II- SPRUING Dr. Waseem Bahjat MushtahaSpecialized in prosthodontics
Definition: The sprue channel is the passage leading from the crucible to the mold cavity. Purpose of sprue: 1- provide an escape way for the molten wax. 2- provide a way for the molten alloy. 3- provide a reservoir for the molten metal to compensate for solidification shrinkage.
Requirements of sprue: 1- should be large enough 2-should allow easy flow of molten metal. 3-should be attached to the thickest part of the wax pattern. 4-should be clean and free of investment or other debris.
TYPES OF SPRUES 1-Wax sprues: 1- it may be supplied either ready made or made by sprue former. 2- it is the best type because, it melts at the same time as the wax pattern.
2- plastic sprues: a) Supplied as ready made only. b) Its disadvantage is that plastic has a melting temperature higher than that of the wax so that, during burning out of wax the plastic sprue will be hard while the wax is soft, this prevents proper burning out of wax.
c) It is better when it is used coated by a thin layer of wax and then it is pulled out of the investing ring by a tweezer before burning out of the wax. 3- metallic sprue: a) Made of rust-proof metal e.g. stainless steel wires, to prevent oxide formation. b) Should be removed from the casting ring before wax burning out.
Size of sprue: 1-should be thicker than the thickest part of casting to avoid shrinkage porosity -thickness of the pattern will determine size of the sprue. 2-the reservoir: -used in case of thick casting. -placed as close to the pattern as possible. -used to compensate for solidification shrinkage.
Length of the sprue -It may vary from 1\4-5\8 inch(6-15mm). -should be long enough to place the pattern, 1\4 inch(6mm) from the end of the casting ring. This is to allow escape of gasses from the mold, and thus prevent back pressure that might cause porosity of the casting.
Location and direction of sprue -should be located at the thickest part of the casting usually on the occlusal surface at the margin ridge. -if accessory sprues are used, they should be waxed and united together with the main sprue
- In the joining the sprue to the pattern, first put a drop of sticky wax into the area to be spread, and then slightly heat the sprue and stick it to the pattern and hold it until it hardens.
Venting Venting. Small auxiliary sprues or vents have been recommended to improve casting of thin patterns. Their action may help gases escape during casting' or ensure that solidification begins in critical areas
1-Cleaning the pattern -The wax pattern should be cleaned with 50\50 mixture of green soap and hydrogen peroxide, then with water and dried. -Application of wetting agent. This to reduce the surface tension of the wax and allow the investment to be adapted closely to the wax pattern.
2-Lining the casting ring: -A clean stainless steel or copper ring of 35mm long and 29 mm internal diameter is prepared. -The ring is lined from inside by one layer of wet asbestos(1mm)thickness.
The asbestose will serve to: a) Regulates expansion of investment. i.e. act as a cushion for investment. b) Regulates distribution of heat. c) Source of water for hygroscopic expansion of the investment. d) Facilitate removal of investment from the ring after casting.
-The asbestose must be short at the crucible end by 1\8inch(3mm), to ensure investment binding itself to the ring at this area.
3-Mounting the pattern on the crucible former -The crucible former should be clean and free from any old set of investment. -in mounting the sprued wax pattern on the crucible former, it should be adjusted so that a space of 1\4 inch (6mm) is left between the bottom of the casting ring and the nearest part of the wax pattern. this allow for the escape of the gasses from the mold, thus preventing back pressure.
4-Pouring the investment: The casting investment is composed of : a) Refractory material (silica): -In the form of quartz or cristobalite -It is responsible for thermal expansion. b) Binder (gypsum products): e.g. calcium sulphate alpha-hemi hydrate (dental stone) it is responsible for setting and hygroscopic expansion.
c) Reducing agent (copper or carbon) Act as a scavenger to remove the formed oxides during casting. d) Coloring agent To differentiate each type of investment - The setting, hygroscopic and thermal expansion of the investment is very important and solidification contraction of the gold alloy.(2.5-0.5%)
-When mixing the powder of the investment with the liquid, the proper w\p ratio should be followed. -A too thin mix will result in weak and brittle investment that might break by force of casting. -A too thick mix will result in improper coating of the investment to the wax pattern producing porosity of the casting
Technique 1-At first fill the wax pattern with the investment, then the ring, by applying the investment in small amounts on the side wall of the ring until all the ring is full, while it is on a mechanical vibrator. Leave it to harden.
2- After the investment hardens, trim its base and then remove the crucible former. 3-remove the metal sprue after heating it and pulling it using a tweezer. 4-If a wax sprue is used, be sure that there are no particles of investment at the sprue hole that might impinge on the burning out of wax.
5-Burning out of wax 1-Done after complete hardening of the investment, (about 30 minutes) and after removal of the sprue. 2-Soack the ring in the water for 3-4 minutes before placing it in the oven. This to prevent cracking of the investment.
3-The ring is placed in the oven so that the sprue channel is facing down to allow for the wax to come down by gravity. 4-Start to raise the temperature for wax burn out.
6- CASTING PROCEDURE Casting equipment is consist of: a) Device for melting the gold. b) Device for forcing the gold in to the mold cavity, such as: 1-Air pressure casting machine. 2-Centrifugal casting machine. 3-Vacuum casting.
Casting flame: It is called gas-air blow pipe. When adjusted, it has 3 zones: 1-inner oxidizing zone: dark blue in colour. 2-middle reducing zone: light blue in colour. 3-outer oxidizing zone: purplish in colour. The middle zone is the only one to be used because: a) It is the hottest zone. b) It is a reducing zone and thus minimize surface oxides.
Flux -It is a reducing agent, that prevents surface oxides on the cast gold. - The flux most used is a mixture of boric acid and borax. It is supplied in the powder form.
Steps of casting procedure 1-After burning out of the wax, take the ring out of the oven. 2-immediately start the casting, by putting the ring in the casting machine according to the technique of casting. 3-Apiece of gold is placed in the crucible space and using the reducing zone of the flame start to melt the gold.
4-When the gold becomes molten and red in colour, put some of the flux and continue the melting till gold becomes cherry red in colour and mirror like shiny. 5-Start then to push the gold to the gold to the mold.
Casting techniques 1-Air pressure technique: Application of the pressure by a metallic disc lined by one or 2 layers of wet asbestos. 2-Centrifugal technique: a) Manually: -The ring is placed in the holder of the machine. -after the gold is molten, sudden rotation of device will force the molten gold into the mold.
b) Mechanically: -The ring is placed in its placed on the machine. -After melting of the gold sudden loosening of the stopper holding the machine will force the gold into the mold. 3-Vcuum technique: -Molten gold is forced into the mold by applying -ve pressure.
7-Cleaning the casting (pickling) -After hardening of the casting and removal from the ring, we remove the investment around it and the sprue using a disc. -Pickling is then carried out by the casting in the pickling solution, which is a warm 50% solution of sulphuric acid (H2SO4)or hydrochloric acid (HCL).
8- Polishing of the casting -Smoothening of the casting surface is carried using: 1-Sand paper disc. 2-Green stone and pumice. 3-Rubber wheel stone and pumice. -Polishing is then done by Iron Rouge and wheel.
1- Incomplete casting: Caused by 1-too small sprue. 2-insufficient alloy 3-insufficient casting pressure 4-alloy not hot enough(150 c increase melting temperature) 5-too cold mold.
2-Round margins 1)Round shiny margins of wax. - Incomplete burnout of wax. 2) Round dull margins caused by: - Insufficient casting pressure. - Back pressure (too short sprue) -inadequate alloy.