A short introduction to investment casting wax
Wax is the oldest thermoplastic material known to man and because it can be cast or formed while in a liquid, semi-liquid or plastic state, its history has been closely linked with the development of arts and craft and the growth of the industry. In early times the craftsmen of China and Egypt used the lost wax process, but the name wax referred only to beeswax. However, today the name, especially in the investment casting industry, is applied to any substance having a wax like property. Modern blends of investment casting wax are complex compounds containing numerous components including.
- Paraffin wax
- Microcrystalline wax
- Hard wax
- Resins
- Polymers
- Filler materials
Paraffin wax is a hydrocarbon based petrochemical or petroleum distillate produced as a by-product in the refinement of crude oil. Structure comprises of short straight chain molecules 20 to 36 carbon atoms in length with relatively organised and ordered properties. Typical melting range is between 32 and 66 °C. Paraffin wax is used to affect the rheological properties, the injection temperature and fluidity of the investment casting wax material.
Microcrystalline wax is also a hydrocarbon based petrochemical produced as a by-product of the distillation of crude oil. Its structure comprises of branched hydrocarbon molecules 31 to 50 carbon atoms in size with disorganised and more random properties. Typical melting range is between 60 and 93 °C. Microcrystalline wax is used to control the flow, the hardness, and the strength of the wax material.
Hard wax materials can be natural esters or modified hydrocarbon compounds. Melting points ranges are from 65 to 120 °C and with typical hardness of less than one tenth of a millimetre. Mechanically they are brittle compounds and exhibit low viscosities. Hard wax is used to influence the hardness and surface finish of the wax material
Resins fit into three main types in the context of investment casting wax, hydrocarbon resins, synthetic resins, and natural resins, each with its own unique advantage and all soften gradually and without sudden expansion. Resins therefore reduce expansion and contraction characteristics associated with the crystalline structure of other materials used to make up the blend. Resins are used to influence solidification, shrinkage, rigidity, and hardness of the wax.
Polymers are added with ethylene-vinyl acetate or EVA the most widely used with melting ranges between 50 and 200 °C and mechanically they usually exhibit high ductility. Polymers can be added to influence many properties including mechanical strength, viscosity, and dimensional stability in the investment casting wax.
Fillers are now used extensively throughout the investment casting industry with types of filler material including cross linked polystyrene or XLPS, terephthalic acid, bisphenol A or BPA and water. The filler material must be inert and not chemically react with any constituents of the wax blend. They should have controlled particle size distribution and specific gravity close to the base wax. Filler materials used must be organic to ensure complete burnout from the mould. Fillers are added to reduce cavitation, assist fluidity, and improve the surface finish of the investment casting wax pattern.
Fundamentally the length and the complexity of the carbon chains of the various components influences the properties of the final wax. Accordingly, many variations are formulated to suit differing foundry requirements and key properties such as congealing point, melting point, hardness, viscosity, expansion, contraction, fluidity and setting rate are all influenced by the structure and composition of the wax compound.
The complex composition of modern wax products manifests itself in a physical behaviour different to that of other substances. Unlike other homogeneous chemical compounds, wax does not melt immediately on heating but passes through several intermediate states. Similarly, the structure and components used in an investment casting wax will influence the expansion and contraction characteristics. Like other materials wax expands on heating and contracts on cooling. In comparison with a metal the expansion is relatively high. Wax expansion and contraction rates are not uniform but vary with phase and structure changes during heating and cooling.
Types of wax
Modern investment casting wax materials can broadly be split into the following types, pattern wax, runner wax, soluble wax, and specialist wax. All are of key importance in the process, but pattern wax will often command the most attention and this type of wax material can be further sub divided into the following categories.
Unfilled pattern wax – These wax materials are complex compounds of many wax and resins products. When using an unfilled wax cavitation can occur on solid sections and chills are sometimes used to overcome this cavitation. Unfilled wax materials have significant advantages in recycling in that once any water is removed, most of the product can be processed for reuse.
Emulsified pattern wax – These have similar base materials to an unfilled wax but are emulsified with water. The surface finish is extremely smooth and because the water acts partially as a filler, cavitation can be reduced. Handling and control of emulsified wax is relatively simple but crucial to maintain the key properties of the material and not lose water. The melting temperature should not exceed 85 °C and the holding temperature must be controlled below 80 °C. Should the temperature exceed these limits, the water will gradually evaporate, and the properties of the wax will change. Use of a non-emulsified wax for the runner system is recommended to avoid spitting of wax particles during the assembly stage.
Filled pattern wax – These types of material are widely used throughout the industry. Here the base materials are like those above, but into the compound is blended a powdered filler material insoluble in the base wax. It is essential the filler used is organic, to ensure complete burnout leaving no ash and there are several different filler materials used. It is also critical to use a fine particle sized filler so that surface finish is not impaired and to have the specific gravity of the filler as near as possible to that of the wax base to ensure that minimum separation takes place when the wax is liquid. The advantage of filled wax is that little or no cavitation should take place even on heavy sections and such materials will give greater stability to large, especially thin walled patterns. Again, process control is of key importance with continuous agitation required once melted to keep the filler in suspension and to avoid filler separation.
Summary
Investment casting wax compounds are complex, they consist of many different components and consequently they exhibit a range of properties. Wax properties influence pattern behaviour in the foundry and ultimately the quality of castings produced. Correct product choice together with strict process and quality control procedures is essential.