ULTRA SONIC WELDING

Ultrasonic (sonic) welding joins plastics by converting high‑frequency vibration into heat at the joint line, melting a thin layer of material, and then letting it solidify under pressure to form a solid bond.

 

How sonic welding works

1. Electrical to mechanical vibration

   • A power supply creates high‑frequency electrical energy (typically 20–70 kHz).herrmannultraschall+1

   • A converter and booster turn this into mechanical vibration and transmit it to the horn (sonotrode).

2. Applying force and vibration

   • Two plastic parts are clamped together; the horn presses on the top part with a set force.plasticsdecorating+1

   • The horn vibrates vertically (longitudinally) a very small distance (about 5–50 micrometers), at ultrasonic frequency.

3. Heat generation in the joint

   • At the contact interface, vibration causes microscopic sliding and internal molecular friction.[telsonic]​[youtube]​

   • This friction and viscoelastic loss generate heat only in a very small region, so the bulk of the part stays relatively cool.

4. Melting and fusion

   • When the interface reaches the melting temperature of the thermoplastic, a thin molten layer forms.dukane+1

   • Vibration is stopped but pressure is maintained while the melt cools and solidifies, fusing the parts together.

5. Energy directors and joint design

   • Often the joint includes a small triangular rib called an energy director on one side; this concentrates stress and heat, so melting begins exactly there.

   • Different joint geometries (shear joints, tongue‑and‑groove, step joints, spot welds) are used to control weld strength, sealing, and appearance.

 

Suitable plastics and their behavior

Ultrasonic welding is best for thermoplastics (materials that melt and re‑solidify) rather than thermosets (which do not remelt).

Typical weldable thermoplastics:

• Amorphous polymers: ABS, polycarbonate (PC), polystyrene (PS), PMMA, PVC.

• Semi‑crystalline polymers: polypropylene (PP), polyethylene (PE), nylon (PA), PBT, PET.

Behavior differences:

• Amorphous plastics

  • Soften over a wider temperature range and generally weld more easily.pluswelding+1

  • Require lower amplitude (smaller vibration stroke) to weld.

• Semi‑crystalline plastics

  • Have sharp melting points and higher internal damping; they absorb more energy before melting.

  • Need higher amplitude and tighter control of joint design and process parameters to get strong welds.

Incompatible materials or large differences in melting temperature, stiffness, or acoustic properties can make welding difficult or impossible, or require special joint design.telsonic+1

 

Frequencies and their differences

Industrial ultrasonic plastic welders typically use discrete frequencies such as 15, 20, 30, 35, 40, and up to about 70 kHz, chosen according to part size, geometry, and material.

General trends:

• 15–20 kHz

  • Used for larger parts, thicker sections, and tougher materials.

  • Horns can deliver higher power and amplitude but are physically larger and louder (audible edge of human hearing at 20 kHz).

  • Suitable for big automotive or appliance components and for semi‑crystalline materials that need more energy.

• 30–40 kHz

  • Used for small to medium‑sized parts where precision and lower vibration transmission into the rest of the assembly are important.

  • Lower amplitude and power than 20 kHz, but better control in delicate applications (electronics housings, medical disposables).

• 40 kHz (e.g., 50–70 kHz)

  • Used for very small, delicate parts and micro‑welding where you want very localized energy and minimal marking.

  • Equipment and horns become more complex and expensive; delivered power is typically lower.

Frequency, amplitude, and power are always balanced: lower frequency → larger horns and more power; higher frequency → smaller horns, finer control, less penetration depth.

 

Simple Process overview

• Step 1 – Power
  AC mains → ultrasonic generator → high‑frequency electrical signal.

• Step 2 – Vibration stack
  Generator → converter (piezoelectric) → booster → horn; electrical energy becomes mechanical vibration.

• Step 3 – Part setup
  Parts placed in a rigid nest; joint includes energy director; horn contacts upper part.

• Step 4 – Weld
  Clamp force applied → ultrasonic ON → friction and internal damping create heat at joint → interface melts.

• Step 5 – Hold
  Ultrasonic OFF → hold pressure during cooling → solidified weld.

 

Plastics vs settings

• Amorphous (ABS, PC, PS): lower amplitude, shorter weld times, broader processing window.

• Semi‑crystalline (PP, PE, PA): higher amplitude, more precise joint design, sometimes more clamp force.

• Large parts or thick walls: lower frequency (15–20 kHz), higher power horn.

• Small precision parts: higher frequency (30–40+ kHz), lower amplitude, micro‑scale joints.

If you tell me your target product (e.g., PP caps, ABS housings, nylon components), I can suggest a specific frequency range, joint design, and basic process window tailored to that application.