by Christopher G. Jeznach, Application Engineer, SPIROL International Corp.

Thermoplastic insert parts can be installed using heat or ultrasonic methods. Here’s a look at the pros and cons of each approach.

As the use of plastic parts has increased in a number of industries, fastening methods have become increasingly important. When screws or bolts are threaded directly into plastic components, failures can occur due to stripped threads or plastic creep. In situations where joint strength and the ability to assemble and disassemble without degradation of components are required, threaded inserts satisfies both needs.

Undercuts, knurls and threads on the outside of an insert help improve its performance.

It is important to note that there are other methods for installing inserts. Installation of inserts after molding (post mold) versus molding-in reduces costs by shortening molding time. Post mold installation also reduces the chance of scrap and potential mold damage resulting from dislodged inserts.

Heat and ultrasonic installation is only used with thermoplastic parts. Thermoplastics are solid at normal temperatures and can be re-melted a number of times, whereas thermoset parts have a one-time reaction in their conversion from liquid to solid and cannot be re-melted.

With both heat and ultrasonic installation methods, the insert is embedded into a molded or drilled hole through re-melting of the plastic. The melted plastic conforms to the external features of the insert, ensuring retention within the hole. A sufficient volume of plastic must be displaced to entirely fill these external features to ensure maximum performance when the plastic solidifies. An accurate way to determine sufficient plastic flow into the knurls, barbs and undercuts of an insert is to take a cross section of the installed insert and check that the features are mirrored in the plastic. It is important to ensure proper plastic flow into the features of the insert as this dictates the torque and pullout performance.

Though they are both dependent on localized plastic melting, heat and ultrasonic installation methods can vary in performance. Both installation methods have advantages and disadvantages.

An ultrasonic insertion machine converts electrical power into a mechanical vibratory output. A pneumatic cylinder typically delivers the downward force while an ultrasonic horn delivers mechanical energy to the metal-plastic interface.

Ultrasonic horns (manufactured from various metals including titanium alloys, stainless steel and aluminum alloys) directly contact the metal insert. As the horn vibrates, the mechanical energy is transferred to the plastic surrounding the insert creating the heat and melting necessary for insertion.

Two methods are available for heat insertion; heat is transferred from the heated tip through the insert to the plastic, or by preheating the inserts and then pressing them in. In both cases, a controlled force is applied to the insert to ensure that the plastic is sufficiently melted before the insert is installed. Since heat installation requires heating the entire insert and not just the metal-plastic interface, the insert material should have excellent thermal conductivity (brass and aluminum are common choices).

Heat installation enables the insert to efficiently transfer heat to the plastic. (It also enables the insert to cool down quickly after installation). Once the plastic reaches its melting temperature, it begins to fill the retention features of the insert and then solidifies while inducing minimal stress.

Due to faster insertion and shorter cooling time, ultrasonic insertion typically has a shorter cycle time than heat insertion when installing a single insert that has not been preheated.

However, heat equipment that preheats the insert will have comparable installation time as compared to ultrasonic equipment. In addition, when installing multiple inserts simultaneously, heat insertion will offer faster throughput.

You can use several methods to install inserts. Installation of inserts after molding (post mold) versus molding-in reduces costs by shortening molding time. Post mold installation also reduces the chance of scrap and potential mold damage resulting from dislodged Inserts. As shown in this table, heat and ultrasonic installation is only used with thermoplastic parts. Thermoplastics are solid at normal temperatures and can be re-melted a number of times, whereas thermosets have a one-time reaction in their conversion from liquid to solid and cannot be re-melted.

Disadvantages of ultrasonic installation One disadvantage is insufficient melt, which results in:

An ultrasonic insertion machine converts electrical power into a mechanical vibratory output. The downward force is typically provided by a pneumatic cylinder.

Another disadvantage is metal particulates and flakes may appear when the ultrasonic horn vibrates against the insert and chips off the insert material. Results of this issue include:

Heat insertion of inserts is accomplished by transfer of heat from the heated tip through the insert to the plastic, or by preheating the inserts and then pressing them in.

Potential disadvantages of heat installation Heat insertion’s slightly longer process time for installation of a single insert, when it is not preheated, is balanced by its advantages over ultrasonic installation.

The flexibility, consistency, high performance, and price of heat insertion make it a good choice for installing inserts into plastic for many applications.

As much as 75% of an insert’s performance is a direct result of how well it was installed, therefore all of the factors that affect installation must be carefully controlled to maximize performance.

With so many different combinations of insert types, plastic types, and performance requirements, it is recommended that manufacturers partner with industry experts in fastening and assembly of insert products. Proper choice of the insert and the installation process can be the difference between part failure in the field and part integrity for the intended life of the assembly.

SPIROL International Corp. U.S.A. www.spirol.com

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