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SECTION 1: INTRODUCTION TO EXTRUSION ➤ SINGLE SCREW EXTRUSION ➤ TWIN SCREW EXTRUSION ➤ EXTRUSION PROCESSES ➤ SAFETY ➤ SINGLE SCREW EXTRUSION Extrusion may be defined as a process for making a product (an extrudate) by forcing a material through an orifice or die to form a shape, or alternatively, the production of finished, or semi-finished products, using an extruder. Scope of the Handbook This book will be mainly concerned with the extrusion of thermoplastic prod- ucts because of their importance in extrusion processes. Thermoplastics are by far the largest group of plastic materials extruded; approximately 65% of all plastics pass through an extruder. As the most popular type of extruder is the single screw machine, this book will mainly concentrate on processes based on that type of machine. Single screw machines are the most popular because they are relatively simple, cheap and easily give a continuous output. Extrusion and Thermoplastics In extrusion of a thermoplastic, heating first softens the material so that it can be shaped. The extrusion machine, or extruder does this process. This heat softening is called by various names, such as ‘plastication’, ‘plasticization’ or ‘thermal softening’. Most extruders are single screw machines. It is the screw which forces the material towards, and then through, the die. Shape is impart- ed by the die, and/or by post-extrusion forming, and then the product is set to shape by cooling it while maintaining its shape. The equipment that does this is called the post extrusion equipment, while the whole system is called an extrusion line. Extruder Classification Extruders may be classified by three figures, for example, 1-60-24. The first number states how many screws the machine has, the second number speci- fies the screw diameter in millimeters (mm) and, the third number specifies the effective screw length as a multiple of the screw diameter. In the example given therefore, a single screw machine is being described that has a screw of diameter 60 mm and a length of 24 screw diameters (that is, a L/D ratio of 24/1). Machine Construction A cut-away drawing of a simple single screw machine is shown in Figure 1. This shows the arrangement of the different parts of the machine. The screw and barrel are the two units that interact to convey the plastic material, melt the material and then force it through the die. The electric motor drive unit and gearbox rotate the screw at a predetermined speed. Temperature controllers 1 section 1: introduction to extrusionPDF Image | Dynisco Extrusion Processors Handbook
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