Plastic Fastener Threading: what do you want to know?
No two screws are created equal. Just take a look at all the different kinds of screw heads there are. Just like there are different kinds of screw heads that are used for different reasons, the threading on those screws can also vary depending on the application.
Metal, drywall, and stone require different products, but perhaps the most challenging product of all is plastic. It is very lightweight, which presents a wide range of design possibilities. It is also a much lower density material than metal. These two features make fastening a challenge.
When fastening plastic materials, you need to think carefully about the fastener threading on the screws you choose.
Choose a narrow flank angle
Type A and type AB screws are specifically made for applications that involve metal. They have a wide thread profile, or flank angle, of 60 degrees. This creates a high radial force, creating outward stress and potential damage when screwing into a softer material.
Plastic fastener threading is designed with narrower threading profiles that range from 29 degrees to 45 degrees, depending on the plastic being used. Although they are able to displace the same volume of material as screws with a 60-degree thread profile, fasteners with a smaller thread profile can reduce the radial force by half. Tri-lobed screws with slender thread angles are popular in this regard because they form deep internal threads while generating less hoop stress.
With the reduction in radial stress, fasteners made for plastics can utilize smaller bosses, which can reduce the overall costs associated with material usage and molding cycle times.
Maximize stripping torque and increase resistance to pullout
When it comes to plastic materials, it’s also important to maximize stripping torque, which is the torque needed for the fastener’s thread to strip the plastic. Self-tapping screws are usually effective in this regard, especially if the holes are pre-drilled. The thread spacing should be wider than a traditional screw used with metal.
Traditional screws can also cause problems when it comes to the potential for pullout. A greater area of thread engagement can help ensure this doesn’t happen.
Plastic fasteners generally have a smaller minor diameter and a higher thread profile, allowing the screw to contain a larger volume of material, making the screw less resistant to pullout.
The type of plastic matters
When it comes to plastic fastener threading, different types of plastics have different properties, so the type of plastic you’re using in your application matters. When it comes to commercial-grade plastics, fasteners with blunt points are best. A fastener with a slotted shank works well with plastics like phenolics, while the hardest plastics may require a slotted-shank screw with a conical point.
Even screws with plastic fastener threading can cause problems for some materials. A nail-type faster might be the solution because it has barbs and rings that more effectively bite into the plastic than standard threads.
Optimal performance with reduced cost
Ultimately, choosing a fastener with the right threading can ensure optimal performance and reduced costs. The proper fastener can provide you with higher strip-out torque values, an increased resistance to loosening, and higher pullout values. In addition to the reduced costs in regards to material usage and cycling times, you can streamline the assembly process and eliminate the need for inserts and adhesives.