During the pipe extrusion process, gas needs to vent from the melt. If these gases cannot discharge in time, defects such as pores, bubbles, and surface dullness may appear on the surface or inside the pipe products, seriously affecting the physical and mechanical properties, chemical properties, and electrical properties pipe.
There are 1 to 2 vents between the feeding port and the machine head to remove the moisture and other volatiles in the molten extruded material. However, some openings of steel cylinders often appear. A common problem is a material ejection from 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 downtime.
Generally, there are two reasons for pipe extruder feeding. One is the unreasonable screw design, which causes the material to flow back at the exhaust port; the other is the exhaust port’s narrow design. The molten material is “hanged” when it passes through the exhaust port.
First of all, you can see whether the material in the screw flows back from the vent. In most vented extruders, you can see the melt moving forward in the screw. Under normal circumstances, the degree of filling of the screw groove with materials does not exceed 50%. If it exceeds, it will affect the exhaust effect and cause the fabric to expose at the exhaust port; when it is less than 50%, the screw can work typically. Poor material may cause by the unreasonable design of the exhaust port or the shunt component.
Factors Affecting Material Evasion of Pipe Extruder and Solutions
1. Screw factor
Exhaust screw mainly adopts multi-stage design. This design’s advantage is that the exhaust port is under normal pressure, and the material will not flow out. An extruder with one vent requires a 2-stage screw, and two vents need a 3-stage screw. Each stage has an intermediate pressure section, a compression section, and a metering section. The initial phase of the first stage is the standard pressure feeding section, and the second stage is the normal pressure exhaust section, which is where the exhaust port located.
For the screw design of the vented extruder, there are two problems: when the material reaches the vent section, the material must completely melt to discharge the volatiles; second, the feeding amount of the second-stage screw must be more significant than that of the first-stage screw to make The screw groove at the beginning of the second stage is not complete, so that the air outlet can maintain normal pressure.
When the feed rate of the first-stage screw is more significant than that of the second-stage screw, the melt in the extruder will flow back. To solve this problem, it is necessary to reduce the feeding amount of the first stage or increase the second stage’s feeding amount.
2. Process conditions
The quick and easy way to solve the problem is to change the process conditions. For instance, I am increasing the friction and shear stress and the steel cylinder or screw and increasing the friction or viscosity along the steel cylinder’s surface to improve the delivery volume.
3. Exhaust port
If the screw at the exhaust port is only partially filled and the exhaust port is still feeding material, there is a particular problem with the exhaust port design. The exhaust port should be wider than the rolling stream to ensure that the melt does not block the exhaust port. Simultaneously, the opening of the exhaust port should not be too large, which can reduce the residence time of the melt and the expansion time of the material flow.
As a pipe extruder manufacturer, Jinwei’s experience is: Whether the vented screw is out of the material is directly related to the extrusion process, plastic performance, and the design of the screw and vent. Therefore, the manufacturer is using the vented extruder. When leaving the machine for production and processing, it is necessary to understand the processing technology, resin characteristics, and equipment performance to make the vented extruder usually operate and stably.