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How does the reverse screw element play a role in a twin screw extruder?

Dec 28, 2017

Reverse screw element reverse screw element and the shape of the forward screw element of similar shape, just opposite direction of spiral groove. Because the reverse thread conveyed the material to the opposite direction, the positive thread conveyed the material to the extrusion direction. Therefore, the material was set up high pressure in front of the anti threaded section entrance to overcome the resistance produced by the reverse flow in the reverse thread, so that the material could be transported forward through the gap of the reverse thread. In extruder, sometimes different pressure zones are formed in different positions or quite short distances of the screw according to the need of extrusion process. The pressure difference is sometimes large to several MPa, so we can use reverse thread element to achieve this goal. For example, in a position of screw axial adding liquid additives or foaming agent, adding position must be in the area of low pressure, at this point in the area before adding set screw elements to form a high pressure anti sealing function, prevent liquid additives or foaming agent to reverse flow, but in the reverse screw element is formed after adding low pressure area to the additive or foaming agent. A reverse screw element is set before the exhaust port to form high pressure in front of the exhaust area and sudden depressurization in the exhaust area to facilitate the exhaust. Reverse screw in the pressure drop reverse screw element has no positive transport capacity, positive materials is a pressure loss. The size of the pressure drop of the reverse screw element is an important factor that must be considered when designing and selecting such components. The reverse thread element is a resistance element. When the pressure increases, the positive thread conveying element should be set up in front of it, so that the resistance can be overcome and the material will be transported to the die. A pressure peak will appear when adding a reverse screw element. When adding two sections of anti screw element, there will be two pressure peaks. In this way, multiple anti thread elements can be set up according to need.

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