Seven common granulation problems and solutions of rubber and plastic granulation

1. Continuous granulation problem
Connected particles refer to the situation that a series of particles are connected with each other, that is, in some cases, particles are connected with each other through the film end to the end face or in a tangential way. In the process of processing, the following problems can individually or jointly cause grain cutting and material connection:
The cooling water is too hot during processing. In this case, reduce the water temperature to make the particle surface fast enough to quench.
If the water flow speed is too low, it will cause the particles to leave the particle cutting chamber at a slower speed, and make the particles continue to feed.
If the distance between the die and the hole is too close, the expansion of the outlet during processing will cause particle contact, so the die with large spacing and small number of holes is used.
2. Dragging problem
Trailing is that the edge of the particle is somewhat prominent, and the cutting edge is like the shape of a hockey stick, which looks like a pollutant or tear at the bottom of the cut.
The main reason for this is that the cutting device failed to cut cleanly at the cutting point. In general, the cutting particles coming out of the lath cutter should be a right angle cylinder, and the cutting particles coming out of the underwater cutter should be a nearly perfect sphere.
Generally, materials that are not prone to material powder will also produce material powder due to tailing. For lath cutting, the solution is to replace the hob and bottom knife to provide a new and sharp cutting edge; Or re-determine the equipment spacing as required. For the underwater cutting line, it is necessary to check the template and blade to ensure that there are no nicks, because nicks and grooves often cause tailing.
3. End of material problem
For most crystalline materials, such as GP-PS, powder is a common and unique hazard. The powder will change the bulk density of the material and is easy to degrade or burn in the extruder barrel.
The main goal of the resin manufacturer is to produce uniform particle shape, that is, it has a fixed length and diameter, and it is free from pollution from the powder or foreign substances. To solve this problem, the end of material can be reduced by adjusting the equipment and controlling some important process parameters. When the material enters the cutter, the temperature of the drawing production line should be as close as possible to the Vicat softening point of the material to ensure that the material strip is hot cut as much as possible to avoid cracking.
For specific polymers, the selection of hobs with proper cutting angle plays an important role in reducing the amount of particles. For unfilled polymer, keep the edge of hob and bottom knife sharp to avoid breaking the polymer. For the subsequent equipment of pelletizing, no matter pressurized or vacuum equipment, air should be avoided.
For the underwater cutting line, it is necessary to ensure that sufficient tool pressure is maintained against the mold surface during the processing, and the residence time after cutting is properly adjusted to ensure that the particles are hot when entering the dryer.
4. Bottom knife fracture
The bottom knife of the pelletizing equipment is a hard carbonized steel sheet. Generally, the bottom knife will break after the blade of the bottom knife rotates. In this process, it needs to be adjusted and operated in strict accordance with the process parameters. The threaded invar core rod is fixed in place by silver welding. It has a shear limit and is easy to be damaged by excessive torque during installation. In addition, in the process of rotation or installation, the broken bottom knife is easy to shift, and will fly away in the granulator, damaging the blade of the hob and increasing the maintenance cost.
5. Shrinkage gap problem
Shrinkage voids and hollow particles indicate that the tempering of the strip is not proper. When the shrinkage gap is slight, it may only be a small pit on the particle end face, but when it is serious, it may produce hollow particles. The core temperature of the strip is close to the melting state, and the strip shrinks immediately after being granulated. For the correctly tempered bar, the temperature gradient at its 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 shrinkage gap is that when the processing water is too cold for a specific polymer, the outer layer of the strip is frozen, resulting in a hard shell, leaving heat in the core of the wire; In addition, the strip does not have enough immersion time in air or water, resulting in that the heat of the core of the wire can not be transferred to the surface of the wire, so that good section cooling can not be carried out.
Particles produced by underwater granulation will also have shrinkage voids due to trapped volatiles in the melt. An effective preventive measure is to check the vacuum hole on the extruder.
6. Bar drift problem
The bar drift is the tendency of the bar to cluster to one side on the feeding platform, which will cause the quality of the particles to become poor, there are long and thin bars and processing disorders. If the cutting plane of the pelletizer is not parallel to the extrusion template of the extruder, the strip will tend to be crowded to the left or right, resulting in the drift of the strip.
In addition, other causes of strip drift include the inconstant gap between the lower feeding roller and the scraper, and the inconsistent diameter of the lower feeding roller.
7. Strip control problem
Slim strip is a kind of abnormal product produced by the granulator, mainly because its length is longer than the conventional particle size, and the extra size usually varies within a few inches. The appearance of thin strip indicates that the strip attitude control is not good when feeding the hob. Specifically, because the strip is not at a vertical angle when feeding the hob, there will be a tilt angle at the end of the strip during cutting.
The distance between the feed roller (bite point) and the hob (cutting point) is called the press distance. There is nothing in this span to control the wire material. If the feeding roller is not installed correctly or the working condition is poor, the rubber and plastic strips will not be fed into the cutting device at an angle perpendicular to the cutting surface. In this way, the wire starts to cross each other, causing further deterioration of the cutting quality, and eventually causing serious problems. The cross strip will force the two feeding rollers to separate from each other, causing the strip to lose tension, thus causing the strip to sag temporarily, making the strip deflect to both sides of the feeding roller. The warning signal of the above problems is that the upper feeding roller is in bad working condition, with grooves, cracks or discoloration (hardening caused by aging or heat).
Other common problems in strip control include: wear of lower feeding roller, which will cause loss of traction; Incorrect wire quenching process will cause the material bar to bend violently like a snake; There are also worn strip templates, which will produce various wire materials with different diameters. Not only that, manufacturers should also be alert to the extremely worn hob and the bottom knife that holds the strip, because the bottom knife is responsible for pushing the line material to the cutting point to prevent the cutter from running at the ultra-high speed, because this ultra-high speed will cause the strip to swing.