Crystal point, also known as "fish eye", is probably the most common problem in the production of polyethylene film, but also the most headache. Although the crystal point is very common, it is not clearly defined. Generally speaking, we call the granular defects protruding from the surface of the film as the crystal point problem. Although the crystal point problem is very common, it is not easy to solve. This is mainly because there are too many factors that can cause the crystal point problem. It is difficult to take measures to improve or solve the problem if the cause of crystal point cannot be determined. Therefore, what we need to know first is the classification and origin of crystal dots, and the methods to identify different types of crystal dots. And then find out the causes, naturally there are corresponding solutions. Generally speaking, the crystal point can be divided into three categories: foreign pollutants, crosslinking / oxidation and poor plasticization. The naked eye is not enough to distinguish the type of crystal point. A simple method is to carry out hot stage analysis under a polarizing microscope. More complex and sophisticated detection methods need professional testing instruments, such as frozen section microscope, micro infrared, EDX spectrum and so on. Because those professional testing equipment are not ordinary film manufacturers can equip, so here we only introduce a simple method based on the hot stage polarizing microscope.
Foreign pollutants
There are not a few crystal spots caused by foreign pollutants. After all, the environment of film production workshop is complex, there are many potential external pollution sources. For example, fibers dropped from work clothes, dust / dirt on the surface of resin bags, and even the bodies of insects / mosquitoes. These potential pollutants may enter the mixer, followed by the hopper and extruder. When they come out of the die, they naturally become the crystal points on the film.
The following three photos are microscope photos of the sum of room temperature, heated to 130 ℃ by hot stage and re cooled to room temperature. Before heating, fibrous material can be seen in the crystal point; after the main body of the film is melted by the hot stage, the fiber becomes very obvious; after cooling to room temperature, the fiber is still obvious. It can be concluded that the crystal point is actually caused by fiber pollution. However, not all the foreign contaminated crystals can be easily seen by hot stage microscope. If the contaminants are other types of polymer resins (such as nylon, EVOH), the hot stage polarizing microscope is only used as an auxiliary means. By continuously increasing the temperature of the hot platform, the melting point of the contaminated resin can be roughly observed, and then it is necessary to check the potential pollutant resin in the actual production process to make a better judgment.
In any case, once it is determined that foreign pollutants are the main problem of crystal point, corresponding measures can be taken. For example:
Improve the cleanliness of workshop;
Improve the material storage and transportation process, keep the resin packaging bag clean;
Strictly control the production order, avoid cross contamination between different types of raw materials.
Crosslinking / oxidation
Crosslinking is the most common type of crystal point. After all, the production of polyethylene resin and film will go through the melting extrusion process. At high temperature, polyethylene molecules will cross-linked, and then form a large molecular weight of the hard plasticizing part, which is shown in the film is the crosslinking crystal point. The crystal point of mild crosslinking is generally colorless. However, when the crosslinking degree is very high, the polymer will even degrade seriously, thus forming the so-called oxidation crystal point. This kind of crystal point is generally yellowish brown or even dark brown.
Hot stage polarizing microscope is still a simple and easy method to determine the crosslinking or oxidation point. Highly crosslinked or oxidised crystals are difficult to melt. Therefore, when the main body of the film has been completely melted by the hot stage, the obvious crosslinking crystal points and even some brown degradation products can be seen under the polarizing microscope. The following figure shows the oxidation point which is not melted under the microscope at 145 ℃ in the hot stage.

Different from the highly crosslinked or oxidized crystal point, the slightly crosslinked crystal point will disappear with the increase of temperature under the microscope. It seems to melt with the main body of the film (or disappear at a slightly higher melting point of the main body). But when the temperature drops, it still appears. The following figure shows the slightly crosslinked crystal point that can still be seen after cooling.

Solutions to different crystal points
We introduced the main types of crystal point, identification methods, causes and Countermeasures in detail. It can be summarized as follows:
Identification of different types of crystal dots by hot stage polarizing microscope
Under the microscope, we can see the crystal point of foreign pollutants;
After the main body of the film is heated and melted, clear crystal point or degradation product - highly crosslinked / oxidized crystal point can still be seen;
When the main body of the film was heated and melted, the crystal point disappeared, but the crystal point slightly cross-linked could be seen again after cooling;
When the main body of the film is heated and melted, the crystal point disappears, but the crystal point does not recover when the temperature drops.
How to deal with the crystal point of foreign pollutants
Improve the cleanliness of workshop;
Improve the material storage and transportation process, keep the resin packaging bag clean;
Strictly control the production order, avoid cross contamination between different types of raw materials.
How to deal with crosslinking / oxidation point
Change the raw materials with less crystal point;
Reduce the processing temperature;
Add additional antioxidants;
Optimize the equipment and replace it with screw with excellent design.
Countermeasures for poor plasticizing crystal point
Avoid mixing granular materials with powder materials;
Pay attention to the pumping process of raw material particles to avoid raw material powder due to friction;
Avoid mixing raw materials with very different melting points;
If the formula requires, the raw materials with big difference in melting point must be mixed; the temperature of the first zone and the second zone of the screw need to be reduced to avoid the premature melting of the low melting point raw materials; at the same time, the filter screen can be densified to increase the shear of the metering section.
Dongguan Beite packaging materials Co., Ltd. - located in Dongguan City, the world's manufacturing city, is a subsidiary of Beite packaging (China) Co., Ltd., with a plant area of 6000 square meters. It is a flexible packaging enterprise integrating R & D, production and sales. It has a complete set of domestic advanced production equipment, a scientific and humanized management system and a group of experienced technical personnel For backing, to provide customers with quality products and first-class service. The company produces composite bags, roll film, fruit bags, aluminum foil bags, suction mouth bags, self-supporting bags, zipper bags, anti-static bags, special-shaped bags, etc., which are suitable for the internal and external packaging of food, daily chemicals, electronics, toys, medicine, cosmetics, industrial products and other industries. We uphold the integrity of cooperation, good quality, good service business philosophy, won the industry's approval, products sold to all over the world; welcome friends from all walks of life to inquire, visit our factory to investigate guidance and business negotiations.