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Pilot Hole Calculator for Plastic Thread-Forming Screws
A thread-forming screw for plastic needs a pilot hole of roughly 0.70 to 0.85 x its diameter, depending on the plastic. A 4 mm screw in PP takes a 2.8 mm hole, in PA6, POM or PBT 3.0 mm, in ABS 3.2 mm, and in PC 3.4 mm (Bossard ecosyn®-plast table). Pick the diameter and plastic below.
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Calculators & Converters
Optimizing Fastener Integrity in Plastics
Fastening into thermoplastics requires a specific approach to handle material flow and prevent boss bursting. The ecosyn®-plast thread geometry features a unique 30° flank angle designed to reduce radial stress during the thread-forming process. To achieve the maximum strip-to-drive torque ratio, your pilot hole diameter must be precisely matched to the specific resin type and boss geometry.
Incorrect pilot hole sizing leads to immediate assembly failure. A hole that is too narrow creates excessive hoop stress, causing the boss to crack. Conversely, a hole that is too large reduces thread engagement, leading to stripped joints and poor pull-out resistance. Our calculations help you find the “sweet spot” for reliable, repeatable plastic assembly.
The Boss Geometry Rule
For most thermoplastics, a boss outside diameter of 1.8 to 2.5 x the screw diameter (Bossard: PA6 1.85, PP 2.0, PC 2.5) is recommended to ensure the assembly can withstand the expansion forces of thread forming.
Interactive ecosyn®-plast Calculator
Input your screw diameter and material class to determine the ideal pilot hole size and boss dimensions. The factors follow the Bossard ecosyn®-plast pilot hole table for common commercial resins.
Plastic Thread-Forming Screw Pilot Hole Calculator
Calculate optimal pilot hole diameters for thread-forming screws in various plastics.
Recommended Pilot Hole (d_h)
Millimeters
PP / PE-LD hole (0.70 x d)
--
PA6 / POM / PBT hole (0.75 x d)
--
ABS / PVC-U hole (0.80 x d)
--
Boss OD to screw dia (D/d)
1.8 to 2.5
Pilot hole (scientific notation, m)
0.00e+0
*Guide values from the Bossard ecosyn®-plast pilot hole table (hole 0.70 to 0.85 x d by material). Confirm with the screw maker's table and a trial assembly.
Hole diameter dh = F x d. Specific material tests are recommended for critical assemblies.
How to Use the Plastic Pilot Hole Calculator
- Enter the screw nominal diameter in millimeters.
- Choose the plastic group: soft (PE-LD, EVA), standard (PP, PE-HD, PVC-U), engineering (ABS, PA6, POM) or high strength (PC, PBT, PET).
- Read the recommended hole and the values for the other plastic groups.
- Confirm on a trial boss: check drive torque, strip torque and that the boss does not crack.
Formula and Worked Examples
Hole = d x F, where F is 0.70 for PP and PE-LD, 0.75 for PE-HD, PA6, POM, PBT and PET, 0.80 for ABS, ABS/PC and PVC-U, and 0.85 for PC, PMMA and PPO (Bossard ecosyn®-plast table). Stiffer plastics get a larger hole because they flow less around the thread and crack more easily.
| Screw | PP, PE-LD (0.70) | PE-HD, PA6, POM, PBT (0.75) | ABS, ABS/PC, PVC-U (0.80) | PC, PMMA, PPO (0.85) |
|---|---|---|---|---|
| 3 mm | 2.10 mm | 2.25 mm | 2.40 mm | 2.55 mm |
| 4 mm | 2.80 mm | 3.00 mm | 3.20 mm | 3.40 mm |
| 5 mm | 3.50 mm | 3.75 mm | 4.00 mm | 4.25 mm |
| 6 mm | 4.20 mm | 4.50 mm | 4.80 mm | 5.10 mm |
These are guide values for screws made for plastics. Glass fiber filled grades usually need the upper end, and each screw maker publishes its own hole table, which takes priority. A boss outside diameter of about 2 x d and a thread engagement of about 2 x d are common starting points.
Joint Performance Factors
| Design Metric | Engineering Impact | Application Goal |
|---|---|---|
| Drive Torque | The force required to form the thread in the plastic boss. | Ensures efficient assembly. |
| Strip Torque | The threshold where the formed plastic thread fails. | Maximizes safety margins. |
| Radial Stress | Outward pressure exerted on the boss walls during installation. | Prevents boss cracking. |
| Engagement Depth | The total amount of material being displaced by the fastener. | Increases pull-out strength. |
Frequently Asked Questions
What size pilot hole for a 4 mm screw in plastic?
About 2.8 mm in PP or PE-LD, 3.0 mm in PE-HD, PA6 (nylon), POM or PBT, 3.2 mm in ABS and 3.4 mm in polycarbonate, following the Bossard ecosyn®-plast table.
Why does my plastic boss crack when I drive the screw?
Usually the hole is too small for a stiff plastic, the boss wall is too thin (aim for an outside diameter near 2 x d), or the screw is driven too fast. Try the next larger hole first.
How deep should a screw go into a plastic boss?
About 2 x the screw diameter of thread engagement, plus a little extra hole depth below the screw tip.
Can I use a standard metal screw in plastic?
It works poorly. Machine and sheet metal threads have a 60 degree flank that creates high radial stress; screws made for plastics use a narrower flank to reduce cracking and raise pull-out strength.
Next Steps
- Pilot Hole Design for Metric Thread Formers: pilot holes for forming taps in metal
- Thread Insert Types: when a threaded insert beats a screw in plastic
- Threaded Inserts: inserts for plastic and soft metal
- Request a quote for the fasteners in your joint.
Related Calculators
More free tools for the same job:
- Pilot Hole Design for Metric Thread Formers: Size the pilot hole for metric thread-forming screws.
- Tap Drill Size Calculator: Find the tap drill size before tapping a thread.
- Thread Engagement Length Calculator: Find the minimum thread engagement for blind and through holes.
- Steel Weight Calculator: Estimate fastener weight by type, size and material.
Developing a New Plastic Component?
Our engineering team can perform application testing to determine the exact torque specs and hole designs for your specific thermoplastic resin and assembly requirements.