First, the reason for the black spots
The quality of the raw materials itself is poor, and the black spots are excessive;
Local overheating of the screw causes the material to become carbonized, and the char is brought into the strip, causing more points;
The local shear of the screw is too strong, causing the material to become carbonized, and the char is brought into the strip, causing more black spots;
The pressure of the machine head is too large (including blockage, too much filter screen, too low temperature of the machine head, etc.), too much reflow material, carbonization of the material is increased, and charcoal is brought into the material strip, causing more points;
The machine has a long service life, the gap between the screw and the barrel increases, and the adhesion of the barrel wall increases. As the extrusion time passes, it is gradually brought into the strip, causing more black spots;
The natural exhaust port and the vacuum exhaust port are not cleaned for a long time, and the accumulated carbonized material is increased, and the continuous extrusion is carried into the strip in the subsequent period, resulting in a larger number of points;
The external environment or artificially caused other impurities to mix in, resulting in more black spots;
The die (including the discharge port and the internal dead angle) is not cleaned, resulting in more black spots;
The discharge port is not smooth enough (for example, some shallow grooves and potholes, etc.), and materials may accumulate for a long time. As the extrusion time elapses, it is gradually carbonized and then carried into the material strip, causing more black spots;
Part of the thread original damage (corner angle, wear, etc. form a dead angle), resulting in carbonization of the material at the dead corner, in the subsequent continuous extrusion process, is gradually brought out to the strip, resulting in more black spots;
Natural exhaust and vacuum exhaust are not smooth, causing carbonization of the material in the screw, resulting in more black spots.
Second, the analysis of the processing of finished products
Broken bars produce the original deficiency:
Increase the mesh number or the number of sheets;
Appropriately reduce the host speed or increase the feed speed;
Properly reduce the extrusion processing temperature (head or other zones).
External impurity
Check whether the dead angle of equipment in all aspects of mixing and discharging is cleaned up and impurities are mixed in;
Minimize the addition of broken material or artificially screen the broken material to remove impurities;
Increase the number of mesh screens and the number of sheets;
Try to cover the holes (solid covers or net covers) that may have fallen debris.
Internal impurity
The head pressure is too high (including the blockage of the die, too much filter, the temperature of the head is too low, etc.), causing the increase of the reflux to cause the carbonization to increase, and the char is taken out to the strip, causing a broken strip under the traction force;
The extruder is partially overheated, causing the carbonization to increase, and the char is taken out to the strip, causing a broken strip under the traction force;
The shearing of the screw is too strong, causing the local carbonization of the material to be aggravated, and the char is taken out to the strip, causing a broken strip under the action of traction;
The machine has a long service life, the screw and the barrel are worn, the gap is increased, the backflow is increased, and the carbon carbide adhering to the barrel wall is increased. As the extrusion time is extended, the char is gradually taken out to the strip, causing a broken strip under the traction force. ;
Vacuum or natural vents (including gaskets and dead ends) are not cleaned for a long time, and the existing char is brought to the strip, causing broken strips under the action of traction;
The die of the machine head (including the discharge port and the inner dead angle of the machine head) is not cleaned, and the die contains carbonized material or impurities which are brought to the strip, which causes a broken strip under the traction force;
The time interval for replacing the filter is too long, the filter screen is blocked, and the material does not come out, causing broken bars.
Poor plasticization of materials:
The extrusion temperature is too low or the screw shear is too weak, the material is not fully plasticized, and the material is defective. Under the traction force, the broken strip is caused.
Low-melting-point auxiliaries (including EBS or PETS, etc.) in the formulation system cause poor plasticization and broken strips under the premise that the screw shear is weak or the gap between the screw and the barrel is increased and the shear is weak.
Material properties change:
The blending components are too different at the same temperature, and the fluidity is not matched or not fully compatible (including physical entanglement and chemical reaction). Theoretically, this is called "phase separation" and "phase separation". Generally, blending extrusion does not occur, and it occurs more frequently in the injection molding process. However, if the MFR difference is too large, a broken strip may occur under the premise that the screw is relatively weakly sheared;
Viscosity change of blended components: For the same material, if the MFR is reduced, the hardness, rigidity and notch become larger, it is possible that the molecular weight of the batch is larger than before, resulting in a larger viscosity, at the original processing temperature. Under the action of the process, the plasticization is poor, and at this time, the extrusion temperature can be increased or the screw speed of the main machine can be reduced.
The strip is trapped or exhausted:
The processing temperature is too high or the local shear of the screw is too strong or the screw is partially overheated, causing the decomposition of some flame retardants and other additives, releasing the gas, the vacuum does not extract the gas in time, the gas is trapped inside the strip, under the traction force Causing a broken strip;
The material is seriously damp, the processed water vapor is not removed by natural exhaust and vacuum in time, and the steam trap is trapped in the strip, which causes a broken strip under the action of traction;
Natural exhaust or vacuum exhaust is not smooth (including blockage, air leak, gasket is too high, etc.), causing gas (or steam) trapped in the strip, under the action of traction, causing broken strips.
The material is rigid, water-cooled or over-watered, and the traction is not matched:
The material is too rigid, the water temperature is too low, too much water, the discharge of the machine head is very soft, and the water immediately becomes very hard. Under the effect of mismatching traction, the broken strip is caused. This phenomenon often occurs in PBT or PET fiber, PC fiber, AS fiber, ABS fiber and other materials with very fast crystallization speed or very high rigidity, especially for small machines, the experiment is more serious. At this time, increase the water temperature, reduce the water volume, and let the cut The strip of the granulator keeps a certain degree of softness and can be solved.
The filter mesh is too low or the number of sheets is not enough:
This phenomenon often occurs when the above-mentioned machine head is under pressure, external impurities and internal impurities.
Even the grain problem:
A series of pellets that stick to each other and are often referred to as duplexes or chains.
The reason may be that the processing water temperature is too high or the water flow rate is too low.
A granule refers to a situation in which a series of particles are connected to each other, that is, in some cases, the particles are joined together by end faces of the film or tangentially. During the processing, several process problems may cause this phenomenon alone or together. For example, too much heat in the processing water is one cause of the granules. In this case, the water temperature should be lowered to give the surface of the particles sufficient quenching; in addition, the low water velocity is also a cause of granules, which causes the particles to be cut. The particle chamber velocity is slowed down, and particle agglomeration occurs. In addition, if the hole distance of the die is too close, the expansion of the outlet will cause the particles to touch during the process. The solution is to replace the existing die with a die with a large pitch and a small number of holes.
Trailing problem:
The so-called tailing is that the edge of the particle is somewhat prominent, and the cutting edge is like the shape of a hockey stick. It looks like a contaminant or tear at the bottom of the incision. The reason for this is that the cutting device is not able to perform a crisp cutting here. In general, the correct cutting particles from the strand pelletizer should be a right-angled cylinder. The correct pellet from the underwater pelletizer should be a nearly perfect sphere.
In general, materials that are not prone to materialization will also result in tailing due to tailing. Assuming all processing parameters have been checked, the smearing may generally be diagnosed as a cutting problem. For the wire pelletizing line, the solution is to replace the hob and the bottom knife to provide a new and sharp cutting edge; or to re-determine the equipment spacing according to the values ​​specified in the manufacturer's manual. For underwater pelletizing, the template and the edge need to be inspected to ensure that there are no scores, as the scores and grooves often cause tailing.
Powder problem:
For many crystalline materials, such as general purpose polystyrene, the end of the material appears to be a common and characteristic hazard. They have become a problem for processors because they change the bulk density of the material and degrade or burn it in the barrel of the extruder, causing trouble for the conveying process. The main goal of the resin manufacturer is to produce a uniform grain shape, ie having a defined length and diameter, without contamination from the end of the material or foreign matter.
In response to this problem, the purpose of reducing the end of the material can be achieved by adjusting the equipment and controlling some important process parameters. When entering the cutter, the temperature of the strand line should be as close as possible to the Vicat softening point of the material to ensure that the strand is hot cut as much as possible to avoid cracking.
For a specific polymer, choosing a hob with the appropriate dicing angle plays an important role in reducing the end of the material. For unfilled polymers, Stellite or tool steel hobs should be used as much as possible, and the hob and bottom knife edges should be kept sharp to avoid breaking the polymer. For subsequent equipment after pelletizing, avoid air entrapment, whether pressurized or vacuumed.
For underwater pelletizing lines, ensure that the knife pressure against the die face is maintained during processing and that the residence time after pelletizing is properly adjusted to ensure that the particles are hot as they enter the dryer.
Bottom knife rupture problem:
The bottom knives of the pelletizing equipment are a hard, carbonized steel sheet that is welded with an invar alloy in its proper position to enable it to be threaded onto the bracket. Normally, the bottom knife will rupture after the blade of the bottom knife rotates. For this reason, appropriate measures can be taken to avoid this problem. In this process, it is necessary to follow the recommended method in the manufacturer's equipment manual. Here, it is important to emphasize that the threaded Invar core rod is fixed in place by silver welding, which has a shear limit and is easily damaged by excessive torque during installation. In addition, during rotation or installation, the broken bottom knife is prone to displacement and will scatter in the pelletizer, destroying the cutting edge of the hob and increasing maintenance costs.
Wire drift problem:
Wire drift is a tendency of strands to be bundled on the feed platform, which causes problems such as poor grain quality, slender strips, and processing turbulence. If the cutting plane of the pelletizer is not parallel to the extruder extrusion template, the strand will appear to be crowded towards the left or right, eventually causing the strand to drift. In addition, other causes of wire drift include that the gap between the lower feed roller and the scraper is not constant, and the diameter of the lower feed roller is inconsistent.
Linear control problem:
Slender strips are a type of abnormal product produced by a pelletizer. As the name suggests, the length is longer than conventional particle sizes, and the length that grows typically varies within a few inches. The appearance of slender strips (also known as bevel cut particles) indicates that the wire material attitude control when feeding the wire feed is not good, specifically because the wire material is not at a vertical angle when feeding the hob, so When cutting, an angle of inclination will appear at the end of the wire.
The distance between the feed roller (biting point) and the hob (cutting point) is called the press-in distance, and nothing is used to control the strands in this span. The pelletizer is different from the wood planer. If the feed roller is not installed correctly or the working condition is poor, the plastic strand will not be fed into the cutting device at an angle perpendicular to the cutting surface. As a result, the strands begin to cross each other. , causing further deterioration in the quality of the cut, which eventually causes serious problems. The crossed strands will force the two feed rolls apart from each other, causing the strands to lose tension, which in turn causes the strands to hang temporarily, deflecting the strands toward the sides of the feed rolls. An early warning signal to the above problem is that the upper feed roller is in a poor working condition, with grooves, cracks or discoloration (aging or heat-induced hardening).
Common problems with other wire control include: lower feed roller wear, which will cause loss of traction; incorrect wire quenching process, which will cause the wire to bend like a snake; and worn wire The template will produce a variety of strands of different diameters. Not only that, manufacturers should be wary of extremely worn hobs and bottom knives that hold the wire, because the knives are responsible for pushing the wire to the cutting point, preventing the cutter from operating at ultra-high speeds because of this high The speed will cause the wire to sway.
In the underwater pelletizing system, the main reason for the sliver is that the feed speed does not match the cutter speed. In this case, it is necessary to increase the cutter speed to match the feed speed or reduce the feed speed to match. Cutter speed. In addition, ensure that there are enough inserts on the cutting tip during the process to ensure that the particles have the correct geometry and to check for slow movement or blockage of the polymer stream.
Third, analysis of granulation hollow problem
Poor exhaust:
Exhaust (or steam) is not smooth: due to natural exhaust or vacuum exhaust (may be too heavy in the material itself, it may also be decomposed by additives such as flame retardants, there may be vacuum blockage or vacuum is too small or leaking or The vacuum gasket is too high, etc., causing a gas (vapor) body to form a hollow.
Poor plasticization:
The processing temperature is low, the material is not completely plasticized, the light (small hole) causes the particles to be hollow, and the heavy one (large hole) causes the broken strip;
There are too many low-melting auxiliaries (including EBS or PETS), and the shearing of the screw is weak (for example, the production of ordinary ABS on the 2nd line, the EBS cannot be too much, and too many "pores" appear), resulting in poor plasticization of materials. Forming a hollow;
There are too many low-melting auxiliaries (including EBS or PETS), and the gap between the screw and the barrel is increased (for example, ordinary wire ABS is produced on line 1, sometimes EBS is not too much, too many "pores" appear) or screw shear Under the premise of weaker conditions, plasticization is poor and hollowness is formed.
The water temperature is too low: the cooling water temperature is too low, the material shrinks with water, causing shrinkage holes, such as PP products - such phenomena are mainly for crystalline plastics; in general, crystalline plastics (such as PP, PA, PBT, etc.) Low water temperature should be used, and non-crystalline plastics (such as ABS, PC/ABS, HIPS, etc.) use high water temperature.
Shrinkage void problem
The presence of shrinkage voids indicates that the strand retraction is not appropriate.
Shrinkage voids and hollow pellets indicate that the strands are not properly tempered. When the shrinkage gap is slight, it may be just a small pit on the end face of the particle. In severe cases, hollow particles may be generated, just like a bartender. This phenomenon occurs when the core temperature of the strand is close to the molten state and the line The material is shrunk immediately after being pelletized. For a properly tempered strand, the temperature gradient at the interface will remain constant and it will not respond to the cooling medium (air or water) when it is cut.
The specific reason for the occurrence of shrinkage voids is that when the process water is too cold for a particular polymer, the outer layer of the strands freezes, creating a hard shell that leaves heat in the core of the strand; in addition, the strands are There is not enough immersion time in the air or water, so that the heat of the core of the wire cannot be transferred to the surface of the wire, so that good cross-section cooling cannot be performed.
Underwater pelletizing particles, due to the presence of trapped volatiles in the melt, shrinkage voids also occur. An effective precaution is to inspect the vacuum holes in the extruder.
Fourth, the natural exhaust port, vacuum material
Natural vents:
The feed speed does not match the host speed, and the feed speed is appropriately reduced or the host speed is increased;
The temperature in the region from the feeding section to the natural exhaust port is too low, the material is not plasticized, and under the action of the screw extrusion, the raw material is generated;
The temperature near the natural exhaust port is too high, the viscosity of the material is seriously degraded, the screw is slipped in this section, and the material cannot be transported to the front section in time, and the clinker is caused by the subsequent crushing of the flow;
The position of the natural exhaust port of the screw does not match the position of the natural exhaust port of the barrel, resulting in a raw material;
There is no reverse conveying element or reverse meshing block here, which can not reduce the pressure of the natural exhaust port groove, which will cause the material to flow under the subsequent crushing action.
Vacuum material:
The vacuum pumping force is too large, and the material is sucked into the vacuum pipe to cause the raw material;
The screw vacuum position is not provided with the reverse conveying element or the reverse meshing block, and the pressure of the vacuum section screw groove cannot be reduced, and the vacuum is caused by the vacuuming;
The temperature of the vacuum section is too high, the viscosity of the material is seriously degraded, the screw of this section is slippery, the material cannot be transported to the front section in time, and under the action of vacuum pumping, the raw material is caused;
The extrusion processing temperature is too low, the material is not plasticized or the flame retardant and the like are not sufficiently dispersed in the resin, and under the action of vacuum pumping, the raw material is caused;
The screw combination is also reasonable, the position is also matched, the temperature and the main machine and the feeding speed are also matched, the vacuum gasket is low, and the material is caused by the material extrusion and the vacuum pumping force;
The pressure on the machine head is too large (the reasons include: blockage of the die, too much filter net, too low temperature of the machine head, etc.), causing an increase in the backflow, causing a flue under the action of vacuum pumping.
Five, hopper bridge
Too much filler, moisture absorption, agglomeration, resulting in increased friction between the mixture and the hopper wall, adding liquid lubricants such as “white oil†to reduce the friction between the mixture and the hopper wall and the mixture, which can be solved;
Mixture agglomeration (including high temperature agglomeration and liquid additives added too much to agglomerate), reduce drying temperature or high mixing time, reduce the amount of liquid additives added, add powdery resin that absorbs "oily" substances Or additives (such as high rubber powder, AS powder, PP powder, etc.) can be solved.
Sixth, the material bridge
Mainly because the melting point of some processing aids is too low, the processing temperature of one zone and two zones is too high, the material softens in the feeding silo, and adheres to the equipment wall, which makes the subsequent material feeding difficult, and appropriately reduces the processing of one zone and two zones. Temperature can be solved.
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