FOUNDRY AND FOUNDRY DEFECTS ( CASTING TECHNIQUE )
Introduction :
Foundry is a process for shaping alloys from the liquid state, enabling the production of metal frames for PPAM and infrastructures for crowns and bridges.
1-Foundry of alloys :
1-1 Prerequisites:
Lost wax metal casting or precision casting requires the creation of a wax model of the desired metal part. After fixing a casting rod, the model is invested in a refractory material (or coating) which will perfectly match its shape. After solidification of the coating, the model is completely eliminated by heating.
The molten alloy is then injected into the void thus created. After solidification of the alloy and destruction of the mold, the metal part obtained must be the most accurate reproduction possible of the original model.
Example of a metal frame:
- Parallelizer study
- Preparation of primary models for the production of the duplicate
NB: In fixed prosthesis the working model is cast in extra hard plaster, then this model is split in order to obtain unitary positive models which allows to separate the prepared teeth from the rest of the model. It is on this model that the model of the prosthesis will be made.
- The production of the duplicate in coating.
- Production of the model of the metal frame on the coating duplicate
NB: In fixed prosthesis, the working model is varnished at the level of the prepared teeth, the wax is then sculpted according to the anatomy of the teeth.
- Setting up casting accessories and coating
- Heating of refractory molds and removal of wax
1-2 Foundry of the alloys itself:
The alloy used in PPAM is “Chrome Cobalt”, that used in fixed prosthesis is “Nichel-Chrome”.
Currently, several methods are used to melt alloys before casting them.
- Flame fusion (with blowtorch)
- Fusion by Joule heating
- Fusion by electric arc heating
- Induction heating fusion
Centrifugal casting
The molten mass of alloy is propelled into a mold rotating around an axis, which produces, by centrifugal force, the flow of the alloy into the finest parts of the mold.
Centrifugal force is increased by a greater amount of alloy.
The horizontal rotation sling is the most widely used system in all laboratories
Pressure-vacuum cast :
An inert gas, usually argon, is injected under pressure into the chamber containing the molten alloy. This is then projected into the cylinder where the vacuum has been created by the molten wax.
This casting method is relatively recent. It has some particularities.
It is not necessary to put as much alloy as for a conventional casting since the thrust of the alloy is done by gas pressure/depression.
The tank is placed under vacuum, the molten alloy mass flows under its own weight into the mold by pressure. This results in the cylinders being filled with a penetrating alloy without any brutality, avoiding any crushing and deformation of the coatings and a complete filling.
2-Casting defects:
Even if all precautions are taken, it may happen that the prostheses present certain defects.
Chemical heterogeneity
- It is represented by a difference in chemical composition within the prosthetic part.
- Casting from metallurgically stable alloy blocks makes the presence of chemical heterogeneity in the cast prosthetic parts virtually impossible.
- It should be noted that during melting, the non-precious alloy reacts with oxygen which modifies its chemical and microstructural properties. This is why the reuse of recovered weights from old castings produces porous prosthetic parts whose mechanical and chemical properties have changed.
Defects due to contraction
- Volumetric contraction is a phenomenon that occurs during the cooling of alloys.
- The transition from the liquid state to the solid state is accompanied by a significant loss in volume
- The empty spaces caused by hardening contraction are called shrinkage or micro shrinkage, depending on their size.
Fig.: Shrinkage in an alloy
Incompletely cast part
- Insufficient alloy quantity
- Too low or too high temperature of the alloy during casting
- Cylinder preheating time too short: (final temperature must be maintained 30 to 50 minutes before casting)
- Thickness of the wax layer too thin (it must be at least 0.4 m)
- Poor preparation of the sprue system (length and diameter of the sprues, positioning and diameter of the die as well as the diameter of the feed channels must allow rapid filling of the mold)
- Increased oxide content in the alloy: when reusing
weights.
Fig.: Incomplete casting in relation to a reinforcement in
wax too thin
Solid or gaseous inclusions in the alloy
- Investment inclusions: Waxes must be smooth and have no angles. Investment must be strong enough and have finished setting. Pouring must not be too violent.
- Inclusion of ceramic and/or graphite particles: crucibles must be well maintained and changed regularly.
- Gas residues within the investment: Phosphate-bonded investment materials are particularly dense. To allow air to escape during casting, the smooth surface of the cylinder must be removed.
- Improper flame adjustment: When melting with an oxypropane flame, the alloy must be melted using the reducing zone of the flame.
- Overheating the alloy: Do not overheat the alloy as it can absorb excessive gas which causes porosities in the casting.
Fracture of the cast object
- Fracture during demolding: demolding must always be done after the alloy has cooled.
- Sulfur embrittlement: Gypsum-bonded coatings should not be heated above 750°C or they will decompose, releasing sulfur which makes alloys containing platinum and palladium brittle.
Surface defects
- Insufficient vacuum when mixing the coating: If the vacuum is not sufficient, air bubbles are incorporated into the coating.
- Excessive vibrations when pouring the coating: the intensity of the vibrations must be limited, or for certain coatings, no vibrator must be used to avoid the formation of bubbles.
- The coating temperature rises too quickly and cracks appear.
- Position of the cylinder in the preheating oven: the cylinder must not be too close to a wall because uneven heating of the cylinder can cause cracking.
- Too low thickness of the coating layer: a minimum thickness of 6 mm at all points of the model must be left to guarantee sufficient mechanical resistance of the coating during casting and to avoid leakage of the alloy through the bottom of the cylinder.
- Coating too dry: Some moisture in the coating is necessary to achieve good wax removal.
Conclusion :
The foundry chain is a successive series of steps based on knowledge of basic metallurgical principles.
The practitioner’s mastery of these concepts is a rule in order to ensure obtaining a prosthetic part that meets the expected chemical, microstructural and mechanical requirements.
FOUNDRY AND FOUNDRY DEFECTS (CASTING TECHNIQUE)
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