The Casting Process

Investment casting is an industrial process based on and also called lost-wax casting, one of the oldest known metal-forming techniques. From 5,000 years ago, when beeswax formed the pattern, to today’s high-technology waxes, refractory materials and specialist alloys, the castings allow the production of components with accuracy, repeatability, versatility and integrity in a variety of metals and high-performance alloys.

The Casting Process
Finishing with Subcontractors

McKenna Group maintains excellent relationships with a range of sub-contractors supplying a wide variety of high quality finishing services.

These include…

  • Machining
  • Painting
  • Anodising
  • Surface Treatment / Coating
  • Heat Treatment
  • Hot Isostatic Processing
  • X-Ray
  • Passivation
  • Laser Marking

Working with our sub-contractors, McKenna Group can meet all of our customers’ finishing requirements in full.

The casting process is generally used for small castings, but has produced complete aircraft door frames, steel castings of up to 300 kg and aluminium castings of up to 30 kg. It is generally more expensive per unit than die casting or sand casting but with lower equipment cost. It can produce complicated shapes that would be difficult or impossible with die casting, yet like that process, it requires little surface finishing and only minor machining.

Casts can be made of the wax model itself, the direct method; or of a wax copy of a model that need not be of wax, the indirect method. The following steps are for the indirect process, which, in total, can take two days to one week to complete.

1. INJECTION

The casting process begins with the production of a wax pattern, which is made by injecting wax into a metal die. At McKenna Group, most dies are either supplied by our customers or manufactured in-house to specific customer requirements. However, we do offer a generic medical product range for which we use our own tooling.

casting process injection
2. ASSEMBLY

Once the requisite number of wax patterns has been produced, the next stage is to mount these patterns onto a runner system. The runner system is made up of a number of passageways and reservoirs. These not only serve to guide the molten metal during casting, but also provide additional metal to compensate for the contraction that inevitably results from cooling.

casting process assembly
3. SHELL

At this stage, the entire pattern cluster is dipped into a ceramic slurry, drained, and coated with a fine ceramic sand. After drying, this process is repeated several times, using progressively coarser grades of ceramic material, until a self supporting shell is formed.

casting process shell
4. DEWAX

Once suitably coated, the cluster is turned upside down and placed in a steam boiler clave where the wax melts and pours out through the gates, runners and pouring cup. This leaves a ceramic shell containing cavities of the desired casting shape, together with a suitable runner and feeding system.

casting process dewax
5. CAST

The ceramic shell mould is fired to burn out the last traces of wax, to develop the high temperature bond of the ceramic system, and to preheat the mould in preparation for casting. Once the mould is ready, molten metal is poured in and left to cool.

casting process cast
 6. KNOCK-OUT
Once the metal has cooled and formed, excess ceramic material is removed pneumatically.
casting process knock out
7. SPECTRO ANALYSIS

As an added measure McKenna Precision Castings perform a chemical analysis of the mould. By cutting away a test disc, which forms part of the cast mould, and placing it under the very accurate Spectro Analysis Machine instant results show whether the alloy falls within the specification agreed with the customer.

spectro analysis
8. CUT-OFF

The individual patterns are now removed from the runner system in a laser controlled automatic cut-off machine. Alternatively, arc air or plasma is used.

casting process cut off
9. BLASTING

The patterns are then blasted to remove remaining ceramic material. Aluminium clusters are water blasted, whilst clusters formed in other alloys tend to be grit-blasted.

casting process blasting
10. BELT-GATE

Remaining protrusions are finally sanded away using belt grinding equipment.

casting process bell gate
11. FINISHING

The casting is now ready for secondary operations such as heat treating, straightening, machining and whatever inspection is specified. McKenna Group inspects all castings to stringent quality standards, and also subjects castings to non-destructive testing where necessary.

casting process finishing

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