Casting Structures: Sprues, Burnout, and Metal Alloys

• These study notes cover the preparation of structures for casting, classification of sprues, casting cylinders, wax burnout, metal alloys, and various issues related to casting.

Sprue and Riser Preparation

• Sprues act as entry points for molten metal into the casting structure, connecting the crucible to the wax pattern.
• Proper sprue placement ensures casting success.
• Poor sprue placement can lead to porosity and defects.
• Risers facilitate the escape of gases formed during casting.
• They have a smaller diameter than sprues to avoid weighing down the wax structure, directing gases to the walls or base and never to the surface.

Sprue Classification

• Materials used are wax, plasticized wax, and metal.
• Primary sprues connect the outside cylinder to the metal reservoir, feeding metal from the crucible to the reservoir (3-4mm).
• The reservoir stores and distributes metal to the structure (4-5mm).
• Use transverse or circular reservoirs.
• Jitos connect reservoirs to the structure.
• These should be short to keep the metal from cooling (2-3mm).
• Position sprues in the center of the part or interproximally.
• Avoid areas with active interocclusal contacts.
• Secondary sprues address casting difficulties in certain areas (2-2.5mm)
• They connect primary sprues with structures, using acute angles and sigmoid shapes to facilitate flow.

Sprue Placement Technique

• Add wax to the cone, position the sprue, and seal with wax.
• Areas with thicker wax are cusp tips.
• Add spruess to the cusp tips, but not on active cusps.
• For single crowns:
  • Cut sprues and shape them
  • Position and attach the pattern to the sprue
  • Create a reservoir 2mm from the coping.
  • Join the cone with wax, leaving the coping facing upward
  • Seal it.
• For bridges:
  • Cut the sprue and shape it into a "V".
  • Cut another sprue, attach it horizontally over the "V", and join them.
  • Add secondary sprues as needed or create a horizontal bar across the entire wax structure.
  • Place the structure on the cone and seal with wax.

Sprue-Union Zone

• Funnel-shaped sprues decrease pressure as speed increases, reducing the need for rework.
• Inverted funnels allow metal to enter gradually, which is advantageous for increased entry.
• Cylindrical sprues don't change the union zone but create right angles that may cause metal rebound.

Casting Cylinders

• Molds are used to introduce wax patterns and investments.
• Some remain through casting, others are removed.
• During casting, ensure the reservoir is within the thermal center and that the structure is outside it.
• Maintain a 5-7mm safety space and cylinder walls.
• Structures should be well-attached to the cone
• Add vents on the sides, apply vaseline to the inner cylinder wall, seal the cylinder base, pour the investment, and allow it to set.
• Some cylinders are removed before casting after the investment has set and before oven placement.
• They cool faster and are more exposed to impacts.
• Metal cylinders use an internal coating to allow for investment expansion, and silicone cylinders use an internal vaseline coating.

Thermal Behavior

• Thermal center areas of the preheating furnace reach maximum temperature and cool slowly.
• Place the reservoir in the thermal center to maintain a higher temperature than the structure.
• Insufficient temperature can lead to porosity, defects, and casting failure.

Wax Weight and Metal Calculation

• Calculated by multiplying the structure's wax weight by the metal density plus 10% for operation.

Factors Influencing Casting

• Heating the die, sprue thickness, placement errors, reservoir existence, and cylinder base to wax pattern distance.

Investment Materials

• Used in dental casting to create a mold.

• Components Include:

• Refractory materials

• Binders like sulfates, phosphates, or silicates

• Modifiers to improve properties.

• High-fusion investments have a melting point between 700-1000°C.

• The casting cylinder and conical base are placed in a humidifier for setting at room temperature.

• Low-fusion investments have a melting point between 350-700°C, submerging the ring in water to expand the wax pattern.

• Investments should have a smooth surface, allow for expansion, provide ample working time to detach easily, be non-flammable, allow gas release, and maintain a thin layer.

Investment Preparation and Cylinder Filling

• Use techniques to reduce surface tension.
• Apply an anti-adherent material like vaseline inside the cylinder.
• Then mix liquid and powder, spatulate manually or with a vacuum mixer.
• Place well-mixed investment in the cylinder and vibrate to remove bubbles.
• Pincel with care to control vibrating speed and time, avoiding direct pouring onto the wax pattern.

Expansion of Investment and Cylinder

• Setting expansion: investment expands by 65% during setting.
• Wax pattern expansion from the heat of the investment can distort shape. Hygroscopic expansion requires cylinder immersion in water.
• Thermal expansion is the volume increase in some materials with heat.
• Coefficient of Expansion:
• Describes volume increase with temperature.
• It refers to changes in height, width, and thickness for solids.
• It refers to volume changes for liquids or gasses
• Expansion relies on total investment and includes 65% setting and 35% thermal expansion (oven).

Furnaces

• Come in gas and electric types.
• They are programmed or non-programmed.
• Furnace Parts Include:
• Chimney
• Muffle
• Electric resistance
• Pyrometer
• Door
• Display screen
• Sensors

Preheating

• High-temperature preheating hardens the investment.

Phases begin in a cold oven:

• Warm the oven to 200°C and place the cylinder inside. Heating melts the wax, releases toxic gases, and leads to revestment expansion.
• Temperature is gradually increased until the cylinder is ready for casting.

Lost Wax Technique

• Introduces molten metal into the internal cylinder structure (evaporated wax pattern).
• The cylinder contains entry holes and an internal wax structure.
• Once the wax melts, it produces combustion residue and the metal is then cast into the cavity.

Equipment and Material

• Preheating furnace
• Centrifuge
• Cylinder
• Alloy
• Crucible
• Torch
• Gloves
• Mask
• Welding mask
• Apron
• Tongs

Metal Casting

• Alloys melt within a range based on the metal used.

• Overheating causes volatilization, while underheating causes incomplete melting.

• Direct methods use a torch.

• Indirect methods use electrical resistance, not needing precision, and involve a temperature programmer, resistance, and crucible support.

• Induction casting transforms 220 volts into electric power to create a serpentin-induced heat.

• Electric arc casting is also employed.

Casting Machines

• Centrifugal Casting:
• The cylinder is positioned on one side, with a counterweight on the other, enabling rotation.
• Pressure casting incorporates a vacuum, with the metal entering the cylinder and the metal fully entering as compressed air collaborates

Crucibles

• These containers hold metal for melting, made of refractory material.
• They’re used for melting, casting, burning, and calcining substances.
• Always burn borax before using them, dedicate crucibles to a specific alloy, and maintain a cold state for cleaning.
• Types include porcelain, graphite, or silicon carbide, with vertical crucibles used in machines with pressure and horizontal crucibles in centrifugal machines.

Alloys

• Precious metals: gold
• Base metals: chromium and cobalt (<25%) are cost-effective, with high elasticity, density, durability, and resistance.
• Characteristics include, elasticity, malleability, limit of proportionality tensile strenth, crystal sizes, low contration, biocompatibility minimal reactivity, adhesion, and easy casting.

Button Examination

• Check the casting button after removing the investment to asses casting success.

• The button is located at the cylinder base, where metal enters the structure.

• A red button means that the metal is still hot.

• Rough, irregular, or burned-out buttons mean the metal has been over casted..

• Lack of button indicates

  • Insufficient metal
  • Ruptured investment
  • Obstructed crucible.

Cleaning and Sandblasting

• Remove the cylinder by, gently tapping or using cutting pliers to avoid damaging the structure.
• Tungsten burs help remove investment near the structure, while sandblasting removes residue.
• Sandblasting projects aluminum oxide powder at 4-5 bars of pressure, using 2-3 second intervals to prevent damage.
• Types Include:
  • Water blasters
  • Air blasters
  • Dry filter blasters
  • Vacuum blasters

Finishing

• Carbide burs are used to remove the coating.
• Polish with aluminum, align holes with burs and buff with various pastes, gels, and brushes.

Casting Retrieval

• Remove the the gating and the vents attached to the metallic structure by using a carborundum disk.
• Perform a visual inspection to identify color, shape, and any imperfections.

Fit Control

• Check the alignment of the material.
• Inspect undercuts and internal areas.
• Methods depend on the material:
• Metal crown: examine mesial and distal contact points for porosity using rubber wheels and polishing paste.
• Veneer crown: use initial sandblasting to remove any residual investment
• Metal-ceramic crown: sandblast for removal, cut sprues, and adjust the crown on the die
• Post and core casting: make sure there is retention in the intraradicular area as well as the abutment zone

Structures

• Use instruments such as devasted (black), remade (green), leveled (red) and polished (yellow).

Errors

• Non stable expanding liquid, bad coating mixture, liquids which don't release tension and wrong preheating and setting programmes.
• Not calculating metal correctly, and not using the right technique.
• Problems:
• Incomplete filling: caused from low metal temperature, or too slow filling and must be centrifuged more and be heated adequately.
• Cavities from metal contraction: cause an inadequate thermal coating. Use a metal with an adequate thermal coate to prevent this.
• Microporosities: cause a mis-alloying. Use the correct alloy.
• Metal outside the cylinder, caused from exceeded temperatures. Ensure the cylinder is well positionsed.
• Emptiness: caused by low metal, an inadequate temperature. Calculate the mass well and test the flame force.

CAD/CAM Systems

• CAD/CAM support the design and manufacturing of dental prosthetics.
• CAD assists designing, analyzing, and optimizing.
• CAM assists planning, managing, and controlling operations.
• They may be:
• Direct (on-site)
• Indirect, by shipping designs to a milling center.
• Design Foundations:
• Digitization of impression
• CAD
• CAM
• Operation follows phases creating design systems which should be fast, precise and create good productivity and comunication and avoid storing.
• The inconveniences:
• Hard to learn method
• Expensive

Digitization

• Intraoral scanning rounds out the ends and polished area that have reflection, allowing an increase of comfort.
• Model scanning only scans the die.
• They can be tactile or with optic (using laser) which is the fastest method.
• The scan is then transported to a design program and has the following phases:
• Set a definition through all details for the patient
• Documentation and selection.
• Milling final product

Milling

• It's an industrial process where the raw materials are shaped.
• They can be highly productive and precise and produce non-chip material.
• When sintered, a solid piece is created through lasers with thermal formed materials.

Machine Systems

• Some companies use intraoral cameras, other use Laser and other different methods.

CAD/CAM Materials

• Sintering the ceramic material produces maximal hardness.
• Ceramics can either be:
• Aluminous
• Feldspathic
• Lithium dissilicate: with high aesthetics and hardness
• Titanium: used in implants

Zirconium Oxide

• Has a high fracture resistance, low thermal conductivity, and extreme hardness.
• Is a light and cheap alternative, but it is fragile and has a high price.
• This excellent characteristics makes them most adequate for ceramics.

Feldspathic Ceramics

• Are composite of calcium, quarts and caolin which are not brittle
• Plastics can be made of PEEK and PMMA.

Explore casting structures, sprue classification (primary, reservoir, jitos), and materials like wax and metal. Understand the importance of sprue placement for successful casting and avoiding defects. Learn about wax burnout processes, metal alloys, and addressing common casting issues.