GB622456A - Improvements in or relating to patterns for moulds - Google Patents

Abstract

For the casting of articles such as dentures in a mould in which the pattern is formed of a material which can be destroyed or removed by heat without leaving objectionable ash, separate parts of the pattern such as clasps and reten <PICT:0622456/IV (a)/1> <PICT:0622456/IV (a)/2> <PICT:0622456/IV (a)/3> <PICT:0622456/IV (a)/4> tion bars are formed of standardized preforme parts of thermoplastic material capable of being joined to the model by pressure but having sufficient resistance to distortion with respect to cross-sectional dimension and shape so as not to become permanently distorted under the pressure required for joining to the model. Labial and lingual portions of patterns for dentures may be similarly formed. If desired, the heat-destructible pattern parts may be made of a harder plastic composition but coated with a pressure-sensitive adhesive composition. The thermoplastic material may be a cellulose derivative, synthetic resin, rubber, chlorinated rubber, or asphalt composition, containing a soft resin or plasticizer in sufficient proportion to impart the required adhesive properties. In an example, the pattern parts are made from a composition comprising a polymerized mixed ester of ethylene glycol with sebacic and maleic acids, pentaerythritol abietate butyral, and ethyl cellulose. Instead of the pentaerythritol compound there may be used as plasticizer or softener for rendering the composition tacky, dibutyl sebacate, tricresyl phosphate, diphenyl phthalate, trichlorocumene, or a hydrogenated methyl ester of rosin or abietic acid. In another example, the pattern parts are made from a composition comprising ethyl cellulose, ester gum, cyclicized rubber in micro-wax, a terpene polymer resin, and an oil-modified alkyd resin of drop melt point 68-75 DEG C. In another example, the composition comprises ethyl cellulose, ester gum, maleic rosin glyceride, terpene polymer resin, and an alkyd resin. Another composition comprises asphalt, coumarone-indene resin, ceresin, and a vulcanizable mixed polyhydric alcohol ester of a resin acid and polymerized aliphatic acids which occur in drying or semi-drying oils. The composition may also contain micro-wax, and soft resins to increase the pressure-adhesive property. The asphalt used may be a simple petroleum asphalt or a blown petroleum asphalt, or Wurtzilite asphalt. Rosin pitch, stearin pitch, ozokirite, or a paraffin wax, may be used with or instead of asphalt. As shown in Fig. 1, a plurality of preformed patterns 5 for forming clasps for dental appliances is obtained by forcing the plastic composition into the cavity between the closed mould parts 2, 3, at room temperature or elevated temperature. The preformed patterns are formed with a connecting rib 6. They are preferably formed against a sheet material 7 from which they can be stripped or cut when required. Such sheet material may be of cellulose acetate or of regenerated cellulose or paper coated with polyvinyl alcohol or sodium silicate. The pattern parts 10 (Fig. 3) are for forming palatal or lingual bars for dental restorations. The pattern material may also be moulded into sheet form. As shown in Fig. 4, a sheet of pattern material 14 is moulded between two protecting sheets 15, 16. The sheet may be used as such or cut into strips or pattern parts. As shown in Fig. 6, the pattern material is applied to a refractory model 20 which is preferably coated with a composition which facilitates adhesion of the pattern material. As coating composition there may be used a mixture of an oil-modified alkyd resin, a terpene polymer resin, a hydrogenated methyl ester of rosin, and ester gum, and it may be applied in molten condition. Another coating composition which may be used comprises micro-wax and a high melting-point wood rosin. The coating composition may be applied as a lacquer or emulsion. The pattern parts are applied to the model or to other pattern parts by gentle pressure. Pattern bars 10 (Fig. 3), are employed as palatal bars 24, and clasps, arms, or rests 25, of the pattern shown in Fig. 6. Tapered parts are used for clasps 26. The completed pattern may be smoothed by means of a flame, radiant heat, hot spatula, or solvent. The pattern is then invested in refractory material, which forms a refractory mould about the pattern. The mould is heated until the pattern material is completely burned out or eliminated and then molten metal or alloy is introduced into the mould cavity and cast to shape.