Understanding and Characterizing the Drilling of Recycled Plastics

Abstract

Recycled plastics are increasingly being used to manufacture planks used in large-volume applications, including decks, garden, and cloakroom chairs. These products, although manufactured near-to-net shape, often require drilling for assembly purposes. There are very limited data on the machining of plastic material. Manufacturers often rely on data and models established for metals. The machining of plastics, although limited to assembly purposes, or to the removal of excess materials, requires an understanding of the behavior of these materials during the machining in order to obtain better quality parts. It is even more important for recycled plastics, which are inhomogeneous, contain pores, and most often, are made with more than one type of product. This work analyzes the machining of recycled plastics in order to establish and compare their machining models with those traditionally used for metals, and to better understand the behavior of the plastics during machining. The workpiece is drilled at different process conditions and at different temperatures. The process performance indicators such as cutting forces, chip formation, and chip form are analyzed. The originality of this work resides in its study of chip formation and the effects of the preset workpiece temperature on the drilling mechanisms. It is found that there is a range of critical temperatures of transition for plastics similar to the Charpy impact ductile-brittle temperature separating the domain of low cutting force and long and spiral chip from that of high cutting force corresponding to the accordion-type of chip. A parameter describing this phenomenon is defined. It is also found that for low- to moderate-speed operations, the cutting speed has very little effect on the cutting forces, which depend mainly on the feed rate and the workpiece temperature. The relationship between the drilling forces and the feed rate established for metals remain valid, but the exponent of the feed rate for the thrust force is lower. The thrust force and the tangential force are proportional to the feed rate exponent 0.4 compared to 0.8 for metals when drilling workpiece at room temperature or below.