Why does the extruder’s vacuum port have the problem of material leakage?

In the extrusion process, the gas needs to be discharged from the melt, if these gases can not be discharged, the product surface or internal defects such as pores, bubbles and surface darkness may appear, which can seriously affect the physical and mechanical properties of the product, chemical properties and electrical properties. Between the feed port and the head set 1 or 2 exhaust port can remove the molten extruded material in the water and other volatile components. But the barrel opening will often appear some, the most common problem from the exhaust port bubble material, a small amount of material will affect the discharge of volatile components, so that product quality is affected; a large number of bubble material will block the exhaust port, and even lead to downtime.
There are generally two reasons for the material leakage, one is the screw design is not reasonable, resulting in the backflow of material at the exhaust port; the second is the exhaust port design is not reasonable, the molten material through the exhaust port is “hanging”. Find the reason first from the exhaust port to see whether the material in the screw backflow, in most of the exhaust extruder, you can see the screw in the rotating forward motion of the melt. In general, the screw groove is filled with no more than 50% of the material, if more than that, not only affect the exhaust effect, may also cause the exhaust port bubble material; less than 50%, the screw can work normally. The bubbling material may be caused by the exhaust port or diversion element design is not reasonable.
Factors and solutions that affect the material leakage

01 Screw

Exhaust screw mostly adopts multi-stage design.The advantage of this design is that the exhaust port is under normal pressure and the material does not flow out.An extruder with one exhaust port requires a 2-stage screw, and a 3-stage screw is required for 2 exhaust ports. Each stage has an atmospheric section, a compression section and a metering section. The first stage starts with an atmospheric pressure charging section and the second stage is the atmospheric pressure exhaust section, where the exhaust port is located.
There are 2 main issues with venting extruder screw design: first, the material must be fully molten by the time it reaches the venting section to expel volatiles; second, the feed rate of the 2nd stage screw must be greater than that of the 1st stage screw so that the screw groove at the beginning of the 2nd stage is not filled so that constant pressure is maintained at the vent. When the feed volume of the 1st stage screw is larger than the 2nd stage screw, the melt in the extruder will backflow. This problem can be solved by reducing the feed of the 1st stage or increasing the feed of the 2nd stage.

02 Process Conditions

The easiest and fastest way to solve the problem of material leakage is to change the process conditions. Such as lowering the temperature, increasing the friction and shear stress along the barrel or screw, and increasing the friction or viscosity along the surface of the barrel to increase the amount of material delivered.
The feed rate of the 1st stage can be reduced in several ways.
(1) Raising the temperature in barrel zones 2 and 3.
(2) Cooling the first stage screw.
(3) Feeding by starvation method.
(4) Adjusting the temperature of the feed bin (repeated experiments are required).

Increasing the feed of the 2nd stage can be done as follows.
(1) Lowering the temperature of the 2nd stage barrel.
(2) Increase the temperature of the 2nd stage screw.
(3) Increase the head temperature.
(4) Increase the head gap or reduce the head resistance.
(5) Reduce the number of filters.
(6) Use a filter with a larger gap.
If changing the processing conditions still does not solve the problem, other methods have to be used, such as redesigning the screw, reducing the head resistance, lengthening the screw and barrel or installing a gear pump between the extruder and the head. Installing a gear pump can solve the problem of bubbling material, but it costs more than a new screw.

03 Exhaust port

If the screw at the exhaust port is only partially filled and the exhaust port is still bubbling material, then there is some problem with the design of the exhaust port. The exhaust port should be wider than the rolling flow to ensure that the exhaust port is not blocked by the melt. Also, the vent opening should not be too large to reduce the melt residence time as well as the expansion time of the flow.
Under normal operating conditions, the screw groove is half full, and the exhaust port is at atmospheric pressure, but in fact there is still pressure in the rolling melt, the pressure is about 0.21 ~ 0.35MPa or higher, which is enough to make the melt at the exhaust port expand. This exhaust port in the design should take into account the normal viscoelastic expansion of the material, otherwise, part of the rolling material flow will be “hanging”, piled up at the diversion element. How much the melt stream expands is determined by the time it passes through the exhaust port. The longer the dwell time, the greater the expansion. The dwell time is controlled by the screw speed and the size of the exhaust port, and increasing the screw speed can reduce the dwell time. This is the reason why low speed extrusion is more serious than high speed extrusion.

However, the larger the vent opening, the longer the dwell time. When the melt builds up at the vent, it will block the vent, and the solution is to change the vent opening to meet the normal expansion of the melt at the vent. If the rolling material flow expands 5 to 10mm, the depth of the vent opening should be at least 5 to 10mm.

04 Exhaust manifold element

As a real-life example, a 150mm freshly polished single screw extruder with a venting port was extruding sheet material with serious spillage and was unable to produce a satisfactory product. The operator wanted to check the melt in the screw groove at the exhaust port but could not see the screw at all, which meant that the design of the diverter element was very faulty and the operator had to remove the diverter element to check how full the screw groove was. The operator had to remove the manifold element to check how full the screw groove was. After checking, the screw groove was only 40% full, indicating that the screw design was reasonable.
The failure then focused on the manifold element. Although this was a well-known extruder manufacturer, the design of the manifold element was seriously flawed. Upon inspection of the manifold element, it was found that the exhaust port not only had a bottom cut in the upper stage that allowed for proper expansion of the material, but also in the lower stage. There was no need for a bottom cut in the upper stage screw, and it was extremely detrimental to have this cut because it increased the residence time of the melt at the vent and caused the melt to expand more, increasing the melt pressure build-up at the vent.
Another problem is that flat and rectangular exhaust ports are more difficult to clean than straight ones. If the manifold element is properly designed, the exhaust port should be perpendicular to the radius of the barrel. With the above analysis, the manifold element can then be repaired in two ways: first, by filling the upper step bottom cutout with metal; second, by changing the rectangular opening to radial, which allows the operator to see the melt in the screw. With the improvement of the above ways, the problem of bubbling material was solved and a high quality product was produced.

05 Conclusion

The venting screw is directly related to the extrusion process, the plastic properties and the design of the screw and the venting port.Therefore, when using the exhaust extruder for production and processing, manufacturers should have a thorough understanding of the processing process, resin characteristics and equipment performance in order to make the exhaust extruder operate properly and stably.

Contact：Michelle Jiang/Andrew Shan
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