Self-tapping screws for plastic as the name implies form or tap its threads when screwed into plastic and are installed into pre-drilled or moulded pilot holes. These provide an economical method of joining two plastics or plastic to metal and allows easy disassembly and re-assembly.
They are commonly mistaken to self-drilling screws, but self-drilling screws have drilling tip at the end and used without a pilot hole.
Thread profile angle, also called a flank angle of self-tapping screws are smaller as shown in the figure below which reduces the internal radial stresses created during the removal and deformation of plastic. Also, the thread engagement (CM) diametrically is larger as the root diameter is smaller than the standard fasteners, which increases the holding power on soft plastic compared to metal.
Types of self-tapping screws
They are usually divided into two groups, viz., thread forming and thread cutting. The selection of screw-type will depend on its application requirement, material type and properties. There are a lot of trademarked and patent designs from various companies such as Stanley, EPOK, TR fasteners etc. There are also various thread profile angles (30-45-48 degrees) available.
Thread forming screws don’t have pointed tip and would need a pilot hole. They form threads by deforming and displacing the plastic material which would flow around the screw threads. Hence the overlapping amount and thread profile angle are crucial.
Since the material is deformed rather than removed, large internal stresses are introduced. Hence these screws can only be used in soft plastics with flexural modulus below 200,000 psi (1380MPa).
These are sometimes referred to as thread tapping and have a sharp cutting edge which removes plastic material as it is screwed into the pilot hole. These produce reduced internal stresses compared to thread forming screws making it suitable for stiffer plastic material with flexural modulus between 200,000 – 1,000,000 psi (1380MPa and 6900MPa).
It is important to make the hole the correct size and deeper than the screw engagement length, so the removed material has space to move. For fibre or glass-filled plastics, these provide high thread engagement, high clamp loads & reduced stress. The biggest drawback of the thread cutting screw is thread stripping during disassembly.
Flexural modulus of plastics
Flexural modulus or bending modulus measures the stiffness the plastic during the initial step of the bending process and computed as the ratio of stress to strain in flexural deformation. In simple terms plastics tendency to resist bending.
Flexural modulus of plastics can be used as an indicator, on how different types of plastics will respond when self-tapping fasteners are used. Low flexural modulus means, that more material will flow and allow the formation of threads.
|Materials||Flexural Modulus (psi)|
|ABS, 0-20% glass fill||350000|
|Polyamide 66 (PA)||350000|
|Polypropylene, 40% talc fill (PP40)||500000|
|Moderate||ABS, 20% glass fill||650000|
|Polyamide 66, 12% glass fill||800000|
|Polycarbonate, 20% glass fill (PC20)||850000|
|Stiff||Polycarbonate, 30% glass fill (PC30)||1100000|
|Polybutylene Terephthalates 30% glass fill (PBT30)||1100000|
|Polyamide 66, 30% glass fill (PA30)||1200000|
|Liquid Crystal Polymer (LCP)||1400000|
|Thermosets||Polyphenylene Sulfide, 40% fill (PPS40)||1700000|
|Phenolic, 20% glass fill||1750000|
|Polyester, 50% glass fill||2100000|
Advantages and Disadvantages of using self-tapping screws for plastic
- Self-tapping screws for plastic joining is reliable if used properly
- They are cheap compared to other means of joining
- Easily disassembled
- It doesn’t require pre-moulded threads
- Good impact and vibration resistance
- No curing time or settling time as the full strength is achieved immediately
- No special tool required
- Only up to 10 disassembly and reassembly cycles
- Stress concentration or raiser is likely
- Very difficult to manage the tightening torque
- Increases the part count
- Might affect the aesthetics of the end product