Causes And Solutions for Material Leakage From Vacuum Port of Extruder

During the extrusion process, it is necessary to discharge gas from the molten material. If these gases cannot be discharged, defects such as pores, bubbles and dark surface may appear on the surface or inside of the product, which may seriously affect the physical, mechanical, chemical and electrical properties of the product. One to two exhaust ports are set between the feed port and the die head to remove moisture and other volatiles in the molten extruded material. However, there are often some problems at the opening of the steel cylinder. The most common problem is that the material leaks out of the exhaust port. A small amount of material will affect the discharge of volatiles and affect the product quality; a large amount of material will block the exhaust port and even cause shutdown.

There are generally two reasons for material overflow. One is that the screw design is unreasonable, resulting in material backflow at the exhaust port; the other is that the exhaust port design is unreasonable, causing the molten material to be "hung" when passing through the exhaust port. To find the cause, first check whether the material in the screw flows back from the exhaust port. In most exhaust extruders, you can see the melt rotating and moving forward in the screw. Under normal circumstances, the degree of material filling the screw groove does not exceed 50%. If it exceeds, it will not only affect the exhaust effect, but also may cause material overflow at the exhaust port; when it is less than 50%, the screw can work normally. Material overflow may be caused by unreasonable design of the exhaust port or diversion element.

Factors affecting leakage and solutions

1. Screw factors

Venting screws are mostly designed in multiple stages. The advantage of this design is that the vent is at normal pressure and the material does not flow out. An extruder with one vent requires a 2-stage screw, and an extruder with two vents requires a 3-stage screw. Each stage has a normal pressure section, a compression section, and a metering section. The first stage is the normal pressure feeding section, and the second stage is the normal pressure exhaust section, which is where the vent is located. There are two main problems with the design of the venting extruder screw: first, when reaching the exhaust section, the material must be completely melted to discharge the volatiles; second, the feeding amount of the second stage screw must be greater than that of the first stage screw, so that the screw groove at the beginning of the second stage is not full, so that the vent can be kept at normal pressure. When the feed rate of the first stage screw is greater than that of the second stage screw, the melt in the extruder will flow back. To solve this problem, you need to reduce the feed rate of the first stage or increase the feed rate of the second stage.

2. Process conditions

The simplest and fastest way to solve the problem of material overflow is to change the process conditions. For example, lower the temperature, increase the friction and shear stress along the steel barrel or screw, and increase the friction or viscosity along the surface of the steel barrel to increase the material delivery.

The feed rate of the first stage can be reduced by the following methods:

(1) Raise the temperature of the second and third zones of the steel drum.

(2) Cool the first stage screw.

(3) Feed by starvation method.

(4) Adjust the temperature of the feeding bin (repeated experiments are required).

The feed rate of the second stage can be increased by the following methods:

(1) Lower the temperature of the second stage steel drum.

(2) Raise the temperature of the second stage screw.

(3) Raise the temperature of the die.

(4) Increase the die gap or reduce the die resistance.

(5) Reduce the number of filters.

(6) Use filters with larger gaps.

If changing the processing conditions still cannot solve the problem, other methods must be used, such as redesigning the screw, reducing the die resistance, lengthening the screw and steel barrel, or installing a gear pump between the extruder and the die. Installing a gear pump can solve the problem of material overflow, but it costs more than a new screw.

3. Vent

If the screw at the vent is only partially filled and material still comes out of the vent, then there are some problems with the design of the vent. The vent should be wider than the rolling material flow to ensure that the vent is not blocked by the melt. At the same time, the opening of the vent should not be too large, which can reduce the residence time of the melt and the expansion time of the material flow. Under normal operation, the screw groove is half full, and the vent is at normal pressure. In fact, there is still pressure in the rolling melt, which is about 0.21 to 0.35MPa or higher, which is enough to expand the melt at the vent. In this way, the normal viscoelastic expansion of the material should be taken into account when designing the vent, otherwise, part of the rolling material flow will be "hung" and accumulated at the diversion element. How much the melt flow expands is determined by the time it passes through the vent. The longer the residence time, the greater the expansion. The residence time is controlled by the screw speed and the size of the vent. Increasing the screw speed can reduce the residence time. This is why low-speed extrusion causes more severe material bubbling than high-speed extrusion. However, the larger the vent opening, the longer the residence time. When the melt accumulates at the vent, it will block the vent. The solution is to change the vent opening to meet the normal expansion of the melt at the vent. If the rolling flow expands 5 to 10 mm, the depth of the vent should be at least 5 to 10 mm.

4. Diverter element at the vent

Take a real-life example. When extruding a sheet with a 150mm single-screw extruder that had just been polished and equipped with a vent, the extruder had serious material overflow and could not produce qualified products. The operator wanted to check the melt in the screw groove at the vent, but could not see the screw at all. This showed that there was a big problem with the design of the diverter element. The operator had to remove the diverter element to check the filling degree of the screw groove. After inspection, it was found that the screw groove was only 40% full, indicating that the screw design was reasonable.

The focus of the fault was on the diverter element. Although this is a well-known extruder manufacturer, there are serious defects in the design of its diverter element. When checking the diverter element, it was found that the vent was not only provided with a bottom cutout on the upper stage to allow the material to expand normally, but also on the lower stage. It is completely unnecessary to design a bottom cutout for the upper stage screw, and it is extremely disadvantageous to set up this cutout because it increases the residence time of the molten pool at the vent and causes the molten pool to expand more, increasing the melt pressure accumulation at the vent. Another problem is the flat and rectangular exhaust ports, which are more difficult to clean than the straight ones. If the diverter element is designed properly, the exhaust port should be perpendicular to the radius of the steel cylinder. Through the above analysis, the diverter element can be repaired in the following two ways: first, fill the upper step bottom cut with metal; second, change the rectangular opening to radial, so that the operator can see the molten material in the screw. After the above improvement, the problem of material overflow was solved and high-quality products were produced.

Conclusion

Whether the venting screw leaks material is directly related to the extrusion process, plastic properties, and the design of the screw and vent. Therefore, when manufacturers use venting extruders for production and processing, they must have a detailed understanding of the processing technology, resin characteristics, and equipment performance to ensure that the venting extruder can operate normally and stably.

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