What are the reasons for the overflow of plastic extruders

The extrusion method of an extruder generally refers to melting plastic at a high temperature of around 200 degrees, and the melted plastic is then passed through a mold to form the desired shape. Extrusion molding requires a deep understanding of plastic properties and rich experience in mold design, making it a technically demanding molding method. Extrusion molding is a method in which materials are continuously molded through a die in a flowing state by heating and pressing in an extruder, also known as "extrusion molding". Compared with other molding methods, it has the advantages of good speed and decent unit cost.

Extrusion method is mainly used for the molding of thermoplastic plastics, and can also be used for certain thermosetting plastics. The extruded products are continuous profiles, such as pipes, rods, wires, plates, films, wire and cable cladding, etc. In addition, it can also be used for plastic mixing, plasticizing granulation, coloring, blending, etc. The extruded product can be called a "profile", but due to its irregular cross-sectional shape, it is also known as a "profile".

During the use of an extruder, it is necessary to release gases from the molten material. If these gases cannot be released, there may be pores, bubbles, and uneven surfaces on or inside the product, which can greatly affect the physical, mechanical, chemical, and electrical properties of the product. There are 1-2 exhaust ports between the feed inlet and the machine head, which can remove moisture and other volatile components from the molten extruded material. However, the cylindrical opening of steel often occurs, and the main problem is that it comes out from the exhaust port. A small amount of material will affect the emission of volatile matter, resulting in a decrease in product quality; A large amount of material loss can block the exhaust vent and even cause shutdown.

There are usually two reasons for material spillage: firstly, improper screw design leads to material backflow at the exhaust port; The second issue is that the exhaust port design is incorrect, causing the molten material to be suspended when passing through the exhaust port. To identify the cause, first check whether the material inside the screw is flowing backwards from the exhaust port. For most exhaust extruders, the molten material can be seen rotating and moving forward on the screw. Generally speaking, the filling degree of the spiral groove should not exceed 50%. If it exceeds this limit, it will not only affect the exhaust effect, but also lead to material leakage from the exhaust port; When the time is less than 50%, the spiral operates normally. The outflow may be caused by improper design of exhaust holes or diversion components.

1. Spiral factor:

Exhaust screws often adopt a multi-stage design, which has the advantage of maintaining normal pressure at the exhaust port and preventing material leakage. Extruders with exhaust ports require two-stage screws, while those with two exhaust ports require three-stage screws. Each stage has a normal pressure section, a compression section, and a metering section. The first stage starts with the normal pressure feeding section, and the second stage is the normal pressure exhaust section, which is where the exhaust port is located.

There are two main issues with the screw design of exhaust extruders: firstly, at the exhaust port, all raw materials need to be melted and volatile emissions need to be released; 2, The feed rate of the second layer screw needs to be greater than that of the first stage in order to fill the screw groove at the beginning of the second stage, which will keep the exhaust port at normal pressure. If the feed rate of the first-order screw is greater than that of the second-order screw, the melt on the extruder will be poured out. To solve this problem, the first-order feed rate can be reduced or the second-order feed rate can be increased.

2. Process conditions:

The simple way to solve the problem of material leakage is to change the process conditions. For example, cooling increases the friction and shear stress on the steel cylinder or screw, increases the friction or viscosity on the surface of the steel cylinder, thereby increasing the conveying capacity.

The first-order feed rate can be reduced through the following ways:

(1) Raise the temperature of Zone 2 and Zone 3 of the steel cylinder.

(2) Cool the first stage screw.

(3) Use hunger based feeding.

(4) Adjust the temperature of the feeding bin (requires repeated testing).

The following methods can be used to increase the second-order feed rate:

(1) Reduce the temperature of the second-order steel column.

(2) Raise the temperature of the second-order screw.

(3) Raise the temperature of the head.

(4) Increase head clearance or reduce head resistance.

(5) Reduce the number of filters.

(6) Use filters with larger gaps.

If the processing conditions cannot be solved by changing, other methods can only be used, such as redesigning the threads, reducing head resistance, extending the length of the screw and steel cylinder, or installing a gear pump between the extruder and the head. The installation of a gear pump can solve the problem of water ingress, but it costs more than a new screw.