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SECTION 2: MATERIALS AND THEIR FLOW PROPERTIES ➤ POLYMERS AND PLASTICS ➤ THERMOPLASTIC MATERIALS ➤ VISCOSITY AND VISCOSITY TERMS ➤ FLOW PROPERTIES MEASUREMENT ➤ ELASTIC EFFECTS IN POLYMER MELTS ➤ DIE SWELL ➤ MELT FRACTURE ➤ SHARKSKIN ➤ FROZE-IN ORIENTATION ➤ DRAW-DOWN ➤ POLYMERS AND PLASTICS A polymer is a chemical compound made up of a large number of identical molecular repeating units. A plastic material is a polymer, generally modified with additives, which is capable of being shaped or molded under reasonable conditions of temperature and pressure. It generally is solid at room tempera- ture and is distinguished from a rubber/elastomer by having a higher stiff- ness/modulus and a lack of reversible elasticity. A thermoplastic is a plastic material that will soften (melt) on heating and harden on cooling. This soften- ing (or melting) by heat and hardening by cooling can be repeated. This allows thermoplastic materials to be recycled. Synthetic Polymers Most commercial polymers and rubbers/elastomers are composed of mole- cules based on the element carbon; that is to say, they are organic. They are synthesized, or made, from simple, oil-based raw materials called monomers. A process, known as polymerization, connects these simple low molecular weight materials together to form polymers, which consist of long chains con- taining many identical repeat units. Polymers and rubbers based on one monomer are known as “homopolymers”, while those based on two (or more) monomers and are known as ‘copolymers’. They may be linear, branched or “cross-linked” (i.e., two or more polymer chains are connected). Usually, poly- mer chains of a wide range of lengths (chains of varying molecular weight) are present in commercial materials. Since the final molecular weight (and size) is generally large, these materials are frequently referred to as “high polymers” or “macromolecules”. Although a high molecular weight is necessary to obtain useful properties (such as stiffness and strength) in a plastic material, it also means that the material is difficult to process. 15 section 2: introduction to extrusionPDF Image | Dynisco Extrusion Processors Handbook
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