Shengbang Four Seasons
Apr 17,2026
Classification and Properties of Dental Plaster Materials
In the field of dental prosthetics, dental plaster serves as the core material for creating basic models such as dental casts and master casts.
In the field of dental prosthetics, dental plaster serves as the core material for creating basic models such as dental casts and master casts; its physical properties and handling characteristics directly influence the precision and fit of dental restorations. To meet diverse clinical needs, dental plaster has been developed into several categories, each with distinct priorities regarding setting characteristics, hardness, and expansion control.
I. Classification of Materials: Categorized by Application and Properties
Dental plaster is primarily classified based on its physical strength, setting expansion rate, and working time to suit different stages of fabrication:
Standard plaster: Commonly used in the initial stages of model fabrication, such as the initial casting of impressions. This type of plaster has good flowability but relatively limited surface hardness and scratch resistance, making it suitable for models that do not require fine margins
Hard Plaster: By reducing the water-to-powder ratio and optimizing the crystal structure, the surface hardness of the cured plaster is enhanced. It is suitable for working models requiring trimming and marking, such as base models for removable partial dentures
Extra-hard plaster: Possesses high compressive strength and surface abrasion resistance, enabling more accurate reproduction of the subtle morphology of oral tissues. It is commonly used for working models of fixed restorations, such as the fabrication of master models for crowns and bridges
High-expansion extra-hard plaster: Modifies the setting expansion characteristics of extra-hard plaster to compensate for the shrinkage of impression materials within a certain range. It is used as a master model material in precision casting processes
II. The Curing Process: A Combination of Physical and Chemical Changes
The setting of dental plaster is a process in which hemihydrate calcium sulfate reacts with water through hydration to form dihydrate calcium sulfate:
Hydration reaction: When hemihydrate calcium sulfate powder is mixed with water, it forms a slurry. After a brief induction period, needle-like or columnar dihydrate calcium sulfate crystals begin to grow and intertwine, forming a dense solid structure
Exothermic reaction: Heat is released during the reaction; the setting time depends on ambient temperature, the water-to-powder ratio, and the presence of additives.
Volume change: Crystal growth causes slight volume expansion; expansion rates vary among different types of plaster, which is one of the factors affecting model accuracy.
III. Key Performance Indicators: Determining Clinical Performance
The performance of dental plaster is evaluated based on several parameters that influence every stage of model fabrication:
Setting time: The time required from the start of mixing to complete setting. An appropriate setting time ensures the operator has sufficient time for pouring and trimming without requiring an excessively long wait that could compromise efficiency.
Compressive strength: The ability of set plaster to resist external compression. Insufficient strength may cause the model to break during removal or trimming.
Surface hardness: The model’s resistance to abrasion and scratches, which affects the clarity of margins during trimming and marking.
Setting expansion rate: The degree of volume increase caused by the hydration reaction. Appropriate expansion helps compensate for the shrinkage of the impression material, but excessive expansion may result in poor fit between the restoration and the abutment tooth.
Detail reproduction capability: The ability of the plaster to clearly reproduce the fine structures on the impression surface, which is a key indicator for evaluating model quality.
IV. Precautions for Handling and Storage
There are several key points to note regarding the use and storage of dental plaster:
Water-to-powder ratio: The amount of water used during mixing must be proportionate to the amount of powder; too much or too little water will affect the strength and expansion properties after setting.
Mixing method: Mixing can be done manually or using a vacuum mixer; vacuum mixing reduces air bubbles and improves the smoothness of the model surface and the reproduction of fine details.
Storage conditions: Unused plaster powder should be stored in a sealed container in a dry environment to prevent pre-hydration caused by absorbing moisture from the air, which can affect setting properties and performance.
As a fundamental material in dental restoration, dental plaster’s classification system and performance characteristics provide appropriate material options for various restoration needs. Understanding the properties of different types of plaster helps in making appropriate decisions at every stage of model fabrication.
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