Self Tapping Screws for Plastic : Thread Cutting vs Thread Forming Screws

Plastics are widely used to manufacture consumer electronics, telecom and automotive parts. Various joining techniques such as screws, snap joints and plastic welding techniques are used to join plastic parts. But each plastic joining technique has its own challenges. In this article we will discuss Self tapping screws : Thread Cutting and Thread Forming screws that are used to join plastic parts.

Selection of the right type of fastener or screws for plastic parts is very important to get the following key features in plastic part designs.

  • High pullout force.
  • Avoid cracks in plastic bosses.
  • Increase resistance to screw loosening.

Properties of Plastic affecting the performance of Screw Joint

Following properties of plastic materials have an impact on the performance of screw joints in plastic parts.

  • Flexural modulus or stiffness of the material.
  • Proportion of filler material such as glass added into plastic.
  • Creep rate.
  • Impact of injection molding.

How Screws for Plastic are different from Standard screw?

Screws for plastics are different from standard screws in the following ways.

  • Fasteners for plastics have a narrow thread profile to reduce the radial stress. As a result of reduced radial stresses, plastic boss thickness can also be reduced. This reduced plastic thickness helps in reducing injection molding defects and reduces weight.
  • Radial Force required to tighten a screw with a narrow thread profile is also less compared to standard screws.
  • Fasteners designed for plastics have a smaller minor diameter and large thread profile. As a result, a larger volume of screw is covered by the plastic material increases the pullout resistance of the screw.

Various Types of Self Tapping Screws for Plastic Parts

Self tapping screws for plastic parts are broadly classified into two categories. Each of these types of screws has their own advantages, limitations. and applications. Various types of screws are available in these categories.

  1. Thread-Cutting Self Tapping Screws
  2. Thread-Forming Self Tapping Screws
1. Thread Cutting Self Tapping Screws

Thread cutting self tapping screws have a sharp cutting edge that removes plastic material when the fastener is screwed to the pilot hole.

this image shows Self Tapping Thread cutting screws
Self Tapping Thread cutting screws

Compared to thread forming screws, small internal stresses are produced in thread cutting self tapping screws. Thread cutting screws are recommended with stiffer or fiber / glass filled materials.

Advantages of Thread Cutting Screws
  • Low tightening Torque.
  • Best Choice for stiffer materials.
  • Reduced radial stresses.
  • Increased load carrying capacity and pullout force.
Disadvantages of Thread Cutting Screws
  • Stripping may occur during disassembly.
2. Thread Forming Self Tapping Screws

Thread forming self tapping screws don’t have a sharp cutting edge. These screws form threads in the pilot hole by deforming and displacing plastic material around the screw threads.

This image shows an example of thread forming screws for plastic

Due to deformation of plastic material large internal stresses are produced in the material. Therefore thread forming self tapping screws are recommended for softer materials such as ABS.

Following are the examples of thread forming self tapping screws that are used to join plastic parts.

  • Delta PT Thread Forming Fasteners
  • PT Thread Forming Fasteners
  • DST Thread Forming Fasteners
Advantages of Thread Forming Screws
  • Self locking screws.
  • Best used for softer materials.
  • High loosening torque.
Disadvantages of Thread Forming Screws
  • High tightening torque.
  • Increased radial stresses.

Thread Cutting vs Thread Forming Self Tapping Screws

PropertiesThread Cutting ScrewsThread Forming Screws
DefinitionHave a sharp cutting edge that removes plastic material when the fastener is screwed in the pilot hole.Don't have a sharp cutting edge and they form threads in the pilot hole by deforming and displacing plastic material around the screw threads.
Internal StressesLowHigh
Tightening TorqueLowHigh
Plastic Material RecommendationsStiffer hard materials such as glass filled plastic parts.Softer materials such as ABS

We will keep adding more information on self tapping screws for plastic. Please add your comments or questions on various types of screws for plastic in the comment box. Click this link!! to know the difference between screw and bolt.

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