Lecture 9: Abrasion and Polishing PDF

Lect. Elaf Jamal Harb Dental Material Prosthetic Dental Department/College of Health and Medical Technology MSc., BSc.: Prosthetic Dental Technology (Iraq). Lecture 9: Abrasion and polishing Before any restoration or appliance is placed permanently in the...

Lect. Elaf Jamal Harb Dental Material Prosthetic Dental Department/College of Health and Medical Technology MSc., BSc.: Prosthetic Dental Technology (Iraq). Lecture 9: Abrasion and polishing Before any restoration or appliance is placed permanently in the mouth it should be highly polished. In spite of all the care taken during processing, many restorations and prostheses usually require further trimming, smoothing and finally polishing. A rough or unpolished surface may: Be uncomfortable to the patient. Cause food and other debris cling to it and makes it unhygienic. Lead to tarnish and corrosion. Abrasion It occurs when a hard, rough surface slides along a softer surface and cuts a series of grooves. Defined as The wearing a way of a substance or structure through a mechanical process, such as grinding, rubbing or scraping. TYPES OF ABRASION Abrasion may be: A two body process, e.g., action of a diamond bur on enamel.( abrasive are bonded to the instrument). A three-body process, e.g., pumice applied with a bristle brush. (abrasive particles stay free). Lect. Elaf Jamal Harb Dental Material Prosthetic Dental Department/College of Health and Medical Technology MSc., BSc.: Prosthetic Dental Technology (Iraq). Erosion: A process of material removal achieved by air-pressure–propelled abrasives (air-particle abrasion). Abrasive: Is a hard substance used for grinding, finishing or polishing of a less hard surface, or the material that causes wear is called abrasive. Substrate: The material being abraded is called a substrate. Rate of abrasion The rate of abrasion of a given material by a given abrasive is determined primarily by the followings: 1.Hardness hardness of an abrasive is directly proportional to the rate of its abrasion.The harder the abrasive than the substrate the more abrasion will be produced. 2. Particle size: Larger, coarse abrasive particles will abrade a surface more readily than smaller particles, but they tend to leave more coarser scratches in the substrate. 3.Particleshape Sharp, irregular shaped particles will abrade a surface more rapidly than will rounded particles having dull cutting angles. However the former will produce deeper scratches than later. 4.Speed and pressure Both speed and pressure are directly proportional to the rate of abrasion. At higher speed greater friction is produced, which tends to Lect. Elaf Jamal Harb Dental Material Prosthetic Dental Department/College of Health and Medical Technology MSc., BSc.: Prosthetic Dental Technology (Iraq). produce higher temperatures. Similarly, greater pressure causes higher temperatures &possibly patient discomfort. Lubrication Lubricants are used during abrasion for two purposes: 1.to reduce heat buildup. 2.to wash away debris to prevent clogging. But too much lubrication can reduce the abrasion rate by preventing the abrasive from coming in contact with the substrate. Abrasive instrument design: 1.Abrasive grits: these are derived from materials that have crushed and filtered through a series of mesh screens. The types of abrasive grits are based on particle size which are coarse, medium and fine. 2.Bonded abrasives: these are bonded or incorporated into a grinding tool.The bonded abrasive tend to disintegrate rapidly since the particles detach with time. 3.Non–bonded abrasives: these involve adding a paste or a water soluble medium to a non-abrasive instrument that is made from rubber or cloth brushes. CLASSIFICATION OF ABRASIVES Abrasives used in dentistry can be classified into the following three types: 1. Finishing abrasives: These are generally hard, coarse and used primarily to produce the desired contours of a restoration or tooth preparation. 2. Polishing abrasives: Have finer particle sizes and are less hard than the finishing abrasives, they are used to smooth surfaces roughened by finishing abrasives. Lect. Elaf Jamal Harb Dental Material Prosthetic Dental Department/College of Health and Medical Technology MSc., BSc.: Prosthetic Dental Technology (Iraq). 3.Cleansing abrasives Cleansing abrasives are soft materials with small particle sizes and are intended to remove soft deposits that adhere to enamel or a restorative material. Types of abrasives: Many types of natural and synthetic abrasives are available. Generally, synthetic abrasives are preferred because of their more predictable physical properties. 1. Arkansas stone is a semi translucent, light gray rock mined in Arkansas. It is dense, hard & contains uniformly textured microcrystalline quartz. It is used for fine grinding of tooth enamel and metal alloys. 2. Chalk: A white abrasive composed of calcium carbonate. Chalk is used as a mild abrasive paste to polish tooth enamel, amalgam, and plastic materials. 3.Corundum: This is a white mineral form of aluminum oxide. Corundum is most commonly used in an instrument known as a white stone. Corundum has largely been replaced by synthetic alpha aluminum oxide in dental applications. 4.Diamond: A transparent, colorless mineral composed of carbon. It is the hardest known substance and is called a super abrasive because of its ability to abrade any other known substance. Synthetic diamond abrasives: are used farm or commonly than natural diamond due to its consistent shape, size and low cost. Diamond are mostly used on tooth structure, ceramics and resin-based composite materials. POLISHING It is the production of a smooth mirror like surface without much loss of any external form. If the particle size of an abrasive is reduced sufficiently, the scratches become extremely fine and may disappear entirely. The surface then acquires a smooth shiny layer known as a polish. Difference between a abrasion and polishing The difference between an abrasive agent and a polishing agent is difficult to define.The terms are generally inter changeable. Particle size a given agent having a large particle size acts as an abrasive, Producing scratches.The same abrasive with a smaller particle size is a Polishing agent. Lect. Elaf Jamal Harb Dental Material Prosthetic Dental Department/College of Health and Medical Technology MSc., BSc.: Prosthetic Dental Technology (Iraq). Material removed very little of the surface is removed during polishing—Not more than 0.005mm(0.002inch). Speed the optimum speed for polishing is higher than that for abrading. Non abrasive polishing Polishing is usually achieved by an abrasive process. However a smooth shiny Surface can also be achieved through non abrasive means.These include: 1.Application of a glaze layer like glazing of composites and ceramics. 2.Electrolytic polishing. 3.Burnishing. Methods of abrasion Abrasion may be carried out 1. Manually: Proximal stripping of enamel using abrasive strips. 2. Rotary instruments: Burs, wheels, cups, discs, cones, etc. 3. Blasting: the object is blasted with a steady stream of abrasive, sandblasting to remove Investment of castings.