Companies are increasingly struggling to eliminate the batch manufacturing process in lieu of continuous flow or lean manufacturing. The reasons for the change are obvious: shorten lead times, decrease WIP, decrease inventory and scrap, increase flexibility and increase productivity.
|Fig. 2. Induction vacuum chamber (IVC) with controls|
|Fig. 2. Induction vacuum chamber (IVC) with controls|
One process that has historically been locked into batch mode is the vacuum furnace (i.e. vacuum brazing, hardening, etc.). Until recently, there hasn’t been an alternative to traditional batch vacuum-furnace processing. Historically, all vacuum processing has been conducted in relatively large vessels that require long pump-down and heat cycle times, thereby necessitating the inevitability of batch processing. Due to the heat source (typically radiation/convection) traditionally available in vacuum processing, heat cycle times have been the limiting factor in process cycle time. Therefore, the answer was to go to larger vessel sizes, increasing throughput with larger quantities of parts while still maintaining similar heat cycle times.
The solution to eliminate batch vacuum processing is to reduce the heat cycle time needed. By choosing an alternative heat source, the cycle time can be dramatically reduced. Through the use of induction, the necessary heat is produced quickly, efficiently and directly into the affected area of the part itself (IVC) or into a small graphite susceptor (IVF), which radiates the heat uniformly into the part.
Induction heating produces heat in metals and other conductive materials (graphite) through the application of an electromagnetic field. The imposed electromagnetic field produces a current flow in the part (IVC) or susceptor (IVF). The current flows against the resistivity of the part material or the graphite in the susceptor and produces heat. With induction heating, it is possible to specifically design the current flow that is induced into the part or susceptor (hot zone), thereby precisely controlling the heat pattern and therefore the amount and area of heat.
Through the use of induction and small vacuum chambers, cycle times are greatly reduced, and parts may be processed individually or in small quantities as needed very quickly.
Vacuum and/or atmospheric processing is required for high-temperature nickel brazing of aerospace and other superalloys, copper brazing of steel parts, and processes requiring clean finished parts (medical-device hardening, for example). Generally, these processes are conducted in vacuum furnaces that can be from 12 inches to 12 feet in diameter. Selection of the vacuum furnace is dependent upon the part geometry, vacuum/atmosphere needed and batch size desired for processing.
Current practices are better suited to run similar high-volume parts. Repair and reworked parts (orphans) are typically uneconomical to run in a large, high-volume vacuum furnace.
The most desirable parts for vacuum processing are those that can be self-fixtured. It is undesirable to have parts that require fixturing because the fixturing as well as the part must be heated during the heat cycle. Typically, these parts are tack welded or press fit for traditional vacuum processing.
There are several limitations to conventional batch vacuum-furnace processing:
|Fig. 3. IVC with optical pyrometer|
1. Increases part cycle time – Batches of parts are taken out of the manufacturing cell and sent to the furnace for heating. Moving from a continuous-flow system to a batch process slows production and causes significant inefficiencies in the manufacturing process (additional inventory, packaging, transportation costs, tracking, shrinkage and scrap). This is exponentially true when the parts are sent off-site for processing.
2. Limits quality control – There is no control over individual parts and no way to verify that an individual part reached the desired processing temperature.
3. Produces high reject rates – When braze material melts, it flows to the hottest areas of the part, which may not be the joint. This can result in poor cosmetics, poor joints and poor quality. Many firms rely on braze stop-off materials, which generally require manual application. If a process problem occurs, the entire batch is suspect and could be scrapped.
4. Limited temperature-range flexibility – Typical vacuum furnaces utilize tungsten filaments for low-temperature applications. High-temperature applications require an expensive hardware change to a molybdenum filament.
5. High fixturing costs – Brazing operations that require parts to be held (angle parts that are not press-fit) require special fixturing, which may not be cost-effective to develop and manufacture. In precision heating applications, close tolerances must be kept on the fixture to allow for thermal expansion. In these applications, fixturing cost will be significant. Since the fixturing is heated along with the parts in a batch system, the life of the fixturing is limited, and the fixturing cost is higher due to the type of materials that must be used.
6. Non-selective heating – In a batch vacuum furnace and continuous atmospheric furnace the entire part is heated, not just the joint area. In many applications (e.g., microwave components, electronics, semiconductors, etc.), it is not desirable to heat the entire part.
7. High energy costs – To maintain production rates, furnaces and ovens are generally kept at processing temperature between batches or even left on continuously. This results in significant energy cost and waste.
8. Process development – Generally, an entire batch must be run to accurately model the effect of process parameters. Therefore, process development becomes costly due to the high volume of parts that may ultimately need to be scrapped and the inherent cycle time needed to process the batch.
|Fig. 4. Induction vacuum furnace (IVF)|
By integrating induction heating with a small vacuum chamber, virtual continuous-flow manufacturing can be realized. The use of induction heating eliminates the long heat cycle-time requirements in a conventional vacuum furnace. The use of several smaller vacuum chambers maintains throughput by reducing pump-down cycle times. One operator can load and unload a series of chambers, thereby keeping the principles of continuous-flow manufacturing in practice.
While there are numerous suppliers of both induction heating equipment and vacuum/inert gas systems, there are several critical factors that will determine success of the system. In vacuum, voltage break downs occur at a lower threshold than in air, resulting in arching within the vacuum chamber. Therefore, when choosing an induction heating system, one of the characteristics that needs to be considered is the voltage on the coil without sacrificing operating efficiencies. GH Induction Atmospheres’ induction vacuum chamber (IVC) and induction vacuum furnace (IVF) systems employ the use of a low-voltage work coil with a small chamber to provide a flexible platform for various types of atmospheric/vacuum processing, including brazing and hardening.
|Fig. 5. IVF series induction vacuum furnace|
Advantages of IVC/IVF Processing
1. Wide temperature range – IVC/IVF systems provide complete system flexibility to handle a wide range of parts and temperatures from 1450°F (788°C) for silver brazing to 2200°F (1205°C) for nickel brazing and 2200°C (3992°F) for special material processing.
2. Cellular manufacturing – With a small, compact footprint and built to withstand the rigor of 24/7 operation, IVC/IVF systems have been designed to fit in a manufacturing cell. As throughput requirements increase, additional vacuum chambers can be integrated – powered and controlled separately or by a single power supply.
3. Improved quality control – Because IVC/IVF systems quickly and accurately heat one joint (part) or small quantities of parts at a time and the part temperature is controlled, a quality part is assured each time. Real-time monitoring and SPC (statistical process control) are available through the PLC, HMI and digital chart recorder. Data may be stored and transferred directly to a computer. This is critical in any aerospace and medical application. For precise temperature control, each IVF system includes type-S thermocouples for control and over-temperature and five type-K workload thermocouples. A two-color optical pyrometer is recommended for the IVC system.
4. Ease of fixturing – Even in precision heating applications, fixture design is simplified because only small quantities of parts are heated at a time with the IVF. Fixture life is substantially increased because the fixture is not subjected to heat during the direct heating part process of the IVC.
5. Selective direct-joint heating (IVC) – Because induction heating affects only the joint area, other components are not exposed to heat. This results in faster cooling times and the ability to easily braze joints, which previously have not been practical. Because only the joint area is heated, the braze stays in the joint area. This vacuum-chamber system improves cosmetics, reduces the amount of braze material needed and produces better quality joints. Additionally, firms that previously had to rely on braze stop-off have been able to eliminate it, reducing the manual labor involved in both applying and removing the stop-off.
6. Indirect part heating (IVF) – The IVF is designed to heat parts of virtually any shape in a high-temperature, high-vacuum environment. With a hot-zone diameter of 12 inches by 12 inches high, the quick, clean induction heating system can achieve 1900°F in less than eight minutes.
7. Lower energy costs – Because IVC/IVF systems use induction as the heating method, energy usage is significantly lower than a typical vacuum furnace. The energy used with induction is over 80% efficient. As a result, over 80% of the energy used goes into heating the part/braze joint.
8. Ease of process development – Due to the fact that a limited number of parts are processed at one time, process parameters can be easily determined, adjusted and finalized.
The majority of industries that currently use a vacuum furnace for brazing or heat treating – aerospace, medical, HVAC and industrial components – are looking for continuous-flow solutions in all of their manufacturing operations. GH Induction Atmospheres has designed and supplied IVC and IVF systems that satisfy the continuous-flow criteria for nickel brazing of superalloys, copper brazing of steel, silver brazing of copper and brass assemblies, material processing, and hardening of precision medical devices.
Through the use of induction as the heat generator, a small chamber size and integrated controls, effective vacuum chamber and furnace processing is achievable in a continuous-flow environment. The flexibility of the GH Induction Atmospheres’ induction vacuum chamber (IVC) and induction vacuum furnace (IVF) can be easily integrated to process various parts in one turnkey system. IH
For more information: Contact Chip Laskowski, director of sales, GH Induction Atmospheres, 35 Industrial Park Circle, Rochester, NY 14624; tel: 585-368-2120; e-mail: firstname.lastname@example.org; web: www.inductionatmospheres.com