Manufacturing techniques practice questions.

In investment casting the wax pattern is melted out of a one-piece ceramic shell before the metal is poured. Which consequence follows directly from this mechanism?

• The mould can be reused for thousands of identical castings
• Cooling is rapid enough to refine the grain of the alloy
• Undercuts can be cast because there is no parting line
• Machining allowances of several millimetres must be added

Answer: Undercuts can be cast because there is no parting line

The shell is formed around a single wax pattern and is broken away afterwards, so the mould has no split line and needs no draft. Sand and die casting both work in two mould halves, which is exactly what limits their ability to reproduce undercuts.

Selective laser sintering builds a component by fusing successive layers of polymer powder. Compared with milling the same component from solid stock, which classification statement is correct?

• It removes material, so swarf must be recycled
• It adds material, so unfused powder is the main waste
• It forms material, so the grain follows the part profile
• It joins material, so a filler is consumed at each layer

Answer: It adds material, so unfused powder is the main waste

Laser sintering is an additive technique: material is placed only where the part exists, and the loose powder around the build is the waste stream. Milling is subtractive, so the entire volume around the finished shape leaves the machine as swarf.

Compression moulding is normally used for thermosetting polymers rather than for thermoplastics. Which property of thermosets makes the process appropriate?

• They cross-link irreversibly when heated in the mould
• They soften repeatedly, so the mould can be cooled quickly
• They melt at a lower temperature than thermoplastics
• They can be reground and fed back into the hopper

Answer: They cross-link irreversibly when heated in the mould

Heat and pressure in the closed tool cure the charge by cross-linking, so the moulding is finished in the mould and can never be re-melted. Thermoplastics soften and flow repeatedly, which is why they suit injection moulding instead.

Two mild steel tubes are joined by drawing a molten bronze filler into the gap between them at about 900 °C, while the steel itself stays solid. Which joining process is being used?

• Fusion welding
• Soft soldering
• Sintering
• Brazing

Answer: Brazing

Brazing melts a filler above 450 °C and draws it into the joint by capillary action while the parent metal remains solid, so the tube geometry is undistorted. In fusion welding the parent metal itself melts, and soft soldering works below 450 °C with a much weaker joint.

In laminating, thin timber veneers are coated with adhesive and clamped in a former until the glue cures. Which statement explains why the curve is retained after the clamps are removed?

• The lignin softens and re-sets in the new shape
• The cured adhesive stops the veneers sliding back
• The moisture content rises and swells the outer fibres
• The wood is compressed beyond its elastic limit

Answer: The cured adhesive stops the veneers sliding back

Each thin veneer bends elastically and would spring back on its own, but the cured glue lines prevent the layers sliding over one another, so the assembly holds the former's shape. Softening the lignin is the mechanism of steam bending, not of laminating.

In injection moulding the cooling time varies approximately with the square of the wall thickness of the part. The wall thickness of a moulding is doubled and nothing else is changed. State the effect on the cooling time.

• It falls to one quarter
• It is unchanged
• It doubles
• It rises to four times

Answer: It rises to four times

A square-law relationship means that doubling the thickness multiplies the cooling time by two squared, that is by four. Cooling dominates the moulding cycle, so a thicker wall costs cycle time out of proportion to the extra material used.