No two screws are created the same. Simply check all the extraordinary varieties of screw heads there are. Just like there are one-of-a-kind varieties of screw heads which can be used for one-of-a-kind motives, the threading on those screws also can vary depending on the utility.
Drywall, stone and metal all require different screw solutions, but possibly the most difficult material of all is plastic. While plastic is very lightweight (which affords a wide range of layout opportunities), it also has a much lower material density than the others. These two properties make fastening plastics a challenge.
When fastening with plastic materials, consider the screw threading.
Select a narrow flank angle
Common screw types Type A and type AB are specifically made for applications that involve metal. These have a wide thread profile, aka "flank angle", of around 60 degrees. This creates outward stress – called high radial force – and can potentially damage when screwing into a softer material.
Plastic fastener threading is designed with a much narrower threading profile, that can range from 29 degrees to 45 degrees, depending on the type of plastic material 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 pullout resistance
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.
Plastic type 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 / 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.
Post time: 12-15-2016