Ultrasonic Welding Horn Design: Shapes, Tuning and Failures

Introduction

An ultrasonic welding horn, also called a sonotrode, transfers mechanical vibration from 

the converter and booster to the plastic part.

A properly designed horn must match the system frequency, provide the required amplitude, 

contact the part evenly and withstand repeated production cycles. Poor horn design can cause

 weak welds, surface marking, uneven amplitude, high generator load, overheating and cracking.

Frequency and Horn Tuning

Common ultrasonic plastic welding frequencies include 15Khz, 20Khz and 35Khz.

Each horn must be designed for the operating frequency of the converter and generator. A 20 kHz

 horn cannot be used directly on a 35 kHz system, even when the mounting thread appears compatible.

Horn length is initially calculated according to the acoustic wavelength of the selected material:

Wavelength = acoustic velocity ÷ frequency

Higher-frequency horns are generally shorter. However, the final resonant frequency is also affected by:

• horn material

• cross-sectional changes

• slots and cavities

• mounting threads

• working-face geometry

• coatings and replaceable tips

The finished horn must therefore be measured and tuned after CNC machining. Repair, resurfacing, 

wear and coating can also change its frequency.

Incorrect tuning may result in abnormal noise, unstable vibration, excessive current or high generator load.

Common Ultrasonic Horn Shapes

Horn geometry affects amplitude gain, working area and internal stress.

Straight, Stepped and Exponential Horns

Straight horns are suitable when little amplitude gain is required. They are relatively rigid and simple 

to manufacture.

Stepped horns increase amplitude through a change in cross-sectional area. They provide higher gain

 but also create greater stress near the transition. Sharp corners should be avoided to reduce the risk 

of cracking.

Exponential horns use a smoother change in cross-section. They normally provide lower peak stress 

but less gain than stepped designs.

Large and Multi-Point Horns

Bar and block horns are commonly used for long or wide products. As horn size increases, maintaining

 equal amplitude across the complete working face becomes more difficult.

Slots may be added to control lateral vibration and divide a large horn into narrower vibrating sections. 

Slot position, depth and end radius should be determined through analysis and testing.

Composite and multi-tip horns are suitable when several separated areas must be welded at the same time. 

Each working point must have consistent height, contact pressure and amplitude.

Contoured Horns

A contoured horn is machined to match a curved, stepped or irregular product surface.

Accurate contact helps distribute pressure and reduce surface damage. However, deep cavities, thin sections 

and sharp transitions can weaken the horn or create unwanted vibration modes.

Horn Face and Contact Design

The horn face must transfer vibration and pressure without damaging the plastic part.

The horn does not always need to cover the entire weld area. Contact should be positioned where vibration

can reach the joint effectively without pressing on unsupported walls, decorative surfaces or fragile components.

Poor contact may cause:

• uneven weld strength

• part tilting

• whitening or dents

• excessive flash

• inconsistent collapse distance

• high generator load

Horn contact should be evaluated together with fixture support and ultrasonic welding joint design.

A smooth face is generally preferred for cosmetic parts. Textured surfaces can reduce slipping but may 

leave marks. Knurled surfaces are more suitable for staking, insertion and non-cosmetic applications.

Thin-wall parts require sufficient fixture support. Increasing welding pressure cannot compensate for 

poor support or incorrect horn contact.

Amplitude Distribution and FEA

Large horns do not always produce the same displacement at every position.

Uneven amplitude may cause strong welds in the center and weak welds near the edges, or the opposite. 

It can also result in inconsistent staking height, local overheating, flash and sealing failures.

Finite element analysis can help evaluate:

• resonant frequency

• axial vibration mode

• lateral or torsional vibration

• stress concentration

• amplitude distribution

• slot geometry

• weak sections

FEA is particularly useful for large, asymmetrical, composite and multi-tip horns.

However, simulation cannot replace physical testing. After machining, amplitude should be measured

 at several positions across the working face, especially at the center, edges and individual tips.

How Horn Material Affects Design

Aluminum is lightweight, easy to machine and suitable for prototypes and many large horns.

Titanium provides higher fatigue strength and is commonly used for high-amplitude or high-cycle 

production.Steel offers high hardness and wear resistance but is generally more suitable for lower-amplitude 

applications, staking tips and abrasive materials. For a detailed comparison, see guide to titanium,

 aluminum and steel ultrasonic horn materials.

Common Ultrasonic Horn Failures

Cracks commonly begin near sharp corners, deep slots, threads and sudden changes in cross-section. 

A cracked horn should normally be replaced because continued operation may damage the booster, 

converter or generator.

When welding is uneven, simply increasing amplitude or weld time may over-weld the stronger area. 

Horn contact, amplitude distribution, fixture support and molded-part consistency should be checked first.

Horn Validation Before Production

A new or modified horn should pass the following checks:

Frequency Test

Confirm that the resonant frequency matches the ultrasonic system.

No-Load Test

Assemble the converter, booster and horn, then check frequency tracking, generator load, noise 

and temperature.

Amplitude Measurement

Measure several positions across large, composite and multi-tip horns.

Contact Inspection

Confirm that the horn contacts the plastic part evenly without pressing on unsupported areas.

Sample Welding Test

Use actual molded parts to evaluate:

• weld strength

• collapse distance

• appearance

• flash

• deformation

• leakage

• cycle consistency

A horn may pass the no-load test but still require contact, fixture or parameter adjustments

during actual welding. Ultrasonic welding sample testing can verify the complete process

before equipment production.

When to Repair or Replace a Horn

Minor face wear may sometimes be corrected by resurfacing, recoating or replacing a removable tip.

The horn should normally be replaced when:

• a crack is detected

• threads are seriously damaged

• deep wear changes the contact profile

• frequency becomes unstable

• amplitude remains uneven

• repeated repairs have removed too much material

Any machining or repair changes the mass of the horn. The repaired horn must be retuned and tested 

before returning to production.

Information Required for Custom Horn Design

A horn supplier normally needs:

• 3D and 2D product drawings

• actual molded samples

• plastic material and filler content

• weld joint position

• required strength or sealing standard

• ultrasonic frequency

• production cycle

• cosmetic requirements

• machine and booster information

• fixture support details

Actual samples are important because molded parts may differ from the original CAD dimensions.

These inputs allow the supplier to design custom ultrasonic welding horns that match the product

geometry, operating frequency and production requirements.

Conclusion

Ultrasonic welding horn design must balance frequency, amplitude, geometry, contact area and tool life.

Large or irregular horns may require slots, FEA and multi-point amplitude testing. Before production, 

the horn should be verified through frequency testing, contact inspection, amplitude measurement and

sample welding.Correct horn design improves welding consistency, reduces generator load and extends

tooling life.

Need a Custom Ultrasonic Welding Horn?

Send us your product drawings, plastic material, welding area and production requirements. TIMEAST

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