Rapid Shell Build For Investment Casting: Revolutionizing an Ancient Process. IRC in Materials Processing, University of Birmingham, UK.

Transcription

1 Rapid Shell Build For Investment Casting: Revolutionizing an Ancient Process S Jones, C Yuan and S Blackburn IRC in Materials Processing, University of Birmingham, UK. Abstract Investment casting is a time-consuming, labour intensive process, which can produce complex, high value-added components for a variety of specialised industries. Drying and strength-development of each coat during shell mould production is the most significant rate-limiting factor in the reduction of lead times and production costs for industry. As such, improvements which reduce cost and cycle times, open up new opportunities for product development. A new shell build technique using a super absorbent polymer additive has been developed for mould production, such that moisture removal is not required to cause binder gellation for each individual mould coating. Therefore the drying time before a subsequent coat can be applied is dramatically reduced. All coats can be applied using the same novel additive thus leading to a large cumulative saving in overall mould processing time. This technology may allow the industry to successfully compete for the first time with other high volume forming methods as the traditional barrier of long processing times will be reduced. This paper introduces the technique, details shell development and results and follows the process through various iterations to the final proven concept. Key words Investment, superabsorbent, gellation, colloidal, binder 154/1

2 Introduction Investment casting is a time-consuming, labour intensive process, which can produce complex, high value-added components for a variety of specialised industries. The principles can be traced back to 5000BC [1] when Early Man employed the method to produce rudimentary tools. This was followed by centuries of use for jewellery and artistic components [2] before the military requirements of the 2 nd World War saw the process developed for aerospace and subsequently engineering components [3]. The term investment casting derives from the characteristic use of mobile ceramic slurry, or 'investments', to form a mould with an extremely smooth surface [4], replicated from precise component patterns and transmitted in turn to the final metal casting itself. The process allows dimensionally accurate components to be produced and is a cheaper alternative than forging or machining since waste material is kept to a minimum [5]. The technique itself has tremendous advantages in the production of quality components and key benefits of accuracy, versatility and casting structural integrity. As a result, the process is one of the most economic methods of forming a wide range of metal components. Environmental [6] and economic [7] pressures have, however, resulted in a need for the industry to improve current casting quality, reduce manufacturing costs and explore new markets for the process. Optimisation of the mechanical and physical properties of the ceramic shell, whilst reducing process/material costs, will be instrumental to achieving these aims. Following the introduction of the Environmental Protection Act in 1992, the degree of waste gases allowed from industrial processes was drastically reduced and resulted in the increased use of water-based (colloidal) binders within the UK industry to reduce volatile organic compound (VOC) emissions. Unlike previous alcohol based binders, water-based colloids are not chemically set and require sufficient moisture removal between coat applications to achieve gellation. Drying conditions, such as temperature, humidity and air speed need to be carefully controlled to avoid shell cracking and as a consequence the inter-coat dry times vary between 2 and 24 hours dependant upon which layer is being applied. Increased automation, worldwide foundry competition, rapidly increasing energy costs and the need to recycle constituent materials, has placed increasing pressures upon the industry. The incentive to reduce inter-coat dry times is tremendous and a variety of new processes aimed at reducing production times have been suggested and developed. However, the need to remove sufficient moisture to gel the colloidal binder and develop sufficient ceramic strength for re-dip has remained a fundamental time consuming and rate limiting feature. Here work carried out to explore the possibility of rapid gellation of the binder by alternative means, which at the same time produces shells which have sufficient strength to be redipped almost immediately removing the need for costly and time consuming moisture removal at this stage of the process is reported. The aim was wax to de-wax in less than 60 minutes, significantly reducing production costs by an estimated 75%. 154/2

3 As the basic technique itself is inherently expensive, improvements that reduce cycle times open up new product opportunities for foundries through a reduction in processing costs, lead times and an increased ability to be involved in rapid component prototyping. A new technique using super absorbent polymer additives [8] has been developed for mould production such that conventional moisture removal is not required to cause binder gellation and the drying stage is dramatically reduced. All investment coats can be carried out using the same novel additive thus leading to a large cumulative saving in processing time. Conceptual Background Gelcasting [9] was developed as a shaping method for advanced ceramics to overcome long production times associated with more traditional methods. A concentrated slurry of ceramic powder in a solution of organic monomer is poured into a mould and polymerised in situ to form a green body in the shape of the mould cavity. Extending this concept of gel solidification to include colloidal sol binders is achieved by utilising the effects of polyacrylamide and related absorbent polymers [10] upon water containing bodies. Colloidal silica can be rapidly flocculated [11] using polyacrylamide, with the added advantage that the resultant gel absorbs moisture and locks it within the structure whilst maintaining a degree of rigidity. Consequently, if the water absorbing properties of these materials could be used produce a ceramic body with sufficient strength to be autoclaved immediately then an advantageous process would be forthcoming. Moisture could then be removed during the standard firing schedule of the moulds. Superabsorbant polymers are essentially synthetic hydrophilic non-toxic cross linked polymers that can absorb up to 800 times their own weight in water, but cannot dissolve because of their three-dimensional polymeric network structure. All superabsorbers are based upon the acrylate monomer [12] (Figure 1), which is an ethyl chain with a carboxylic acid side chain. The polymerisation of the acrylate monomers produces a linear molecule that has a very high molecular weight usually greater than one million monomer units, with high quantities of carboxylic side chains forming the polymeric network and giving the superabsorbant properties. In the dry powdered state the chains of the polymer are coiled and lined with carboxyl groups or (-COOH). When hydrated with water, the carboxyl groups dissociate into negatively charged carboxylate ions (COO -1 ) which form bonds with the hydrogen atoms in water, locking the water and preventing the polymer being taken into solution. The carboxylate ions repel one another along the polymer chain, thereby widening the polymer coils, allowing more water to move into contact with more of the carboxyl groups. As the polymer continues to uncoil the swelling forms a suspension with a gel-like consistency. The different polymers available are modifications of polyacrylate achieved by changing of the chain length or by modifications of the carboxylic side 154/3

4 group. Two common examples are sodium polyacrylate and polyacrylamide. The difference between sodium polyacrylate (Figure 2) and the polyacrylamide polymer (Figure 3) is the ion used to replace the hydrogen bonded onto the oxygen of the carboxylic acid, sodium polyacrylate has a sodium ion (Na + ) while the polyacrylamide has an amide (NH 2 ). When these polymers are hydrated with water, the amide group /sodium ion dissociates to form positive ions which form bonds with the OH - ions in the water, locking the moisture from within the water and preventing the polymer being taken into solution. Again the dissociated ions repel one another along the polymer chain, thereby widening the polymer coils. The sodium ion works more efficiently due to the greater ionic charge and allows the polymer to absorb approximately 800 times its weight in distilled water. The amide ion is considerably less efficient and such polymers will only absorb around times their weight in water. This lack of ionic strength in the polyacrylamide polymer greatly reduces its water absorbing characteristics. In the Rapid Shell Build technique [13], a superabsorbant polymer is incorporated into the stucco material before application. This material then rapidly uptakes moisture from the previous slurry coat upon contact to immediately gel the colloid, thus eliminating the time needed for controlled drying between each individual coating to achieve the same effect. Instead of removal, in essence the binder/slurry moisture is retained within the stucco until 300 C [14,15] and above when the polymer will release the bound water during the mould firing cycle. The use of these polymers for this technique is novel and innovative, even though the materials themselves were developed for widespread use in disposable nappies in the early 1980 s [16]. Currently, the major uses for superabsorbant polymers are in medical science, production of soft contact lenses, disposable nappies, moisture retentive materials in agriculture and as filtration media for paper manufacture [17]. This represents a significant redevelopment of the basic principles of a casting technique that has remained largely unmodified for centuries. This paper introduces the technique, details shell development and follows the process through various iterations to the complete patented concept. Experimental Shell Development Colloidal silica can be rapidly flocculated using polyacrylamide, with the added advantage that the resultant gel absorbs moisture and locks it within the structure whilst maintaining a degree of rigidity. As such, the water absorbing properties can be tailored to rapidly gel individual layers of ceramic moulds. Any retained moisture can then be removed during controlled firing after complete mould production giving much reduced process times overall. To test this hypothesis, standard aluminium ceramic slurries were prepared and ceramic mould samples built by repeated dipping and stucco coating according to standard processing conditions. This gave a total preparation time of 2750 minutes (46 hours). At the same time a 10wt% polyacrylamide addition, in the form of a coarse particulate 154/4

5 material (0.3 to 1 mm), was made to the secondary stucco material (30/80 mesh alumino-silicate refractory) and comparable samples were prepared. The primary coat was produced conventionally (1240 minutes), however, to prevent excessive shell cracking and reduction of overall strength. Here the total preparation time was 1280 minutes (21 hours) and shells with reasonable structural integrity were formed (Figure 4), but there appeared to be high levels of re-absorption of moisture both during manufacture and de-wax. De-lamination and particle swelling occurred in this initial system, which led to a loss in strength and failure of the primary coat during dewax and firing (Figure 5). Firing also appeared to induce extensive breakdown of the ceramic structure, possibly due to pyrolysis products emanating from the polymer particles weakening the ceramic through void formation. The green shell was absorbing large quantities of excess water from the slurry, leading to unacceptable swelling and structural damage during shell build. In order to reduce this an alternative polymer was sought. Polyacrylamide was originally developed to grow plants in arid environments and has been refined to last longer and absorb water at a high rate than other classes of absorbant. Polyacrylate is a closely related member of the same polymer group but is capable of absorbing greater amounts of liquid and yet breaks down easily on heating. The greater absorption allows the particle size of the additive to be reduced significantly, whilst at the same time allowing a more controllable polymer removal stage. Further, because of the superior absorbency less material was required (2.5% by weight) to achieve the desired result. Polyacrylate addition modified ceramic mould samples were prepared. The polyacrylate had a average particle size of <300 mm and the primary coat was also polymer modified to further reduce shell preparation time by removing the need for the slow, controlled drying of this individual coat. This gave a total preparation time of 22.5 minutes (Figure 6). Though the shell build time was rapid, delamination of the primary coat still occurred during the shell firing schedule, which would be unacceptable for an investment mould. The de-lamination and stripping (removal of previous gelled coats) during shell manufacture and de-waxing was considered to be due to the volume expansion of the individual polymer particles as water is absorbed and the particles swell. The stripping effect may also have been due to the polymer being introduced as a discrete particle, preventing all the moisture from the slurry layer being removed from the colloidal binder before the next coat was dipped. There is a limit to the rate of moisture transport through a capillary network and the use of a small number of large polymer particles would have increased the average depth through which water molecules would have to travel before being locked into the polymer structure. As the next layer is dipped, there would be an excess of unlocked moisture within the colloidal network, preventing binder gellation and promoting breakdown of the already gellated structure. The delamination and cracking of the shell structure during firing was possibly 154/5

6 due to a thermal mis-match between ceramic/colloid/polymer addition or the promotion of excessive expansion due to volatilisation of the polymer particles. Discrete particle additions would have a high concentration of polymer in one particular location leaving holes as this is removed, leading to weakening of the ceramic structure. Coating the individual stucco particles with the polymeric material, rather than having discrete particles within the shell structure can achieve a more even distribution of the polymer. Such a distribution would reduce expansion cracking and thermal mis-match during production and firing. This would also distribute a larger percentage of polymer throughout the entire coating structure, reducing the average transport distance for water molecules, preventing re-wetting and stripping of previous coats during rapid dipping. A mixture of polymer, stucco and deionised water was produced in a high shear mixer and force dried to remove excess moisture. The acrylate content (0.25 wt%) was an order of magnitude less than that used in previous trials where discrete particles were added. The coating was accomplished by using the polymers ability to form a liquid gel by hydration and dehydration. Once in gel form the stucco is added and the resultant mixture agitated. Dehydration of the gel produces a solid mix of stucco in polymer. This material is re-graded into the original particle distribution (by light grinding) to produce a stucco material, which is now partially covered by superabsorbant material (Figure 7). Ceramic mould samples were dipped in 40 minutes with another 1080 minutes (18 hours) of a final dry before de-wax to stabilise the shell system. With the much reduced polymer content in the stucco phase, the modified samples did not crack at all during de-wax (Figure 8). The entire shell, both primary and secondary layers were intact as a result of the new polymer addition process. After firing the shell structure (including the primary coat) was intact, producing a structurally sound mould for investment casting. Conclusions Drying and strength-development of each coat in the manufacture of investment shell moulds is one of the most significant rate-limiting factors in the reduction of lead times and production costs. As such, improvements which reduce cost and cycle times, open up opportunities for product development, cost savings and the environmentally sound practice of reduced energy consumption. An alternative method of individual slurry coat colloidal binder gelation, using a super absorbent polymer additive to rapidly remove binder moisture has been developed for investment mould production, such that time consuming removal by individual coat drying in controlled atmospheres is no longer required. The system has been proven as an industrial alternative, requiring little capital cost or equipment replacement, as current systems can easily be adapted. There is potential for decreased labour requirements and material costs and the current lead times from wax/casting can be greatly decreased allowing current components to be produced faster. Improvements that reduce cycle times open up new product opportunities for foundries 154/6

7 through a reduction in processing costs, reduced energy consumption and an increased ability to be involved in rapid component prototyping. This represents a significant redevelopment to the basic principles of a casting technique that has remained largely unmodified for centuries. References 1. Taylor P R, An Illustrated History of Lost Wax Casting, Proc. 17th Annual B. I. C. T. A. Conference, September, Kotzin E L, Metalcasting and Molding Processes, American Foundrymens Society Inc., Illinois, USA. 3. BARNETT S O, Investment Casting - The Multi Process Technology, Foundry Trade Journal International, 11 (3), 1988, pp Beeley P R and Smart R F, Investment Casting, Institute of Materials, 1 st Edition, 1995, ISBN Jones S and Marquis P M, The Role of Silica Binders in Investment Casting, British Ceramic Transactions, 94, No 2, Environmental Protection Act, Crown Copyright Rosskill Information Services, 2004, 8. Jones, S., Patent Filing: Improved Investment Casting Process, O published 19 th February Omatete O O, Janney M A and Nunn S D, Gelcasting: From Laboratory Development Toward Industrial Production, J. Europ. Ceram. Soc., 17 (1997), pp Dalian Guanghui Chemical Company Ltd, Literature, ghhx.com.cn/e_noname2.htm 11. Baltar C A M and Oliveira J F, Flocculation of colloidal silica with polyacrylamide and the effect of dodecylamine and aluminium chloride pre-conditioning, Minerals Engineering, 11, no. 5, 1988, pp Mukerjee M, 'Superabsorbers' article, Jones, S, Patent Application: GB , filed 5 th February Klug, F, Method for removing volatile components from a gel-cast ceramic article, US Patent US2002/ A1 August 15 th J Boisvert J, Persello J, Castaing J and Cabane B, Dispersion of alumina-coated TiO 2 particles by adsorption of sodiumpolyacrylate Colloids and Surfaces A: Physicochemical and Engineering Aspects,178, (1-3), March 2001, pp Butterworth G A M and Elias R T, Disposable Absorbent Pad, US Patent 3,967,623 July Macro Galleria. The University of Southern Mississippi. Acknowledgements The authors wish to gratefully acknowledge EPSRC (GR/R81480/01 ROPA Realising Our Potential Awards) for funding this project. 154/7

8 Figures [ C H 2 C H ] n C = O O H Figure 1: The simplest acrylate Poly (acrylic) acid --CH 2 --CH(CO 2 Na) Figure 2: The monomer for sodium polyacrylate H H H C=C C=O N H H [CH 2 CH] n C=O O NH 2 Figure 3: The monomer for sodium polyacrylate (a) (b) Figure 4: Comparison of initial aluminium shell build (a) Standard ceramic shell (b) Initial development shell (10 wt% acrylamide particles) 154/8

9 Figure 5: Initial development shell (10 wt% acrylamide particles) showing extensive delamination after de-wax (a) (b) Figure 6: Comparison of initial aluminium shell build (a) 2.5wt% shell made in 22.5 minutes and de-waxed immediately (b) 2.5wt% shell made in 22.5 minutes and de-waxed after 12 hours 154/9

10 Figure 7: Optical image of partially coated stucco particle (a) (b) Figure 8: (a) green 0.25% modified shell (b) fired 0.25% modified shell 154/10