Spruing Flashcards
What is the definition of sprue?
a channel through which molten alloy can reach the mold in an invested ring, after the wax has been eliminated
What are the functions of spruing?
- forms a mount for the wax pattern
- create a channel for wax elimination
- forms channel for entry of molten alloy
- provides a reservoir of molten metal to compensate the alloy shrinkage
What is the selection of sprue based on?
- sprue former diameter
- sprue length
- sprue former position
- angulation (direction)
- attachment
What are the requirements of sprue diameter?
- it should be approximately the same size as the thickest area of the wax pattern, but In general, a relatively large diameter sprue is recommended because this improves the flow of molten metal into the mold
- for centrifugal casting machine
sprue should be thicker than the thickest portion of the wax pattern
2.6mm (10 gauge)for molar and metal ceramic restorations 2mm (12 gauge) for premolar and partial coverage - if sprue diameter is too small, this area would solidify and leads to suck back porosity
- reservoir sprues are used to overcome this problem
What is a resorvoir?
a small amount of additional wax added to sprue former 1mm below wax pattern
What is the function of reservoir?
compensate for the shrinkage occurs during solidification of the casting
What is the recommended location of sprue?
- the ideal area for the sprue former is the point of greatest bulk in the pattern away from margins and occlusal contacts
- placement sites are occlusal surface, proximal wall or largest non functional cusp (better)
What should the direction of sprue be?
- it should not be at right angle leads – turbulence
- it should be directed away from any thin or delicate parts of the pattern
- ideal angulation: 45 degrees
What should the length of sprue be?
- depends on length of casting ring
- length of sprue former should be such that it keeps pattern away from casting ring: 6-8mm for gypsum bonded or 3-4mm for phosphate bonded
- to provide adequate bulk of investment to withstand forces
What happens if sprue lenght is too short?
gases cannot be escaped to permit complete filling of the mold with alloy
What happens if sprue lenth is too long?
- fracture of investment, molten alloy gets soldified
- the pattern should be placed as close as possible to the center of ring
Where is the sprue attachment supposed to be?
- sprue point of attachment should be carefully smoothed to minimize the risk of turbulence
- the attachment area should not be restricted because it increases cast porosity and reduce mold filling
- patterns are sprued directly and indirectly
What are the types of sprue attachment?
- direct: provides a direct connection between pattern area and sprue base (the flow of molten metal is straight)
- indirect: using a connector or runner bar (reservoir bar) where the wax pattern sprue is attached (mainly used for multiple single units and fixed partial dentures)
What is the number of sprues?
- single
- double
- multiple
What are the types of sprue?
- wax
- plastic
- metallic
What is the preferred type of sprue to use?
wax
What are the characteristics of wax sprue?
preferred because they melt at the same rate of the wax pattern and thus allow easy escape of the molten wax
What are the characteristics of plastic sprue?
- soften at a higher temperature than wax pattern and may block the escape of wax, which results in increased casting roughness
- the plastic sprues are useful when casting fixed partial dentures because of the added rigidity
What are the types of metal sprues?
solid hollowed
What are the characteristics of metal sprue?
- must be made from non-rusting materials to avoid contamination of the interface area between the wax and investment (casting)
- often hollow to increase contact surface area and strengthen the attachment between sprue and the pattern
What are accessory sprues?
small auxiliary sprue or vent
What are accessory sprues recommended for?
- improve casting of thin patterns
- their action may help gases escape during casting
- ompensate for the shrinkage during solidification by acting as a heat sink
What is crucible former?
constitute the base of the casting ring during investing and the
sprue is attached to it
What can the crucible former be made of?
- rubber
- metal
- plastic
What is casting ring and liners?
serves as a container for the investment while it sets and restricts the setting expansion of the mold
What is the aim of liners?
- act as cushion
- venting (regulate heat transfer through the investment material)
- facilitate removal of investment after finishing of casting
- creates a space to allow for investment expansion
- share in hygroscopic expansion when it is placed wet
What happens if lienr is dry?
may absorb water from the investment which cause undesired increase of the total setting expansion
What should the thickness of liner be?
should be around 1 mm not less (one or two layers)
What are the types of rings according to shape?
- round
- oval
What are the types of ring accorsing to complete ring?
- rigid: metal and plastic
- flexible: rubber
What are the types of ring accorsing to split ring?
- metal
- plastic
What is theproblem with asbestos liners?
carcinogenic
What is the disadvantage of ceramicpaper (aluminium silicate)?
cause lung tumour
Is cellulose paper safe?
yes
What isthe sprue in CAD/CAM sustems (milling machines)?
- the sprue in milling machine is the connection between the crown and block
- no need for the conventional sprue because there is no casting
