4. The flow state of plastic in the extrusion process

In the extrusion process, due to the rotation of the screw, the plastic is pushed, and the barrel is not moving, which produces a relative motion between the barrel and the screw, and this relative motion has a friction effect on the plastic, so that the plastic is dragged forward. In addition, due to the resistance of the mold, porous screen and filter in the head, the plastic produces a reaction force in the advance, which complicates the flow of plastic in the screw and barrel. The flow state of plastics is usually regarded as consisting of the following four flow forms:

1) Positive flow - refers to the flow of plastic along the screw groove to the head direction. It is produced by the pushing force of the screw rotation and is the most important of the four flow forms. The size of the positive flow directly determines the extrusion amount.

2) Backflow - also known as countercurrent, its direction is the opposite of the flow direction of the positive flow. It is caused by the pressure generated in the head area (the reaction force of the plastic forward) caused by the mold, screen plate, and filter in the head that hinder the positive movement of the plastic. From the head to the feed port, a "return under pressure" is formed, also known as "reverse pressure flow". It can cause a loss of productive capacity.

3) Cross flow - it is the plastic flow along the direction of the axis, that is, perpendicular to the thread groove. It is also formed by the pushing of the screw when it rotates. Its flow is subject to the resistance of the side wall of the threaded groove, due to the mutual resistance of the threads on both sides, and the screw is in rotation, so that the plastic in the spiral groove produces a turnover movement, forming a circular flow, so the cross-flow is essentially a circulation. Circulation is inseparable from the effect of circulation on the mixing and plasticizing of plastics in the barrel. Circulation makes the material stir and mix in the barrel, and is conducive to the heat exchange between the barrel and the material. It has important significance for improving the extrusion quality, but has little effect on the extrusion flow rate.

4) Leakage flow - it is also caused by the resistance of the mold, screen plate and filter in the head. However, it is not the flow in the screw groove, but the reverse flow formed in the gap between the screw and the barrel. It can also cause a loss of productive capacity. Because the gap between the screw and the barrel is usually very small, under normal circumstances, the leakage flow is much smaller than the positive flow and the reverse flow. In the extrusion process, the leakage flow will affect the extrusion amount, the leakage flow will increase, the extrusion amount will decrease, the four flow states of plastics will not appear in a separate form, for a plastic particle, there will be no real backflow, nor will there be a closed circulation. The actual flow of melt plastics in the threaded groove is a synthesis of the above four flow states, and a flow forward in a spiral trajectory.

5. Extrusion quality

Extrusion quality mainly refers to whether the plasticization of the plastic is good, whether the geometric size is uniform, that is, whether the radial thickness is consistent, and whether the axial outer diameter is uniform. In addition to the plastic itself, the main factors that determine the plasticization are temperature, shear strain rate and action time. High extrusion temperature not only causes the fluctuation of extrusion pressure, but also leads to the decomposition of plastics, and may even lead to equipment accidents. While reducing the depth of the screw groove and increasing the length-diameter ratio of the screw, although it is conducive to the heat exchange of the plastic and the extension of the heating time to meet the uniform requirements of plasticization, it will affect the extrusion amount and cause difficulties for the manufacture and assembly of the screw. Therefore, an important factor to ensure plasticization should be to increase the shear strain rate generated by the screw rotation on the plastic, so as to achieve uniform mechanical mixing and balanced extrusion heat exchange, and thus provide protection for uniform plasticization. The size of this strain rate is determined by the shear stress between the screw and the barrel, so it can be seen that the depth of the screw groove can be increased under the condition of increasing the rotational speed under the requirement of guaranteeing the extrusion volume. In addition, the gap between the screw and the barrel also has an impact on the extrusion quality. When the gap is too large, the backflow and leakage of the plastic increase, which not only causes the extrusion pressure fluctuation, but also affects the extrusion quantity. And due to the increase of these backflows, the plastic overheats and causes the plastic to scorch or form difficult.


Qingdao Tianxing Cable Co., Ltd. is a technology enterprise specializing in the production of wire and cable in Shandong Province, with more than 150 sets of domestic excellent cable production equipment and testing equipment, excellent production technology, modern testing equipment, to ensure stable product quality and timely supply. The company's main products are power cable, mineral cable, high life wire and cable, radiation crosslinked cable, control cable, computer cable, photovoltaic cable, communication cable, data cable, low smoke halogen free cable, flame retardant series wire and cable.