Fundamentals of gating and feeding nonferrous alloys.
The primary function of a gating and risering system is to introduce clean, dross free, tranquil metal into the mold cavity and then continue to feed the casting as it solidifies. In nonferrous casting, gas, inclusion and shrinkage defects can be attributed to inefficient gating and risering systems. When these problems arise, it is the job of the foundryman to examine the complex network of sprees, runners, ingates and risers and determine the cause of the defects. At the heart of this analysis is an understanding of the fundamental principles of gating and risering system design.
This article reviews some of the fundamentals of nonferrous gating and risering system design. Although absolute rules can't be applied to all nonferrous alloys nor to castings of all shapes, sizes and complexities, general guidelines can provide the foundation for gating and risering systems that produce sound cast components.
The first concern in gating for nonferrous alloys is the non-turbulent introduction of metal to the mold cavity at the lowest possible velocity to maintain an optimum casting fill rate. The optimum fill rate for a particular alloy cannot be considered a fixed value because it depends on many factors such as casting weight, section thickness and casting shape. Optimum stream velocities vary with the alloy and can range from 75 mm/sec for aluminum bronze alloys to 500 mm/sec for aluminum alloys. Excessive stream velocities result in increased turbulence, the potential for breakup and entrapment of oxides and sand inclusions on the metal front, a reduction of the casting's mechanical properties, and often, rejected castings.
As a solution, the conflicting requirements of high fill rate and low stream velocity (particularly with short freezing range and strong dross-forming alloys) often result in gating systems that appear excessively large and exceed recommended gating ratios based on surface area. Nonferrous alloys must be cast through non-pressurized gating systems with ingates taken off the cope side of the mold to ensure the runner is running full at all times. In addition, the distance between the spree and ingates should be maximized to allow for dross to float out and be trapped against the upper surface of the runner bar [ILLUSTRATION FOR FIGURE 1 OMITTED]. Whenever possible, nonferrous castings should be gated into or near the bottom of the casting to minimize turbulence within the mold cavity.
The use of a pouring basin (properly designed) is recommended on all but the smallest of non ferrous castings. The pouring basin must be designed so the pourer can fill the sprue quickly and maintain a near constant head throughout the pour. An off-set design incorporating a dam achieves this objective [ILLUSTRATION FOR FIGURE 2 OMITTED]. The pouring basin must be rectangular in shape so the upward circulation during pouring will assist in dross removal, and the exit from the basin must be raised and match up with the sprue entrance. A manually lifted pouring stopper is sometimes used in the basin (with short-freezing alloys such as aluminum bronze) to allow the pourer to fill the basin completely for a quick gating system and mold fill and to provide time for the dross to float to the top before the stopper is removed.
The practice of pouring directly down the sprue or the use of conical-shaped basins with direct flow down the sprue isn't recommended for nonferrous castings. Without the basin, air and dross are entrained in the metal and carried down into the system, and the high velocity of the metal stream will result in excessive turbulence in the gating system.
The sprue controls the fill rate of the casting and, as a result, is the single most important part of the gating system. Whenever production practices permit, the sprue should be tapered with the smallest controlling area at its base. All subsequent parts of the gating system are determined from the sprue exit area.
Many formulas exist to determine the correct taper for the spree, however, it is sufficient to provide a 5 [degrees] taper from the controlling area (the pouring basin). When the spree height is more than 300 mm, the sprue's section diameter or length must be increased by 50% [ILLUSTRATION FOR FIGURE 3 OMITTED].
The cross section of the sprue can be round, square or rectangular, although evidence suggests that a rectangular shape is preferred because it reduces the tendency for vortex formation and subsequent air aspiration. In addition, a square or rectangular shape may be easier to construct than a round, tapered section.
Once the metal stream has reached its peak velocity and flowed to the end of the sprue - the sprue well - it is important to cushion the stream and allow the metal flow to change from vertical to horizontal with a minimum of turbulence. Recommended sizes of the spree well are: a diameter 2-3 times the sprue exit diameter and a depth equal to twice the depth of the runner bar [ILLUSTRATION FOR FIGURE 3 OMITTED].
Runners and Gates
As previously mentioned, non ferrous alloys must be cast with a non-pressurized gating system with the runner in the drag and the ingates in the cope. The area of the runner bar must be 2-4 times the area of the sprue base, and the total area of the ingates must be at least equal to or up to twice the area of the runner. This ensures the required fill rate is achieved at the lowest possible velocity. Alloys that are particularly susceptible to drossing, such as aluminum bronze, may require even larger runners and gates to ensure stream velocities are kept to a minimum.
The runner cross-section ideally must be rectangular with a width to depth ratio of 2 to 1. The wider upper surface maximizes the potential of the runner bar to collect dross and inclusions. When multiple gates flow from a runner bar, the area of the runner must be reduced by the area of each gate it passed to ensure that the metal flow from each gate is uniform [ILLUSTRATION FOR FIGURE 4 OMITTED]. In addition, a dross trap at the end of each runner can remove the initial, heavily oxidized metal from the system.
Ingates must enter the mold cavity at the lowest possible level to avoid the turbulence of the falling metal stream. As with the runner bar, ingates must be rectangular in cross-section (instead of square) to avoid hot spots and subsequent porosity at the casting contact. The exact width-to-thickness ratio must be determined by the solidification time of the casting. As a rule of thumb, the thickness of the ingates must be less than 33% the thickness of the casting at the point of contact.
Solidification mechanisms (risers) in the feeding systems of nonferrous alloys vary widely because the freezing ranges of nonferrous alloys can vary from a few degrees for an aluminum bronze to more than 392F (200C) for a tin bronze. As a result, to discuss alloy feeding practices, it first is necessary to understand how the alloy solidifies. Nonferrous alloys cannot be simply classified by name because they can vary greatly in freezing range. Instead, alloys are classified according to their solidification ranges (liquidus and solidus isotherms).
Short-freezing range alloys cool in a sand mold as the portion of the liquid metal that first reaches the liquidus temperature begins to solidify. This usually occurs at the mold interface where the heat extraction is greatest. The chilling action of the mold wall results in the formation of a thin skin of solid metal surrounding the liquid. With further extraction of the heat through this shell of solid metal, the liquid begins to freeze onto it, and the wall of solid increases in thickness.
The solid and liquid portions are separated by a sharp line of demarcation - the solidification front - which advances toward the center of the casting. The crystal growth on the solidification front is short and corresponds to the start of freeze at the apex and the end of freeze at the base. Short-freezing range alloys encourage directional solidification even at low thermal gradients.
With long-freezing range alloys, the development of directional solidification is difficult. Although a thin skin may initially form on the mold walls, solidification does not proceed progressively inward. Instead, solidification begins by advancing from the mold walls toward the interior of a nucleation wave corresponding with the liquidus isotherm. At a later point, a second end-of-freeze wave corresponding with the solidus isotherm moves away from the mold wails and pursues the nucleation wave toward the center of the casting. Therefore, freezing begins at each location in the casting when the nucleation wave passes it and ends when the end-of-freeze wave reaches it.
In general, there are three distinct zones during the solidification of a long-freezing range alloy: a completely liquid zone at the thermal center of the casting; a solid metal zone next to the mold walls; and a region of partial solidification between the liquid and solid zones. In extreme examples of long-freezing range alloys, such as heavy-section tin bronze that has a freezing range more than 392F and high thermal conductivity, there can be liquid and solid phases coexisting throughout the casting section.
Solidification Mechanism and Shrinkage
The variable freezing modes that are displayed by nonferrous alloys result in different forms of porosity within the casting and riser. Generally, short-freezing range alloys show deep pipes in the risers as feed metal is supplied to the casting through the entire solidification interval. Internal porosity within the casting can take the form of small open cavities (centerline shrinkage) that occur near the end of solidification when feed metal is cut off by the merging of parallel solidification fronts, or, open cavities at inadequately fed thermal centers and isolated heavy sections [ILLUSTRATION FOR FIGURE 5A OMITTED].
With long-freezing range alloys, risers often show minimal pipe as the "mushy" solidification mode only will allow liquid flow for part of the total solidification time. Finely dispersed porosity can exist throughout the casting section, with coarser concentrations in slower cooling areas such as junctions and riser heads. Under normal foundry conditions, it is almost impossible to achieve absolute soundness in long-freezing range alloys such as tin or phosphorus bronze. It is common for up to 60% of the liquid and solidification shrinkage of this alloy to be dispersed through the entire casting section [ILLUSTRATION FOR FIGURE 5B OMITTED].
To produce sound castings with short-freezing range alloys, the metal must begin to solidify at points away from the riser heads and proceed progressively toward the risers (the last part to solidify). As a result, all liquid and solidification shrinkage is contained in the risers, while the casting is sound. This type of continuous solidification is called directional solidification, which is defined as the establishment of conditions that develop a solidification front that is substantially V-shaped in a longitudinal cross-section (with the large end of the V directed towards the risers).
This ideal scenario, however, usually is not achieved in practice due to the complexity of casting design and the difficulties in establishing adequate thermal gradients across the casting section. General riser guidelines for the successful feeding of short-range alloys are:
* risers must be placed over thermal centers of the casting;
* risers must solidify after the part of the casting they are connected to (modulus ratios more than one are used);
* risers must be of sufficient volume to compensate for the liquid and solidification shrinkage of the alloy that is influenced by the alloy constitute, the degree of pouting superheat, the shape of the casting and the gas content of the alloy.
Consideration must also be given to the feeding range of a particular alloy to determine the distance a feeder will modify the thermal gradients in a uniform casting section to promote directional solidification.
For long-freezing range alloys, directional solidification has little relevance and can adversely affect casting soundness by concentrating porosity into localized areas. For example, in long-freezing copper and aluminum alloys, the difficulty in feeding the porosity is aggravated by the alloys' high thermal conductivity, which maintains nearly uniform thermal gradients through casting solidification. In addition, the high specific heats and latent heats aggravate this condition.
The goal in feeding long-freezing range alloys is not to eliminate porosity, but to ensure it is dispersed evenly throughout the casting section. An example of this practice is with leaded bronze, which is often cast without risers so that thermal gradients are kept as uniform as possible.
It is best if risers only compensate for superheat and a portion of solidification shrinkage. To avoid extending solidification time, modulus ratios from 0.6-1 are commonly used. The shrinkage volume that risers must compensate for is influenced by the alloy constitution, the degree of pouring superheat, the section thickness of the casting and the gas content of the alloy. Due to the alloys' small feeding range, the absolute elimination of porosity in normal foundry conditions is almost impossible.
This article was adapted from an article that appeared in the January-February 1995 issue of Metal-Casting and Surface Finishing.
RELATED ARTICLE: Variables in Gating and Riser System Design
A gating and risering system is an expensive part of a casting. It must be designed by trial and error, fabricated and added to the pattern, molded into the sand or cut into the die, separated from the casting via a mechanical process, and remelted as scrap. Since the aluminum casting industry has a 55% casting yield, 45% of everything it pours is part of the gates and risers system. But, this system is critical to the production of a sound cast component.
The goals of a gating and risering system are to:
1. Reduce melt turbulence by maintaining its velocity with set limits.
2. Avoid air entrapment in the melt.
3. Prevent incomplete castings.
4. Promote directional solidification.
5. Clean the metal.
6. Allow consistent pouring.
7. Compensate for shrinkage.
Other factors, however, are often built into the system that can compromise the basic purpose of a gating and risering system. For example, foundries commonly demand that their gating and risering systems:
* be easy to remove;
* fit the contours of a molding box or permanent mold setup;
* aid with knockout, shakeout and machining operations;
* have the highest possible yield;
* work the first time;
* be simple and consistent with other jobs in the foundry.
But, these compromises in the system are demanded for good economic reasons. For example, when a foundry designs its gating system for easy removal in shakeout and knockoff, it also may incorporate up to 20% more metal. From the foundry's perspective, its savings on labor costs by providing for easy removal of the gating system outweigh the remelting cost of the metal. In this case, the foundry examined its options in the design of its gating and risering system and made a decision based on economics and functionality.
Gating System Components and Their Design Variables Gating Component Variable Ladle Pouring rate; height of metal pour; lip design. Pouring basin Shape; cross-sectional area; dam height/profile. Sprue Profile; cross-sectional area; taper; height. Sprue well square or round; depth; base contour; cross-sectional area. Runner bars Tall, short or square; drag or cope. Runner extension Length; shape. Runner reduction Steps; taper; ratio. Ingates Number; shape. Change of direction Radius. Vents Number; location. Choke Size; location. Gating ratio 1 to 4 to 4; other options. Filling time Varies by casting.
The table below lists the various components of a gating system and the design options that are available. It is vital for foundries to understand that it is difficult to predict how changes to two interacting parameters will affect the efficiency of a rigging system. Therefore, when a suggestion is made for a rigging change, which at face value makes economic sense, look out for the functionality of that change as it can return to harm you in post-casting processing.
Philip Sandford, Foseco, Inc., Cleveland
This sidebar was adapted from a presentation at the 1998 International AFS Conference on Molten Aluminum Processing. Conference proceedings are available from AFS Publications at 800/537-4237.
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|Title Annotation:||includes related article on gating and riser system design|
|Date:||Feb 1, 1999|
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