IMPRESSION COMPOUND

Characteristics of Impression Compound

Composition

Impression compound is composed of various materials that contribute to its properties and functionality. These include:

1. Matrix Materials:
   • Natural Resins: Such as colophony and shellac, or waxes, are used to form the matrix of the impression compound. These materials provide thermoplasticity, allowing the compound to soften when heated and harden when cooled.
2. Fillers:
   • Fillers, such as soapstone or talc, are added to increase the viscosity of the compound at temperatures above that of the mouth. They also enhance the rigidity of the compound at room temperature, ensuring that it retains its shape and structure.
3. Lubricants:
   • Lubricating agents, like stearic acid or stearin, are included to improve the workability of the impression compound. These lubricants help in the manipulation and shaping of the material.
4. Pigments:
   • Pigments are added to provide color to the impression compound. This helps in visualizing and distinguishing the material during use.

Functions of Components

• Natural Resins: Form the base of the compound, providing it with the necessary plasticity to be molded into impressions.
• Fillers: Increase the viscosity of the compound when heated, ensuring that it maintains its shape and stability during impression making.
• Lubricants: Improve the handling characteristics of the compound, making it easier to manipulate and shape.
• Pigments: Confer color to the compound, aiding in visualization and identification during the impression procedure.

Manipulation Technique

The manipulation of impression compound involves several steps to ensure proper softening, molding, and setting. Here is a detailed procedure:

1. Softening the Material:
   • The impression compound cakes are placed in a lined bowl containing water heated to a temperature range of 55-60°C (131-140°F).
   • The cakes are allowed to soften uniformly in the warm water bath for approximately 4-5 minutes. This process ensures that the compound reaches the desired softening consistency, as depicted in Figure 5.1.
2. Molding the Tray:
   • Once adequately softened, the impression compound is removed from the water bath and molded into a tray shape using appropriate instruments.
   • The tray is then inserted into the patient’s mouth and gently held in position. It is crucial to maintain gentle pressure to ensure accurate impressions without causing discomfort to the patient.
3. Cooling and Hardening:
   • The tray is left in the mouth until the impression material cools below its fusion temperature and hardens. This usually takes a few minutes, during which the impression is formed.
   • Once the compound has solidified into a rigid mass, the tray can be carefully removed from the mouth.
4. Separating from the Cast:
   • To separate the impression from the cast, the assembly is immersed in a hot water bath. The heat helps to soften the compound, making it easier to remove from the cast without damaging the impression.

Setting Reaction

The setting reaction of impression compound is a thermoplastic process. Here’s how it works:

• When heated above its fusion temperature, the impression compound softens and becomes plastic, allowing it to be molded.
• During the impression-making process, the compound remains in a softened state.
• Upon cooling, the polymer solidifies into a rigid mass, capturing the details of the oral tissues in the impression.

This physical setting reaction enables the impression compound to retain the shape and form of the oral structures, ensuring accurate impressions for dental procedures.

Properties

1. Non-Elasticity:
   • Impression compound is non-elastic, meaning it lacks the ability to stretch or flex. As a result, it can distort upon removal from undercuts, potentially causing tissue trauma.
2. Lack of Fluidity:
   • While impression compound is plastic and moldable, it lacks the fluidity needed to capture fine details. This can affect the accuracy of the impression, particularly in intricate areas.
3. Coefficient of Thermal Expansion:
   • Impression compound exhibits a high coefficient of thermal expansion, resulting in a volumetric contraction of approximately 1.7% upon cooling. This property can lead to dimensional changes in the impression.
4. Poor Thermal Conductivity:
   • Due to its poor thermal conductivity, impression compound behaves unevenly when heated. The outer layer softens more quickly than the inner layers, resulting in differential softening.
5. Softening and Hardening Temperatures:
   • At room temperature (approximately 37°C), impression compound is hard and rigid. However, it becomes soft and plastic at temperatures around 45°C. This temperature sensitivity is crucial for its manipulation during the impression-making process.
6. Dimensional Stability:
   • Impression compound is susceptible to dimensional changes, especially with temperature fluctuations. To minimize this, it should be allowed to thoroughly cool in the mouth before removal. Additionally, casting should be done promptly to capture the impression accurately.