Several problems should be paid attention to in the design of new screw

Update:11 May 2019
Summary:

The so-called new type of screw is a screw that uses va […]

The so-called new type of screw is a screw that uses various mixing sections on the screw or changes the conventional full-thread three-stage screw structure.

They are based on conventional full-thread three-stage screws, and although they are different from conventional full-thread three-stage screws, their central problem revolves around increasing extrusion throughput and extrusion quality.

They are in the same position in the extruder as conventional full threaded screws, therefore. The criteria for evaluating the screw and the factors that should be considered in the design of the screw we introduced in the previous section apply equally to the new screw.

However, in order to properly select and design various new types of screws. It is necessary to emphasize the following points.

1. First of all, we must clearly understand the working principle of various new types of screws, as well as the applicable occasions.

This is because different new types of screws have different functions and thus have different applications.

1) A new type of screw based on mixing, such as pin screw and DIS screw. Obviously, this new type of screw is suitable for mixing to obtain a uniform melt. 2) A new type of screw based on shearing. Such as barrier type screw. This one is suitable for plasticizing materials (but not for the processing of heat sensitive materials).

2. Select the position of the ideal mixing and shearing elements

It can be seen from the several new types of screws introduced above that the mixing element and the shearing element are mostly disposed in the metering section (or a part of the compression section), and should not be too close to the feeding section. This is because prematurely setting the mixing element hinders the movement of the solid material, increases the flow resistance, and reduces the discharge amount.

In general, the position of the mixing element is preferably about a solid phase distribution function x/w = 0.3. Because this can play the biggest role of the mixing element.

3. The screw melting capacity must match the homogenization and conveying capacity.

1) After adding the mixing element on the screw, it is likely to increase the melting rate. At this time, the conveying efficiency should be increased. Otherwise, the material may have a longer residence time in the barrel and exceed the required temperature and may be decomposed by overheating. On the contrary, it will cause poor plasticization.
2) Only when the conveying efficiency is high, we can use strong mixing elements and shearing elements to ensure sufficient plasticizing ability;
3) When the conveying efficiency is low, only medium-strength mixing elements and shearing elements can be used;
4) When the conveying efficiency is very low, the addition of the mixing element and the shearing element may cause the material temperature to be too high to cause deterioration and decomposition of the material.

When setting up mixing and shearing components, it must be noted that each component has its own optimal operating conditions and it works well under (or in the vicinity of) the ideal operating conditions. Therefore, it is not sufficient to use a mixing element and a shearing element without analysis or to blindly add a plurality of elements to a single screw to obtain a good effect.

The design of the new screw and the selection of the best working conditions are not an easy task. However, under the guidance of extrusion theory. Through theoretical analysis, experimental verification and production practice, it is still possible to achieve this goal.