How to improve the blending performance of twin screw extruder?(1)

There are many methods for improving the performance of twin-screw compounding extruders, which are well known to operators and plant engineers. However, many of these methods do not appear in the device manual. Paying attention to small details can go a long way in increasing extruder performance.

1. Thermal insulating gasket

The feeding section of the barrel is almost always water-cooled. However, since this barrel is flange-to-flange locked next to the second (heated) barrel, heat can be continuously conducted from the hot barrel to the cold barrel. As a result, the 2# cylinder could not maintain a sufficient temperature. Typically, the operator sets a temperature of 180°C in the 2# cylinder, but the 2# section never reaches a temperature above 135°C due to heat loss in the feeding section.

The simplest solution is to add a thermal insulation gasket between the feeding section and the 2# section cylinder flange. This gasket is generally 1mm thick. We recommend stacking two spacers for better thermal insulation. There is still some heat conduction lost through the bolt and screw shaft, but the presence or absence of thermal insulation has no major effect. New extruders often provide adequate thermal insulation. But many operators don't realize the importance of thermal insulation and throw them away when disassembling the barrel. Insulation gaskets should be replaced after a few years, as these materials have a tendency to degrade and shrink with age.

2. Powder side feeding

Side feeds are commonly used to feed filler material into twin screw extruders. Many processors require very high loadings of fillers, which tend to have very low bulk densities. The final fill volume is governed by two factors:

A: The volume feeding capacity of the side feeding and the volume feeding capacity of the extruder main screw.

The volumetric feeding capacity depends on the free volume of the side feeder and main screw screws, and the rotational speed of these two screws. If a material can flow into the storage drum without the resistance of the main screw in the test, the material feeding rate can be achieved very well. If the side feeder is attached directly next to the main screw, the amount of material that the auxiliary screw can accept will be limited. Therefore, the host screw preferably has long pitch thread elements extending 2D-4D lengths downstream of the side feed ports. This will allow the melt to pass quickly through the side feed ports, maximizing the intake of filler material by the main screw. If the host screw design causes any build-up of the side feed material, it will severely limit the amount of filler fed.

B: The venting capacity to allow gas to be discharged from the extruder.

The purpose of the exhaust is to allow the air to escape easily, while also protecting the packing from escaping a lot from the exhaust port. The best configuration is to open an upwardly vented exhaust port upstream of the side feed port. Sometimes a small half-slit vent is opened in the upper part of the side feeder.

C: There are some other factors to consider:

---The blanking height of the feeder

Ideally, the feed screw should be mounted as close to the top of the side feed port as possible to reduce the drop height. If some fluffy material flows through the air, it may become filled with air, making it much less false density after it falls.

---The stirring type of the feeder

Make sure the agitator in the filler feeder does not fluidize the filler. Many agitator manufacturers have designed special agitators for filler feeding. The accumulator will introduce air into the extruder along with the filler. An open top chute hopper aids in venting. If your chute feed hopper has a solid lid with a round tube and a flexible connection to the feeder, it is important to have a vent.

---The hopper chute should be grounded to prevent static electricity.

Some materials generate static electricity due to friction. Static electricity causes the powder to hang on the wall of the hopper, resulting in a cake-like powder. An easy solution is to ground.

---Compressed air blaster

If the cake-like powder persists, special measures are often required. Hopper shakers can be used, but they take up space and require installation. An alternative is to install a blaster with an air nozzle in the right position in the chute to blow the cake before it grows.

3. High pressure water pump

It is known that turbulent flow in a water pump can result in more heat exchange from the tube wall than laminar flow. Laminar flow occurs under low flow and low pressure delivery conditions. In the case of laminar flow, a stable boundary layer is established, as is an insulating layer between the main stream and the tube wall. The stream layer slides along the adjacent stream layer without mixing and removing more heat. Turbulent flow in high pressure transport has a high degree of lateral momentum exchange, which breaks the boundary layer. As a result, the violent fluid motion causes a much greater heat exchange between the tube wall and the fluid.

The extruder cooling circuit typically provides 20-60 psi of pressure. In order to achieve turbulent flow, a pressure of 120 PSI is best achieved. Just change the water pump of the circulation system to a high pressure pump. Almost all cooling system components (hoses and valves) of the extruder must be upgraded to 150PSI in order to meet the safety factor of 120PSI. The advantage is that a large amount of heat is removed immediately during processing.

4. Flush the cooling water channel with acid

The water-cooled extruder barrel is a heat exchanger, and like all heat exchangers, the water channel is prone to scaling and deterioration. Many operators notice that the cooling performance of a new extruder is much better than that of an extruder that is three or four years old. This is due to the new extruder barrel having smooth, newly drilled cooling water passages. Older machines have a layer of scale that adheres to the inner surfaces of the waterways, acting like a thermal barrier. Layers of scale can cause serious problems if left in disrepair. In the end, the cooling water channel was completely blocked and no water flowed. At this point, the only way to do this is to remove the barrel from the extruder and take out all the cooling water passages—a time-consuming procedure.