Ultrasonic Welding Intro

What is ultrasonic welding ?• Ultrasonic welding is a type of plastic joining process which uses high frequency

vibrations to create frictions to weld two parts together• Two plastic parts are held under pressure between the oscillating horn (high

frequency between 20kHz to 60kHz) and immobile fixture.• Two type of the ultrasonic welding - Near field ultrasonic welding (Less than 6mm / from source to joint) Suitable for crystalline and low stiffness material- Far field ultrasonic welding (More than 6mm / from source to join Suitable for amorphous and high stiffness material

Advantage of ultrasonic welding• It is a fast (Cycle time for weld is less than 2 seconds). • It is clean process (No solvents, adhesives, mechanical fasteners or external heat

are required). • Ultrasonic welded joints are made of strong bonds.• The rapid dissipation of heat makes this process considerably faster than other

methods of assembly therefore no ventilations are needed.• Horn can be changed quickly (~ 30 minutes) increased flexibility• Thin sheet can be welded together without any defects.

Limitations of ultrasonic welding• It cannot be welded for a workpieces larger than 250mmx 300mm using current

technology in a single operations• If settings are not set correctly, the welded plastics may stuck to the horn itself• The horn sizes are limited to the part designs constraint• Initial tooling costs for horn is quite high for complex shape• The by-products of the ultrasonic welding is high pitched sounds which can

causes hazards of hearing losses through the long operation uses. (Can be mitigated through the usages of proper PPE)

Ultrasonic Process Diagram

1. The two thermoplastic parts to be joined are placed together, one on top of the other, on a supportive surface called a fixture.

2. A titanium or plated aluminium component called a horn is brought into contact with the upper plastic part.

3. A controlled pressure is applied to the horn, clamping the two plastic parts together against the fixture.

4. The horn is vibrated at certain frequency times per second for a predetermined amount of time is called weld time. Through careful part design, this vibratory mechanical energy is directed to limited points of contact between the two parts.

5. The clamping force is maintained for a predetermined amount of time to allow the parts to fuse as the melted plastic cools and solidifies. This is known as hold time.

6. Once the melted plastic has solidified, the clamping force is removed and the horn is retracted. The two plastic parts are now joined as if moulded together and are removed from the fixture as one part.

What happened to the material when it undergoes ultrasonic welding ?

Type of joints used in U/S Weld

Components in the ultrasonic welding machine

Transducer• Transducers convert the electrical energy received from a generator into

mechanical energy in the form of high frequency vertical vibrations. They accomplish this by using the principles of piezoelectricity, which is the generation of electricity or electric polarity by compressing a crystalline substance.

• The table below shows the frequency of frequency commonly found in the ultrasonic welding machine

FREQUENCY PEAK TO PEAK AMPLITUDE

15 kHz 31 microns

20 kHz 20 microns

30 kHz 13.5 microns

40 kHz 9 microns

50 kHz 8 microns

Booster• It functions as to modify the amplitude of the vibrations• The boosters are either made of titanium or aluminium. • Titanium boosters while cost more are more robust• Aluminium boosters are recommended in continuous applications where heat

dissipation is a benefit

Welding Press• The primary purpose of the ultrasonic welding press is to apply the horn, and

consequently the ultrasonic energy, to the workpiece in a controlled, repeatable manner• Mechanical stop adjustment is usually provided on the press to prevent the horn

from accidentally contacting an empty fixture which would shorten the horn life. The bottom/mechanical stop adjustment is also used to repeatably weld parts to a finished height.

Welding Horn• The purpose of the horn is to properly transmit the ultrasonic vibrational energy

to the workpiece so that it localizes in the area where the melt is to take place• The horn shape design will affect the gain(The amplitude of horn displacement

during oscillations) . Below is some of the commonly used shapes in u/s welding

Welding Horn• The horn actually stretches and shrinks in length when oscillating, much like a

spring. Since the horn must stretch, the material from which a horn is made is a major consideration• Aluminium – (GOOD) Good acoustic properties, low material cost, and ease of machining relative to other materials (BAD) Poor surface hardness and moderate fatigue strength can make it unsuitable for long-term, high-wear production applications• Titanium – (GOOD) Excellent acoustic properties, good fatigue strength, and good surface hardness (BAD) Not readily available, most expensive to machining

Welding Parameters- Frequency• Higher frequency advantages includes less audible noises, smaller horn sizes, less

indirect heating outside the joint area, faster welding speed as well as lower weld forces• Disadvantages are the far field ultrasonic weld are hard to perform due to

reductions in a amplitude

Welding Parameters- Weld Time• Weld time refers to actual time of horn contacting the plastic parts• Increasing weld time will increase the joint strength until the certain limit.• Furthermore, the more increment beyond optimum points will actually decrease

weld strength as well as produce flashing and marking the parts

Welding Parameters- Weld Force• Weld force are force that needed to hold two parts of plastics so the ultrasonic

energy can be introduced • Too low weld forces results in poor ultrasonic energy transmissions causing

incomplete melt flow• Too high weld forces results in greater melt volumes which may causes alignment

of parts to be slide off and decreasing weld strength