Black spots and impurities are characterized by small PFA particles, dark brown, and generally non-reflective,PFA GranulesLarge impurities are layered, brittle, fragile, broken porous, its distribution has two major characteristics:

1, some of the overall irregular scattered distribution, some of the local irregular scattered distribution, sometimes only occasionally in a local area;

2. Such black spots and impurities sometimes only appear on the surface of PFA particles, and sometimes they exist regardless of the depth of the surface. However, the internal black spots closer to the surface are lighter in color than the black spots on the surface, and the deeper black spots cannot be seen at all. It is interesting to link these two points, it will be found that the impurities that only appear in a local area must be the black spots on the surface, and the distribution of black spots must be the overall irregular and scattered distribution regardless of the depth of the surface and inside. This is due to the fact that the impurities present in the interior are necessarily present before the granulation, and the impurities present only during the granulation are necessarily distributed only on the surface. In this way, impurities are divided into two categories before and during granulation:

Causes of black spots and impurities before 1. granulation:

1. When PFA particles are processed, due to various reasons, the foreign matter is not clean and the PFA particles appear black spots;

2. Black spots caused by impure granulation;

3.PFA particles mixed with color masterbatch or with spots of crushed pieces, scrap;

4. The material is not pure, and the low melting point material is mixed with high melting point material particles;

5. Impurities may be mixed in the process of packaging, transportation and storage. It is obviously characterized by PFA particles. After opening the package, careful observation shows that there are foreign bodies and impurities on the surface of PFA particles;

6. Except for 5 and 11, which will not directly pollute the hopper material, other parts may pollute PFA particles in the feeding process, resulting in black spots and impurities of PFA particles. Pollution sources include: dust in the air, material-like suspended solids, different material scraps, different material particles, powdered color masterbatch, dyeing agent, etc. In order to prevent black spots caused by foreign matter and impurities, management must be strengthened to control all links (including reuse process) from PFA particles entering the factory to feeding. When changing materials, the parts where the original materials or chips may stay must be carefully cleaned, especially the material preparation box, hopper, funnel, bottom clamp fastening place, etc. During normal production, special attention should be paid to the cleaning of the feeding source-the preparation box. After the production is stopped, the feeding port of the feeding pipe, which is exposed to the external environment, should be sealed to prevent pollution, so as to achieve interlocking and closed management.

7.PFA particles are carbonized. The black spots are generally larger in volume. The appearance diameter of large black spots can reach 1-2mm. Most "black spots" are thicker, and one or two layers are thinner. This situation is the long-term accumulation of PFA particles or local high heat, decomposition, coking, carbonization into blocks. It is formed after the flow is broken by shear at the screw or nozzle.

PFA particles carbonization reasons are:

1) the melt temperature is too high, the material temperature is too high will cause overheating decomposition, the formation of carbide, especially for some heat sensitive materials temperature range is narrow, must control the barrel taste part temperature can not be too high.

2) Coking of accumulated materials: If molten PFA particles stay somewhere for too long, coking accumulated materials will occur, causing black spots, and areas that may cause material retention include the connection between the shooting head and the screw barrel, the screw barrel wall, the melting glue ring, the contact part between the nozzle and the gate, the corner of the hot runner, the dead angle of the main runner, etc.

3) The gap between the barrel is too large, and the gap between the barrel and the screw is too large, which will cause the material to stay in the barrel, and the retained material will be decomposed by long-term overheating, resulting in black spots.

8. Additives drop, decompose and change color. Additives include antistatic agents, purple/infrared absorbers and general dyeing agents. Their properties are generally more active than PFA particles. Under the action of shear force at processing temperature, when PFA particles are not decomposed, they have decomposed and become dark, yellowish brown and even black. Black spots and impurities are formed during granulation.

Black spots and impurities caused by external factors in 2. production are very common and very stubborn after they appear.

1, PFA material quality is not good, parting surface or granulation surface or touch through surface will drop iron powder, resulting in black spots.

2. The thimble is rough and easy to burn, and iron powder is dropped, causing black spots.

3. The slider grinds iron foam powder, causing black spots.

4. Leakage rust or other stains inside the slider. Rust and stains are flown out by the slider and fall on PFA particles to form black spots.

Black spot identification:

If black spots appear on the surface of the whole PFA particle and there are black spots in the deep part, they should belong to the black spots before granulation; if black spots only appear on the surface, they should belong to the black spots in granulation. If it is only distributed in a specific area on the surface at the same time, it is undoubtedly a black spot in granulation. If the black spot is large (generally 0.5~1mm), it should belong to PFA particle carbonization black spot; If the black spot is dark, brittle and bulky and porous at the same time, it can be determined as carbonization black spot: if the black spot density is extremely large, check that PFA particles have no obvious impurities, and generally belong to the original clean after just changed, otherwise, the cause of the material shall be checked.

3. solutions:

1. For granulation impurities, black spots are formed due to foreign matter mixed into the material. The cleaning of all links of production, packaging, storage, transportation, newspaper opening and mixing to the barrel must be strictly controlled.

2. For carbonized black spots, the processing temperature should be strictly controlled. In actual production, the processing temperature and the actual processing temperature are different. For the same equipment, different back pressure, different cycle time, different one-time injection, different thermocouple insertion position, will cause the same processing temperature material carbonization, degradation trend is different. Specifically, when the temperature is the same, the back pressure is small, the cycle is short, and the one-time injection is large. When the thermocouple is inserted into the upper part of the screw barrel, the melt drop/decomposition trend is weak, and it is not easy to produce carbonized black spots.

To prevent carbonized black spots, it is necessary to strictly prevent material accumulation, eliminate dead corners in the screw cylinder, nozzle and flow channel, make smooth transition at the turning, and eliminate the area where material may stay.

3. For additives degradation deposition or existing carbonized material deposition on the thread surface and the black spots on the wall of the screw barrel. Generally, with the production, these black spots will be discharged from their original attachment surface into the melt under various strong shear effects in the screw barrel. This "discharge" process is the process of black spots and impurities in the PFA particles. For example, in the production of material and color change, this "discharge" of impurities is inevitable, and we should try our best to shorten this process. It is "cleaning". The following are two cleaning methods:

1) Clean the screw for the empty melt glue (air shot after melting glue).

2) Melt glue into the injection table, spray glue into the air, then melt glue into the injection table, and spray glue into the air. This is repeated. Practice has proved that 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 also long. This effect is good, clean up the foreign body quickly. Note: There is a principle when the back pressure is not fixed-as high as possible, so high that the screw barrel will not heat up automatically.

3) For the black spots in granulation, the black spots in granulation must be attached to the surface of the cavity first, and then fixed on the surface of PFA particles by melt, so this black spot removal method and make it not in the cavity. To determine the poor material PFA, but also to determine the cause, determine the location. All relative movement of the cavity, mold core including slider, neutron, thimble/block, are likely to burn, determine the burn, to repair the damaged parts, in addition to reduce the sliding parts of the friction, add lubricating oil. In addition, the relative movement speed of the sliding part should be reduced, such as pushing forward/backward, opening and closing the mold, the slider should move as slowly as possible, rust and other stains between the slider and the template should be removed, rust and other stains should be removed, and the water nozzle should be tied tightly. For oil or water splashed on the smooth granulation surface of the mold cavity, black spots should be wiped frequently to prevent the appearance of oil and water on the granulation surface.

This article comes from:www.tflpfa.com

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