Core-Making Process

Core making is an important branch in any foundry and the choice of core making depends on various factors. To name them are depending on type of metal to be cast, depending on the size of casting, choice based on complexity involved in a casting process, depending on the requirement of quality in final product, depends on equipment used for production and energy source.There are six most common technologies involved in core making. They are

• Oil as the sand binder
• Green sand
• Hotbox
• Coldbox
• Nobakes
• Shell process

In each of the above technologies there are various advantages and drawbacks.

Requirements and Features of Common Technologies in Core Making :

When each of the above technology is ranked based on the usage of energy per unit of product ranking from greatest to lowest amount the order is oil as the sand binder being the highest followed by green sand, hotbox, coldbox, nobakes and shell process being lowest usage of energy in ranking.

That is when rated in percentage the technology of oil as the sand binder uses the maximum energy of hundred percentage and green sand used 85% followed by hotbox with 75%, coldbox with 58%, nobakes with 38%. The least energy usage is made by shell process technology with 35% recorded as usage.

The oil as the sand binder as well as the green sand technology uses high energy because it needs high temperatures for the process of curing and refractory coating and pasting has to be carried out on cores. The hotbox technology gives output as solid cores and needs hot curing for the process of binder setting. The technology of coldbox is carried out with the usage of heated sand and amine gas mixture.

Nobakes technology operates at accurately controlled setting. The least usage of energy is by shell process technology in which no coating is necessary.

Process change to any foundry must be taken only after careful projection on various factors being taken into account. Some to name in this direction are processing change results on costs like fixed and variable, investment or capital cost, payback needed and so on. In a similar way per actions towards energy consumption must be made for achieving system efficiency, which must also focus on step towards consistent usage of such energy consumed to be used efficiently.

Some of the steps suggested for workforce in this aspect are avoiding wastages of shell sand, usage of equipments producing heat only when needed, analyzing and taking steps towards vital defects, making economical usage by switching of unwanted machineries.

Some of the process improvements that could be made at reduced cost towards this direction are taking steps for oven insulation, going for moisture sensors installation, taking steps towards process improvements like reviewing for reusable products like steel cores which increase productivity and helps in achieving greater quality with cost cutting.

The high cost up gradations that could be taken in the direction for system efficiency are going for improvements in drying process by making use of either infrared heating or microwave heating, making use of technique of continuous drying, reuse of heat recovered, taking steps towards heat recovery like recovery of heat from exhausts attached in the drying or baking oven.

DETAILED DISCUSSION ON GATING SYSTEM

A system that leads molten metal to flow through a ladle to mold cavity consisting of pouring basin, Sprue, Sprue base, runner, runner extension, in-gates, riser called as gating system.
The design consideration of gates are given below:
1) The mold cavity should be completely filled within a smaller time without having to rise the metal temperature as huge higher metal head.
2) The molten metal should flow smoothly into the mold without any turbulence.
3) Unwanted materials such as slag and other mold materials should not be allow to enter the mold cavity.
4) The metal velocity should be controllable so as to avoid any aspiration of air ,erosion of mold cavity.
5) Should promote directional solidification.
6) Casting yield should be maximized.
The different elements of gating system are discussed below:
1) Pouring basin/cup
2) Sprue
3) Sprue base
4) Runner
5) Runner extension
6)In gates
     POURING BASIN/CUP:: It is the first step where the molten metal is poured and act like reservoir from which it moves smoothly into the Sprue. The molten metal is not directly poured in Sprue because it may erode the wall of sprue and it also does not allow to enter the slag into mold cavity. The pouring basin should be full otherwise atmospheric air and slag may enter the mold cavity. The main function of a pouring basin is to reduce the momentum of the liquid flowing into the mold cavity.
   SPRUE:: Sprue is the channel through which the molten metal is brought into the parting plane where it enters the runners and gates to ultimately reach the mould cavity. The molten metal when moving from top of the cope to the paring plane gains velocity and as a consequences  requires a smaller area of cross section for the same amount of metal to flow at the top. To eliminate this problem of air aspiration, the sprue is tapered to gradually reduce the cross section as it moves away from top of the cope.
SPRUE BASE WELL:: This is the reservoir for the metal at the bottom of the sprue to reduce the momentum of the molten metal. The molten metals it moves down the sprue gains in velocity, some of which is lost in the sprue base well by which the mold erosion is reduced
 RUNNER::  It is generally located in the horizontal plane , which connects the sprue to its in-gates , thus allowing the metal enter the moud cavity.The runners are normally made trapezoidal in cross section. It is a general practice for ferrous metals to cut the runners in the cope and the in-gates in the drag. The main reason for this is to trap the slag and dross,which are lighter and thus trapped in the upper portion of the runners.
  RUNNER EXTENSION:: The runner is extended a little further after it encounters the in-gate. The extension is provided to trap the slag in the molten metal. The metal initially comes along with the slag floating at the top of the ladle and flows straight, going beyond the in-gate and then is trapped in the runner extension.
 GATES::  These are the opening through which the molten metal enters the mold cavity. The shape and the cross section of the in-gate should be such that it can readily be broken off after casting solidification and also that it allows the metal to enter quietly into the mold cavity.