Sprue Former

Treesa Mary george

2nd year

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 prev. occupied by wax pattern

Definition

FunctionsoForms a mount for the wax pattern.oCreate a channel for wax elimination.oForms channel for entry of molten alloy.oProvides a reservoir of molten metal to compensate the alloy shrinkage.

Sprue Diamete

r

Sprue postion

Sprue attachm

ent

Sprue directio

n

Sprue length

Selection of sprue

It should be approximately the same size as the thickest area of the wax pattern

Ie; if the pattern is small,the sprue former must also be small coz a large sprue former to a thin,delicate pattern could cause distortion

If sprue diamtr is too small,this area would solidfy and leads to suck buck porosity.

Reservoir sprues are used to overcome tis problem

I. SPRUE DIAMETER

The ideal area for the sprue former is the point of greatest bulk in the pattern:

to avoid distorting thin areas of wax.

permit complete flow of alloy into mold

cavity. Placement sites are occlusal

surface,proximal wall or non functional cusp to minimize subsquent grinding of occlusal anatomy and contact areas.

II. Sprue Position

III. Sprue Attachment

•It should be attached to the portion of the pattern with the large cross sectional areas.•Alloy flow from thick section-thin area to minimize the risk of turbulence.•Flared for high density gold alloys & restricted for low density alloys.•Patterns are sprued directly and indirectly.

Direct Spruing

•Sprue former provides a direct connection btwn pattern area and sprue base.

Indirect Spruing•a connector or reservoir bar is used.•Mainly used for multiple single units and fixed partial dentures.

Reservoir prevent localized shrinkage.

IV.Sprue Direction Directed away from thin parts of pattern. It should not be at right angle leads –

turbulence Ideal angulation : 45 degrees.

Sprue Length Depends on length of casting ring. Length of sprue former should be such

that it keeps pattern abt 6-8cm away from casting ring.

Short sprue length: gases cannot be adq. vented to permit the molten alloy to fill the ring leading to back pressure porosity.

Long sprue length: fracture of investment,molten alloy gets soldify.

6mm-gypsum bonded 3-4mm phosphtae bonded.

Types of sprue

I. Wax-mainly prefered II. Plastic- 2 stage burnout technique is used.