Views: 3 Author: Site Editor Publish Time: 2024-06-24 Origin: Site
The template of the extruder consists of a template body with a discharge hole and a heat insulation layer, and is fixed to the die head with a threaded rod. Only when the discharge speed of each die hole is consistent can particles of uniform size be cut out.
During use, there are many factors that affect the smooth flow of the template. When the template is severely blocked, it will affect the appearance of the particles, the service life of the cutter, and cause shutdown.
Blockage Factors
Blockage occurs during equipment shutdown, startup, and normal production
If the products in the processing section and the discharging section are subjected to excessive thermal degradation during the equipment shutdown period, critical precipitation or carbonization may occur. This extreme thermal degradation of the product may lead to long-term problems during startup and production. If carbonization occurs in the template, it will cause inevitable die hole blockage.
During the startup process, if the template is not flushed properly, it will also affect the smooth flow of the die hole. At the same time, the time setting of the product, pellet water and pelletizer blade reaching the template is critical to ensure uniform product flow and produce uniform pellets. If the water flowing to the template starts too early compared to the product time, up to 30% of the template holes will be "frozen".
During normal production, the blockage mainly occurs at the contact surface between the template and the cutting water. Due to the rapid change of temperature and the influence of the cutting knife, the viscous polymer in this surface rapidly undergoes a high elastic transition and a glass transition, and then continuously forms blockage and punching holes. As time goes by, some oligomers and impurities will settle down, forming a similar "hanging wall" that reduces the flow area of the die hole. This kind of blockage generally has no effect during normal production, but when cooling down to produce high melt index products, this characteristic will directly affect the operation of the extruder due to the high fluidity of high melt index products. Therefore, the template and pelletizing water temperature play a key role in preventing blockage. Too low pelletizing water temperature will reduce the template temperature and increase the blockage rate. However, if the pelletizing water temperature is too high, the pellets cannot achieve the cooling effect, which may cause sticking.
The influence of product characteristics on die hole clogging
Low melt index products
Polypropylene itself is a mixture of molecules of various molecular weights. High molecular weight parts are easy to exist in low melt index products. These high molecular weight polymers have low melt index and are not easy to mix completely. When the granulation output is low or the production fluctuates, especially when the granulator is running at a low speed, the die hole may be blocked due to the low discharge flow rate.
High melt index products
The flow of polypropylene melt in the barrel can be regarded as the flow of viscous fluid, and the flow equation can be obtained according to fluid dynamics.
Q=K△P／μ（1）
Q is the flow rate of the extruder; K is a constant, which is related to the structural size of the screw; △P is the melt pressure difference; μ is the melt viscosity. When the extrusion flow rate remains unchanged, as the melt index increases, that is, the melt viscosity decreases, the melt pressure difference will decrease accordingly. When the melt index remains unchanged, as the flow rate increases, the melt pressure difference will increase accordingly.
The increase in melt pressure difference is conducive to the cleaning of low melt index products and impurities adhering to the die hole, while the high melt product itself reduces the pressure on the template, which is not conducive to its own flushing ability on the die hole. During production, special attention should be paid to the changes in die head pressure. Although the die head pressure is mainly affected by the pressure drop at the load, melt index, screen changer and other components, severe blockage of the die hole will directly lead to a rapid increase in die head pressure.
Impact of production conversion scheme on die hole blockage
In the flexible production process, the modified factory often needs to switch brands, which causes changes in the production conditions of the pelletizer: the polymer MFR changes greatly in a short period of time, resulting in uneven polymer flow rate in the extruder barrel, which leads to unstable discharge and some die holes are "frozen" and blocked. When the controlled rheology (CRPP) scheme is converted to high melt index products, if the product is not mixed evenly in the mixing section, some low melt index products enter the die plate, and because the barrel and die plate temperatures drop too quickly when converting to high melt index products, the low melt index part cannot be completely melted and is very easy to be blocked at the die hole.
The influence of load on die hole blockage
The production load needs to be adjusted frequently during the production process. High load and low temperature will inevitably lead to incomplete mixing and thus die hole blockage. Low load and high temperature will cause the material in the barrel to be extruded too fast at the die plate, which is easy to cause knife entanglement and knife padding. Load adjustment should also avoid too fast and too large amplitude. In daily production, except for special needs, the load adjustment amplitude should not exceed 5% of the current load.
The impact of equipment conditions on die hole blockage
The inner wall of the die hole itself is smooth, but under the action of external forces, such as mechanical damage to the die hole caused by improper maintenance, or impurities brought by additive feeding, although the filter will filter out most of the impurities, fine impurities such as metal impurities will also cause the die hole to "hang on the wall" or "scratched" after passing through the filter. At the same time, when the filter is blocked, the pressure drop at the filter will increase, and the flushing pressure on the template will be reduced.
The law of the existence of the hole blocking phenomenon
When some of the die holes are "frozen" and blocked, that is, the cross-sectional area of the die plate is reduced, the remaining die holes will have a fast discharge speed. As the extruder runs longer, some of the die holes will be blocked or even completely blocked. The particle shape often changes, tending to be long and thin, and a large number of large and small particles appear.
(1) After the extruder is started normally, the die hole blocking will reach a relatively stable level within a certain period of time. Without major adjustments, as the running time increases, some die holes will become blocked.
(2) Cooling down and replacing additives due to production needs will aggravate the blockage of the die holes in a short period of time. After heating up, some die holes will be flushed open.
In actual production, the template operation status should be analyzed in time according to the analysis results, and a solution should be established. For example, before switching to high melt index products, the template temperature and pelletizing water temperature should be increased in advance to improve the die hole blockage caused by various factors in the production stage of low melt index products, and then the temperature should be lowered and the production should be switched according to the conversion plan.
Product impact and solution measures
When the die hole is blocked, a large amount of fine particles will be produced, resulting in uneven product particle size and affecting product appearance. During the conversion and production of high melt index products, the template is not smooth and it is easy to cause the cutter, pad knife and even cause the pelletizer to stop due to high torque, which will affect production, long-term operation of the unit and device efficiency.
In view of the above situation, the following measures can be taken:
(1) Appropriately increase the temperature of the granulation cylinder and the template to enhance the flow performance of the material; before converting to high melt index products, the template and pelletizing water temperature can be appropriately increased to melt or improve the blocked die hole.
(2) Appropriately increasing the load is conducive to flushing the die hole, but the cylinder temperature and template temperature must be appropriately increased to ensure that the product is fully mixed. At the same time, if the cutter torque fluctuation has occurred, the load should be reduced in time and the speed should be increased to avoid high torque interlocking stop.
(3) Close the pelletizing throttle valve to increase the mixing degree of the pelletizer, especially during the production of high melt index products, it is necessary to ensure that the product is fully mixed.
(4) If a long shutdown occurs, the temperature of the processing section must be lowered, especially in the area with a high working temperature, so as to avoid excessive thermal degradation of the products in the processing section during the shutdown period, resulting in carbonization precipitation.
(5) The template should be thoroughly checked and cleaned before starting. When flushing the template, make sure that the template has been flushed before closing the trolley. During the start-up process, in order to prevent the pelletizing water from contacting the template and causing a sharp change in temperature, the template temperature can be appropriately increased in advance.
(6) The impurities in the raw materials and the melting points of all subsequent additives should be strictly controlled. At the same time, fluctuations in the amount of additives added will directly cause the die hole to be blocked.
In actual production, the die hole blockage varies depending on the changes in production conditions. Especially when producing products with different characteristics, it is necessary to grasp the main contradiction that causes the instability of the extruder unit and determine the best process plan based on the specific characteristics of the extruder pelletizer to improve product quality and achieve long-term operation of the extruder.
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