Casting Patterns, Allowances, and Process Analysis

Introduction to Metal Casting

• Casting is a primary manufacturing process of producing metallic parts by pouring molten metal into a prepared mould cavity and allowing the metal to cool and solidify into the desired shape.

• Basic Sequence of the Casting Process:

  • Mould preparation using a pattern.

  • Heating metal in a furnace until it reaches a temperature above its melting point.

  • Pouring liquid metal into the mold cavity.

  • Solidification of liquid metal into the internal shape of the mold.

  • Removal of the solidified casted part from the mold.

  • Post-casting finishing operations:

  • Cutting off gating systems and risers.

  • Clean up (shakeout, surface cleaning).

  • Machining to obtain final dimensions.

Detailed Sand Casting Process Steps

• Pattern Making: Crafting the physical replica (pattern) of the target component.

• Core Making: Preparing sand cores to form internal cavities or hollow spaces within the casting.

• Mold Assembly & Gating System:

  • Moulding sand is packed around the pattern within moulding boxes (flasks).

  • Constructing the gating system (sprue, runner, gates, and risers) to allow liquid metal entry and feed volume contraction.

• Melting & Pouring:

  • Metal is melted in furnaces above its melting temperature.

  • Molten metal is poured directly into the mold cavity.

• Solidification & Cooling: Metal transitions from liquid phase to solid phase while filling the cavity.

• Shakeout & Cleaning:

  • Solidified casting is separated from the sand mold during shakeout.

  • Removal of risers and gates.

  • Cleaning and surface finishing.

• Secondary Processing & Quality Assurance:

  • Additional heat treatment to achieve required mechanical properties.

  • Inspection stage to check for:

  • Internal and external casting defects.

  • Pressure tightness.

  • Dimensional accuracy.

Pattern Concept and Construction

• Definition: A pattern is a replica of the casting to be made. By using a pattern, impressions (mould cavities) are formed in damp sand to construct a mold.

• Primary Functions:

  • Forms the mould cavity of precise dimensions.

  • Accommodates allowances for metal contraction, machining, and pattern removal.

Pattern Materials

• Wood:

  • Easy to work with and readily available.

  • Can be cut easily into required shapes.

  • Used primarily when a small number of castings is required.

• Metal:

  • Does not change shape when subjected to moist sand conditions.

  • Provides high dimensional stability and long service life.

  • Used for producing a specified volume of castings.

• Plastics:

  • Non-hygroscopic (does not absorb moisture from sand).

  • Possesses a smooth surface finish.

  • Can be withdrawn easily from the mold without damaging mold walls.

• Plaster:

  • Possesses high compressive strength.

  • Ideal for precise detail replication.

Green Sand Mold Setup and Operations

• Molding Components:

  • Flask: Outer frame holding the molding sand. Divided into upper half (Cope) and lower half (Drag). When a middle section is required, it is called a Check Box.

  • Molding Board: Base board supporting the drag and pattern during ramming.

  • Sprue Pin: Tapered pin used to form the vertical passage (sprue) for metal entry.

  • Riser Pin: Pin used to create vertical reservoirs (risers) to feed molten metal during solidification.

  • Pouring Basin: Funnel-shaped cavity at the top of the sprue to receive liquid metal.

  • Parting Line: Interface plane dividing the cope and drag sand sections.

  • Weight: Placed on top of the cope during pouring to resist buoyancy forces exerted by molten metal.

  • Vents: Small holes pierced through sand to allow gas escape.

  • Lifter: Tool used to clean loose sand from the bottom of the mould cavity.

  • Runner & Gate: Channels delivering liquid metal from sprue base into mould cavity.

• Progressive Operations:

  • First Operation: Place drag on molding board with pattern inverted; fill and ram molding sand.

  • Second Operation: Flip drag, assemble cope on drag, place sprue pin and riser pin, introduce parting sand, and ram sand into cope.

  • Final Assembly: Remove sprue/riser pins, separate cope from drag, extract pattern, insert core if required, cut runner/gate channels, reassemble cope and drag, place weight on top.

Classifications and Types of Patterns

• Single Piece Pattern:

  • Made from a single piece of material; contains no loose parts.

  • Simplest and least expensive type of pattern.

  • Used for simple casting geometries.

  • Elements: Single pattern block creating mold cavity in sand flask.

• Split Pattern (Two-Piece / Multi-Piece Pattern):

  • Most widely used pattern for intricate castings.

  • Pattern is split into two or more parts so that one part rests in the cope and the other in the drag.

  • Uses dowel pins to align pattern halves precisely.

  • For three-part flasks, middle sections use a check box.

• Cope and Drag Pattern:

  • Similar to split patterns, but cope and drag halves of pattern—along with gating and riser systems—are prepared separately.

  • Attached separately to metal or wooden mounting plates along with alignment pins.

• Match Plate Pattern:

  • Extension of cope and drag pattern.

  • Cope and drag pattern halves, including gating and risering systems, are mounted on opposite sides of a single matching metal or wooden plate.

  • Highly efficient for production lines.

• Loose-Piece Pattern:

  • Used when casting contour makes direct pattern withdrawal from mold impossible due to overhangs or obstructions.

  • Obstructing projections are designed as loose pieces attached to the main pattern, which remain in sand during main pattern withdrawal and are subsequently extracted separately.

• Skeleton Pattern:

  • Constructed from wooden rib strips forming a framework or skeleton of the desired shape.

  • Final shape is achieved by packing sand inside and around the frame.

  • Economical for very large castings.

• Gated Pattern:

  • Sections connecting individual pattern units act as runners and gates.

  • Fabricated from metal to increase strength and prevent warping.

  • Allows manufacture of multiple castings simultaneously in a single pouring.

  • Employed extensively in mass production systems.

• Sweep Pattern:

  • Consists of a sweep arm holding a pattern profile (red profile) rotated around a central axis to sweep out the mold cavity in sand.

  • Eliminates the need for full three-dimensional patterns in axisymmetrical parts.

• Segmental Pattern:

  • Rotated segmentally to form large circular mold cavities step-by-step.

Molten Metal Cooling Curve

• The cooling curve tracks liquid metal temperature against time during solidification.

• Phase 1: Liquid Cooling:

  • Starts at Pouring Temperature ($T_p$).

  • Sensible heat is extracted until temperature drops to the point where freezing begins (Liquidus temperature).

• Phase 2: Freezing Phase (Solidification):

  • Coexists as liquid and solid ($L + S$ phase region).

  • Isothermal or narrow-range phase change occurs while latent heat of fusion is released.

  • Extends from Freezing Begins to Freezing Completed.

  • Interval represents Total Solidification Time.

• Phase 3: Solid Cooling:

  • Starts once freezing is completed.

  • Sensible heat extracted from solid casting until room temperature is reached.

Pattern Allowances

• Pattern dimensions must differ from final casting dimensions to account for volumetric changes, machining operations, and mold extraction dynamics.

1. Shrinkage Allowance

• Metals undergo thermal volumetric contraction when cooling from pouring temperature down to ambient temperature.

• Stages of Contraction:

  • Liquid Contraction: Volume drop during liquid cooling to liquidus.

  • Solidification Shrinkage: Volume drop during liquid-to-solid phase transition.

  • Solid Contraction: Linear thermal contraction from solidus temperature to room temperature.

• Shrinkage allowance specifically compensates for solid contraction by expanding pattern dimensions.

• Shrinked casting without allowance results in undersized parts; providing positive shrinkage allowance yields desirable casting size.

• Contraction magnitude varies according to metal composition.

2. Machining (Finishing) Allowance

• Added to pattern surfaces to increase metal thickness in regions intended for post-casting machining.

• Compensates for material loss during surface finishing, turning, grinding, or milling.

• Value depends on machining method, casting size, and surface quality requirements.

• Added cumulatively on top of shrinkage allowance.

3. Draft or Taper Allowance

• Slight taper given to vertical surfaces of pattern parallel to direction of mold withdrawal.

• Applied to internal and external vertical faces.

• Prevents pattern walls from dragging against and damaging delicate sand mould walls during extraction.

• Without draft: Removal tears sand walls leading to distorted mold cavities.

• With draft: Pattern breaks contact immediately upon initial lift.

4. Shake or Rapping Allowance

• Negative allowance applied to pattern dimensions.

• Striking or rapping the pattern laterally to loosen it before withdrawal enlarges the sand mold cavity.

• To ensure finished mould cavity dimensions match design values, pattern dimensions are reduced by the rapping allowance.

5. Distortion (Camber) Allowance

• Castings with irregular or asymmetrical cross-sections experience non-uniform cooling rates, leading to uneven thermal contraction and structural distortion.

• Distortion Mechanism in Structural Shapes (e.g., U-channels, Flange-Web structures):

  • Thin sections (flanges) have higher surface-area-to-volume ratio, causing them to cool faster and solidify first.

  • Thick sections (web) cool slower and solidify later.

  • Early contraction of flanges combined with delayed contraction of web generates internal thermal stresses, causing casting to bend/distort.

• Remedy (Camber Provision):

  • Pattern is deliberately manufactured with an initial distortion in the opposite direction (cambered pattern).

  • As unequal cooling and internal bending occur during cooling, casting distorts back into the desired straight geometry.

Computational Analysis & Scilab Sample Problems

Sample Problem 1: Solidification Time and Surface Temperature Calculation

• Parameters:

  • Mould face temperature: \theta_F = 1540\n^\circ\text{C}

  • Initial mould temperature: \theta_0 = 28\n^\circ\text{C}

  • Latent heat of iron: $L = 272 \times 10^3\,\text{J\,kg}^{-1}$

  • Density of iron: $D_m = 7850\,\text{kg\,m}^{-3}$

  • Specific heat of iron: $C_s = 0.67 \times 10^3\,\text{J\,kg}^{-1}\text{K}^{-1}$

  • Specific heat of copper: $C = 0.376 \times 10^3\,\text{J\,kg}^{-1}\text{K}^{-1}$

  • Thermal conductivity of iron: $K_s = 83\,\text{W\,m}^{-1}\text{K}^{-1}$

  • Thermal conductivity of copper: $K = 398\,\text{W\,m}^{-1}\text{K}^{-1}$

  • Density of copper: $D = 8960\,\text{kg\,m}^{-3}$

  • Height dimension: $h = 1\,\text{m}$

• Mathematical Formulations & Steps:

  1. Thermal Diffusivity of Solid Iron ($\alpha_s$):      $\alpha_s = \frac{K_s}{D_m \cdot C_s}$

  2. Transcendental Roots Evaluation for Base Condition ($\zeta_1$):      $\zeta_1 \cdot e^{\zeta_1^2} \cdot \text{erf}(\zeta_1) = \frac{(\theta_F - \theta_0) \cdot C_s}{\sqrt{\pi} \cdot L}$      Solving yields: $\zeta_1 = 0.98$

  3. Solidification Time ($t_{s1}$):      $t_{s1} = \frac{h^2}{16 \cdot \zeta_1^2 \cdot \alpha_s} \quad \text{(in seconds)}$      Dividing by $3600$ converts $t_{s1}$ to hours.

  4. Dimensionless Mold Property Factor ($\Phi$):      $\Phi = \sqrt{\frac{K_s \cdot D_m \cdot C_s}{K \cdot D \cdot C}}$

  5. Transcendental Roots Evaluation with Mold Coupling ($\zeta_2$):      $\zeta_2 \cdot e^{\zeta_2^2} \cdot [\text{erf}(\zeta_2) + \Phi] = \frac{(\theta_F - \theta_0) \cdot C_s}{\sqrt{\pi} \cdot L}$      Solving yields: $\zeta_2 = 0.815$

  6. Solidification Time under Modified Cooling Condition ($t_{s2}$):      $t_{s2} = \frac{h^2}{16 \cdot \zeta_2^2 \cdot \alpha_s}$

Sample Problem 2: Gating System Filling Time Calculation

• Parameters:

  • Sprue height: $h = 15\,\text{cm}$

  • Cast length: $l = 50\,\text{cm}$

  • Cast width: $w = 25\,\text{cm}$

  • Cast height: $h_1 = 15\,\text{cm}$

  • Gravitational acceleration: $g = 981\,\text{cm\,s}^{-2}$

  • Cross-sectional gate area: $A_g = 5\,\text{cm}^2$

  • Molten iron density: $D_m = 7800\,\text{kg\,m}^{-3}$

  • Viscosity: \eta = 0.00496\,\text{kg\,m}^{-1}\text{s}^{-1}\n - Flow angle: \theta = 90^\circ\n - Equivalent length-to-diameter ratio: \text{Eq} = 25\n - Channel length: L = 0.12\,\text{m}\n- Mathematical Formulations & Steps:\n 1. Theoretical Liquid Velocity at Gate Base (v_3):\n     v_3 = \sqrt{2 \cdot g \cdot h} \times 10^{-2}\n 2. Gate Equivalent Diameter (d):\n     d = \sqrt{\frac{4 \cdot A_g}{\pi}} \times 10^{-2}\n 3. Reynolds Number (Re):\n     Re = \frac{D_m \cdot v_3 \cdot d}{\eta}\n 4. First Approximation Friction Factor (f):\n     f = 0.0791 \cdot Re^{-1/4}\n 5. Discharge Coefficient (C_d):\n     C_d = \left[1 + 0.45 + 4 \cdot f \cdot \left(\frac{L}{d} + \text{Eq}\right)\right]^{-1/2}\n 6. Corrected Gate Velocity (v_3'):\n     v_3' = C_d \cdot v_3\n 7. Iterative Reynolds Number Correction (Re'):\n     Re' = \left(\frac{v_3'}{v_3}\right) \cdot Re\n 8. Second Approximation Friction Factor (f'):\n     f' = 0.0791 \cdot (Re')^{-1/4}\n 9. Refined Discharge Coefficient (C_d'):\n     C_d' = \left[1 + 0.46 + 4 \cdot f' \cdot \left(\frac{L}{d} + \text{Eq}\right)\right]^{-1/2}\n 10. Final Liquid Velocity (v_3''):\n      v_3'' = C_d' \cdot v_3$$