Review of Investment Casting Wax
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 • Pattern wax • 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.
PROPERTIES OF INVESTMENT CASTING WAX AND THEIR INFLUENCE ON QUALITY
Investment casting wax materials are blend of numerous complex compounds. Each compound has been included to influence the final properties of the wax in some way. A few points that affect the quality of a casting wax and hence pattern production are listed. 1. Contraction and cavitation: Stable results on contraction and cavitations of a casting wax are extremely important to the foundry. We have already discussed how structure and composition affects contraction. This highlights the importance of both the wax manufacturer and foundry’s quality control tests. 2. Congealing point or melting point: Congealing point and melting point are temperatures at the beginning and end of the semi-liquid state respectively. They have a major influence on the injection temperature & pressure settings of the injection machine. 3. Ash content: Most foundries would be aware of the importance of using and maintaining wax with low ash content and of the detrimental effect of ash. The limit generally recommended is 0.05% maximum. 4. Hardness and elasticity: Casting wax must have sufficient hardness and elasticity to help reduce the possibility of rejects due to breakages, bending or other undesirable phenomena during the subsequent processing of the wax pattern. 5. Viscosity: The viscosity of a casting wax compound is critical to successful pattern production. Where large fine sections need to be produced then often a low viscosity wax is required to enable the wax to penetrate into the finest spaces in the die. For heavier sections a less fluid wax may be preferred. Viscosity is generally directly related to injection temperature. 6. Good surface finish: A good surface finish is an important property for successful pattern production. In general, unfilled wax has a glossy surface, emulsified wax has more surface smoothness, whereas filled wax has a slightly rough surface. Surfaces that could prove detrimental, are the ‘soft easily damaged’ surface or the ‘pitted’ surface usually associated with coarse particle sized filler being used. 7. Setting rate: On one extreme, some production parts require a very fast set and release from the die, whereas on the other extreme a slower setting wax is an advantage. 8. Oxidation stability: Oxidation or breakdown of certain compounds in wax due to the action of heat or simply ageing will markedly change the overall properties and the wax may become unsuitable for use. It is necessary for the manufacturer to use antioxidant materials where this could occur and foundries must be aware of this. 9. Reclaimibility: The Reclaimibility of wax are important economic and ecological issues. While stating it is possible to reclaim and reconstitute all three categories of wax, strict quality control over the process is recommended. The above points considered should cover the majority of properties of an investment casting wax and how these can affect quality of wax and wax pattern production.
WAX PATTERN PRODUCTION AND THE MONITORING OF FAULTS
If problems with wax pattern production are being encountered, it is very important to consider with the wax supplier a number of fault guidelines. The most common faults encountered during wax injection are: 1. Flow lines: are usually associated with; a) Cold die b) Cold wax c) Incorrect injection pressure d) Injecting a thick section through a thin section 2. Trapped air: is usually associated with; a) Wax too hot – causing turbulence during injection b) Flow rate too high – the wax flowing into the die faster than the air escaping through the joints, thus becoming trapped. c) Air entrapped in the wax in the machine, causing air bubbles to be injected with wax. d) Air trapped in the patching wax when filling in slots in ceramic cores. 3. Lubricant marks: can be associated with over – lubrication of the die, allowing wax to push lubricant into the folds or creases, giving the appearance of flow lines. 4. Chill breakthrough: is usually associated with; a) Chill too large b) Distorted chill c) Chill too small (floating to one side) d) Pips missing from the chill e) Sinks on the chill in pip location area f) Chill movement due to force of wax, especially if located near the sprue. 5. Incomplete coverage of chill: is associated with; a) Too much lubricant on chill b) Trapped air around the chill (injection rate too fast) c) Insufficient injection pressure 6. Orange peel effect is associated with; a) Die too cold b) Wax too cold c) Insufficient injection pressure 7. Misrun is usually associated with; a) Cold wax b) Cold die c) Injection rate too low d) Wax flow restriction in the die, predominately with thin wall sections 8. Cavitation is usually associated with; a) Die temperature too high b) Wax temperature too high c) Insufficient injection pressure d) Sprue too small e) Sprue in wrong position a. Chill left out of die b. Chill required c. Injecting a thick section through a thin section. This long list only highlights the many variables that exist during wax injection technology and the object is to illustrate how important it is for the foundry to check each area thoroughly.
A LOOK AT POSSIBLE FUTURE TRENDS
In the future the industry is likely to become more sophisticated and therefore wax and its quality control will increase in sophistication also. The wax of the future should be a low price, high quality material that can be reclaimed. In a competitive world it would be good to think wax manufacturers could aim to achieve this ideal. However, the reality of the situation is that with increased emphasis on understanding properties and quality control a compromise must be made on cost depending largely on the nature of the casting to be produced, the process used and the market the foundry is operating in. Let us consider in a little more detail some of the trends that could develop in the future. Quality control of investment casting wax With further future development it would seem essential even greater emphasis is placed on quality control of wax. In previous section the properties of wax and their importance to wax pattern production was discussed. Now we can say it equally important to monitor these properties by both manufacturer and foundry using a strict quality control procedure. When a foundry produces wax patterns it will usually do so against set machine and die parameters for specific patterns. • Wax temperature in injection machine • Nozzle temperature • Die and/ or platen temperature • Injection pressure • Flow control • Injection and hold time etc. If there is a variation in material specification such as congealing point, penetration and viscosity and the foundry has not been informed, then a considerable amount of time can be wasted producing reject patterns before the machine variables are adjusted satisfactorily. A concise quality control system should overcome this and help to reduce any wasted time and cost. Most associations or institutes would issue their own recommended test methods. They are sometimes varied by individual manufacturing companies but as long as the manufacturer and foundry are looking at the same test methods, this is not critical. Melting point (drop point): “The melting point is the temperature at which a drop of the sample detaches itself from the main bulk.” Congealing point: “Congealing point is that temperature at which molten wax, when allowed to cool under prescribed conditions, ceases to flow.”
The results give a variation in temperature but for practical purposes they give a picture of what is happening to the wax. Most important is that for the foundry it gives guide to temperatures required in the injection machine tank and the injection temperature. Ash content: It represents the percentage of non combustible solids contained in the compound and providing the figure is below the required limit, it is accepted by the customer and manufacturer. Penetration: “The distance that a standard needle penetrates vertically into a sample of the material under fixed condition of loading, time and temperature.” Penetration gives the hardness of the wax. If the penetration figure has increased but is still within the limit, then the wax is slightly softer, and it may be necessary to increase the hold time in the die to maintain dimensions. If the penetration has decreased then the converse applies. For the manufacturer the test is again a further check on materials used. Viscosity • Kinematic viscosity is a measure of the time for a fixed volume of liquid to flow through a capillary. The kinematic viscosity is expressed in either stokes (st) or centistokes (cst) or millimeters squared per second (mm2/s). • Dynamic viscosity is numerically the product of kinematic viscosity and the density of the liquid, both at the same temperature. The unit of dynamic viscosity is expressed in either poise (P) or milliPascal-second (mPa.s). For Newtonian fluids, the absolute (dynamic) viscosity is defined as “a quantitative measure of the tendency of a fluid to resist shear.” The results of these tests give the foundry a guide to the flow ability of the wax, the required injection pressures and the size of the injection channel required to maintain pressure applied. Finally, there are a number of other tests sometimes applied to a wax. These include dimensional, volumetric contraction or expansion, linear contraction or expansion, strength, specific gravity etc.
Materials for the future There are from time to time discussions about alternative materials to wax. Polystyrene and expanded Polystyrene are used as pattern materials, urea is used and there is the process of the totally injected shell. As discussed earlier in the review, casting wax blends are complex compounds of many different components. Wax is a loose definition of their form as they are basically chemical compounds and it would be difficult to see other materials totally replacing these, as again they would only be chemicals or blends of chemicals themselves. It would seem more logical to suggest that as the industry moves forward so wax manufacturers will continue to work with foundries and continue to expand on their existing knowledge to produce further wax-type materials to suit specific requirements. Reclaiming and reconstitution Traditionally investment casters have tended to use reclaim wax mainly for runner systems or certain patterns unfilled or emulsified wax. Now with the advance in reclaim technology coupled with strict quality control measures it is possible for a foundry to consider the use of reclaim and reconstituted wax irrespective of whether they use unfilled, emulsified or filled wax.
Such technology offers a foundry the opportunity of considering their autoclaved or used wax being reclaimed and reconstituted within specification of that of virgin wax. When following this route a number of critical points need to be considered 1. It is necessary to ensure there is only one base wax material in the wax system. 2. It is unsatisfactory to mix different pattern wax materials. 3. A separate runner wax should not be used in the system. 4. All wax for reclamation should be processed in-house or at one wax reclaimer’s plant to avoid contamination. 5. It is important to have a general appreciation of wax reclamation and quality control. 6. A foundry must develop controls on the quality of wax it generates for reclamation and reconstitution. For example: a. Waste products must not be mixed with the wax. b. The amount of silicon used should be reduced as far as possible. c. Water mixed with the wax should be reduced as far as possible. d. A filter cloth placed over the autoclave tray can prevent ceramic sand entering the wax during dewaxing. e. The size of autoclaved blocks should be considered to enable easy packing and optimum use of transport. f. The wax blocks should be strapped and wrapped to further reduce the chance of contamination whilst being stored. If such guidelines are adopted and by working closely with the wax reclaimer, a foundry can have large volumes of autoclaved or used wax reclaimed and reconstituted to a specification of that of virgin wax. With economic and environmental considerations always likely to be important, future emphasis on reclaim and reconstitution of wax is likely to become increasingly important also. Economics of wax: With existing wax compounds on the market it is unlikely that major reductions in cost would occur. However, cheaper wax compounds can be formulated and supplied in certain cases. Wax can be designed with specific requirements in mind. Lower cost raw materials can be looked at with the aim of maintaining the major characteristics of wax, but the cost saving needs to balance the cost of testing and changing. We can consider the following two examples where changes in formulae were made to counteract high cost. Firstly, in the 1970s carnauba wax was widely used in wax formulations.
There was an acute shortage and consequently the cost rose astronomically having a great effect on the cost of certain casting wax compounds. Substitute compounds without carnauba wax were manufactured, approved and used by numerous foundries thus overcoming some very large cost increases. When carnauba wax returned to a lower price, so the original compounds could be reduced in the price. Secondly and more recently polystyrene filler, used in numerous filled wax compounds for many years, rose steeply in price due to high increases in price of the feedstock of raw styrene. The various options were to a pay a much higher cost for filled wax using the material, to use a compound with a substitute filler material at a much lower cost or to consider reclaiming and reconstituting used wax. Some foundries opted to change to the lower cost compound or the reconstituted wax. What we are trying to highlight here is if an increase in price reaches a limit the foundry cannot tolerate, then by working with the supplier it is often possible for cheaper alternatives to be offered. However, the overriding factor should be not to detract from the quality of compound needed by the foundry to produce patterns successfully, especially with an increasing emphasis being placed on quality and quality control as the industry moves forward. 6 Conclusion In conclusion it is hoped that the review has provided a suitable summary of the situation regarding investment casting wax to date. Also it is hoped that the review provides some thought on various factors that could influence wax within the investment casting industry in future.