Vật liệu đàn hồi polyurethane nhiệt dẻo (TPU) được ép tiêm như các loại nhựa nhiệt dẻo thông thường. Các thông số hoạt động như nhiệt độ, thời gian và áp suất tiêm ảnh hưởng nhiều đến tính chất của chi tiết tạo thành. Phần tài liệu dưới đây trình bày các thông số hoạt động cụ thể của quá trình ép tiêm TPU.

Operating parameters of temperature, time, and pressure affect the properties of the finished part to high degree. A homogeneous and off-white to beige color melt should be obtained with the proper processing conditions.

Barrel temperatures vary with different grades of TPUs (different hardness), with part design, ratio of shot volume to barrel volume, and cycle time. Typical temperature profile ranges from 350°F to 450°F (177°F to 232°C).

Injection pressure in the range 6,000–15,000 psi (41–103 MPa) is adequate for most parts made from TPUs. Low injection molding pressures may not fill the mold completely with the material. Too much pressure may cause the material to overfill the mold and create flash.

Hold pressure. Typically, hold pressure is about 60–80% of the injection pressure.

Injection cushion (resin feed). Best results are obtained when the cushion does not exceed 0.125 in. (3.175 mm). Too much cushion can lead to overpacking of the part.

Screw backpressure below 200 psi (1.4 MPa) is needed for sufficient homogenization.

Screw speed. Since low shear is generally required for processing TPUs, screw speeds of 20–80 rpm are typical, with 20–40 rpm being preferred.

Injection speed. It is of advantage to fill the mold as fast as possible to minimize the appearance of weld lines, improve weld-line strength, improve surface appearance, and lower the injection pressure. Thicker section parts require a slower injection speed; thinner ones require a faster injection rate. Overall injection time depends on the machine and the part geometry.

Mold temperature. The optimum mold temperature varies with the thickness of the part and hardness of the TPU being processed. Thicker parts require a lower temperature to effectively cool the resin within a reasonable cycle time. Softer resins require lower mold temperature than harder ones. The range is typically 50–150°F (10–66°C). Proper mold temperature ensures a proper release of parts.

Cycle time. The optimum cycle to produce quality parts includes a fast fill, a hold time just long enough for the gates to freeze, and cooling time long enough so that the part ejectors do not penetrate the part. Cooling time is the major part of the total molding cycle.

Trích đăng từ sách Handbook of Thermoplastic Elastomers, Jiri George Drobny, William Andrew, trang 227 – 228, 2007

Nguồn: www.books.google.com.vn

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