Housings of household appliances, electronic devices, luminaires and power tools are increasingly made of plastic. A screw driven directly into a plastic boss is therefore one of the most common joints in series production – and one of the most common sources of trouble on the assembly line: cracked bosses, stripped threads and unstable tightening torque.
This article summarises what to look for when choosing a screw for plastics and designing the boss, so that the joint stays reliable across millions of parts.
Why an ordinary screw does not work well in plastic
A metric screw needs a nut or a pre-made thread, and a self-tapping sheet-metal screw is designed for hard, thin material. Plastic behaves differently: it is softer, slowly deforms under load (creep) and is sensitive to the radial pressure created by the thread. A screw with the wrong geometry will spread and crack the boss, or strip the thread in the plastic during tightening.
Thread forming or thread cutting
For most thermoplastics (e.g. ABS, PC, PA, PP, POM), screws that form the thread are used: the material is displaced during screwing and closely wraps around the thread, with no chips. Such a joint handles vibration well and also allows re-screwing.
In very hard, brittle or heavily filled materials (e.g. with a high glass-fibre content, or thermosets), the radial pressure can be too high. There, a screw that partly cuts the thread, or an adapted point and thread geometry, is more suitable.
Thread geometry is key
Screws for plastics differ from standard screws mainly in the thread form:
- A smaller flank angle (often around 30° instead of 60° for a metric thread) reduces radial pressure on the boss and therefore the risk of cracking.
- A larger pitch and deeper thread engage more material, increasing the pull-out strength of the joint.
- The point shape ensures centring and an even start of screwing – especially important in automated screwdriving.
- The right screw hardness ensures the thread is formed in the plastic rather than the screw itself deforming.
Designing the boss
Even the best screw will not perform if the boss is poorly designed. These are guideline values, which we then confirm by testing in the actual material:
- Hole diameter is usually around 0.7–0.9 × the nominal screw diameter – smaller for softer materials, larger for harder and glass-filled ones.
- Outer boss diameter should be roughly 2–2.5 × the nominal screw diameter so that the boss can withstand the radial pressure.
- Engagement depth should be at least around 2 × the nominal diameter; the hole should be slightly deeper than the screw length.
- A chamfer or counterbore at the entry of the hole reduces stress at the edge of the boss and helps with centring.
- Watch out for weld lines from injection moulding and for sharp transitions – these are where bosses most often crack.
Tightening torque and automated assembly
When screwing into plastic, two torques matter: the torque needed to form the thread, and the torque at which the thread in the plastic strips. There must be a large enough window between them in which to set the screwdriver. The larger this window, the more stable the assembly and the less scrap.
The window is affected by the screw geometry, hole diameter, material and also screwing speed, as plastic heats up at high speed. On automated lines, tight tolerances and batch repeatability therefore matter as much as geometry – every deviation shows up in the torque.
The most common mistakes
- Cracked boss – outer diameter too small, hole too small, or a screw with too large a flank angle.
- Stripped thread – hole too large, torque too high or engagement depth too short.
- Loss of preload – plastic relaxes over time; the right thread geometry and material choice help.
- Problems when re-screwing – during servicing, the screw should find the existing thread rather than form a new one.
How we can help
At Interwolf we make screws for plastics for manufacturers of electronic devices, household appliances and other technical products, in sizes from 2 to 8 mm. In addition to standard designs, we also develop custom screws made to customer drawings, with geometry adapted to the specific material and assembly process.
For a new project, we review the drawing of the screw and the boss, propose optimisations and produce samples for torque and pull-out testing. More on the Custom screws, Electrical industry and household appliances and Technical support pages.
Working on a project with a plastic housing? Send us your drawing – we reply within 1–2 working days.