Fastener selection plays a vital role in determining long-term part integrity, assembly repeatability, and structural performance. Among the most widely used fastening methods for thermoplastics are heat set inserts and ultrasonic inserts. Both methods deliver durable, threaded metal connections, but they differ in how they interact with molded materials and production environments.
What Are Heat Set Inserts?
Heat Set Inserts are threaded metal components, commonly made from brass, aluminum or stainless steel, installed using conductive heat and pressure. The insert is pressed into a pre-molded hole while heated, causing the surrounding plastic to soften and flow into the knurled and undercut features of the insert. As the plastic cools and resolidifies, it locks the insert into place mechanically and thermally.
This method is widely used in various engineering plastics like PBT, PA6, and PP. Heat set installation allows deeper plastic flow and often results in higher pull-out and torque strength in soft or toughened plastics.
Ultrasonic Inserts: Speed and Consistency
In contrast, Ultrasonic Insertion uses high-frequency vibrations delivered via a horn or sonotrode to melt the plastic locally at the insert interface. The vibrations generate heat through friction, allowing the insert to be embedded quickly without external heating tools.
Ultrasonic insert installation is especially advantageous for automated and high-speed production lines. The equipment is compact and cycle times are extremely short, making ultrasonic installation ideal for high-volume applications in electronics housings, automotive trim panels, and appliance components.
However, ultrasonic insertion may be less effective in high glass-filled resins or semi-crystalline plastics due to inconsistent energy transfer and melt behavior.
Insert Boss Geometry: Why One Size Doesn’t Fit All
One of the most critical aspects of successful insert performance is boss geometry. The size, shape, and tolerance of the hole into which the insert is installed play a major role in joint integrity, regardless of the installation method.
For heat set inserts, bosses must accommodate material flow around retention features like knurls and undercuts. Taller bosses provide better heat distribution and reduce the risk of sink marks or part warping. A generous fillet at the base and minimal draft angles help maintain surface contact and minimize installation stress.
With ultrasonic inserts, precise alignment between the horn and insert boss is essential. Since the process relies on vibrational energy coupling, any irregularity in the boss shape, draft angle, or wall thickness can lead to inconsistent bonding and decreased performance. Shorter bosses with consistent wall thickness are generally preferred to ensure efficient ultrasonic energy transfer.
In both methods, undersized holes can increase hoop stress and cause cracking, while oversized holes reduce retention strength. Guidelines recommend a hole diameter that accounts for the insert’s OD, material shrinkage, and thermal expansion characteristics.
Installation Speed and Process Control
Heat set inserts can be slower to install, especially in manual or semi-automated production. That said, heat set inserts offer high repeatability and more control over thermal behavior. Repeatability and control are critical in components with tight tolerances.
Ultrasonic insertion is significantly faster and more scalable, but requires consistent part geometry, resin density, and horn contact. Engineers must carefully design for vibration coupling to avoid uneven melting or surface defects.
Holding Strength Comparison
Holding strength depends on the insert design, host material, and process control. Heat set inserts tend to produce higher pull-out and torque resistance in soft plastics due to deeper material flow into the insert’s retention features.
Ultrasonic inserts can match this performance in amorphous plastics like ABS and polycarbonate, where energy transfer is uniform. However, variability in horn contact or part geometry can lead to inconsistent retention in some cases.
Application Suitability and Engineering Considerations
- Heat set inserts are preferred for:
- Low- to mid-volume runs
- Prototype parts or variable part thickness
- Thermoplastics with high crystallinity or fiber reinforcement
- Ultrasonic inserts are ideal for:
- High-throughput production lines
- Consumer and automotive assemblies with consistent geometry
- Applications requiring rapid assembly cycles
Regardless of method, correct hole sizing, thermal profiling, and insert geometry are critical for success. Early collaboration between design, molding, and fastening engineers improves joint reliability and overall assembly efficiency.
Conclusion
Heat set and ultrasonic inserts each bring distinct advantages depending on the resin system, production scale, and application environment. From durable threaded bosses in control modules to fast-turnaround components in industrial enclosures, choosing the right insert method ensures higher part integrity, fewer failures, and better long-term product quality.
Where Does CFI Fit In?
For over 30 years, Components for Industry has been a worldwide provider in industrial components and a partner for industrial plastics and composites companies in a number of industries such as Automotive, Aerospace, Appliance and Heavy Industrial. If you want more information about CFI, our industrial components including threaded inserts, contact us at 847-918-0333 or sales@componentsforindustry.com.
