The strand pelletizer is regarded as the most traditional and simplest pelletizer. It is normally used for a wide range of strong melt, strengthened polymers.
While those applications require a bigger base frame for the pelletizer, the polymer melt comes from the extruder die head, falls into the water cooling bath vertically, pulled into the pelletizer after passing through the air knife/air wiper. The cooling time in the water bath is decided by the characteristics of different polymers.
Nevertheless, after decades of improvement, there are several new kinda pelletizing systems were introduced on the market. One of them is the under water pelletizer, it is now the priority choice for polymer reaction, compounding and recycling applications. From point of operation, the under water pelletizer is the most complicated system compared to other options. But generally speaking, it can continuously work without workers attending unless the under water pelletizer is stopped by accident. It can be controlled automatically and alarm by itself.
When starting the under water pelletizer, the water will be pumped into the cutting chamber, the cutter will cut the material coming out from the extruder or reactor by touching the die head. The uniformed pellets cut by the pelletizer emerged in the water and cooled immediately, being pumped to the centrifugal dryer for dewatering and further cooling. The whole pelletizing process is working in a closed loop without air. The pellets will be classified by a vibrator, and then transported to the silo by a vacuum pump or air blower. A packing system might be used afterwards.
Whatever strand pelletizing or under water pelletizing system will give some unique challenges to resin producers, masterbatch producers or recycling producers as followings:
Improve Strand Control
Long strands: the length could be variable by several inches more than the strand pellets size 3x3mm. It shows that the strands are not well controlled when fed into the strand pelletizer, they are not fed into the pelletizer vertically with the rotor.
The distance between the feeding roller and the rotor is called pressing distance, in this distance, there is nothing to control the strands. If the roller is well fixed or in a bad condition, then the plastic strands will not be fed into the pelletizer vertically with the rotor. The strands will come across each other, which makes the result even worse.
The crossing strands will enlarge the gap between the rubber roller and the steel roller, the strands lose its tension, thus the strands will fall down or twist, causing the strands to go to the side of the rollers. The roller must be well fixed and in good condition to get a quality pellet.
The reasons for the problem above mentioned are probably
- The rubber roller is in a bad condition, with grooves, and cracks on the rubber roller.
- The steel roller was worn out. That will lead to the loss of pull strength.
- Wrong strands water cooling process, making the strands bend.
- The strand die head wore out. Strands come out from the die head with different diameters.
- The worn out rotor and fixed knife. For under water pelletizer, the long pellets issue origins from the melt feeding speed does not match the speed of the under water pelletizer. Need to increased the under water pelletizer speed to match the melt feeding speed or reduce the melt feeding speed to match the max. under water pelletizer speed. Make sure there are enough number of cutting knives to cut the pellets into the desired shape and check if there is any hole block on the die plate.
Strands drifting means that the strands gather to one side of the pelletizer, which will make the pellets’ quality bad. If the cutting flat of the pelletizer is not in a parallel position to the extruder die head, then the strands will rush to the right or left side of the pelletizer. Other reasons for the strands drifting are clearance between the steel roller and rotor is not universal or the diameter of the steel roller is not the same.
Avoid the Shrinking Hole
The shrinking hole most possible reason is that the cooling is not correct. It could a very small hole on the pellets, sometimes it might be a hollow hole formed. The core of strands is not sufficiently cooled and the pellets shrink after cutting. The solution is to have enough cooling for the strands, when it is cooled completely, can feed it to the strand pelletizer to cut.