22-Mec-A4 Design and Manufacture of Machine Elements · May 2013

Question 4 of 8: Casting defects in a no-bake sand mould for a cast-iron tee

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

Paper format. National Examination, 07-Mec-A4 Design and Manufacture of Machine Elements, May 2013 — 3 hours, open book, any non-communicating calculator permitted. Eight questions on six pages, divided into Part A (manufacturing processes, Q1–Q4) and Part B (machine-element design, Q5–Q8). The rubric asks for three from Part A and two from Part B, five questions constituting a complete paper, all of equal value (20 % each). All eight questions are solved here, because this document is a study resource rather than an examination script.

Reference texts.

Kalpakjian & Schmid, Manufacturing Engineering and Technology, 7th ed. — grinding (Ch. 26), sheet-metal forming and the forming-limit diagram (Ch. 16), sand casting and casting defects (Ch. 11–12).
Budynas & Nisbett, Shigley's Mechanical Engineering Design, 10th/11th ed. — combined stresses and static failure theories (Ch. 3, 5), fatigue (Ch. 6), shaft design (Ch. 7), bolted joints in shear (Ch. 8), welded joints (Ch. 9).
Hibbeler, Mechanics of Materials, 10th ed. — stress transformation and combined loading.
ASM Handbook Vol. 15, Casting — gas, penetration and mould-wall-movement defects in no-bake sand systems.

Check: Part B is figure-driven. Every dimension used below was read from the printed figures (Fig. S4–S7). Where the drawing dimensions a distance from a face rather than from a bolt centre (Q7), the reading is stated explicitly in Given so a grader can substitute a different interpretation without redoing the method.

Question 4: Casting defects in a no-bake sand mould for a cast-iron tee (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) Gas bubbles confined to one location in the base of the tee

The controlling fact is that no-bake (air-set) sand is bonded with an organic resin, typically a furan or phenolic-urethane system cured by an acid or amine catalyst. When molten iron at roughly 1400 °C contacts that binder, the resin pyrolyses within seconds and evolves a large volume of gas — carbon monoxide, hydrogen, water vapour and hydrocarbons. Every surface of the mould and core generates this gas. The question is therefore not "why is gas generated" but "why can it only escape everywhere except here."

Gas generated in the mould walls escapes outward through the permeable sand to atmosphere; the path is short and unobstructed. Gas generated in the core, however, is generated inside a body that is completely surrounded by liquid metal. Its only escape route is axially, along the core, out through the core prints where the core is supported by the mould. If the core prints are undersized, if they are sealed by metal flash, or if the core has no vent passage running to a print, then the core gas pressure rises until it exceeds the metallostatic head plus the surface tension of the melt — at which point gas is injected into the liquid iron.

Once injected, buoyancy takes over. Bubbles rise until they reach the highest point of the local metal volume and are trapped there by the advancing solidification front. In the base of the tee, the metal section is thick, it is the last region to solidify, and the roof of that section is a re-entrant pocket formed by the underside of the core. The bubbles collect immediately beneath that roof and are frozen in as blowholes. Everywhere else in the casting the metal section is thinner, solidifies sooner, and lies below a mould surface that vents freely to atmosphere — so no bubbles accumulate. That is precisely why the defect appears at one location and nowhere else.

The remedies follow from the mechanism: increase core-print size and vent the core to a print with a wax or rope vent; reduce the binder addition to the minimum that gives adequate strength (gas evolution scales directly with resin content); ensure the sand is dry and the catalyst level is correct, since over-catalysed resin evolves more gas; and reposition or add a riser so that the last-to-solidify pocket is outside the casting.

(b) Factors causing the penetration defect near the bottom of the inside diameter

Metal penetration is the infiltration of liquid metal into the pore spaces between sand grains, producing a rough, sand-encrusted surface that must be ground off — or, in the extreme, a fused sand-metal layer that cannot be removed at all. It occurs when the metallostatic pressure exceeds the capillary pressure that keeps the melt out of the intergranular pores:

$$P_{\text{metal}} = \rho g h \;>\; P_{\text{cap}} = \frac{2\gamma\cos\theta}{r_{\text{pore}}}$$

Every factor in that inequality points at the bottom of the core's inside diameter as the most vulnerable spot, and the following contribute:

Maximum ferrostatic head. The bottom of the bore is the deepest point below the pouring cup, so $\rho g h$ is greatest there. Cast iron has a density near 7000 kg/m³, so even a modest head produces substantial pressure.
Loss of core strength from all-round heating. A core is heated from every side simultaneously and has very little heat-sink capacity. Its binder burns out early, so the sand loses cohesion and the pores open just as the pressure peaks. Mould walls, heated from one side only, retain their strength far longer.
Coarse sand and/or low compaction. Pore radius scales with grain size, so a coarse AFS grain-fineness number, a wide grain distribution or poorly rammed sand around the core all increase $r_{\text{pore}}$ and lower the capillary barrier.
Absent or inadequate refractory coating. No-bake cores for iron are normally coated with a zircon or graphite wash precisely to seal the surface pores. A missed, thin or incompletely dried coat — and the bore of a core is the hardest surface to coat properly — removes the main defence.
High pouring temperature and slow solidification. Superheat keeps the metal fluid and lowers its surface tension, and a long local solidification time gives it more opportunity to infiltrate. The thick base section adjacent to the bore aggravates both.

(c) Factors causing the enlargement at "C"

An enlargement, or swell, means the mould cavity itself grew during pouring: the mould wall moved back under the pressure of the liquid metal. Three things must combine for that to happen, and all three should be listed:

Inadequate mould rigidity or compaction at that location. Air-set sand develops its strength through chemical cure. If the sand at C was under-rammed, if it was placed after the working time of the binder had expired (so the resin had already begun to set and the grains could not bond), or if the catalyst was poorly mixed there, the local hot strength is far below specification and the wall simply yields.
High metallostatic pressure and insufficient clamping or weighting. The pressure at C is $\rho g h$ acting over the local area. If the cope is not adequately weighted or the flask not clamped, the cope lifts and the parting line opens, producing both a swell and the associated flash. This is the classic cause when the enlargement sits near a parting line, as it does in Fig. S3.
Graphite expansion during eutectic solidification. This factor is specific to cast iron and is the one most often missed. As grey or ductile iron solidifies, graphite precipitation causes the solidifying metal to expand, generating substantial internal pressure against the mould wall. In a rigid mould that expansion is harnessed to feed the casting; in a weak or unclamped mould it pushes the wall outward and creates exactly the enlargement described.

Mould erosion should also be considered and then largely excluded: erosion by the metal stream would produce a rough, sand-inclusion-bearing enlargement close to the gate, whereas a swell is a smooth, dimensionally generous region. The remedies are to increase compaction and check binder work time and catalyst distribution, to weight and clamp the flask adequately for the calculated buoyant force, and to keep the mould rigid enough to exploit rather than suffer graphite expansion.

Topic: Gas (blowhole), metal-penetration and mould-wall-movement defects in no-bake sand casting of iron.

Key relations: Metallostatic pressure $P = \rho g h$; capillary resistance to penetration $P_{\text{cap}} = 2\gamma\cos\theta / r_{\text{pore}}$; cope buoyant lift force $F = \rho g h A_{\text{proj}}$, which sets the required weighting; core gas must vent through the core prints because a core is fully enclosed by metal.

Why this works: All three defects are pressure-balance problems, which is what makes them a single coherent examination question. A blowhole occurs when core gas pressure beats metal pressure plus surface tension; penetration occurs when metal pressure beats capillary pressure; a swell occurs when metal pressure beats the hot strength of the mould. Once a candidate frames each defect as an inequality, the list of contributing factors and the list of remedies both write themselves, and the location of each defect follows from where its governing pressure is largest.

Common pitfall: Attributing the gas bubbles to moisture in the sand. No-bake systems are chemically bonded and essentially dry; the gas comes from binder pyrolysis, and the answer must say so. The second pitfall is forgetting graphite expansion in part (c) — it is the mechanism that makes swelling a specifically cast-iron problem and it is what the examiner is looking for.

Source: Kalpakjian & Schmid, Manufacturing Engineering and Technology, 7th ed., §11.4–11.6 and Table 12.1 (casting defects); ASM Handbook Vol. 15, Casting, sections on no-bake binder gas evolution, metal penetration and mould-wall movement.

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