Nut Insertion

Thermoplastics, thermosets and other polymeric materials are great for light-duty applications, but do not perform well under compression or shear loads. That is to say, bolting or screwing plastics together using threaded fasteners often leads to damaging the plastics beyond repair. Many plastics in fact are too soft or too brittle to have threads tapped into them to begin with. To address these issues, threaded inserts are used to add threaded connection points to plastics as a means of gaining higher fastening strengths than could be achieved otherwise.    

Installing threaded inserts into plastic parts starts with a cursory review of the materials involved. Different threaded inserts work best with various plastic types, and are further selected based on the temperature, load rating, life expectancy and operating conditions of the parts in question. Depending on the different criteria described above, the best threaded insert style can be chosen, ranging from press-in, mold-in, self-tapping, ultrasonic and heat insertion methods. 

Certain plastics are not conducive to having threaded inserts molded into their bodies during forming. In these cases, inserts can be installed using a post-process procedure in order to protect the base plastic materials, though this can also be done as a cheaper alternative to cast-in insertion when high-strength fastening is not needed. In addition, common variances in the plastic-forming process can leave small dimensional differences between parts, and post-process thread insertion can adjust as needed around these variances. 

Threaded inserts should be selected through careful consideration of the material types involved and the fastening load ratings desired. In particular, threaded inserts are rated by their pull-out strength, torsional strength, shear strength and thermal expansion coefficient – all of which are technical details that govern how well (or poorly) the insert will perform while in use. The higher resilience expected, the larger the insert needed and the higher the cost. Beyond the insert itself, there are more considerations around how the insert will be installed as well, including details such as the density and deformation resistance of the base plastic, insertion force, insertion temperature (for hot-press and ultrasonic inserts) and insertion speed. 

Adding threaded inserts to plastic parts is a great way to obtain high-strength fastening points in otherwise soft, pliable plastic materials. Brittle, dense plastics usually prefer self-tapping inserts installed after initial forming, whereas lower-density plastics prefer mold-in inserts that can be directly cast in place while the plastic part is being formed. Vacuum cast and 3D printed thermoplastic parts often prefer cold-press or hot-press inserts, including ultrasonic inserts. 

Heat staking – or inserting threaded nuts into a plastic base material using heat – is an excellent way to add high-strength threaded connections to plastic assemblies. Knurled nut inserts are pressed into pre-drilled holes in the plastic base, with the press arm adding a small amount of heat to deform and melt the plastic into the insert’s knurled profile. This fuses the insert into the plastic base, offering great resistance to pull-out and torsional failure. In addition to knurled nuts, other common styles include groove nuts, O-ring sealing nuts, smooth wall nuts and expansion nuts. 

Proper heat-staking relies on several key factors: uniform heat distribution, very precise temperature, smooth insertion speed, accurate surface alignment and tightly controlled cooling while maintaining insertion pressure. Though using a soldering iron may work to a degree, this method is not able to achieve and repeatably control all of these technical factors mentioned above. In order to attain high-quality, completely fused, consistent results, a proper heat-staking machine or station is strongly recommended.