Underwater strand pelletizing unit is a new type of polymer semi-finished product processing machine, named after its cutting process carried out in water.
The entire working process of underwater strand pelletizing unit is as follows: when a high-temperature polyester melt with a certain pressure is extruded from the casting head, the casting strip first uses gravity to immerse itself in the overflow water on the starting plate of the granulator for cooling, and then flows through the guide plate and cutting plate with grooves. During this process, it is further cooled and solidified by spraying water. Finally, the casting strip is introduced into the gap between the moving and fixed knives through the front and rear feeding rollers, and is cut off by the rotation of the spiral blade of the moving knife. Due to the fact that the center of the cut off particles has not yet fully cooled and is in a semi molten state, water is also provided to prevent adhesion between particles. After the conveying water enters the cutting head, it is divided into two streams by the distribution plate: one stream is used to cool the newly cut particles, and the other stream is used to transport the particles out of the pelletizer. This achieves the goal of continuous cutting through the continuous operation of the pelletizer, and the size of the particles depends on production needs. The typical cutting size is 4 * 4 * 25, and how this size is achieved should be understood by process personnel, The following is the calculation method for cutting particles.
From a mechanical perspective, in order to ensure a certain length, the feeding line speed and cutting line speed must maintain a certain proportional relationship, which can be represented by the following equation:
L=d π/Zr * i=75 π/30 * (85/43)=4 (mm)
In the formula: L-slice length
D-diameter of rear feeding roller
Zr – Number of teeth on the moving blade
I – Speed ratio between the rear feeding roller and the moving knife
The thickness of particles is determined by the feed gap, and the usual calculation method is:
δ=δ 1+（ δ 1 * K)=1.6+(1.6 * 0.5625)=2.5 (mm)
In the equation: δ- Particle thickness
1- Feed gap
K-expansion coefficient at 200 ℃
Specific structure of the granulator
1. Casting strip guide part
The main component of the guide part of the casting belt bundle is a longitudinal groove guide plate, with a pneumatically operated starting plate installed at the top. At the inlet of the water connection, water evenly flows through the entire width of the guide plate. When the device starts and stops, the last starting plate is opened, and the starting plate is also opened when the drive is terminated or the cooling water is cooled. The bottom of the guide part of the casting belt bundle is equipped with a stopping plate, When the gap between the feeding rollers in the pulling part (also known as the feeding part) is blocked, the cutting machine stop plate is interlocked and stopped. In order to facilitate the maintenance of the cutting head, the replacement of the rotating roller, and the adjustment of the cutting, the guide plate of the casting belt bundle can be tilted to a horizontal position after the cover of the cutting chamber is opened.
2. Cutting head section
The feeding unit consists of two feeding rollers, both of which have the same linear speed, and one is smooth (rear feeding roller). The latter is a stainless steel roller with spiral teeth (front feeding roller) with a spiral angle of 2, and the axial groove improves the feeding performance. Install the front feeding roller as a cantilever
It is movable and can be pneumatically bounced to adapt to uneven thickness of the cast belt bundle. To adjust the cutting gap and correct any abnormalities, the roller can be manually pulled back.
The cutting device consists of a moving knife (spiral rotary knife) and a fixed knife, with the fixed knife being a tungsten carbide cutting knife. The moving knife is made of a special alloy steel.
3. Water system
In the cutting chamber, the water flow is divided into two streams, one of which forms a vertical curtain and flows to the moving knife immediately after the cutting point. Commonly known as radial water flow, its function is to prevent newly cut materials from sticking to each other. Another stream enters the cutting chamber along the tangent direction of the rotating blade, commonly known as tangential water flow. Its function is to cover the bottom of the cutting chamber and maintain a water depth of about 5mm at the bottom, in order to further cool the sliced particles and transport the materials out of the cutting machine. In addition, the overall structure also includes the motor drive part and instrument control system, which will not be described anymore.