Causes and countermeasures of black spots and impurities in plastic molding

1. Some of the holistic irregular distribution, some are local random scattered distribution, and sometimes only occasionally appear in a certain local area;

2. Such black spots and impurities sometimes appear only on the surface of the product, sometimes regardless of the depth of the surface, but the internal black spots near the surface are lighter than the black spots on the surface, and the deeper black spots are seen at all. Not enough. It is interesting to associate these two points and find that the impurities appearing only in a certain local area must be the black points of the surface layer, and the distribution of black spots, regardless of the depth of the surface, must be a random distribution of the whole. This is because the impurities present inside are necessarily present before the molding, and the impurities which are only formed during the molding are necessarily distributed only on the surface. In this way, impurities are divided into two categories before and during molding:

1. Black spots and impurities before molding:

1. When processing raw materials, for various reasons, foreign matter is not clean and black spots appear in the raw materials;

2. Impure granulation causes black spots;

3. The raw materials are mixed into the color masterbatch or the pulverized block with spots and the chips;

4. The material is impure, and the high melting point material is mixed into the low melting point material;

5. In the process of packaging, transportation and storage, impurities may be mixed in. The obvious feature is that after the raw materials are opened, carefully observed, it can be seen that there are foreign matter and impurities on the surface of the material particles;

6. Except for 5 and 11, it will not directly pollute the material of the hopper. Other parts may contaminate the raw materials in the process of feeding, resulting in black spots and impurities. Sources of pollution include: dust in the air, suspended solids in the material, crumbs in the foreign material, particles in the foreign material, powdered color masterbatch, dyeing agent, and the like. In order to prevent the black spots caused by foreign matter impurities, it is necessary to strengthen management and control all aspects from the raw materials entering the factory to the feeding (including the recycling process). When refueling, it is necessary to carefully clean the parts that may be retained by the original materials or scraps, especially the material storage box, the hopper, the funnel, and the hem of the hem. In normal production, special attention should be paid to the source of the feed – the cleaning of the preparation tank. After the production is stopped, the feeding system should be sealed to the external environment exposed place - the loading port of the feeding pipe to prevent pollution, so as to achieve interlocking and closed management.

7. Carbonization of raw materials, such black spots are generally large in size, and the diameter of large black dots can reach 1-2 mm. Most "black spots" are thicker, and there is also a thin one or two layers. In this case, the raw material is accumulated for a long time or locally subjected to high heat, decomposed, coked, and carbonized into a block. The flow is formed by shearing and breaking at a screw or a nozzle.

The reasons for raw material carbonization are:

1) The melt temperature is too high, and the material temperature is too high, which will cause superheat decomposition and formation of carbides. Especially for some heat sensitive materials, the temperature range is narrow, and the temperature of the barrel taste must be controlled to be too high.

2) Accumulation of accumulated material: If the molten plastic stays somewhere for too long, coking material will accumulate, causing black spots, areas that may cause material retention, joints between the head and the barrel, the wall of the barrel, and the rubber ring , the contact part of the nozzle and the gate, the hot runner turn, the dead corner of the main road, and the like.

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 to generate black spots.

8. Auxiliary drop, decomposition discoloration, additives include antistatic agent, violet/infrared absorbing agent and general dyeing agent, its properties are generally more active than raw materials, under the action of processing temperature shear force, when the raw materials are not decomposed, They have been decomposed into dark, yellowish brown and even black, forming black spots and impurities during molding.

Second, black spots and impurities generated by external factors in production are very common, and they are very stubborn after emergence.

1, the mold material is not good, the parting surface or the molding surface or the wearing surface will drop the iron powder, causing black spots.

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

3, the slider grinds iron foam, causing black spots.

4, the slider inside the leaking rust or other stains, rust and stains are flying out by the slider activity, falling on the product will form black spots.

Black point discrimination:

If black spots appear on the surface of the whole product, and there are black spots in the deep part of the part, it should belong to the black spot before molding; if the black spots appear only on the surface, it should be a black spot in the molding. If it is only distributed in a specific area of ​​the surface at the same time, it is undoubtedly a black point in the forming; if the black point is large (generally 0.5~1mm), it should belong to the raw material carbonized black spot; if the black point is dark, brittle, bulky and porous, Can be determined as carbon black spots: If the black dot density is particularly large, check the raw materials without obvious impurities, generally should be the fresh material after the fresh material is fresh, or the reason should be checked.

Third, the solution:

1. For the forming impurities, black spots are formed due to the foreign matter mixed into the material, and the cleaning, such as production, packaging, storage, transportation, reporting, and mixing to the barrel must be strictly controlled.

2. For carbonized black spots, the processing temperature should be strictly controlled. In actual production, it means that the processing temperature and the actual processing temperature are different. 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 portion of the screw barrel, the melt drop/decomposition tendency is weak, and carbonized black spots are not easily generated.

To prevent carbonization of black spots, it is necessary to prevent accumulation of materials. Eliminate dead angles in the barrels, nozzles, and runners, so that there is a gentle transition in the corners to eliminate areas where material may remain.

3. Black spots deposited on the threaded surface and the wall of the barrel for the auxiliary degradation deposit or existing carbonized material. Generally, with the production, these black spots will be discharged from the original attachment surface into the melt under various strong shearing actions in the barrel. This "discharge" process is the process of black spots and impurities in the product. In the process of converting color change, this process of “discharging” impurities is inevitable. We must try our best to shorten this process, which is “cleaning”. There are two cleaning methods:

1) Clean the screw against the molten metal (air jet after melting).

2) The injection gel melts the glue, the air injection glue, and then the injection gel, the air-laid glue, and so on. It has been proved that the back pressure plays a big role in the cleaning, and the melt and the screw are strongly sheared. . The melt speed is slow, so the glue time is also long. This effect is good, and the foreign matter is cleaned up quickly. Note: There is a principle of back pressure in the absence of timing - as high as possible, so high that the barrel does not automatically heat up.

3) For the black spots in the molding, the black spots in the molding must be attached to the surface of the cavity first, and then fixed by the melt to the surface of the product, so the black point removal method and the way it is not in the cavity. For the mold to determine the poor material, we must first determine the cause and determine the position. All the relatively movable cavity, the core including the slider, neutron, thimble / block, are likely to burn, to determine the burn, repair the damaged part, in addition to reduce the friction of the sliding part, add lubricant. In addition, to reduce the relative movement speed of the sliding part, such as jacking/retracting, opening and closing the mold, the slider should move as slowly as possible, rust and other stains between the slider and the template, to remove the slider, rust and other Remove the stain and tighten the water nozzle. For oil or water splashing on the smooth molding surface of the cavity, the black spots should be wiped frequently to easily produce oil and water, so as to prevent its appearance on the molding surface.