Carbonization of plastic products can have serious adverse effects on the quality and production cost of the products.
For plastics manufacturers, it is extremely important to effectively prevent and solve the carbonization of plastic products.
So how is the carbonization of plastic products formed? How can we prevent this problem?
Today, Yankang will give a detailed introduction to the reasons and solutions for the formation of carbonization of plastic products.
1.Reasons and processes of carbonization of plastic products
In the actual production of plastic products, the feed port is generally isolated before shutdown, the plastic melt in the extruder and the machine head is removed, the temperature of each zone is turned off, and the power is turned off.
Due to the strong adhesion of the plastic melt to the metal, it is impossible to completely eliminate it every time it is stopped. Finally, there will always be a thin layer of plastic melt firmly adhered to the barrel and machine of the extruder. The inner wall of the head and the screw. After the machine is shut down, the equipment naturally cools down and stays in the high temperature state for a long time during the next heating process, and obvious thermal deterioration occurs, gradually turning yellow and zooming into carbide.
According to the conventional shutdown method, no effective sealing measures are taken at the die mouth and the feed port of the machine head, which causes the air to enter the machine to generate oxidation, which causes the residual plastic heat deterioration in the machine to increase, which provides favorable conditions for carbonization.
Since the equipment is a metal structure, the thermal expansion rate of the plastic is greatly different from that of the plastic. The adhesion of the carbonized plastic to the metal is reduced, and it is easy to fall off from the inner wall of the barrel of the equipment, the inner wall of the machine head and the screw, and is mixed into the plastic melt, resulting in There are many black spots on the inner and outer walls of the product, which are prone to many quality problems such as hole penetration or leakage.
2.Solutions for carbonization of plastic products
2.1 In-machine cleaning
After a long period of downtime, heating and then turning on, plastic parts are prone to many black and white spots, which can not meet the quality requirements of the products. These black spots are the carbides after the thermal deterioration of the residual plastic in the machine.
It is usually used to clean the machine by continuously extruding with plastic to remove the plastic melt with black spots, and the washing time is up to 3-5 hours. If the original ultra-high molecular weight plastic is carbonized and the original color product is produced, the difficulty of washing the machine will be greatly increased, the washing time will be longer, and the washing machine cost is higher.
To quickly clean the carbides in the machine, it is necessary to use a relatively harder plastic or its return material and white mineral oil to clean, the temperature is appropriately increased by 10 ~ 15 °C. At this time, it is necessary to consider the plasticizing ability of the small extruder for the plastic.
For large and medium-sized extruders that can plasticize ultra-high molecular weight plastics, they can be cleaned with ultra-high molecular weight plastics or their recycled materials plus white mineral oil.
Because the ultra-high molecular weight plastic is more rigid, and when the temperature is higher, the cleaning object is softened, and thus becomes easy to fall off, which is favorable for the carbide to be taken out by the plastic melt.
During the cleaning process, the speed of the extruder is repeatedly switched between low speed and high speed.
If there is a margin in the motor load, the extruder can be repeatedly stopped and restarted multiple times during operation to further improve the cleaning effect.
When the carbides (black spots) in the melt are significantly reduced, the usual plastics are replaced and cleaned until they are properly adjusted to a suitable temperature.
2.2 Disassembly cleaning
It is easier to clean the carbide in the extruder by the internal cleaning method, and it is difficult to clean the carbide in the storage head of the hollow blow molding equipment.
Because the structure of the extruder is simple, the structure of the hollow blow molding equipment storage head is complicated, and the equipment is used for too long, which is more difficult to clean.
In this case, it is imperative to disassemble the cleaning. After the machine head is heated to a sufficient temperature, turn off the power and remove all the components of the machine head. Use a piece of copper to clean the plastic melt inside the machine head, use a metal brush to remove the charred carbide, and then polish the assembly with a sandpaper or flower impeller to assemble.
Although the disassembly and cleaning effect is very good, it takes a long time and is laborious, and it is generally not easy to use this disassembly cleaning method.
3.Precautions for carbonization of plastic products
3.1 Temperature and time control
The processing temperature of plastic products is determined according to the plasticization requirements of raw materials and products, and cannot be too high or too low.
Too low a temperature will result in an excessive load, and defects such as water marks and weld lines on the appearance of the product may occur. If the temperature is too low, the fluidity of the melt will be somewhat inferior, and the glossiness of the surface of the product and the fullness of the corners will also be greatly affected.
If the temperature is too high, and sometimes temporarily shut down for two hours, or even several hours, it is easy to produce melt carbonization in the machine.
In this case, the constant temperature is between 100 and 120 ° C, and it is necessary to warm up to the required temperature before starting the machine, so as to prevent carbonization caused by a sudden increase in temperature or a long holding time.
3.2 heating and boot operation control
A one-step heating method is usually used. That is, after the temperature is set, the temperature is continuously increased to the set temperature, the size of the device is divided, and the temperature is maintained for 1 to 3 hours before being turned on. This method easily causes carbonization of the residual plastic in the machine.
If the step heating method is adopted, the temperature is increased in a time period, and the effect is much better.
For example, the initial set temperature is 100 °C, the temperature is set to 130 °C after constant temperature, and the temperature is set to 170 °C after constant temperature, and the temperature is gradually increased according to the time period, and the temperature is raised to the required temperature. Medium and small equipment can be turned on when the required temperature is reached. Of course, it is necessary to prepare relevant preparations in advance. After the large equipment reaches the required temperature, it needs to be thermostated for 1 to 2 hours.
At the beginning of the machine, it must be noted that the speed of the extruder is slowly accelerated from 1 to 5 rpm, and the feed port is slowly opened. Keep an eye on the current value displayed on the extrusion ammeter. If the load is too large, increase the constant temperature time appropriately, and increase the temperature step by step. It can be turned on as early as possible when starting the machine to prevent the residual plastic in the machine from colliding with the high temperature and carbonizing in the long waiting time.
3.3 shutdown and temperature control operation control
Usually the shutdown operation is to close the inlet of the extruder before stopping, and eliminate the plastic melt in the machine. As the current of the extruder decreases, the plastic melt in the machine is gradually eliminated, and then the temperature is stopped.
Due to the high temperature inside the machine and the unsealed die, a small amount of plastic adhered to the inner wall of the machine is most likely to be thermally degraded during shutdown and heating, plus oxygen entering the air from the die. The thermal deterioration is exacerbated into carbides.
If you stop using a new method of cooling and closing, the effect is very good. Since the carrier-plastics of recycled materials and masterbatches are less resistant to oxygen than the new pure plastics, no matter whether the recycled materials or the ingredients containing the masterbatch are used before the shutdown, they should all be excluded. After the plastic melt in the machine, add a new pure plastic low-speed washing machine. While washing the machine, the temperature is lowered by 25 to 35 ° C. As the temperature is lowered, the extruded plastic melt gradually eliminates the latent heat in the machine. However, special attention should be paid to observing the current value of the extruder to avoid accidents in the drive motor overload. When the actual temperature is lower than the normal use temperature of about 25 °C, according to the actual situation of the melt flow, close the feed port, dismantle the plastic melt inside the machine, close the die, prevent air from entering, and then stop the temperature. It is also possible to stop the washing machine when the actual temperature is lower than the normal use temperature by 25 to 40 ° C in the above process, so that the semi-flowing plastic melt fills the inner space of the machine to be closed. In this case, pay special attention to the next heating and holding time should be properly extended to ensure that the melt in the machine is warm enough before starting.
Inspection and selection of plastic raw materials
In the production process of plastic granulation manufacturers, plastic carbonization also occurs. Sometimes, when used, it is found that there are more black spots in the plastic particles, especially for the quality of the original color products, which requires strengthening the inspection of the raw materials of the plastic materials. If necessary, select plastic particles with black spots and try not to reuse them as much as possible to avoid the quality of the whole batch.
To avoid the harm of carbonization to plastic products, we must not only understand the nature of carbonation, but also be good at using effective solutions. The key is to prevent and minimize losses. Every step in the solution and prevention of carbonization of plastic products requires strict control.
The above is the knowledge about the carbonization of plastic products shared by Yankang. Learn more about blow molding machines, you can click on the YanKang blog to view.The blog link is:https://www.yankangmachine.com/category/blog/.