Putting screws into plastic without stripping it

Three ways to fasten into plastic

• Thread-forming screw straight into a moulded boss — cheapest, fine for something opened once or twice.
• Heat-set brass insert — a small brass sleeve pushed in with a soldering iron. Gives a real metal thread you can reuse forever.
• Screw and captive nut on the far side — strongest, but needs access to both sides.

Pilot hole rules

For a thread-forming screw in hard plastic, the hole is roughly 80% of the screw diameter. In soft plastic, about 75%.

The boss wall around the hole should be at least 0.6 times the screw diameter thick, or it splits when the thread forms.

Depth should be about twice the diameter, with a little extra space at the bottom so the debris the screw pushes down has somewhere to go.

Heat-set inserts, step by step

Drill or print the hole to the insert maker's size — usually a fraction under the insert's outside diameter.

Fit a soldering iron tip that matches the insert, set it around 220 to 250 °C for PLA or 250 to 280 °C for PETG and ABS.

Press slowly and straight down until the insert is flush, then let it cool before touching it. Rushing tilts it.

How tight

Stop the moment the head touches. Small plastic bosses strip somewhere around a third of the torque the same screw takes in steel.

Use fingertips on the driver handle, not a full grip, on anything M3 and under.

Common questions

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