The marketplace is flooded by offerings of some format of high performance selftapping screw that claims to work better than anything else for threading into plastic. Most seem to have more than just general buzz going for them; different flank angles, tailored pitches, graduated point diameters, etc. Sometimes it appears as though they’re all overlapping versions of the same technology.
The plastic industry continues to develop dynamic alloys and new applications; fasteners have maintained pace. By definition the materials we are concerned with are called thermoplastics, all of which exhibit varying levels of flexibility and memory; for example, ABS (acrylonitrile), PC (polycarbonate), and Nylons. These and many others have “cold flow” properties and are ideal for self-tapping screws of the thread forming or thread rolling styles.
Given that, let’s focus on a thread design that isn’t novel, but material, equipment and tooling have evolved to permit pitch, diameter and thread form combinations that generate remarkable results when threading into most thermoplastic alloys.
What we are defining is a 30 Degree thread form called Type FT. By comparison, a standard Type B thread profile is 60 Degrees Included. A Plastite Twin Lead profile is 48 Degrees Included. The obvious advantages of a 30 Degree thread form are:
- The narrower profile moves into the receiving plastic with less resistance.
- Lower driving torque is required.
- Less heat is generated.
- Sharp threads at the major diameter permit a “knife” versus “wedge” action as the
thread form moves into the receiving material.
- The cutting process of this sharp thread displaces receiving wall plastic with less
outboard pressure. Splitting and distortion are eliminated; bosses can be thinner and
permit greater component density.
- Joints are stronger; the height of this thread form moves further into the receiving
nut material. The displaced material flows inboard to permit high clamp loading; with
this deep footprint there’s more surface contact at the thread interface.
- This thread form is a full diameter which permits uniform material loading the entire
grip length; therefore, maximum efficiency of the loading potential of the plastic –
the weaker component in the joint.
Typically the receiving plastic fails when a tapping screw is over-torqued. Developing application loading torque values entails tightening to failure. Due to the greater engagement of this 30 Degree thread form it isn’t unusual for the screw head to break off rather than strip the plastic.
Again, the deeper footprint moving into the plastic geometrically distributes the loading over a greater area, thus the increased tensile performance of the receiving material. Screw head breakage as a failure mode isn’t the norm, but it’s not an issue because loading torque should be determined by joint behavior and not the performance of one component only.
A target percentage of thread engagement for plastic commonly runs from 40% to 75%. This result is impacted by mechanics like hole diameter, depth, working (grip length) threads, boss diameter and/or section and the behavior of the specific plastic.
Dimensions and material are basic design choices that can be predicted theoretically only to a point. Final designs should only be frozen by satisfactory results via testing.
The advantages offered by this self-tapping screw in areas of design, installation and application performance are powerful reasons to consider Type FT 30 Degree thread rolling screws. The mechanical advantages utilized can correct problems on existing applications and apply the listed benefits to new designs.
If you have new products on the drawing board or sporadic problems involving threading into plastics Type FT thread-rolling screws will save you time, money and grief on what should be an inexpensive component. Give The Federal Group the opportunity to work for you and improve your results with this user and application friendly high performance
We sincerely hope you found this article informative and valuable. We welcome your comments or suggestions regarding this article or any other subjects you would like to see us write about.
The Federal Group USA
By Robert Levy – CEO TFGUSA
© Copyright 2017