Investment Casting Wax
Good slurry adhesion during application of primary coats is critical to achieving excellent surface finish of precision investment castings. Non-adherence of the primary slurry onto the wax surface due to presence of residual mold release leads to lifting or buckling of the prime coat and subsequent bulging on the surface of castings. The consistency and cleanliness of wax pattern surface requires good pattern wash practices and maintenance of the solvent.
Presented to the ICI 59th Annual Technical Conference on 9th October 2012.
In both the waxes and the slurries employed in investment casting, the control mechanisms involved in the contention between kinetically and thermodynamically driven processes are of joint scientific and practical interest. Particularly during the injection of a wax, the temperature the material experiences and changes in temperature profile during the injection process can have effects on the flow and dimensional characteristics of a wax. An understanding of these mechanisms allows strategies to be developed that lead to improved process control and surface definition of wax patterns. Controlled stress rheometry and dynamic mechanical analysis (DMA) have been employed to study the behaviour of waxes at the onset of congealing under flow conditions in an attempt to measure these effects. Characterization of the wax using controlled stress rheometry reveals phase changes that occur along with linear viscoelastic behaviour. Temperature modulated DMA allows for the investigation of reversible and non-reversible phenomena in the melting and crystallization region of waxes. Temperature modulated DMA also provides high sensitivity for small and slow changes in crystallinity, e.g. during re-crystallization. The combination of controlled stress rheometry and DMA yields new information about local processes at the surface of polymer crystallites.
Wax based blends are an essential part of the investment casting process. They are complex mixtures of natural and synthetic organic compounds and, as such, can present a variety of health hazards if handled incorrectly.
In this paper, the author takes a detailed look at the health and safety aspects of investment casting waxes, focusing on the physical and chemical hazards these materials can present in the wax room and the correct handling and processing techniques that should be used to counter them.
Reference is made to current health and safety legislation, examining how it impacts on both the manufacturers and users of casting waxes
The mechanical properties of casting waxes play a vital part in the investment casting process. Wax patterns must be sufficiently tough to resist breakage during assembly and must not distort if dimensional tolerances are to be maintained. Equally, runner bars must not fracture or sag when assemblies are handled by robot dipping lines.
In this paper, the author describes a straightforward approach for characterising these mechanical properties. The method, based around a three point bending technique, uses inexpensive and readily available equipment to generate numerical and graphical information on wax properties such as toughness and flexibility.
The paper goes on to describe how this information is directly relevant to the efficient running of the wax room and the investment casting process in general. The paper contains many illustrations showing the equipment and techniques used as well as the results generated.
The investment casting of titanium alloys is growing in importance and economic value. The reactive nature of these alloys forces the foundry to be more critical in selecting the key materials – the pattern and runner waxes, and the primary and second coat binders, flours and stuccos – used in the process.
This paper reviews Wax, Binder and Refractory options available to the investment caster, discusses the factors to be considered when making the selection and provides system recommendations that will enable a foundry to begin investment casting of titanium alloys.
It is the intent of this paper to provide the foundry man with a broader understanding of soluble cores from a functionality stand point by reviewing their chemistry and analyzing their physical and chemical behavior in the foundry.
In addition, guidelines will be provided to enhance surface finish, dimensional consistency and dissolution characteristics that in turn could help generate a better pattern wax thus a better final casting.
Remet is evaluating a new method for assessing the rheological properties of its waxes on both a heating and cooling basis with the aim of providing a greater insight into wax performance under injection and dewax conditions. This paper describes the testing that has been done to date and provides interpretative comments regarding wax performance in the foundry.
Precision investment casting waxes are a complex blend of several different carefully selected ingredients. Fillers, additives, resins, microcrystalline and paraffin waxes are used to develop the desired wax to meet the specific requirements of the investment caster. The blend is then analyzed and the results are used to characterize the end product. This paper will look at the various properties that are tested and evaluated. The different properties of a wax will be explained in detail, with an emphasis on how each property influences the investment casting process. In addition, the interaction that occurs at the wax/primary slurry interface will be discussed.
A large proportion of the investment casting waxes in current use contain fillers of one type or another. In helping to control excessive pattern shrinkage and sinkage, these fillers facilitate the production of the large, complex and tightly dimensioned castings required in today’s markets.
In this paper, the author explores the effects these fillers have on the physical properties of the wax blends in which they are incorporated.
Four commonly used fillers are blended, at different levels, into a common base wax. The resulting filled blends are laboratory tested for a variety of physical properties, including viscosity and volumetric expansion. The “real world” performance of the blends is also evaluated by injecting them into a standard test piece and performing dimensional analysis on the resulting patterns.
The work shows that fillers play a vital role in controlling levels volumetric sinkage of injected parts and that they have a major influence on the basic physical properties of filled casting waxes.
Investment Casting Binder
Remasol Adbond SP-3301 and LP-3301 are enhancements of the commercially successful Remasol Adbond BV binders to promote faster setting shell systems. These systems are particularly viable where parts contain deep pockets that are difficult to dry. Because it can be used at low silica concentrations it has potential in aluminum casting.
A significant number of investment casting foundries add organic polymers to their backup slurry systems to make process improvements. These improvements can be in added green strength or permeability, better rheology, and thicker shells. etc. This paper will look at the relationship between polymer selection, use level, and viscosity to previously mentioned properties. The individual foundry can then use these results to optimize the polymer type, level and viscosity for their specific needs.
The increasingly restrictive limits on alcohol emissions has led to the development of modified colloidal silica binders – otherwise known as enhanced binders - as a replacement for traditional ethyl silicate based ceramic shell systems. The first enhanced binder systems were developed for primary applications, which function to reduce surface defects such as buckle, cracking and lifting and to promote adhesion to the wax pattern. In order to achieve these goals a primary binder has different requirements to that of a backup binder, as it should have longer term stability due to its slower turnover rate and provide excellent coating characteristics.
Various types of materials such as latexes, polyvinyl alcohols, methyl cellulose, acrylic resins, etc., have been evaluated in colloidal silica to develop enhanced primary and backup binders. This has resulted in the creation of a new generation of products, such as Remasol Adbond BV and Ludox SK that have opened the door for even more research into these binder types. The enhanced primary binders have been shown to reduce typical primary type drying defects, while the backup binders have provided faster processing and higher strengths versus standard colloidal silicas.
Since its market introduction in late 1992, Remasol Adbond BV binder has successfully replaced both ethyl silicate and other silica binders in foundries where fast processing times are required. Because of its good green strength, a reduction in autoclave cracking can also be realized.
Due to the variety of use conditions, additional work has been done with Remasol Adbond BV binder to determine the effect of dilution with respect to shell properties and improved cost performance. This paper updates that work.
Colloidal silica is the most popular binder used in the precision casting industry today. It is safe, economical, easy to use and performs well. All of these desirable attributes tend to lull us into a sense of well being when we use colloidal silica. We frequently forget that this work horse of the industry is sensitive and needs some attention to keep it working at its best. This paper describes colloidal silica, its properties and those things that will affect its performance.
The paper discusses the chemical properties and bonding characteristics of new commercially available binders of aqueous colloidal alumina, monohydrate and zirconium oxide. Potential applications are indicated, together with some physical property data derived from bonded refractory systems.
Investment Casting Refractory
Zircon has historically been seen as an important material in the contemporary Precision Investment Casting Industry. In this paper the authors discuss the prevailing market situation for Zircon on a world-wide basis. They go on to discuss the technical nature of the material and the resulting benefits of its usage, and suggest possible substitute materials. Materials such as alumino silicates, ferruginous felspars, corundums and forsterite-rich olivines are currently well known in the general foundry industry, and can, under certain circumstances, be used in the zircon’s stead. The authors consider the challenges presented by these materials, and methods by which these may be resolved.
During the past several years, the zircon market has experienced volatile periods of supply and demand with correlated pricing activity. These volatile periods were associated with a supply and demand imbalance that caused price spikes and material allocations, making it difficult for foundries to accurately plan production and price castings. Since mid-2010, we find ourselves in another volatile period that has caused foundries to re-evaluate their processes with a goal of reducing or eliminating zircon usage. By reducing dependence on zircon, foundries minimize or eliminate their exposure to this volatile market. This paper will discuss testing performed on different refractory material and binder systems to identify viable and economical zircon sand and flour substitutes.
A new material, which has the potential to replace zircon sand for secondary prime dips, and possibly the first prime for some casters, has been developed. Remasil 60 RG100, based on a 60% alumina, alumino-silicate material, is a high mullite product which has been fired to a high temperature. It is then sized to provide the optimum match to zircon sand (the distribution is slightly coarser than zircon.) Laboratory testing (MOR, permeability and thermal expansion) was performed to determine how closely the RG100 sand performed versus zircon. The test results revealed some small differences between the two sands. The method of application (fluid bed or rainfall sander) resulted in only minor differences in the permeability or MOR results.
Ceramic Shell Cracking
The drying of recessed areas can contribute to significant time delays in the processing of shell molds, as well as leading to rework and scrap issues. This paper will present data from various testing to evaluate the effect of different binders (polymer and non-polymer) to try to overcome this all too common problem.
This paper will discuss in detail the various properties that can be quantified for a ceramic slurry and how they influence the ceramic shell. The raw materials (colloidal silica, polymer, refractory, wetting agent, antifoam) used to make a slurry play a major role in determining the overall final ceramic shell characteristics. There are many factors like silica type and level, polymer type and level, plate weight, solids loading, viscosity, flour particle size distribution, refractory type, etc that play a role on the final ceramic shell properties. These factors will affect the ceramic shell with respect to strength (green, hot, fired), permeability, creep, thermal conductivity, and thermal expansion.
Shell cracking is the product of stresses applied during the shell building process during this paper factors that contribute to the stresses are identified and discussed.
There also practical methods described which produce maximum green strength of shells and these include; (1) the process control of slurries and stucco and (2) Mechanics of shell building and drying.
We also include twelve guidelines to ensure optimum shell strength and therefore resist the stresses that cause cracking.
This paper forms a discussion focused on shell formulation, control and process to prevent stress on the ceramic shell, which can cause shell cracking.
Reclaimation of Investment Casting Waxes
Today, the reclamation of spent investment casting wax forms an integral part of most casting wax supplier’s business activities. The economic and environmental pressures existing in the current business climate have created an upswing in the volumes of wax being reprocessed and subsequently reused by investment casting foundries.
In this paper, the author considers the various wax reclamation techniques in use today. By focusing on the methods used by Remet UK Ltd., he gives a detailed description of how wax returned from the foundry can be dried, cleaned and processed to yield a variety of cost effective, reclaim based products. In many cases these products have performance characteristics that rival those of virgin waxes.
The paper considers the technical quality of these reclaim based products and also reflects on the steps a foundry can take to help the wax reclaimer make the best of the wax returned for reprocessing.
Remet - "A better way"
Remet design and supply full consumable solutions to the investment casting industry. Specialists in the advanced materials used in the modern foundry, our technical services provide our customers innovation, technical support and quality assurance.
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