®
`
`DuPont Engineering Polymers
`
`Electrical/Electronic
`Thermoplastic Encapsulation
`
`Toroids from Standex Electronics
`Solenoid by Dormeyer
`Step Motor from Pacific Scientific
`
`Start
`with DuPont
`Engineering Polymers
`
`® DuPont registered trademark
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`1 of 20
`
`FITBIT EXHIBIT 1026
`
`

`
`Introduction
`Electrical coils and components have for some time
`been encapsulated or potted with thermosets for protec-
`tion from operating environments and to provide elec-
`trical insulation and thermal dissipation. With their
`good electrical properties, mould flow characteristics,
`and low cost, thermosets such as epoxies, phenolics,
`and thermoset polyesters were until recently virtually
`the only encapsulation resins used in coil / component
`encapsulation.
`However, encapsulation materials are now shifting in
`the direction of thermoplastics. This is occurring for
`reasons of:
`• productivity and component integration;
`• better physical properties of thermoplastics in thin
`sections compared with thermosets;
`• extensive IEC 85 and UL 1446 Electrical Insulation
`Systems recognitions for thermoplastics for encapsu-
`lated motors, solenoids, and transformers;
`• the virtual elimination of volatile organic compounds
`(VOCs) generated in thermoset potting or encapsula-
`tion.
`
`Environmental Considerations
`Thermoplastic encapsulation processes use only solid
`materials that become a melt when heated. The volatile
`organic solvents used in varnish impregnation are not
`present, so environmentally harmful solvent emissions
`are eliminated. Also, parts encapsulated with thermo-
`plastics come out of the moulds ready to assemble
`without requiring the inherently “dirty” deflashing or
`trimming operations so often associated with ther-
`mosets. The high-impact strengths of engineering ther-
`moplastics in thin sections compared with thermosets
`also contribute to lower cost because there is signifi-
`cantly less part breakage.
`When thermoplastic parts do break or for some reason
`are not usable, they can be ground up, remelted, and
`used again in the moulding process. Many of the DuPont
`engineering thermoplastic resins used in encapsulation
`have been approved by UL to be moulded using up to
`50% regrind.
`
`Global Availability
`With technology and production increasingly becoming
`globally sourced, it is important to use encapsulation
`resins that are available worldwide and are recognized
`to internationally important standards, e.g., UL and
`IEC. The DuPont resins discussed here are generally
`available worldwide and meet the leading national and
`international standards.
`
`Fig. 1. Hydraulic cartridge valve from HydraForce. Coil form
`in RYNITE® 530 (Foremost Plastic Products Co.). RYNITE®
`encapsulated solenoid produced by Warsaw Coil Co.
`
`Cost Comparisons with Thermosets
`On a weight basis, thermosets can cost substantially
`less than thermoplastics. However, any cost compari-
`son between the two material types must be done on
`the basis of “finished encapsulated part ready for ship-
`ment.” Savings found in faster moulding cycles, higher
`product yields, and fewer secondary operations gener-
`ally favor thermoplastic encapsulation over thermosets.
`This is particularly true in high-volume automotive
`component operations and in encapsulated coils and
`components where brackets and connectors can be
`moulded-in during the encapsulation step as value-
`added features.
`The inherently better toughness of engineering thermo-
`plastics used in encapsulation compared with thermo-
`sets allows use of thinner walls (Table 1, page 3).
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`Fig. 2. Solenoid coil encapsulated with ZYTEL® glass-reinforced
`PA66 nylon resins (ATR).
`
`Fig. 3. Antenna coil for automotive auto-theft system.
`Coil form and encapsulation in ZYTEL® 77G33 HS1L
`(Standex Electronics).
`
`Table 1 Comparative Impact Strengths of Selected Thermosets and Engineering Thermoplastics
`30% Glass-
`Reinforced
`FR PET
`Thermoplastic
`Polyester
`91,0
`
`Thermoset
`Polyester
`(Electrical)
`53,4
`
`33% Glass-
`Reinforced
`PA66
`Thermoplastic
`Nylons
`107
`Dry as Moulded
`
`Izod Impact J/m
`ASTM D256
`
`Phenolic
`Electrical
`15,5
`
`Alkyd
`17,1
`
`The impact strength of thermoplastics is also a factor
`in the UL 1446 / IEC 85 Insulation Systems recognitions
`now required in many encapsulated solenoid, motor, and
`transformer applications. Such recognitions are listed by
`thickness of the encapsulation layer. The required test-
`ing (see page 17) includes a vibration step that favours
`the stronger thermoplastics over thermosets in the thin-
`ner sections.
`Thermoplastics also generally mould faster than thermo-
`sets. Cycle reductions of over 50% are frequently found
`in moving to thermoplastics. Thermoplastics are also
`more stable in contrast to thermosets, many of which
`have shelf lives of six months or less. Some thermoset
`grades must even be refrigerated during storage before
`moulding. Thermoplastics are also more easily coloured,
`either through cube blending with concentrates or
`through full compounding. Some thermosets do, how-
`ever, offer two advantages compared with thermoplas-
`tics. Epoxies, for example, can be used to impregnate
`thin-wire, high-voltage coils before setting up, and
`
`some thermosets transfer heat better than conventional
`thermoplastics. Recently developed thermally conduc-
`tive thermoplastics used in transformer encapsulation
`are even better in heat transfer. The thinner walls and
`encapsulation layers made possible using thermoplas-
`tics also help with heat transfer (see pages 13–15).
`
`Moulding and Tooling Techniques
`Thermoplastic encapsulation is basically an insert
`moulding operation. The wound coil or electrical com-
`ponent is inserted into the mould, and the thermoplastic
`material is injected while lead wires or terminals are
`clamped off from the resin flow. The object being
`encapsulated can be held either with stationary pins
`or hydraulic pins that are withdrawn before the melt
`freezes. This latter technique was developed by DuPont
`engineers to encapsulate golf balls with SURLYN®
`ionomer resins and is now used extensively in the
`encapsulation of sensors and transformers (see
`Figures 4, 5 and 6).
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`
`While some encapsulation is done using horizontal
`moulding equipment, the preferred encapsulation
`moulding process is based on vertical clamp machines,
`which use gravity to hold the coil or insert in place
`during the moulding process.
`The use of shuttle or rotary table moulding presses with
`two or more lower mould halves leads to maximum
`productivity. While a device is being encapsulated at
`the moulding station, an operator or a robotic device
`can remove finished parts and insert coils at the load-
`ing / unloading station(s).
`
`Moulding machines that are convertible from horizontal
`to vertical modes of operation are also available (see
`Figure 5). This allows, for example, solenoid coil bob-
`bins to be moulded in the horizontal mode and solenoid
`encapsulation to be run in the same press in the vertical
`mode. In each case, care should be taken to size the
`barrel carefully to the shot size and the clamping pres-
`sure to part surface area.
`
`Holding Pins Forward
`
`Holding Pins Retracted
`
`Fig. 5. Vertical-clamp injection moulding machines are ideal
`for encapsulation. Unit shown is located at the DuPont
`Application Technologies Center, where it is used for
`development and testing of encapsulation materials,
`tooling, and methods. This unit is also convertible to
`horizontal operation.
`
`Fig. 4. “Golf ball encapsulation” using hydraulic pins. As the
`molten polymer fills the cavity, the hydraulic pins are
`retracted, leaving behind a perfectly round, encapsulated
`golf ball. This same technique is used in the encapsula-
`tion of sensors and transformers.
`
`Fig. 6. “Golf ball encapsulation” showing uniform wall sections
`achieved with the use of hydraulic pins (DuPont).
`
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`Productivity Tip
`For faster cycling of bobbin moulds, we recommend
`core pins made of Ampcoloy* 940 copper alloy. The
`core pins generally do not have internal cooling; but
`because Ampcoloy 940 conducts heat six times faster
`than tool steel, the pins usually run only 1,6 to 2,2°C
`hotter than the rest of the mould. This leads to shorter
`cycle time than with core pins made of tool steel.
`
`Tooling
`Tooling for encapsulated coils and components consists
`of two different types. The first is for the coil forms used
`in many encapsulated applications. The second is for
`the encapsulation process itself.
`In designing and moulding coil forms, great care must
`be taken to produce fully crystallized products having
`uniform flanges that are tapered slightly for ease of
`part ejection. Uniform flanges are important to help
`safeguard against voids or distortion caused by the
`shrinkage of the coil form during the encapsulation
`process.
`A modular type of mould can save time and expense
`in making and modifying prototype bobbins. DuPont
`engineers developed the design shown in Figures 7
`and 8 using a mould frame from Master Unit Die Prod-
`ucts, Inc. It combines a standard frame, side-action
`mechanism and cooling system with provisions for
`interchangeable cavity inserts having their own cooling,
`side-action wedges and various other components that
`differ according to bobbin size and configuration.
`
`Fig. 9. Jim Patterson and Tom Boyer of DuPont Engineering
`Polymers setting up an encapsulated coil moulding trial.
`
`Gating
`To minimize coil distortion, it is essential to ensure
`equal pressure on windings by filling the mould cavity
`through two or more gates (Figure 13).
`
`Gate Design
`The rounded-end design of tunnel gate (Figure 15)
`is a necessity for successful encapsulation. It prevents
`premature freezing of material at the gate, permits
`effective packing out to compensate for shrinkage in
`the relatively thick wall sections often used for encap-
`sulated components, and ensures a good surface finish.
`
`* Registered trademark of Ampco Metal, Inc., U.S.A.
`
`Fig. 7. Modular mould for solenoid coil bobbins.
`
`Fig. 8. Modular mould for solenoid coil bobbins (continued).
`
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`Venting
`Good venting on the mould’s parting line ensures max-
`imum strength in weld line areas as well as preventing
`surface defects. It is also helpful to vent the runners to
`reduce mould deposit.
`
`Fig. 13. Modular mould for encapsulating a solenoid coil
`in a vertical-clamp press (top view).
`
`Fig. 10. Automotive engine temperature sensor encapsulated
`with ZYTEL® glass-reinforced PA66.
`
`Fig. 11. Danfoss magnet valve encapsulated in CRASTIN® PBT
`polyester.
`
`Fig. 14. Modular mould for encapsulating a solenoid coil
`in a vertical-clamp press (side view).
`
`Fig. 12. Electrovalve encapsulated with CRASTIN® T805 PBT
`polyester (CEME, Italy).
`
`Fig. 15. Gate design for encapsulation recommended by DuPont.
`
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`
`Encapsulation Applications
`Thermoplastic encapsulation is used in many applica-
`tions that require some special manufacturing techniques
`and materials of encapsulation. These include solenoids,
`sensors, self-supporting coils, transformers, motors,
`and electronic components of various types.
`
`Solenoids
`Solenoids are generally made by encapsulating coils
`wound on coil bobbins (Figure 16). The number one
`requirement for coil bobbins used in solenoids is that
`they be fully crystallized, because the subsequent hot
`encapsulation process can cause additional coil bobbin
`shrinkage and distortion such as the “bubbles” shown
`in Figure 17. To achieve the full crystallisation needed,
`the typical minimum recommended mould temperatures
`for coil bobbin moulding are 100°C for RYNITE® PET
`thermoplastic polyester resins and 80°C for ZYTEL®
`PA66 nylon resins.
`Level or precision-wound solenoid coil winding is rec-
`ommended whenever possible (Figure 18). This allows
`smooth flow of the encapsulation material over the sur-
`face of the windings. Random winding can cause flow
`problems and magnet wire bunching except in those
`coils using very fine wire.
`
`Fig. 16. Cadillac load leveling system solenoid in ZYTEL® 70G33 HS1L
`(top) being produced by René Miller and Joey Champion
`of Multicraft Electronics (bottom).
`
`7
`
`Fig. 17. Result of encapsulating a wound coil bobbin that is not
`fully crystallized.
`
`Fig. 18. Smooth surface of level-wound coil eases flow during fill
`of encapsulation mould and avoids problems of bunching
`of magnet wire.
`
`Taping wound coils prior to encapsulation is not neces-
`sary and is not recommended. Also, it is important that
`good quality magnet wire be used in all encapsulated
`coils. Any defect in the coating of the magnet wire
`used will be magnified by the hot melt temperature,
`high pressure, and shearing action of the thermoplastic
`encapsulation process.
`The metal cans used in many cases with solenoids for
`magnetic flux reasons are sometimes used as a “shell”
`into which the thermoplastic encapsulant is injected.
`Alternatively, the can may be included in the encapsu-
`lation layer.
`Proper termination of the wound magnet wire on pins
`inserted into the bobbin flange is also important.
`The encapsulation process can cause coil malfunctions
`if terminations are loose.
`
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`
`Fig. 20. Double solenoid for fuel pump flow-meter encapsulated
`in CRASTIN® T805 (Sirai).
`
`Fig. 21. Yumiko Nakaniwa and Hajime Shiozawa of DuPont KK
`(Japan) testing the insulation resistance of encapsulated
`solenoids.
`
`Fig. 19. Appliance solenoid coil encapsulated with RYNITE® PET
`polyester (Dormeyer).
`
`The types and thicknesses of magnet wire enamels are
`important for successful solenoid manufacturing.
`For example, thermoplastic-overcoated, polyurethane-
`enameled wire works far better in thermoplastic encap-
`sulation than wire coated only with a single layer of
`polyurethane. Also, the thermoplastic encapsulation
`should be matched to the solenoid end-use environ-
`ment. For example, in some automotive applications,
`polyamides function far better than polyesters. Solenoid
`terminals can be encapsulated in place using a pre-
`moulded insert. The high strength of the engineering
`plastics used in solenoid encapsulation help retain the
`terminals in place as well as permit compact solenoids
`to be made with encapsulation layers significantly thin-
`ner than is possible with thermosets.
`Types of encapsulating resin used, moulding conditions,
`and coil bobbin flange design are all key to manufac-
`turing quality solenoids. In extensive testing at our
`Yokohama, Japan, research facility, different solenoids
`having various resin combinations of coil bobbin and
`encapsulant are tested by first heating them at 80 °C for
`an hour, immersing them for an hour in a 0°C, 5%
`NaCl solution, rinsing them off, and then measuring
`solenoid insulation resistance values. From this work,
`we are able to tailor encapsulation resins to meet
`specific application requirements (Figure 21).
`
`Fig. 22. Deltrol Controls solenoid with a board mounted bridge
`rectifier chip encapsulated with RYNITE® 415HP.
`
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`
`Our testing also shows that minor changes in the flange
`design of solenoid coil forms can improve solenoid
`performance dramatically. For example, switching
`from a flange design with straight ends to one with
`tapered edges leads to dramatically better adhesion
`between coil bobbin and thermoplastic encapsulant,
`because the tapered flange tip melts more easily during
`the hot encapsulation step than does a straight flange
`edge (Figures 23 and 24).
`
`Sensors
`The rapid proliferation of electrical and electrical-
`mechanical systems in automotive, appliance, and
`industrial applications has dramatically increased the
`demand for sensors. Sensors are used in such systems
`to measure variables such as speed, position, tempera-
`ture, or fluid level. Many are encapsulated to provide
`insulation and to protect them against moisture, dirt,
`or mechanical damage. An example is the wheel speed
`sensor, shown in Figure 25, encapsulated with a glass-
`reinforced PA66, ZYTEL® 70G33 HS1L. Automotive
`sensors like this one used to require a two-step process:
`potting the coil in epoxy, followed by overmoulding.
`Today, the same part is encapsulated in a single step.
`Note that the metal inserts in the mounting flanges are
`moulded in. The cable is provided with a grommet
`and the encapsulation mould is designed to support
`the grommet and prevent flashing down the cable.
`
`Fig. 23. Vertical flange.
`
`45°
`
`30°
`
`Fig. 24. Tapered flange.
`
`Thinner, tapered coil bobbin flanges improve solenoid performance
`by melting more easily during encapsulation and forming a strong
`bond with the encapsulant upon cooling. The flange design shown
`in Fig. 24 is far superior to that in Figure 23.
`
`Finally, solenoids used in appliances and some control
`systems may have to meet both UL94-V0 flammability
`and UL 1446 and IEC 85 Electrical Insulation Systems
`standards. It’s important to recognize that both standards
`have thickness guidelines. Meeting these standards
`requires that both coil form thickness and encapsulation
`layer thickness be taken into account. (See Table 3,
`page 18.)
`
`Fig. 25. Wheel speed sensor encapsulated with glass-reinforced
`ZYTEL® PA66.
`
`Another major type of sensor being used in increasing
`numbers is the “Hall Effect” or electronic sensor.
`This type of sensor is used in ignition and transmission
`control applications as shown in Figure 26.
`The primary challenge in encapsulating “Hall Effect”
`sensors is to avoid damage or displacement of their
`integrated circuit chips, other delicate components, and
`connections among them. In relatively simple sensor
`constructions, success has been achieved using con-
`ventional polyester resins and slow injection rates.
`The hydraulic pin configuration discussed (page 4)
`in golf ball encapsulation is also used in encapsulating
`electronic sensors.
`
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`
`For encapsulated sensors having to take very high tem-
`perature spikes, e.g., during soldering processes, both
`ZYTEL® HTN high temperature nylon and ZENITE™
`liquid crystal polymers (LCP) can be used. (Some
`additional sensors are shown below in Figures 27, 28
`and 29.)
`
`Fig. 26. Typical “Hall Effect Sensor.”
`
`Glass-reinforced (GR) PA66 resins are widely used
`in sensor encapsulations. However, in more complex
`constructions involving multiple components, crimped
`connections, or other delicate assemblies, prototype
`work has shown that (GR) PA612 can produce even
`better encapsulated sensors than those encapsulated
`with either (GR) PA66 or PBT polyester. The reason is
`that the (GR) PA612 has a slower crystallisation speed
`than either of the other two resins. It also has higher
`flow and lower moisture absorption than (GR) PA66
`and excellent elongation at low temperatures. The net
`result is an encapsulated sensor with much better adhe-
`sion between coil bobbin and encapsulation layer and
`better resistance to thermal shock cycles.
`
`“Wire friendly nylons”
`The abrasion of magnet wire enamel during winding
`or handling can open the way to electrolytic corrosion
`of the wire. Given the right conditions of moisture,
`temperature, time, and certain additives contained
`in many thermoplastic resins, electrolytic corrosion
`of any exposed conductor wire can occur, sometimes
`quite rapidly. However, as a practical matter, wire cor-
`rosion is rarely an issue for sensors that use 30 AWG
`or larger magnet wire because surface oxidation is
`insignificant compared to the volume of copper that
`must be corroded to cause failure. However, wires used
`in many sensors are much finer, typically AWG 39 to
`50, and these can be affected more easily.
`To prevent this destructive wire corrosion, new DuPont
`encapsulation resins called “Electrical / Sensor Resins”
`have been formulated for fine-wire encapsulations.
`They virtually eliminate the potential for electrolytic
`corrosion of magnet wire with punctured coatings.
`Both (GR) PA66 (ZYTEL® FE5389 BK-276) and (GR)
`PA612 (ZYTEL® FE5382 BK-276) resins of this type
`have been introduced for the encapsulation of sensors
`and other fine-wire electrical components (see pages 18
`and 19).
`
`Fig. 27. Motor “knock” sensor encapsulated in ZYTEL® 70G25.
`
`Fig. 28. Wheel speed sensor for ABS system encapsulated
`in RYNITE® 530 PET.
`
`Fig. 29. Temperature sensor for outside of car, encapsulated
`in DELRIN® 107 acetal resin.
`
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`
`Self-Supporting or Bonded Coils
`A particularly noteworthy type of coil now being used
`in thermoplastic coil encapsulation is the “self-support-
`ing coil.” Self-supporting coils are just that: they do
`not depend on an independent coil form for support.
`Where feasible, assembly costs are lowered and manu-
`facturability is enhanced (Figures 30 and 31).
`The self-supporting or bonded coils are formed by
`winding a bondable magnet wire on a mandrel. The
`coils are then charged electrically to melt the adhesive
`coating, compressed, cooled, and ejected from the
`rotary head winder used in their manufacture. A major
`supplier of such coils is Alcoils located in Columbia
`City, Indiana, U.S.A.
`
`Toroids
`Toroids used for power filtering applications are another
`type of coil well suited to thermoplastic encapsulation.
`Conventionally potted with epoxies in either thermoset
`or thermoplastic cups, toroids encapsulated instead
`with engineering thermoplastic resins can generally
`be produced more quickly at significantly lower
`costs. In the case of the Standex encapsulated toroids
`(Figure 32), encapsulation with ZYTEL® glass-reinforced
`PA66 or PA612 nylon resins is done in multiple-cavity
`tooling in less than a minute; epoxy potting in a thermo-
`set cup can take up to 24 hours of epoxy cure time.
`
`Fig. 32. ZYTEL® nylon resin encapsulated toroids for surface-mount
`applications (Standex Electronics).
`
`Motors
`Motors are another area in which thermoplastic encap-
`sulation is beginning to expand rapidly, particularly
`in stator insulation. Being replaced are the tapes, films,
`etc., used in conventional motor insulation. Complexity
`of encapsulation can vary from the small Hansen stator
`coils shown in Figure 34 to the larger and more intricate
`Cadac (Figure 35) and Pacific Scientific (Figure 36)
`encapsulated stators. Using the steel laminate covers as
`an insert, the stator is made in a one-step overmoulding
`operation. Encapsulation provides slot and end insula-
`tion, termination holders, contour supports, and guide
`posts for windings – all in a single moulding step.
`
`Fig. 30. Warner Electric’s “Mag Stop” clutch and brake solenoid
`for riding lawn mowers encapsulated with RYNITE® PET.
`The clutch solenoid is wound without coil bobbins, with
`the wire insulation being heat bonded to retain shape
`during encapsulation by Alcoils.
`
`Fig. 33.
`Steve Fecanin
`of DuPont Automotive
`working on another
`coil project.
`
`Fig. 31. Warner Electric’s self-supporting clutch coil for automotive
`air conditioners encapsulated in ZYTEL® 70G33 L.
`Interchangeable mould permits use of the same tool
`to encapsulate coils terminated with either leads
`or connectors.
`
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`
`Fig. 34. Encapsulated stator coil replaces a tape-insulated unit
`in a hysteresis synchronous motor that operates timing
`devices, vents, and valves in heating, ventilation, and air
`conditioning equipment. Both coil bobbins and encapsula-
`tion are in ZYTEL® PA66. Encapsulated coils withstand
`3000 V versus a test limit of 2200 V for the taped coils
`being replaced (Hansen Corp.).
`
`Fig. 35. When used in a washing machine, this very energy-
`efficient Cadac 1,3-HP (1-kW) DC motor eliminates the
`need for a gearbox. Stator components in RYNITE® PET are
`part of an UL 1446 Class F (155°C) Electrical Insulation
`System.
`
`Fig. 36. Key to the Pacific Scientific step motor encapsulation is the replacement of the aluminum rear end bell, an 8-pin connector and
`epoxy potting with RYNITE® 530 polyester in one moulding operation. This thermoplastic encapsulation also eliminates eight
`connectors and a circuit board, reduces production time from the 2 hours spent on epoxy potting to 45 seconds and forms an end
`bell that runs significantly cooler than the aluminum end bell used before.
`
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`
`UL flammability and insulation system recognitions are
`also as important in motors as they are in transformers.
`DuPont qualifies all of its open UL 1446 insulation
`systems using motorettes so that the results are applic-
`able to both motors and transformers. Encapsulated
`insulation systems are qualified using encapsulated
`coils for use in both solenoids and motors (see UL 1446
`insulation systems, page 17).
`
`Transformers and Lighting Chokes
`Encapsulating transformers and lighting chokes with
`thermoplastics is basically the same as for solenoids
`and sensors and generally follows the same principles.
`As in the encapsulation of solenoids and sensors, trans-
`former connectors or terminals can be moulded in. Coil
`forms with stepped configurations or the usual tapered
`flanges are used to ensure a secure bond with the
`encapsulation material, and coil windings are wound
`smooth to ease thermoplastic flow during encapsula-
`tion. Differences found in transformer encapsulation
`compared with the others include the large size of
`some of the encapsulated transformers, the use of con-
`ductive resins to remove heat from the coils / lamina-
`tions, and the use of thermoplastic compression mould-
`ing compositions to encapsulate transformers larger
`than 3 kVA. RYNITE® thermoplastic PET polyesters are
`generally used in transformer encapsulation because
`continuously operating transformers require encapsula-
`tion resins having higher relative thermal indices (RTI)
`than those of polyamides.
`
`While transformers and lighting chokes have been
`encapsulated with potting thermosets for years, the same
`needs for higher productivity, cleaner environmental
`processing, and better product designs driving the
`thermoplastic encapsulation of sensors and solenoids
`are also applicable here. The growing importance of
`UL 1446 insulation system recognitions is also a driv-
`ing force, because thermoplastics are able to achieve
`such recognitions at thinner encapsulation thicknesses
`than are possible using thermosets. Thermoplastic
`encapsulation also leads to a better locking of the
`lamination plates and less noise than can be achieved
`with thermosets.
`
`We divide encapsulated transformers into three classes,
`based on size: 0 to 500 VA, 500 VA to 3 kVA, and 3 to
`>100 kVA.
`
`0 to 500 VA
`These are relatively small transformers that are encap-
`sulated just like sensors and solenoids using conven-
`tional thermoplastics and moulding techniques. A good
`example is shown in Figures 37 and 38 from DuPont
`U.S. Patent 5,236,779. In comparing small, epoxy-pot-
`ted 20–50 VA transformers with the same transformer
`
`fully encapsulated with RYNITE® FR530, we found that
`the operating temperatures for the encapsulated trans-
`former were 10°C lower than those for the potted
`transformer. By adding a layer of thermally conductive
`polyester resin containing carbon, we were able to
`reduce the transformer operating temperature yet
`another 14°C.
`
`Fig. 37. This example from U.S. Patent 5,236,779 to DuPont shows
`the effect on transformer operating temperature by first
`encapsulating with RYNITE® PET polyester and then
`overmoulding that with a thermally conductive RYNITE®
`containing carbon.
`
`Fig. 38. Cross section of the transformer above.
`
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`
`While heat transfer through the RYNITE® PET polyesters
`is rather low at about 0,23 W/ m-K, Philips found in
`1988 that replacing the PA 66 encapsulating their 250 W
`and 400 W lighting chokes with RYNITE® 935 still
`resulted in a 2–3°C lower heat rise (Figure 41).
`To minimize the unit heat rise further, Philips lighting
`chokes are encapsulated only on five faces. The
`exposed laminations on the base face are attached
`to the metal base plate to maximize heat transfer.
`
`Fig. 39. Rakesh Puri, DuPont Americas Encapsulation Leader,
`planning another project.
`
`500 VA to 3 kVA
`Transformers of this size can be encapsulated through
`injection moulding; however, heat buildup begins to
`become a problem. One possible solution here is the
`use of conductive thermoplastics such as RYNITE®
`CR503 PET polyester which has a thermal conductivity
`of 1,5 W/ m-K, some six times that of normal RYNITE®.
`Because the conductivity is enhanced through selected
`carbon additives, these compositions are electrically
`conductive as well. Therefore, a transformer encapsu-
`lated with this resin has to be insulated electrically in
`various areas before overmoulding with the conductive
`layer. This is illustrated in Figure 38. Note, too, the use
`of a three-flange coil bobbin with the tapered flanges
`used to get a good bond with the insulating encapsula-
`tion layer of RYNITE® FR530 PET polyester (white).
`
`Resin selection for transformer encapsulation is parti-
`cularly important because of product requirements
`involving heat transfer, thermal cycling, insulation sys-
`tems recognitions, and long-term thermal stability. In
`addition, if both conductive and insulating resin layers
`are used, the interface between the layers must be void-
`free for optimum heat transfer. These requirements
`favor the polyesters over polyamides for transformer
`encapsulation.
`
`Fig. 40. Valentine Technologies Class II transformer with coil
`bobbins and encapsulation in RYNITE® FR 530 polyester.
`
`Fig. 41. Philips lighting choke transformers (250 and 400 W)
`for gas discharge lighting encapsulated in RYNITE® 935,
`a PET polyester with glass and mineral reinforcement.
`
`14
`
`14 of 20
`
`

`
`Also, a 3 kVA transformer contains 20 kg of coils and
`laminates and requires 2 kg of thermoplastic encapsu-
`lation resin. Dealing with this size of shot in injection
`moulding requires prohibitive investments in both tool-
`ing and moulding equipment. In addition, experiments
`carried out at DuPont show that it is unlikely that, for
`this size transformer, injection moulded encapsulation
`layers can survive heat cycles to 200 °C without crack-
`ing.
`In order to fulfill these requirements, DuPont has
`developed a proprietary thermoplastic, compression
`mouldable composite sheet (MCS) approach to the
`encapsulation of large transformers. In the thermo-
`plastic encapsulated transformer concept shown in
`Figure 43, the transformer is overmoulded with a layer
`of electrically insulating, thermoplastic SC 140 and
`a layer of thermally / electrically conductive SC 500.
`Properties of the DuPont thermoplastic MCS materials
`are shown in Table 2.
`
`Fig. 42. A 400-ton compression moulding press at DuPont’s Appli-
`cation Technologies Center (ATC) holding a mould and
`a 3 kVA transformer being encapsulated with SC 125
`mouldable composite sheet. (See U.S. Patents 5,236,779
`and 5,338,602 to DuPont).
`
`3 to >100 kVA
`The thermoplastic encapsulation of larger transformers
`(3 to >100 kVA) is particularly challenging for a variety
`of reasons. Incumbent dry-type transformers of this
`size go through heat cycles up to 200°C. Therefore,
`any thermoplastic encapsulation layer of such a trans-
`former has to be able to withstand significant thermal
`cycling without cracking and be able to dissipate heat.
`
`Metal Core
`
`SC 125 Coil Form
`
`Wire Windings with
`NOMEX®‚ Aramid Fiber
`Paper Interlayers
`SC 140 (Insulation layer)
`Overmoulded with
`Thermally Conductive
`SC 500
`
`Fig. 44. DuPont Distribution Transformer Encapsulation Team of
`Bob Ward, Dr. Lana Sheer, and Gary Kozielski examine an
`SC 500 encapsulated transformer and the ZYTEL® HTN coil
`bobbin from Miles Platts that goes into it.
`
`Fig. 43. Concept drawing of a thermoplastic encapsulated distri-
`bution transformer. Such transformers in the 100 kVA
`range are now under development.
`
`15
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`15 of 20
`
`

`
`Table 2 DuPont Mouldable Composite Sheet Preliminary Physical Properties (23°C)
`Property
`SC 125
`SC 140
`Thermal Conductivity (W/m-K)
`0,3
`0,3
`1 · 1016
`1 · 1016
`Volume Resistivity (ohm-cm)
`Tensile Strength (MPa), ASTM D638
`180
`226
`Elongation (%), ASTM D638
`2,3
`2,0
`Flexural Modulus (GPa), ASTM D790
`8,3
`11
`Specific Gravity, ASTM D792
`1,56
`1,69
`
`SC 500
`>3,4
`0,05
`64
`1,4
`9,4
`1,81
`
`Electronic Component Encapsulation
`Thermoplastic encapsulation of active electronic com-
`ponents such as integrated chips is extremely difficult
`and rarely done. The reason is that even at low pres-
`sures, thermoplastic encapsulation can easily result
`in fine wire distortions. Also, getting thermoplastics
`to penetrate tightly wound coils or very fine spaces
`is quite difficult. Components such as rectifier chips,
`resistors, etc., can be encapsulated.
`Some small assembled circuit boards have also been
`encapsulated with thermoplastics. However, this can be
`done only with thermoplastics that have melting points
`low enough that solder on the circuit boards is not
`affected and the electronic components are not damaged.
`An example of an encapsulation resin used here is
`DELRIN® acetal resin (Figure 45). Other resins that can
`be used here include the low melting point HYTREL®
`polyester elastomers.
`
`Housings for Potted Coils and Components
`For high-voltage coils used in such applic