Edited by Stephen J. Mraz [email protected]
Plastic parts and assemblies are becoming more common as engineers work to reduce costs and weight in their designs. But plastics often aren’t strong enough to support fasteners such as screws or bolts. Using screws to attach parts to plastic housings, for example, often ends with stripped threads, failed housings, and assemblies that fall apart.
To get around this limitation, engineers first install threaded metal inserts into plastic parts. They give screws and bolts stronger metal threads to mate with, letting plastic parts be easily assembled, taken apart, and repeatedly reassembled.
Two of the most common methods for installing inserts into thermoplastic parts are heat and ultrasonics. (Thermoplastics can be remelted a number of times. Thermosets, on the other hand, have a onetime reaction and cannot be remelted, making them unsuitable for heat or ultrasonics.)
With both heat and ultrasonic installation, remelted plastic firmly holds inserts in molded or drilled holes by conforming tightly to the knurls, barbs, and undercuts around the outside of the inserts. But the installation process must create enough remelted plastic to entirely fill these external patterns to get maximum performance, including pullout and torque resistance, when the plastic solidifies.
Both heat and ultrasonic installation depend on melting the plastic around the inserts, but results can vary with each method. Engineers should consider the advantages and disadvantages of both methods before purchasing installation equipment or finalizing manufacturing plans.
During ultrasonic installation, a relatively small downward force, typically generated by a pneumatic cylinder, presses the insert into a predrilled or molded hole while an ultrasonic horn converts electric power into high-frequency vibrations. Those vibrations get delivered to the insert-plastic interface through direct contact with the insert. The vibrations generate enough heat to melt the plastic around the insert. Equipment needed for ultrasonic installation includes an electronic power supply, cycle controlling timers, an electrical or mechanical energy transducer, and an ultrasonic horn.
Here are some of the advantages of ultrasonic installation:
• It is generally fast for inserts under 0.250-in. OD, leading to short cycle times. These times increase with insert size.
• The machines can be changed to accommodate different sized and shaped inserts.
• Ultrasonic-installation machines can also be used for plastic-to-plastic welding and other processes.
Now here’s a look at the downside of ultrasonic installation:
• Insufficient melt, or not enough plastic being melted, can result in inserts being reamed into solid plastic. This so-called cold pressing can damage inserts and the plastic parts. Cold pressing can be caused by several factors. For example, bad fixturing/clamping of the plastic part can dissipate vibrations, preventing enough heat from being generated around the insert. Slight size variations of inserts or holes can also lead to cold pressing. And if inserts are driven too quickly, the plastic does not have time to fully melt, creating high stresses and poor retention in the plastic. This can lead to part failure during installation or, even worse, in the field.
Although sophisticated controls can prevent cold pressing, the cost of such controls nearly doubles the price of an already expensive ultrasonic installation machine.
Here are some of the other downsides of ultrasonic installation:
• Ultrasonic vibrations can chip metal particulates and flakes off of inserts. These flakes could prevent the ins er t f rom seating properly, thus weakening the plastic-to-insert connection. The flakes are also unsightly.
• Ultrasonic installation machines are loud due to the metal-on-metal contact between the insert and ultrasonic horn. And the bigger the insert, the louder the noise.
• It is difficult and costly, if not impossible, to install multiple inserts simultaneously, depending on the design.
• Using the wrong vibrational frequencies or downward force damages inserts.
• Extra caution needs to be taken when using nonheaded inserts to ensure proper contact is made between insert and the horn. Otherwise, it is likely that the inserts’ internal threads will be damaged.
• Ultrasonic horns are expensive. They are also subject to wear and often need to be replaced. It is common for them cost over $1,000.
Machines that rely on heat to install inserts use one of two approaches. In some, a metal tip transfers heat to the insert. In the others, inserts are preheated and pneumatically pressed into predrilled holes in the plastic. In both approaches, inserts are pushed into the plastic by a controlled force, usually less than 50 lb. Heat installation also requires the entire insert be heated, not just the metal-plastic interface. So for proper installation, inserts should have enough thermal conductivity so that plastic around the insert quickly heats and melts. That’s why two of the most common insert materials are brass and aluminum. Once the plastic melts, it fills the insert’s retention features, then solidifies while inducing minimal stresses.
Good thermal conductivity also lets inserts cool quickly after installation. Here are some of the advantages of heat installation:
• Reliability and consistency. Lower installation forces let inserts be installed into thin-walled parts which would be destroyed by ultrasonic equipment. With stable and adjustable temperature, force, and depth settings, installed inserts have predictable pull-out and torsionfailure forces, which can be tailored for applications.
• Noise levels. Heat installation is quieter than ultrasonic installation.
• More economical. Heat installation machines cost about half of ultrasonic equipment because they are less complex and do not require as many components.
• Easy insertion into deep recesses. Longer heat tips can place inserts in a part’s deep recesses which would be inaccessible to an ultrasonic horn.
• Versatility. Platen-style machines can install several inserts on different planes. And prototyping or lowvolume applications can be handled by manual machines. The same machine can also install inserts of different sizes by switching out the interchangeable heat tips. Heat installation also works with headed and nonheaded inserts.
• Automation. Heat-insertion equipment can be equipped with vibratory bowl feeders so operators never need to touch the inserts during installation. This is important for small inserts, which are difficult to handle and orient.
• Minimal maintenance. Heat machines seldom need maintenance (if ever). Maintenance and sparepart costs are low, and heat tips cost approximately $55.
• Better performance. Completely heating inserts lets melted plastic fully flow into all retention features. Ultrasonically installed inserts sometimes are insufficiently heated.
One problem with heat installation is that it takes longer if inserts aren’t preheated. In fact, ultrasonic installation has quicker insertion and shorter cooling times, giving it shorter cycle times than heat insertion when installing a single unheated insert. However, preheated inserts will have comparable installation times compared to ultrasonic equipment. In addition, when installing multiple inserts simultaneously, heat insertion offers faster throughput.
As much as 75% of an insert’s performance depends on how well it was installed, therefore all the factors that affect installation must be carefully controlled to maximize performance. In general, heat installation gives users more flexibility, consistency, and performance at a lower cost. But with so many different combinations of inserts, plastics, and performance requirements, manufacturers should partner with fastening and assembly experts. After all, choosing the right insert and installation process can be the difference between failure in the field and integrity for the life of the assembly.