Induction Heating Consultant

Induction Heating

Stanley Zinn

Stanley Zinn • Induction Consultants • Tel: 585-737-8824
15307 Strathearn Drive, Unit 11202, Delray Beach FL 33446 •
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Stanley Zinn

Induction Heating as a Source of Heat

by STANLEY ZINN, Induction Heating Consultants

Proceedings of the 22nd Heat Treating Society Conference and the 2nd International Surface Engineering Congress,15-17 September 2003, Indianapolis Indiana, USA


Abstract

Induction is basically a source of heat similar to radiant, convection or conductive heating. However, its unique features of controllability, localized and rapid heating are basic requirements for many manufacturing processes.


Examining the benefits of induction simply as a heat source can greatly expand its use as a tool for manufacturing.


Induction heating is well known as a heat processing tool used for heat treating of metals. Speed of heating is limited only by the power available and the coupling between a well-designed coil and the workpiece. For mass heating applications, as found in forging and warm forming, the ability to heat deeply and rapidly enables the user to increase production while cutting costs. In all these instances, an improved product is also one of the major benefits.


There are few manufacturing processes that do not utilize heat in some form. Induction has the ability to place highly-controlled heat in a well-defined area. For this reason alone, induction should be perceived as something other than a heat treat tool. Rather, it should be used as one of the basic heating systems along with radiant, convection and conductive heating.


Typically, while capital costs of using induction are high, the savings in operation greatly reduce operating costs in most situations:

  1. Electrical energy costs are limited to the power actually used, for the time duration of the heating cycle. There are no warm-up or continuous energy usages as experienced with ovens or furnaces.
  2. The heat is generated solely in the area of the part being processed. There is no external heat. This means that the system can be integrated directly into production lines or lean manufacturing cells.
  3. The heat is controlled and rapid. Processing is highly repeatable lending itself to automated systems.

When you consider the huge variety of heating applications in industries as varied as fiber optics, automotive components or medical devices, you get some idea of the range of heating problems that engineers are forced to deal with in industry. By considering induction heating as a possible alternative to radiation, convection and conduction, many of these unique problems can be solved. Induction should be considered as just another source of heat. However, it also presents opportunities to perform heating functions that are impossible with other techniques.


The key to successful application rests in the unique ability to put the heat solely, where you want it, when you want it and as rapidly as possible.


Shrink fitting of objects as diverse as bicycle pedals and aircraft bearings benefit from the application of induction heating. In this case (Fig. 1), a forged aluminum bicycle pedal requires the insertion of a bearing for mounting on the pedal shaft. In order to reduce the number of components required to secure the bearing in place, shrink fitting was selected for this operation. Using high frequency and a 15-second heating cycle, the aluminum forging was heated to approximately 400°F providing enough expansion to allow the bearing to be dropped in place and secured as the aluminum cooled.


A similar problem is encountered in the manufacture of aircraft bearings (Fig. 2). Here, the outer race must be expanded to allow the last ball in the bearing to fall in place. Since the race has been heat treated prior to this operation, it must be kept below 400°F to prevent changes to the alloy structure. In this case, the induction system is controlled by an optical pyrometer, which reads surface temperature of the outer bearing. The temperature at this point is maintained, through the closed loop, as heat progresses through the thick section of the bearing until the expansion is sufficiently expanded to perform the operation. The technique is fast (approximately 40 seconds) on the largest bearing and there is minimal WIP as would be required in an oven.


Further, in the bearing industry, the new, small, portable heat stations are coming into play as possible hardening stations, in situ, directly on the automatic screw machines that manufacture the bearing races.


Rapid local heating for straightening processes is used with automatic machines in the manufacture of twist drills. In this application, the balancing machine determines the location that the drill is out of required TIR. An induction coil is rapidly moved into position, the part is heated and the automatic straightening is done with minimal, if any, change to the drill,s metallurgical structure.


The automotive world, with its desire to move to lighter weight materials, is utilizing greater quantities of plastic components. One limitation of plastics, however, is their inability to maintain a thread during repeated removal and resetting of fasteners. Accordingly, a relatively new approach is the use of internally-threaded metal inserts (Fig. 3). These are heated by induction and when at temperature are inserted in pre-positioned holes in the molded plastic. The plastic flows around ribs or grooves in the outer surface of the insert and serves to lock the insert, rigidly, as the material cools. Any number of inserts can be placed simultaneously, with the proper coil, with a single time cycle in the order of five seconds.


The manufacture of optical fibers presents a unique application for induction heating. To form the individual fiber, a large tube of glasslike material (a boule) is heated locally to approximately 1650°C. As glass increases in temperature, it becomes conductive. The area to be drawn into fiber is heated locally by induction and the fiber is pulled from this area, providing miles of material from a single boule.


In order to raise the glass to a temperature at which it is conductive, a carbon graphite slug is lowered in to the boule. The graphite heats by induction, and its radiation causes the temperature of the glass sufficiently to bring it to the required temperature. The carbon graphite is removed once the glass can be heated directly by the induction field. The carbon graphite in this application is used as a susceptor.


A susceptor is a material which can be easily heated by induction and which passes its heat to another material by radiation, convection or conduction. This is a technique which can be used in any number of applications, where the material itself will not heat directly by the induction field. In the application of induction to plastics and other non-conductive materials, the one most apparent (and unrecognized) is the cap-seal on pharmaceutical products. In this case, the susceptor is the aluminum foil.


In practice, the foil is bonded to a thin layer of plastic, commonly polyethylene. The plastic bottle cap is fitted with a waxed insert to prevent bonding to the cap and then a layer of the composite aluminum foil/polyethylene This assembly is then used to cap the bottle, with the polyethylene layer close to the bottle rim (Fig. 4A).


As the capped bottle then proceeds through a coil providing an induction field (Fig. 4B), the aluminum foil heats from the field, melting the plastic polyethylene layer to the rim of the bottle. Since neither the bottle nor the pharmaceutical contents will heat with an induction field, the product is unaffected. However, the seal is now fully bonded to the rim of the bottle. This same technique is now used for packaging, on a variety of food products such as peanut butter, to prevent contamination.


This multi-layer composite, is used as well, in the manufacture of toothpaste tubes. Here, the foil/plastic material is wrapped around a mandrel. The induction field heats the overlap of the foil, at the seam, creating a permanent bond. This technique enables machines to manufacture in excess of 150 tubes per minute!


In many cases, the base material itself acts as a secondary induction heated susceptor for another process. Metal assemblies are coated with paint or powder which must be cured at high speeds. By induction heating the base metal, the solvent is driven from the paint or the powder is brought to flow temperature by heating the assembly with induction. With radiation or conduction, the outer surface of the coating forms a skin, trapping the volatiles. With the induction technique, the solvents are driven from the inside of the coating out to the air, providing a stronger bond with the base metal.


In the manufacture of measuring tapes (Fig. 5), there are a number of coating processes that require curing. These include: base coat, paint coat, marking ink and a protective cover of mylar. Normally, each of these operations require the use of a 30-foot oven. The induction system dries these coatings in a 6-foot length at speeds upward of 400 feet per minute.


More recent developments in the field include fluxless brazing of aircraft components. As illustrated in Figure 6, using an induction coil in a vacuum chamber (which can be filled with an inert gas if necessary) rapid, localized heating can be restricted to the braze area. Particularly, where aircraft engine components are being brazed, it is important that the structure of the adjacent metal be affected as minimally as possible. This calls for highly local, closely-controlled temperature at the joint. In addition, because of hidden cavities and channels, fluxless brazing is desired.


Flux would be highly corrosive and cleaning to remove all traces of flux residue would be highly expensive and not necessarily foolproof. Localized induction used in a vacuum achieves all of these requirements.

The same problems occur in many automotive engine assemblies and systems for this purpose are easily automated.


At the other end of the scale is the desire to produce extremely high temperatures on a massive scale. Here, the initial cost of a vacuum furnace, as well as the opening cost, would be prohibitive. This is the case in the manufacture of certain silicon carbide components


This technique utilizes a carbon graphite susceptor under a flow of inert gas to prevent oxidation. Insulation blankets between the coil and the graphite reduce the losses from the surface of the susceptor due to radiation.


Radiation losses at 2300 °C. from a black body are 1000 watts per square inch of surface. For the susceptor shown in Fig. 7, (36"D x 15ft OAH) normal radiation loss would be in the order of 20 megawatts. Proper application of insulation reduces this so that less than 300 kW of power is actually required.


Conclusion


These few examples, from the flea power required to seal a tamper-evident pharmaceutical cap to the firing of the silicon carbide show the range and versatility of induction heating.


Induction is highly competitive, on a case-by-case basis, with other forms of process heating, be they radiation, conduction or convection.


Induction offers many benefits that are unobtainable by other heating methods. The key factors to remember are:

  1. Extremely rapid heating
  2. Selectively localized heating
  3. Highly repeatable processes
  4. Cost-effective application

Recognize induction's capabilities and you have another tool for your heating processes.


As the saying goes, "THINK OUT OF THE BOX".

Fig. 1: Aluminum bicycle pedal
with inserted bearing
Fig. 2: Induction heated bearing races are maintained at temp. via optical pyrometer during fitting. Fixture accommodates various coil diameters to match bearing sizes.
Fig. 3: Molded plastic assembly with brass insert. Note serrations on OD of insert to allow molten plastic to trap insert upon cooling.
Fig. 4A: Plastic bottles with tamper-evident seals bonded to bottle rim.
Fig. 4B: Top - assembly of cap with aluminum/polyethylene laminate; Bottom - cap assembly showing induction field.
Fig. 5: Measuring tapes and metal strappings with induction-cured coatings decrease plant floor requirements from 30 ft. OAL to 6 ft. OAL. and provide a superior bond between metal and coating.
Fig. 6: Self-contained vacuum brazing system including power supply, plumbing system, infrared pyrometer and programmable controller.
Fig. 7: Induction heated firing furnace for silicon carbide components. Coil dimensions are 4 ft ID by 20 ft overall height. Furnace operates at 2300°C