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Why is plastic fastener threading important?

Brightly coloured plastic threaded fasteners

When fastening plastic materials, you need to think carefully about the fastener threading on the screws you choose. Plastic is the most difficult material to fasten, as it’s lightweight and has a much lower density than other materials. Unless you want to make costly mistakes, pay attention to your fasteners’ threading.

Which plastic?

Let’s start with the plastic you use. Different plastics have different properties, of course. Consider your application. For commercial-grade plastics, fasteners with blunt points work best. If you’re using plastics like phenolics, go for a fastener with a slotted shank. For the hardest plastics, a slotted-shank screw with a conical point is your best bet.

Use a narrow flank angle

Type A and Type AB screws are designed for applications involving metal. They have a wide thread profile, or ‘flank angle’ of around 60 degrees. This means they can create a high radial force, causing outward stress that results in damage when screwing into a softer material.

Plastic is a very different matter, with fastener threading possessing a narrow threading profile. Its flank angle ranges anywhere from 29 to 45 degrees, depending on the type of plastic you’re using. This reduces radial stress and expansion, which in turn optimises fastener performance.

Because of the reduction in radial stress, fasteners for plastics can use smaller bosses. This in turn cuts your overall costs, thanks to less material needed and moulding cycle times.

Maximise stripping torque

The deeper the penetration, the more friction increases, so maximising stripping torque is crucial. This is the torque needed for the fastener’s thread to strip the plastic. You’ll find self-tapping screws effective, especially if the holes are drilled beforehand. Ensure that the thread spacing is wider than a traditional screw used with metal.

Keep in mind that traditional screws are also problematic when it comes to pullout. You can get around this with a greater area of thread engagement, which is the distance through which the fully formed threads of both the nut and bolt are in contact.

Your rewards: reduced costs

Fasteners with the right threading for your application cuts out waste, which helps control your costs and reduces cycling times. The right choice also lets you streamline the assembly process and avoid the need for inserts and adhesives.

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