Cores and Casting of Metals

Meaning of Cores: 

Core is a pre-prepared mould shape. It is used to provide the casting with internal cavities, recesses, or projections. It is usually placed in a mould after the pattern has been removed.

Usually a core is made of the best quality sand and placed in the mould cavity in the desired position. Core prints are added to the pattern on both sides to create impressions that support and hold the core at both ends.

It flows around the core when the molten metal is poured and fills the rest of the mould cavity. Cores are subject to extremely severe conditions and therefore must have very high erosion resistance, exceptionally high strength, good permeability, good refractory and adequate collapse.

To allow gasses to escape easily, special vent holes are provided on the core. Cores are sometimes reinforced with low carbon steel wires or even cast-iron grids (in large cores) to ensure stability and shrinkage resistance.

Types of Cores:

Generally, cores are of two types:

1. Green Sand Core:

A core formed by the pattern itself is known as the green sand core in the same sand used for the mould. The pattern is designed to provide the green sand core. The pattern's hallow part produces the core of green sand. It's displayed in Fig. 3.11 (a).

2. Dry Sand Core:

In key boxes a core is ready individually and frozen, recognized as the heart of dry sand. Also known as process cores are the dry sand cores. They are available as required in various sizes, shapes and designs. In Fig, the heart of dry sand is shown. 3.11. (b).

Some common types of dry-sand cores are:

(i) Horizontal Core:

The horizontal core is the most prevalent form of heart and is placed horizontally on the mould's dividing sheet. The core finishes rest in the positions given on the chart by the key marks. This sort of core can resist the molten metal poured's turbulence impact. Figure shows a horizontal nucleus for blank equipment mould. 3.11 (c).

(ii) Vertical Core:

With some of their part lying in the sand, the vertical nucleus is put vertically. Top and bottom of the core are usually kept tapered, but at the bottom of the top I d, taper. Figure shows a vertical center. 3.11 (d).

(iii) Balance Core:

Only one part of the mould expands the equilibrium center. Only one core print can be found on the equilibrium key model. This is best suited to having only one side opening for casting. This is used in the painting to produce blind holes or recesses. In Fig, a equilibrium nucleus is displayed. 3.12 (e).

(iv) Hanging Core:

The hanging nucleus in the mould is placed vertically. This can be accomplished either by swinging cables or by resting the key collar in the collar pocket produced at the top of the mould. There is no upper support for this kind of heart. In Fig a hanging nucleus is displayed. 3.11 (h).

(v) Drop Core:

The fall center is used when placing the core either above or below the dividing line. In Fig a fall nucleus is displayed. (J) 3.11. Also regarded as the heart of the wing, core of the tail, core of the seat, etc.

(vi) Kiss Core:

When a number of holes of less dimensional accuracy are required, the kiss core is used. No key prints are given in this scenario, and consequently no place for the heart is accessible. The nucleus is kept roughly between the handle and the drag and is therefore referred to as the touch heart.

A kiss core is shown in Fig. 3.11 (g):

Core Materials:

The combination of sand, binders and additives is the ingredients of key fabric. The key rocks are silica, zircon, olivine, etc., and the key binders are key oils, resins, molasses, dextrin, etc.

Sand includes more than 5% clay that not only decreases permeability but also collapse and is therefore not appropriate for key production.

The commonly used core sand is a mixture of following items:

(i) Core Sand:

For smaller casts, the sand can be green sand and a mixture of fire clay, green sand and heavier casting betonies. The core is supported by an oven to wash its moisture away. The cores of dry sand are stronger than those of green and cores. The sand with rounded grains is also best suited for base production because they are more permeable than the sand of angular grains.

(ii) Oil Sand:

It is possible to use oil sand for almost any implementation of sand casting.

A typical composition of oil sand is:

Sand 95 — 96%

Cereal flour 1 — 1.05%

Core oil 1 — 1.5%

Water 1 — 2%

Bentonite 0.1—0.3%

Oil sand is very popular in core making because:

(a) They get good strength.

(b) They provide excellent surface finish.

(c) They have better collapsibility after baking.

(c) The backed oil sand cores are very hard and not easily damaged in handling of mould.

(iii) Resin Sand:

These are thermosetting or the use of thermoplastic binders such as rosin, phenol, urea, furan, formaldehyde, etc. to achieve excellent sand bonds. Because of their elevated strength, small gas formation, great collapseability, moisture absorption resistance, greater casting precision, etc., they are becoming prevalent in use.

(iv) CO2 – Sodium Silicate Sand:

In the heart, silica concrete and potassium silicate (3-4%) are pressed and then CO2 gas passes through water to create the rock difficult. For very big castings, such kinds of cores are used. They don't need to dry, so it's a very quickly core making technique.

(v) Core Binders:

Natural sand has insufficient binding characteristics and therefore some binders are used to enhance key sand binding power. Binders have the function of holding the sand particles together and providing the nucleus with greater force.

There are two types of binders used are:

a. Inorganic Binders:

They include aluminum oxide, fire clay, bentonite, limonite, silica powder, iron oxide, etc. They are popularly used and very fine powder.

b. Organic Binders:

They include key oils such as petroleum oil, vegetable oil, linseed oil, maize oil, dextrin and malasses. Organic binders are quickly becoming difficult and providing excellent resistance.

(vi) Core Additives:

Besides key sand and key binder, certain additives are used to enhance the core's unique characteristics.

The additives are:

(a) Kaolin or fire clay to improve stability.

(b) Iron oxide (Fe2O3) and aluminum oxide (Al2O3) to improve hot strength.

(c) Zircon flour and pitch flour to improve refractoriness.

(d) Molasses to improve binding properties.

(e) Organic additives to improve collapsibility like raw dust.

(f) Silica powder, paints and graphite bonded with resin are used to improve the surface finish.

Properties of Good Core Materials:

A good dry sand core must have the following properties in order to successfully use in casting process:

1. Strong:

It should be powerful enough to resist molten metal's turbulence strength. It must be resistant to erosion.

2. Hardness:

It should be capable, of being baked to obtain good hardness strength.

3. Permeability:

It must be permeable to allow the easy escape of the gases formed.

4. Refractoriness:

It must be highly refractory in nature to withstand high temperature of the molten metal.

5. Dimensional Stability:

It should be stable in dimensional accuracy, shape and size during pouring and solidification.

6. Minimum Gas-Formation:

Core material should generate minimum gases, while subjecting to molten metal in casting process.

7. Good Surface Finish:

Core surface should be smooth enough to provide good surface finish of the casting.

8. Sufficiently Collapsible:

Cores must be sufficiently collapsible i.e., easy removal of the core from the casting after solidification.

Core Prints:

The key prints are additional drawings on the structure that shape a recess in the mould to maintain and place the core in its correct place. Several kinds of key prints are available, such as vertical, horizontal, balancing, swinging and dropping key prints.

Core Shifting:

While comprising a metal, the nuclei change their stance because of the liquid metal's chaotic movement. Also, slender cores tend to float readily and shift from their correct place owing to the upward thrust of the molten metal.

In order to prevent moving, the key weight is improved during key production by embedding steel rods, steel cables, slender tubes, etc. This is regarded as key strengthening.

Core Chaplets:

If the duration of the core is long and the end bases are at greater altitudes to each other, the core will decay when warm liquid metal is poured.

Chaplets are used in such instances to resolve these flaws. Chaplets are intended in such a way that the nucleus is supported and restricted from decay.

The chaplets are produced from the same material as the winding metal to form an essential portion of the structure. Some chaplets that are frequently used are shown n Fig. 3.14.

Core Chills:

The key chills are the parts of metal that are either inserted or positioned to contact the cutting surface to speed up the solidification cycle where it is delayed. The smaller region strengthens more quickly, generating stress and casting distortion.

Therefore, a means to standardized the solidification (freezing) frequency at all casting segments must be provided.

The chills are of following two types:

(i) Internal Chill:

In a mould where the region is relatively big, an inner draft is put in location to assist standardized solidification throughout the casting.

This is shown in Fig. 3.15:

(ii) External Chill:

There is an internal cold around the mould that just touches its bottom.

This is shown in Fig. 3.16: