This document provides information on various plastic joining processes including ultrasonic welding, vibration welding, spin welding, and induction welding. It discusses the principles, advantages, disadvantages, applications, suitable materials, and design recommendations for each process. The key points are: 1. Ultrasonic welding uses high-frequency vibrations to melt plastic surfaces and join them. It is fast and economical but requires specifically designed joints. 2. Vibration welding generates heat through high-amplitude, low-frequency vibrations. It can join large parts but has initial high equipment costs. 3. Spin welding friction welds circular joints by rotating one part at high speed. It is simple and energy efficient but limited to circular

1. IIT Bombay
Module
4
Design for Assembly

2. IIT Bombay
Lecture
6
Design for Joining of Plastics

3. IIT Bombay
Instructional objectives
By the end of this lecture, the student will know
1. principles of different welding processes used for plastic joining,
2. advantages and limitations of different plastic joining processes,
3. recommended designs to achieve good quality joints in polymers.
Joining of moulded plastic parts is required when the finished assembly is too large or
complex to mould in one piece, requires disassembly and reassembly is necessary, and often
to reduce cost to produce a single large moulded plastic component. The plastic parts about to
join can be of same or dissimilar materials. Thermoplastics are generally joined by welding
processes, in which the part surfaces are melted, allowing polymer chains to interdiffuse. Few
important welding processes used for thermoplastics welding are ultrasonic welding,
vibration welding, spin welding, and induction welding.
Ultrasonic Welding of Plastics
Principle
Ultrasonic plastic welding is, in principle, the joining of thermoplastics through the use of
heat generated from high frequency mechanical motion. It is accomplished by converting
high frequency electrical energy into high frequency mechanical motion. The mechanical
motion along with the applied force creates the frictional heat at the mating surface of the
plastic components. In effect, the plastic material at the joint surface melts and forms a
molecular bond between the parts. The frequency of the frequency is around 20 to 40 kHz.
Figure 4.6.1 depicts the line diagram of an ultrasonic welding machine. The electrical power
supply provides high-frequency electrical power to a typical piezoelectric based transducer
creating a high frequency mechanical vibration at the end of the transducer. Next, the booster
amplifies the mechanical vibrations produced by the transducer and supplies to the horn.
Further, the horn (also known as sonotrode) transfers the amplified mechanical vibrations to
the workpiece.

4. IIT Bombay
Figure 4.6.1 Illustration of the ultrasonic welding for polymers [1].
Advantages and Disadvantages
Table 4.6.1 explains the advantages and the disadvantages of ultrasonic welding of plastics
Table 4.6.1 Advantages and disadvantages with ultrasonic welding of plastics
Advantages Disadvantages
• Fast, economical and easily automated.
• Mass production, upto 60 parts per
minute is possible.
• Increased flexibility and versatility
• Possibility to join large structures.
• Used in health care industries due to
clean welds.
• Produce the high strength joints
consistently.
• Large joints (>250 x 300 mm) can
not be welded in a single operation.
• Specifically designed joints are
required.
• Ultrasonic vibrations can damage
electric components.
• Tooling costs for fixtures are high.

5. IIT Bombay
Applications of Ultrasonic Welding of Plastics
(1) Ultrasonic welding is used in the automotive industry to fabricate headlamp parts,
dashboards, buttons and switches, fuel filter, fluid vessels, seat-belt locks, electronic
key fobs, lamp assemblies and air ducts.
(2) In electronic appliances like switches, sensors and data storage keys are fabricated
using ultrasonic welding.
(3) Ultrasonic welding is also used to make medical parts like filters, catheters, medical
garment and masks.
(4) Packing applications like blister packs, pouches, tubes, storage containers and carton
spouts can be fabricated using ultrasonic welding.
Suitable Plastic Materials for Ultrasonic Welding
Most of the thermoplastic materials can be ultrasonic weldable. Teflon with low coefficient
of friction and high melting temperature is impossible to weld using this process. The list of
the thermoplastic materials suitable for ultrasonic welding is given elsewhere [1, 2].
Design Recommendations for Ultrasonic Welding of Plastics
(1) Surfaces of the plastic workpieces to be joined should be free of distortion and
warpage.
(2) Bead or narrow raised sections called energy indicators are molded on one of the
surfaces of the workpieces. This smallest possible surface area increases the frictional
heat and in turn the melting rate [Figure 4.6.2(b)].
(3) Step joints are more preferable to reduce the unwanted flash and to increase the joint
strength [Figure 4.6.2(c)]. Even a greater strength is possible by using the tongue-and-
groove joint as shown in Figure 4.6.2(d). But moulding this type of joint is more
difficult.
(4) Figures 4.6.2(e)-(h) show few more recommended joint designs.
(5) It is not recommended to bevel one surface of the joint [Figure 4.6.2(i)]. The problem
associated with this joint is the expulsion of the large volume of the material beyond
the joint.

6. IIT Bombay
Figure 4.6.2 Design recommendations for ultrasonic welding of plastics [2].
Fig. no. Joint design
Name of the joint and its
performance
a
Normal butt joint;
Bad
b
Butt joint with energy indicator;
Good
c
Step joint;
Better
d Tongue-and-groove joint;
Best

7. IIT Bombay
Figure 4.6.2 Design recommendations for ultrasonic welding of plastics (continued) [2].
Fig. no. Joint design Name of the joint and its
performance
e
Typical ultrasonic joint designs;
Better
f
g
h
i
Butt joint with bevel edge;
Bad
Vibration Welding of Plastics
Principle
In vibration welding process, the weld joint is produced at the interface of the thermoplastic
workpieces due to the heat generated by the high magnitude (3-5 mm) vibrations at low
frequencies (120 Hz). The molten materials flow together under pressure, forming a weld
upon cooling. Figure 4.6.3 depicts the movement of the parts undergoing vibration welding.
The direction of the vibration is parallel to the plane of the joint rather than perpendicular as
in the case of ultrasonic welding. Moreover the vibration can be linear or angular.

8. IIT Bombay
Figure 4.6.3 The movement of parts in vibration welding can be either linear or angular [2].
Advantages and Disadvantage
Table 4.6.2 explains the advantages and the disadvantages with vibration welding of plastics
Table 4.6.2 Advantages and disadvantages with vibration welding of plastics
Advantages Disadvantages
• Welds are produced in short cycle time.
• Large parts can be welded with ease.
• Insensitivity to surface preparation.
• No additional materials are required to
place between the workpieces.
• Parts can be welded regardless of how
they processes (injection molded,
extruded, vacuum formed, etc.).
• Produce high strength, pressure-tight
hermetic seals.
• Initial high capital cost of the
equipment and tooling compared to
ultrasonic welding.
• Sound generation, which is typically of
90-95dB is a drawback.
• Generation of fine particulates or fluff
at the joint line is a drawback for some
end-use applications.
• Sometimes it is difficult to lock large
workpieces into the supporting fixtures.
Applications of Vibration Welding
(1) Extensively used in the appliance industry for assembling washer and dishwasher
pumps, particulate-filled soap dispensers, and dishwasher spray arms.
(2) Automotive applications include all-plastic automotive bumper, headlight, taillight,
instrument panel assemblies [Figure 4.6.4(a)], acetal gasoline reservoirs, dash-and-
trim components, air conditioning and heater ducts etc.

9. IIT Bombay
(3) Manufacturing of the automotive air intake manifold. The manifold is made in two
or three injection molded parts and linear vibration welding is used to assemble the
final manifold [Figure 4.6.4(b)].
(4) Welding chain saw motor housings made of 30% glass filled nylon, butane gas
lighter tanks, batteries and pneumatic logic boards.
(5) Widely used in making high quality joints in polyethylene (PE) gas distribution
pipes.
Figure 4.6.4 (a) automotive instrument panel assembly (b) vibration welded automotive air
intake manifold [1].
Suitable Plastic Materials for Vibration Welding
Most of the thermoplastic materials can be vibration welded. The only polymers that are
difficult to weld are fluoropolymers, due to their low coefficient of friction. The list of the
thermoplastic materials suitable for vibration welding is given elsewhere [1, 2].
Spin Welding of Plastics
Spin welding is a form of friction welding used to join the thermoplastic parts/workpieces
having circular joint line. To spin-weld, one part of the assembly is rotated at high speed and
presses against the other stationary part. This results in the generation of the frictional heat at
the mating surface and subsequent melting. When the rotary motion is stopped, pressure is
retained until the molten material solidifies and forms the final joint. Depending on the
workpiece material to be joined, the rotational velocities ranging from 3 to12 m/s and
pressure ranging from 2000 to 4800 kPa are required to bring it to the melting temperature.
(a) (b)

10. IIT Bombay
Advantages and Disadvantages
Table 4.6.3 explains the advantages and the disadvantages with spin welding of plastics
Table 4.6.3 Advantages and Disadvantages with spin welding of plastics
Advantages Disadvantages
• Simple and highly energy efficient
process.
• Suitable for automation.
• Strong hermetic joints can be produced.
• No need of introducing any foreign
material between the parts.
• Parts over one meter diameter can also
be spin welded.
• Limited to parts with a circular joint line.
• Protrusions on the rotatable part of the
system or any eccentric part restrict the use
of the process.
Applications of the Spin Welding
1. Successfully used in assembling structural components, connecting ventilation pipes
to blow-molded fuel tanks, and welding tops and bottoms on containers.
2. Used for the joining and repair of polyethylene (PE) pipes.
3. Manufacturing of fuel filter, check valves, truck lights, aerosol cylinders and floats.
Suitable Plastic Materials for Spin Welding
Almost all thermoplastic materials can be spin welded. The list of the thermoplastic materials
suitable for spin welding is given elsewhere [1,2].
Design Recommendations for Vibration and Spin Welding of Plastics
1. One part of the assembly must be free to move relative to the other in the plane of the
weld in both vibration and spin welding
2. Adding a flange [thickness (t) equals to 2 to 3 times of the part thickness (w)] to the
joining surface of the parts improves the rigidity that limits flexure, applies uniform
pressure close to the weld joint and improves the strength of the weld joint [Figure
4.6.5(a)].
3. Special joint designs are required to contain the flash that is squeezed to the outside of
the part during the welding process [Figures 4.6.5(b) – (d)].

11. IIT Bombay
4. Breakaway stud or socket can be incorporated into the part halves for easy assembly
alignment [Figure 4.6.6].
Figure 4.6.5 Recommended designs for vibration and spin welding of plastics [2].
Fig. no. Recommended joint design
a
b
c
d

12. IIT Bombay
Figure 4.6.6 Breakaway studs to aid preassembly of parts prior to vibration welding [2].
Induction Welding of Plastics
Principle
Induction welding uses electromagnetic induction to heat the workpiece. The welding system
usually contains an induction coil that is energised with a radio frequency electric current. A
high frequency electromagnetic field is generated that acts on an either electrically
conductive or ferromagnetic workpiece. In electrically conductive workpiece, the main
heating effect is resistive heating due to induced current also referred to as eddy current. In
ferromagnetic materials, the heating is primarily caused due to hysteresis effect as the
electromagnetic field distorts the magnetic domains of the workpiece.
In the case of joining plastics, an additional metallic or ferromagnetic material (implant) is
placed between the parts to be joined. This implant is a composite of the thermoplastic with
either metal fibers or ferromagnetic particles. Next this assembly of the parts with implant is
placed within or in the proximity to an induction coil, through which a high-frequency
alternating current is passed. The electromagnetic field resulting from the current in the coil
generates heat in the metal particles from eddy current. The hot metal particles melt their

13. IIT Bombay
thermoplastic binder and, in turn, the surfaces of the parts to be joined. Figure 4.6.7 depicts a
typical installation for welding the top to a cosmetics cartridge using induction welding
process.

14. IIT Bombay

15. IIT Bombay
Figure 4.6.7 Schematic view of induction welding [2].

16. IIT Bombay
Advantages and Disadvantages
Table 4.6.4 explains the advantages and the disadvantages with induction welding of plastics
Table 4.6.4 Advantages and disadvantages with induction welding of plastics
Advantages Disadvantages
• Strong and hermetic pressure tight joints can
be produced
• The implant fills molding irregularities or
cellular voids at the joint interface.
• Multiple joints can be welded simultaneously.
• Welded joint can be reopened for repair
purpose.
• Production rate of the weld joints is high.
• Additional cost of the implant.
• Additional work of preplacing
implant.
• The presence of implant can
sometimes affect the mechanical
performance of the joint.
Applications
(1) Frequently used for welding large or irregular shaped parts made by injection-
moulding, blow-moulding, rotational moulding or thermo-formed [Figure 4.6.8].
(2) Used in the manufacturing of a glass-filled PA 6 injection moulded automotive intake
resonator.
(3) Extensively used in sealing plastic coated metal caps to plastic bottles, joining of cross
linked PE pipes, welding metal grills to the front of loud speaker units etc.
Figure 4.6.8 Induction welded products [1]

17. IIT Bombay
Suitable plastic materials for induction welding
Almost all thermoplastic materials can be induction welded. The list of the thermoplastic
materials suitable for induction welding is given elsewhere [1, 2].
Design Recommendations for Induction Welding
(1) The coupling distance, i.e., the space between the work coil and the bond line should
remain constant.
(2) The joint line should be as close as possible to the work coil [Figure 4.6.9(a)]. The
irregularities that prevent the work coil from being located close to the joint line
should be avoided.
(3) Joints should be designed in shear rather than in peel or butt [Figure 4.6.9(b)].
Figure 4.6.9 Recommended and not recommended joint designs for induction welding [2].
(a) (b)

18. IIT Bombay
Exercise
1. What are the design recommendations for the laser welding of plastics?
2. What are the limitations in using hot gas welding for plastic joining?
References
1. M. J. Troughton, “Handbook of plastics joining”, William andrew Inc, 2nd
2. J. B. Bralla, “Design for manufacturability handbook”, McGraw hill handbooks, 2
edition,
New york.
nd
edition, New York.