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.
…
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.
…
Trích
đăng từ sách Handbook of Thermoplastic Elastomers, Jiri George Drobny, William Andrew, trang 224, 2007
Nguồn:
www.books.google.com.vn
(vtp-vlab-caosuviet)
