In the world of metal assembly, “torque control” is the undisputed golden rule: we set a target torque, the tool stops when it reaches that value—simple, direct, effective.
Yet when we copy this strategy verbatim to the case of driving self-tapping screws into plastic parts, trouble usually follows: either the screw is “floating” (not fully seated) or we over-do it and strip the threads. Yield rate never improves.

The core issue is: for self-tapping screws in plastic, looking at torque alone is far from sufficient; it is actually risky. Let’s dig into why and reveal a more reliable solution.

Torque is essentially a measure of “force”, the resistance that must be overcome to rotate the screw. In metals this resistance mainly comes from the clamping force created by bolt elongation, and the two are fairly proportional. In plastics, however, things get complicated:

Soft material, high tapping torque

A self-tapping screw has to cut its own thread in the plastic. This “tapping” phase produces very high friction, so torque shoots up. If you watch torque only, the tool may reach the target value before the screw has generated any useful clamping force and stop prematurely. Result: the screw “floats” above the surface and the joint is loose.

Brittleness after the yield point

Once tapping is finished the screw enters the “seating-tension” phase. Plastic yields at a low stress and, after a sharp torque peak, enters failure almost without plastic deformation. A torque-only tool cannot tell whether that peak is “healthy clamp load” or “the prelude to stripping”. It easily over-tightens and destroys the thread.

To escape torque’s blind spot we must add a second dimension: angle.

Angle records the entire travel of the screw. A typical torque-angle curve for a plastic self-tapping screw contains several key stages:

A. Run-in: screw spins freely, torque is negligible.
B. Tapping: screw cuts the plastic, torque rises quickly and steadily.
C. Seating point (snug): screw head touches the plastic surface—this is where clamping really starts.
D. Elastic tension: screw shank stretches slightly, building preload; torque and angle increase together.
E. Peak-torque point: maximum torque is reached and preload is close to optimum.
F. Failure: material yields, torque collapses, thread strips.

Angle monitoring tells us exactly which step the screw has reached.

Modern high-precision tightening strategies therefore combine torque and angle. Through testing we determine an optimal “torque-angle window”.

A joint is accepted only if both conditions are met:

• Torque OK: the measured torque falls inside the specified range.

• Angle in window: the rotation counted from the seating point lies within the predefined angular limits.

How does this solve the earlier problems?

• Prevents floating: if torque is reached but the angle is far below the lower limit, the controller flags “insufficient angle”. This usually means tapping is incomplete or the screw is not yet seated, so an alarm is raised.

• Prevents stripping: if torque is reached but the angle is already beyond the upper limit, the controller flags “excessive angle”—a strong sign of over-tightening and imminent stripping—and stops immediately.

Torque-angle combined control is the only route to reliable plastic joints, the elimination of mass-quality incidents, and ultimately high yield rates.