Post 580 –by Gautam Shah
Sheets are surface materials with stiff to flexible body. Sheets have high proportion of surface area in comparison to volume. A sheet (mainly metal) is thinner than 6mm when compared to the counterpart, the plate. A film is a thinner formation than a sheet. Sheets, Plates and Films, all have different properties and uses. A sheet is used for packing tins, air-conditioning ducts, automobile bodies, furniture, appliances, utensils, pipes, purlins, etc. A plate is used for heavier structural entities like ships, tunnel sections, pressure storage tanks, chemical reaction vessels, reactors etc. A film is an independent entity applied as a cover or foil, formed by material deposition (gas, liquid or solid), or one integrated to a substrate (surface alloying, surface ceramic formation or molecule deposition).
Metal sheets are formed chiefly by hot or cold rolling. Polymer, glass, cement, paper, gypsum sheets are extruded or cast. Foams can be classed as cast sheets. Soft sheets such as fabrics, mats etc. are formed by weaving, netting or pressing the mass of fibres. Palm leaves, leathers, skins, timbers, stones, mica, are naturally formed composite sheets.
In modern age substantial volumes of sheets are manufactured by sheet lamination or layering, particulate forming, impregnating woven or non-woven structures, or by sandwiching various types of sheets. A sheet could have skins formed from other or same materials (that constitutes the core). Sheets formed by rolling or extrusion could be in long lengths or small width strips. Sheets, otherwise, are formed and sized to ‘standard’ sizes. The ISO standard sizes (width and length) are in multiples of 300mm. Sheet forms like fabric are manufactured and sized in multiples of 100mm. Paper is supplied in sizes (ISO standard) of series A & B (such as A0, A1, etc.). Standardization of sheet sizes encourages standardization of manufacturing plants, processes, transport, storage and wastage rationalization.
Plates, Sheets and Films, basically have distinctive processing methods, though some may carry common terms. Plates processing, require greater effort to shape, cut or punch through due to their thickness. Plates have characteristic flexural stiffness. So sheets like plywood and other composites behave like plates. Plates may show different stress behaviour on two faces, such as seen in rolling, bending and welding processes. Plates are assembled by welding, rivetting, forging, mechanical joining and occasionally situational fixing. Sheets are processed by methods such as cutting, punching plain bending and deformation bending. Metal sheets formed by cold-forming. Sheets are assembled by butt welding, adhesion joining, seaming, rivetting, screwing and situational fixing (e.g. dove tail joining of wood materials). Film materials in sheet forms are applied by static charging, adhesion or heat melting.
Malleable metal sheet products are manufactured mainly by stamping or pressing and drawing. Some drawn parts go through changes in body thickness (such as the kitchen pressure cooker forming). Large plastic deformation is an advantageous property for metals’ sheets. During stamping or micro drawing often there are no changes so some processes like corrugation or furniture are done on pre-coated sheets. Stiffer sheets like wood veneers and plywoods, stone (for cladding), acrylics, etc. must be sawn and in few instances shear cut. Sheets show anisotropy or directional variation of mechanical properties where the material reacts differently in different directions. Metal and composites take advantage of this characteristic while creating geometric compositions.
Sheets are further processed like folding, corrugation, embossing, and perforation to impart geometric qualities. Sheets are engraved or etched by mechanical, laser and plasma-based processes. Simple processes like grinding, rubbing, ironing, burnishing, flame treatments are applied on wood, paper, leather, fabrics and plastics to remove or suppress surface irregularities. Chemical treatments include Linear Plasma-based processes, nitridation, oxidation electroplating, zinc coating, chromate and phosphate treatments, coating, painting, material deposition, surface hardening, surface alloying and cementation. These alter the surfaces for polarity, wettable, electrical conductive, weldable or solderable, corrosion resistant, tarnish proof, chemical resistant, high wear, hardness and remove surface irregularities.