Twin-screw extruders are commonly used equipment for modified plastics manufacturers. Today we introduce some minor details that can improve the performance of twin-screw extruders. The techniques described in this paper include equipment modification, equipment maintenance and operating specifications.
Equipment modification
One, add insulation gasket between the feeding section and the second flange
The feeding section of the barrel is mostly water-cooled and is locked with a second section of cylinder (heated) with a flange, and the heat can be continuously transmitted from the high temperature barrel to the cold barrel. This results in the failure of the second section of the barrel. Maintain sufficient temperature. Usually, the operator sets the temperature of 180°C in the second section of the barrel, but the actual stability is usually not more than 135°C due to heat loss in the feeding section.
The simplest solution is to add 1-2 1mm thick insulation gaskets between the feed section and the second section of the cylinder flange, and replace the insulation gasket once every few years because the insulation gasket has aging. Degradation and shrinkage tendency.
Second, replace the extruder cooling system with a high pressure cooling system
As we all know, turbulent flow in a water pump can cause more wall heat exchange than laminar flow. The turbulent flow in high pressure transmission has a high lateral momentum exchange, it breaks the boundary layer. The result of intense fluid movement has caused the wall and A much larger heat exchange between the fluids. The extruder cooling circulation system typically provides a pressure of 20-60 psi. To achieve turbulence, a pressure of 120 PSI is preferred. Extruder Almost all cooling system components (hose and valve) ) We must upgrade to 150PSI to meet the safety factor of 120PSI. After the transformation, the cooling system can discharge a large amount of heat in time.
Third, the use of appropriate conversion to increase the amount of feed
Side feeds are usually used to feed the fillers to twin-screw extruders. Many processors require very high filler loading. These fillers often have a very low bulk density. The final fill volume is limited by the following factors:
1, The volume feeding capacity of the side feeding and the volume eating capacity of the extruder main screw. The volume feeding capacity depends on the free volume of the side feeder and the main screw and the speed of the two screws.
If a material in the test can flow to the storage drum without the resistance of the main screw, the material feed rate can be very good. If the side feeder is directly connected to the main screw, then the auxiliary screw The amount of material that will be accepted will be limited. Therefore, the main shaft screw preferably has long pitch threaded elements that extend 2D-4D downstream of the side feed port. This will allow the melt to quickly pass through the side feed port, maximizing packing. Ground is eaten by the main screw. If the main-screw design causes any side feed material to accumulate, it will severely limit the amount of material being fed.
2. Allows gas to discharge from the extruder. The purpose of the exhaust is to make the air easy to escape, while also protecting the packing from large escapes from the exhaust outlet. The best configuration is to open the up-exhaust exhaust outlet upstream of the side feeder. Sometimes, on the side The upper part of the feeder opens a small half-slot vent.
3, there are other factors to consider:
(1) Feeder's blank height
Ideally, the feed screw should be mounted as close as possible to the top of the side feed port to reduce the height of the blank. If some fluffy material flows through the air, it may become full of air, causing it to become After dropping, the false density is greatly reduced.
(2) Feeder agitation type
Make sure that the agitator in the packing feeder does not fluidize the packing. Many agitator manufacturers have designed a special agitator to feed the packing. The hopper will introduce air into the extruder along with the packing. An open top The chute hopper helps venting. If your chute feed hopper has a solid cover with a round tube and is flexible to the feeder, then you need to design a vent.
(3) The hopper chute should be grounded against static electricity
Some materials cause static electricity due to friction and cause the powder to hang on the wall of the hopper, resulting in a powdery cake. A simple solution is to ground the hopper chute.
(4) Compressed air blaster
If cakes continue to exist, they often require special measures. The hopper shaker can be used, but it takes up space and needs to be installed. An alternative is to install a blaster with an air nozzle and install it in the chute in the proper position. The cake is blown before it grows.
Equipment maintenance
First, flush the cooling water with acid
A water-cooled extruder barrel is actually a heat exchanger. Like all heat exchangers, waterways tend to foul and deteriorate. Many operators notice that a new extruder has better cooling performance than three or four. The extruder is much better in the year. The old machine has a layer of scale attached to the inner surface of the water channel, which affects the heat transfer efficiency. The use of a certain concentration of the acid cleaning cylinder can remove the scale and maintain a good heat transfer effect of the barrel.
Second, the use of synthetic gear oil
The gearbox is the heart of the twin-screw extruder. Therefore we have to prevent problems with the gearbox and use synthetic gear oils to better protect the gears.
Synthetic gear oil worry point:
More creamy, resulting in less friction;
Gears, bearings and seals will be more durable;
Gears operate cooler and quieter
Synthetic oils do not lose their viscosity due to machine shear;
It maintains a high viscosity at high temperatures.
Third, often clean the machine
The purpose of the cleaning machine is to remove the remaining material from the extruder and the die. It avoids the time-consuming dismantling cleaning. The cleaning materials are often divided into two categories: chemical cleaning agents and mechanical friction cleaning agents. Many cleaning agents also have chemistry And friction function. Chemical cleaning agent will react with plastic residue to make it out of mechanical surface.
The principle of mechanical friction cleaning is to rub and flush away residual material from the metal surface, using a friction and shear method. The processor can only find suitable cleaning materials and methods through multiple tests. There is no cleaning material for all materials. There are cleaning effects. Processors need to clean materials cheaply and effectively.
For example, many operators in the Midwestern United States use corn as a material for cleaning machines because it is readily available and has a good cleaning effect on the machine.
Here are two more effective cleaning agents:
1# mixture: 50% HDPE + 50% diatomaceous earth
2# mixture: HDPE + water + detergent powder. The HDPE is added to the screw at the usual speed at the melting temperature. The detergent powder is slurried with water in a bucket and slowly added to the slurry from the main feed port. These pulps are foamed. And impact, friction, cleaning the screw.
Operation specification
One, the correct way to switch colors and materials
Extruders always want to drive machines for a long time. However, in reality, due to the needs of customers, we often need to constantly switch to produce 80 kg of product A, and then produce 200 kg of product B, etc.
1. The batch size of the product should correspond to the size of the machine. This seems self-evident. But many processors have no plan to produce 500 pounds of orders using 75 machines, resulting in 300 pounds of waste.
2. Write out procedures for cleaning or disassembling each product. This helps the operator plan the time and eliminate their controversy about how much heading material will be generated.
3, to help the operator, put the clean operating tool around the extruder, if there is frequent refueling, then engage a workbench around the extruder to get everything you need.
4. Study the order of raw materials and finished products flowing into and out of the operation site. Arrange so that the operators spend less effort on moving vehicles.
5, Clear the next product's requirements for machine cleanliness. If you know that the next product requires a thorough manual cleaning, then do not waste time and materials to fool the machine.
6, if possible, to schedule in order to reduce the cleaning machine. This means to do dark colors from light. On the contrary, you have to disassemble several times.
Second, the correct way to start and stop the extruder
Common operating methods:
The operator used to leave the extruder open in the morning but went to other things. The extruder was heated to full scale for several hours. This would allow the polymer to stick to the screw and barrel, and then these The polymer material will continue to degrade and charring. The machine cannot remove these materials. What's worse is that once the machine produces the product, black spots will periodically run out of the screw and barrel, polluting the product and causing customer complaints.
The correct boot method:
Plan when you really need to operate the extruder, and then start heating, as long as the temperature reaches a predetermined value, then the barrel charge.
The correct shutdown method:
Before shutting down, maximally fill the extruder with an inert polymer (such as HDPE) to seal the gap in the machine. HDPE removes oxygen and coats the metal surface, greatly reducing the formation of carbon oxides. After the machine is 'sealed', it is best to quench the extruder instead of leaving it in the air for 18 hours. The so-called chilling is to set all temperatures to zero and turn on all cooling water valves. Open the cooling water pump so that The cooling water passes through all the parts of the barrel and cools down quickly. Rapid cooling will prevent polymer degradation and charring.