Twin screw extruder is widely used by modified plastic manufacturers. Today I will talk about some small details that can improve performance of a twin screw extruder. The skills presented in this article include three aspects: equipment modification, equipment maintenance, and performance.
1. Hardware Upgrade
1.1. Add a thermal pad between feed section and flange of second section
The barrel feed section is mainly water cooled and flange is locked with second part of barrel (heated) so that heat can be continuously transferred from high temperature barrel to cold barrel. As a result, second cylinder section cannot maintain sufficient temperature. Usually operator sets temperature to 180°C in second section of cylinder, but due to heat loss in feed section, actual stability usually does not exceed 135°C.
The easiest solution is to add 1-2 1mm thick heat pads between feed section and second cylinder flange and replace heat pads every few years as heat pads of tablet tend to age and break. and shrink.
1.2. Replace extruder cooling system with high pressure cooling system
It is known that turbulent flow in a pump can lead to greater wall heat transfer than laminar flow. The turbulent flow in high pressure transmission has a high degree of lateral momentum exchange, which destroys boundary layer. The resulting vigorous movement of fluid causes a much greater heat transfer between pipe wall and fluid. The circulating cooling system of extruder typically provides a pressure of 20-60 psi. To achieve turbulent flow, it is best to achieve a pressure of 120 psi. Almost all components of cooling system (hoses and valves) of extruder must be upgraded to 150 psi to meet 120 psi safety factor. After transformation is completed, cooling system can dissipate a large amount of heat in time during processing.
1.3. Use appropriate modification to increase amount of feeding
Side feeders are typically used to supply fillers to twin screw extruders. Many processors require very high loadings of fillers, which often have very low bulk densities. The final filling volume depends on following factors:
1.3.1, side feed volumetric feed and main screw volumetric feed of extruder. The volumetric feed rate depends on free volume of side feeder and main auger, as well as speed of rotation of these two augers.
If, during test, material can enter storage drum without resistance from main screw, then material feed rate can be achieved very well. If side feeder is attached directly next to main auger, amount of material that secondary auger can take will be limited.
Therefore, main screw of engine preferably has a threaded element with a large pitch, stretching2D-4D length downstream of side feed port. This will allow melt to quickly pass through side feed opening, maximizing intake of filler by main auger. If design of main auger results in accumulation of material fed from side, this will severely limit amount of filler being fed.
1.3.2, venting capacity to allow gas to escape from extruder. The purpose of exhaust is to facilitate exit of air and at same time protect packing from leaking in large quantities from outlet. The best configuration is to open an upstream outlet upstream of side feed port. Sometimes a small semi-slotted ventilation hole is made in upper part of side feeder.
1.4, there are other factors
1.4.1. Feeding Height
Ideally, feed auger should be mounted as close to top of side feed port as possible to reduce drop height. If fluffy material is flowing through air, it can fill with air so that it has a much lower false density when it falls.
1.4.2. Feeder stirring type
Check that agitator in filler feeder is not fluidizing filler. Many agitator manufacturers develop special agitators for feeding filler. The hopper will supply air to extruder along with filler. Open top funnel for easy ejection. If your trough feed hopper has a sturdy round tube lid with a flexible connection to feeder, then an air vent will need to be designed.
1.4.3. Hopper chute must be grounded to prevent static electricity
Some materials cause powder to hang on wall of hopper due to static electricity generated by friction, resulting in formation of a powder cake. A simple solution is to ground hopper chute.
1.4.4. Compressed air jet
If crust persists, special measures are often required. A hopper vibrator can be used, but it takes up space and requires installation. Alternatively, you can install a blaster with air nozzles placed in chute in right place to blow cake before it rises.
Second, equipment maintenance
2.1. Flush cooling water passage with acid
The water-cooled barrel is actually a heat exchanger. Like all heat exchangers, water channel is subject to fouling and wear. Many operators have noticed that cooling efficiency of new extruder is much better than that of an extruder that has been in use for three or four years. The old machine has a layer of scale attached to inner surface of water channel, which affects heat transfer efficiency. Cleaning barrel of a machine with acidThe tenth concentration can remove scale and keep good heat transfer effect of machine barrel.
2.2. Use synthetic gear oil
The gearbox is heart of twin screw extruder. Therefore, we need to prevent problems with transmission, and use of synthetic gear oil can better protect gears.
Synthetic gear oils:
More creamy, resulting in less friction;
Gears, bearings and seals will last longer;
Drive runs cooler and quieter;
Synthetic oils do not lose viscosity due to machine shearing;
Retains high viscosity at elevated temperatures.
2.3. Clean your car often
The purpose of machine cleaning is to remove residual material in extruder and die, thus avoiding time-consuming disassembly and cleaning. Cleaners often fall into two categories: chemical cleaners and mechanical friction cleaners. Many cleaners perform both chemical and abrasive functions. Chemical cleaners react with plastic residue and remove it from mechanical surfaces.
The principle of mechanical friction cleaning is to abrade and wash away material residues from metal surface using a friction and shear method. Processors can find suitable cleaning agents and methods through many trials. No cleaning material is effective on all materials. Recyclers need to clean materials cheaply and efficiently.
For example, many operators in American Midwest use corn as a cleaning agent for their machines because it is readily available and cleans machines well.
Here are two more effective cleaners:
1#blend: 50%HDPE+50% diatomaceous earth
2# mix: HDPE+water+cleaning powder.
Inject HDPE into screw at normal speed at melting temperature. Mix scouring powder with water in a bucket to form a slurry and slowly add slurry from main supply port. These slurries attack, rub and clean auger due to foaming.
3. Operation of equipment
3.1, correct way to switch colors and materials
People involved in extrusion always want machine to run for a long time. But in fact, due to needs of customers, we often have to constantly switch, producing 80 kg of product A, then producing 200 kg of product B, and so on.
3.1.1 The batch size of product must match size of machine. This seems self-evident. But many recyclers are planning for unscheduled production, using 75 machines to produce 500-pound orders, resulting in 300-pound scrap.
3.1.2. Record cleaning or dismantling procedures for each product produced to help operators plan their time andhurts their arguments about how much machine head material will be produced.
3.1.3. Place clean work tools around extruder to help operator. If material changes frequently, set up a workbench around extruder to prepare everything needed.
3.1.4 Review flow of raw materials and finished products to and from site. Designed so that operator spends less effort to move car.
3.1.5, clean next product from cleaning process of machine Degree requirements. If you know your next product will require extensive manual cleaning, don't waste time and materials on cleaning.
3.1.6 If possible, organize production in descending order of cleaning machines. This means transition from light to dark. Instead, you will have to parse it multiple times.
3.2. Extruder and right way to stop
3.2.1, general working methods
Operators are accustomed to leaving extruder running when they arrive at plant in morning, but they are busy with other things. Let extruder heat up to full power and let it sit for a few hours. This will cause polymer to stick to screw and barrel, which will then continue to decompose and char. Cleaning machines cannot remove these materials, and worse, after machine has finished producing, black dots will occasionally come out of auger and shaft, contaminating product and causing customer complaints.
3.2.2, correct loading method
Schedule when you really want to use extruder, then start heating, just need temperature to reach set point, and then fill barrel.
Before shutting down, fill extruder with an inert polymer (eg HDPE) to maximum extent to close gap in machine. HDPE repels oxygen and coats metal surfaces, significantly reducing formation of carbon oxides. After extruder has been "sealed", it is best to cool extruder rather than leave it in air for 18 hours. The so-called rapid cooling consists in setting all temperatures to zero and opening all cooling water valves. Turn on cooling water pump so that cooling water passes through all parts of cylinder and cools down quickly. Rapid cooling will avoid degradation and charring of polymer.