Reasons for the Outflowing of Materials on the Vacuum Port of Extruder

In the process of using the extruder, the gas in the molten material must be discharged first. If these gases cannot be discharged, there may be pores and bubbles on the surface or inside the product. The uneven surface has a great impact on the physical and mechanical properties, chemical properties and electrical properties of the product. There are 1 to 2 exhaust ports between the inlet and the head to remove the moisture and other volatile components from the molten extrudate. However, in the actual production process, the outflowing of material on the vacuum port of extruder often happens, which may be summarized as the following reasons.

1. Screw factors of the extruder

Exhaust screw generally uses a multi-stage design. The advantage of this design is that there is atmospheric pressure on the exhaust port, so the material will not flow out. Extruders with exhaust ports use two-stage screw, and two exhaust ports need a third-stage screw. Each stage has an atmospheric section, a compression section and a metering section. The starting section of the first stage is the atmospheric pressure charging section, and the second stage is the atmospheric pressure exhaust section, where the exhaust port is located.

There are two main problems with the screw design of a venting extruder. By the time the exhaust port is reached, the raw material must be completely melted, discharged and volatilized; the feed of the second stage screw must be greater than the first stage in order to fill the screw groove at the beginning of the second stage, which will keep the exhaust port at atmospheric pressure. If the feed of the first-stage screw is greater than the second-stage screw, the melt on the extruder will be poured out. To solve this problem, the feed to the first stage can be reduced and the feed to the second stage can be increased.

2. The technological conditions of extruder

A simple and quick way to solve the problem is to change the technological conditions. For example, improve the cooling capacity, increase the friction, shear, viscosity on the barrel or screw, so as to increase the delivery volume.

If changing the technological conditions of extruder still does not solve the problem, other methods can be used, such as redesigning the screw structure to reduce head resistance, extending the length of the screw and barrel, or installing a gear pump between the extruder and the die. The installation of a gear pump can solve the problem of water ingress, but it is more expensive than a new screw.

3. The exhaust port of the extruder

If the thread on the exhaust hole is only partially filled and the exhaust pipe is still bubbling, then there are some problems with the design of the exhaust hole. The exhaust hole should be wider than the rolling material flow to ensure that the outlet will not be blocked by the melt. At the same time the opening of the exhaust pipe should not be too large, thus shortening the melt residence time and material expansion time.

4. Diverter element of extruder exhaust pipe

Take the actual production, when using 150mm just polished single-screw extruder with exhaust hole to extrude sheet, the outflowing of material is hard to solve, so it cannot produce qualified products. The operator has to check the melt in the screw groove at the exhaust port, but actually cannot see it at all. Therefore, there is a big problem with the design of the diverter element, and the operator has to remove the diverter element to check if the screw groove is full. Upon inspection, the spiral groove is only 40% full, explaining the reasonableness of the spiral design.

Then, the focus of the failure is on the diverter element. When testing the diverter element, it is found in the upper stage that not only is there a bottom cutout on the exhaust port that would allow the material to expand normally, but the lower stage is also equipped with an inlet, eliminating the need to design the upper stage screw bottom cutout at all, and this is extremely unfavorable. Due to the increased residence time of the melt pool on the exhaust port and further expansion of the melt pool, it increases the build-up of melt pressure at the exhaust port.

Extruders from jwell extrusion machinery are renowned for their superior quality and performance, making them an excellent choice for any production line. They are designed to be easy to operate and are energy efficient, making them a cost-effective choice.