GATING METHODS AND TYPES OF GATING SYSTEMS

In choosing the method of gating and the design of a gating system, it is necessary to keep in mind that the molten metal must steadily flow into the mold, without swirling or impinging on the projecting parts of the mold and cores. The pouring methods must also permit gradual escape of the air and gases from the mold and enable directional solidification of the casting, taking into account its shape and the properties of the alloy cast.

In gating of gray iron casting, it is common to supply the metal to a narrow portion of the casting in order to level off the rate of cooling in its individual portions. As it passes through a narrow portion of the casting, the metal heats up the mold at the inlet section and goes on moving now cooled to a certain extent into a massive part of the casting. The thin portion in the heated part of the mold and the massive part filled with a somewhat cooled metal solidify at the same rate, which makes for a higher quality of the casting, decreases internal stresses

However, in running of casting with massive members, along with the method of supplying metal to a casting through its narrow portion, in use is the gating method by which the gate delivers the metal to a massive part, into the base of the riser, in order that the metal in the riser may long stay in the liquid state and thus feed the casting.

The melt that fills the mold should not flood cope seats or impede the escape of gases from the cope and mold. The pouring rate (rise of metal in the mold per second) must be sufficient for the gases being evolved and the air found in the mold cavity to leave freely the mold. On the other hand, a slow pouring may lead to a decrease in the fluidity of metal, to poor filling of the mold, and, in large molds with a developed metal surface, to excessive heating of mold walls due to thermal radiation of the melt, and slag spots on the surface of the casting.

The types of running used with each of the chief gating methods are bottom gating, mold-joint gating, top gating, step gating, stack gating, side gating, composite gating. Each of the gating methods has its own advantages and disadvantages. The choice of a gating method depends on the shape of the casting, the purpose it must serve, and the metal employed.

The gating method and the design of a gating system and its elements depend not only on the shape and size of the casting, but also on the properties of the alloy cast.

GATING SYSTEMS

Mold-joint gating. The systems of this type of gating are the simplest and find wide use for running various castings in molds having a depth of the cavity from the parting line to its lowermost point up to 200 mm (the allowable height from which the incoming metal falls on the bottom of the mold without causing perceptible damage to the walls). When using such gating systems, one should take account of the mass of melt and its pressure on the mold walls, which depends on the sprue height.

Bottom gating. The bottom gating systems allow the quiet flow of metal into the mold. In casting a gear, for example, the metal enters the casting through the gate that runs to the bottom of the hub, but not to the teeth. Bottom gating is applicable for running both small and large castings. In the mass production of small castings, however, molding machines are rarely used to prepare molds for bottom pouring since the gating system here calls for the production of special cores.

Quiet entry of metal into the mould cavity is best achieved by its introduc­tion at the lowest level. Using this method the metal rises steadily through the mould, splashing is eliminated and dislodged moulding material tends to be carried to the surface. If bottom gates are used with top feeder heads the resulting temperature gradients are opposed to feeding, but various measures are available to mitigate this effect. Despite the greater complexity of moulding the method is much used for heavy castings.

Top gating. The gating systems of this type are employed to run both medium-size and large castings such as flywheels, gears, and cylinders. In the shower-type top gating system, the metal runs into the mold from the top through a shower gate that consists of several small-diameter pencil gates cut in the pouring cup or in a special core. The shower gate has the disadvantage of producing a splashing effect: as the stream of metal falls on the mold bottom or on the surface of the melt, it splashes and thus forms on the mold walls beads (buttons) of solidified metal that does not weld up with the basic metal of the casting. A combination of the bottom gate and the shower gate used in practice does away with this drawback.

An important feature in favor of the shower-type gating system is that the metal uniformly fills the mold without overheating its individual portions. This results in a sound casting free of porosities and shrinkage cavities.

When metal is poured through a top gate or directly into an open feeder head, the stream impinges against the bottom of the mould cavity until a pool is formed; this is kept in a state of agitation until the mould is filled. The erosive effect of the unconfined stream can be severe, whilst the associated splashing gives opportunity for oxidation. The mould surface can, however, be protected at the point of impact by preformed refractory tiles; the intensity of erosion can also be reduced, where fluidity of the metal permits, by the use of pencil gates. This method and others which divide the metal stream are unsuitable for alloys which are sluggish or prone to rapid oxidation, but are used with success, for example, in the gating of cast iron.

The principal advantages of top gating are its simplicity for moulding, its low consumption of additional metal and, above all, the generation of temperature gradients favourable to feeding from top heads; this arises from the proportionately rapid cooling of the first metal poured, followed by the progressive accumulation of metal from above until the mould is full.

Composite gating. The gating systems of this variety serve for running intricately shaped, high, thin-walled castings. The system is a combination of the bottom gate and the shower gate. The mold is first poured through the bottom gate and then through the shower gate. This pouring method prevents mold erosion and metal splashing. As the metal fills the mold, its level in the sprues gradually rises, and, starting at a certain moment, the upper gates begin to feed the casting.

Step gating. Step-gate systems are used for running large and heavy castings. The step-gate construction enables subsequent feed of the casting through the gates from the bottom upwards. The molten metal flows from a basin down a sprue and into a runner through a choke whose cross section determines the rate of pouring. The metal then runs through branched vertical runners into gates. Because the runners have a larger cross section than the choke, the melt first goes into the lower part of the casting and then into its upper part through the next gates. Such a system excludes overheating and erosion of the mold at the inlet to the cavity and enables the production of high-quality castings.

Stack gating. This system of gating is employed in stack molding of small castings and also in casting of small pieces on automatic flaskless molding lines producing molds with a vertical parting joint. The ingates for every row of castings differ in cross section: the higher the position of a gate, the larger its cross-sectional area, with the result that the pouring time for every casting becomes the same.

Side gating. Moulding can be simplified by the discharge of metal into the side of the mould cavity through ingates moulded along a parting plane; this practice frequently offers the best compromise between moulding convenience and the ideal gating arrangement. Using side gating, progressive mould filling can be achieved by tilting the mould towards the ingates to provide uphill casting conditions.

Many variations of bottom and side gating find practical application. Apart from the multiple systems to be separately considered, the horn gate is one widely applied form of bottom gate. This type of gate, with its smooth curves and progressive change of dimensions, is designed to minimize erosion and oxidation. Since the normal horn gate is prone to jet effects, the reversed horn gate has frequently been adopted for those alloys such as the aluminium bronzes which are especially susceptible to skin formation, although opinion is not unanimous as to its effectiveness.

These fundamental differences in running and gating practice have been the subject of such attention in the production of aluminium alloy castings, given the now well-recognized dangers of insidious oxide film defects arising from surface turbulence and air entrainment.

Multiple gating systems. Whilst small castings are commonly either top poured or gated through a single sprue and ingate. The latter method becomes increasingly unsuitable for castings of larger dimensions, because of the danger of overheating the mould adjacent to the point of entry and because of the long flow distances within the mould cavity.

The solution lies in the use of more complex gating systems in which the metal is directed through separate gating elements to different parts of the mould. Multiple systems can be used either to introduce the metal at widely separated points in the same horizontal plane, or at higher levels as in step gating. The choice or combination of these methods depends upon the shape and orientation of the casting. In multiple gating it is necessary to control the distribution of metal between the separated elements of the system.