What is Ultrasonic Welding? | Ultrasonic Welding for Injection Molded Parts | Common Welding Problems and Solutions | Ultrasonic Welding Advantages

What is Ultrasonic Welding?

Ultrasonic Welding is a process for joining or reforming injection molding parts using mechanical vibrations above the audible range. The mechanical vibrations are created by converting high-frequency electrical energy into high-frequency mechanical motion. Generally, the vibrations are produced using a welding sonotrode or horn. The applied force or vibration creates friction heat that joins the parts in a molecular bond normally at 20 or 40 kHz.

The design of equipment and the material’s mechanical properties ( to be welded) plays a crucial role in the quality of welding the components.

Here, the welding times are short, hence can successfully be implemented for mass production. Ultrasonically welded parts are used in various applications like consumer goods, automobiles, and electronics.

How to Choose the Right Material?

To join or create a molecular bond between two thermoplastic parts, the materials must be chemically compatible, or the molecular bond will not form.

Taking an example, let’s try to join polypropylene and polyethylene parts. Both the thermoplastics have a semi-crystalline structure, similar appearance, and identical physical properties. However, different chemical properties make them chemically incompatible, making them unable to join with each other.

Only thermoplastics with similar chemical properties can be welded together. For example, PBT plastic can be welded with PBT. Unalike thermoplastic can be welded only if they have a melting temperature of about 40ºF (6ºC).

Generally, similar amorphous polymers have supreme ultrasonic welding capabilities. Exceptionally, ABS can be welded with acrylic because of similar chemical properties.

Several other factors and attributes can heavily affect the weldability of parts – fillers, lubricants, flame retardants, resin grades, pigments, plasticizers, hygroscopicity, etc.

Ultrasonic Welding Machine – Construction: 

Ultrasonic welding equipment is comprised of many parts. Such as:

1. Machine Press
2. Generator 
3. Converter
4. Booster
5. Horn
6. Support Tooling 

1. Machine Press:

The machine’s primary function is to hold the welding mechanism and apply adequate force for welding. It comprises a pneumatic cylinder to apply the force and a base plate to hold the tooling jig.

The machine is equipped with a pressure gauge and regulator to regulate the welding force. Pressure on each ultrasonic welding machine may differ from one another.

To avoid shape and size-related differences in welded parts, a load cell must be scaled so a clear differentiation of welding forces can be made from machine to machine.

A flow control valve is equipped to control the welding head’s speed approaching the component being welded.

Some manufacturers have introduced an electromagnetic force application mechanism replacing the traditional pneumatic cylinder completely. That gives extra accuracy while welding tiny and delicate parts by improving the movement ratio and speed.

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2. Generator:

The generator modifies the electrical energy into the correct frequency and voltage to convert it into mechanical vibrations—the microprocessor unit controls the welding cycle and provides key information to the operator (visible in the user-interface). The user interface makes the operator’s job quite easy as it helps to add serval operating parameters.

3. Converter:

A converter is used to convert the generator’s electrical energy to mechanical vibrations critical for the welding process. It has multiple piezo-electric ceramic discs stuck between two metal blocks made up of titanium.

A thin metal plate is crammed between the discs which form the electrode. The sinusoidal electrical signal will be fed to the transducer via the electrodes; reacting to that, the discs will expand and contract, producing an axial, peak-to-peak movement of 15 to 20µm.

Converters easily break; they are sensitive in design, so they need to be taken care of.

4. Booster:

A booster is used for two purposes; first, to intensify the converter’s mechanical vibrations and transfer them to the welding horn. Secondly, it provides a place for fitting the stack on the welding press.

The booster, similar to other elements in the ultrasonic welding machine, is a tuned device. Thus, it needs to oscillate at a specific frequency to transmit ultrasonic energy from the converter to the welding horn.

Usually, the booster is one-half of the wavelength of the material from which it is manufactured.

5. Horn:

The welding horn is an integral part of the ultrasonic welding system. Its primary purpose is to dispense energy to the part being welded. Similar to the booster, a horn is also a tuned device that also boosts the mechanical gain to the machine.

The tip of the horn is responsible for transmitting the ultrasonic energy to the part being welded. To attain maximum energy transfer between the horn and the part, the tip should be specifically designed to suit the part.

The horn length should also be one-half of a wavelength of ultrasound in the material from which it is manufactured. That will help in generating enough pressure at the end of the welding horn.

The pressure must be between 30 and 120µm. The welding horn’s geometry is critical as the street caused by expansion and contraction of the horn could rupture in high-pressure applications.

The horn is manufactured with aluminum or titanium. Aluminum-made horns are mostly used for low-volume applications as constant wear can be a problem of the material.

6. Support Tooling:

The name says it all. The support tooling supports tooling that supports the component in the welding phenomenon. Its primary function is to support and keep the part steady during welding. It is usually machined before the operations begin.

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Joint Design Considerations –

The joint design choices are a critical part of the welding process. It’s something people overlook and don’t take seriously but can change the dynamics of assembling in the blink of an eye.

The factors to pay attention to when talking about a particular part’s joint designs – the type of thermoplastic, the requirement of the weld, and part geometry.

The Type of Thermoplastic:

Protection joint AKA energy director is made up of a small triangular section joint with the part and running the joint perimeter length. The energy directer’s prime function is to guide the ultrasonic energy into the apex, producing a lot of heat in a contained area.

Reacting to that, the triangle-shaped section starts to melt across the joint surface, carving a weld. Here starts the selected thermoplastic role; it is crucial for determining the triangular energy directer’s form.

Typically, amorphous materials need a right angle triangle with the 90° angle of the apex. Semi-crystalline materisl requires a 60° equilateral projection. The energy director’s height ranges from 0.2mm to 1.00mm( can be changed depending on the material).

There is a term called far-field welding implemented for a scenario where the difference between the joint one and the contact surface is greater than 6mm. The welding mechanism is compatible with amorphous materials like PMMA, ABS because they have great ultrasound properties.

Talking about semi-crystalline materials(PP is a good example), which are poor conductors of ultrasonic energy that required the joint to be as close as possible to the welding horn area. That phenomenon is better known as field welding.

 Weld Requirments:

One of the most critical aspects of ultrasonic weld plastic is to set accurate welding parameters for achieving good results. It includes Amplitude, welding modes, trigger pressure, downspeed, hold time, and weld time.

Amplitude:

Accurate amplitude setting n integral part of successful welding. The amount of vibration at the tp of the welding horn can make or break things.

Amplitude selection completely depends on the thermoplastic being welded. Booster or horn combination amplitude is standard.

Generally speaking, semi-crystalline materisl consume more energy than amorphous materials; thus, more amplitude to the horn is necessary.

Modern ultrasonic machines come with digital process control making the amplitude more outlined. In the beginning, the amplitude is higher to melt ht material, and then it is slowly decreased to control the density of the molten material.

Ultrasonic Welding Plastics – Advantages:

1. It is a fast, easy, reliable, and repeatable process for producing string bonds between objects.
2. The cycle time is very low, making the part assembling a faster process. That’s because the energy transmission occurs seamlessly and is contained to the immediate joint area.
3. Difficult materials are easy to assemble ultrasonically.
4. No requirement for additional fasteners, solvents, adhesives, and external heat.
5. Faster than other assembly methods thanks to the rapid squandering of heat.
6. Inexpensive set-up and maintenance costs.
7. Wide range of verticals applications like automotive, communications, electronics, electrical, consumer products, packaging, textile, toys, etc.
8. Ultrasonic welding assembly mechanism can be quickly modified depending int eh application. That kind of flexibility is not found in any other assembly methods.
9. The acceptance of automation for welding has made the overall process even leaner, repeatable, accurate.

Some Ultrasonic Welding Problems and (How To Solve Them):

There are mainly four segments or areas with potential problems which affect the production in multiple ways. Let’s talk about them.

1. Equipment
2. Process parameters
3. Materials 
4. Part Design 

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FAQs – 

1. What is the difference between Ultrasonic welding and vibration welding?

Ans. Vibration welding works by vibrating one component relative to another in a linear side-by-side motion. Teh friction between the two parts generated by vibration creates heat for welding. On the other hand, ultrasonic welding vibrates one component perpendicularly to another much like a jackhammer or pavement.

2. What is Ultrasonic Vibration?

Ans. Ultrasonic vibrations are sound waves of frequencies above the perceptible or audible range. The general physics laws as sound apply to them too. Their high frequency and short wavelength make them very useful for applications that depend on energy transmission and on directional control of this transmission.

3. What are the two types of ultrasonic welding?

Ans. There are two types of Ultrasonic welding – Plastic ultrasonic welding and Metal ultrasonic welding.

4. Are there any waterproof ultrasonic welding machines?

Ans. Sonikel Ultrasonics, a company based in Turkey, designs and manufactures waterproof ultrasonic welding machines utilized for ABS and Polycarbonate waterproof welding. It is specially designed for mass production and automation. In addition, there is no need to use any chemicals for sealing plastic parts to each other.

Suggested Read – 

• What is Nylon? | Different Types of Nylon | How is Nylon Made? | Key Properties | Applications
• What is Shore Hardness Scale? | The Complete Guide
• Types of Gates for Injection Molding – A Detailed Guide
• How to Select the Right Plastic Material? | Mechanical Properties | Special Properties | Types of Plastics
• 7 Best UV Resistant Plastics For Outdoor Applications

The Takeaway – 

Thus, was my take on the Ultrasonic welding process. Ultrasonic weld plastic is useful in many applications, and it is a must for your shop floor to boost client satisfaction and, most importantly, profits.

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