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`Thermoftìrming
`
`CRC Press
`
`Taylor(cid:3)&(cid:3)Francis(cid:3)Group(cid:3)
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`■ € @ 1 M 0 E © D T I1 ® IM
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`Thermoforming
`
`A Plastics Processing Guide
`
`0004
`
`

`

`Thermoforming
`aTECHNOMlC^ublication
`
`Fubluhed in the Wutem Hemisphere by
`Ifechnomic Publishing Company, Inc.
`851 New Holland Avenue, Box 3535
`Lancaster, PEonsylvania 17b04 U S.A .
`
`Distributed in the Rest € i the ftbrid by
`Ibchnomic Publishing AG
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`C o p y ri^ © 1998 by Technomic Publishing Company, Inc.
`All rights reserved
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`
`Printed in the United States of America
`1 0 9 8 7 6 5 4 3 2 1
`
`Main entry uixler title:
`Tbennofi^nning; A Plastics Processing Guide, Second Edition
`
`A Technomic Publishing Company book
`Bibliography: p.
`Includes index p. 231
`
`Library of Congress Catalog Card No. 98-60402
`ISBN No. 1-56676-625-7
`
`0005
`
`

`

`1 3
`
`35
`
`27
`
`Table of Contents
`
`Foreword
`ix
`Preface
`xiii
`L Introduction.
`2. Heating of the P lastic................................... ......
`Means of conveying heat to the plastic
`3
`Physics of radiation heating
`5
`15
`Thermal properties of plastics
`18
`Heating equipment for plastic sheets
`Judging the correct temperature of the heated sheet
`Heater controls
`31
`Clamping of sheets
`33
`5. Thermoforming M olds....................................................
`Reduction in wall thickness: male and female molds
`35
`Computer-aided engineering for thermoforming
`45
`Part shrinkage and dimensional tolerances
`48
`Warpage
`50
`Draft in the mold
`Surface appearance
`Mold materials
`54
`Mold-cooling provisions
`Air passage holes
`56
`Increasing stiffness
`59
`Mold plugs
`59
`4. Vacuum, Air Pressure, and Mechanical Forces..................... 61
`Measuring vacuum and pressure forces
`61
`Vacuum sources
`62
`Vacuum accumulators or surge tanks
`Application of vacuum forces
`64
`Pressure forming
`65
`Mechanical forming
`
`52
`53
`
`56
`
`62
`
`66
`
`0006
`
`

`

`VI
`
`Table o f Contents
`
`5. Cooling of Thermoformed P arts.............................................. 69
`Means o f cooling the formed part
`69
`Non-conventional cooling methods
`72
`
`6. Trimming of Thermoformed P arts.......................................... 73
`Tools for trimming
`74
`
`7. Thermoforming E quipm ent.................................................... 79
`Single-station thermoformer
`80
`Shuttle thermoformer
`82
`Rotary thermoforming equipment
`Continuous in-line thermoformers
`In-line thermoformer
`97
`Linear thermoformers
`100
`102
`Pneumatic thermoformers
`Hydraulically operated thermoformers
`Mechanically operated thermoformers
`Skin packaging equipment
`104
`Blister packaging equipment
`104
`Snap packaging
`104
`Vacuum packaging
`Packaging machinery
`Control mechanisms
`
`82
`85
`
`103
`103
`
`104
`105
`109
`
`111
`
`116
`116
`
`8. Thermoforming-Related Material Properties.
`Glass transition temperature
`111
`Heat deflection temperature
`112
`Softening range and hot strength
`Specific heat
`115
`Thermal conductivity
`Thermal expansion
`Heat of fusion
`117
`118
`Thermal diftusivity
`118
`Thermal stability
`119
`Water absorption
`Orientation and crystallization
`Manufacture o f starting materials
`Coextrusions and laminates
`129
`Mechanical properties
`130
`Material economics
`131
`Regrind utilization
`133
`
`112
`
`120
`128
`
`0007
`
`

`

`Table o f Contents
`
`139
`
`9. Thermoforming M aterials’ Chemical Descriptions
`Acrylics
`136
`Ceilulosics
`136
`Polyolefins
`137
`Styrene polymers
`Vinyl resins
`140
`141
`Engineering plastics
`Copolymers, blends, and alloys
`Fiber-reinforced thermoplastics
`Transparent materials
`145
`Barrier materials
`146
`Electrical properties
`153
`Plastics recycling
`Flammability of plastics
`Toxicity of plastics
`157
`
`143
`144
`
`152
`
`155
`
`159
`
`10, Thermoforming Processes ..
`Billow, bubble, or free forming
`Cavity forming
`161
`Drape forming
`162
`164
`Plug-assist forming
`165
`Billow drape forming
`166
`Snap-back forming
`167
`Air slip forming
`Reverse draw with plug-assist forming
`Trapped sheet pressure forming
`168
`Twin-sheet forming
`168
`Pressure forming
`171
`172
`Mechanical thermoforming
`Plug^and-ring forming or ridge forming
`Slip forming
`174
`Matched-mold forming
`175
`Rubber pad and fluid pressure or diaphragm forming
`Other thermoforming processes
`175
`Adjusting process parameters
`177
`Thermoforming troubleshooting guide
`
`167
`
`173
`
`181
`
`11. Design C onsiderations.............
`Assembly and bonding
`185
`Snap-fits
`185
`Mechanical bonding
`Forming around inserts
`
`186
`186
`
`VII
`
`135
`
`159
`
`175
`
`183
`
`0008
`
`

`

`VUI
`
`Table o f Contents
`
`Welding
`186
`Solvent bonding
`186
`Adhesive bonding
`187
`Rigidizing thcrmoformed parts
`Bonding multiple parts
`187
`Foaming-in-place
`188
`Fiber-reirrforced structural supports
`188
`Finishing and decorating thcrmoformed parts
`
`187
`
`190
`
`72. Related and Competing Forming Processes___
`Forming processes performed at lower temperatures
`Packaging container forming
`196
`Limitations for thermoforming
`197
`Injection and extrusion molding
`198
`Blowmolding and rotomolding
`199
`Plastisol or slush molding
`199
`
`193
`
`194
`
`Appendices
`A Exemplary properties of thermoforming materials
`B Exemplary properties of film materials
`205
`C Trade names and materials manufacturers
`209
`D Conversion factors
`225
`231
`
`Index
`
`201
`
`0009
`
`

`

`Foreword
`
`Th e r e m a r k a b l e a c h i e v e m e n t s of plastics engineers, chemists,
`
`and technicians in the field of thermoforming have long been recog-
`nized by those industries that have so greatly benefited through the
`increas^ profitability engendered by attractive, convenient packaging
`of their products. Packaging is by far the most visible advertisement for
`thermoforming, but it is only one indication of the breadth of this
`fast-growing, ever-changing technology. Indeed, it may well be that a
`new era approaches in which past promises unfulfilled and radical
`technologic^ innovations forthcoming will impose increasing demands
`on the proven ingenuities of those who have pioneered, and those who
`continue, the development of thermoforming technology. Again, these
`new technologies will be most visible in packaging as vastly improved
`processes for food preservation emerge in the forms of irradiation
`(gamma), super-pasteurization and other novel sterilizing techniques. It
`just may be that the long awaited fuel cell for powering automobiles may
`be close to reality; if so, will thermoformed automobile bodies not be far
`behind? New construction methods appear to be in the offing that will
`demand thermoformed composites providing structural strength, insula-
`tion, and weather resistance. Moreover, it would seem that in the not too
`distant future a group of new plastics materials will emerge that should
`find many applications in thermoforming; these materials will be of
`substantidly lower cost than those of today in that their monomers are
`produced by direct partial air oxidation of essentially unwanted crude oil
`and gas oil components in a simple process of almost negligible capital
`and operating costs.
`A comprehensive treatise on thermoforming has long been needed; for
`too long this most important technology has been retained (closely held)
`by a relative few. This does not suggest some kind of intentional cabal,
`for there are numerous publications describing single processes. But an
`all-encompassing treatise on thermoforming technology such as this by
`Dr. Gruenwald is long overdue. He has ingeniously compiled an enor-
`mous amount of information and references, reformed them without loss
`ix
`
`0010
`
`

`

`Foreword
`
`of details, and organized the material with an “overlay” of the scientific
`principles involved. Hence, his book is an ideal text for all plastics
`engineers and technicians and for polymer chemists. Yet it is at least
`equally valuable to thermoformers and those who utilize thermoformed
`products. The generic terminology employed is especially appreciated
`in that students are not lost in a maze of unfamiliar, nondescriptive
`jargon. Dr. Gruenwald has, however, provided an invaluable delineated
`description of thermoforming processes replete with practical “how-to”
`problem-solving guides for even the most sophisticated professional in
`the field. And these experts will be every bit as comfortable with his
`terminology as novices.
`Thermoforming is a highly interdisciplinary technology encompass-
`ing chemistry, physics, materials, mechanical engineering, and thermo-
`dynamics competence. Its breadth, already grandiose, may range even
`to metals; several alloys are now known to have remarkable plasticity,
`and probably many more will be discovered. Indeed, it is quite likely that
`glasses will be commonly thermoformed. But the great growth in ther-
`moforming will undoubtedly continue in plastics materials. Dr. Gruen-
`wald has indicated the desirable properties of polymeries for differing
`applications; thus, his text is especially useful for polymer chemists who
`must “tailor” plastic materials for specific groups of applications. Engi-
`neers in extruding and calendering film and sheet will benefit from the
`intimate relationships elucidated between processing parameters im-
`posed upon stocks employed in thermoforming and the products thereof.
`Mold designers are provided with a complete guide that will enable them
`to avoid the less obvious pitfalls and wasted effort so often experienced
`in the evolution of molds for (especially) complex parts. This book is
`especially useful for mechanical engineers in that it delineates all of the
`factors, including thermodynamics, important in the design and con-
`struction of thermoforming systems. Operators will find “everything you
`ever wanted to know” about the technology of thermoforming, enabling
`them to improve productivity, minimize down-time, and greatly reduce
`scrap. Quite likely. Dr. Gruenwald’s suggestions will lead to considerable
`benefits to those who read and practice by this remarkable exposition of
`thermoforming technology.
`Dr. Gruenwald’s 35 years of experience in plastics and thermoforming
`with Farbwerke Hoechst and General Electric, combined with the innu-
`merable suggestions and additional information provided by other ex-
`perts in this field, has resulted in a remarkably complete compilation on
`the subject. And his decade of teaching at Behrend College and Gannon
`University is reflected herein by clarity of expression, systematic organi-
`zation of materials, and penetrating analyses. Combined with the com-
`plete exposition, this volume is “must” reading for all in the plastics field
`
`0011
`
`

`

`Foreword
`
`XI
`
`and most assuredly for those in thermoforming or with an interest therein.
`I thoroughly enjoyed reviewing the manuscript and to it assign my
`highest recommendation.
`
`R o b e r t K. Jo r d a n
`Director—^Metalliding Institute
`Director—Engineering Research Institute
`Scientist in Residence
`Gannon University
`University Square
`Erie, PA 16541
`
`0012
`
`

`

`Taylor & Francis
`Taylor & Francis Group
`http:/ItayIorandfrancis.com
`
`0013
`
`0013
`
`

`

`Preface
`
`Th e d i v e r s i t y o f equipment and the multitude of parts formable by
`
`these methods make it difficult to describe thermoforming compre-
`hensively. Therefore, for a long time the subject of thermoforming
`appeared only in articles for periodicals or short chapters in handbooks.
`In 1987 nearly simultaneously three books came on the market solely
`devoted to thermoforming. The book by J. Florian, Practical Thermo-
`forming: Principles and Applications, is published by Marcel Dekker,
`Inc. New York (1987) and James L. Throne’s Thermoforming by Carl
`Hanser Verlag (1987). The latter book recently was expanded to nearly
`900 pages and appeared in 1996 by Hanser/Gardner I^blications, Inc.
`under the new title Technology o f Thermoforming. This book contains a
`very large collection of data tables and charts and covers mathematically
`any aspect of thermoforming. Numerical examples guide the less mathe-
`m atic^y inclined reader to their useful appUcation. Another way to
`acquire knowledge about thermoforming is to attend seminars or confer-
`ences on thermoforming, which are regularly scheduled at different
`locations by the Society of Plastics Engineers, Inc., Brookfield, CT.
`The first edition of the book Thermoforming—A Plastics Processing
`Guide appeared in 1987 by Technomic Publishing Company, Inc. (Lan-
`caster, PA). As the subtitle indicated, this book was mainly addressed to
`the practitioner who is or plans to get involved in the production of
`thermoplastic parts outside the ubiquitous injection molding process.
`Every effort has been made to express material properties and behaviors
`in language understandable by both the specialist and the neophyte.
`Therefore, in general, all plastic materials are referred to by their generic
`names. However, a list of trade names (Appendix C) is provided for those
`more accustomed to using commercial terminology.
`Thermoformed parts have become important in two main areas: (1)
`structural and functional parts and (2) low-cost, high-performance pack-
`aging applications. There is no doubt that significant advances in both
`areas are yet to come. This guide made an effort to describe the thermo-
`forming processing conditions with particular attention to behavioral
`changes in the plastic materials, inie book portrayed the types and
`xiii
`
`0014
`
`

`

`XIV
`
`Prrface
`
`operational sequences of standard equipment, the properties of normally
`used raw materials, and the peculiarities and advantages of major and
`minor forming processes. Also examined were cognate forming proc-
`esses that compete with thermoforming.
`This guide is intended to be comprehensive, a goal whose attainment
`is limited by the breadth of the field and the rapid evolution of new
`products. Several areas of intense development have been emphasized
`in the original book and have bœn further expanded and brought up to
`date for the second edition. These are as follows: (1) The vast expansion
`in regard to the availability of polymeric materials. This relates not only
`to the widening in copolymer and alloy compositions but also—as seen
`especially in polyolefíns—^in the increased multitude of polymerization
`processes. (2) The remarkable expansion of the materials’ properties has
`further been amplifíed by the application of orientation and crystal-
`lization processes. (3) V^ere desirable properties have not become
`obtainable by these two means, one has found ways to obtain them by
`laminations or coextrusions of different materials or by surface treat-
`ments. These types of materials can be expected to gain the fastest growth
`rate in barrier packaging materials.
`After studying the main chapters, the reader should glance at Chapter
`Twelve, “Related and Competing Forming Processes.” At that stage,
`imagination plus the information derived from a few experiments should
`enable the reader to envision the most efficient processes and operational
`stages. It is hoped that the reader can join the successful businesses that
`use expedient production methods to manufacture thermoformed parts
`at favorable costs. Many of these methods were considered impossible
`by experts 25 years ago.
`The limitations and disadvantages of thermoforming processes are
`explicitly discussed in this book, not to discourage, but to sharpen the
`technician’s mind in fînding ways to minimize them.
`Some tables and an extended index are found on the concluding pages
`to help the casual reader fínd numerical data and additional information
`on subjects of interest. The reader should be cautioned that, in many
`cases, wide variations exist between materials supplied under the same
`generic names. It is therefore, always advisable to obtain data sheets from
`the materials’ manufacturer for the specific grade of plastic to be used.
`
`0015
`
`

`

`Introduction
`
`Th e r m o f o r m i n g w i l l a l w a y s represent only one part of all the
`
`possibilities for shaping plastics items, but the total market is so large
`that if only a fraction of it goes to thermoforming, sizeable business
`opportunities exist. In 1997 the growth rate for the next 5 years is
`anticipated to maintain 4 to 6%. The Business Communications Co., Inc.
`(Norwalk, CT) writes that “packaging of all types is a $100 billion
`business in the U.S. and that the market value of plastics used to package
`medical, pharmaceutical and specialty health-care products was $801
`million in 1995.” The Rauch Guide (Impact Marketing Consultants,
`Manchester Center, VT) to the U.S. Plastics Industry states that “the U.S.
`sales of plastic resins and materials reached $150.5 billion in 1996.
`Among the several (five) technological advances that will contribute to
`future growth of plastics, the further development and use of barrier
`plastics for packaging applications occupies the first rank, the expansion
`of coextrusion film in flexible end uses were mentioned as second.” Both
`are pointing to a respectable growth in thin film thermoforming. The
`third reason mentioned, “the further development of plastic blends,
`alloys and copolymer technology,” will attain benefits also for the
`heavy-gauge thermoformer.
`The various thermoforming processes are based on the recognition that
`rigid thermoplastics become phable and stretchable when heated but will
`return to their original rigidity and strength when cooled. For most
`plastics molding processes the temperature of the material is raised until
`it turns into a liquid-like but highly viscous material. This makes it
`possible to achieve any shape as long as a suitable mold can be built for
`it.
`
`In thermoforming we begin with an already preformed part, in most
`cases, a thermoplastic sheet or film. We limit the supply of heat so that
`the plastic to be formed becomes highly flexible and stretchable but still
`retains sufficient strength to withstand gravitational force. Only rela-
`tively weak forces are required to make the sheet conform to the surface
`of a mold. In most cases a fraction of the atmospheric pressure is
`sufficient to do all the forming. This can be readily attained by utilizing
`
`0016
`
`

`

`Introduction
`
`a partial vacuum between the sheet and the mold or by pressurizing an
`air chamber to which the sheet has been sealed. In either case the air
`passes through small air holes distributed over the mold surface. The
`most widely used thermoforming processes are commonly designated as
`vacuum forming. There are many thin-walled articles that cannot be
`produced profitably by any other way than thermoforming.
`Two further steps are required to complete production of a useful part.
`The formed sheet must be cooled to become rigid again, and, finally, the
`excess material around the circumference must be trimmed off. The
`cooling and heating processes represent the most time-consuming steps
`and must, therefore, be painstakingly arranged.
`The first important plastic utilized in the thermoforming process—as
`in injection molding—was celluloid, a highly flammable cellulose ni-
`trate-based plastic, which many years ago was made obsolete by more
`suitable alternatives. The twin-sheet forming process was used, in which
`two sheets of celluloid were mounted between two complementing or
`identical mold halves. These sheets were then heated and expanded into
`the molds by pressurized steam. The objects produced consisted primar-
`ily of children’s toys and small low-cost containers.
`Unlike all other plastic forming processes, thermoforming can be
`adapted to so many variations that it becomes difficult to cover all aspects
`in a single text. If all the practiced process combinations were described,
`too many details would have to be repeated.
`In most other processes for forming plastic parts, the possibilities for
`variations of the part’s wall thickness lie within quite narrow ranges. This
`is especially true for injection molding and blow molding. For thermo-
`forming no such limitations exist. A thin film, 0.001” thick, as well as a
`sheet r thick can be thermoformed. Because the time frames for heating
`and cooling in these instances can vary more than 1000-fold, it should
`become obvious that many of the other processing details would vary
`also. Therefore, thermoforming has become divided into thin-gauge (less
`than approximately 1/16”) and heavy-gauge (greater than approximately
`1/8") thermoforming processes. For practical purposes this division
`should rather be made between processes utilizing a film fed to the
`former continuously from a roll or an extruder on one hand and a process
`in which a precut flat sheet is individually clamped in a frame and
`formed. The resulting products similarly can be parted into mass-pro-
`duced, nearly no-cost, thin wall, short-lived packagings, and the struc-
`turally strong, durable, utilizable objects or parts of appliances.
`
`0017
`
`

`

`Heating of the Plastic
`
`Th e h e a t in g p r o c e s s can be one of the most time-consuming steps
`
`in thermoforming. Although the scientist isolates heat transfer into
`three distinct phenomena—conduction, convection, and radiation—^in
`practice they will mostly be concurrent. The differentiation between thin
`film and heavy sheet thermoforming is in no other processing step more
`evident than during the heating of the plastic. TTie peculiar physical
`properties of plastics dictate the appropriate selection to obtain the most
`efficient heating method.
`In June 1995 a 42-page report on “Heating Technologies for Thermo-
`forming” was issued by the Electric Power Research Institute (Center
`for Materials Fabrication, Columbus, OH). It presents a comprehensive
`overview of present infrared heating systems.
`
`Means of conveying heat to the plastic
`
`Contact with a uniformly hot metal plate, which is conduction heating.
`is preferred for the heating of very thin films. Heating is accomplished in
`a fi-action of a second, which is of great importance for forming biaxially
`oriented films. This method is used especially for the mass production of
`smaller, thin-walled articles and is called trapped sheet heating because
`the sheet is held to a hot plate by a slight vacuum or by applying a slight
`air pressure from the outside. Such heating plates consist mainly of
`aluminum and have a nonstick fluorocarbon polymer coating. They
`contain a number of small holes and are generally located directly above
`the mold but could also be used as sandwich heaters. Conduction heating
`provides the most uniform and consistent temperature distribution in film
`or sheets and by far the best energy efficierKy. Where sandwich contact
`heaters are used, instant heating takes place and gauge fluctuations of the
`material will have no effect on the forming process.
`Due to the low infrared absorption of polyolefins and especially the
`fluoropolymers only a small amount of radiant energy would be utilized
`if these thin films were heated by radiation.
`
`0018
`
`

`

`Heating of the Plastic
`
`Steam heat represents an ideal convection heat source both in regard
`to heat output and uniform temperature distribution. Its convection heat
`transfer coefficient is 1000 times greater than that of air. However, the
`application of steam as a heat source is limited to cases where the
`generated condensed water poses no problem and where this very limited
`temperature range (212°F) can be tolerated. Presently, steam heat domi-
`nates the production of polystyrene foam parts and foam boards. Steam
`heat had its day at the beginning of thermoforming, when toys etc. were
`fabricated from celluloid using the twin-sheet steam injection process
`that provided heat and pressure simultaneously. It is no longer utilized
`for thermoforming, because most of today’s plastics require higher
`temperatures.
`Present-day convection heating of plastics is accomplished in two
`different ways. Heating the plastic by submerging it in a heat transfer
`liquid is very efficient due to the 100 times higher convection heat
`transfer coefficient of liquids versus air. However, its use is restricted to
`heavy-gauge parts because the removal of the liquid poses problems. Its
`main advantage is that here also a very uniform heat distribution can be
`obtained in a relatively short time.
`Air convection ovens are used extensively for heavy sheets because
`they provide quite uniform heating. Such ovens, when set at lower
`temperatures, are also suitable for drying sheets that have been exposed
`to moisture. If the temperature of these ovens is kept at the forming
`temperature of the sheet, heating times are long, but a wide margin of
`safety in regard to heating time variations is obtained. Because the
`effective convection heat transfer coefficient from turbulent air to solid
`plastic is low (0.002 to 0.010 W/sq in. x °F) and the heat conductivity of
`the plastic is also very low, heating times are long and increase with the
`square of the sheet tMckness.
`With all these methods, a considerable preheating time is required for
`the equipment before the heat can be transferred to the first piece of
`plastic. On the other hand, some infrared radiation heating equipment
`can supply instant heat and, therefore, needs only to be turned on for the
`seconds the plastic sheet is heated.
`Microwave heating methods have found only few applications in
`thermoforming due to the great expense of such equipment and the
`difficulty of obtaining satisfactory heat distribution over large areas.
`Besides, dielectric heating is unsuitable for most thermoplastics. Al-
`though heating times can be cut to 10%, the extended cooling time
`requirements would still remain unchanged. Internal heating methods
`are applicable to forming processes where only localized heat is required,
`e.g., edge forming of materials having high dielectric loss factors, such
`as polyvinyl chloride sheets.
`
`0019
`
`

`

`Physics o f radiation heating
`
`Physics of radiation heating
`
`Radiation heating occupies the largest share of plastics heating for
`thermoforming. The energy density, which can be transferred by infrared
`radiation from the heat source to the plastic sheet, depends on many
`variables. The higher the radiation energy density—usually expressed in
`watts per square inch—the less heating time will be required. The limit
`for heat flux for thin films is approximately 0.02 to 0.05 W/sq in. x °F
`to obtain short heat-up times. For films the heater times increase propor-
`tionally with thickness. Due to the low thermal conductivity of the plastic
`the surface temperature of thick sheets may quickly surpass the thermal
`stability of the plastic, making it necessary to lower the radiation energy.
`This can be achieved on roll-fed thermoformers by lowering the wattage
`to the second heater bank or on individually heated sheet formers by
`reducing the tubular quartz heater output in one or two steps. Slow-re-
`sponse heaters can be kept throughout the whole heating cycle only at
`the lower energy setting.
`The surface temperature of practical radiant heaters ranges between
`600 and 1800°F. Figure 2.1 illustrates the relative emissive power
`radiated by an ideal black body surface with an emissivity of 1. In Figure
`2.1 the areas enclosed between the baseline and the respective tempera-
`ture curves express the amount of energy emitted by an ideal infrared
`heater. The heat energy of available heaters may amount to only 75 to
`95% of that amount due to lower emissivities.
`As a general rule for assessing emissivity values, it can be stated that
`all heater surfaces have a high value of approximately 0.95. This in-
`cludes, besides oxidized stainless steels, other oxidized metal surfaces,
`such as iron, nichrome wire, and galvanized steel. Glass and ceramic
`surfaces, including white glazed porcelain, exhibit equally high values.
`On the other hand, highly polished metal surfaces are usually at or below
`0.05 and can, therefore, advantageously be utilized as reflectors of heat
`radiation. All metals, including gold, silver, copper, aluminum, nickel,
`and zinc, fall into this group. The difference lies in their capability of
`retaining the shiny polished surface. Absorptivity should theoretically be
`identical to the emissivity, but it relates in thermoforming processes to
`the cold materials exposed to radiation. Therefore, different temperatures
`and wavelength ranges dominate. Most nonmetallic materials have a
`value of 0.90, and this applies to wood, glass, plastics, textiles, and paints.
`There is only a small difference between a black and a white paint or
`plastic because the binder resin or polymer controls the absorption.
`It can be seen that at higher temperatures the bulk of radiation occurs
`at lower wavelengths, 3 to 4 microns. By contrast, at lower temperatures
`the considerably lower radiation energy is spread out over a broader and
`
`0020
`
`

`

`Heating of the Plastic
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`F ig u re 2.1. B lack b o d y rad iation (co u rtesy o f Irco n , In c., N iles, IL 606 4 8 ).
`
`W AVELENGTH. M IC R O N S
`
`higher wavelength range, 4 to 7 microns. This is important when ther-
`mofoiming thin sheets or films because each plastic material absorbs
`infrared radiation in distinct regions. Only absorbed radiation is utilized
`for heating the plastic directly. The remainder will just heat up the oven
`enclosure and thus indirectly heat the sheet or will be lost to the outside.
`The portion of radiation that is reflected by the sheet is insignificant
`and nearly independent of the wavelength and naturally independent of
`sheet thickness. Unless one wishes to thermoform a metallized plastic
`
`0021
`
`

`

`Physics o f radiation heating
`
`sheet, reflection losses, which normally account to no more than 4%, can
`be ignored. Energy losses due to transparency in certain wavelength
`regions, however, can be signiflcant for very thin films. From transpar-
`ency in the visible region one cannot conclude whether the sheet will be
`opaque or transparent at the frequency of maximum heat radiation. As
`seen in the graphs of Figure 2.2 (pages 8 and 9), transparency increases
`markedly as the thickness of the film decreases. Sheets of 1/8" thickness
`will absorb practically all infrared radiation. On the other hand, thin films
`of less than 5 mils will be inefficiently heated when irradiated in certain
`infrared regions, e.g., fluorocarbon polymer films are transparent up to
`7.5 microns. This is similarly true for thin polyolefin (polyethylene,
`polypropylene) films. In these cases radiation passes through the film
`and heats the surrounding chamber, which then indirectly heats the film
`at a higher wavelength. But the rise in heater temperature results in higher
`energy losses. On the other hand, a radiation in a less suitable frequency
`range may be applied to advantage by letting the energy penetrate deeper
`into the sheet material, thus avoiding scorching the sheet surface and
`making it possible to irradiate at a higher wattage.
`The relationship between heater surface temperature and heat energy
`transfer can be estimated using the Stefan-Boltzmann relationship:
`
`E = a t
`
`^heater
`
`100 )‘- ( M
`. W
`.
`Btu
`.
`.
`^
`E = Radiation energy per unit area