Engineering & Technology · Manufacturing Processes & Process Selection

Casting, moulding and solidification behaviour

A liquid fills a mould perfectly, yet the cooled casting contains a cavity inside a thick junction. Filling the shape and producing a sound solid are different tasks. The material must also change state, lose heat and reach its final dimensions without creating unacceptable defects.

Material Structure & Properties and Engineering Design Methods & Trade-offs supply deformation, properties, requirements and trade-offs. This chapter recalls volume, stress and percentage arithmetic before using them. The last lesson introduces mean and standard deviation in context. All numerical prices, dimensions, shrinkage factors, times, yields, distributions and resource quantities are original teaching assumptions. They are not supplier data, machine settings, qualified procedures or allowable service values.

Back to the chapter

After reading, you can

Explain how filling, cooling or curing and mould release affect a shaped part and its defects.

Follow the material beyond filling

In metal casting, liquid enters a cavity and solidifies as heat leaves. Sand moulds are normally broken away; a permanent mould is opened and reused. A core occupies a region intended to remain hollow. These choices affect complexity, tooling, surface detail and how the part can be removed. A permanent tool must release the geometry; a feature trapped behind another may require a separate moving or removable element.

Metal can contract during solidification and further cooling, though behaviour depends on the alloy and stage. A feeder is an additional liquid reservoir intended to supply contracting regions while a liquid connection remains. If that route freezes too early, a region can be isolated. Gas and entrained material offer other routes to defects: not every pore has the same cause.

Notice where the last liquid can become trapped

The original cross-sections show a thick region with a narrow connection to a feeder. In the first state the connection is liquid; in the second it has frozen while a pocket remains liquid. The picture explains a possible shrinkage mechanism. It does not calculate temperatures, solidification times or the size of an actual defect.

Abrupt thickness changes affect heat flow and local contraction. A pattern may include allowances for contraction and later machining, but one global enlargement cannot correct every local distortion. Verify dimensions and internal quality using methods suited to the required features and defects. A visually complete exterior cannot exclude internal porosity.

Each original state contains an upper feeder connected by a narrow neck to a larger part. Earlier, an orange liquid path links feeder to liquid in the part inside blue solid. Later, the neck is solid blue while an orange pocket remains in the thick part. The pocket can contract without liquid replenishment. Dimensions and times are intentionally unspecified; this is a mechanism diagram, not a simulation.

Distinguish thermoplastic cooling from reaction curing

In thermoplastic injection moulding, heated polymer flows into a cavity and is cooled before removal. Flow, packing and cooling interact; thick regions may cool and contract differently from thin walls, creating sinks, internal voids or warpage. A flow simulation is a model to compare with measured parts, rather than proof that a moulded part meets requirements.

A thermosetting resin can instead form a network through a curing reaction. That chemical change is not simply reversible melting and freezing. Cure history and reaction heat can matter. “Moulding” therefore describes a way of giving shape, while the material and process determine how shape becomes stable. Polymer extrusion produces a continuous profile; it does not use a closed cavity to make each separate object.

Keep a linear allowance separate from volume change

Assume a deliberately simple model: after cooling, each length is 98% of its mould-cavity length. A 100 mm cavity then gives 98 mm. To target a final 100 mm, the model cavity is 100/0.98 = 102.0408… mm, approximately 102.04 mm.

Adding 2 mm instead would give 102 × 0.98 = 99.96 mm. The difference arises because the assumed 2% reduction is measured from the cavity length. Neither value is a recommendation for a real alloy or mould.

If the same 0.98 factor applied in three independent perpendicular directions, volume would scale by 0.98³ = 0.941192: a 5.8808% reduction, not 2%. Real constraints, feeding and anisotropic shrinkage can make that simple uniform model unsuitable.

Try the reasoning

Why may a feeder fail to prevent a shrinkage cavity?
Show the answer and reasoning
Its liquid supply path can freeze before the region has finished contracting. Reservoir volume alone is not sufficient.
Does a smooth outside surface establish a sound interior?
Show the answer and reasoning
No. Internal pores or inclusions can exist without an obvious surface opening; use appropriate evidence for the required quality.
Is a cured thermoset equivalent to a thermoplastic that has cooled?
Show the answer and reasoning
No. Curing forms a chemical network, whereas thermoplastic cooling stabilises a different material state. Their processing and reuse routes differ.

What to carry forward

Mould filling, material transformation, heat flow and removal must work together. A cavity shape alone does not determine final quality.

References

Teaching resources and research supporting this lesson’s explanations and examples.