7 common cutting problems and corresponding solutions (plastic products)

Connected particles are a series of particles connected to each other, that is, in some cases, the particles are connected by the membrane end to end or in a tangential manner. During the processing process, several process problems alone or together may cause this phenomenon to occur. For example, the processing water is too hot is a cause of the particle, in this case, the water temperature should be lowered to give sufficient quenching of the particle surface; In addition, the water flow speed is too low is also a cause of granulation, it will cause the particle cutting chamber speed slowed down, and then the particle agglomeration. In addition, if the hole distance of the die head is too close, the export expansion will cause particle contact during processing, and the solution is to replace the existing die head with a large spacing and a small number of holes.

Trailing problem

The so-called tail is that the edge of the particle is somewhat prominent, the cut edge is like a hockey stick shape, it looks like a pollutant or tear at the bottom of the cut. The reason for this is that the cutting device has not been able to make a clean cut here. In general, the correct cut particle coming out of the line pelletizer should be a right Angle cylinder, and the correct cut particle coming out of the underwater pelletizer should be a nearly perfect spherical shape.

Usually, materials that are not prone to fining will also produce fining due to tailing. Assuming that all machining parameters have been checked, tailing can generally be diagnosed as a cutting problem. For the line cutting line, the solution is to replace the hob and bottom knife to provide a new and sharp cutting edge; Or re-determine the equipment spacing according to the values specified in the manufacturer's manual. For underwater grits, the template and blade need to be checked to ensure that there are no nicks, as nicks and grooves often cause tailing.

Material waste problem

For many crystalline materials, such as general-purpose polystyrene, fining 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 burn in the extruder barrel, and cause problems for the transportation process. The main goal of the resin manufacturer is to produce a uniform grain shape, that is, with a given length and diameter, without contamination from material residues or foreign substances.

To solve this problem, the purpose of reducing the powder can be achieved 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 Veka softening point of the material to ensure that the wire is subjected to as much heat as possible, thus avoiding breakage.

For a particular polymer, selecting a hob with the right cutting Angle plays an important role in reducing the amount of material. For unfilled polymers, Stellite alloy steel or tool steel hobs should be used as much as possible, and the edge of the hob and base knife should be kept sharp to avoid breaking the polymer. For the subsequent equipment after cutting, whether pressurized or vacuum equipment, it is necessary to avoid wrapping air.

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

Bottom knife rupture problem

The base knife of the granulator is a hard, carbonized steel sheet with Invar alloy welded in place to enable it to be threaded into the support. In general, the bottom knife breaks when the blade of the bottom knife is turned, and appropriate measures can be taken to avoid this problem by carefully following the recommended methods in the manufacturer's equipment manual. Here, it is particularly important to emphasize that the threaded Invar mandrel is fixed in place by silver welding, which has a shear limit and is prone to being destroyed by excessive torque during installation. In addition, during rotation or installation, the cracked bottom knife is prone to shift and will fly in the granulator, damaging the blade of the hob and increasing maintenance costs.

Shrinkage gap problem

Shrinking voids and hollow particles indicate improper tempering of wire material. The shrinkage gap may be only a small pit on the particle end surface in mild cases, and in severe cases it may produce hollow particles, like a mixstick. This phenomenon occurs when the core temperature of the wire is close to a molten state and the wire shrinks immediately after it is cut. When the wire is properly tempered, the temperature gradient at the interface remains constant, and it does 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 particular polymer, the outer surface of the wire material freezes, creating a hard shell, and trapping heat in the core of the wire material. In addition, the wire does not have enough time to soak in air or water, resulting in the heat of the wire core cannot be transferred to the wire surface, so that good cross-section cooling cannot be performed.

Particles produced by underwater pelleting, due to the presence of trapped volatiles in the melt, will also appear to shrink voids, an effective preventive measure is to check the vacuum hole on the extruder.

Line drift problem

Linear drift is the tendency of linear material to cluster to one side on the feeding platform, which will cause the quality of the grain to deteriorate, there are slender strips and processing disorders. If the cutting plane of the grain cutter is not parallel to the extruder extrusion template, the line material will tend to be crowded to the left or right, and eventually lead to the line material drift. In addition, other causes of line drift include the gap between the lower feed roller and the scraper is not constant, the diameter of the lower feed roller is inconsistent, etc.

Wire control problem

The long strip is an abnormal product produced by the grain cutter. As the name suggests, its length is longer than the regular particle size, and the size of the long strip usually varies within a few inches. The appearance of thin strips (also known as bevel cutting particles) indicates that the wire attitude is not well controlled when the wire is fed into the hob, specifically because the wire is not at a vertical Angle when fed into the hob, so the end of the wire will appear at a slant Angle when cutting.

The distance between the feed roll (bite point) and the hob (cut point) is called the press distance, and there is nothing to control the wire material at this span. The grain cutter is different from the wood planer, if the feeding roller is not properly installed, 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 material begins to cross each other, causing further deterioration of the cutting quality, and eventually causing serious problems. The crossed wire material will force the two feeding rolls apart from each other, causing the wire to lose tension, which in turn causes the wire to temporarily drop, causing the wire material to be biased to both sides of the feeding roll. An early warning sign of these problems is that the upper feed roller is in poor condition, with grooves, cracks, or discoloration (hardening due to aging or heat).

Other common problems with wire control include: lower feed roller wear, which causes loss of traction; Incorrect wire quenching process, which will cause the wire to bend like a snake; There is also a worn wire template, which will produce a variety of wire with different diameters. Not only that, manufacturers are also wary of extremely worn hobs and bottom knives that hold the wire to the cutting point, because the bottom knife is responsible for pushing the wire to the cutting point and preventing the cutter from operating at ultra-high speeds, which can cause the wire to sway.

In the underwater pelleting system, the main reason for the production of slender strips 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 the cutter speed. In addition, during the processing, it is necessary to ensure that there are enough blades on the cutting head to ensure that the particles have the correct geometry, and to check whether there is a slow movement or obstruction of the polymerized material flow in the die hole.