The Sprue :

Its a channel through which molten alloy can reach the mold in an invested ring after the wax has been eliminated. Role of a Sprue: Create a channel to allow the molten wax to escape from the mold. Enable the molten alloy to flow into the mold which was previously occupied by the wax pattern.

FUNCTIONS OF SPRUE

1 . Forms a mount for the wax pattern .
2 . Creates a channel for elimination of wax .
3 .Forms a channel for entry of molten metal
4 . Provides a reservoir of molten metal to compensate for the alloy shrinkage .

SELECTION OF SPRUE

Sprue former gauge selection is often empirical, is yet based on the following five general principles:
1.   Select the gauge sprue former with a diameter that is approximately the same size as the thickest area of the wax pattern. If the pattern is small, the sprue former must also be small because a large sprue former attached to a thin delicate pattern could cause distortion. However if the sprue former diameter is too small this area will solidify before the casting itself and localized shrinkage porosity may result.
2.   If possible the sprue former should be attached to the portion of the pattern with the largest cross-sectional area. It is best for the molten alloy to flow from the thick section to the surrounding thin areas. This design minimizes the risk of turbulence.
3.   The length of the sprue former should be long enough to properly position the pattern in the casting ring within 6mm of the trailing end and yet short enough so the molten alloy does not solidify before it fills the mold.
4.   The type of sprue former selected influences the burnout technique used. It is advisable to use a two-stage burnout technique whenever plastic sprue formers or patterns are involved to ensure complete carbon elimination, because plastic sprues soften at temperatures above the melting point of the inlay waxes.
5.   Patterns may be sprued directly or indirectly. For direct sprueing the sprue former provides the direct connection between the pattern area and the sprue base or crucible former area. With indirect spruing a connector or reservoir bar is positioned between the pattern and the crucible former. It is common to use indirect spruing for multiple stage units and fixed partial dentures.

Cement liners

Applications (if remaining dentin thickness is <0.5 mm)

o    Used for thermal insulation where cavity preparation is close to the pulp
o    Used for delivering medicaments to the pulp

•    Calcium hydroxide stimulates reparative dentin or
•    Eugenol relieves pain by desensitizing nerves
•    Used to deliver F ion to enamel and dentin

Components

o    Paste of calcium hydroxide reactant powder, ethyl toluene sulfonamide dispersant, zinc oxide filler, and zinc stearate radiopacifier
o    Paste of glycol salicylate reactant liquid, titanium dioxide filler powder, and calcium tungstenate radiopacifier

Reaction

Chemical reaction of calcium ions with salicylate to form methylsalicylate salts Moisture absorbed to allow calcium hydroxide to dissociate into ions to react with salicylate Mixture sets from outside surface to inside as water diffuses

Manipulation

Dentin should not be dehydrated or material will not setMix drop of each paste together for 5 secondsApply material to dentin and allow I to 2 minutes to set

Properties

o    Physical-good thermal and electrical insulator
o    Chemical-poor resistance to water solubility and may dissolve
o    Mechanical-low compressive strength (100 to 500 psi)
o    Biologic-releases calcium hydroxide constituents, which diffuse toward the pulp and stimulate
o    reparative dentin formation

POLYCARBOXYLATE CEMENT 

Use:. The primary use of polycarboxylate cement is as a cementing medium of cast alloy and porcelain restorations. In addition, it can be used as a cavity liner, as a base under metallic restorations, or as a temporary restorative material. 

Clinical Uses

Polycarboxylate cement is used in the same way as zinc phosphate cement, both as an intermediate base and as a cementing medium. 

c. Chemical Composition. 

(1) Powder:. It generally contains zinc oxide, 1 to 5 percent magnesium oxide, and 10 to 40 percent aluminum oxide or other reinforcing fillers. A small percentage of fluoride may be included. 
(2) Liquid. Polycarboxylate cement liquid is approximately a 40 percent aqueous solution of polyacrylic acid copolymer with other organic acids such as itaconic acid. Due to its high molecular weight, the solution is rather thick (viscous). 

d. Properties. 

The properties of polycarboxylate cement are identical to those of zinc phosphate cement with one exception. Polycarboxylate cement has lower compressive strength. 

e. Setting Reactions: 

The setting reaction of polycarboxylate cement produces little heat. This has made it a material of choice. Manipulation is simpler, and trauma due to thermal shock to the pulp is reduced. The rate of setting is affected by the powder-liquid ratio, the reactivity of the zinc oxide, the particle size, the presence of additives, and the molecular weight and concentration of the polyacrylic acid. The strength can be increased by additives such as alumina and fluoride. The zinc oxide reacts with the polyacrylic acid forming a cross-linked structure of zinc polyacrylate. The set cement consists of residual zinc oxide bonded together by a gel-like matrix. 

Precautions. 
The following precautions should be observed. 
o    The interior of restorations and tooth surfaces must be free of saliva. 
o    The mix should be used while it is still glossy, before the onset of cobwebbing. 
o    The powder and liquid should be stored in stoppered containers under cool conditions. Loss of moisture from the liquid will lead to thickening. 
 

WETTABILITY
To minimise the irregularities on the investment & the casting a wetting agent can be used .

FUNCTIONS OF A WETTING AGENT

1 . Reduce contact angle between liquid & wax surface .
2 .Remove any oily film left on wax pattern .