Getting the pattern out of the shell – a VIAM review of dewaxing methods

This VIAM review covers a single investment-casting operation – removing the pattern material from the shell. The authors say it largely determines casting quality. A crack that forms in the shell during dewaxing opens up during firing, metal runs into it at pouring – and the result is either a defective casting or a failed mould. The review is also of interest because it is written by people who make pattern waxes themselves, and it summarises every dewaxing method with its pros and cons.

A little history

The authors start from afar. Gas turbine engine building in the country has developed since the 1930s and is linked with academician Arkhip Lyulka. At first turbine blades were forged from bar and then machined at length, turning metal into chips. When alloying with refractory metals raised high-temperature strength to the point where the alloys could no longer be hot-worked, another technology was needed.

Cast turbine blades came from the work of academician Sergei Kishkin: in the 1950s, at the All-Union Institute of Aviation Materials, he founded the casting technology for hot-section superalloy parts. Casting gave a blade of the required profile with a complex internal cavity, with the required accuracy and finish and almost no machining. The first chief designer to use a cast superalloy in an engine was Nikolai Kuznetsov.

Since the gas temperature ahead of the turbine has long exceeded the melting point of the alloys themselves, cooled blades with alumina ceramic cores, removed with hydrofluoric acid, and thermal barrier coatings followed. VIAM went from equiaxed casting to directional solidification and single crystals, built high-gradient furnaces with automatic control, and developed mould and core compositions.

Fig. 1. Cast gas turbine engine blades. Fig. 1 from Forostovich et al., Trudy VIAM, 2026, CC BY

How the process works

Investment casting – "precision" casting – remains the main technology for turbine blades. Patterns of the part and the gating are made from pattern material, most often wax-based, in dies; a ceramic core for the cooling cavity is set inside, and patterns are assembled into clusters. The cluster is coated layer by layer with a slurry of binder and refractory powders, stuccoed with grit of different sizes, and each coat is dried. Then the pattern is removed and the shell fired. The fired mould withstands the temperature of molten superalloy. After vacuum pouring the castings are knocked out, gates and risers are cut off, and the cores are removed, usually by leaching.

Since the mid-2010s, for import substitution, VIAM has produced its own pattern waxes of two types – "VIAM" and "Salyut" – from domestic raw materials, products of organic synthesis.

Why the shell cracks

The requirements for dewaxing are simple: the wax must come out completely, and the shell must keep its strength, surface quality and integrity. It also matters that the wax can be reused without costly regeneration.

The main hazard is tensile stress in the shell wall. Its cause is the difference in thermal expansion coefficients: when heated, the wax expands faster than the ceramic. Hence the first and main requirement for good dewaxing: get heat into the surface layer of the pattern as fast as possible by raising the temperature of the heating medium. A steep temperature gradient forms between the pattern surface and its core: the surface melts and drains cleanly while the inner layers have no time to warm up. The gap that opens between shell and pattern takes up the expansion of the bulk.

With slow heating the whole pattern warms through and presses on the shell from inside. The cracks this causes are not visible at once: they open up during firing.

Reference points from the review

about 95 °Cwater in hot-water dewaxing, 15–20 minutes

80 °Cabove this wax melting point hot water does not work

about 200 °Cworking temperature of polyglycols

90 + 10%urea and potassium nitrate in the MON-10K salt pattern

Eight methods

Hot water, about 95 °C. The wax drop point is below the water temperature, so the wax floats up in the tank. 15–20 minutes, medium cracking risk. Water can react with wax components and degrade both the shell and the reclaimed wax; the shell must be dried afterwards. Not suitable for waxes melting above 80 °C or with some binders. About 0.1% surfactant is added to prevent saponification and tank damage.
Hot air. Suits waxes melting above 80 °C and does not wet the shell, but it is slow and energy-hungry: air heats more slowly than other media, and the risk of cracking and mould failure is higher.
Superheated wax. Impregnates the shell pores, raising green strength, and burns out during firing. The drawbacks are working conditions and the quality of reclaimed wax.
Polyglycols. Around 200 °C. They dissolve in some waxes, change their properties, impregnate the shell and are consumed.
High-boiling organic liquids. Few cracks, but toxic vapours, fire and explosion hazards, protective equipment for staff, and longer firing to burn out residues in the pores.
Microwave heating. Polarisation and conduction currents heat the whole volume of the dielectric – pattern and shell – at once. A good shell surface, a sharp drop in scrap, nearly full wax recovery and no drying. VIAM tested a batch laboratory unit with two magnetrons in the early 2000s and obtained high-quality moulds with full wax recovery. The drawbacks are equipment cost and protecting staff from radiation leakage.
Superheated steam in a boilerclave. Steam's high heat capacity melts the pattern out quickly, scrap is low, and the method is practical and productive. But the reclaimed wax takes up steam and is harder to regenerate, and a boilerclave is a hazardous pressure installation requiring trained and certified staff.
Dissolving salt patterns in water. The MON-10K composition (90% urea and 10% potassium nitrate) is dissolved in water at 20–25 °C and the shell rinsed with running water. Cracks are almost absent, shrinkage is low, and the material is cheap and readily available. But the binder and shell must be water-insoluble, the patterns are brittle and hygroscopic, and clusters must be protected by a film inert to the slurry. Dissolution is sped up by channels inside the patterns and by additives.

Burning out costly heat-resistant expanded-polystyrene patterns is also mentioned – done together with firing, with decomposition products released and no material recovery. Every method requires supply-and-exhaust ventilation with emission cleaning.

Fig. 2. A hot-water dewaxing unit: 1 – body, 2 – electric heater, 3 – wax collector, 4 – pattern clusters, 5 – basket. Fig. 3 from Forostovich et al., Trudy VIAM, 2026, CC BY

What is being researched

The goals are fewer cracks through faster removal, less residual wax in the mould, lower losses and regeneration costs, and recommendations for series production. Among current Russian research lines the authors list:

the regularities of the process, the conditions for stress and cracking, and the stress state of the shell during dewaxing;
control of water hardness in the tank – calcium salts can lead to non-metallic inclusions in the casting;
pH control with hydrochloric acid so that high alkalinity does not raise the ash content of repeatedly used wax;
choice of surfactant against stearin saponification and tank corrosion;
practical techniques to reduce cracking during dewaxing;
reinforcing, insulating and pore-forming additions to the shell, and the effect of the ceramic's thermal properties on dewaxing time;
methods for calculating full melt-out time from the heating medium and the wax melting point;
trials of microwave dewaxing.

The authors' conclusion and how to use the review

Sound blades with stable dimensions and a clean airfoil surface require high-quality moulds. That calls for pattern waxes with minimal shrinkage and thermal expansion, a well-designed gating system and an effective dewaxing cycle. Reliable methods that remove the wax quickly and completely and allow it to be reused, in the authors' view, open the way to lower scrap.

This is a review with no new experimental data: its figures are for reference, and the cycle for a given wax–shell pair is still set on site. It is useful as a summary of arguments when choosing or changing a dewaxing method and as a list of Russian literature on the subject – the paper cites 30 sources.