The objective of this review is to have a look at how investment casting wax has developed, with a summary of structure, categories of casting wax available, properties and wax pattern production. The review then moves on to look at the possible direction wax may follow in the future considering quality control, choice of a wax, future materials, reclaim and reconstitution and cost.
STRUCTURE OF INVESTMENT CASTING WAX
Modern blends of investment casting wax are complex compounds containing numerous components such as natural hydrocarbon wax, natural ester wax, synthetic wax, natural and synthetic resins, organic filler materials and water. Many variations of such compounds have been formulated to suit various requirements; properties such as melting point, hardness, viscosity, expansion and contraction, setting rate etc are of course all influenced by the structure and composition of any wax compound. Hydrocarbon wax, natural ester wax, synthetic waxes and some resins are aliphatic compounds (straight chained carbon atoms). However, some of the resins and filler materials are of ring structured carbon atoms (aromatic compounds). The short chain wax compounds have lower melting point and low hardness. With increasing chain length both hardness and melting or congealing point rise. The chain length will also influence viscosity and solubility of the wax. The casting wax is a mixture of a large number of compounds of different chain lengths resulting in physical properties different from other substances. Wax does not melt immediately on heating like other homogeneous chemical compounds, but passes through an intermediate state. With gradual heating, solid wax first becomes softer, then plastic and then semi plastic. At higher temperature it acquires the consistency of a semi liquid and finally to a Newtonian liquid. It should be noted here that filled wax is not a true Newtonian liquid. This change in state occurs as short chain fractions melt first while longer chains remain solid. With further increase in temperature the latter melt progressively until the liquid state is reached. Structure or components of casting wax will also affect expansion and contraction. Wax expands like other materials under the influence of heat and on cooling it contracts. In comparison with a metal the expansion of a wax is relatively high. In this brief look at structure we have a simplified view of how or why numerous components are added to a wax blend and the properties that result. We can now consider the types of investment casting available and how these are categorized.
CATERGORISATION OF INVESTMENT CASTING WAX
Investment casting wax is broadly classified as shown
-Reclaim or reconstituted wax
-Water soluble wax
-Other special wax – including dipping, patching and adhesive.
Pattern wax can be further divided into the following three main areas:
-Straight or unfilled pattern wax
-Emulsified pattern wax
-Filled pattern wax Unfilled pattern wax is a complex compound of many waxes and resin components.
The surface finish is glossy and the wax can be reclaimed and reconstituted for use. Emulsified pattern wax is similar to unfilled wax compounds, but is emulsified with 7-12% water. The surface finish is smooth and the water acts partially as a filler. This wax can be reclaimed and reconstituted for use. Filled pattern wax again is similar to unfilled wax compounds, but is blended with a powdered, inert filler material, insoluble in the base wax, to give the compound greater stability and less cavitation. It is essential that the filler used is organic to ensure complete burnout leaving no ash and there are a number of different filler materials used. It is also critical to use 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 the base wax to ensure minimum separation takes place when the wax is liquid. Here again filled wax is widely used and with advance reclaim technology can usually be reclaimed and reconstituted for use. Reclaim or reconstituted wax is a service carried out by the wax manufacturer, whereby a foundry’s used wax can be thoroughly cleaned and blended or reconstituted to an agreed specification. The material is then returned for use on runner systems or patterns again. Unfilled wax, emulsified wax and filled wax can all be reclaimed and reconstituted in this way. Water soluble wax is designed to produce internal shapes which are difficult to produce by other means. The wax is soluble in water or a mildly acidic solution. Other special wax grades are unfilled wax compounds used in dipping, patching or repair and adhesive applications. We now move on to look at how the general properties of investment casting wax influence quality.