Vật liệu polyurethane nhiệt dẻo (TPU) có khả năng hút ẩm nhanh khi tồn trữ. Lượng ẩm thu vào làm cho sản phẩm có cơ tính thấp, bề mặt có vết phồng, nứt gãy. Phần tài liệu dưới đây trình bày quá trình sấy khô TPU trước khi gia công ép tiêm tạo sản phẩm.
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Thermoplastic polyurethanes absorb moisture rapidly upon exposure to atmospheric air. The moisture pickup and equilibrium level are humidity dependent. Since the optimum moisture content of the material to be processed is less than 0.05%, it has to be dried. Excessive moisture content in the material being processed results in molding and extrusion difficulties. In extrusion process bubbles, poor surfaces, wave forms, surging, and degradation are observed. In injection molding, defects due to excessive moisture are blisters, splay marks, bubbles, porous (foamy) melt, nozzle drool, and poor physical properties of the finished parts. One possible drying procedure is on trays in an oven by circulating air heated to 80–100°C (176–212°F) for several hours. In such a case, pellets of the TPU should be spread 1 inch deep maximum. A better method is using desiccant hopper driers capable of providing hot air with a dew point of –18°C (0°F) maximum. In this drying equipment, softer grades of TPUs are dried typically at temperatures 82°C (180°F), harder grades at 104°C (220°F) with residence times 1–3 hours. Prolonged drying should be avoided to prevent material discoloration.
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Trích đăng từ sách Handbook of Thermoplastic Elastomers, Jiri George Drobny, William Andrew, trang 224, 2007
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