Have you ever tightened a screw into a plastic part...

And instead of locking tight, it suddenly strips and just keeps spinning?

You probably blamed the plastic for being weak. But you actually just used the wrong screw. 👇

When new engineers design plastic assemblies, they often grab whatever screw is lying around the lab.

But plastics are wildly sensitive to how threads are created inside a hole.

There are two main types of screws designed for plastic, and mixing them up will destroy your parts.

𝟭. 𝗧𝗵𝗿𝗲𝗮𝗱-𝗖𝘂𝘁𝘁𝗶𝗻𝗴 𝗦𝗰𝗿𝗲𝘄𝘀 (𝗧𝗵𝗲 𝗦𝗵𝗮𝘃𝗲𝗿𝘀)

These have a sharp slot or flute cut into the tip.

As they drive in, they act like a drill bit, physically shaving away the plastic to carve a threaded path.

Use these ONLY for hard, brittle plastics (like Polycarbonate or Acrylic).

Brittle plastics cannot stretch. If you try to force a screw into them, they shatter. So you must cut a path instead.

𝟮. 𝗧𝗵𝗿𝗲𝗮𝗱-𝗙𝗼𝗿𝗺𝗶𝗻𝗴 𝗦𝗰𝗿𝗲𝘄𝘀 (𝗧𝗵𝗲 𝗦𝘁𝗿𝗲𝘁𝗰𝗵𝗲𝗿𝘀)

These have a smooth, blunt tip with no cutting slot.

They physically push and compress the plastic out of the way, forcing it to flow around the threads.

Use these ONLY for soft, ductile plastics (like Polypropylene, Nylon, or ABS).

Soft plastics easily stretch. As they try to bounce back to their original shape, they create a massive, permanent grip on the screw.

💥 𝗧𝗵𝗲 𝗙𝗮𝘁𝗮𝗹 𝗠𝗶𝘀𝘁𝗮𝗸𝗲𝘀:

If you put a Thread-Cutting screw into a soft plastic, it carves away too much material.

When you apply the final twist of your screwdriver, there isn't enough plastic left to hold the load. The threads strip out instantly.

If you put a Thread-Forming screw into a brittle plastic, the screw acts like a giant wedge.

The outward pressure (hoop stress) skyrockets, and the plastic mounting boss literally explodes.

💡 𝗣𝗥𝗢 𝗧𝗜𝗣: When using a Thread-Forming screw in a soft plastic, the hole diameter in your CAD model should be sized so the screw threads overlap the plastic by about 𝟳𝟬%.

This guarantees maximum pull-out strength without snapping the screw.

Have you ever had a plastic prototype crack the second you put a screw into it? Let me know below!