Thread rolling screws are designed for fastening material such as plastic. The hex head washer has an external hex head with a washer-style flange attached underneath the head which creates a large surface connection area. Hex heads protrude above the surface. The externally driven head allows greater torque to be applied to the screw compared to other screws that are internally driven. Indentation on the surface of the head is preferred for applications involving power-driving tools. The external hex drive is designed to be used with a ratchet. Tri-Round, Tri-Lobular, screws are designed with coarse machine screw threads. This allows the screw to produce less chips then other points. Zinc plated steel offers good corrosion resistance in applications not exposed to excessive moisture.