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Word of mouth! 10 ways to improve the performance of twin-screw extruders (part one)

Word of mouth! 10 ways to improve the performance of twin-screw extruders (part one)


Word of mouth! 10 ways to improve the performance of twin-screw extruders (part one)

Use of insulated gaskets

The twin-screw extruders feed section is mostly water-cooled. However, because the twin-screw extruders barrel is bolted to the flange and then to the extrusion barrel section being heated, heat is constantly transferred from the pellet extruder hotter barrel section to the cooler barrel section.

As a result, plastic extruder Barrel #2 usually does not maintain a high enough temperature. In a typical case, the screw extruder operator sets the temperature of barrel 2 to 180°C, but the extruder machine temperature never exceeds 135due to heat loss from the plastic extruder feed barrel.

The simplest solution is to install an insulating washer between the flange of the extruder feed barrel and barrel 2. These extruder washers are usually 1 mm thick and we recommend stacking two washers extruder together to get better insulation. There will still be some heat transfer through the threaded element and extruder screw shaft, but not as much as without the insulation.

Insulation is often included with new extruders, but many extruder operators do not realize its importance and discard it when the extruder barrel is disassembled. After a few years, the insulating washer should be replaced because the material degrades and crumbles.

Side feeding of extruder powders

Plastic extrusion machine side feeding is widely used to feed fillers into twin-screw extruders. Many extrusion machine processors require very high filler fillings, which are usually low bulk density (fluffy) materials. The achievable ultimate load is usually limited by two parameters.

1. the volume capacity of the plastic extrusion machine side filler and the main extruder screw.

The volume capacity is based on the free volume of the twin screws of the side feeder and the main extruder screw, and the speed of both extruder screw sets. If the material can pass through the extruder side feeder that unbolts from the extruder and into the conveying section, it may easily feed a high rate of material. However, when the filler is attached to the extruder, the capacity is usually limited by the amount of material that can be accepted by the main extruder screw.

Therefore, it is preferable for the main extruder screw to combine a long pitch (long thread advance) threaded element at the filler position, extending from 2D to 4D downstream of the filler. this is to keep the melt in the extruder moving forward quickly enough to allow maximum free volume for filler entry. If the screw design causes any material "plugging" downstream of the filler, it will severely limit the amount of filler that can be fed.

2. The ability to allow air to escape from the extruder.

The purpose of venting is to allow air to escape easily while preventing large amounts of filler from being lost through the vent. The best configuration is to have the top vent of the extruder barrel immediately upstream of the extruder side filler. Sometimes a small half-slot vent insert can also be used at the top of the extruder side filler.

The following are some other factors to consider.

Extruder feeder drop height: Ideally, the extruder feeder should be as close to the top of the extruder side filler as possible to minimize the drop. If fluffy material is allowed to fall through the air, it will aerate to the point where the stack density is significantly reduced. This can limit the throughput rate of the entire line.

Type of feeder agitation: Ensure that the agitator in the filler feeder does not aerate the material and reduce bulk density. Many extruder feeder manufacturers have specialized powder agitator designs.

Ensure Filler Hopper Ventilation: Along with the filler, the filler also introduces a significant amount of air into the twin screw extruder. The top of the opening on the side filler chute will be responsible for ventilation. If you see a solid cover on the chute with a short round tube and a flexible connector to the extruder feeder, it will be important to have another vent.

Ground all hoppers/chutes to discharge static electricity: Some materials can generate static electricity due to friction. Static charges can cause extruder powders to adhere to the inner surfaces of the hoppers and chutes, leading to problems such as caking. A simple solution is to connect a ground wire (10 gauge wire recommended) from the chute to a known good ground point on the frame.

Compressed air jets: If clumping persists, sometimes a special solution is needed. Hopper vibrators can be used, but are difficult to size and install. Another option is a "blaster" with air jet nozzles, which are cleverly placed inside the chute walls to break up any agglomerates before they become too large. The air nozzles are connected to a solenoid valve that is activated by a repeat cycle timer. This allows you to set the spray cycle and the dwell time between jets. It is a good idea to install a small air reservoir upstream of the solenoid valve to provide a sharp pulse of air.

Extruder High Pressure Pumps

It is known that turbulent flow in pipelines causes greater heat transfer than from the pipe wall and laminar flow. The laminar flow phenomenon occurs at low fluid velocities caused by low pressure transport. In the case of laminar flow, a stable boundary layer is formed, which acts as an insulator between the main fluid flow and the pipe wall. The fluid "layer" slides over the adjacent layers without mixing and without taking much heat.

In turbulent flows caused by high pressure transport, there is a high degree of lateral momentum exchange, which destroys the boundary layer. As a result, the violent fluid motion leads to a much larger heat transfer from the tube wall to the fluid.

The easiest way to induce extruder turbulence in the barrel cooling orifice is to increase the conveying pressure. The supply pressure of an extruder cooling recirculation system is typically 20 to 60 psi. To achieve extruder turbulence, a pressure of about 120 psi is required. This can be achieved relatively easily by changing the extruder pump in the recirculation system to a high pressure type. Almost all extruder cooling system components (hoses and valves) are rated for at least 150 psi, so 120 psi still provides a margin of safety.

The advantage will be immediately apparent in highly exothermic processes, where temperature coverage can often be greatly reduced or even eliminated. Another benefit of turbulence is that it prevents cooling orifices from scaling due to fouling.

Acid flushing of the extruder machine cooling orifices

Water-cooled extruder barrels are heat exchangers, and like all other heat extruder exchangers, coolant holes can scale due to fouling. Most plastic extruder operators notice that the cooling performance of a new extruder is much better than an extruder that has been in operation for three or four years. This is because newer extruder barrels have smooth, shiny, newly drilled cooling holes. Older machines have holes lined with a crusty mineral deposit that acts as an insulator.

If allowed to develop, scale buildup can lead to more serious problems. Eventually, the extruder cooling holes may become completely clogged, so the water flow is zero. If this happens, the only solution is to remove the barrel from the extruder and drill out all the cooling holes - a time-consuming process.

Therefore, just like removing scale from a domestic water heater with acid, the extruder cooling holes can be acid cleaned at regular intervals.

Use synthetic gear oil

The gearbox is the heart of the twin-screw extruder. If it is damaged, it can be expensive to repair and can take quite a long time. There is one thing that everyone can easily do to avoid extrusion machine gearbox problems: switch to synthetic gear oils. Synthetic oil is a huge advance in lubrication technology.

It is more "slippery" and reduces extrusion machine friction.

It makes extrusion machine gears, bearings and seals last longer. 

Extrusion machine transmissions run cooler and quieter.

Synthetic oil does not lose viscosity due to extrusion machine mechanical shear.

Extrusion machine maintains a high viscosity at high temperatures. 

It improves the overall efficiency of the extrusion machine transmission.

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