Practice questions on riser design, freezing ratio, solidification, and foundry processes
Q1. Let A and V represent the surface area and volume of components under subscripts. A special measure for riser design, freezing ratio, R, is defined as
✅ Correct: Freezing ratio R = (A/V)casting ÷ (A/V)riser, ensuring riser solidifies last.
Q2. For proper functioning of the riser, freezing ratio must be
✅ Correct: Riser must solidify after casting, so freezing ratio > 1.
Q3. The shrinkage of metals during cooling in molds includes
✅ Correct: Metals shrink in both liquid and solid states, so both types occur.
Q4. From design point of view, the ratio of the minimum volume of riser to that dictated by the liquid shrinkage consideration is kept at
✅ Correct: Riser volume is usually about 3 times the liquid shrinkage requirement to ensure feeding.
Q5. Generally, risers are not required in molds for casting of gray cast iron because
✅ Correct: Graphite expansion during solidification compensates liquid shrinkage in gray cast iron.
Q6. Based on the Chvorinov’s rule, Caine developed which of the following empirical relationship (x is freezing ratio, y is the ratio of volumes of riser and casting, and a, b, c are constants)
✅ Correct: Caine’s relation is x = a / (y − b) − c, derived from Chvorinov’s rule for riser design.
Q7. A spherical casting of 20 mm diameter undergoes volumetric solidification shrinkage and volumetric solid contraction of 2% and 4%, respectively under uniform cooling. The diameter of the casting after solidification and contraction is
✅ Correct: Total volumetric shrinkage = 6%. Equivalent linear shrinkage ≈ 2%. Final diameter = 20 × 0.98 = 19.59 mm.
Q8. The decision on the volume of the design riser is based on
✅ Correct: Riser volume is designed using Chvorinov’s rule, which relates solidification time to volume-to-surface area ratio.
Q9. With a solidification factor of 1×10⁶ s/m², the solidification time (in seconds) for a spherical casting of 100 mm diameter is
✅ Correct: Using Chvorinov’s rule, t = (V/A)² × solidification factor. For 100 mm sphere, result ≈ 277.78 s.
Q10. While cooling, a cubical casting of side 50 mm undergoes 1%, 2% and 3% volume shrinkage during the liquid state, phase transition and solid state, respectively. The volume of metal compensated from the riser is
✅ Correct: Only solidification (phase change) shrinkage is fed by riser. That is 3% of volume.
Q11. The purpose of chaplets in molding is
✅ Correct: Chaplets are small metal supports used to hold cores in position inside the mold cavity.
Q12. The velocity of the molten metal stream at the base of a sprue of height h is equal to
✅ Correct: By Torricelli’s theorem, velocity at the base of a sprue of height h is √(2gh).
Q13. In shell molding, the shell is made of
✅ Correct: Shell molding uses fine sand coated with a thermosetting resin binder, hardened by heating.
Q14. Shift is a casting defect caused by shifting of
✅ Correct: Shift occurs when mold halves, pattern, or cores are misaligned, so “all of the above.”
Q15. Directional solidification can be ensured by
✅ Correct: All listed methods help control heat flow and promote directional solidification.
Q16. A casting defect formed when two metal streams meet without complete fusion is known as
✅ Correct: Cold shut occurs when two metal streams meet but fail to fuse properly, leaving a weak joint.
Q17. A casting defect caused by low green strength of the sand when a portion of the sand breaks away from the mold is known as
✅ Correct: Drop defect occurs when weak sand breaks away from the mold surface and gets embedded in the casting.
Q18. Select the wrong statements about casting defects
✅ Correct: Both scar and blister definitions are accurate, so “none of the above” is the wrong statement option.
Q19. Core prints are prepared in molds to support
✅ Correct: Core prints are recesses in the mold that hold and support **cores** in position.
- **Pattern**: gives the cavity shape, not supported by prints.
- **Chaplets**: small metal supports used only when prints are insufficient.
- **Chills**: promote faster cooling, unrelated to core support.
Q20. Solidification of casting with complex details is not uniform but is accompanied by warping distortions. This factor is compensated in pattern as
✅ Correct: **Camber allowance** is built into patterns to counteract warping/distortion during uneven solidification.
- **Solidification allowance**: accounts for shrinkage, not warping.
- **Draft**: taper for easy pattern withdrawal.
- **Shake allowance**: compensates for rapping of pattern, not distortion.
Q21. Excess turbulence in the stream in molding process results in
✅ Correct: Turbulence causes **all of these problems**.
- It traps **slag/dross** in the metal.
- It pulls in **air**, leading to porosity.
- It can **erode mold walls**, contaminating the casting.
Hence, gating systems are designed to minimize turbulence.
Q22. The casting process which uses rotating mold is
✅ Correct: **Centrifugal casting** spins the mold so molten metal distributes by centrifugal force, producing dense, defect‑free parts.
- **Slush casting**: for hollow thin‑walled shapes, no rotation.
- **Die casting**: high‑pressure injection into dies.
- **Continuous casting**: solidifies metal continuously through a mold, not rotating.
Q23. Water-based refractory slurry is used in
✅ Correct: **Investment casting** coats wax patterns with a water‑based refractory slurry to build a ceramic shell.
- **Centrifugal casting**: uses metal molds, no slurry.
- **Shell molding**: uses resin‑coated sand.
- **Die casting**: uses steel dies, no slurry.
Q24. Approximate value of the thickness of the shell in shell molding, although dependent upon curing time, is
✅ Correct: In **shell molding**, the resin‑coated sand forms a thin shell around the pattern.
- Typical thickness is about **5 mm**, enough to provide strength while keeping the mold lightweight.
- **10–20 mm** would make the shell unnecessarily heavy and defeat the purpose of the process.
- The exact thickness depends on curing time and sand quality, but ~5 mm is the standard.
Q25. The cross-section of all the elements of a gating system should normally be
✅ Correct: A **circular cross‑section** minimizes surface area for a given flow rate, reducing friction and turbulence.
- **Rectangular/oval** sections are easier to make in sand molds but cause uneven flow and higher turbulence.
- “Any of the above” is not ideal because only circular provides the most efficient, uniform flow.
Q26. Pressure, anywhere in the molten metal stream, should not fall below the atmospheric pressure. This is essential to avoid
✅ Correct: If pressure drops below atmospheric, **air is sucked into the molten stream** — this is the **aspiration effect**.
- It leads to gas porosity and weak castings.
- “Atmospheric air effect” and “respiration effect” are not standard terms in casting.
- Hence, the precise technical term is **aspiration effect**.
Q27. To avoid aspiration effect, the flow area for stream of the molten metal in vertical sprue should vary with respect to head in
✅ Correct: The sprue cross‑section must **decrease parabolically** with height to maintain a full stream and prevent aspiration.
- **Linear** reduction is insufficient, leading to partial vacuum.
- **Exponential** is too steep and impractical.
- “None” is wrong because parabolic is the correct design.
Q28. The stream velocities are reduced, thus the turbulence and aspiration are minimized in
✅ Correct: In a **non‑pressurized gating system**, the total runner area is larger than the sprue base, so velocity drops and turbulence is minimized.
- **Pressurized systems** keep velocity high to ensure mold filling but increase turbulence.
- “Both” is wrong because the two systems behave differently.
- “None” is incorrect since non‑pressurized is the right answer.
Q29. In a gating system, the ratio of sprue base area : runner area : ingate area is 1:2:4. Thus, it should be a
✅ Correct: A ratio of **1:2:4** means the total flow area increases as metal moves downstream.
- This reduces velocity and turbulence, typical of a **non‑pressurized system**.
- **Pressurized systems** have decreasing areas (e.g., 1:0.75:0.5) to keep velocity high.
- “Both” and “none” are incorrect since the ratio clearly matches non‑pressurized design.
Q30. During cooling and solidification, the pure metals and alloys have
✅ Correct: **Pure metals** solidify at a fixed temperature, giving a sharp freezing point.
- **Alloys** usually solidify over a temperature range, forming dendritic structures.
- Hence, their solidification behavior is **different**, not similar or unknown.
Q31. When alloys solidify over a short range of temperatures, they form
✅ Correct: Alloys with a **narrow freezing range** solidify quickly, producing a **columnar grain structure**.
- **Wide freezing range alloys** form dendritic or mixed structures.
- “Wholly dendritic” and “partially mixed” apply to longer freezing ranges.
- “None” is wrong since columnar is the right answer here.
Q32. According to Chvorinov’s rule, solidification time of a casting is
✅ Correct: **Chvorinov’s rule** states:
\[ t_s = C \left(\frac{V}{A}\right)^n \]
where \(t_s\) = solidification time, \(V\) = volume, \(A\) = surface area, \(n \approx 2\).
- So, time is proportional to the **square of the volume‑to‑area ratio**.
- Not directly proportional to volume or area alone.
- Hence, option (a) is the closest correct interpretation.
Q33. Risers should be designed to
✅ Correct: A **riser** acts as a reservoir of molten metal to feed shrinkage.
- It must **solidify last** (longer time than casting).
- It ensures **directional solidification** from casting to riser.
- It maintains a **temperature gradient** to avoid shrinkage cavities.
- Therefore, all of the above are correct.
Q34. In cold chamber die casting process,
Q35. Permeability is
Q41. Light impurities in centrifugal castings are
Q42. Porosity results in castings due to
Q43. Hot chamber die casting is suitable for
Q44. Swell is caused by
✅ Correct: Swell is a casting defect caused by the deformation of vertical mold face due to the pressure of molten metal.
The mold wall bulges outward, producing an oversized cavity and hence an oversized casting.
Q45. Select the correct statement for riser design
✅ Correct: A riser must remain molten longer than the casting to feed shrinkage cavities.
Hence, it should be of minimum possible volume (to reduce waste) but with a solidification time longer than the casting.
Q46. Sprue is
✅ Correct: The sprue is the vertical channel through which molten metal flows from the pouring basin down to the runner system.
It controls the flow and minimizes turbulence during mold filling.
Q47. Using disposable pattern in investment casting, the molten metal should be poured
✅ Correct: In investment casting with disposable wax patterns, molten metal must be poured rapidly.
This prevents premature solidification and ensures the thin sections of the mold are completely filled.
Q48. If volume of casting is V and surface area is A, the solidification time will be proportional to
✅ Correct: By Chvorinov’s rule, solidification time t ∝ (V/A)².
- V = volume of casting (heat to be removed).
- A = surface area (heat transfer area).
Thus, larger volume-to-surface ratio means longer solidification time.
Q49. Hot tears are
✅ Correct: Hot tears are cracks formed in the casting due to residual stresses when the metal is weak and brittle at high temperature during solidification.
They occur because the casting cannot accommodate contraction stresses while cooling.
Q50. The investment casting process uses patterns made of
✅ Correct: In investment casting, disposable patterns are made of wax (often with polystyrene filler).
These patterns are coated with refractory slurry, then melted out to leave a cavity for pouring molten metal.
Q51. The height of the down-sprue is 175 mm and its cross-sectional area at the base is 200 mm². The cross-sectional area of the horizontal runner is also 200 mm². Assuming no losses, indicate the correct choice for the time (in seconds) required to fill a mold cavity of volume 10⁶ mm³. (Use g = 10 m/s²).
✅ Correct: Velocity at sprue base = √(2gh) = √(2×10×0.175) ≈ 1.87 m/s.
Flow rate = A × v = (200×10⁻⁶ m²) × 1.87 ≈ 3.74×10⁻⁴ m³/s.
Mold volume = 10⁶ mm³ = 10⁻³ m³.
Time = Volume / Flow rate = 0.001 / 3.74×10⁻⁴ ≈ 2.69 s.
Q52. A mold has downsprue of height 185 mm and cross-sectional area at base 100 mm². This sprue feeds a horizontal runner of cross-sectional area at ingate 50 mm² to fill the mold cavity of 10⁶ mm³. The time required to fill the mold cavity will be
✅ Correct: Velocity at sprue base = √(2gh) = √(2×10×0.185) ≈ 1.92 m/s.
Flow rate at ingate = A × v = (50×10⁻⁶ m²) × 1.92 ≈ 9.6×10⁻⁵ m³/s.
Mold volume = 10⁶ mm³ = 10⁻³ m³.
Time = 0.001 / 9.6×10⁻⁵ ≈ 5.24 s.
Q53. Bottom gating system is preferred over vertical gating because
✅ Correct: A bottom gating system introduces molten metal from the bottom, so the mold fills smoothly upward.
This avoids splashing and turbulence, reducing oxidation and defects compared to vertical gating.
Q54. Disposable patterns are made of
✅ Correct: In expanded polystyrene (EPS) or lost‑foam casting, disposable patterns are made of polystyrene.
They vaporize when molten metal is poured, leaving the cavity for the casting.
Q55. The solidification time ts of a cubical casting is related to its side, a, as
✅ Correct: By Chvorinov’s rule, t ∝ (V/A)².
For a cube of side a: V = a³, A = 6a² → V/A = a/6.
So, t ∝ (a/6)² ∝ a².
Q56. Which of the following materials requires the largest shrinkage allowance, while making a pattern for casting?
✅ Correct: Carbon steel has the highest solidification shrinkage among the listed materials.
Therefore, the largest shrinkage allowance must be provided in the pattern design.
Q57. Light impurities in the molten metal are prevented from reaching the mold cavity by providing a
✅ Correct: A skimbob is used to skim off light impurities (slag, dross) from molten metal before it enters the mold cavity.
Strainers and wells help control flow, but skimbob specifically prevents impurities from entering.
Q58. Directional solidification in castings can be improved by using
✅ Correct: Chills accelerate cooling in desired regions, while padding slows cooling in others.
Together, they promote directional solidification from thin to thick sections.
Q59. Which one of the following methods are used for obtaining directional solidification for riser design?
1. Suitable placement of chills
2. Suitable placement of chaplets
3. Employing padding
✅ Correct: Directional solidification is achieved by placing chills (to promote faster cooling) and using padding (to slow cooling).
Chaplets are used for core support, not for controlling solidification.
Hence, the correct methods are 1 and 3.
Q60. Gas holes are the casting defects which occur due to