Today, there are an estimated 13,000 varieties of plastics with more introduced every day. Fillers including fiberglass, talc, stainless steel, mica and carbon fibers are being used to boost the performance of plastic. And, while each additive gives plastic a different level of performance, they also change the fastening characteristics of the fastener as well as the design requirements of the end product (including the length of engagement, the hole diameter and the boss diameter).

Ideally, this planning should start as far back as the product design stage. At this point, product designs can be altered to accommodate the required fastener. The problem is that most manufacturers are not afforded this luxury. A new material is often introduced to a standing design, and cost usually prevents a product re-design even if it's actually the most prudent alternative in the long run.
The staggering number of new variations makes the final selection of a fastener design a complex process. However, the following considerations can be a good place to start:

Flexural Modulus
This is a measure of a material's stiffness, and should be the first factor considered in fastener selection. In typical cases, a flexural modulus under 1.3 million p.s.i. will dictate the use of a thread-forming screw. If the material's flexural modulus is over 1.3 million p.s.i., a thread cutter is usually recommended because the material is too stiff to allow thread-forming.

The "borderline" applications, those with.a flexural modulus between 1.2 and 1.4 million p.s.i., are usually the toughest because their fastening characteristics are unique to each application. This is another case where planning and fastener testing is imperative to ensure that all application parameters are satisfied.

Filler Content and Type
The next item to be considered is the type and amount of filler added to the plastic. For example, fiberglass and talc might increase the stiffness of a plastic, but they also increase the friction encountered by a fastener. This could dictate a different fastener design, such as a switch to a different thread profile or a thread cutter. Lubricants may also be added. They present problems because they lower the required drive torque, but rarely respond with the clamp load you would expect.

Stainless steel and carbon fibers are also tricky because they do not add rigidity to the plastic, but still cause problems with increased friction. Same thing with impact modifiers - they increase the material's rigidity but not the stiffness, and will also dictate a specific fastener design.

Joint Requirements

Does the joint require re-assembly? If so, it might not be a candidate for a thread-cutter. On the other hand, the material might not lend itself to any other fastener design. This is another situation where it's better to spend the time at the planning stage to ensure a successful application.