14 Common Problems and Solutions in Modified Plastic Granulation Processing

1. Insufficient Extrusion Material

■ Increase the mesh size or number of filter screens;

■ Appropriately reduce the main extruder speed or increase the feed speed;

■ Appropriately reduce the extrusion processing temperature (die head or other zones).

2. External Impurities

■ Check whether the equipment's dead corners in the mixing and discharging stages are clean and whether any impurities have entered;

■ Minimize the amount of crushed material added or manually perform initial screening of the crushed material to remove impurities;

■ Increase the mesh size and number of filter screens;

■ Cover any holes where debris may fall in (with solid covers or mesh covers).

3. Internal Impurities

■ Excessive die head pressure (including die blockage, excessive filter screen, excessive die head temperature, etc.) increases backflow, leading to intensified carbonization. Carbonized material is carried into the extruder strip, causing strip breakage under traction force.

■ Localized overheating of the extruder intensifies carbonization, causing carbonized material to be carried into the extruder strip, resulting in strip breakage under traction force.

■ Excessively strong screw shearing in certain areas intensifies localized carbonization of the material, causing carbonized material to be carried into the extruder strip, resulting in strip breakage under traction force.

■ Long machine lifespan leads to screw and barrel wear, increased gaps, and increased backflow. More carbonized material adheres to the barrel wall, and as extrusion time increases, this carbonized material is gradually carried into the extruder strip, causing strip breakage under traction force.

■ Vacuum or natural venting ports (including gaskets and dead corners) are not cleaned for extended periods, allowing existing carbonized material to be carried into the extruder strip, causing strip breakage under traction force.

■ 4. Poor Material Plasticization

■ Inadequate cleaning of the die head (including the discharge port and internal dead corners) allows carbonized materials or impurities to be carried into the material strip, causing breakage under traction.

■ Infrequent filter replacement leads to filter blockage, preventing material flow and causing breakage.

■ Low extrusion temperature or weak screw shear results in insufficient plasticization, leading to lumps and breakage under traction.

■ Low-melting-point additives (including EBS or PETS) in the formulation, combined with weak screw shear, increased screw-barrel clearance, or weak shearing, result in poor plasticization and breakage. 5. Changes in Raw Material Properties

■ Significant differences in flowability between blended components at the same temperature can lead to "phase separation" due to flowability mismatch or incomplete compatibility (including physical entanglement and chemical reactions). Phase separation generally doesn't occur in blend extrusion but is more common in injection molding. However, if the molecular weight flow rate (MFR) differs greatly, breakage may occur under conditions of relatively weak screw shear.

■ Changes in blended component viscosity: For the same material, a decrease in MFR leads to increased hardness, rigidity, and notch size. This may indicate a higher molecular weight in the batch, resulting in increased viscosity. Under the original processing temperature and process conditions, this can cause poor plasticization. In this case, increasing the extrusion temperature or decreasing the main screw speed can resolve the issue.

6. Steam Trapping or Poor Venting in the Material Strip

■ Excessively high processing temperature, excessive local shearing of the screw, or localized overheating of the screw can cause the decomposition of certain flame retardants and other additives, releasing gases. If the vacuum fails to remove these gases in time, they become trapped inside the material strip, leading to strip breakage under traction force.

■ Severely damp material; if processing moisture is not promptly removed through natural venting and vacuum, the gas becomes trapped in the material strip, causing breakage under traction force.

■ Poor natural or vacuum venting (including blockages, leaks, excessively high gaskets, etc.) can cause gas (or steam) to become trapped inside the material strip, resulting in breakage under traction force.

7. High Material Rigidity, Excessive Water Cooling or Flushing, Mismatched Traction

If the material is too rigid, the water temperature is too low, or too much water is applied, the output from the pellet mill will be very soft, but after watering, it will immediately become very hard. Under the mismatched traction force, this can cause breakage. This phenomenon often occurs with materials like PBT, PET with fiber, PC with fiber, AS with fiber, and ABS with fiber, which have very fast crystallization rates or very high rigidity. This is especially serious in small-scale experiments. In this case, increasing the water temperature and reducing the water flow rate to maintain a certain degree of softness in the pelletizer strips can solve the problem.

8. Insufficient Filter Mesh Count or Number of Filter Sheets

This phenomenon often occurs when there is insufficient pressure at the pellet mill head, or when there are external or internal impurities.

9. Pellet Chaining Problem

A series of pellets that stick together is usually called a double-linked or chained pellet. This can be caused by excessively high processing water temperature or too low water flow rate.

Pellet chaining refers to a situation where a series of particles are connected to each other; that is, in some cases, particles are connected together by end-to-end contact of the film or tangentially. During processing, several process issues may individually or collectively lead to this phenomenon. For example, excessively hot processing water can cause pellet clumping; in this case, the water temperature should be lowered to provide sufficient cooling to the particle surface. Additionally, excessively low water flow velocity can also cause pellet clumping, as it slows down the pelletizing chamber speed, leading to particle agglomeration. Furthermore, if the die orifice spacing is too close, exit expansion during processing will cause particles to collide; the solution is to replace the existing die with one featuring a larger spacing and fewer orifices.

10. Tailing Problem

Tailling refers to the protruding edge of the pellets, resembling the shape of a hockey stick. It looks like a contaminant or tear at the bottom of the cut. This occurs because the cutting device fails to make a clean cut at this point. Generally, correctly cut pellets from a wire feed pelletizer should be right-angled cylinders, and those from an underwater pelletizer should be near-perfect spheres.

Materials that don't usually produce fine particles may also generate fine particles due to tailing. Assuming all processing parameters have been checked, tailing is generally diagnosed as a cutting problem. For wire pelletizing lines, the solution is to replace the rollers and bottom cutters to provide new, sharp cutting edges; or to re-determine the equipment spacing according to the values ​​specified in the manufacturer's manual. For underwater pelletizing lines, the die and cutting edge need to be inspected to ensure there are no scratches, as scratches and grooves often cause tailing.

11. Powder Issues

For many crystalline materials, such as general-purpose polystyrene, powder seems to be a common and unique hazard. It becomes a problem for processors because it alters the bulk density of the material, degrades or burns in the extruder barrel, and causes problems in the conveying process. The primary goal of resin manufacturers is to produce uniform pellets, i.e., pellets with a predetermined length and diameter, free from contamination from powder or foreign matter.

To address this problem, powdering can be reduced by adjusting the equipment and controlling some key process parameters. When entering the cutter, the temperature of the wire pellet production line should be as close as possible to the Vicat softening point of the material to ensure the wire is subjected to maximum thermal shearing, thereby avoiding breakage.

For specific polymers, selecting a cutting roller with an appropriate pelletizing angle plays a crucial role in reducing material waste. For unfilled polymers, Stellite or tool steel cutting rollers should be used whenever possible, and both the roller and the bottom cutter edges should be kept sharp to avoid crushing the polymer. For subsequent equipment after pelleting, whether pressurized or vacuum-operated, air entrapment must be avoided.

For underwater pelleting lines, ensure sufficient pressure is maintained against the die during processing and adjust the post-pellet dwell time appropriately to ensure the particles enter the dryer hot.

12. Bottom Cutter Breakage Problem

The bottom cutter of the pelletizing equipment is a hardened carbide steel sheet with Invar alloy welded to it in appropriate locations, allowing it to be threaded onto a support. Typically, bottom cutter breakage occurs after the cutting edge rotates. Appropriate measures can be taken to avoid this problem, and the manufacturer's recommended methods should be carefully followed. One point that needs special emphasis here is that the threaded Invar alloy mandrel, fixed in place by silver solder, has a shear limit and is easily damaged by excessive torque during installation. Additionally, during rotation or installation, a broken bottom blade is prone to displacement and can scatter within the pelletizer, damaging the cutting edge of the roller and increasing maintenance costs.

13. Wire Drift Problem

Wire drift is the tendency of wire to bunch to one side on the feed platform. It causes problems such as poor pellet quality, the presence of long, thin strips, and processing disorder. If the pelletizer cutting plane is not parallel to the extruder extrusion die, the wire will tend to crowd to the left or right, ultimately leading to wire drift. Other causes of wire drift include inconsistent gaps between the lower feed roller and the scraper, and inconsistent diameters of the lower feed roller.

14. Linear Control Problem

Long, thin strips are a type of abnormal product produced by the pelletizer. As the name suggests, their length is longer than the normal particle size, with the extra length typically varying within a few inches. The appearance of thin, elongated strips (also known as angled-cut particles) indicates poor control of the wire's posture when fed into the cutter. Specifically, the wire is not at a perpendicular angle when fed into the cutter, resulting in a tilted end during cutting.

The distance between the feed roller (bite point) and the cutter (cutting point) is called the feed distance, and nothing is used to control the wire within this span. Unlike a wood planer, if the feed roller is improperly installed or in poor condition, the plastic wire will not be fed into the cutting device at an angle perpendicular to the cutting surface. This causes the wire to cross, further deteriorating the cutting quality and ultimately leading to serious problems. The crossed wire forces the two feed rollers apart, causing the wire to lose tension and temporarily sag, deflecting the wire to either side of the feed rollers. Warning signs of this problem include poor condition of the upper feed roller, such as grooves, cracks, or discoloration (aging or heat-induced hardening).

Other common issues in wire control include: worn bottom feed rollers, which cause loss of traction; improper wire quenching processes, which can lead to severe, snake-like bending of the wire; and worn wire die plates, which produce wires of varying diameters. Furthermore, manufacturers must be wary of extremely worn rollers and bottom cutters that hold the wire in place, as the bottom cutter is responsible for pushing the wire to the cutting point and preventing the cutter from operating at excessively high speeds, which can cause wire wobble.

In underwater pelletizing systems, the primary cause of elongated strips is a mismatch between the feed rate and the cutter speed. In this case, the cutter speed needs to be increased to match the feed rate, or the feed rate decreased to match the cutter speed. Additionally, during processing, it is crucial to ensure that the cutter head has sufficient blades to guarantee the correct particle geometry and to check for slowing or blockages in the polymer flow through the die orifices.