First, the reason for the black point is too much
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 carbonized material is brought into the strip, causing more black spots;
The head pressure is too large (including blockage, too many filters, the head temperature is too low, etc.), too much material is recirculated, the material is carbonized, and the char is brought into the 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 wall of the barrel increases. As the extrusion time elapses, 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 over a long period of time. As the extrusion time elapses, it is gradually carbonized and then carried into the strip, causing more black spots;
Part of the thread originals are damaged (corner angle, wear, etc. form a dead angle), which causes the carbonization of the material at the dead corner to be aggravated. In the subsequent continuous extrusion process, it is gradually taken out to the material strip, causing 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 areas).
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 crushed material or manually screen the crushed material to remove impurities;
Increase the number of mesh screens and the number of sheets;
Try to cover the holes (solid cover or net cover) that may have debris falling.
Internal impurity
The head pressure is too high (including the clogging of the die, too many filters, 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 traction force;
The machine has a long service life, the screw and the barrel are worn, the gap is increased, the backflow is increased, and the 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 materials or impurities that 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 is blocked, and the material does not come out, causing a broken strip.
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 generated. Under the traction force, the broken strip is caused.
Low melting point auxiliaries (including EBS or PETS, etc.) in the formulation system, under the premise of weak screw shear or increased gap between the screw and the barrel and weak shear, resulting in poor plasticization and broken strips.
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. At this time, increasing the extrusion temperature or reducing the screw speed of the main machine can be solved.
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 broken bars;
The material is seriously damp, the processed water vapor is not removed by natural exhaust and vacuum in time, and the vapor is trapped in the material strip, causing broken strips under the action of traction;
Natural exhaust or vacuum exhaustion (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 does not match:
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 mismatched traction, the broken strip is caused. This phenomenon often occurs in PBT or PET fiber, PC fiber, AS fiber, ABS fiber, etc., which have very fast crystallization speed or very high rigidity, especially for small machines. The water temperature is increased, the water volume is reduced, and the water is cut. The strip of the machine 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 machine head is under pressure, external impurities and internal impurities.
Even the grain problem:
A series of granules that stick to each other and are often referred to as duplexes or granules.
The reason may be that the processing water temperature is too high or the water flow rate is too low.
A granule is a condition 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. In the process, several process problems may be alone or Together, this phenomenon occurs. For example, too much processing water is a cause of granules. In this case, the water temperature should be lowered to give the particle surface sufficient quenching; in addition, the water flow rate is too low to cause granules. One reason is that it causes the particle cleavage chamber to slow down and particle agglomeration. 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 use a large pitch. The die with a small number of holes replaces the existing die.
Trailing problem:
The so-called tailing is that the edge of the particle is somewhat prominent. 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 be crisp and neat here. Cutting. 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 tailing may generally be diagnosed as a cutting problem. For the wire pelletizing line, the solution is solved. The method 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 blade need to be inspected to ensure that there are no nicks. Because nicks and grooves often cause tailing.
Powder problem:
For many crystalline materials, such as general-purpose polystyrene, the end of the material seems to be a common and characteristic hazard. The reason they become a problem for processors is that they change the bulk density of the material. Degradation or charring in the exit barrel, causing trouble for the conveying process. The main goal of the resin manufacturer is to produce a uniform granular shape, that is, with a given 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 production line should be as close as possible to the Vicat softening point of the material to ensure that the line is exhausted. May be eager to avoid rupture.
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, try Stellite or tool steel hobs. Keep the hob and the bottom knife edge 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 knife of the pelletizing machine is a hard carbonized steel sheet, which is welded with Invar in its proper position to enable it to be threaded onto the bracket. Usually, the bottom knife is broken after the blade of the bedknife is rotated. In this regard, appropriate measures can be taken to avoid such problems. 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 The rod is fixed in place by silver soldering. It has a shear limit that is easily damaged by excessive torque during installation. In addition, during rotation or installation, the broken bottom knife is prone to displacement and will be pelletized. The machine flies, destroys the blade of the hob, and increases maintenance costs.
Wire drift problem:
Wire drift is the tendency of strands to be bundled on one side of the feed platform. It causes the quality of the pellets to deteriorate, and there are problems such as slender strips and processing disorders. If the cutting plane of the pelletizer is not parallel to the extrusion If the machine is extruded, the wire will appear to be crowded to the left or right, which will eventually cause the wire to drift. In addition, other reasons for the drift of the wire include that the gap between the feed roller and the blade is not constant, and the feed is lower. The diameter of the rolls 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 of the strip usually varies within a few inches. Slender strips (also known as bevel cut particles) The appearance of the wire indicates that the wire material attitude control when the wire feed is fed is not good, specifically because the wire material is not at a vertical angle when feeding the hob, so when cutting, an inclination angle will appear at the end of the wire material. .
The distance between the feed roller (biting point) and the hob (cutting point) is called the press-in distance. There is nothing on this span to control the strand. The pelletizer is different from the plank planer, if the feed roller If the installation is not correct, or the working condition is poor, the plastic wire will not be fed into the cutting device at an angle perpendicular to the cutting surface. As a result, the wires begin to cross each other, causing further deterioration of the cutting quality, 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, causing the strands to deflect toward the sides of the feed rolls. The early warning signal to the above problem is that the upper feed rolls In poor working conditions, there are 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, which will produce a variety of different diameters. Not only that, but manufacturers also have to 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 cutting The knife is running at ultra high speed, because this ultra high 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. Cutting speed. In addition, ensure that there are enough blades on the cutting head during the machining process to ensure that the particles have the correct geometry and to check for the 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.), resulting in the presence of gas (vapor) in the particles, forming a hollow.
Poor plasticization:
The processing temperature is low, the material is not completely plasticized, the light (small hole) causes the hollow of the particles, 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 the too many stomata appear), resulting in poor plasticization of the material. Forming a hollow;
There are too many low melting point 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 'holes' appear) or screw shearing Under the premise of weaker conditions, the plasticization is poor and the hollow 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. 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 strands are not properly retracted.
Shrinkage voids and hollow pellets indicate that the strands are not properly tempered. Shrinkage voids may be just a small pit on the particle end face, and in severe cases hollow particles may be produced, just like a bartender. In the case where the core temperature of the strand is close to the molten state, and the strand is shrunk immediately after pelletizing. The strand obtained with the correct tempering will have a constant temperature gradient at the interface and it will be cut against the cooling medium ( Air or water) did not respond.
The specific reason for the 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 check the vacuum holes in the extruder.
Fourth, natural exhaust, vacuum material
Natural vents:
The feed speed does not match the host speed, and the feed speed is reduced or the host speed is increased.
The temperature in the area 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, and the viscosity of the material is seriously degraded. The screw of this section slips, 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 reverse vacuum conveying element or the reverse meshing block is not provided in the screw vacuum position, 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, and the viscosity of the material is seriously degraded. The screw of this section is slippery, and the material cannot be transported to the front section in time, causing the raw material under the action of vacuum pumping;
The extrusion processing temperature is too low, the materials are not plasticized or the additives such as flame retardants are not fully dispersed in the resin, and under the action of vacuum pumping, the raw materials are 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, the temperature of the head is too low, etc.), causing an increase in backflow, causing a splash 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 a liquid external lubricant such as 'white oil' to reduce the friction between the mixture and the hopper wall and the mixture, 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' Or additives (eg, high rubber powder, AS powder, PP powder, etc.), can be solved.
Six, 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 feeding difficult, and appropriately reduces one zone and two zones. Temperature, can be solved.