In underwater pelletizing, using the wrong die size can lead to various issues with pellet quality. Pictured here is COWELL underwater pelletizer.
Compounders encounter numerous challenges in today’s industry, with the key to success lying in maintaining consistent product quality. Among the common challenges faced in underwater pelletizing is the issue of pellet inconsistency, which can result from improper die selection or sizing, as well as output fluctuations. This article delves into the causes of pellet inconsistency and proposes solutions to address the problem.
IMPROPER DIE SIZING
Incorrect die sizing in underwater pelletizing applications can result in uneven polymer flow across the die-hole cross-section and polymer solidification within the die hole (also known as freeze-off). To avoid this, it is necessary to maintain a minimum polymer velocity through the extrusion hole and the correct die temperature.
To prevent solidification within the die hole, a recommended velocity of 2.5 to 3 ft/sec through the hole is necessary. This requires the correct number of holes in the die plate for the desired production rate.
For a hole diameter of 0.125 in. (3.2 mm), a rate of 50 to 60 lb/hr per die hole is required to achieve the recommended velocity (V=0.0002122/R2 x rate per hole in lb/hr), assuming a material with specific gravity of 1. However, if there is a significant amount of filler or pigments, the material’s specific gravity will be higher and must be taken into account when sizing the die.
For example, if the material has a specific gravity of 1.8, 1.8 times the rate of 50 to 60 lb/hr per die hole is needed. Adding more holes to the die to reduce pressure drop is not necessary; fewer holes are needed to keep all holes “open” while running. A calculation can determine if all die holes are open while running:
- X = R × 7.6 /(W × S × N), where X is the number of open holes.
- R is the rate in lb/hr, W is the weight in grams per pellet.
- S is the speed of the pelletizer in RPM.
- N is the number of blades on the cutter hub.
Proper management of polymer velocity and heat at the die plate is necessary to achieve cleanly cut pellets. Without this control or understanding, process settings or engineered devices may not provide a full solution to problems causing pellet inconsistency.
IMPROPER DIE TEMPERATURE
This could also lead to die freezing. Generally, the die temperature should be maintained at least 25°F higher than the melt temperature of the polymer, although it may vary slightly for different products. The insulation used on most dies is crucial for keeping the die’s cutting face isolated from the process water and preventing heat loss.
The insulation is typically sealed with a high-temperature RTV silicone, and it’s important to take note of the temperature ratings since the sealant can deteriorate over time, especially at high temperatures. Water can seep into the insulation, which can cause a drop in the die temperature. If the die struggles to reach or maintain the original setpoint temperature despite the control system calling for heat, it’s likely that the insulation on the die plate has degraded, is missing, or has been installed incorrectly.
To determine if any heaters have failed, check the power being used by the die. Many control systems have an ammeter for each die heat zone. Calculate the required amp draw for each zone and compare it to the actual reading. If the reading is lower than optimal, it’s possible that one or more heaters have failed.
Inconsistent pellet size can occur due to physical blockage of the die holes caused by contaminants in the material or poorly dispersed mineral fillers in the matrix. To prevent this, it is recommended to use a screen changer or some form of filtration before the die. In the case of smaller systems, a hand-cleaning tool can remove the blockage from the die hole, while larger systems require removing the die for cleaning. Additionally, reducing temperature profiles, including the die heat zone, during prolonged idle periods is advised.
Although we have discussed the causes and effects on the underwater pelletizer, there are other factors in the process that can lead to inconsistent pellet sizes. For example, feedstock bridging in the hopper can hinder material flow into the extruder, resulting in inconsistent pellets or die freeze depending on the product being processed.
Improper extruder temperature settings can cause surging, leading to varying pellet sizes and possible pellet deformity. It is important to note that a high melt temperature may not necessarily cause inconsistent pellet sizes but can still result in deformities.
FINES AND TAILS ON THE PELLETS
This issue can have various causes, but it is typically the result of wear on the die and/or blade. If the die-plate cutting face becomes grooved, it can prevent a new and sharp blade from cutting the polymer cleanly. As a result, a small amount of the material is pulled through the groove at the point of the cut, creating a tail on the pellet. The same is true for a grooved blade operating on a new die face. It is crucial to choose materials for the die face and blades that are compatible and appropriate for the job. While using a blade that is too hard may extend its lifespan in the short term, it can speed up wear on the die face.
There are various factors that can cause tails, but the main one is usually wear on the die and/or blade. When the die-plate cutting face becomes grooved, even a sharp blade cannot cleanly cut the polymer, resulting in a tail on the pellet. Similarly, if the blade is grooved and operating on a new die face, it can also cause tails. It is important to select compatible materials for the die face and blades to avoid excessive wear. Using a blade that is too hard may provide a longer blade life initially, but it can accelerate wear on the die face.
Other potential causes of tails include a high melt temperature, misaligned die, mechanical wear on pelletizer bearings causing excessive shaft run-out, and hot process water. Additionally, an exaggerated flow gradient within the die hole can occur without proper maintenance of polymer velocity through the die-plate extrusion holes, leading to a sleeving effect and an undesirable polymer shoulder that can make it difficult for the blade to cut cleanly.
As the pellet goes through a centrifugal dryer, the tail can break off, resulting in fines. This can lead to problems in material-handling systems and cause high residual moisture on the pellets. Fines can also stick to the dryer screens, reducing the open area and airflow through the screens.
Higher than desired residual moisture content on the finished pellet can also be caused by melt fracture, a pellet that is porous due to high loadings of filler or fibrous material, irregular pellet shape, or operating and environmental parameters.
Melt fracture creates a rough surface on the pellets, which can trap water as the pellet passes through the centrifugal dryer. The rough surface usually appears on the side wall of the pellet and not on the cut ends. To reduce melt fracture, one option is to cut the pellet slightly thinner to decrease the sidewall area, although this increases the total surface area. Polishing or extrude-honing the dies can also minimize melt fracture by reducing the velocity through the die holes.
Mineral fillers at high levels can create a porous pellet that is challenging to dry. The porosity allows water into the voids, which can be difficult to remove through centrifugal action alone. Fibrous fillers like glass or wood can draw moisture into the pellet like a wick, making mechanical drying almost impossible, so post-drying may be necessary.
Proper operating parameters must be established when using centrifugal dryers, which operate on three basic principles: residual heat in the pellet, centrifugal action, and countercurrent ambient air flow. If the process-water temperature is too low, the resulting pellet temperature will be insufficient to flash off enough surface moisture. Conversely, if the water temperature is too high, tails may be generated. Countercurrent air flow through the dryer is crucial in reducing surface moisture levels and should be checked and adjusted according to the manufacturer’s specifications.
The reduction of surface moisture in pellets can be significantly affected by environmental conditions. In areas with high humidity and when processing melt-fractured or porous pellets, achieving low pellet-moisture levels may be challenging. In addition, if a product is sticky and requires very cold water during processing, and the pellet exits the dryer at a temperature below the current dewpoint, condensation may form, resulting in higher moisture content when the pellet is packaged, even if it was dry when it exited the dryer.
Operators of underwater pelletizing systems commonly encounter these issues. However, by understanding the cause and effect of these problems, they can usually be eliminated. If you are experiencing any of these issues, it is recommended to contact your underwater pelletizer manufacturer for assistance in improving your process.