Manufacturing Companies for the 21st Century

Investment casting converts molten metal in a single operation to precision engineered components with a minimum wastage of material and energy and subsequent machining. It has a versatility approached by few other metal forming processes. Intricate or re-entrant contours can be incorporated. These features offer great freedom of design with the process. The versatility of the technique extends to materials, since virtually any alloy can be cast. Castings of over 250kg and an envelope of 1 cubic metre are commonplace.

The IC process is distinguished by the use of an expendable pattern. A metal die is usually used to produce the pattern, now almost universally of wax. These injection dies are normally made of duralumin or brass. Preformed ceramic or water soluble cores may be used to give precision internal cavities and these are located in the wax die prior to injection. Patterns can be mounted onto a runner system to give an assembly ready for subsequent coating with refractory.

Industrial investment casting is based on the ceramic shell process where the wax assembly is dipped into a thin refractory slurry and after draining, fine grains of refractory are deposited onto the damp surface, providing a primary refractory coating. The primary coat typically contains a zircon based refractory while the binders used are either alcohol based (ethyl silicate) or water based (silica sol) or a hybrid of these. When the primary coat has hardened or set, subsequent cycles of ‘wet’ dipping and dry ‘sanding’ build up the thickness of the invested material to provide a refractory shell that, when fully hardened, is sufficiently strong to hold the liquid metal during casting.

At the end of the investing process, the wax pattern material is removed by thermal means, steam autoclaving being usual. The mould is heated to a high temperature to eliminate any residual wax and to induce chemical and physical change in the refractories that will ensure maximum strength and stability combined with minimal reaction between the mould surface and the liquid metal to be poured.
The majority of investment casting foundries have air melting facilities and cast a wide range of materials. Steel casting furnace’s tend to be of the roll over or tilt induction melting type, whilst for many of the more advanced nickel superalloys, vacuum melting / casting is essential. In the case of aluminium investment castings, melting may be by gas or electricity, while various methods of pouring the molten metal are in use (e.g. gravity, vacuum or pressure assisted).
When the mould has cooled sufficiently, the mould material is removed to leave the castings which are then separated from the running system. Various post–casting operations may be carried out to meet customer requirements.

Benefits of the Process

The benefits of the investment casting process may be summed up by the four words accuracy, versatility, integrity and finish. Few if any alternative metal forming methods can offer such a unique and broad spectrum of advantages. Accuracy and versatility stem from the use of a one piece mould without a joint line or the need for draft angles. These features not only give rise to a component shape that is aesthetic and uniform; they also allow the process to give, on a regular basis, consistent and repetitive close tolerances, intricate and re-entrant contours (many impossible to create economically by alternative manufacturing techniques) and competitive cost ratios. Versatility extends to the choice of materials since virtually all alloys can be investment cast.

Utilising the aluminium die form ensures tooling is relatively cheap and is adaptable should design changes be necessary. It also enables relatively small quantities, typically for research and development trials, to be produced prior to commitment to production quantities.

Casting integrity is an important feature of the process and investment casting has a long history of serving the most demanding sectors of industry. This has promoted a tradition of quality and reliability, an aspect that by recent work to develop production methods of guaranteed integrity has resulted in fatigue performances equal to that given by forgings measured longitudinally. Based on this work, investment castings are now beginning to replace forgings and machined components in fatigue related environments.
Other advantages arise from the high degree of dimensional accuracy, + or - 0.13mm per 25mm with improvements up to + or - 0.08mm per 25mm and the excellent surface finish that can be routinely achieved, typically 1.5 to 3.2 microns with improvements up to 0.8 microns.

Typical minimum wall thickness of 1.5mm with thinner sections of 1mm are possible. Tolerances quoted should be taken as a guide, as they may vary depending on the complexity and configuration of the component. Consistency from casting to casting will generally be within the tolerances indicated with individual foundries being able to advise on this point. These characteristics minimise the requirements for machining, in some cases eliminating it entirely, and this leads to substantial savings in raw materials, labour costs and capital expenditure, reduces and simplifies production control and simplifies assembly operations.
These benefits individually offer great competitive advantages; collectively they suggest an overwhelming case for the consideration of investment casting as the most economic method of forming for a wide range of metal components.

Process Characteristics

FREEDOM OF DESIGN HIGH PRODUCTION RATES HIGH DIMENSIONAL ACCURACY HIGH DIMENSIONAL CONSISTENCY HIGH INTEGRITY CASTINGS EXTREMELY GOOD SURFACE FINISH CAN BE OBTAINED

COMPLEX SHAPES CAN BE CAST LONG/SHORT RUNS CAN BE ACCOMMODATED MACHINING CAN BE REDUCED OR ELIMINATED MINIMUM FINISHING OF CASTINGS REQUIRED ALMOST ANY ALLOY CAN BE CAST ENVIRONMENTALLY FRIENDLY PROCESS

Most industries requiring castings are catered for but investment castings are admirably suited for high technology, high volume orders especially in respect of the aeronautical industry. The petroleum, chemical, electronic, defence, prosthetic and automobile industries are also large users of castings produced by the process.

Future Potential & Development

With the development of rapid prototyping techniques for the production of patterns and shells it is now possible to produce investment castings quickly with lead times reduced to less than two weeks.
Work done in BICTA Committees on high integrity castings utilising vacuum produced ingot and melting and HIPping will ensure the process will further replace welded and forged components extending the field of use of investment castings.
Casting size and weight will increase with 1 cubic metre plus envelopes and 500Kg castings becoming commonplace in steel. With the use of improved melting techniques and larger melting and casting facilities investment castings can be shown to be the fastest growing metal forming technique and investment casting now represents some 15% of all metal cast in the UK.

Contributors include BICTA, with special thanks to Mr David Critchley