| 摘要 |
An extrusion die comprises a die cavity having a shape corresponding to the cross-sectional shape of the required extrusion, and a preform chamber in communication with the die cavity, the preform chamber being of generally similar shape to the die cavity but of greater cross-sectional area, so that regions of the preform chamber communicate with corresponding regions respectively of the die cavity. Each region of the preform chamber has a bearing length which is so determined in relation to its dimensions and position that, in use, extrusion material passing through each region of the preform chamber is constrained to move at a velocity such that the material passes through all regions of the die cavity at a substantially uniform velocity. The die cavity itself is of uniform, preferably zero, bearing length so that the extrusion process is controlled solely by adjustment of the preform chamber, such adjustment then being readily quantifiable and repeatable.
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