Frequent retract blade and shutdown

Frequent retract blade and shutdown? Pay attention to these problems in the process of polypropylene granulation!

What factors contribute to the frequent occurrence of retract stops?

Polypropylene granulation process

The extrusion granulation unit is the key unit of polypropylene converted from powder to pellets, and the powder is formed into the final polypropylene granule product after continuous melting, mixing, homogenization, pressurization, filtration and granulation stages.

Among them, under the close cooperation of the template and the cutter, the pelletizer is responsible for cutting the molten resin into granular products.

During the shutdown, the pelletizer and the template are in a separated state, when driving, the pelletizer according to the set program, the pelletizing trolley advances to the formwork, through the mechanical lock to realize the locking and sealing of the pelletizing water chamber and the template, the cutting shaft advances so that the cutting edge and the template granulation hole fit, through the motor drive the knife shaft rotation to achieve cutting granulation.

The cut polypropylene particles are cooled by pelletizing water and transported downstream for dehydration and drying. However, during the operation of the extrusion granulation unit of the polypropylene plant, the pelletizer frequently has retract and stop failures, which seriously affects the production efficiency and product quality.

By analyzing the phenomenon of high-torque chain stop caused by the retraction of the pelletizer when the pelletizer is driven up and during the normal production process, the fault can be troubled from the aspects of the use of the cutter template, the alignment of the pelletizer and the template, the knife pressure adjustment of the pelletizer, the temperature adjustment of the platen and the pelletizing water, and the operation stability of the hydraulic oil system, and solutions can be proposed.

Poor fit of the cutter to the template causes a retraction

  1. The cutterhead is subjected to the force of the cutter shaft to produce pressure in the direction of the template on the cutter, and the motor drives the cutter shaft and the cutterhead to apply the torque force in the circumferential direction to the cutter, which is also the driving force of the cutter to cut resin;
  2. The resin pressurized by the melt pump produces axial resistance to the cutter through the stencil granulation hole, and the cutting resistance generated by the resin in the circumferential direction of the circumferential knife;
  3. The formwork granulation belt produces axial resistance to the cutting knife, and the frictional resistance in the circumferential direction of the cutting knife is generated by the fitting operation of the granulation belt;
  4. Since the rake face before the cutter is a beveled plane, during the rotational movement of the cutter, the pelletizing water will produce resistance in the circumferential direction of the cutter and pressure in the direction of the template.

Under the combined action of these four forces, when the axial resistance is greater than the axial pressure, it will cause the pelletizer to retract; When the resistance in the circumferential direction is greater than the motor output torque protection value, it will cause the motor to stop over the torque chain.

The knife pressure setting of the pelletizer is unreasonable

The pelletizer is equipped with a hydraulic oil system, the hydraulic oil realizes the knife shaft feed and retract through the hydraulic cylinder, and the knife feed pressure and retract pressure act on both ends of the hydraulic cylinder piston during normal production, and the feed pressure needs to overcome the retract pressure, the resistance of the knife shaft itself, and the template granulation hole discharge resin resistance, so that the cutting edge and the template granulation belt are evenly fitted to ensure that the size of the cut resin particles is uniform and the blade is worn neatly.

When the load of the unit is stable, the resistance of the knife shaft itself and the resin resistance of the template granulation hole are in a stable state, at this time, if the pressure difference between the inlet pressure and the retract pressure is too high, it will lead to serious wear of the cutting knife root, and the gap between the tip part and the formwork granulation belt, and then the pad, retract and high torque interlocking parking situation will occur. 

If the pressure difference between the inlet pressure and the retract pressure is too low, it will lead to serious wear of the cutting tip, and a gap between the root of the knife and the formwork granulation belt, resulting in pads, retracts and high-torque interlocking stops. 

The temperature adjustment of the template and pelletizing water is unreasonable, resulting in the retraction

Under the pressure thrust of the molten pump, the resin is pressed into the feeding cavity of the pelletizing template, heated again through the heating channel of the formwork heat transfer oil, and extruded from the stencil granulation hole into the pelletizing water chamber.

Under the combined action of the granulation water and low-pressure pelletizing water chamber environment, the resin expands rapidly and the surface solidifies. The part of the extruded granulation hole is cut and formed by the rotating cutter (moving knife) and the end face of the blanking hole (fixed knife) with the template granulation belt to complete the granulation.

In this process, if the temperature of the template and pelletizing water is too low, it will cause the problem of small resin particles and more debris, which will affect product quality and capacity;

If the temperature of the template and the pelletizing water is too high, the resin will flow out of the formwork granulation hole too quickly, and the pelletizing water cannot completely solidify the surface of the resin, resulting in difficult pelletizing of the cutting knife, and the occurrence of pad, retract and high-torque interlocking stops.

Through the melting point detection of different grades of products, it is found that the melting point between products varies greatly.

Comparing the products produced when retract stop failures occur less frequently in the production process with frequent retract stop faults, it is found that the melting point of the latter is about 20 °C lower than that of the former.

The corresponding set temperature of the template and pelletizing water is not proportionally adjusted, so the unreasonable temperature adjustment of the template and pelletizing water may be the cause of the retract.

The unstable operation of the hydraulic oil system causes the retraction

The pelletizer is equipped with a hydraulic oil system, the hydraulic oil through the hydraulic cylinder to achieve the knife shaft intake, retract, if the hydraulic oil system during operation can not provide stable hydraulic oil pressure, will lead to the pelletizer cutter shaft force is unstable, and then cause pad, retract and high torque interlocking stop.

Through the combing and study of the oil circuit of the hydraulic oil system of the pelletizer, combined with the inlet and retract pressures set during driving, it was found that the setting pressure of the relief valve controlling the retract pressure was 3 MPa, which was the same as the set retract pressure. According to the structural principle of the relief valve, the setting pressure is achieved by adjusting the amount of spring compression, and when the setting pressure is at the critical point of the setting pressure of the relief valve, the anti-interference ability of the relief valve is weak.

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