11 Induction Welding

11.1 Process Description to hysteresis losses. As the magnetic field (B) increases,
the ferromagnetic material becomes magnetized, and
Induction welding, also called electromagnetic or the magnetic intensity (H) increases. As the magnetic
EMA welding, uses induction heating from radio field decreases, so does the magnetic intensity. How-
frequency (typically 2–10 MHz) alternating current to ever, there is a lag between the two, which results in a
magnetically excite an implant placed at the joint inter- hysteresis loop (Fig. 11.2). This phenomenon results in
face of the two parts being welded. This implant, or dissipation of energy in the form of heat.
gasket, is normally a composite of the polymer to be The induction welding process can be divided into
welded with either metal fibers or ferromagnetic parti- four main steps (Fig. 11.3):
cles. The heat generated melts and fuses the implant
with the surrounding material. It is a reliable and rapid Step 1: Placement of the Implant
technique, ranging from fractions of a second for small Implants are available in many forms, such as sheets,
parts to 30–60 seconds for parts with long (400 cm; extruded profiles, injection molded parts, tapes and
157 inches) joint lines, and results in structural, her- strands (Fig. 11.4), depending on the size and shape of
metic, or high-pressure welds [1, 2]. the parts to be welded, and on the position of the work
The two most commonly encountered mechanisms coil. They can be positioned at the joint line, either by
by which heat can be generated by an induction field hand or by using an automated system [1, 5].
are eddy current heating and heating due to hysteresis Step 2: Application of Pressure
losses. In eddy current heating, a copper induction coil Pressure can be applied to the parts by placing them in
(work coil), which is connected to a high-frequency fixtures attached to a pneumatic cylinder, or the work
power supply, is placed in close proximity to the joint
(Fig. 11.1). As electric current at a high frequency
passes through the work coil, a dynamic magnetic field B
is generated whose flux links the implant. Electric cur-
rents are induced in the implant, and when these are
sufficiently high to heat the conducting material, the
surrounding thermoplastic parts soften and melt. If
pressure is applied to the joint, this aids wetting of the
molten thermoplastics, and a weld forms as the joint
cools [3, 4].
Ferromagnetic materials (such as stainless steel
and iron) also heat up in a dynamic magnetic field due H

Pressure
Implant (gasket)
Thermoplastic part

Work coil
• • • •
• • • •

Figure 11.2. Graph of the magnetic field (B) in a

ferromagnetic material and the magnetic intensity (H) for
an alternating magnetic field. The magnetization curve is
the full line and the hysteresis curve is the broken line. The
area enclosed by the hysteresis loop is equal to the energy
Figure 11.1. Set up for induction welding (Source: TWI Ltd). dissipated in one circuit (Source: TWI Ltd).

113
114 JOINING PROCESSES

Figure 11.3. The induction welding

process: (a) placement of the
implant, (b) application of pressure,
Implant Work coil (c) induction heating, and (d) cooling
(a) (b) (c) (d) under pressure (Source: TWI Ltd).

from 1.02–1.05, depending on the pressure used in

welding and the material being welded: AE ⫽ kAV. The
constant (k) allows for overfilling the cavity and ensures
that any surface irregularities are smoothed out during
welding [1, 2, 6].

11.2 Advantages and Disadvantages

Induction welding provides structural, pressure-
tight welds with most thermoplastics. It can be used to
weld large parts, with bond lines up to 6.1 m (20 ft.) at
one time. It can also be used to weld three-dimensional
Figure 11.4. Various forms of induction welding implants (3-D) joints with complex geometries, and multiple
(Source: Emabond Solutions). joints can be welded simultaneously.
High quality, stress-free welds in highly filled ther-
moplastics can be produced. The implant adds resin in
coil may be embedded in a PTFE or ceramic block, the joint area to ensure good polymer to polymer link-
which applies the pressure. age, even in cases of surface irregularities or wide toler-
ances between parts. Certain incompatible materials that
Step 3: Induction Heating
cannot be welded with other methods can be joined by
Power is applied to the work coil, creating the electro-
induction welding, regardless of melt temperatures. No
magnetic field that heats the implant, which in turn
prior surface treatment is necessary, and no distortion of
heats and melts the surrounding thermoplastic by ther-
the outside surface occurs. Heat is created precisely
mal conduction. Electromagnetic fields become expo-
where needed, without causing thermal stresses in the
nentially weaker as the distance from the work coil
main body of the part, and reject rates are low. Higher
increases, so that joints placed as close as possible to the
reliability of the weld is often obtained, with less sensi-
coil maximize the heating of the implant. During heat-
tivity to changes in temperature and humidity, and weld-
ing, the implant flows to fill the gap between the parts.
ing is less dependent on properties of the materials being
Step 4: Cooling and Removal of the Parts welded, such as color or melt temperature.
After a preset time, the power is switched off and the After welding, the joint can be reopened using the
parts are allowed to cool under pressure for a preset same equipment, in order to repair defective welds or
time. The welded assembly is then removed and the open an assembly for internal repair or recycling.
cycle is repeated. Production cycles in induction welding are fast,
with weld times typically ranging from 1 to 10 seconds,
Weld strength is proportional to the surface area of and the process is economical in its energy require-
contact; however, it is important that the molten flow ments, resulting in lower running costs.
be contained within the joint area. The amount of mol- The main disadvantages of induction welding are
ten material needed to fill the joint cavity can be calcu- the additional cost of the implant, which can be signifi-
lated using the cross-sectional area of the implant (AE), cant, the cost of optimizing the configuration of the
the cross-sectional area of the cavity between the parts work coil, and the additional assembly operation of
being joined (AV), and a constant (k) with values ranging placing the implant at the joint. In addition, the presence
11: INDUCTION WELDING 115

of the implant can sometimes affect the mechanical Induction welding has also been used in the manu-
performance of the joint [1, 2, 5–7]. facture of a glass-filled PA 6 injection molded automo-
tive intake resonator, with complex 3-D joints, where
three separate welds were produced in the same weld-
11.3 Applications ing cycle [2].
In the appliance industry, induction welding has
Induction welding is frequently used for welding been used in the manufacture of steam irons, dish-
large or irregularly shaped parts that have been injection- washer spray arms, and PP kettles (Fig. 11.7) [7].
molded, blow-molded, extruded, rotational-molded or In the electronics industry, sixteen 38.1 cm
thermoformed (Fig. 11.5), or for thermoplastics that (15 inches) structural foam polycarbonate computer
are difficult to weld [6]. consoles were automatically assembled on a conveyor
Probably the largest volume application is in the line. In the packaging industry, PP was welded to PE
sealing of aseptic drink cartons, where an aluminum for a cosmetic container, using a continuous rotary
foil layer in the box wall is heated by induction to sealer to provide a reliable leak proof seal at 150 parts
melt and seal the low density polyethylene (LDPE) per minute. Other applications include welding PP
coating. solar panels with a 4 ft. (1.2 m) bond line in 9 seconds,
In the automotive industry, the technique has been and PC blood oxygenators and arterial filter compo-
used to produce station wagon structural load floors nents for medical devices [6].
and seatbacks composed of 40% glass-mat reinforced Other applications include sealing plastic-coated
PP composite (Fig. 11.6) [2]. metal caps to plastic bottles, jointing of cross-linked PE
(PEX) pipes, welding metal grilles to the front of loud-
speaker units, and welding HDPE lawnmower shroud/
gas tanks (Fig. 11.8) [1, 2, 8].
The feature of induction welds that allows them to
be easily separated was taken advantage of in welding
50 mm (2.0 inches) threaded HDPE fittings directly to
the opening of 95, 114, and 209 liter (25, 30, and 55
gallon) blow-molded drums (Fig. 11.9). For drum recon-
ditioning, the fittings can be easily removed [2, 7].

11.4 Materials
Induction welding is less dependent than other
welding methods on the properties of the materials
being welded. It can be used to weld almost all thermo-
plastics, crystalline, and amorphous, and can weld high
Figure 11.5. Examples of induction welded products performance and difficult-to-weld resins. Dissimilar
(Source: Emabond Solutions). materials or thermoplastics containing glass, talc,

GMT SHELL

Work coil
EMAWELD®
Work coil
GMT PANEL
Dual weld
Single perimeter weld
(a) (b)

Figure 11.6. Induction welded joint designs for panels composed of a 40% glass mat reinforced PP composite used
to produce station wagon structural load floors: (a) dual weld, placed at the interfaces of the flat panel and ribbed shell,
(b) single weld, located at the end of the part (Source: Ref. [7]).
116 JOINING PROCESSES

0.75
5.1 4.8

1.35 Figure 11.7. A polypropylene tea

0.7 Units: mm
kettle joined using induction welding
(Source: Ref. [7]).

Reflector minerals, wood, or other fillers can be welded, and

thermoplastic materials can be joined to nonthermoplas-
Pre Reflector Post tic materials, such as paper. When welding dissimilar
weld weld thermoplastics, the thermoplastic matrix enclosing the
ferromagnetic filler material consists of a blend of the
two materials being welded. In welding filled materi-
als, the amount of thermoplastic resin in the implant
material can be increased to compensate for the filler
content in the parts. A greater volume of melt will be
produced during welding, resulting in a higher-strength
Work coil
bond. Reinforced plastics with filler levels up to 65%
have been successfully welded [6, 7].
Work has also been carried out with thermoplas-
tics such as PPS, PA12, and PP reinforced with glass
and carbon fibers. Here substantially higher joint
strengths were achieved with induction welding
when compared with adhesive bonded and riveted
joints [9].

11.4.1 Implant Materials

The electromagnetic material can be a metallic mesh
Figure 11.8. A polypropylene lawnmower shroud/gas tank or micron-sized ferromagnetic powders of different
welded using induction welding. types (metallic, such as iron or stainless steel; or

Pressure
EMAWELD®
preform

Before
Work coil

HDPE
threaded fitting
After

Figure 11.9. Polyethylene pipe fittings

induction welded to a blow molded drum
(Source: Ref. [7]).
11: INDUCTION WELDING 117

nonmetallic, such as ferrite materials), particle sizes, Induction Generator

and concentrations. For bonding thermoplastics, these The induction generator converts at 50–60 Hz supply
electromagnetic materials are enclosed in a thermo- to a radio frequency (2–10 MHz), with a power typi-
plastic matrix that is compatible with the plastics being cally between 1 and 5 kW, depending on the applica-
joined. In bonding thermosets such as sheet molding tion. The impedance of the loaded work coil must be
compound, an adhesive matrix surrounds the electro- matched with the output impedance of the generator, in
magnetic material. Heat is generated directly in the order to ensure a consistent and efficient operation of
adhesive, providing a rapid cure; gel times as short as 30 the system. This is referred to as tuning the coil.
seconds can be achieved in the cure of epoxies [1, 2, 6].
Heat Exchanger
For bonding thermoplastic parts made from the
During the welding cycle, very high electric currents
same material, the matrix is generally the same mate-
pass through the work coil. Therefore, to prevent it
rial as in the part, and can be matched on melt flow. For
overheating, water is circulated through the coil and is
dissimilar materials, a blend of the two thermoplastics
cooled via a heat exchanger, which is often combined
is used. The implant formulation must be carefully
with the generator in a single unit.
considered for each application, and maximum effi-
ciency may require production of a custom-made Press
material [1, 5]. The system for applying the pressure during welding is
normally a ram connected to a pneumatic (or some-
times hydraulic) cylinder.
11.5 Equipment Fixtures
Fixtures, or placement nests hold the parts together
A typical induction welding machine is shown in during the welding operation; one nest is generally
Fig. 11.10, and consists of five main parts: induction fixed, and the other is movable. Nests are constructed
generator, work or induction coils, heat exchanger, using electrically nonconductive material, such as phe-
press, and fixtures or nests. nolic or epoxy, since the presence of a metal conductor

Induction generator Timer

Placement nests

Pressure

Work Work
coil coil

Figure 11.10. Typical induction

welding machine. The inset shows
EMAWELD® the joint area with placement of work
Heat exchanger
material coils and implant (Emaweld material)
(Source: Emabond Solutions).
118 JOINING PROCESSES

near the work coil would reduce the intensity of the field intensity can develop at the end of the coil leads,
magnetic field. which can be alleviated by use of a reflector. This coil
requires less space than other designs. A multiturn coil
Systems can be designed for in-line or rotary high- (Fig. 11.11b) eliminates the weak field intensity of a
production sealing lines using programmable control- single-turn coil. It can be cylindrical or helical for
lers and semiautomatic or automatic operation of the welding round containers, or rectangular, square or
welding process. With automated equipment, a sealing irregularly shaped, depending on the contours of the
rate of up to 150 parts per minute can be achieved. joint. Because the greatest magnetic field strength is
Equipment costs range from tens to hundreds within the perimeter of the coil, joints should be placed
of thousands of dollars (US), depending on the size of in the center of the coil. Reflectors are necessary for
equipment needed, automation desired, and the maximum efficiency when parts being joined are six
application [1, 5–7]. inches (152 mm) or larger. The number of turns in the
coil is dependent on the total surface area of the weld;
the length of a multiturn coil should not be more than
11.5.1 Work Coils
three to four times the coil diameter. Pancake coils
The function of the work coil is to provide the mag- (Fig. 11.11c) are used for heating large flat areas and
netic field around the joint. It should be compatible they are made by winding tubing in a horizontal plane
with the power output of the induction generator and to a predetermined diameter [1, 2].
designed to reduce tendencies for arcing or overload- Hairpin coils (Fig. 11.11d) are single-turn coils
ing at high frequencies [2]. squeezed together so that the coupling distance between
Work coil design and positioning are important in the turns is equal to the part thickness. The magnetic
achieving high-strength welds and process efficiency. field becomes more concentrated as the coupling dis-
Work coils should follow the contours of the joint and tance is reduced. Hairpin coils can be formed into
can be custom-made for each part. The distance irregular shapes and are used for bonding long flat
between the coil and the joint (coupling distance) sheets or perimeter seals of structural components
should be as small as possible, ideally less than 1.6 mm made from glass mat composites. For large parts,
(0.063 inches); short coupling distances are essential such as pipes or conduits, or for parts with limited
because energy from the magnetic field used for heat access to the joint line, split coils (Fig. 11.11e) can
generation is inversely proportional to the square of the be used. These coils can be opened for easy part
distance from the coil. Coils can accommodate 3-D removal [1, 2].
joints and can weld joints as long as 6.1 m (20 ft.).
Copper reflectors can also be placed in the center of the
coil to concentrate the magnetic field within the joint 11.6 Joint Design
area [2, 10].
Coils are constructed from copper tubing, sheet The five basic types of joint designs used in induc-
stock, or machined blocks; all coils are water-cooled. tion welding are shown in Fig. 11.12. The simplest
Coils made from tubing can be round, square, or rect- design is the flat-to-flat joint, used for continuous weld-
angular; common sizes are 3.2 mm (1/8 inch), 4.8 mm ing operations, or for parts with a long weld line; this
(3/16 inch), 6.4 mm (1/4 inch), and 9.5 mm (3/8 inch). joint produces a structural weld and a static flow air-
Since constricted water flow in the 1/8 inch coil can tight seal. A flat-to-groove joint ensures accurate posi-
contribute to overheating, this size is used only for tioning of the weld and containment of the implant. It
short heating cycles and small parts, which are not is used when the implant material needs to be auto-
affected by slight overheating. Square tubing is pre- matically extruded into the joint interface. Highest
ferred over round tubing, in order to obtain optimum strength welds are obtained with tongue-and-groove
coupling distances. Copper sheet stock (1/16 inches, joints. The implant is completely contained within the
1.6 mm thickness) is used for larger parts, up to 5 ⫻ 20 joint, producing hermetic and pressure-tight seals and
inches (127 ⫻ 508 mm) or 1 ⫻ 80 inches (25.4 mm ⫻ aesthetically pleasing welds. A shear joint is used for
2.03 m), and for sealing large surface areas and irregu- high-pressure container seals; the weld can withstand
lar flat shapes [2]. pressure from both inside and outside the container. A
The simplest coil design is the single-turn coil step joint, a modification of a shear joint, can accom-
(Fig. 11.11a), in which the magnetic field is concen- modate wide variations in part shrinkage and produces
trated around the inner diameter of the coil. A weak a high-pressure, hermetic seal [1, 2].
11: INDUCTION WELDING 119

Before After

(a)

(a)

(b)

(b)

(c)

(c)

(d)

(d)

(e)
(e)
Figure 11.12. Joint designs commonly used in induction
Figure 11.11. Common designs of work coil used in welding: (a) flat to flat, (b) flat to groove, (c) tongue and
induction welding: (a) single-turn, (b) multiturn, (c) pancake, groove, (d) shear, and (e) step (Source: TWI Ltd).
(d) hairpin, and (e) split (Source: Emabond Solutions).
120 JOINING PROCESSES

11.7 Welding Parameters 2. Emaweld: Electromagnetic Welding System for

Assembling Thermoplastic Parts, Supplier techni-
The main welding parameters in induction weld- cal report, Ashland Chemical Company, 1995.
ing are: power, weld time, weld pressure, and cooling 3. Davies J, Simpson P: Induction Heating Handbook,
time. Reference book (ISBN-13: 978–0070845152),
Typical induction powers are in the range of 1–5 kW. McGraw-Hill, 1979.
Higher power output is necessary for larger parts or 4. Harry JE: Plastics Fabrication and Electrotech-
parts with greater joint length. The power output must nology, Reference book, Heydon & Sons, 1971.
also increase as the coupling distance between the joint 5. Stokes VK: Joining methods for plastics and plas-
and the coil increases. tic composites: an overview. ANTEC 1989, Con-
Weld time is dependent on the type and particle ference proceedings, Society of Plastics Engineers,
size of the electromagnetic filler, the cross-sectional New York, May 1989.
area of the electromagnetic filler enclosed in the ther- 6. Thompson R: Assembly of fabricated parts. Mod-
moplastic matrix, power output, frequency, and part ern Plastics Encyclopedia 1988, Reference book
size. These parameters are tailored for each specific (M603.1), McGraw-Hill, 1987.
application [2, 5]. 7. Chookazian SM: Electromagnetic welding of ther-
The weld pressure ensures an even distribution of moplastics and specific design criteria with empha-
the implant inside the joint. Cooling times vary depending sis on polypropylene. ANTEC 1994, Conference
on the application, but can be less than one second. proceedings, Society of Plastics Engineers, San
Other important factors in induction welding Francisco, May 1994.
include the design of the joint and work coil, magnetic 8. Friend S, Harget D, Hauki P: High performance
field frequency, and the type of electromagnetic fusion jointing of PE-X pipes. Plastics Pipes XI,
material. Conference proceedings, Munich, Germany, Sep-
tember 2001.
9. Rudolf R, Mitschang P, Neitzel M: Induction weld-
ing of fabric-reinforced fibre-plastic composites.
References Schweissen und Schneiden, 53(10), p. 690, October
2001.
1. Sanders P: Electromagnetic welding: an advance 10. Elber G: Putting the heat on attachable parts.
in the thermoplastics assembly. Materials & Design, Plastics Design Forum, Trade Journal, Advanstar
Trade Journal, Elsevier, 1987. Communications, 1993.