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
-Die and/ or platen temperature
-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.