Production Technology 2 | Casting in One Shot | GATE 2023

Summary

This GATE-focused lecture covers metal-casting fundamentals, pattern allowances, common defects, riser and gating-system design, green-sand mould properties, and several casting processes. The instructor emphasizes understanding the principles and derivations rather than memorizing formulas.

1. Casting fundamentals and shrinkage

A pattern creates the mould cavity. It resembles the finished part but is not an exact replica, because allowances are added.

Molten metal cools in three stages:

1. Liquid metal cools from pouring temperature to its melting point.
2. The metal solidifies, changing from liquid to solid at the melting point.
3. The solid casting cools from the melting point to room temperature.

The lecture assigns compensation for these stages as follows:

• A riser supplies metal to compensate for shrinkage during the first two stages.
• A pattern allowance accommodates contraction of the solid casting during the third stage.

A riser acts as a reservoir of molten metal. It should solidify after the casting so it can feed the casting as needed.

2. Pattern allowances

The lecture highlights three important allowances:

• Shrinkage allowance: The pattern is made larger so the casting reaches its intended dimensions after cooling.
• Machining allowance: Extra material is added because sand casting may have limited dimensional accuracy and surface finish. The part is then machined to its final dimensions.
• Draft allowance: A slight taper—approximately 0.5° to 2° in the lecture—helps the pattern be withdrawn without damaging the mould.

The instructor also discusses gray cast iron. Graphite formation during solidification can cause expansion. The lecture gives an approximate 2.8% carbon threshold: above it, expansion is expected; below it, contraction is expected. If a problem does not specify the carbon content, the instructor recommends assuming contraction and checking whether the exam’s answer key may have used a different assumption.

3. Pouring temperature and casting defects

Pouring temperature needs to be optimum: both excessively low and excessively high temperatures can cause problems.

• Too low: The metal may not flow or fuse properly through thin sections, leading to defects such as cold shuts and misruns. Lower temperature makes the liquid metal more viscous and less able to fill the mould.
• Too high: The riser’s feeding burden may increase. In a sand mould, heat can also break down moisture and release gases. Gas escaping near the casting surface may produce pinhole porosity.

Poor sand ramming can contribute to defects caused by molten-metal pressure:

• Scab: Sand is displaced from a mould surface, leaving a raised or irregular defect.
• Swell: The mould wall gives way or deforms under metal pressure, enlarging the casting locally.
• Rat tail: A related defect that appears on thinner sections of a casting.

4. Riser design methods

The lecture presents three approaches to riser design.

Solidification-time or first-principles method

The lecture uses the relationship:

[ \text{solidification time} \propto \left(\frac{\text{volume}}{\text{heat-transfer area}}\right)^2 ]

The relevant area is the area through which heat transfer occurs:

• For a side riser, heat transfer is considered across its exposed surfaces.
• For a top riser, the surface in contact with the casting is excluded from the riser’s heat-transfer area.

An optimum riser shape minimizes surface area for a given volume, reducing heat loss and extending solidification time. The key design objective is for the riser to solidify after the casting.

Modulus method

The modulus is:

[ \text{modulus} = \frac{\text{volume}}{\text{surface area}} ]

For the riser to solidify after the casting:

[ \text{riser modulus} > \text{casting modulus} ]

If a problem gives volumetric shrinkage, the lecture recommends starting with a riser volume based on approximately three times the shrinkage volume, then checking whether the resulting riser modulus is sufficient. In a worked example, the instructor compares cylindrical side and top risers for a cube and adjusts the initial dimensions when the riser would otherwise freeze too soon.

Caine’s method

Caine’s method is presented for materials that undergo substantial solidification shrinkage, particularly steel. It uses a Caine curve relating riser and casting dimensions, including:

• A modulus ratio
• A riser-to-casting volume ratio

The lecture gives the curve in the form:

[ X = \frac{A}{Y-B} + C ]

The constants are supplied by the relevant problem or reference.

The instructor’s selection guide is:

• Use Caine’s method when its constants or curve are provided.
• Use the modulus method when volumetric shrinkage is specified.
• Otherwise, use the solidification-time or first-principles approach.

For steel castings, the lecture also mentions using insulating or exothermic material at the riser to reduce heat loss and improve feeding.

5. Gating systems

The lecture focuses on top and bottom gating systems.

• Top gating: Metal enters from above. It can fill the mould relatively quickly, reducing the time available for dendritic growth. However, the falling stream can be turbulent, eroding sand and carrying sand-related inclusions into the casting. These hard inclusions can make machining difficult and increase tool wear.
• Bottom gating: Metal enters from below and rises through the mould. Filling is slower, but turbulence and sand erosion are generally reduced. A pouring basin helps control the incoming stream; a ceramic splash core can limit erosion where the stream strikes the mould.

The lecture also describes gating-system components and their roles:

• The sprue carries metal downward from the pouring basin.
• A tapered sprue helps prevent low pressure and air being drawn into the stream, a problem known as aspiration.
• A runner carries metal horizontally. In a gravity-fed system, the runner may have a diverging area to maintain pressure and compensate for flow losses.
• A skim bob or similar feature helps separate lighter floating material from heavier settled impurities before metal enters the mould cavity.
• The vena contracta near the sprue exit is accounted for in runner design to reduce turbulence.

Typical gating-area ratios, expressed as sprue : runner : ingate, include:

• Gravity-fed systems: Common ratios include 1:2:3 and 1:2:4. The lecture also mentions 1:2:2 for more viscous metal.
• Pressure-fed systems: Areas decrease in the flow direction. Examples include 1:0.75:0.5 and 1:1:1.

6. Filling-time calculations

The lecture derives filling-time relationships using flow rate and Bernoulli’s equation.

For a top gate, filling time is based on a nearly constant entry velocity:

[ t_{\text{top}} = \frac{AH}{A_g\sqrt{2gH_t}} ]

Here, (A) and (H) describe the casting section, (A_g) is the gate area, and (H_t) is the manometric head.

For a bottom gate, entry velocity changes as the metal level rises. For a uniform casting cross-section:

[ t_{\text{bottom}} = \frac{2A}{A_g\sqrt{2g}} \left(\sqrt{H_t}-\sqrt{H_t-H}\right) ]

The instructor works through examples that divide the mould and riser into sections when their cross-sectional areas differ, then add the individual filling times.

One result highlighted in the lecture is that when the manometric head equals the height to be filled, the bottom-gating fill time is twice the top-gating fill time under the stated assumptions.

7. Properties of green-sand moulds

Three sand-mould properties receive particular attention:

• Permeability: The mould’s ability to let gases pass through spaces between sand grains and escape. In the standard test described, the permeability number is 3007 divided by the test time in minutes.
  • Increasing moisture can initially improve permeability by changing how fine particles occupy the sand voids.
  • Beyond an optimum moisture level—about 8% in the lecture—excess water occupies the voids and permeability decreases.
  • The lecture notes a trade-off: greater void space can improve permeability but reduce mould strength.
• Collapsibility: The mould’s ability to give way as the casting contracts during solid-state cooling. Poor collapsibility can restrain the casting and contribute to cracking. Additives such as wood flour or cow dung are mentioned; they burn out when hot metal is poured, leaving space for contraction.
• Strength: The mould must retain its shape under the pressure and heat of the metal while still having suitable collapsibility and permeability.

8. Other casting processes

Die casting

Die casting uses a reusable permanent metal die, commonly cast iron in the lecture. The lecture distinguishes:

• Hot-chamber die casting, suited to lower-melting-point materials. Aluminum is generally avoided in this arrangement because of its reaction with iron.
• Cold-chamber die casting, used for higher-melting-point materials.

Advantages discussed include high production rates—up to roughly 50–60 castings per hour in the lecture—and good strength associated with rapid cooling and fine grain structure. Die surfaces may need cleaning after repeated use because adhered material can degrade surface finish.

Centrifugal casting

The lecture describes three types:

• True centrifugal casting: The mould rotates and metal is forced against its wall. It is commonly used to make hollow products such as pipes; the lecture associates ceramic and cast-iron pipes with this process.
• Semi-centrifugal casting: A mould rotates around a vertical axis. Examples include wheels and pulleys.
• Centrifuging: Several mould cavities are arranged around a central sprue and filled by centrifugal force. The lecture notes its relatively low material yield and mentions its use for producing patterns for investment casting.

The lecture also contrasts related manufacturing routes: steel pipes may be made by resistance welding, while plastic pipes are commonly made by extrusion.

Shell moulding

A heated pattern—about 280°C in the lecture—is exposed to a sand-and-resin mixture. The pattern is inverted so sand adheres and forms a shell, which is then cured in an oven. Ejector pins release the shell from the pattern. Shell sections are joined, metal is poured, and the shell is broken to remove the casting. Shell moulding is presented as a relatively accurate casting process.

Investment casting

Patterns are made from wax or another removable material and assembled into a tree. The tree is cleaned, repeatedly dipped into a slurry, and dried to build a ceramic shell. Heating removes the pattern material; the shell is then fired at about 1150°C to strengthen it and remove remaining pattern material. Metal is poured while the shell is still hot, after which the shell is broken away.

The process is highlighted for its accuracy and use in making gas-turbine blades.

9. Speaker and sources

• Speaker: Dr. Shadesh Singh, the instructor named in the subtitles.
• Sources cited: Handbook of Casting and a casting reference by Rosenthal, mentioned in connection with the gray cast iron discussion. The lecture also refers to the instructor’s Facebook page, where supporting material for that discussion was said to have been posted.