What type of screws engage non-threaded holes mostly on plastic?

The choice of thread forming screws as the correct answer is based on their specific design and application. Thread forming screws are engineered to create their own threads within the material they are driven into. This makes them particularly effective when engaging non-threaded holes, especially in materials like plastic.

When a thread forming screw is inserted into a pre-drilled hole in plastic, it deforms the material around it to form internal threads. This not only provides a secure fit but also helps avoid the risk of cracking or splitting that can occur with standard screws, particularly in more brittle materials like plastic. This self-tapping feature enables a robust connection without the need for pre-existing threads, which is ideal for applications involving plastics.

In contrast, other types of screws, such as wood screws, are specifically designed for use in wood and may not perform well in plastic. Machine screws require pre-tapped holes or nuts to engage effectively, meaning they do not function optimally in non-threaded holes without additional hardware. Thread locking screws are focused on preventing loosening due to vibration and also require existing threads to function properly. Thus, thread forming screws are the optimal choice for engaging non-threaded holes in plastic.