Twin-screw extruder forward and reverse thread element

1. Forward threaded elements

The positive thread element is mainly used for conveying materials. The shape of the screw groove of the forward screw element can be rectangular or other shapes. At present, spiral screw groove shapes are mostly used.

2. Reverse thread element

The shape of the reverse threaded element is similar to the shape of the forward threaded element, except that the spiral direction of the thread groove is reversed. Since the reverse thread conveys the material in the opposite direction and the positive thread conveys the material in the extrusion direction, a high pressure is established in front of the entrance of the reverse thread section to overcome the resistance generated by the reverse flow in the reverse thread, so that the material is conveyed forward through the gap of the reverse thread. In the extruder, sometimes depending on the needs of the extrusion process, different pressure zones are formed at different positions in the axial direction of the screw or within a relatively short distance (the pressure difference is sometimes as large as several megapascals), and this can be achieved by means of counter-threaded elements. For example, a liquid additive or foaming agent is added at a certain position in the axial direction of the screw, and the adding position must be in the low pressure zone. At this time, a reverse thread element is set before the adding zone to form a high pressure seal to prevent the liquid additive or foaming agent from flowing in the reverse direction, while a low pressure zone is formed after the reverse thread element to facilitate the addition of additives or foaming agents. The reverse thread element is arranged in front of the exhaust port, which can form a high pressure in front of the exhaust area, and suddenly reduce the pressure in the exhaust area to facilitate the exhaust. The pressure drop in the reverse thread The reverse thread element itself has no forward conveying capacity, and the forward conveying of material is at the cost of pressure loss. The magnitude of the pressure drop of a reverse threaded element is an important factor that must be considered in the design and selection of such elements. The reverse thread element is a resistance element. When the pressure increases, a positive thread conveying element should be set in front of it to overcome its resistance and convey the material to the die. A pressure peak will appear when a reverse thread element is added, and two reverse thread elements are added.