LEC 9 Dental Material Dr. Hassan Jamil Waxes and Investments

Transcription

1 LEC 9 Dental Material Dr. Hassan Jamil Waxes and Investments Waxes Indications The primary use of waxes in dentistry is to make pattern of appliances prior to casting as many dental restorations are made by the lost wax technique, in which a pattern is made in wax then putted in the mold (investment material). After setting, the wax is burnt out and the space is filled with molten metal or plastic (acrylic). Chemically waxes are polymers consisting of hydrocarbon and their derivatives like esters and alcohol. Dental waxes are mixture of natural and synthetic waxes, gums, fats, fatty acids, oils, natural and synthetic resins, and coloring agents. Requirements of dental waxes: 1- Must conform to the exact size and shape and contour of the appliance which is to be made 2- Should have enough flow when melted to reproduce the fine details 3- No dimensional changes should take place once it is formed 4- Boiling out the wax without any residue 5- Easily carved and smooth surface can be produced 6- Definite contrasting in color to facilitate proper finishing of the margins Classification of waxes according to origin: 1- Minerals a) Paraffin wax; refined from crude oil, has relatively low melting point (50-70 oc ) and relatively brittle. b) Ceresin wax (microcrystalline): refined from petroleum, has medium melting range (60-90 oc ). 2- Plants a) Carnauba: obtained from palm trees. It is hard, tough, and has high melting point (80-85 oc ). b) Candelilla: it is hard, tough, and has high melting point (80-85 oc ), used to increase the melting point and reduce flow at mouth temperature. 3- Animals a) Stearin: obtained from beef fat, has low melting point. 1

2 b) Bees wax: obtained from honey comb, consist of partially crystalline natural polyester. It is brittle, has medium melting temp (60-70 oc ) 4- Synthetic waxes They are used to modify some properties of natural waxes like polyethylene Classification of waxes according to use: 1- Pattern wax a) Inlay casting wax: should be hard and brittle in order to fracture rather than to distort when removed from undercut areas. The wax is blue in color. They are used to make inlays, crowns and pontic replicas. They are mostly paraffin with carnauba wax. There are 2 types: i. Type 1: for direct technique ii. Type 2: for indirect technique b) Denture casting wax: used to produce the metal components of cobalt chromium partial denture. It is based on paraffin wax with bees wax to give softness necessary for molding and stickiness to ensure adhering to an investment material. It is green sheets. c) Denture base plate wax (modeling wax): it is used to form the base of the denture and in setting of teeth. Pink sheets. 2- Processing wax : wax used during processing of the appliance like a) Beading wax: used to make beading around the impression before pouring gypsum to protect the margins. b) Boxing wax: used to make box around the impression to make pouring gypsum into impression easier and more perfect c) Block-out wax: to block-out undercut areas on cast during processing of cobalt chromium (Co/Cr) metal frameworks. d) White waxes: to make pattern simulate veneer facings in crowns e) Sticky wax: to join and stabilize temporarly broken pieces of the denture before repair. 3- Impression wax: previously discussed as they are used to make impression but they distort upon removing from undercuts, they are highly flow. a) Impression wax: used to make the impression b) Corrective wax: used to record selected areas of soft tissues an edentulous arches. 2

3 Properties: 1- They are thermoplastic materials that soften when heated and solid at room temperature. 2- High coefficient of thermal expansion and contraction. It is highest of dental materials 300* *10-6 cm/cm oc. Shrinkage of wax from liquid to solid at room temperature is 0.4%. Thermal contraction of wax is compensated by expansion of investment. 3- Poor thermal conductivity. After softening of the wax, it is allowed to cool which accompanied by contraction and because of poor thermal conductivity only the outer layer will solidify while the inner mass will solidify later producing internal stresses. Reliefing stresses occur later when the temperature increases. Greater stresses may take place if the wax is not properly softened. The best way to soften the wax is to be held in warm raising air area, above the flame and not in the flame itself. Another ideal way for softening the wax is to use annealed which is thermostatically controlled oven, keeping the wax just above the softened temperature and ready to use. 4- Flow: should have high flow when softened but should little or no flow at room or mouth temperature so not to distort. 5- Brittleness: inlay wax should be brittle in order to fracture rather than distort when removed from undercuts of the cavity. 3

4 Investment materials It is ceramic material used to form a mold which an alloy casted in. Requirements of an ideal investment material: 1- Easy to use and reasonable setting time 2- Sufficient strength at room temperature 3- Must not decompose at high temperature 4- Should have enough expansion to compensate for shrinkage of wax and metal 5- Should be porous to allow air and other gases to escape 6- Should produce smooth surface and fine details 7- Should break easily after casting is completed 8- Should be inexpensive Types: 1- Low temperature casting investment: gypsum bonded investment. 2- High temperature casting investment: phosphate bonded investment, ethyl silicate bonded investment. Each type of investment should be composed of: 1- Refractory material which is usually a form of silicone dioxide such as quartz, tridymite, and cristobolite or a mixture of them. 2- Binder: as the refractory material alone does not form a coherent solid mass, the binder is needed. So alpha calcium sulphate hemihydrate is the binder for gypsum bonded investment or phosphate for phosphate bonded investment or ethyl silicate for ethyl silicate bonded investment. 3- Modifiers: to modify the physical properties such as sodium chloride, boric acid, and potassium sulphate Gypsum bonded investment Uses: it is used to form mold for casting gold alloys for crown and bridges. Manipulation: the powder is mixed with water and poured around wax pattern and allowed to set. 4

5 Properties: 1- Three types of expansions may develop: setting, thermal, and hygroscopic 2- Thermal, setting, and hygroscopic expansion is about %. 3- It will decompose to sulfur dioxide and sulfur trioxide when heated above 700 oc tending to embrittle the alloy therefore it is not used for casting Co/Cr or palladium alloys but used for gold alloys Phosphate bonded investment Uses: to form mold for casting high temperature dental alloys like Co/Cr. Also as a fixture for holding dental appliance to be soldered or welded. It is composed of powder which is silica dioxide 80%, ammonium di-acid phosphate 20%, and magnesium oxide. The liquid is colloidal silica suspended in water. The powder is mixed with the liquid and poured around the wax pattern and allowed to set for min, then placed in a furnace to burn out the wax pattern. Then heat the investment and melt the alloy for casting. Ethyl silicate bonded investment It is used for the same indications as phosphate bonded investment. It is composed of powder which is silica dioxide, magnesium oxide, the liquid is supplied into 2-3 liquid systems mixed to form solution that mixed with the powder. The liquid is composed of ethyl silicate and denaturized acid. It expansion is about %. 5