Self tapping screw design

With regard to screw geometry the following requirements should be observed:
- thread flank angle must be as small as possible (30°) in order to obtain small radial and hoop stresses in the boss
- thread core design possibly profiled in order to allow a trouble-free material flow during the thread-forming process

In special cases, the usefulness should be established by means of component tests.

Standard thread cutting screws

The figure below shows some examples of standard thread cutting screws. These screws are provided with cutting slots.

The type BT (formerly known as type 25) screw is the most common standard thread cutting screw due to its wide thread spacing and generous cutting slot.
The type BF screw also has wide thread spacing, but the slotted cutting flights may tend to clog when working with softer materials. The B series screws have been used with materials having a flexural modulus as a low as 1400 MPa.
The type T (or 23) is often useful with very high modulus glass reinforced materials with a flexural modulus greater than 7000 MPa.

Standard thread forming screws

Thread forming screws do not contain cutting slots and do not produce chips. They are generally used with plastics with a modulus smaller than 3000 MPa. The figure below shows some examples.

These standard screws with their 60º thread angle generate relatively high radial and hoop stresses. The wider thread spacing of the types AB and B is recommended over the type C for most applications. The gimlet point of the type AB necessitates extra long bosses as the tapered point does not contribute to the strength of the connection.

Self-tapping screws designed for plastics

There are four commonly used screw types:
HiLo® screws
Plastite® screws
PT® screws
Polyfast® screws

HiLo® screws

HiLo® screws are designed with a double lead, consisting of a high and a low thread. The screw thread configuration has a smaller minor diameter than that of conventional screws and the high threads make a deeper cut into the material between the threads, contributing to greater resistance to pullout and stronger fastening. The high thread has a 30º thread angle and the low thread has a 60º thread angle. The screws are available in thread cutting and thread forming varieties and with different point and head styles.

Plastite® thread-forming screws

Plastite® thread-forming screws have a trilobal cross-section, which reduces the driving torque. After installation, cold flow of the plastic effectively locks the screw in place, increasing the resistance to loosening and making the screw ideal for vibration applications.

Plastite screws feature a tri-lobal cross section

PT® screws

PT® screws, supplied by Ejot, are thread-forming screws with a 30º thread angle and a modified shank, which is said to improve plastic flow during the thread forming operation.

Polyfast® screws

Polyfast® screws have an asymmetric screw profile. The widely spaced flights have a 35º leading edge and a 10º trailing edge.


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