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Gate, runner location critical to quality castings.

The sprue basin is the enlargement at the bottom of the sprue that reduces turbulence in liquid metal entering the runner. Typically, high velocity occurs at the bottom of the downsprue in horizontally parted molds that, left uncontrolled, can cause serious gas and oxide entertainment and possible mold erosion.

Though a rectangular sprue basin is theoretically the optimum shape, a circular shape works as well. Good basin design has a flat or convex bottom rather than a concave shape that can induce metal splashing and trap gases or oxides.

Nonpressurized System

An effective gating system remains full through an entire pour. Because of the gating ratios required in a nonpressurized system, ingate location is critical in creating and maintaining a full system. The largest total cross-sectional area in this system is typically at the ingate(s). Therefore, the choke, located at the bottom of the downsprue, can never provide enough metal to flood the runner and ingates unless both are correctly located.

If one considers a nonpressurized system as a funnel, it is very easy to fill and maintain a certain depth of liquid. Invert the funnel and the opposite is true. The pourer can never introduce enough metal through the downsprue to fill the ingate(s).

To properly fill this system, the correct placement of ingate(s) in relation to the runner is essential. If the runner is placed in the cope with the ingates, a majority of the metal will go directly into the mold cavity without filling the gating system.

Placing the runner in the drag and the ingate(s) in the cope will always assure completely flooded runners before any metal enters the ingate(s). This design is preferred.

Pressurized System

Unlike the nonpressurized system, this system tends to fill quickly because of its gating ratio. The smallest total cross-sectional area will always be at the ingate(s). Because the ingate(s) restricts flow beyond what the sprue and runner can deliver, the pourer should always keep this system full.

The placement of runner and ingate(s) becomes more important as a method to clean molten metal rather than establishing and maintaining a full system. Impurities in the metal stream tend to float, dictating the shape and orientation of the runner.

In a pressurized system, the runner and ingates(s) are located in the cope and the runner is at least twice as high as it is wide. This allows oxides or sand to migrate up and away from the ingate(s) entrance at the parting line. If an equal part of the runner is placed in the drag, then drag ingates may be used. Locating a portion of the runner in the drag keeps the first metal down the sprue from entering the mold cavity.

Metal Distribution

When multiple ingates are used, each ingate should deliver an equal quantity of metal to reduce sand defects and to promote directional solidification. In a straight runner design, momentum of a free-flowing liquid metal will cause it to bypass ingate(s) located close to the downsprue. Not until an equal or greater force acts upon it, will the direction change.

This force is usually created as back pressure caused by the molten metal after it strikes the end of the runner. The ingate farthest from the downsprue will deliver the first and majority of metal into the mold cavity, followed consecutively by ingates closer to the downsprue. Thus, the end ingate would also be the location of sand erosion and expansion and of shrinkage defects.

In an effective system, each ingate will deliver an equal amount of metal and begin filling at the same time. This results from reducing the cross-sectional area of the runner beyond each ingate by the area of the ingate passed. The same flow can be obtained by simply tapering the entire runner. However, a stepped design is more desirable.

Runner Extension

The first metal down the sprue and runner is of the least desirable quality. A runner extension several inches beyond the last ingate will trap and prevent this polluted metal from entering the mold cavity. If adequate room in the flask is unavailable, then some type of sump is effective.

For those wanting to learn more about gate and riser design, CMI will offer courses in July and September 1992. For more information, call CMI at 800/537-4237.
COPYRIGHT 1992 American Foundry Society, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1992, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Title Annotation:part 4
Author:Strobl, Scott
Publication:Modern Casting
Date:Jun 1, 1992
Words:726
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