What To Do If The Die Hole of Underwater Pelletizing Is Clogged? Do These Three Things To Reduce The Frequency of Machine Cleaning

Extruders are increasingly using underwater pelletizing to produce engineering polymers, including resins such as PET, nylon and polycarbonate, which have “difficult” properties such as low viscosity and/or rapid cooling or high melt temperatures.

Compared with strand pelletizing, underwater pelletizing has the advantages of better pellet quality, higher degree of automation and lower cost. However, compared with less sensitive commodity resins such as polyethylene, polypropylene or polystyrene, the risk of mold freezing is greater when producing engineering resins such as those mentioned in the previous paragraph.

Underwater pelletizers typically use a round die with 10 to 20 holes or even hundreds of holes. Systems can be designed to compound anywhere from a few hundred to nearly 20,000 lbs/hr. (Even larger pelletizers are used for extrusion production.)

Causes of die hole freezing

The die hole diameter is usually 0.125 inches, but can be as small as 0.01 inches for smaller particles and as large as 0.20 inches. The output of each hole is about 25-100 pounds per hour, but the average output of a 0.125-inch die hole is about 30 pounds per hour.

The key to underwater pelletizing is to deliver enough hot plastic through the die holes to keep the holes at the melt temperature of the resin while the melt and die are in contact with water that is several hundred degrees cooler. Metal molds are very good conductors of heat, so unless the polymer flow through the die holes is sufficient, the water will cool the holes and the polymer will solidify in them. However, the processor has no way of knowing immediately that this blockage has occurred.

The problem is that if one or more of the die holes become clogged with polymer, the efficiency of the die will decrease. Eventually, the clogged hole will affect the flow of surrounding holes, the finished pellets will be large and irregular in shape, and the output will decrease. Eventually, the line needs to be completely shut down to clean the die holes.

There are three main reasons for mold freezing: improper start-up sequence of polymer and water flow and cutter operation; inadequate mold heating and insulation; and process fluctuations. Not considering the material formulation (resin, filler level and filler type - heat absorbing or heat conducting) can also cause problems. But you can't change the formulation to please the pelletizer, it should be designed to run the majority of formulations.

Adjusting the pelletizer startup process

Most die hole freeze-ups occur during startup, when the resin, cooling water, and cutter must be started in the correct sequence. If the resin begins to flow through the die before the water reaches the die face and the cutter engages, smearing and resin plugging will occur outside the hole in the water box. If the water reaches the die face before the polymer begins to flow, excessive cooling can cause hole freeze-ups.

In early underwater pelletizing systems, a skilled operator controlled this sequence manually. Starting in the early 1990s, startup could be automated and controlled by commercial PLC control systems. Polymer diverter valves and process water bypass systems allow the PLC to more precisely control the timing of polymer and water entering the die. Pelletizers manufactured since the mid-1990s offer automatic cutting systems that use hydraulics, pneumatics, and servomotors to control the cutting pressure. As a result, today's fully automatic systems can control the startup sequence in a fraction of a second.

Optimize mold design

Once the line is up and running, most mold freezes can be avoided by maintaining a very constant flow of material through the die orifice. But some freeze problems are related to the mold design itself.

The mold is heated by electric cartridge heaters or hot oil. Electric heating is not completely uniform because the cartridges are placed primarily in a ring around the mold holes, near the outside edge of the mold plate. As a result, the holes around the outside of the die tend to be hotter than the holes in the center. (Mold manufacturers have tried adding cartridges in the center of the mold, but the engineering proved clumsy.)

Therefore, cartridge heaters may not be suitable for large molds or materials with a narrow melting temperature range, such as nylon, which may tend to freeze in the center of the mold.

Oil heating applies more uniform heat over a larger area than cartridge heaters. Oil systems are configured with up to eight mold zones. But oil heating is more expensive than cartridge heaters. Oil heat is typically used on high-output production lines at resin manufacturers or large compounding plants. More recently, it has been applied on smaller systems to achieve better temperature control.

If the oil valve becomes clogged, the mold bore in the affected area of the mold will become colder and may freeze, but the rest of the mold will be fine. Output may not change at first. Again, as with cartridge heaters, the problem will manifest as defective pellets.

All dies have some type of insulation. Most molds use special plates that are attached to the middle of the mold face. But the plates can become loose. Water then flows behind them, cooling the mold and potentially causing a freeze.

Another insulation method is to use a "hollow" mold design with gaps that contain a vacuum. It can be retrofitted onto most systems if it was not part of the original mold design. The vacuum is a very good insulator and cannot be compromised like solid insulation panels.

Reduce process fluctuations

Any type of interruption in the extrusion process or fluctuation in output can cause freezing. For example, extruding recycled materials with low bulk density can cause fluctuations and swings in the extruder output, which can cause inconsistencies in the polymer flow, freezing the die orifices in a random manner. Polymer melt pumps can be used to prevent these fluctuations and provide consistent die flow.

Using a discontinuous slide screen changer can also cause flow interruptions and entrap air, which can cause a temporary loss of polymer flow to the die orifice. This can cause random die orifice freeze-ups. The solution is to use a continuous screen changer, which can replace dirty screens on the fly without interrupting the process or entrapped air.

NANJING HAISI is professional manufacturer of Plastic Extruder Machine, we supply Twin Screw Extruder, Single Screw Extruder, Plastic Recycling Extruder, Two Stage Extruder, Lab Scale Extruder, Mixing and Feeding Machine, Cooling and Pelletizing Machine, Crushing Machine and so on.