Black spots and impurities are characterized by small and dark brown particles, which are generally not reflective. When the particles are large, the impurities are layered, brittle, fragile and porous after crushing, and their distribution has two major characteristics:
1. Some of them are randomly distributed in the whole, some are randomly distributed in the local, and sometimes only occasionally appear in a certain local area;
2. Sometimes, such black spots and impurities only appear on the surface of products, and sometimes they exist regardless of the depth of the surface and the inside. However, the inner black spots closer to the surface are lighter in color than the surface black spots, and the deeper black spots are invisible at all.
Interestingly, by linking these two points, it can be found that the impurities that only appear in a certain local area must be black spots on the surface, and the distribution of black spots must be overall irregular and scattered regardless of the depth of the surface and the interior.
This is because the impurities existing inside must exist beforemolding, and the impurities appearing only during molding must only be distributed on the surface. In this way, impurities can be divided into two categories: before molding and during molding:
I. Causes of black spots and impurities before molding:
1. When raw materials are processed, due to various reasons, foreign bodies are unclean and black spots appear in raw materials;
2. Black spots caused by impure granulation;
3. The raw materials are mixed with masterbatch or speckled crushed blocks and scraps;
4. The material is impure, and the material with low melting point is mixed with the material with high melting point;
5. Impurities may be mixed in the process of packaging, transportation and storage. Its obvious feature is that after the raw material is unpacked, it can be seen that there are foreign bodies and impurities on the surface of the material particles;
6. Except 5 and 11, they will not directly pollute the hopper materials, and other parts may pollute the raw materials during the feeding process, resulting in black spots and impurities of products. Pollution sources include: dust in the air, suspended solids, foreign material scraps, foreign material particles, powdered masterbatch, dye, etc. In order to put an end to the black spots caused by foreign matters, it is necessary to strengthen management and control all links from raw materials entering to feeding (including the recycling process).
During refueling, the parts where the original materials or scraps may stay must be carefully cleaned, especially the material preparation box, hopper, funnel, and the fastening part of the lower hem clip. During normal production, special attention should be paid to the cleanliness of the feeding source-material preparation box. After the production is stopped, the feeding port of the feeding pipe, which is exposed to the external environment of the feeding system, should be sealed to prevent pollution, so as to achieve interlocking and closed management.
7. Carbonization of raw materials, such black spots are generally large in volume, and the appearance diameter of large black spots can reach 1-2mm. Most of the "black spots" are thicker, and one or two layers are thinner. In this case, the raw materials are accumulated for a long time or locally subjected to high heat, and are decomposed, coked and carbonized into blocks. It is formed after the flow at the screw or nozzle is sheared and broken.
The reasons for carbonization of raw materials are:
1) If the melt temperature is too high and the material temperature is too high, it will cause overheating decomposition and carbide formation. Especially for some heat-sensitive materials, the temperature range is narrow, so it is necessary to control the temperature of the taste part of the barrel not to be too high.
2) Coking of accumulated materials: If the molten plastic stays in a certain place for too long, coking accumulated materials will occur, causing black spots, which may cause material retention areas, such as the joint between the nozzle and the screw barrel, the screw barrel wall, the molten rubber ring, the contact part between the nozzle and the gate, the corner of the hot runner, the dead corner of the main runner, etc.
3) The gap between the barrel and the screw is too large, which will make the material stay in the barrel, and the stranded material will be decomposed by overheating for a long time, resulting in black spots.
8. Additives degrade, decompose and change color. Additives include antistatic agents, ultraviolet/infrared absorbers and general dyes, whose properties are generally more active than raw materials. Under the action of shearing force at processing temperature, when raw materials are not decomposed, they have decomposed and become dark, yellow-brown or even black, and black spots and impurities are formed during molding.
Second, black spots and impurities caused by external reasons are very common in production, and they are very stubborn after they appear.
1. The mold material is not good, and iron powder will fall off the parting surface, molding surface or piercing surface, resulting in black spots.
2. The thimble is rough and easy to burn, and iron powder is dropped, resulting in black spots.
3. The slider grinds iron foam powder, causing black spots.
4. There is water leakage, rust or other stains in the slider. Rust and stains will fly out by the slider and form black spots when they fall on products.
Black spot discrimination:
If black spots appear on the whole surface of the product and there are black spots in the deep part of the product, they should belong to the black spots before molding; If black spots only appear on the surface, they should be black spots in molding. If it is only distributed in a specific area of the surface at the same time, it is undoubtedly a black spot in molding; If the black spot is large (generally 0.5~1mm), it should belong to the raw material carbonization black spot;
If the black spots are dark, brittle, bulky and porous at the same time, they can be judged as carbonized black spots; if the density of black spots is extremely large, the inspection of raw materials is free of obvious impurities, which generally belongs to the phenomenon that the original materials were cleaned at the end just after refueling; otherwise, the reasons for the materials should be checked.
III. Solutions:
1. For the forming impurities, black spots are formed due to the mixing of foreign materials into the material, so it is necessary to strictly control the cleanliness of all links of production, packaging, storage, transportation, reporting and mixing to the charging barrel.
2. For carbonization black spots, the processing temperature should be strictly controlled. In actual production, there is a difference between the processing temperature and the actual processing temperature. For the same equipment, different back pressures, different cycle times, different one-time injection quantities and different thermocouple insertion positions will cause different carbonization and degradation trends at the same processing temperature. Specifically, when the temperature is the same, the back pressure is small, the cycle is short, and the one-time injection amount is large. When the thermocouple is inserted in the upper part of the screw barrel, the melting material has a weak tendency of degradation/decomposition, and it is difficult to produce carbonization black spots.
To prevent carbonization black spots, it is necessary to guard against accumulation of materials, eliminate dead corners in screw barrel, nozzle and runner, make it have a gentle transition at the corner, and eliminate areas where materials may stay.
3. For the black spots deposited by degradation of additives or existing carbonized materials deposited on the thread surface and screw barrel wall. Generally, with the production going on, these black spots will break away from their original attachment surface and enter the melt under various strong shearing actions in the screw barrel and be discharged. This "discharge" process is the process that black spots and impurities appear in products. For example, in the production of changing materials and colors, this process of "discharging" impurities is inevitable, so we should try our best to shorten this process. Is "cleaning". There are two cleaning methods:
1) Clean the screw for empty glue melting (empty shot after glue melting).
2) Melting glue at the entrance table, spraying glue in the air, and then melting glue at the entrance table and spraying glue in the air, which are repeated. Practice proves: Back pressure plays a great role in cleaning, and the melt and screw shear strongly. The melting speed is slow, so the melting time is long. This has a good effect and can clean up foreign bodies quickly. Note: Back pressure has a principle when there is no time-it should be as high as possible, until the screw barrel just won't automatically heat up.
3) For the black spots in forming, the black spots in forming must be attached to the cavity surface first, and then wrapped and fixed on the product surface by the melt, so this black spot removal method can prevent it from being in the cavity. To determine the mold with bad material, we must first determine the cause and position.
All cavities and cores with relative movement, including slide block, neutron and ejector pin/block, are likely to burn. After the burn is confirmed, the damaged parts should be repaired. In addition, the friction of sliding parts should be reduced and lubricating oil should be added.
In addition, to reduce the relative movement speed of sliding parts, such as jacking/retreating, opening and closing the die, the slider should move slowly as far as possible, and rust and other stains between the slider and the die plate should be removed, the rust and other stains should be removed, and the water nozzle should be tied tightly. As for the black spots caused by oil or water splashing on the smooth molding surface of the mold cavity, the places where oil and water are easy to come out should be wiped frequently to prevent them from appearing on the molding surface.
