20: Model and investment materials

Chapter 20 Model and investment materials

Introduction

The last section of this book deals with the materials used in the process of fabrication of indirect restorations and dentures. In all cases the underlying method of production involves a technique that can be traced back many hundreds of years and which was used extensively in the manufacture of jewellery: the lost wax technique. In this technique, a model of the substructure is first prepared. On this the prototype prosthesis is made using materials such as waxes that can be shaped to the required anatomical shape but which can also be destroyed by heating. Once this prototype is prepared it is invested or surrounded in a material which on setting will form the negative of the prototype pattern. The material for the prototype is then removed by heating. This leaves a space in the investing materials, which is filled either by casting or by applying a dough of the material and closing the mould under pressure. The details of this process will be described for each application.

In clinical practice, it is often necessary to make models of the patient’s teeth. The models are used by the dentist to (in conjunction with other information) to plan a course of treatment or to preoperatively design a prosthesis such as a bridge. Once the treatment plan has been decided, the teeth are prepared and an impression is taken for a new model, and the restoration is then constructed in the dental laboratory by the dental technician on the second model.

Both the preoperative model and the working cast are constructed out of a material based on gypsum. This is commonly referred to as (dental) plaster. As such, plaster is one of those ubiquitous materials which is used in many types of clinical dentistry and in the dental laboratory. Dental plaster is also traditionally used to make an impression of the edentulous mouth prior to the construction of a complete denture (see Chapter 15). A special form of plaster may be used when metal restorations are to be cast using the lost wax technique. This chapter discusses all the dental materials used in the construction of dental models and those used as investment materials.

Types of Model

A model may be defined as a replica of the structures in the oral cavity. It is made by pouring a material such as dental plaster into an impression of the area.

There are a range of materials that the technician can use, the choice of which depends on the purpose and use of the cast. In certain cases, for example when the framework for a metal denture is waxed up, the whole model is required. This is called a refractory model and is made out of a special material – a refractory material – so that it may be invested and subjected to high temperature so that the metal framework can be cast on to it. A refractory material retains its shape and strength, that is, it is physically and chemically stable, at high temperatures. This material should also be resistant to thermal shock and have appropriate thermal properties for the intended purpose.

Models may be made out of dental plaster, dental stone or investment material. All these material are based on gypsum but have different properties, which will determine when and how they are used.

Dental Plaster and Dental Stones

Gypsum is calcium sulphate dihydrate and occurs naturally at many sites around the world. It is crystalline in form (Figure 20.1). To be used as a casting material, the crystalline gypsum is heated at 130°C to remove some of the water contained in it. The product is called plaster of Paris, named after the site where this process was first carried out. The plaster is called calcined and the chemical produced is calcium sulphate hemihydrate. Further heating (up to 200°C) will drive off all of the residual water, leaving behind anhydrous calcium sulphate.

Dental plaster is provided in the hemihydrate form. Once water is added to this, the hemihydrates reverts to the dihydrate with the liberation of heat. It is this reaction which occurs with all dental plasters. The form of the crystalline hemihydrate determines the precise type of plaster which is produced although all types are chemically identical and are dissimilar only in structure and form.

Chemical consituents

As indicated above, the setting reaction for all these hemihydrate materials is initiated by mixing with water. The amount of water required to achieve a suitable mix varies with the plaster type. It is possible to calculate the exact amount of water required to mix with a specific weight of water. Due to the porous nature of the powder and its particle irregularities, the amount of water to achieve a suitable mix of plaster of Paris must be increased so that the powder is wetted. The mass of water required for the other two types of stone is reduced in proportion to the porosity of the powder and the shape and density of the particles. The consequence of using more water in the rehydration of the hemihydrates is that the plaster so formed will be weaker and more friable. With all types of dental plaster the amount of water used should be the minimum required to produce a creamy mix that can be effectively manipulated into an impression to produce an air-blow-free model. The manufacturer will provide this information.

Other chemicals may be added to the stone for various reasons:

Using specific combinations of these chemicals, the manufacturer can ‘tune’ the gypsum product to the application for which the material is designed.

Jan 31, 2015 | Posted by in Dental Materials | Comments Off on 20: Model and investment materials
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