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SI® (Standard Inserts), a PEM® brand, offers a great range of threaded inserts. Their high-quality products are designed for high performance in plastic, providing reliable and secure fastening solutions. Inserts such as their Tapered Heat-Staking Inserts can be inserted using ultrasonic and thermal heat staking. But which technique should you choose? Read on to find out about their differences and advantages and disadvantages of employing both.
How Ultrasonic/Heat Staking Inserts Work
Ultrasonic and heat-staking inserts, also referred to as heat-set inserts, often feature both right-hand and left-hand knurls on various parts of the outer diameter to limit both rotational and axial movement. These knurled areas can be used alone or with barbs to enhance the pull-out strength. By melting the plastic around the insert during installation locally, the barbs and knurls are more effectively filled than press-in inserts, resulting in a more efficient interference fit. Inserts designed for tapered holes, such as Straight Wall and Piloted Heat-Staking Inserts, have a one-sided pilot diameter with one-way barbs. In contrast, inserts for straight holes can have symmetrical designs to streamline the production process. Typically made from brass, aluminum, or stainless steel, these inserts are manufactured by cutting them from round bar stock.
Installation Process
Ultrasonic insertion utilizes frictional heat generated by high-frequency vibrations to melt the plastic surrounding an insert, allowing for easy insertion. The insert is placed in a pre-drilled hole, where ultrasonic vibrations are applied. These vibrations cause the plastic to melt around the insert. Once the vibrations stop, the plastic solidifies, locking the insert permanently in place.
When heat-set inserts undergo heat staking, they're pressed into a pre-molded or drilled hole using a thermal press. A heated probe contacts the insert, heating it to or above the plastic's melt temperature. As the insert is driven into place, localized melting occurs, allowing the plastic to flow around it. Upon retraction of the probe, the plastic re-solidifies, securing the insert.
Although they use different methods, both ultrasonic and heat-staking installation machines are created to melt material again after molding by transferring heat through the insert. The plastic surrounding the insert solidifies after reshaping to match the knurls and barbs of the insert, creating a strong bond at the interface. Ultrasonic machines generate vibrations in the insert at a frequency of 20kHz, causing a thin layer of plastic at the insert surface to melt due to the resulting friction. However, some factors need to be adjusted in order to effectively "weld" the part, including amplitude, feed rate, pressure, and welding time.
This method makes amorphous plastics more flexible due to their wide range of softening temperatures during the glass transition phase. In contrast, semi-crystalline thermoplastics need more energy to reach their melting point. Heat-staking machines apply heat directly to the insert, adjusting the temperature to just 50°F (or 20°C) above the melting point of semi-crystalline thermoplastics. This effectively melts the material without causing it to burn, although finding the optimal installation temperature for amorphous plastics may require some trial and error. Thermosets are not compatible with either of these installation techniques.
Applications
Heavy-duty power tools like cordless drills must be sturdy enough to withstand the dirt and wear and tear associated with a construction site and be light enough to use for the entirety of a workday comfortably. This is accomplished using an amorphous, thermoplastic housing with threaded inserts to securely seal the electronics of the drill from its environment. The amorphous plastic is light and extremely impact-resistant, unlike semi-crystalline thermoplastics or thermosets. Furthermore, the glass transition allows ultrasonic inserts to be installed faster and with less energy into the molded bosses than heat-staking inserts.
Advantages and Disadvantages of Ultrasonic Insertion vs. Thermal/Heat Insertion
The ultrasonic insertion method is known for its speed and reliability, delivering consistent and repeatable results. It is highly compatible with most thermoplastics, making it a versatile choice across applications. Additionally, ultrasonic insertion uses standard 20kHz welders, which are widely available, enhancing its accessibility for many users.
However, one of the main disadvantages of ultrasonic insertion is the noise it generates, which can be a concern in certain environments. The process can be more abrasive, potentially damaging the insert. Additionally, it can create metal dust, which may be undesirable in some applications. The method is also limited by the number of inserts that can be installed at once.
Thermal/heat insertion is a gentler method than ultrasonic insertion, reducing the risk of damage to the insert. It promotes better plastic flow during installation, leading to a more secure fit. The process is also clean, as it does not generate metal dust. Multiple inserts can be installed simultaneously, making it efficient for large-scale operations. This method is particularly effective for larger inserts and can simultaneously accommodate inserts on different levels. Furthermore, tooling, equipment, and maintenance costs are generally lower than those associated with ultrasonic processes, making it a cost-effective choice.
Still, thermal/heat insertion can be slower when installing inserts one at a time than ultrasonic methods. The inserts also require a longer cool-down period before handling, which can slow down the overall process.
Conclusion
Overall, the choice between ultrasonic insertion and thermal/heat staking ultimately comes down to the specific requirements of the project at hand. Ultrasonic insertion may be preferred for applications where speed and reliability are crucial. At the same time, thermal/heat staking could be the better option for projects that require a non-abrasive and cost-effective solution. With the advancements in insert technology, such as externally threaded stud inserts by SI®, manufacturers now have more options to choose from to meet their assembly needs with precision and durability.
HTF can help manufacturers choose which approach to use for SI® threaded inserts within the context of their operations. This helps our clients make well-informed decisions to streamline assembly processes and enhance the overall integrity of their products. With HTF's commitment to prompt service, including same-day shipping from a single location when orders are placed by 3 PM Eastern, you'll get efficient procurement and support for your projects. Get a quote for supplying your operations today.
