Casting, Forming and Polymer Processing Laboratory

Introduction and laboratory context

Casting, metal forming, sheet-metal working, and polymer processing are complementary manufacturing routes. Casting produces a near-net-shape component by filling a mould with liquid metal. Forming changes the shape of solid material by plastic deformation, usually without removing material. Polymer processing converts thermoplastic or thermosetting feedstock into useful products through heating, pressure, and controlled cooling or curing.

A laboratory study of these processes connects material science, mechanics, heat transfer, tooling, quality control, and industrial safety. The central engineering task is not simply to produce a part, but to select process parameters that provide the required geometry, soundness, surface quality, dimensional accuracy, productivity, and cost.

The laboratory activities in this chapter should be performed only on equipment approved by the laboratory supervisor. Operating temperatures, loads, speeds, pressures, and material grades must be taken from the machine manual and local risk assessment; they must not be inferred from a textbook example.

Learning objectives

After completing this chapter, a student should be able to:

1. Prepare a pattern and sand mould, identify its elements, and explain the purpose of allowances.
2. Pour a casting safely and diagnose common defects from their appearance and location.
3. Explain rolling, forging, extrusion, and sheet-metal operations using plastic-deformation principles.
4. Measure important process variables and relate them to dimensional accuracy, surface quality, and defects.
5. Process a thermoplastic or polymer specimen safely and explain the influence of temperature, pressure, and cooling.
6. Record observations systematically, distinguish evidence from inference, and recommend corrective action.
7. Apply safe working practices for hot metal, moving machinery, sharp sheet, heated polymers, and compressed systems.

1. Laboratory method: from specification to evidence

Each experiment should begin with a process plan. The plan identifies the material, desired geometry, machine, tooling, process variables, measurements, hazards, and acceptance criteria. A useful laboratory record contains:

• specimen identification, material and initial dimensions;
• equipment identification and calibration status;
• pattern, die, roll, punch, mould, or polymer-tool details;
• set values and actual readings of temperature, force, speed, pressure, and time;
• sketches or photographs of the setup and product;
• observed defects and their locations;
• measurements before and after processing;
• deviations from the planned procedure;
• conclusions supported by observations.

A process variable is controllable or measurable, such as pouring temperature or roll gap. A response is an outcome, such as thickness reduction, filling quality, hardness, or warpage. A defect is an unacceptable departure from specification. A discontinuity may be harmless or harmful, depending on its size, location, and service requirement.

For a rectangular specimen, volume is calculated as:

where L, b, and t are length, width, and thickness in consistent units. Density may be estimated from mass and volume:

where ρ is density, m is mass, and V is volume. These relations assume that dimensions represent the actual specimen geometry and that voids or surface films do not significantly distort the measurement.

Percentage change in a measured quantity is:

The sign should be retained during analysis: a negative value indicates a decrease. For reporting repeated measurements, the arithmetic mean is:

The mean summarizes repeated observations but does not prove accuracy. Random scatter, systematic error, instrument resolution, and operator technique should also be discussed.

2. Pattern and mould preparation

2.1 Pattern functions and allowances

A pattern is a replica used to form the mould cavity. It is not always identical to the final casting. Important pattern features include a parting line, core prints, draft, fillets, and allowances.

Draft is a small taper on surfaces parallel to the direction of pattern withdrawal. It reduces mould damage. Shrinkage allowance compensates for contraction during solidification and cooling. Machining allowance provides extra material on surfaces that will later be machined. Distortion allowance is used when unequal contraction is expected to bend the casting. The correct value depends on alloy, casting size, geometry, mould material, and foundry practice; it should be obtained from a design standard or process data rather than guessed.

A well-designed pattern avoids abrupt section changes and provides generous transitions. Sharp internal corners increase stress concentration and may encourage hot spots, cracks, or incomplete filling. Core prints locate and support cores, which form internal cavities.

2.2 Sand mould elements

A typical mould includes the cope, drag, parting surface, sprue, pouring basin, runner, ingates, riser, and cavity. The gating system carries metal into the cavity with controlled turbulence and heat loss. A riser supplies liquid metal during solidification shrinkage. Vents permit gases to escape. Cores must be adequately supported and, where necessary, vented.

Moulding sand requires sufficient refractoriness, permeability, strength, flowability, and collapsibility. Excess moisture may generate steam and gas defects; insufficient moisture may reduce green strength. Excessive ramming can reduce permeability and make pattern withdrawal difficult. Poorly mixed sand causes local variations in hardness and surface quality.

2.3 Practical mould-preparation sequence

1. Inspect the pattern, flask, tools, and sand condition.
2. Apply a suitable parting material and position the pattern correctly.
3. Fill and ram the drag uniformly, avoiding damage to the pattern.
4. Strike off the surface, invert the flask, and prepare the parting plane.
5. Fit the cope, establish the sprue and riser, and ram the cope.
6. Separate the flask and withdraw the pattern vertically using a draw spike or approved tool.
7. Repair loose edges, cut runners and ingates, place the core if required, and close the mould.
8. Check alignment, vents, gating continuity, and flask security before pouring.

During pattern withdrawal, the operator should avoid levering against the mould cavity. A damaged cavity can produce fins, dimensional errors, or sand inclusions.

2.4 Casting and solidification

Before pouring, the ladle, tools, mould, and charge must be dry and at the prescribed condition. Moisture contacting molten metal can flash to steam and eject metal violently. The operator must wear the specified face shield, heat-resistant clothing, gloves, safety footwear, and other required protection.

Metal should be poured at the specified temperature and at a steady rate. Excessive superheat increases oxidation, gas absorption, mould erosion, and solidification time. A low pouring temperature may produce misruns or cold shuts. The sprue should remain filled where the gating design requires it, while overflow and spillage are avoided.

Solidification involves nucleation and growth. A casting contracts as it cools, and liquid metal must feed the contraction before the feeding path freezes. A riser should therefore remain liquid longer than the region it feeds. Thick sections and isolated hot spots are especially vulnerable to shrinkage cavities and porosity.

A simplified heat balance for a metal cooled from an initial temperature to its melting temperature and then solidified is:

where Q is heat released, m is mass, cₚ is specific heat capacity, Tᵢ is initial metal temperature, Tₘ is melting temperature, and L is latent heat of fusion. This is an idealized expression: mould heating, superheat above the melting point, phase transformations, and heat losses must be added when they are significant.

2.5 Casting-defect diagnosis

Diagnosis should use multiple clues. For example, a cavity near the last-freezing heavy section suggests feeding failure, whereas many rounded internal voids suggest gas porosity. Confirmation may require sectioning, radiography, dye penetrant testing, or density measurement.

3. Metal forming processes

Plastic forming requires stresses above the material's yield level while maintaining sufficient ductility. The material flows into a new shape, so conservation of volume is a useful approximation for solid metals undergoing small changes in density:

where A is cross-sectional area and L is length; subscripts 0 and 1 denote initial and final states. This approximation is appropriate when porosity and temperature-dependent density changes are negligible.

True strain in uniaxial deformation is:

For rolling, thickness reduction is often expressed as:

where h₀ and h₁ are initial and final thicknesses. Percentage reduction is 100r percent. True strain in thickness reduction, assuming constant width and volume, is approximately ln(h₀/h₁). These measures should not be confused with engineering strain.

3.1 Rolling

In rolling, work passes between rotating rolls. The roll gap, friction, reduction per pass, roll speed, alignment, and material temperature influence the result. If the reduction is excessive, the rolls may fail to draw the strip into the bite. If alignment is poor, the strip may curve, twist, or develop wedge-shaped thickness.

Measure thickness at several positions before and after rolling. Do not place hands near the roll bite. Use tongs or approved feeding devices, maintain a clear exit path, and never attempt to pull a jammed specimen while the rolls are moving.

3.2 Forging

Forging uses compressive force through dies, hammers, or presses. Open-die operations include upsetting and drawing out; impression-die forging forms material within a shaped cavity. Forging can refine grain flow and close some internal discontinuities, but it may also produce laps, underfill, cracks, or die mismatch when temperature, lubrication, alignment, or billet volume is incorrect.

Upsetting increases cross-sectional area while reducing length. Drawing out reduces cross-sectional area and increases length. Hot forging lowers deformation resistance but requires control of oxidation, scale, and temperature loss. Cold forging improves surface finish and dimensional accuracy but demands greater force and sufficient ductility.

3.3 Extrusion

Extrusion forces a billet through a die. In direct extrusion, billet and ram move in the same general direction; in indirect extrusion, the die moves relative to the billet container and friction conditions differ. Extrusion ratio is:

where A₀ is initial billet area and A_f is final product area. The ideal geometric strain associated with area reduction is:

Actual force and energy exceed the ideal requirement because of friction, redundant deformation, die geometry, and material strengthening. Defects include surface cracking, piping, internal cracking, die lines, and incomplete profile formation. Die temperature, billet temperature, lubrication, speed, and bearing length must be controlled within equipment limits.

3.4 Sheet-metal operations

Shearing separates sheet by localized fracture. In blanking, the cut-out part is the blank; in punching, the removed piece is generally the slug. Clearance between punch and die strongly affects burr height, fracture zone, force, and tool life. Bending produces elastic recovery, known as springback, so the tooling angle may need compensation.

A simple bending strain estimate for a sheet of thickness t bent to radius R is:

This expression is a geometric estimate for a simple bend and assumes the neutral axis is near the mid-thickness. Actual strain depends on material behavior, bend geometry, and neutral-axis shift. Excessive strain causes cracking, especially when the bend is made transverse or parallel to an unfavourable rolling direction.

Safe practice includes deburring edges, using guards, keeping fingers away from dies, and confirming that the press returns to a safe position before repositioning the work.

4. Polymer processing

Polymers are processed according to their thermal and rheological behavior. Thermoplastics soften on heating and can generally be reheated, although repeated heating may degrade them. Thermosets undergo a curing reaction and cannot be remelted after cross-linking. Elastomers are highly deformable and commonly require vulcanization or another curing process.

4.1 Compression moulding

In compression moulding, a measured charge is placed in a heated open mould, the mould is closed, and pressure is applied until the material flows and cures or cools. Important variables are charge mass, preheat, mould temperature, pressure, closing speed, dwell time, and cooling practice.

A short shot may result from insufficient charge, low temperature, inadequate pressure, or premature cooling. Flash may indicate excessive charge, poor mould closure, or excessive pressure. Voids may arise from trapped air, moisture, or incomplete consolidation. Warpage is commonly associated with nonuniform cooling, uneven pressure, or residual stress.

4.2 Injection moulding and extrusion

Injection moulding involves plasticizing pellets, injecting the melt into a closed mould, packing to compensate for shrinkage, cooling, and ejecting the part. Fill time, melt temperature, mould temperature, injection pressure, holding pressure, and cooling time interact strongly. Sink marks arise when thick regions shrink during cooling; weld lines form where flow fronts meet; burn marks may indicate trapped air or excessive shear heating.

Polymer extrusion continuously forces melt through a die to make profiles, films, tubes, or filament. Screw speed, barrel temperature profile, die pressure, cooling, and haul-off speed determine output and dimensions. Excessive residence time can cause thermal degradation, while inadequate melting produces unmelted particles and surface roughness.

4.3 Polymer safety and environmental control

Heating polymers may release irritating, toxic, or corrosive products, particularly when a polymer is overheated or contaminated. Consult the safety data sheet, provide effective ventilation, prevent contact with hot surfaces, and never process an unidentified polymer. Scrap should be segregated by polymer type where recycling is intended. Thermoset waste generally cannot be remelted and must be managed according to institutional and regulatory requirements.

5. Measurement, quality assessment, and corrective action

A sound laboratory conclusion links a measured defect to a controllable cause. The following sequence is effective:

1. Define the nonconformance precisely: location, size, appearance, and frequency.
2. Check measurement validity and instrument resolution.
3. Compare the actual process record with the prescribed settings.
4. Separate immediate causes from underlying causes.
5. Change one major variable at a time where practical.
6. Produce a confirmation specimen and compare results.

Dimensional inspection may use vernier calipers, micrometers, gauges, height gauges, or coordinate equipment. Surface examination may begin visually and proceed to appropriate nondestructive testing. Hardness, mass, thickness, and density measurements should include units and uncertainty or resolution where known.

A process capability study requires more than one specimen. A single good part does not demonstrate a stable process. Students should avoid changing several variables simultaneously, because the effect of each change then becomes impossible to identify. In production, corrective action should also consider traceability, operator training, tooling wear, maintenance, and statistical monitoring.

6. Integrated illustrative case: correcting a thin-wall aluminium casting

Background

A student team produces an aluminium housing by green-sand casting. The housing has a thin wall, two bosses, and an internal cavity formed by a core.

Problem

The first casting shows incomplete filling at one corner, a rough inclusion near the runner, and a depression above one boss. The team initially proposes increasing pouring temperature substantially.

Analysis

The incomplete corner is consistent with a misrun, but the rough inclusion indicates mould erosion or turbulent flow. The depression above the boss suggests a local hot spot and inadequate feeding. A large temperature increase could improve fluidity but might also increase oxidation, sand attack, and gas absorption. Thus, temperature alone is not an adequate solution.

Approach and solution

The team checks the pouring record, dries the ladle and tools, inspects the mould, strengthens the sand locally, and modifies the runner to reduce direct impingement on the cavity wall. A small riser is positioned over the boss, and a chill is considered at the appropriate location after consultation with the instructor. The pouring temperature is adjusted only within the approved process range. The second casting is sectioned for inspection and its critical dimensions are measured.

Outcome

The corner fills completely, the inclusion is reduced, and the boss depression is removed. The team records that the improvement resulted from coordinated changes in mould integrity, flow control, and feeding—not from temperature adjustment alone.

Learning points

The case illustrates the value of defect location, process records, controlled experimentation, and safe corrective action. It also shows why a casting defect should be treated as evidence requiring diagnosis rather than as a reason for an immediate, unverified parameter change.

7. Best practices, limitations, and contemporary practice

Best practice combines design for manufacture, verified material data, preventive maintenance, safe operating procedures, and documented inspection. Digital temperature logging, machine data acquisition, simulation of filling and solidification, and 3D scanning can improve understanding, but software predictions depend on material properties, boundary conditions, and mesh quality. Artificial intelligence may assist defect classification from images or optimize process settings, yet it does not replace physical validation, operator judgment, or safety controls.

Laboratory equipment often operates at smaller scale and under more ideal conditions than industrial equipment. Results may therefore not transfer directly to high-volume production. Small samples may hide variability, and classroom tools may have greater wear or lower control precision. Conclusions should state these limitations.

Environmental responsibility includes minimizing melt loss, reusing compatible sand where possible, segregating polymer scrap, controlling fumes and dust, and reducing unnecessary energy use. Ethical engineering requires honest reporting of failed trials, accurate labelling of materials, respect for machine safeguarding, and refusal to bypass a safety interlock or prescribed protective equipment.

Chapter Summary

Casting creates components by filling and solidifying metal in a prepared mould. Pattern allowances, mould properties, gating, risers, pouring practice, and solidification control determine casting quality. Defects such as misruns, cold shuts, gas cavities, shrinkage, inclusions, and fins should be diagnosed from their evidence and corrected through controlled changes.

Rolling, forging, extrusion, and sheet-metal operations shape solid material by plastic deformation. Conservation of volume, reduction, true strain, friction, temperature, tool geometry, and material ductility provide the basis for analysis. Safe operation requires attention to roll bites, presses, dies, hot work, sharp edges, and stored energy.

Polymer processing depends on thermal behavior, flow, pressure, curing, cooling, and degradation. Compression moulding, injection moulding, and extrusion each have characteristic defects and control variables. Across all processes, reliable laboratory practice requires accurate measurement, traceable records, controlled experiments, defect analysis, and corrective actions that are both technically justified and safe.

Key Terms

Pattern: A model used to form a mould cavity.

Draft: Taper provided to permit pattern withdrawal.

Core: A mould element that forms an internal cavity.

Gating system: Channels that carry molten metal into a mould cavity.

Riser: A reservoir of liquid metal that feeds solidification shrinkage.

Misrun: An incompletely filled casting.

Cold shut: A line caused by incomplete fusion of meeting metal streams.

Shrinkage cavity: A void caused by insufficient feeding during solidification.

Plastic deformation: Permanent deformation after removal of load.

Rolling: Reduction or shaping of material between rotating rolls.

Forging: Shaping by compressive forces applied through dies or tools.

Extrusion: Forcing material through a die to produce a profile.

Blanking: Cutting a desired sheet-metal piece from stock.

Springback: Elastic recovery after sheet bending.

Thermoplastic: Polymer that softens on heating and can generally be reheated.

Thermoset: Polymer that forms a permanent cross-linked structure during curing.

Flash: Unwanted thin material forced beyond a mould or die boundary.

Warpage: Unintended distortion caused by nonuniform stress or shrinkage.

Permeability: Ability of moulding material to allow gases to pass through it.

Process capability: Ability of a stable process to meet specified requirements.

Review Questions

Short Answer Questions

1. What is the purpose of draft on a casting pattern?
2. Distinguish a riser from a runner in a sand-casting mould.
3. Define rolling reduction and state the variables used to calculate it.
4. What is springback in sheet-metal bending?
5. Why must polymer processing temperatures be controlled carefully?

Long Answer Questions

1. Describe the complete preparation and pouring sequence for a green-sand mould, including safety precautions.
2. Explain the causes and corrective actions for misruns, gas porosity, shrinkage cavities, and sand inclusions.
3. Compare rolling, forging, and extrusion with respect to loading, tooling, products, and common defects.
4. Explain the effects of temperature, pressure, cooling, and charge mass in compression moulding.
5. Discuss how a laboratory team should design a reliable defect-investigation experiment.

Analytical / Application Questions

1. A strip is reduced from 12 mm to 9 mm thickness in one rolling pass. Calculate the reduction ratio and percentage reduction, and explain two additional observations that should be checked experimentally.
2. A casting has a depression directly above an isolated thick boss but no evidence of gas holes. Diagnose the most likely mechanism and propose two process or design changes.
3. An injection-moulded polymer part has sink marks near thick sections and warpage after ejection. Develop a prioritized investigation plan involving process measurements, likely causes, and corrective actions.

Multiple Choice Questions with Answers

Question 1

Which feature primarily helps a pattern withdraw from a sand mould without damaging the cavity?

A. Riser

B. Draft

C. Runner

D. Core print

Correct Answer: B. Draft

Question 2

A depression near an isolated thick section of a casting most strongly suggests which defect mechanism?

A. Solidification shrinkage caused by inadequate feeding

B. Excessive sheet-metal springback

C. Polymer cross-linking

D. Punch-die clearance

Correct Answer: A. Solidification shrinkage caused by inadequate feeding

Question 3

A strip is reduced from 10 mm to 8 mm thickness. What is the percentage reduction?

A. 8%

B. 20%

C. 25%

D. 80%

Correct Answer: B. 20%

Question 4

The principal loading mode in conventional forging is:

A. Tension only

B. Compression

C. Pure torsion

D. Thermal radiation

Correct Answer: B. Compression

Question 5

For extrusion, the extrusion ratio is defined as:

A. Final area divided by initial area

B. Initial area divided by final area

C. Initial length divided by final length only

D. Final thickness divided by initial thickness

Correct Answer: B. Initial area divided by final area

Question 6

In sheet-metal production, springback is caused mainly by:

A. Elastic recovery after unloading

B. Complete melting of the sheet

C. Gas evolution from moulding sand

D. Riser feeding

Correct Answer: A. Elastic recovery after unloading

Question 7

Which polymer class generally softens on heating and can be reheated?

A. Thermoplastic

B. Thermoset after curing

C. Ceramic

D. Fully cured concrete

Correct Answer: A. Thermoplastic

Question 8

Which casting practice increases the risk of gas-related accidents?

A. Using dry tools and ladles

B. Checking flask alignment

C. Allowing moisture to contact molten metal

D. Inspecting the mould before pouring

Correct Answer: C. Allowing moisture to contact molten metal

Question 9

If a casting corner is incompletely filled, the defect is commonly called:

A. Misrun

B. Flash

C. Die line

D. Springback

Correct Answer: A. Misrun

Question 10

Why should a laboratory team change one major process variable at a time when practical?

A. To increase specimen mass automatically

B. To identify the effect of the change more reliably

C. To eliminate the need for measurements

D. To guarantee zero defects in every trial

Correct Answer: B. To identify the effect of the change more reliably