Types of Moulds and Their Characteristics for Casting

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Moulds for Casting

Mould made of ceramic material e.g. sand particles, bound together with a BINDER which may be:

  • Clay: 90% sand, 8% clay, 2% water - green sand mould.
  • Thermosetting resin system - coat sand with two or more reactants which cure to form a resin (typically 1 - 2 parts binder to 100 parts sand):
    • Curing begins immediately components are combined, for a period of time after initial mixing the sand is workable and flowable to allow filling of mould - No bake systems.
    • Phenolic - urethane no bake is most common.
    • Curing occurs upon application of heat – heat cured binder system.
    • Liquid thermosetting binder and a catalyst are mixed with dry sand. Upon heating the catalyst releases acid which induces a rapid cure in 10 – 15 s. Pattern is removed followed by post curing of mould in an oven.
    • Curing occurs upon passing a gas catalyst through the binder-sand mixture - cold box process.

Following solidification the heat given out by the casting decomposes the thermosetting binder and the sand mould material can then be recycled.

ADVS/DADVS:

  • Low cost mould material.
  • High melting temperature alloys can be cast.
  • Wide range of sizes possible.
  • Economical for low numbers or larger numbers if automated.

Castings have poorer dimensional accuracy and surface texture than from permanent moulds because:

  • Wooden patterns are used which have poor dimensional accuracy.
  • Clearances on dowels used to locate cores reduce dimensional accuracy.
  • Sand deforms.
  • Silica as quartz undergoes phase transformations on heating leading to volume changes.

So accuracy and smooth finish met by local or general machining.

Equipment costs are low, process is labour intensive and slow but can increase production rate with multiple moulds and automation.

Improve castings by:

  1. Use of a precision metallic pattern and locating system.
  2. Use of fine sands or refractory facings (coatings) to improve surface finish.
  3. Techniques to harden mould in contact with pattern.
  4. Use thermally stable “sand”, e.g. zircon sand, ZrSiO4.

Shell Moulding

Mould is not a cavity within a solid block but a THIN WALLED MOULD (shell) in which the external surface follows the contours of the pattern.

  • Shells are thin and thus highly permeable to displaced air allowing castings with intricate details to be produced.
  • Sand is silica or zircon with a resin binder.
  • Resin cures in contact with hot aluminium alloy pattern at 200°- 300°C.
  • After curing in contact with the pattern, the shell is stripped followed by post curing in an oven.
  • Two shells are clamped together. May be rigid enough to be self supporting or may be backed up with sand or steel shot in a mould box. Loose sand or steel shot is poured around the mould followed by vibration to consolidate. Process used for components where limited coring is required, e.g. automotive crankshafts.

Characteristics:

  1. Close control of production variables - higher repeatability.
  2. Intricate details can be produced (need a precision pattern - expensive) - shell smooth and highly permeable - so can cast thin sections, minimum wall thickness of casting ~ 2 mm.
  3. Less final machining than for sand castings.
  4. Less “sand” used than in sand casting.
  5. Production costs slightly higher than sand casting but capital costs less.

Permanent Mould:

  • Same mould used for a large number of castings.
  • Each casting released by opening mould rather than destroying it.
  • Typical processes: Gravity die casting, pressure die casting

Moulds (dies)

  • Permanent moulds need to:
  1. Withstand temperature fluctuations
  2. Withstand wear associated with repeated casting operations. Usually made of metal - so process tends to be restricted to low melting temperature alloys.

Tooling costs are very high – this means the process is uneconomic for short production runs or very large components.

Gravity die casting:

  • Metal mould (die) – made from grey cast iron or tool steel.
  • Liquid enters mould by gravity.
  • Split along vertical joint line passing through die cavity, the running, feeding and venting system are dispersed in the same plane.
  • Dies are provided with locating pins, clamping devices and ejection systems for casting removal.
  • Die halves may be hinged, like a book.
  • Form internal cavities from retractable metal cores or from sand.
  • Undercut shapes require that the cores be made in several interlocking parts which are withdrawn in a fixed sequence.

Preheat die to 300 – 400°C - maintain this in production by cycle time and cooling - cooling fins, forced air cooling or water cooling can be used. Coat die with refractory insulating dressing/lubricant. Pour carefully to avoid turbulence. Eject as quickly as possible so as not to hinder contraction on cooling.

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moulds casting sand casting permanent mould