Self-tapping screws are grouped under two groups viz.: thread forming and thread cutting. The selection of screw-type will depend on its application requirement, material type and properties. There are a lot of trademarked and patent designs from various companies such as Stanley, EPOK, TR fasteners etc. All the manufacturers have options with different thread profile angles (30-45-48 degrees) to suit the range of flexural modulus of plastics. Thread forming flank angle with trilobular threads– A trilobular bodied single start thread forming screw with a 45-degree thread angle designed for installing into a wide variety of thermoplastics including glass-filled Nylons and modified Polyphenylene oxide. This is a challenging question to answer and one which can and does puzzle many. Self-tapping screws tap their own thread, which means they require a pilot hole which is slightly smaller than the diameter of the screw. They work on the premise that if you drill a pilot hole into a soft material like wood, as you drive the tapping screw in, its threads will make the material secure by digging its way into it. Designed and manufactured to be used in thin gauge, malleable material, thread forming screws comprise coarse threads. A hole of the proper size is therefore required as, at the point of the fastener being installed, the hole will be made bigger when the material is pushed outward. The most common sheet metal screws are Types A and AB, both with a standard point, alongside Type B, with its blunt point. The Self-Drilling Screw What is a self-tapping fastener and how can it improve the way you work? Able to tap its own hole as it’s driven into the material – be it wood, plastic or metal – the cleverly-designed component can be used in a variety of different applications.

Boss heights should be constructed so that there is no gap between the top of the boss and the component being clamped. The American Society for Testing and Materials (ASTM) conducts analyses of self-tapping screws to be sure the science for the turn is correct. The tests ensure the screws are being manufactured to established standards and will perform as expected. These standards test the screws’ following qualities: flank angle with trilobular threads– A trilobular bodied single start thread forming screw with a 60-thread angle designed for standard performance in Nylon, Acetal, ABS, and some glass-filled nylons.Flexural modulus or bending modulus measures the stiffness of the plastic during the initial step of the bending and deforming process and is the stress to strain ratio in flexural deformation. In simple terms, plastics’ tendency to resist bending. Materials Hex washer head drill points self-drilling screws in a few dozen sizes with zinc-tipped steel composition and the option for painted heads.

Gutters: Self-tapping screws are usually the foundation of any gutter-attachment system. They make for less work, a secure hold and a finish that will withstand a beating from the water and rooftop debris. The right screws will hold the gutters in place, withstand weather, resist corrosion, look neat and not drive too deeply, among other things. Pilot hole diameter and thread engagement – The pilot hole diameter and the diametrical thread engagement will go hand in hand. The amount of thread flank depth filled by the material is referred to as thread engagement and represented as a percentage. Never use 100% thread engagement as it will increase the torque required and does not improve the joint performance. As a rule of thumb, 75%-80% of thread engagement is used on stiff materials to achieve a good joint. This key difference is self-drilling screws have a tip like a drill bit, so they do not need a pilot hole. The fact that they can drill and tap their own hole into harder materials saves you using two tools, when this one component will do both jobs. Key product design factors that can influence a fastener’s ability to perform satisfactorily are pilot hole diameter, thread engagement, length of engagement and boss design. Though product design may impose certain constraints on these four key factors, laboratory testing will establish the best mix of these parameters for your application. So, you must do some testing on sample plastic material with different screws. Self-tapping screws for plastic – fastener interface design (Credit:stanleyengineeredfastening.com)

Screws for plastic – in stock

All polymers will creep or irreversibly distort when subjected to long term stress or heat. This creep will result in a loss of clamp load. The graph shows a standard self-tapping screw for plastic losing clamping load over time. Creep rate of self-tapping screws for plastics (Credit:stanleyengineeredfastening.com) flank angle– A 30 degree round single start thread forming screw with a reduced core and wide thread pitch for high performance in most thermoplastics and some thermosets Thread forming screws for plastic doesn’t have a pointed tip and would need a pilot hole. Screw thread deforms the plastic material to flow around the screw threads. Hence the overlapping amount (thread engagement) and thread profile angle are crucial. Thread forming screws for plastic (Credit:stanleyengineeredfastening.com) Since plastics are softer than metal, the threads can be easily stripped while tightening the screw. Stripping torque of screws for plastic can be calculated using the formula below for a given pulling out force F. Stripping Torque Diameter: When it comes down to holding something in place, the girth of a screw plays a key role. The self-tapping screw diameter should be matched to the job, whether light or heavy.

Recess: The recess is the part of the screw that receives the screwdriver or drill. It can have a variety of shapes such as square, slot, Phillips and hex. Plastic: The technology associated with self-tapping screws for plastic has evolved to the point that the pieces are now specialized and sometimes made of plastic themselves. Self-tapping screw technology has evolved to include more lightweight materials such as plastic and aluminum and shorter thread engagements. Thread forming screws creates high internal stresses because the material is deformed rather than removed. As a result, these screws can only be utilised in soft polymers with lower flexural modulus. Thread cutting The advantage of self-tapping screws for aluminium is that they don't need pre-drilling and are easily removed without damaging the metal. They can also be reused in the same material. These metal screws require pre-drilled or moulded holes and most commonly have a 30° or 45° flank angle, although 60° angles are also available for specialist applications. For screwing into thinner materials, a selection of our FasPlas screws are available with trilobular heads, which lessen the risk of burst out.Why might you need a self-tapping fastener? In some cases, for example when using sheet metal, it isn’t possible to tap a hole as the material may be too thin. You may also require self-tapping fasteners if you can’t gain rear access to install a nut. In both these instances, self-tapping fasteners ensure you can get the job done smoothly and effectively. Understanding types and materials of which they are applied is vital - for example if you are using self-tapping screws for aluminium, they don't need pre-drilling and are easily removed without any damage caused to the metal. And, actually, they can be reused in the same material. Flexural modulus is used to identify the effectiveness of the self-tapping screw joint. Generally, the lower flexural modulus means that more material will flow around the thread profile and allow better thread formation, hence a better joint. Higher flexural modulus thermoplastic would usually need a low helix angle fastener to avoid excessive drive torque. Effect of additives and fillers There’s a huge range of different types of self-tapping fasteners and each can be set apart by a designation which describes its point (tip) and threads. Here are the most common self-fasteners and how you might use them in your applications: The Sheet Metal Screw Now for a few frequently-asked questions relating to self-tapping and/or self-drilling screws: What's the difference between a self-tapping screw and a self-drilling screw?