Seven common granulation problems and corresponding solutions

Based on the two processes of strand pelletizing and underwater pelletizing, we try to analyze seven common granulation problems and the corresponding solutions, and provide some suggestions for continuous mass production of quality products.

Linkage problem

Linkage refers to a series of particles connected to each other, that is, in some cases through the film end to end or tangential connection. During the process, several process problems may cause this phenomenon alone or in combination. For example, if the processing water is too hot, one of the causes of conglutination, in this case, the water temperature should be lowered to allow sufficient quenching of the particle surface; In addition, the low water velocity is also a cause of particle association, which will cause the particle cutting chamber to slow down, and then appear particle agglomeration. In addition, if the hole distance of the die head is too close, the expansion of the outlet will cause particle contact during the process. The solution is to replace the existing die head with a large spacing and fewer holes.

Trailing problem

A tail is a particle with a slightly protruding edge, and the cutting edge is shaped like a hockey stick. It looks like a contaminant or a tear at the bottom of the cut. The reason is that the cutting device has not been able to cut cleanly here. In general, the correct cut particle coming out of a wire pelleting machine should be a rectangular cylinder, and the correct cut particle coming out of an underwater pelleting machine should be a nearly perfect sphere.

Usually, materials that are not prone to tailing will also produce tailing. Trailing may generally be diagnosed as a cutting problem, assuming that all machining parameters have been checked. For the line cutting line, the solution is to replace the hob and bottom cutter to provide a new and sharp cutting edge; Or re-determine the spacing according to the values specified in the manufacturer's manual. For underwater pelleting, the template and blade need to be checked to make sure there are no marks, as marks and grooves often cause trailing.

Powder problem

For many crystalline materials, such as universal polystyrene, residue appears to be a common and specific hazard. They become a problem for processors because they can change the bulk density of the material, degrade or scorch in the extruder barrel, and cause problems in the delivery process. The main goal of resin manufacturers is to PRODUCE a uniform grain shape, that is, a given length and diameter, free of contamination from particles 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 entering the cutter, the temperature of the wire line should be as close as possible to the vicat softening point of the material to ensure that the wire is heated as much as possible to avoid cracking.

The choice of hob with appropriate granulation Angle for a particular polymer plays an important role in reducing the end of the material. For unfilled polymers, use Stellite or tool steel hobs as much as possible, and keep the hob and bottom cutter edges sharp to avoid breaking the polymer. For the following equipment after granulation, whether pressurized or vacuum equipment, should avoid air trapping.

For the underwater pelletizing line, it is necessary to ensure that sufficient knife pressure against the die surface is maintained during processing, and the residence time after pelleting is properly adjusted to ensure that the particles are hot when entering the dryer.

Cutter rupture problem

The bottom cutter of the pelleting equipment is a hard carbonized steel sheet welded with an Invar alloy in place to enable it to be threaded onto the support. Usually, the blade of the base cutter will be broken after the blade is turned. Appropriate measures can be taken to avoid this problem, and in this process, the method recommended in the manufacturer's equipment manual should be carefully followed. It is important to emphasize here that the threaded invar alloy mandrel, which IS held in place by silver welding, has a shear limit that can be easily destroyed by excessive torque during installation. In addition, in the rotation or installation, the broken bottom knife is easy to shift, and will fly away in the granulator, destroy the blade of the hob, improve the maintenance cost.

Shrinkage problem

Shrinking voids and hollow grains indicate improper tempering of strand. When the shrinkage gap is slight, it may only be a small pit on the end face of the particle, and when it is serious, it may produce hollow particles, just like a cocktail stick. This phenomenon occurs when the temperature of the core of the strand material is close to the molten state, and the strand material shrinks immediately after being pelleted. The temperature gradient at the interface of a correctly tempered wire 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 processed water is too cold for a particular polymer, the outer layer of the strand material freezes, creating a hard shell, and leaving the heat in the core of the strand material; In addition, the strand material in the air or water does not have enough soaking time, resulting in the heat of the strand core can not be transferred to the surface of the wire material, thus unable to carry out a good section cooling.

Due to the presence of trapped volatiles in the melt, the particles produced by underwater pelletizing will also have shrinkage voids. An effective preventive measure is to check the vacuum hole in the extruder.

Drift problem

Strand drift is the tendency of the strand material to cluster to one side on the feeding platform, which will cause the quality of the granules deterioration, the existence of elongated strip and processing disorder and other problems. If the cutter of the strand pelletizer is not parallel to the extrusion template of the extruder, the line material will tend to be crowded to the left or right, eventually leading to strand material drift. In addition, other causes of strand drift include the gap between the lower feeding roller and the scraper is not constant, and the diameter of the lower feeding roller is not consistent.

Control problem

A slant is an unusual product produced by strand pelletizer, it is LONGER than the normal particle size, usually varying in size within a few inches. The appearance of elongated strips (also known as bevel cutting particles) indicates that the line feed hob line attitude control is not good, specifically, because the line feed hob is not at a vertical angle, so in cutting, the end of the line will appear a tilt angle.

Fine if the feeding roller is not installed properly, or the working condition is poor, then the plastic wire will not be fed into the cutting device at an angle perpendicular to the cutting surface, so that the wire begins to cross each other, causing further deterioration of the cutting quality, and eventually produce serious problems.

Other common problems in wire control include: The lower feeding cots wear, which will cause the loss of traction force; Incorrect quenching process of wire material, which will cause the wire material to bend violently like a snake; There are also worn strand template, it will produce a variety of diameter different wire. Not only that, manufacturers should be wary of extreme wear hobs and base cutters, which push the strand to the cutting point and prevent the cutters from operating at ultra-high speeds, which can cause the strand to sway.

In the underwater pelletizing system, the main reason for elongated strips is that the feeding speed does not match the cutting speed. In this case, the cutter speed needs to be increased to match the feeding speed, or the feeding speed needs to be decreased to match the cutter speed. In addition, ensure that there is enough blade on the cutting tool head to ensure that the particles have the correct geometry, and check for slow movement or blockage of the aggregate material flow in the die holes.

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