Dielectric Heating: RF and Microwave

TECL m

~ Publishedthe bv EPRl Center for MaterialsFabrication Vol. 4. No. 1. 1987

of many other products, including rub- #bo84 and Dry ber weather stripping, plastic packag- Y-v-ductlve Materials ing, ceramic molds and catalytic converters, fiberglass tire cords, yarn packs, wood drying, and furniture making.

Advantages .1_ . L Conventional heating metho&, such as convection ovensor infrared, conduct heat from the surface inward, which is slow and inefficient for nonconducting materials.The direct tumm of plastics, rubber, ceramics, energy transfer of RF and microwave WxtNeS, and wood products can avoid , heating makes them attractive the great expense and slow produc- methods for heating these materials. tion rates frequently incurred in Quick heating- Heating ,occurs heating and drying operations. throughout the material's mass and is RF and microwave heating is a 2 to 20 times quicker than by conven- popular method for drying pasta and tional methods. Production 'rates in- tempering meats and is coming into crease, sometimes more than 100°/~. widtmpread use in the manufacturing Heating rates are limitedby. the

How Dielectric Heating Works material's ability toheat rapidly he properties of electrical without being damaged. and thermal insulation are llternating Uniform heating- & a T similar. A material thatcon- lectfic+ field homogeneous material, heating is 1 ducts electricity poorlyalso tends A more uniform throughout the cross to be a poor heat conductor. When section, ensuring more uniform some nonconducting materials, material properties. such as plastics or ceramics, are " Selective heatingldrylng- In a introduced into an alternating elec- Molecular tric field, the molecules within rotation, due to the alter. heterogeneous material, each compo- nating electric field, causes heatin nent responds to the energy and these materials rotate and move in the material. laterally several million times a se- heats at a different rate. Drying ap- cond in anattempt to alignwith communications. Microwave f plications illustrate this feature well. thechanging electric field. The quenciesrange from 300 to 3 Water quickly absorbs the energy and molecular rotation andlateral mo- MHz, with 915 and 2450 MHz corr evaporates before the substrate tion quicklygenerate heat within monly used for heating. Theequi overheats. Drying continues at a I the material in a mannersimilar to mentneeded for processing in uniform rate, a process called friction. This phenomenon, called each frequencyrange is diff moisture leveling. dielectric heating,transfers energy A triode oscillatorgenerates Impmud product quality- Heat- directlythe to material. in the RF range,andthe pro sensitive materials are not exposed to Dielectric heating can beachiev- are usually heated between high temperatures for long times, im ed by both radio-frequency and electrodes orthrough a series proving product strength and quality. microwave treatment. Radio fre- rods. A magnetronor klystron quencies for heatingrange from 2 generatesmicrowave power, CombiMtiOn wlth Commtknrl to 200 MHz,and these frequencies these products areusually hea metfmds - RF or microwave heating are alsoused for broadcasting and within a cavity orwaveguide. boosters can be added before, after, or inside existing equipment to speed

mentarylVol. 41NO. 1 1 L

Boosting with Microwave or RF Heating crowave or RF heating units can be added before,after, or inside cor lentional heating systems to speed a drying or heating process. A M;ombination system is particularly economical when the conventional process heats the material surface efficiently but takestoo much .. ment features instant in/off, so no energy and time to heat the interior. RF or microwave heating boosters quicklv heat or drv the entire thickness. The graph below illustrates a typical warm-up and cool-down time is need- RF-boosted drying process. Hot-&/RF combination drying-re- ed. Electronic controls accurately and quires 2'/2 hours, 70% less time than hot air, and the operatins instantly change product temperature. costs are $0.05, $0.03 less than hot-air drying and $0.05 less High energy efficiency- The effi- than RF drying alone. ciency, defined as the energy put into Several factors should be considered in the decision IO the material divided by the power sup- . boost a conventionalprocess with RF ormicrowave energy plied to the equipment, is typically 50 to 70% for RF and microwave heating. Conventional methods are typically 10 to 30% efficient. So, may increase production with little although electric power is more ex- pensive than gas or oil, the benefits of Rf or microwave heating can be nrlized without any increase -, costs. ults -%me processes ts become feasible only .$ crowave heating is ..:ad ceramicinstance, r 6 ic converters can be ' Time, hours &bd at high production ratesonly s and associated r each mas8 or with RF enerav. typic'al hot-air, R< aid hot-airlRF combination drying operaiions.

mlications Heating is necessaryto cure tt Preheating Plastics Many applications take advantage resins and boil off water. Application: RF energy preheats of the benefits offered byRF and Cotnpotlng procear: Fuel-fired, hot- epoxy preforms priorto molding an microwave heating. RF and air circulating ovens at 500 F heat semiconductor encapsulation. microwave heating uses are firmly unevenly and take overan hour to Process: The epoxy preform must established in the textile, food- cure the cores. The long cycle time molded at high pressures and processing, and wood furniture mak- causes the cores to slump and warp. temperatures to encapsulate thc ing industries. Other typical applica- Nonuniform heatingand curing eause semiconductor. tions are highlighted here: stress cracking. Renetit RF energy is virtually the only ..lrlng Foundry Cores Bemflt RF heating bakes the cores acceptable method for preheating epoxy preforms. RF preheating A9p#crtion: RF heating bakes foun- in 5 minutes at 230 F with littleslump ing and warping. Stress cracking is reduces cycle time byf5 to 30% and dry cores used to make internal ensures uniform curing. Molding cavities in metal castings.The cores reduced, and surface appearance is enhanced. pressures are reduced by30 to 40% shown in Figure 2 are used in the pro- and temperatures by15 to 30 F. duction of plumbing fittings. Outlook: Automatic forming, or "blow- Outlook Automatic handling of Roous: The cores are made with ing," is expected to further streamline preforms from preheaterto molder resin binders, water, andsand. core*formingin the next 2 to 3 years. will further speed the process in the next 2 to 3 years.

h.h..tlng Rubber &pbotkn: Microwave energy heats c rubber prior to vulcanization and molding into weather stripping. proclllm: Rubber must be heated under pressure with additivesto in- creasestrength and resiliency and ! reduce stickiness and odor.The entire,i thickness must be heatedto the '3: ,'''.: :& vulcanization temperature. ;-.;,i.,*-:, . :p,,, comp.ting-*: Steam ";:.;.,- '! !'- !I; autoclaves or hot salt baths would ?f-,!i! preheat rubber as batch processes+.':/' Fm2. A 4O.kW RF ova^^ kknfoundty cores for plumblng fittings. salt contamination is a concern. rn of eachmaterial and varies with 4 temperature, frequency, and moistug? content. Table 1 lists loss factors oft^.. ,,I , some common materials at rwm

r ditions under which it is measure published data should be viewed as the next 2 to 3 years. an initial guide only. Amaterial whos the material would arcto the work- loss factor indicates poor heating at Drylng Ceramlor electrode or enclosure. 70 F using 10 MHz may heat effec- Applhtion: Microwave energy heats The loss factor is a unique property tively at 100 F or 2450 MHz. Dielec- and dries molds and castings for bathroom fixtures (soap dishes and - paper holders). 1's IO! Prooar: Ceramic slip is cast into At room temperature molds, which absorb the excess Wlth sensltlzers added water. The ceramic green bodies are -When preheated to processing temperature.- ckW, glazed, and fired to form final t Is shape falrly regular, Indicating that R hours before gas w~llheat unlforrnlv? Unlform thickness air dry for 12 hours Few edges. I These long drying times I

". Is shapevery complex, idicatfng microwave If uniform heatfng is necessary? Varying cross section tting of the piece in the Many curves/edges. celerated, and the heated YB IN0 waW dries in minutes for reuse. The sntiru, processing time is 30 minutes. Can product be heated rapidly? Microwave energy, perhaps At approprlate rate odkdr: To requlred depth. No stress/cracklng in combination in resistancewith 'I No breaking High moistureUniformHigh mate Surface effects. heating, is expected to be used for Shallow fast firing the glaze in about3 years. I " I Teehnieal Considerations "",I, p'"""b1 HCdl a, app4upll rate durlng entire process? To determine whether RF or Dramatlc heatmg rate lncredbtlb microwave heating is a good alter- Dramaticheatlng rate decrease: natk for your heating process, you must evaluate several technical factors. The following conditions are im- pordlnt indkators of a successful ap- ated areas plictHm for Rfor microwave heating, but cnrery condition need not be pre- sent for success: Pure RF/mlcrowave i" RFimicrowave :valuate RI D Ttm material has an appropriate microwave unllkely, evaluateboostlng I tam factor (described below). heatlng. Ipart of heatlng process J D Ttw material is sensitive to ex- I"' . c(388/v8 emreto heat. The meterial must be heated to a Hot/cold spots uniform temperature throughout its Unheatedareas thickness. 1 D The current (conventional) heating process is inefficient and time- consuming. Shallow D Increased automation and electronic control of the heating process are desired. Evaluate RF/mcrowave I m The desired effect is difficult or im- l-L-1 icrowave/conventionaI RF/conventional passible to achieve by conventional L methods. 1 Laea --The loss factor is a material's ability to absorb energy at

TmhCommeniarylVol. 41No. 1 t Cotton (7% 0.03 humidity) Dry wood 0.04 (yellow poplar) Paper 0.16 Polyethylene 0.000’ Pyrex@glass 0.015 Rayon 0.07 Rubbe 0.08 .03 Teflon 0.00 Waterat 25 C 0.36 .0°03 I I atIce -12 C 0.07 ,003