Shaping, Casting and Moulding

Four Distinct Families

Shaping, casting and moulding all give material a new form without removing it as waste chips — but the mechanisms are completely different. A common exam error is mixing stages from different processes. Keep these four families separate from the start:

• Press forming — squeeze a sheet between two solid dies to change its shape in the solid state.
• Injection moulding — melt polymer granules and inject under pressure into a closed metal mould.
• Sand casting — pour liquid metal into a cavity formed by compressing sand around a pattern.
• Pressure die casting — inject liquid metal under high pressure into a precision reusable steel die.

Press Forming (and Related Sheet-Metal Operations)

Press forming places a flat sheet of metal between a punch (upper male tool) and a die (lower female tool). The press ram drives the punch down with enormous force — hundreds of tonnes for structural pressings — deforming the sheet permanently into the die cavity. The car body panel on every modern car is made this way: the outer skin of a vehicle door takes less than a second to press from a flat steel blank. Related sheet-metal operations include:

• Bending and folding — a press-brake bends sheet along a straight line; a box-and-pan folder creates enclosure cases and duct sections.
• Punching and blanking — a punch shears a hole (punching) or cuts a shaped blank (blanking) from sheet stock in a single stroke.
• Stamping — a high-speed press stamps (embosses or cuts) repeated features — such as the tabs on a battery connector — into sheet at production rates of thousands per hour.
• Composite lay-up — individual layers of glass-fibre or carbon-fibre cloth pre-impregnated with resin (“prepreg”) are laid by hand into a mould, then cured under heat and pressure in an autoclave. Used for F1 monocoques and wind-turbine blades where weight is critical.

Sand Casting

Sand casting is one of the oldest manufacturing processes still used today. A replica of the desired part — the pattern — is placed inside a two-part wooden or metal moulding box (the cope and drag). Sand mixed with a bonding agent is rammed tightly around the pattern. The pattern is then removed, leaving a cavity. If the part requires an internal void (such as a pipe bore), a separate core of sand is placed inside the cavity. The two halves are clamped together.

Molten metal is poured in through the sprue (the vertical channel), flows along the runner (horizontal channel) and enters the mould cavity through the gate. A riser (also called a feeder) is an additional reservoir of molten metal that compensates for shrinkage as the casting cools. Once solidified and cooled, the sand is broken away and the casting is removed. A draft angle on the pattern walls is essential to release the pattern cleanly from the sand without crumbling the cavity walls. The casting is then fettled (excess metal from sprues and runners is cut away) and may be machined to final dimensions.

Pressure Die Casting

Pressure die casting injects molten metal — typically aluminium, zinc or magnesium alloy — under high pressure (10–200 MPa) into a hardened steel die. The die is precision-machined and can be reused for tens of thousands of cycles. The high pressure forces metal into fine detail and thin sections that gravity alone cannot fill. The cooling rate is much faster than sand casting (the steel die conducts heat quickly), giving a finer grain structure and better mechanical properties.

Advantages over sand casting: better surface finish, tighter dimensional tolerance, faster cycle time (seconds per part vs minutes), reusable die. Limitations: the steel die costs tens of thousands of pounds to make, so die casting is only economical at high volumes. It is also limited to non-ferrous metals — the high melting point of steel would destroy a steel die. Gearbox housings, engine covers and tap bodies are typical die-cast products.

Injection Moulding

Injection moulding is the dominant process for producing plastic parts at high volume. Polymer granules are fed from a hopper into a heated barrel where a reciprocating screw melts and homogenises them. When sufficient melt has accumulated, the screw acts as a plunger, injecting the melt under high pressure through the gate into the closed mould cavity. The mould is water-cooled, so the part solidifies in seconds. The mould then opens and ejector pins push the part free. Cycle times are typically 15–60 seconds for most consumer products.

Critical design features for injection moulding: the mould must have a draft angle on all walls so the part releases cleanly; wall thickness should be uniform to avoid sink marks and warping; internal ribs increase stiffness without adding bulk. The mould tool costs £20,000–£200,000 but is amortised across millions of parts. This process produces everything from mobile phone cases to car dashboards.