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
(vtp-vlab-caosuviet)
