Best Plastics for Ultrasonic Welding

Ultrasonic welding is a fast, clean, and reliable method for adhering plastic parts. But while the process is versatile, not all plastics are well-suited for ultrasonic welding. Some resins bond easily under the vibrations, while others resist welding altogether or result in weak, unreliable joints.

The key to a strong weld lies in material selection. Choosing the right plastic resin can mean the difference between a strong weld and a costly manufacturing headache. In this blog, we’ll break down which plastics work best for ultrasonic welding, which ones you should be cautious about, and why certain material properties make such a big difference.

How Ultrasonic Welding Interacts with Plastics

Ultrasonic welding works by converting high-frequency electrical energy into mechanical vibrations. These vibrations, typically at 20 to 40 kHz, are transmitted through a sonotrode into the plastic parts at the joint interface. The friction created by this rapid vibration generates localized heat, causing the plastic to soften and flow. Once the vibrations stop, the material quickly solidifies under pressure, forming a solid bond.

But not all plastics respond to this process in the same way. A resin’s molecular structure directly impacts how it converts vibration into heat. Amorphous plastics, for example, have a wide softening range and respond well to ultrasonic energy, which helps create consistent welds. Semi-crystalline plastics, on the other hand, melt at a sharper temperature point and may not distribute heat as evenly, making the process more difficult to control.

Other factors like stiffness, surface energy, and even moisture content also play a role. Stiffer materials transfer vibrational energy more effectively, while low surface energy plastics may resist bonding even when melted. That’s why understanding the interaction between ultrasonic energy and your selected plastic is critical when designing parts for this process.

Best Plastics for Ultrasonic Welding

Some plastic resins are naturally more compatible with ultrasonic welding due to their molecular structure, thermal behavior, and ability to transmit high-frequency vibrations. Generally, amorphous plastics perform best because of their broader softening range and greater energy absorption. Certain semi-crystalline plastics can also be welded effectively when process parameters are optimized. Below are some of the top-performing materials:

ABS (Acrylonitrile Butadiene Styrene)

ABS is one of the most commonly used plastics for ultrasonic welding. It responds quickly to ultrasonic energy, softens evenly, and forms strong, clean welds. It’s ideal for consumer electronics, enclosures, and structural parts where strength and appearance both matter.

Polycarbonate (PC)

PC offers excellent strength, clarity, and impact resistance. It welds well ultrasonically, although it may require higher amplitude settings than ABS. PC is often used in applications like medical housings, optical devices, and high-performance casings.

Polystyrene (PS)

PS is another amorphous resin that welds cleanly and consistently. Its ease of processing makes it suitable for disposable or low-cost applications, including packaging components, medical disposables, and consumer products.

SAN (Styrene Acrylonitrile)

SAN combines the clarity of polystyrene with improved chemical resistance and mechanical strength. It’s often chosen for transparent components that still need strong welds, such as fluid reservoirs, dispenser parts, and consumer containers.

Nylon (Polyamide, PA6 and PA66)

Though semi-crystalline, nylon can be welded successfully, especially when it's dry and the joint design includes energy directors. Nylon is widely used in automotive connectors, industrial housings, and parts requiring high wear resistance.

PBT (Polybutylene Terephthalate)

PBT is another semi-crystalline plastic that performs well in ultrasonic welding when moisture is controlled and parts are properly designed. Its rigidity and chemical resistance make it ideal for electrical connectors and automotive parts.

All these plastic resins have favorable characteristics that make them well-suited to ultrasonic welding. However, even among these materials, joint design and process optimization are key to achieving repeatable results.

Plastics That Are Challenging for Ultrasonic Welding

While ultrasonic welding is effective for many plastics, some materials pose significant challenges due to their physical or chemical properties. These resins either don’t transmit ultrasonic energy efficiently, resist heat buildup, or struggle to form strong molecular bonds. Below are a few common examples that often require extra care or may not be suitable at all for ultrasonic welding:

HDPE (High-Density Polyethylene)

HDPE is known for its toughness and chemical resistance, but these same characteristics make it difficult for ultrasonic welding. It has a low surface energy, which hinders bonding, and its high flexibility absorbs vibration instead of converting it into heat. Even with optimized joint design, ultrasonic welding HDPE often results in weak or inconsistent bonds, making it an unreliable choice for this process.

Polypropylene (PP)

Polypropylene shares many of the same challenges as HDPE. While it can be welded under the right conditions, it has a narrow process window and is sensitive to joint geometry. To improve results, energy directors or specially formulated copolymers may be required, but even then, the results may not match those of more weldable plastics like ABS or PC.

PTFE (Polytetrafluoroethylene)

PTFE, commonly known as Teflon®, is nearly impossible to weld ultrasonically due to its extremely low surface energy and high thermal resistance. It resists almost all bonding methods, including ultrasonic welding, and typically requires mechanical fastening or adhesive bonding instead.

TPEs (Thermoplastic Elastomers) and Other Soft Elastomers

Soft, rubber-like plastics such as TPEs absorb ultrasonic energy without producing the frictional heat needed to form a weld. Their flexibility and low modulus make it difficult to generate consistent joint strength. In most cases, alternative welding methods, such as heat staking or adhesive bonding, are better suited for these materials.

These challenging plastics don’t necessarily rule out ultrasonic welding entirely, but they demand more process development, specialized joint designs, and often carry greater risk for inconsistent weld quality. When selecting materials for a project that will use ultrasonic welding, it's best to consult with your manufacturing partner early to validate material compatibility and avoid downstream production issues.

Tips for Selecting the Right Resin for Ultrasonic Welding

Choosing the right plastic resin isn’t just about mechanical properties or resin cost, it’s also about how well that material responds to the ultrasonic welding process. A resin that performs poorly in ultrasonic welding can lead to weak joints, scrap parts, and higher manufacturing costs. Here are a few key tips to help you select the right material from the start:

Tip 1: Work with Your Manufacturing Partner Early

Involving your contract manufacturer during the design for manufacturing and material selection phase can save time and prevent costly mistakes. Experienced manufacturers understand how different resins behave under ultrasonic welding and can recommend materials that suit your design and performance requirements.

Tip 2: Prioritize Amorphous Plastics When Possible

If ultrasonic welding is your preferred joining method, opt for amorphous resins like ABS, PC, or PS. These materials typically provide more predictable welding behavior, easier setup, and stronger, cleaner joints.

Tip 3: Use Energy Directors with Semi-Crystalline Plastics

If your application requires semi-crystalline materials like nylon or PBT, design your parts with energy directors, which are small, triangular features at the joint interface that concentrate ultrasonic energy to help initiate the melt. These features can significantly improve weld consistency and strength.

Tip 4: Account for Moisture Sensitivity

Some plastics, especially nylons, absorb moisture from the air, which can create steam pockets and jeopardize the weld quality. Ensure proper drying before welding and plan for storage conditions that minimize moisture exposure.

Tip 5: Test Before You Commit

Don’t just rely on this blog and data sheets; run sample welds with the actual materials and part geometry. Testing gives you insight into how the resin performs under real-world conditions and allows time to adjust welding parameters or tweak the design before scaling up production.

By being thoughtful in your resin selection and validating performance early, you’ll increase the likelihood of achieving strong, reliable welds, without delays or redesigns once you're in production.

Conclusion: Best Plastics for Ultrasonic Welding

Ultrasonic welding is a powerful and efficient method for joining plastic components, but its success depends heavily on material selection. Not all plastics respond the same way to ultrasonic energy, and choosing the wrong resin can lead to poor weld strength, process instability, and increased scrap rates.

By selecting plastics that are known to perform well with ultrasonic welding, like ABS, polycarbonate, and other amorphous resins, you can ensure consistent quality. And if your application requires a more challenging material, early testing, smart joint design, and collaboration with your manufacturing partner become even more important.

In short, don't just choose a resin based on cost or mechanical specs—choose one that fits your process. A well-matched material can save time, reduce risk, and lead to a stronger, more reliable final product.

Do you have a project that needs to be ultrasonically welded? Reach out. We would be happy to speak with you.