Ultrasonic welding is one of the most widely used methods for joining thermoplastic components, offering speed, repeatability, and clean processing without adhesives or fasteners. However, it is not universally suitable for every plastic material. For design engineers selecting a joining process early in product development, understanding which materials weld well ultrasonically and why can save significant time and cost downstream.
This guide provides a practical framework for assessing whether ultrasonic welding is appropriate for your thermoplastic, with reference to material structure, additives, and the design and process implications that follow from your material choice.
The Fundamental Requirement: Thermoplastic Structure
Ultrasonic welding generates heat through the rapid vibration of molecules at the joint interface. For this to work, the material must be a thermoplastic: a polymer that softens when heated and solidifies on cooling. Thermosets and elastomers cannot be ultrasonically welded because they do not undergo this reversible phase change.
Within the broad category of thermoplastics, two structural families behave very differently under ultrasonic energy.
Amorphous Thermoplastics
Amorphous polymers have a random, disordered molecular structure with no distinct melting point. Instead, they soften progressively above their glass transition temperature (Tg). This gradual softening allows energy to be transmitted efficiently through the material from the sonotrode to the joint, even when the weld interface is relatively far from the contact point (far-field welding). Common amorphous plastics that weld well include ABS, polycarbonate (PC), acrylic (PMMA), polystyrene (PS), and SAN.
ABS is generally considered the benchmark for ultrasonic weldability, combining excellent vibration transmission with reliable molecular bonding and broad process latitude.
Semi-Crystalline Thermoplastics
Semi-crystalline polymers have ordered molecular regions (crystallites) alongside amorphous zones. They have a sharp melting point, which means energy must reach the joint interface before significant softening begins. This makes far-field welding difficult and unreliable. Near-field welding, where the sonotrode contacts the part within 6 mm of the joint, is typically required.
Common semi-crystalline plastics include polyamide (nylon/PA), polypropylene (PP), polyethylene (PE), acetal (POM), and polyester (PBT, PET). These materials can be welded ultrasonically, but they demand more precise process control, careful joint design with appropriately sized energy directors, and often higher amplitude settings.
Material Compatibility: Can You Weld Dissimilar Plastics?
Ultrasonic welding fundamentally relies on molecular interdiffusion at the weld interface. For a strong bond to form, both parts must be of the same or a closely compatible polymer family. The general rule is like welds to like.
Some compatible combinations exist, notably ABS with PC, and ABS with PMMA. However, combinations such as PP with ABS, or nylon with acetal, will not produce reliable structural bonds. Where dissimilar materials must be joined, ultrasonic staking or inserting may be more appropriate than welding, as these operations rely on mechanical interlocking rather than molecular fusion.
The Influence of Additives and Fillers
The base polymer family is only part of the picture. The specific grade of material, including its additives, colourants, fillers, and processing aids, significantly affects weldability.
Glass Fibre Reinforcement
Glass fibre content in the range of 10 to 20 percent generally improves ultrasonic welding performance, particularly for semi-crystalline plastics, by increasing the transmission of vibration energy through the material. However, filler content above approximately 35 percent reduces the available thermoplastic matrix at the joint interface and can lead to brittle, inconsistent welds. Engineering grades with very high filler loadings require modified joint designs and process parameters.
Flame Retardants
Flame retardant additives, which can comprise up to 50 percent of material weight in some grades, significantly reduce the thermoplastic content available for bonding. Materials with high flame retardant loadings typically require higher power equipment, increased amplitude, and modified joint geometry to achieve acceptable weld strength.
Pigments and Colourants
Most colourants have negligible effect on weldability. However, white pigments containing titanium dioxide can act as a lubricant at the joint interface if present in excess of approximately 5 percent, reducing bond strength. This is a common source of unexplained weld quality variation when colour variants of the same product are compared.
Moisture Content
Hygroscopic materials such as nylon, ABS, and PMMA absorb moisture from the atmosphere. If these materials are welded without adequate pre-drying, water at the joint interface reaches boiling point during the weld cycle, producing steam that causes voids and a foamy weld interface with significantly reduced strength. Parts should be welded immediately after moulding or dried to the manufacturer's recommended moisture content before processing.
Practical Decision Criteria
Use the following questions to assess whether ultrasonic welding is likely to be suitable for your application:
- Is the material thermoplastic? If not, consider vibration welding, hot plate welding, or adhesive bonding.
- Is the material amorphous or semi-crystalline? Amorphous materials offer more process latitude; semi-crystalline materials require near-field welding and precise joint design.
- Are both parts made of the same, or a compatible, polymer? If not, consider mechanical joining methods or ultrasonic staking.
- What is the filler content? High filler loadings above 30 to 35 percent may require joint design modifications and higher-powered equipment.
- Does the material contain flame retardants, lubricants, or high pigment loadings that could interfere with bonding?
- Is the material hygroscopic, and has it been stored or handled in a way that may have introduced moisture?
- What are the wall thickness and part geometry? Very thin walls or large far-field weld distances may point towards alternative technologies.
When To Consider An Alternative Process?
Ultrasonic welding is not always the optimal choice. The following scenarios typically favour alternative joining methods:
- Large, flat weld interfaces with significant joint area: vibration welding or hot plate welding often delivers better results.
- Highly filled or rubber-toughened semi-crystalline materials with poor vibration transmission: infrared or hot plate welding may be more reliable.
- Dissimilar material combinations without molecular compatibility: mechanical fastening, ultrasonic staking, or adhesives should be considered.
- Applications where vibration or ultrasonic energy could damage sensitive internal components such as electronics or fragile subassemblies: hybrid vibration welding using infrared pre-treatment may reduce the required frequency to safe levels.
Conclusion
Material selection and joining process selection are interdependent decisions that should ideally be made together, early in the design process. Choosing an amorphous thermoplastic with no problematic additives gives you the broadest ultrasonic welding process window. Semi-crystalline materials and highly filled grades are workable but demand more attention to joint design and process control.
Early engagement with a specialist welding engineer can prevent costly design iterations. At Xfurth, our consultancy and design service supports design engineers from initial material assessment through to prototype trials and production process validation.
Contact Xfurth today to discuss your material and application requirements: www.xfurth.com or +44 (0)1582 436000.
