Developments in Vacuum Furnace Design for Investment Casting for

(Equiax / Directional Solidification / Single Crystal)

Noel Guilliard

Consarc Engineering Ltd

Japan Foundry Society, Inc.

13th WORLD CONFERENCE ON INVESTMENT CASTING

Paper: T18

Copyright reserved: Neither the Japan Foundry Society, Inc. nor its officers accept legal responsibility for information, advice given or opinions expressed.

DEVELOPMENTS IN VACUUM FURNACE DESIGN FOR INVESTMENT CASTING (EQUIAX / DIRECTIONAL

SOLIDIFICATION / SINGLE CRYSTAL)

Noel Guilliard, MEng

Consarc Engineering Ltd

Abstract

Modern vacuum furnaces for investment casting come in a variety of configurations dependant on the application. This paper provides a review of modern vacuum furnace configurations, the options open to foundries when selecting appropriate equipment and discusses the advantages and applicability of each. Further, there are a number of developments in furnace technology in recent years which include a number of design features to:

Aid process control Provide excellent repeatability of casting Highest possible quality of casting Highest productivity rates This paper will also describe some of these features and developments included in the

latest generation of vacuum furnaces, detailing the significant and distinct advantages that they can offer. 1. Introduction

The use of vacuum technology in precision casting provides a number of key benefits in terms of quality and consistency in the castings produced. The primary factor is eliminating the environment that would promote the reaction of the liquid metal with oxygen/nitrogen creating inclusions in the casting. This is most beneficial in the casting of critical components such as Nickel based alloys in turbine components. Also processing under vacuum significantly reduces porosity created by gas entrainment within the casting.

Moreover, vacuum casting provides the ability to directly influence the metal solidification through control of the cooling mechanism. Processing under vacuum removes the influence of convection cooling and allows cooling through a single axis (as with directional solidification casting) to be promoted.

Vacuum Precision Casting Furnaces (VPCF) in the investment casting industries come in a variety of designs, the configuration for which are driven by a number of factors, including:

Type of process (Equiax vs. Directionally Solidified / Single Crystal) Size of charge / mold Layout constrictions

Furnaces are categorised by the type of structure in the casting that is required. In

general the three categories are:

Equiax (Equ) – Furnaces where the rate of cooling of the casting is relatively uncontrolled. Typically with these furnaces, the casting is left to solidify in vacuum (through radiation to a cold surface i.e a chamber wall), in an atmosphere of inert gas or in air. Solidification is controlled by the customers mold design and thermal insulation placed around the mold in the form of ceramic fibre thermal insulating blankets. The cooling of the casting promotes an equiaxed grain structure.

Directionally Solidified (DS) – Furnaces where the solidification and structure of the

casting is closely controlled through the use of mold heating technology and progressive cooling. The process sees the casting withdrawn from the heated zone into a cold zone such that solidification occurs progressively in the component through a single axis. Therefore grain growth is promoted in a single direction encouraging improvements in mechanical properties of the casting.

Single Crystal (SX) – A variation on the DS casting furnace where the design of the

mold and use of a starter seed crystal is such that the grain growth is initiated from a single nucleation point. This configuration allows for growth of a single grain in the casting which provides the optimum in casting mechanical properties by eliminating grain boundaries.

Figure 1: Typical Aerospace Blades, Equiax, Directionally Solidified and Single Crystal (from left)

In general VPCF can be considered in terms of either batch systems (generally limited to smaller scale research and development furnaces or low throughput processes) or semi-continuous systems (standard industrial process). 2. Furnace Configurations 2.1. Batch Furnaces

The simplest design of VPCF employs a single vessel containing an induction coil/crucible assembly and facilities to locate molds. An access door or lid allows the melt unit to be charged with pre-alloyed bars and a pre-heated mold inserted into the chamber. The chamber is then closed and the air evacuated by a series of vacuum pumps. Once evacuated, the charge is melted down and cast into the mold by the conventional tilt pouring method.

This is a batch process and is limited by the fact that the shell mold will have inevitably cooled down by the time evacuation and melting are completed. Additionally

production rates of such units are low due to the handling time and evacuation time for each charge.

Mold heaters can be installed inside the batch chamber to compensate for heat loss, however the use of internal mold heaters for Equiax castings may influence the solidification characteristics of the casting and therefore may not replicate exactly the characteristics of high volume production units (semi-continuous processes).

Batch furnaces are therefore most commonly utilised in low production / research type applications or where a high temperature of the shell mold may not be essential or desirable. One such application is investment casting of Titanium with Induction Skull Melting furnaces. In these applications mold filling is assisted by centrifugal casting which is well suited to a batch furnace design.

Figure 2: Typical Batch VPCF (with centrifugal casting capability)

2.2. Semi-Continuous Equiax Operation Most production VPCF’s for equiaxed casting are of the semi-continuous variety. In

this case, the furnace consists of three chambers isolated by vacuum valves. One chamber (melt chamber) contains the melting coil and crucible assembly and another is used as a mold loading and unloading chamber. A third charging chamber is connected to the melt chamber with a vacuum isolation valve to allow it to remain under vacuum for a series of melt cycles.

With the melting chamber under vacuum, the three-chamber design allows a pre-heated ceramic mold to be loaded into the mold chamber and the chamber rapidly evacuated. The interconnecting valve between the chambers is then opened and the mold transported into the melt chamber, where pouring is carried out immediately.

The filled mold can then be retracted into the mold chamber and the interconnecting valve closed. The charging chamber is then used to load the next bar into the melt crucible and melting can therefore continue uninterrupted in the melting chamber (without breaking vacuum) independently of mold handling. This maximises the production rate of the furnace with the rate-limiting factor being the melting rate of the induction power supply.

Transport times from inserting the mold into the mold chamber to pouring are kept as short as possible. This ensures mold temperature loss is minimised and temperatures are maintained at the correct level for casting quality.

Other operations in the melting chamber, such as recharging the melt unit with pre-alloyed bars, taking immersion thermocouple readings, etc., are also carried out using vacuum locks so there is no requirement to break the vacuum in the melting chamber. This has the additional advantage that vacuum conditions inside the melting chamber are maintained at the optimum level. 2.3. Equiax Furnace Configuration

Since the Equiax furnaces do not in general require the use of mold heating technology, the flexibility in design is greatly increased. As such these furnaces can be found in horizontal configuration, vertical configuration or a combination of the two. The selection of a vertical or horizontal furnace is usually the individual customer’s

choice and both configurations are proven for most applications. The customer’s choice may be influenced amongst others by historical preference, building constraints and mold geometry/handling considerations.

2.3.1. Vertical Furnace Configuration

Vertical furnaces are the most widely used and offer an excellent technical solution for the vacuum casting process. These furnaces are characterised by the mold loading mechanism that sees the mold transported in a vertical plane into the melt chamber for casting.

Typically these furnaces are used for molds of up to 1.0m diameter and have the advantage that:

Small overall furnace foot-print. Can accommodate various mold heights with short metal drop and ability to

manipulate mold relative to crucible lip prior to and during pour. Fast mold pumpdown and transfer. Figure 3 below shows a sketch of a typical vertical furnace configuration.

Figure 3: Drawing of Vertical Equiax VPCF

2.3.2. Horizontal Furnace Configuration

These furnaces are characterised by a mold loading mechanism that sees the mold transported in a horizontal plane into the melt chamber for casting. Typically these furnaces are used for molds of a larger capacity and have the advantage that:

Lower head-room requirement than vertical arrangement. No requirement for a furnace pit for mold lifting mechanism. More suitable design of mold transfer system and isolation valves for larger

molds. Possible to design for very large molds i.e. >1.0m diameter.

Figure 4 below shows a sketch of a typical horizontal furnace configuration.

Figure 4: Drawing of Horizontal Equiax VPCF 2.3.3. Combination Furnace Configuration

Combination furnaces are a development of the typical horizontal furnace concept that overcome some of its limitations and are characterised by the mold loading mechanism that sees the mold transported in a horizontal and vertical plane into the melt chamber for casting.

Typically these use a vertical ram provided in the base of the melt chamber of a large horizontal furnace which permits a very wide range of mold sizes to be charged and the metal pour height automatically adjusted during pouring.

These furnaces have the advantage that they can accommodate various mold heights and provide ability to manipulate mold relative to crucible lip prior to and during pour to maintain a short metal drop.

Melt Chamber / Mold Chamber Isolation Valve

Melt Chamber

Mold Chamber Mold Chamber

Loading Door

Mold Loading Drive Mechanism

Induction Melt Coil

Figure 5: Drawing of Combination Equiax VPCF 2.3.4. Vertical Pitless Furnace Configuration

The Vertical Pitless Furnace is a development of the typical vertical furnace concept where the requirement for a pit below the furnace is removed using a cantilevered support assembly off an inverted ram, whilst offering all the same functionality as typical vertical configuration VPCF.

Figure 6: Drawing of Pitless Vertical Equiax VPCF 2.4. Semi-Continuous Directional Solidification (DS) and Single Crystal (SX) Furnace

Configuration Directionally Solidified (DS) and Single Crystal (SX) castings have also become

common place in advanced turbine technology due to their improved mechanical

Mold chamber

Bulk Charger

Mold Transfer Unit

Dip Thermocouple

Melt Chamber

Mold Raise / Lower

properties at very high temperature in service. Due to the requirement to insert the mold into the heater within the melt chamber,

the design of the DS/SX furnace almost exclusively utilises the classic vertical furnace configuration.

Figure 7: Drawing of Vertical DS/SX VPCF 2.4.1. Process

The process sees the pre-heated investment casting mold loaded into the mold chamber and, upon evacuation, raised through an isolation valve into a mold heater within the melt chamber. A separate charging mechanism, utilising a vacuum lock, is used to place a charge and ceramic crucible into an induction melt coil in the melt chamber where it is melted. Concurrently, the mold is heated within a mold heater assembly (resistance or induction) to a temperature above the liquidus of the charge material such that when the metal is poured, the mold is filled and remains molten.

The mold sits on a water cooled plate which provides the initiation of cooling and solidification of the metal in the base of the mold. An insulation baffle plate is held in the base of the mold heater, profiled to match the mold geometry, with a view to misimining heat loss from the mold heater. The mold is then slowly withdrawn out of the mold heater creating a thermal gradient across the casting to facilitate the directional grain growth.

2.5. Mold Heater Technology in Directional Solidification and Single Crystal Furnaces

In principle there are two main, mould heater designs for DS and SX furnaces in industry, resistance and induction heated. The type of mold heater employed in DS and SX furnaces has been a topic of much debate over many years however the fact remains that both techniques can work and are employed within the industry to produce blades of equivalent quality.

In both resistance and induction designs the heaters are often zoned, where multiple zones at varying heights and can be accurately controlled to different power and temperature set-points. 2.5.1. Resistance Mould Heater

Modern resistance heaters are constructed with Carbon Fibre Composite (CFC) heater elements where low voltage power is applied from thyristor controlled transformer power supplies. These heaters are characterised by the following advantages:

Light weight CFC heaters prove very responsive to power application. Multiple heating zones are easy to provide however two zone control has proven

optimum and there is little proven benefit from extra zones. Lower initial capital cost than induction. Fast cool-down. More exposed to molten metal damage.

However they have the following limitations:

Heater connections and insulators require more regular maintenance. Ideally suited to small heater applications.

2.5.2. Induction Mould Heater

Modern induction heater assemblies are constructed with a single or two zone induction coil that couples with a graphite susceptor element. The susceptor is located close to the mold and radiates evenly onto the mold. The design of the induction coil, power supply and susceptor (with respect to power, induction frequency and geometry) are carefully balanced to ensure efficient transfer of the electromagnetic field to the susceptor.

These heaters are characterised by the following advantages: Solid susceptor - very robust and tolerant of molten metal splash. Two-zone control switching induction power supply. Requires very little maintenance. Operates very well at high temp with no internal connections. Generally the industry standard for large production furnaces.

However they also have the following limitations:

Higher initial capital cost. Slower cooldown – though recent developments have improved this. In summary, resistance heaters are a low cost flexible heating system but have

proven to require more maintenance in production. In particular, the maintenance requirements appear to increase in proportion to the heater size and therefore often these heaters are, in general, limited to smaller applications. Induction heating systems, although requiring higher capital expenditure initially, generally require less maintenance and prove to be very reliable and robust in production.

3. Pouring Techniques Employed in Vacuum Precision Casting Furnaces

In principle there are two main pouring techniques that are employed in the design of VPCF furnaces that of Tilt pour and Bottom pour. Both pouring styles can be successfully utilised within the Equiax, DS and SX furnace technologies and both offer different advantages and limitations as described below. 3.1. Tilt Pour

Tilt pour technology sees the charge material melted within a fixed base ceramic crucible rammed within an induction coil. At the point of pouring the melt coil, crucible and charge material rotate around an axis such that the metal pours over the lip of the crucible into the mold. This technique is characterised by the following advantages:

Excellent control and repeatability of process allowing:

o Accurate metal temperature measurement (though use of immersion thermocouples and optical pyrometer technology).

o Allows metal to be effectively stirred prior to pouring to ensure alloy composition and temperature uniformity.

o Ability to control to exact levels of superheat in to metal prior to pouring. o Ability to optimise the casting process and effectively minimise the time the

mold has been cooling whilst waiting for the metal to cast. Allows (in Vertical or Combination Equiax VPCF design) the pour stream height

to be minimised by aligning the movement of the mold with the rotational movement of the crucible.

Allows absolute repeatability in the pouring process against a bench-marked ‘perfect pour’ using Teach Pour technology.

Flexible design that can be utilised for all charge sizes.

However it also has the following limitation:

Requires larger chambers sized to allow the induction coil mechanism to rotate. 3.2. Bottom Pour

With bottom pour technology, the crucible is not fixed within the induction coil, but rather is loaded pre-charged with the metal either with the mold or through an auxillary mechanism. The base of the crucible contains a pouring hole which can be sealed with a ‘penny’, a small disc of charge alloy. The pouring hole/penny is located relative to the induction coil such that the induced field is insufficient to melt it, rather it is melted through conduction from the molten charge above.

This technique is characterised by the following advantages: Compact furnace design that minimises furnace overall foot-print. Simple furnace concept that does not require complexity of crucible tilting

mechanism. Suited to an automated process where operator intervention and process

flexibility are not required.

However it also has the following limitations:

Generally suited to smaller charge weights. Limited control of superheat and pouring temperature.

4. Technology Developments to Support Key Process Requirements

In the investment casting industry there are a number of process drivers, often both interlinked and competing, which require to be accommodated in any vacuum casting furnace design. The designs see a number of features and technologies that have been specifically developed to meet these key process drivers.

These drivers and technologies include: Product Quality through Process Accuracy and Consistency:

o The ability to maintain all process variables from cycle to cycle to ensure absolute consistent product output.

o The ability to identify when a process has deviated from known or anticipated cycle setpoints.

o Process environment (leak rates)

Productivity: o Productivity of the furnace in terms of minimising cycle time, minimising

times between casting operations, maximising furnace up-time and maximising the volume of parts per cycle.

Reliability:

o The failure of any individual process cycle due to equipment failure is likely to lead to a relatively high cost of reprocessing or waste.

o Maintain high furnace productivity. 4.1. Multi-Axis Tilt Pouring System Developments

Modern tilt assemblies are provided with multiple electromechanical drive systems for highly accurate and controllable pouring operations. They can include the following control axis:

Tilt motion Traverse motion Mold raise / lower motion (Equiax only) Along with the standard rotational tilt motion, a traverse servo motor allows the melt

coil to move laterally during the pour process. When coupled with the rotational coil movement, this allows the pour stream of metal to be central in the mold pour cup preventing splash, ensuring the metal stream fills the mold as designed and ensuring that the pour height from crucible lip to mold can remain virtually constant.

Within an Equiax casting VPCF, the furnace is supplied with 3-axis pouring which

provides control of the melt coil rotation, of the melt coil traverse position and of the mold

height during the pour. The mould ram assembly allows the mold to be raised or lowered during the pour to minimise the metal fall between the crucible lip and the mold pouring cup to avoid excessive turbulence and excessive contact with the pouring cup / ceramics.

Coupling together all 3 pouring axis motions with the ability provided in the control

system to create, record and save each axis motion separately, provides a highly accurate and consistent stage of the investment casting process.

Pour initiation, coil rotation and traverse motions started

Pour complete, mold withdrawal commences Figure 8: Equaix VPCF multi-axis pour illustration

4.2. Control System Developments

As the maturity of the VPCF application in the investment casting industry has increased, the control system has developed greatly with a view to accommodating the drivers of accuracy and consistency in the process.

4.2.1. Teach Pour Facility

Of great importance is the accuracy of pour with a view to consistent pour profiles from casting to casting and identical metal cooling effects during the pour process, thus leading to common solidifying dynamics within the mold and identical metallurgical grain structures.

The modern control systems shall allow the tilt pour process to be carried out automatically against a pre-recorded ‘perfect’ recorded pour profile. Essentially the control system shall record the ‘perfect’ manual pour as a set of positional feedback data from tilt, traverse and raise/lower servo motors against a set of regular time intervals (for example every 0.1 seconds during the pour) and uses this data to replay the same profile for future cast.

This functionality therefore allows absolutely consistent pour profiles to be conducted cycle to cycle.

4.2.2. Optical Pyrometer to Immersion Thermocouple Cross Correlation

The main advantage of the tilt pour system compared with bottom pour is that it allows the melt bath to be controlled to a set temperature prior to pouring. This control is generally completed via a feedback loop between an optical pyrometer looking at the temperature of the melt bath surface and the induction power supply.

The accuracy of the optical pyrometer is dependant on the emissivity constant used for the molten metal and as with all instruments the accuracy is known to drift over time. To provide a reference to which the emissivity constant can be adjusted, and therefore the optical pyrometer ‘recalibrated’, an immersion thermocouple is provided to cross reference the melt temperature.

Modern control systems now provide the facility for this cross calibration automatically with a view to ensuring the accuracy of the alloy emissivity constant and consistency of the casting process from cycle to cycle. 4.2.3. Online Statistical Process Control

One of the most powerful process monitoring tools that has been developed, with a view to preventing the unrecognised process variation that can occur from cycle to cycle, comes from a form of Online Statistical Process control and process tracking. This process tracking works by setting maximum and minimum acceptable limits for any key process variable at any point through a cycle. These limits:

Can be absolute (i.e. +/- 5°C) or relative (%) Can be employed at specific points in a cycle or for a full cycle Can be variable throughout a cycle (i.e. such that tighter control is applied to

critical points in the process).

This tracking functionality can be applied to power, temperatures, vacuum levels and any excursion from acceptable limits for the key variable can trigger a variety of control system responses, for example alarm notification, cycle abort etc.

Figure 9: Example of Upper and Lower Limits Set to a ‘Target Cycle’ Against an Applied Power Variable

Upper and lower alarm levels are set following a cast of known good quality – this becomes “the recipe” for tracking

Kw High level

Kilowatts Actual Kw Low level

Lower Zone Kw Tracking

Figure 10: Example of an Alarm Condition Identified Where Applied Power Exceeds Acceptable Limits

For example, should an over-temperature thermocouple in a DS/SX hot zone exceed a programmed allowable temperature limit due to temperature drift of a control thermocouple, this could lead to an abnormally low temperature reading inducing the control system to apply additional power. The system could identify such an occurrence and automatically revert to a system of power control where the system applies power to the same kW / time profile as a previously recorded ‘good’ cycle and as a result the casting could potentially be saved. 4.2.4. Furnace Fault Diagnostics

With a view to maximising the productivity of a VPCF, furnace reliability and down time minimisation is of absolute importance. The complexity of the equipment and their control systems is such that the ability to diagnose and recover from any fault scenario becomes more difficult and potentially time consuming.

To facilitate the resolution of such issues and to aid in the diagnosis of faults by non-specialist technicians, modern VPCF now include simple intuitive fault finding and self-help screens on the control system HMI. These function to highlight to operators and non-specialists the specific conditions that have not been made in an interlock to function a particular process (i.e. valve movement etc).

Further to this, furnaces shall have more accurate self-diagnostic leak detection systems that are built into the control of the furnace, such that leaks on specific furnace areas can be identified more readily for rapid resolution. This includes leak detection in the static and dynamic conditions of each furnace area for greater targeted testing.

With ancillary equipment within the furnace assembly becoming more advanced (for example in the mechanical vacuum pumps), many now include features relating to condition monitoring. Integration of the condition monitoring equipment into the furnace control system allows areas of concern to be identified to the operator prior to failure and provides specific targeted information for rapid issue resolution. Finally all VPCF’s can be provided with online support facility such that issues can

be investigated remotely over an internet connection by the equipment manufacturer and resolved quickly or targeted advice offered.

ALARM Level reached Operator can select KW control from historical Recipe to complete cast

Kw Tracking Out of Control

4.2.5. Consumable Tracking

Systems have been developed to track the usage of all furnace consumables including:

Mold Heater Thermocouples Mold Heater Insulation Materials Insulation Baffles Immersion Thermocouple Crucibles Induction Coils Life

This allows the operator to identify and record the installation of the consumable, set

maximum life cycles, set mandatory inspection timescales, track number of uses, add comments and interlock the furnace operation to applied limits. Further to this, similar systems have been developed to manage furnace calibration (including vacuum gauges, thermocouples, optical pyrometers etc.) and furnace maintenance (including vacuum pump maintenance, scheduled inspections etc.)

All such developments are provided with a view to maximising consistency of operation from cycle to cycle and to prevent the furnace operation taking place (unwittingly or otherwise) with the furnace in a deteriorated condition, at the expense of product quality.

4.2.6. Vibration Monitoring

Specifically with respect to the DS/SX process, extreme care is taken to minimise the vibration that may be propagated from the various vacuum furnace systems to the mold leading to defects occurring during the solidification process.

With a view to maximising consistency and product quality, a vibration monitoring system (utilising piezoelectric velocity transducers) on the mold support has been integrated in modern furnace design that can be monitored on the control system and even fed into the Online Statistical Process Control functionality such that castings that experience excessive vibration during the solidification process can be identified and quarantined. 4.3. Baffle Clamping and Hot Baffle Change (DS/SX VPCF Only)

Within a DS/SX mold heater, for reasons of fine process control and energy efficiency, it is very important to maximise the thermal insulation properties of the mould heater, ensuring thermal uniformity and therefore providing the ability to control the heater temperature closely. Features of the design include:

Use of high quality insulation rings (typically vacuum formed carbon bonded

carbon fibre) with tight fitting stepped joints. Concentric rings of insulation Actuated pour hole cover to reduce heat losses from the top of the mold heater

assembly. Independent multi-zone heating to allow the mold to be heated uniformly and

heat loss to be compensated for without overheating full mold.

Further to these measures, typically an insulation ‘baffle’ (either fixed or removable)

is included in the base of the hot zone. This is included to minimise the gap between the mold heater and the edge of the mold to reduce heat loss radiated out the base of the assembly and facilitate very large thermal gradients on the casting as it exits the mold heater.

This baffle is often designed from the same material as the mold heater insulation, and can be enhanced using graphite foil to produce a matrix that has superior insulation properties. The typical clearances between the mold and the edge of the mold heater are ~5mm, however this can be reduced further through the profiling of the baffle to match the molds and produce complex baffle geometries.

The insulation properties of the baffle will inevitably change with age, due to thermal cycling, metal splash and through other damage mechanisms and therefore with a view to maintaining consistency in the mold heater performance from cycle to cycle the baffle will require periodical replacement. Also the baffle will need to be changed to suit any change in mold size.

With the typical fixed baffle assembly, the baffle replacement would require the furnace to be fully cooled and physical access to the melt chamber. This leads to excess energy usage in the furnace cool down and subsequent heating up process and removes the furnace from production for a period of many hours.

With a view to increasing furnace productivity a system has been developed that allows the baffle to be installed and changed whilst the furnace is hot. The design utilises pneumatic clamping assemblies and specially designed clamping elements that are employed once the insulation baffle has been brought into the furnace on a loading jig. As such a baffle can be changed with the mold heater remaining hot and in a period of 20 minutes, as opposed to many hours.

Further to the productivity advantages, this allows more regular baffle changes / inspections and helps ensure the performance of the furnace mold heater and maximise consistency.

Figure 11: Sketch of baffle clamping mechanism in DS/SX mold heater design

4.4. Modern Raise/Lower Design (In Vertical VPCF Configuration) (DS/SX Only)

The modern raise lower design has been developed to overcome the inherent issues that effect the standard hydraulic system and offer an electro-mechanical drive system for the ultimate in smooth, fully speed and positionally controlled movements.

Such designs see a mechanical actuator to the mold ram, and driven with a servo motor drive and gearbox. The servo motor is fully integrated with the furnace control system with direct feedback from the servo motors encoder to allow direct control of the ram position and speed of movement.

Typically positional accuracy of 0.025 mm and speed control between 0.2mm/min (ideal for DS/SX VPCF applications where slow accurate speed of movement is required) and 5000mm/min (ideal for Equiax VPCF applications where rapid speed of movement is required for furnace productivity) can be achieved.

A further advantage of such a drive system, especially in DS/SX applications, is the ability to accurately vary the speed of mold movement allowing general smoothness of operation and the ability to ramp up or down of speed of movement. Any potential mold vibration caused by jolting mold acceleration or deceleration may potentially lead to post-cast defects (such as hot-tear) within the solidifying casting.

Analysis has been conducted on a DS/SX casting furnace where the existing hydraulic raise/lower system was upgraded to an electromechanical system. Both prior to and following the upgrade the movement of the ram was assessed with the position against time analysed by a system of laser interferometry. Electromechanical Withdrawal System Hydraulic Withdrawal System Figure 12: Example Graph of Electromechanical Withdrawal Systems vs Hydraulic – Laser Interferometry Measurements

It can be seen that both profiles are accurate to the targets, however there is a

distinct ‘saw tooth’ profile in evidence for the hydraulic withdrawal with the variation in withdraw speed oscillating around the set point. Whereas the electromechanical drive can be seen to provide almost exact control mirroring the set point profile.

It is a fact that the original hydraulic system on this particular furnace were of a older generation and modern hydraulic control valve systems have improved the oscillating profile however it was not dissimilar to many systems in current operation. This said, the excellent level of control and repeatability exhibited by the electromechanical system suggests a lower inherent ‘vibration’ on the solidifying mold

and potential improvements on the solidification structure of the casting. With a view to accuracy and consistency, an independent positional transducer can be

used to allow the control system to monitor the position of the mold and correlate to the servo motor encoder such that any deviation is highlighted to the operator and appropriate action taken. 5. Conclusion and Future Developments

Vacuum Precision Casting Furnaces for investment casting come in a variety of configurations and are hugely application specific. As designs mature, however, the common requirements for all of consistency, reliability, product quality and productivity are driving the development and complexity of all designs to higher levels.

Future developments that can be expected going forward in the design of these furnaces will be driven by these drivers and others including:

5.1. Energy Efficiency in VPCF Design

With energy costs continuing to increase, the economics for electrical induction furnaces become more dependent on overall power consumption.

Developments in ancillary equipment, with respect to energy consumption and efficiency, will drive their specification (for example as recently seen in vacuum dry pump technology driven by the semi-conductor markets).

Further to this, developments in the control system with energy efficiency features shall become of greater requirement. These includes a system of Online Analysis of Mold Heater Insulation. This is a feature where intelligence can be built into the furnace control system such that, for any repeated cycle, the power application to the mold heater can be accurately monitored. This allows identification of any deterioration in the insulation properties of the furnace mold heater.

Tracking such power application over time can help the operator define optimum timescales for insulation consumable replacement and allows a balance to be struck between costs of additional power required and cost of lost productivity in furnace downtime for consumable replacement. Such analysis can then be fed into the Consumable Tracking functionality as described above.

Finally, the control system design shall increasingly account for the general method of operation for a furnace such that it shall be tailored to suit the operation style and minimise furnace systems operation whilst unproductive. For example this shall include the use of ‘shut down mode’ where auxiliary equipment such as water cooling and vacuum systems cease operation when the furnaces are not in production.

5.2. Process Integration and Automation

Similarly to energy price pressures, labour prices can be expected to become more prevalent in the economics of the VPCF. Therefore the ability to automate the full casting process in terms of loading, unloading and cycle operation, along with integration into overall factory processes and monitoring systems will continue to grow.

For example the use of robotics in the handling of Equiax molds (pre- and post-cast) removes an operator operation that may lead to fewer operators per group of furnaces. An example of this is indicated in figure 13.

Figure 13: 3D Model of Loading Robot and Mold Pre-Heat Oven

Similarly integration of pre-heat ovens into the furnace operation (in Equiax or DS/SX) with integrated control systems can offer similar advantages of potentially fewer operators, and can promote the overall incorporated cell approach.

The overall cell approach could be further enhanced with systems including the

ability to automatically download production planning direct to the furnace to schedule production automatically with the overall production facility and to feed back directly to over-arching monitoring, analysis and data acquisition systems. 5.3. Process Modelling

With developments in thermal and electro-magnetic modelling and the ability to couple these together, analysis of the important furnace processes may lead to developments / refinements in terms of induction systems design, mold heater design to optimise coupling, reduce power requirements and generally further optimise the casting process. References

(1) Kay,: Overview of Vacuum Furnaces For Investment Casting, 2008