Formica is a registered trade mark of Formica Corporation or its subsidiaries.Technical Services Department April 2004.