Q. What do you understand by directional solidification? What is gating system? Describe, giving sketches, various types of gates. What are the advantages and disadvantages of pressurized gating system?
Ans. Directional Solidification : As the molten metal in the mould cools, it solidifies and contracts in volume. Since all the parts of a casting do not cool at the same rate due to varying sections, varying rate of heat loss to adjoining mould walls etc., voids and cavities are liable to be formed in certain regions of the casting.
In good casting design these voids are filled up with liquid metal from the portion of the casting which is still liquid. Thus solidification should continue progressively from the thinnest section which solidifies first towards the risers which should be the last to solidify. This process is known as 'Directional Solidification', which is aimed at for producing sound castings.
Directional solidification can be ensured by designing and positioning the gating system and risers properly, increasing the thickness of certain sections of the casting by the use of padding, using exothermic materials in the risers or in the facing sand around certain portions of the casting, using chills in the moulds.
The impurities can be prevented from traveling into the casting by observing the following :
(i) Provision of pouring basin of adequate size helps in breaking down the eroding force of the stream of molten metal as it is being poured from a ladle. A proper design of pouring basin regulates the rate of metal entry, allows the metal to flow into the sprue smoothly and prevents turbulence.
(ii) Provision of ceramic strainer in down sprue helps in preventing dross from the ladle entering the casting.
(iii) The maximum impact is felt at the bottom of vertical sprue from where san is likely to be eroded and enter into mould. This can be prevented by providing ceramic splash core at bottom of vertical sprue.
(iv) Sharp corners in metal flow path should be streamlined to avoid turbulence and dead pockets.
(v) Provision of skim bob helps in trapping both heavier and lighter impurities flowing towards the casting.
Gates, depending on their position may be top, parting and bottom type. In the case of top gating he molten metal is poured down the head or riser. Thus erosion of mould by dropping metal should be ensure by making hard mould. In this case hot metal remains at top and thus proper temperature gradients are established for directional solidification towards the riser. Top gate may be made to serve as riser.
Gating Provision : A sand casting is produced by pouring the molten metal into the mould through a port called "gate". It is the conventional practice to locate the gate either at parting line or in the lower most portion of the casting.
The gating system (comprising of pouring basin, sprue, runner, gates etc.) achieves the following purposes :
(i) To direct the molten metal into the mould with minimum turbulence. Excessive turbulence causes aspiration of air and formation of dross.
(ii) To fill the mould system completely. (It should do so with least disturbance, thereby promoting cleanliness and reducing oxidation).
(iii) To distribute the metal with the least disturbance in order to reduce erosion of the mould material and consequent sand inclusions.
(iv) To skim or separate dross or other foreign matters like, as the metal flows through the gating system, loose sand, oxides and slag should be prevented from entering the cavity of mould by providing skimming action. (It is desirable that the appendages which hinder the flow of metal should be used. Also the thin cores or dividing walls, which might spall when subjected to hot metal be avoided in design).
Heavier sections must be fed with sufficient hot metal through heads and risers to compensate for the shrinkage allowance.
Types of Gates : The molten metal can be directed into mould cavity in various ways. Each gating system depends upon its primary objectives. Thus, a gate may be designed for ease of moulding, to avoid turbulent flow, to prevent washing of sand from the mould walls and to avoid inclusion of slag with the metal entering the mould as discussed above. The various types of gates are discussed below :
(i) Paring line gates : The runners and gates which are formed along the parting line separating the cope and drag portions of the mould, are called paring line gates.
They may contain devices such as skim bobs or relief sprues, as shown in above figure, to collect slag or relieve pouring pressure. The use of a pouring basin to serve this function is shown in above figure. A shrink bob serves the dual function of slag collector and metal reservoir to feed the casting as it shrinks.
The use of core inserts to filter the metal and to prevent erosion of the mould is shown in above figure.
(ii) Bottom gates : The gates which enter into the mould cavity near the bottom of the drag are called bottom gates. A bottom gate is particularly used for steel casting or reduce erosion and gas entrapment and to prevent splashing.
(iii) Horn gate : A horn gate is a type of bottom gate. There is one disadvantage in its use that it has a tendency for producing a fountain effect in the casting. However, it is means of bottom gating without the necessity of a core for the gate as shown in figure.
(iv) Branch gate : A branch gate is designed either to feed a single casting at several points or a number of individual castings as shown in figure.
(v) Top gate : A top gate with strainer core, opens directly into the cavity from above. These gates are not used so often but occasionally used to feed the molten metal more efficiently. The wedge gates, finger gates and pencil gates are all types of top gating system.