Module 4 Lecture 6 Final

7/30/2019 Module 4 Lecture 6 Final

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Module

4

Design for Assembly


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Lecture

6

Design for Joining of Plastics


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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.


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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 jointsconsistently.

Large joints (>250 x 300 mm) cannot be welded in a single operation.

Specifically designed joints arerequired.

Ultrasonic vibrations can damageelectric components.

Tooling costs for fixtures are high.


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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 fabricatedusing ultrasonic welding.

(3) Ultrasonic welding is also used to make medical parts like filters, catheters, medicalgarment and masks.

(4) Packing applications like blister packs, pouches, tubes, storage containers and cartonspouts 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 thesurfaces 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 jointstrength [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.


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Figure 4.6.2 Design recommendations for ultrasonic welding of plastics [2].

Fig. no. Joint designName of the joint and its

performance

aNormal butt joint;

Bad

bButt joint with energy indicator;

Good

cStep joint;

Better

d Tongue-and-groove joint;

Best


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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

iButt 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.


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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


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 howthey processes (injection molded,

extruded, vacuum formed, etc.).

Produce high strength, pressure-tighthermetic seals.

Initial high capital cost of theequipment and tooling compared to

ultrasonic welding.

Sound generation, which is typically of90-95dB is a drawback.

Generation of fine particulates or fluffat the joint line is a drawback for some

end-use applications.

Sometimes it is difficult to lock largeworkpieces into the supporting fixtures.

Applications of Vibration Welding

(1)Extensively used in the appliance industry for assembling washer and dishwasherpumps, 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.


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(3) Manufacturing of the automotive air intake manifold. The manifold is made in twoor 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 gaslighter tanks, batteries and pneumatic logic boards.

(5)Widely used in making high quality joints in polyethylene (PE) gas distributionpipes.

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 PlasticsSpin 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)


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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


Simple and highly energy efficientprocess.

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 alsobe 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 pipesto 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 Plastics1. 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 thejoining 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 ofthe part during the welding process [Figures 4.6.5(b) (d)].


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4. Breakaway stud or socket can be incorporated into the part halves for easy assemblyalignment [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


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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 coilgenerates heat in the metal particles from eddy current. The hot metal particles melt their


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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.


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Figure 4.6.7 Schematic view of induction welding [2].


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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


Strong and hermetic pressure tight joints canbe produced

The implant fills molding irregularities orcellular 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 cansometimes 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 intakeresonator.

(3) Extensively used in sealing plastic coated metal caps to plastic bottles, joining of crosslinked PE pipes, welding metal grills to the front of loud speaker units etc.

Figure 4.6.8 Induction welded products [1]


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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 shouldremain constant.

(2) The joint line should be as close as possible to the work coil [Figure 4.6.9(a)]. Theirregularities 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)


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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.

Module 4 Lecture 6 Final

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