The EICF welcomed international delegates to its latest technical workshop for foundry engineers entitled ‘Process Materials for Tomorrow’s Challenges’ held in Bordeaux in September. The event was sponsored by Imerys which also provided a visit by delegates to its Chamotte plant just outside Bordeaux.
In the technical sessions, the different raw materials used in investment casting were considered, beginning with waxes. After a welcome by EICF president Steve Leyland and VP commercial for Imerys, Yan Hoffstetter, Steve Pilbury, managing director of Remet UK, introduced the topic for the session, setting the scene by explaining the complexity of waxes and asking the question what next for wax manufacturers and users?
The complexity of waxes
Waxes, he said, should be considered as thermosetting polymers. Around 12 to 15 components go to make up a wax blend including hard waxes, polymeric waxes, paraffins, resins and fillers. In paraffin and waxes there are wide variations in the products sometimes due to production supply issues, such as a recent fire in Alexandria (Egypt).
The hard waxes control temperature stability, surface and strength. Polymers affect temperature stability, flexibility and homogeneity whilst paraffins influence melting behaviour and viscosity. The most common resins are carbon 5 and carbon 9 but these grades in turn cover a wide range of modifications, including tall oil resins tapped from trees and gum resins. Worldwide, resins are used in road marking paints and in inks so investment casting is a small user.
Fillers contain water, cross linked polystyrene, acrylics etc. and it is necessary to consider ash content and volatiles as well as particle size. Finally, modifiers are used to change the physical properties, or to add colour or change the smell of the waxes.
Jörg Abraham of Romonta Bergweks Holding AG spoke about the properties required in a wax which include hardness and strength, low ash content, temperature stability, defined melting behaviour, and shrinkage characteristics.
Harmonisation of test methods
Typical specifications provided for pattern waxes include congealing point, drop point (for solid to liquid phase transition) viscosity (behaviour in autoclave), needle penetration (strength and hardness at room temperature) and ash content (residues after burning out) but he felt that none of this information is really useful to the foundry.
There are 20 parameters and different test methods for any one parameter, he argued, and this does not really provide performance capability information. The parameters only give hints and no guarantees of suitability for a particular foundry, which will often have its own specific requirements. There is a preoccupation with raw material sources, pricing pressures (cheap!) and availability rather than actual performance in the foundry. Suppliers should inform the customer of any changes but there needs to be a better dialogue between the supplier and the foundry around performance rather than the recipe.
His company offered to support selection and he called for a harmonisation of test methods so that all can “learn the same language”.
This point was reinforced by Grant Bradley, of Remet UK. He spoke about the fact that testing and characterisation focuses only on a reduction in variation, and is usually based on differential scanning calorimetery, DSC. The fast heating rates available in this technique are useful to show what is happening during wax injection and cooling. His company is now using dynamic mechanical analysis, DMA, a dynamic process which enables free expansion to be measured using the application of very low forces, compared with what is possible using volumetric testing process.
He noted that shrinkage is due mainly to volume changes in crystalline components – the paraffins, but that the ‘law of mixtures’ does not apply and additives can have a disproportionately greater effect on shrinkage. He showed how shifts in crystallisation for waxes can be seen with different cooling rates. There is non-linear change in shrinkage for small changes in resin contents, up to 15% resin.
Tests have revealed differences in shrinkage patterns from materials of notionally the same specification. Cloud point, which was developed by the oil industry which needed to look at crystallisation in a pipeline, is being used to study crystallisation in the mould and effects on shrinkage.
There is a need to move from diagnostic to predictive tests to enable a new generation of PIC waxes and he also called for a standardisation in testing methods.
John Machielse, of Paramelt B V discussed surface defects in medical implants (ceramic inclusions) that can have many root causes, and cannot be simply defined as ash inclusions.
He described how to start an analysis - looking at materials, batch, checklists and changes in process. He stressed the need to consider the composition of the ash as this can help to identify the source and the root cause. Ash-related problems are to be expected in dead end sections or thin trials, not in areas that are subjected to high wax flushing. He called for a systematic approach and he said the use of SEM EDX to analyse the chemical composition of the defect can be very helpful in looking beyond the percentage ash content to ash composition and the morphology of wax residue, in the case of wax-related defects.
There was some discussion on best practice with respect to release agents and he recommended not to use silicon-based release agents as these can give rise to ash problems and problems with the adhesion of the shell. During burn out a 10% excess of oxygen is desirable so using gas with an inefficient gas burn is beneficial compared with electricity. In the case of sticky wax (a delamination issue) it is necessary to consider surface tension; this is not usually available from the data sheet but can be obtained from a simple surface tension test.
Phil Hancock, quality manager Blayson Olefines, outlined the benefits of a systematic approach to quality assurance and control and discussed a process based testing approach and a design of experiments technique to identify main parameters. He noted that complex casting geometries require a change in thinking - how does a material flow, how does melting and friction affect flow, what about distortion? Some of the newer techniques include FTIR (fourier transform infrared) spectroscopy can be used to analyse composition and provides a fingerprint trace for different materials, DSC to detect material changes and to look at crystallisation and cloud points and rheometry, which can provide a unique fingerprint for flow, solidification, visco-elastic strength and resistance to deformation and is good for modelling pastes and gels. There are also now non-contact measurements - based on laser measurement - to look at expansion and contraction and avoid, or minimise, hysteresis loops.
Data gathering in itself is not sufficient. Something must be done with the data and then it is necessary respond to results or changes. He predicted that in the future there will be increased modelling of waxes to examine the effect of process conditions on final shape, as has been applied to the metal (using ProCast and Magmasoft) where a CAD model is used to predict the final shape and defects, and apply this approach to waxes. Raw data is needed to improve the models, then simulation can be used to reduce the guess work.
Limitless possibilities
In an enthusiastic presentation, Kevin Smith, UK sales, Voxeljet Technology GmbH, discussed developments in 3D printing materials for investment casting. Additive manufacturing includes printing, sintering, fused deposition modelling (FDM), and direct metal laser sintering, (DMLS).
He spoke about the production of investment casting patterns in poly methyl methacrylate (PMMA). A clean process, using cold printing, so little contraction and low distortion, it can be used to produce printed masters or printed tooling in a highly repeatable, industrial scale process with very low ash contents (<1%). There is minimal pattern expansion, so no cracked shells, and the workspace capacity is 1m x 600mm x 500mm.
He explained that the process is ideal for problem jobs that are difficult, delicate and small, with internal structures, and offers the possibility to “seek out the added value for the customer and prove the concept.” Flexible, the process can be used to join complex cores into one piece and can combine with traditional tooling where appropriate to maximise the opportunities and benefits.
He also discussed direct printing of sand moulds to give high accuracy and fine surface finishes using different grades of sand to produce moulds or complex sand cores of up to 4m x 2m x 1m.
Directional and design freedom
The design flexibility means that runner bars can be any shape to reduce turbulence, gates can be anywhere, methods can be pressurised or unpressurised, runner systems can be joined to existing tooling, hollow sections can be incorporated.
Benefits, he explained, include better casting yield, better energy efficiency and the technology opens up capacity for the more simple jobs, offering time compression. Production tooling is created once and is correct.
It enables the customer to “have a go” with a design change that would normally be too expensive to try and validate. He offered an open invitation to delegates to visit the production facility in the Augsburg, factory.
Remco de Jong, Imerys VP minerals for refractoriness, spoke about his company and how it supplies the industry and sees the foundry sector as an important customer. Presently the company’s overall production of minerals for refractory is 40m tonnes per annum but this is increasing to 47mt pa over the next few years. In terms of foundry minerals, production is 35m tonnes pa with silica being 83% of this. Whilst the tonnage for investment shells is low, the value is high since high value parts are being produced. Europe depends on imported products in refractories, with two-thirds being imported. However this then leads to issues with quality control and political instabilities. Instability in raw material pricing is known and this led, in 2008, to the launch of a raw material initiative in the EU to secure supply and to increase collaboration across the supply chain.
Robert Brown, Remet UK, spoke in general terms about developing a new product and the need for risk reduction measures based on a strategy.
Zircon and the issues of supply, price volatility, and some of the new mines that are being considered viable as a consequence of the price increases, was the subject of Patrick Hegarty’s presentation (Mineral & Chemical Services Ltd).
The subject of Superwool® products was discussed by Craig Freeman of Morgan Advanced Materials, with a particular emphasis on health issues. The aim is to make fibre based products that degrade if ingested to make them safer to handle (to replace refractory ceramic fibres, RCF). He explained the fibre making process and the fact that to be considered safe and declared non-carcinogenic, tests require that half the fibres are removed naturally from the lungs within 40 days. Superwool® products are being developed that have modified silicate structures which breakdown readily in the lungs. This is tested in the lab using a solubility test.
Ladislav Tromek, Lanik SRO, spoke about work that his company is undertaking on ceramic core development. It is difficult to remove long cores from castings so the company has been working through a development project to improve core removal without leaching. The solubility of core must enable it to be removed easily from the casting, but factors that need to be considered include dimensional stability and mechanical strength so there needs to be a balance between shrinkage, strength and solubility.
The core needs protecting in the autoclave due to presence of water vapour so the company has been testing the application of coating to the core to protect it during the process. The coating is applied and is absorbed into the surface layers to provide a water resistant surface with no change in core dimensions. The effectiveness of the coating was demonstrated using a short video in which the untreated core was completely broken down once after some seconds of immersion in water.
Surface defects were observed in initial trials with castings at Foundry MSR Engines SRO, Brno (Czech Republic) but further trials with easy blastable cores (EBC) have shown core removal times reduced from 50 minutes to10 minutes.
Christoph Tontrup, Evonik Industries AG, described a collaborative project with Imerys to develop new alumina binders for high stability investment casting slurries for use for single crystal blades and for directional solidification of superalloys.
High energy milling gives a well-milled agglomerate with a smaller particle size, which contributes to lower viscosity. The use of high purity alumina oxide, with small particle size and low viscosity, produces a binder that is VOC free and with increased shelf life of binder and slurry and a homogeneous shell with superior creep resistance.
Two products are now available including one with a low sodium content suitable for higher temperature casting alloys. Tests have been carried out with no negative effects on shell properties for either the face coat and on full shells.
The latest trends in refractory materials for prime and back-up coats, was the topic discussed by Thomas Krumrei, Imerys CARRD and Danilo Frulli, Imerys Foundry.
The company is the number one global producer of minerals with 250 industrial facilities in the world
and has the Centre for Abrasives and Refractories Research and Development (CARRD) in Austria, which is the R&D centre for Imerys refractory, abrasives and foundry with 30 technical engineers. It also acts as back-up for technical support and is the innovation centre.
In terms of the primary coat, white fused alumina seems the most promising – it is very pure so has high thermal, mechanical and physical properties. The F90 grade is preferred as it is close to zircon sand already used by foundries for steel, stainless and Ni-Co based superalloys.
In terms of the back-up shells, to provide strength and to support face shell, fine stuccos are needed for fine sections and to avoid bridging but coarse stuccos speed up coating and provide greater strength. The shape of particle will affect inter-layer adhesion, coverage of edges, porosity and gas permeability, and filling of cavities and de-dusting is important.
A further consideration is dimensional stability due to the shell moving under its own weight during firing, or due to the additional weight of metal during casting. The new material showed low deformation. If resistance to creep is too high resistance leads to problems of removal after casting.
The importance of plate weight was also noted. How much slurry is on the plate and what is the draining time (how does the slurry coat edges and cavities)? It is not enough to consider only viscosity and the plate weight should also be measured (on a flat surface) to determine the slurry thickness on the wax.
Again the specifics of the foundry and its products need to be considered to tailor the slurry accordingly.
Olivier Joubert, technology director, Imerys Foundry, explained the production process in preparation for the visit by delegates to the Chamotte plant, Imerys Clérac. The process is very energy intensive and includes mining of the alumino-silicate clay, blending and milling, pelletising or extrusion, drying (or pre-calcining to avoid shrinkage during firing) and then calcining to produce the permanent change to Chamotte.
After dinner at the congress centre, the following day delegates were able to visit the Imerys Chamotte plant near to Bordeaux, followed by a wine tasting at the local St Emelion Chateaux.