Spruing a Wax

Objective

Review a basic direct feed sprue configuration for a small bust (roughly 8 inches tall) shown in the bottom figure.

Description

Sprue systems are generally characterized as direct or indirect depending on whether the flow of bronze moves down or up (resp.) into the wax positive. It is common to use a direct feed for bust or bowl shaped piece and indirect for a flat piece with reliefs on one or more sides (generally with sprues feeding into the backside of the wax).

The top figure shows a tree like (branched) gate system oriented with the pour cup upwards. A 1" main gate, to which a cup will be attached, is shown branching into three 1/2" sprues that tapper into a triangular flat section of micro-crystalline wax.

So as to minimize the total height of the investment, the pour cup will be placed roughly 1" above the highest branch.

The gas vent is a 1/4" sprue attached in a similar manner as the secondary gates and worked to be rounded for roughly 1/2" distance from the top. A flexible straw will be attached to this gas vent and secured into the pour cup. After investment, the top bend of this straw will be cut off, leaving a gas vent hole roughly 2-3" from the pour cup.

A 1/4" sprue rod was threaded and attached inside the bust. Once cast, the threads can be cleaned by re-threading the bronze, and the piece will be mounted using this post.

Since the wax positive will be suspended by the sprue system and filled with wet investment material during the investment process, a reasonable test is to fill the wax piece with water and slowly and in a secure manner, lift the wax and water by the main gate. Better the sprues break off long before the investment day while time remains to strengthen them.

Art by Tom Patras (c) 1980, cast in 2000