Spruing, Casting And Investing

Spruing, Casting And Investing

Write a short note on sprue and sprue former.
Answer:

Sprue can be defined as a channel created in the investment through which molten metal enters the mold space which is created due to the burnout of wax pattern. Sprue formers are the materials that form sprues. Sprue former can be made up of wax, metal, plastic/resin.

Functions of Sprue:

• Sprue forms a mount for the wax pattern.
• To create a channel for the elimination of wax during burnout.
• Forms a channel for entry of molten alloy during casting.
• Provides a reservoir of molten metal that compensates for alloy shrinkage during solidification.

Dental Casting Alloy

Dimensions of Sprue Former:

• Sprue Diameter: This depends on the thickness of the wax pattern. The diameter should be equal to the thickest portion of the wax pattern.
• Sprue Location: Sprue should be attached to the widest part of the wax pattern to allow proper flow of molten metal. Sprue should also be placed towards the center to wax pattern to allow molten metal to be directed at all parts of the mold.
• Sprue Direction: Sprue former should be attached at 450 to the thickest part of the wax pattern. The direction of the sprue should also remain away from a thin portion of the wax pattern because molten metal can fracture investment material in this area leading to failure of casting.
• Sprue Attachment: The dimension of connection between the sprue former and wax pattern depends on the density of the metal that has to be cast. Flaring of sprue should be done as it is attached to a wax pattern to allow the entry of molten metal freely into the mold. As the sprue gets attached to the widest part of the pattern, the molten metal will flow from a thick section of mold to thin areas.
• Sprue Length: The length of the sprue solely depends on the length of the casting ring. The length of the sprue former should be adjusted so that the upper end of the wax pattern is within 6 mm of the open end of the casting ring for gypsum-bonded investments and 3 to 4 mm for phosphate-bonded investments. This gives a good strength.