How are black spots and impurities formed in plastic products?

Analysis of causes and countermeasures of black spots and impurities in center console injection molding. Defects of black spots and impurities are the most important factors that cause scrap rates in normal production processes. They mainly affect the appearance of products and lead to scrapping. Impurities and most black spots are foreign substances and have nothing to do with the raw materials themselves, while a small number of black spots and impurities are caused by the raw materials themselves. The characteristics of black spots and impurities are that the particles are small, dark brown, and generally do not reflect light. When the particles are large, the impurities are layered, brittle, and fragile. After breaking, they are porous. There are two major characteristics of their distribution:

1. Some are scattered and irregularly distributed as a whole, some are scattered and irregularly distributed locally, and sometimes they only appear occasionally in a certain local area;

2. This type of black spots and impurities sometimes only appear on the surface of the product, and sometimes they appear regardless of the depth of the surface or inside. However, the internal black spots closer to the surface are lighter in color than the surface black spots, and the deeper black spots cannot be seen at all. Interestingly, if we connect these two points, we will find that impurities that only appear in a certain local area must be black spots on the surface, while black spots distributed inside and outside must be scattered and irregular as a whole. This is because the impurities that exist inside must have existed before molding, while impurities that only appear during molding must only be distributed on the surface. In this way, impurities are divided into two categories: before molding and during molding: 1. Causes of black spots and impurities before molding: 1. When processing raw materials, due to various reasons, foreign matter is not clean and black spots appear on the raw materials; 2. Impure granulation causes black spots;

3. The raw materials are mixed with masterbatches or spotted crushed blocks and scraps;

4. The material is impure, and high-melting-point particles are mixed with low-melting-point materials;

5. Impurities may be mixed in the packaging, transportation, and storage process. Its obvious feature is that after the raw materials are unpacked, careful observation shows that there are foreign matter and impurities on the surface of the particles;

6. Impurities and foreign matter in the feeding process. The following figure is a schematic diagram of the suction or feeder and the hopper; except for 5 and 11, which will not directly pollute the hopper material, other parts may pollute the raw materials during the feeding process, causing black spots and impurities in the finished product. Pollution sources include: dust in the air, material-like suspended matter, foreign material scraps, foreign material particles, powdered masterbatches, dyes, etc. In order to prevent foreign matter and impurities from causing black spots, management must be strengthened to control all links from the entry of raw materials into the factory to the addition of materials (including the recycling process). When changing materials, the parts where the original materials or chips may be retained must be carefully cleaned, especially the preparation box, hopper, funnel, and the fastening of the hem clamp. During normal production, special attention should be paid to the cleaning of the feeding source – the preparation box. After stopping production, the feeding system should be exposed to the external environment – the feeding port of the feeding pipe to prevent pollution, so as to achieve interlocking and closed management.

7 Carbonization of raw materials. This kind of black spot is generally large in size. The appearance diameter of large black spots can reach 1-2mm. Most “black spots” are thicker, and there are also one or two layers of thinner ones. This situation is that the raw materials are accumulated for a long time or are locally subjected to high heat, decomposing, coking, and carbonizing into blocks. It is formed after the flow is sheared and broken at the screw or nozzle. The reasons for raw material carbonization are: 1. The melt temperature is too high. Too high material temperature will cause overheating and decomposition, forming carbides, especially for some heat-sensitive materials with a narrow temperature range. The temperature of the barrel must be controlled not to be too high. 2. Coking of accumulated materials: If the molten plastic stays in a certain place for too long, coking and accumulation of materials will occur, causing black spots. The areas that may cause material retention include the connection between the nozzle and the barrel, the barrel wall, the melt ring, the contact between the nozzle and the gate, the hot runner bend, and the dead corner of the main runner. 3. The gap between the barrel and the screw is too large, which will cause the material to be retained in the barrel, and the retained material will be decomposed after long-term overheating, resulting in black spots.

8. Additives degrade and discolor due to decomposition. Additives include antistatic agents, ultraviolet/infrared absorbers and general dyes. Their properties are generally more active than raw materials. Under the action of processing temperature and shear force, they have decomposed when the raw materials are not decomposed, turning into dark, yellow-brown or even black, forming black spots and impurities during molding.
Black spots and impurities caused by external reasons in production are very common, 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 penetration surface, causing black spots. 2. The ejector is rough and easy to burn, and iron powder will fall off, causing black spots. 3. The slider grinds iron powder, causing black spots. 4. The slider leaks water, rusts or other stains, and the rust and stains are thrown out by the slider and fall on the product to form black spots. Identification of black spots: If black spots appear on the entire surface of the product, and there are black spots deep in the product, they should be black spots before molding; if black spots only appear on the surface, they should be black spots during molding. If they are only distributed in specific areas of the surface, they are undoubtedly black spots in the molding process; if the black spots are large (generally 0.5~1mm), they should be carbonized black spots of the raw materials; if the black spots are dark, brittle, fluffy and porous at the same time, they can be determined as carbonized black spots: if the density of black spots is particularly large, and there are no obvious impurities in the raw materials, it should generally be a phenomenon that the original materials have not been cleaned up just after the material change, otherwise the cause of the material should be checked.
9. Solutions: 1 For molding impurities, black spots are formed due to foreign matter mixed into the material. The cleaning of each link from production, packaging, storage, transportation, opening, mixing to the barrel must be strictly controlled. 2 For carbonized black spots, the processing temperature should be strictly controlled. In actual production, the indicated processing temperature and the actual processing temperature are different. For the same equipment, different back pressures, different cycle times, different one-time injection volumes, and different thermocouple insertion positions will cause different carbonization and degradation trends of the raw materials at the same processing temperature. Specifically, it means that when the temperature is the same, the back pressure is small, the cycle is short, the one-time injection volume is large, and when the thermocouple is inserted in the upper part of the screw barrel, the melt has a weak tendency to fall/decompose, and it is not easy to produce carbonized black spots. To prevent carbonized black spots, it is necessary to strictly prevent material accumulation, eliminate the dead corners in the screw barrel, nozzle, and flow channel, make the turning point have a smooth transition, and eliminate the area where the material may be retained. 3 For the black spots on the thread surface and the wall of the screw barrel where the additives are degraded and deposited or the carbonized materials are deposited. Generally, as the production proceeds, these black spots will break away from their original attachment surface under various strong shearing effects in the screw barrel and enter the melt and be discharged. This “discharge” process is the process in which black spots and impurities appear in the product. For example, in the production of material transfer and color change, this “discharge” process of impurities is inevitable, and we must try our best to shorten this process. It is “cleaning”. The following are two cleaning methods: 1) Cleaning the screw with air melt (air injection after melt). 2) Melt glue on the injection table, spray glue into the air, then melt glue on the injection table, spray glue into the air, and repeat this process. Practice has proved that back pressure plays a big role in cleaning, and the molten material and the screw are strongly sheared. The melting speed is slow, so the melting time is also long. This is a good effect and foreign matter can be cleaned quickly. Note: There is a principle for back pressure when it is not fixed – as high as possible, until the screw barrel does not heat up automatically. 3) For black spots in molding, the black spots in molding must first attach to the surface of the cavity, and then be fixed to the surface of the product by the molten material, so this method of removing black spots will not keep it in the cavity. For molds with poor materials, the cause and location must be determined first. All cavities with relative movement, mold cores including sliders, neutrons, ejector pins/blocks, may be burned. After confirming the burn, the damaged parts should be repaired, and the friction of the sliding parts should be reduced and lubricating oil should be added. In addition, the relative movement speed of the sliding parts should be reduced, such as pushing in/out, opening and closing the mold, and the slider should move as slowly as possible. If there is rust and other stains between the slider and the template, the slider should be removed, the rust and other stains should be cleaned, and the nozzle should be tied tightly. If oil or water splashes onto the smooth molding surface of the mold cavity, black spots should be caused. The oil and water spots should be wiped frequently to prevent them from appearing on the molding surface.