Lost Wax Casting Techniques

The process of attaching a sprue (a channel) to a pattern (a mold) to direct molten metal into the mold cavity during casting.

A connecting bar that distributes molten metal from a single sprue to multiple casting units.

Investment materials for dental casting are categorized based on their binder type: Gypsum, Phosphate, or Silica.

A type of investment material with gypsum as the binder, suitable for low melting point alloys. Not stable at high temperatures.

A type of investment material with phosphate as the binder, suitable for high melting point alloys, stable at high temperatures.

The expansion of an investment material during the setting process, ensuring a tight fit for the casting.

The process of choosing the right casting alloy for dental restoration, based on factors like intended use, physical properties, and biocompatibility.

Choosing the best investment material based on the selected casting alloy, desired setting expansion, and required temperature stability.

A ring liner that covers the entire circumference of the investment ring, ensuring a consistent wall thickness and preventing the investment from sticking to the ring.

Mixing the investment material for an extended period, leading to increased expansion due to heat generated during the mixing process.

Storing the investment material in an environment with high humidity levels, leading to increased expansion due to the absorption of moisture.

Using less water relative to powder in the investment mix, resulting in increased expansion due to less water filling the spaces between particles.

A ring liner that does not absorb water, preventing the transfer of moisture to the investment and potentially affecting its expansion.

A method of applying investment material to a wax pattern using a brush. The mix is manually spatulated and vibrated to ensure proper flow and coverage.

A method of applying investment material to a wax pattern using a vacuum to remove air bubbles and ensure proper flow and coverage.

Ovens designed for eliminating wax from investments. They come in manual, semi-automatic, and fully programmable varieties to control the heating process.

The sprue must allow molten wax to escape the mold, enable molten metal to flow with minimal turbulence, and remain molten longer than the alloy filling the mold.

Sprue types include wax, plastic, and metal. Prefabricated plastic and wax sprues are preferred because they are eliminated during heating.

A 2.5 mm (12 gauge) diameter is recommended for molar metal-ceramic patterns, while a 2.0 mm (10 gauge) is recommended for premolars and partial coverage restorations.

The sprue should be attached to the bulkiest non-functional part of the pattern, ideally on the occlusal surface in accurate occlusion or in the proximal contact area.

The attachment point should be carefully smoothed and unrestricted, avoiding necking to prevent casting porosity. Widening the attachment can lead to shrink spot porosity.

Vents are recommended to allow gases to escape during casting and act as heat sinks.

The casting ring liner holds the investment in place, restricts its expansion, and aids in preventing bubble formation by using a wetting agent (debubblizer).

This technique utilizes paper or plastic casting rings, allowing for unrestricted expansion and being particularly advantageous for high-melting alloys that shrink significantly.

The maximum temperature an investment can withstand before cracking. It varies depending on the type of investment material.

The first step in the casting process where the crucible is heated to a specific temperature before melting the alloy.

The process of melting the alloy in the crucible. The casting ring is then placed in the cradle.

The crucible is positioned so that its orifice (opening) aligns with the sprue, creating a smooth flow path for the melted alloy.

After the pin drops, the casting arm is pulled forward, causing the melt to be spun into the mold cavity by centrifugal force.

A defect in the casting that appears as a rough surface. It can be caused by excess surfactant, improper water/powder ratio, or excessive burnout temperature.

Lumps or bumps on the chewing surface of a cast restoration. This is usually caused by excessive vibration, increased water-to-powder ratio, pattern too close to the edge of the investment, too-rapid heating, or a dropped mold.

Thin, flat pieces of metal that form on the cast surface. This could be from an increased water-to-powder ratio, the pattern being too close to the edge of the investment, rapid heating, or the mold being dropped.

When the casting is not completely filled with metal. This can happen if the mold is too cool, the metal is not melted hot enough, or there is inadequate metal for the casting.

An incomplete cast with a smooth, shiny area, often caused by incomplete wax elimination. If the metal in the sprue solidifies before the metal in the mold, this can create a 'suck-back' defect. Other causes include improper pattern position and a narrow, long sprue.

Air pockets or bubbles trapped within the casting. This can result from a particle of investment becoming dislodged during casting.

Dental alloys are classified into four types based on their intended use. Type I is for simple inlays, Type II for complex inlays, Type III for crowns and fixed dental prostheses, and Type IV for partial removable dental prostheses.

Dental alloys are also categorized based on their hardness. Type I is soft, Type II is medium, Type III is hard, and Type IV is extra hard.

The color of a dental alloy is not a reliable indicator of its gold content. A 9 Carat jewelry alloy (37.5% gold) can appear more yellow than a metal-ceramic dental alloy with 85% gold.

For an alloy to be accepted by the ADA for dental restorations, it must meet specific criteria, including resisting tarnish and corrosion.

Dental alloys vary in cost. Base metal alloys are generally less expensive. Alloys containing approximately 50% gold offer some economic advantage. Palladium-rich alloys with a small percentage of gold are alternatives for metal-ceramic restorations.

Evaluating the clinical performance of dental alloys considers biologic and mechanical properties. Biologic factors include gingival irritation, recurrent caries, plaque retention, and allergies. Mechanical properties include wear resistance, strength, marginal fit, ceramic bond failure, connector failure, and tarnish and corrosion.

The laboratory performance of a dental alloy is also crucial. Key factors include casting accuracy, surface roughness, strength, sag resistance, and metal-ceramic bond strength.

The handling properties of an alloy are important for ease of manipulation in the dental laboratory. This can influence the selection of a particular alloy.