Precious alloys

Shrinkage in noble alloy casting

To obtain a precise casting from any metal, including noble alloys, the size of the internal cavity of the casting mold must exceed the dimensions of the original model. This is necessary to compensate for thermal contraction.

The shrinkage process occurs in three stages:

1. Reduction in the volume of the molten metal during cooling down to the solidification point.
2. Direct shrinkage at the moment of crystallization.
3. Contraction of the finished casting during further cooling from the crystallization temperature down to room temperature.

The shrinkage of gold alloys is 1.25%. Unlike stainless steel (which has a rate of 2.7%), gold alloys can be cast using classical gypsum-bonded investment mixtures, as their expansion fully covers the loss of the noble metal's volume.

Investment materials: ensuring casting precision

Investment materials play a key role in dental casting. Their main task is to accurately replicate the model and compensate for casting shrinkage through their own expansion. A quality material must set within 7–10 minutes, not enter into a chemical reaction with the casting, have a fine-grained structure for surface smoothness, and form a porous shell to allow gases to escape during pouring.

According to the type of binding agent, investment materials are divided into:

• Gypsum-bonded (gypsum acts as the binder, while silicon dioxide provides heat resistance and expansion).
• Phosphate-bonded (phosphoric acid or anhydride reacts with zinc, aluminum, or magnesium oxides; used for steels with a melting point of about 1300 °C).
• Silicate-bonded.

In gypsum-bonded materials, silicon dioxide is represented either by quartz (allows mold heating up to 700 °C and provides linear expansion up to 1.4%) or cristobalite (heating up to 450 °C, expansion up to 1.8%). Cristobalite is considered optimal for gold alloys, as its significant expansion easily compensates for gold shrinkage.

Regulation of gypsum mold properties

The properties of molds can be modified using special chemical additives:

• Boric acid and sodium chloride (2%) — increase mechanical strength.
• Sodium sulfate — accelerates setting time, but reduces the degree of expansion.
• Borax — retards setting time and also decreases expansion.

The standard expansion of the material during hardening is 0.1–0.45%. However, if water enters during the initial setting phase, hygroscopic expansion occurs, which can reach 1–2.5%. This effect can be enhanced by a thick mix, an increase in the proportion of silicon dioxide, and immersing the mixture in water at a temperature of 38–42 °C.

The finished mold must withstand a pouring pressure of at least 55 kg/cm². It should be noted that gypsum-bonded materials are thermally unstable: at 1000 °C they decompose with the release of sulfur and calcium oxides, which is why they are strictly contraindicated for cobalt-chromium alloys and steel, but ideal for noble metals with lower melting temperatures.

Auxiliary and base polymers

Prior to the casting stage, the future design is modeled from special materials. In prosthetic dentistry, polymers are divided into primary (base, elastic, veneering, for artificial teeth) and auxiliary (impression materials, individual trays, protective caps).

To protect polymers from aging, antioxidants are added to them, and to prevent bacterial growth, antimicrobial agents effective in small doses are used.

Polymer compositions are evaluated by such physical and mechanical parameters as modulus of elasticity, tensile and flexural strength, and impact toughness. An important factor is water absorption in the humid environment of the oral cavity, which can lead to distortion of the prosthesis's geometric dimensions and contribute to infection.

Clinical application of plastics

The basis of modern rigid denture bases is most often polymethyl methacrylate. Acrylic materials are low-toxicity, durable, chemically resistant, and aesthetic.

According to their behavior when heated, polymers are classified into:

• Thermoplastics — soften upon reheating without chemical reactions taking place (reversible). They are rarely used in practice due to complex casting technology and poor bonding with acrylic teeth.
• Thermosetting plastics (thermosets) — a chemical curing reaction takes place during molding. Upon reheating, the material permanently loses its plasticity (irreversible).

Elastic acrylic compositions (e.g., "SR-Ivocap") are supplied as a powder with liquid or as ready-made plates. To increase their elasticity, plasticizers (dioctyl phthalate) or polymer chain growth regulators that reduce the molecular weight of the polymer may be present in the liquid. There are also elastic polyvinyl chloride materials, such as the domestic "Eladent-100", whose plasticity is achieved through external plasticization.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of hygroscopic expansion of investment materials.
• Characteristics of phosphate-bonded materials for high-temperature casting.
• Directions for improving acrylic denture base materials.
• Properties and disadvantages of polyvinyl chloride materials in dentistry.

Other topics in "Chapter 8. Dental Materials"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.