Mold injection black spot and impurities are always not cleaned up, we tells you truth!

Time：2020-07-31 07:58:27 / Popularity： / Source：

Black spots and impurity defects are the most important factors for rejection rate in normal production process. It mainly affects appearance of product and leads to scrap. Impurities and most black spots are foreign substances, have nothing to do with raw material itself, while a small part of black spots and impurities are caused by raw material itself.
Characteristics of black spots and impurities are that particles are small, dark brown, and generally not reflective. When particles are large, impurities are layered, brittle, fragile, and porous after being broken. Their distribution has two major characteristics:
Some are distributed randomly and scattered as a whole, some are distributed randomly and scattered locally, sometimes only occasionally appear in a certain local area;
Such black spots and impurities sometimes only appear on the surface of product, sometimes regardless of depth of surface and inside, but inner black spots closer to surface are lighter than surface black spots, and deeper black spots are not visible at all .
What is interesting is that if you link these two points together, you will find that impurities that only appear in a certain local area must be black spots on the surface, and distribution of black spots, regardless of whether surface is deep or shallow, is inevitably distributed randomly and scattered as a whole.
This is because internal impurities must exist before molding, and impurities that only appear during molding must only be distributed on the surface.

In this way, impurities are divided into two categories: forming before forming and during forming:

1. Black spots and impurities before molding:

During processing of raw materials, due to various reasons, foreign matter is not clean and raw materials appear black spots;
Impure granulation causes black spots;
Raw materials are mixed with color masterbatch or speckled crushed pieces and scraps;
Material is not pure, and high melting point pellets are mixed into low melting point material;
Impurities may be mixed in during packaging, transportation, and storage. Obvious feature is that after raw materials are opened, carefully observe that there are foreign bodies and impurities on the surface of material particles;
Impurities and foreign bodies in feeding process;
When raw materials are carbonized, such black dots are generally large in size, and appearance diameter of large black dots can reach 1-2mm. Most of black dots are thicker, and there are one or two layers of thinner.
Raw materials are accumulated for a long time or locally subjected to high heat, decomposed, coked, carbonized into agglomerates. Flow is formed by shearing and breaking in screw or nozzle. Reasons for carbonization of raw materials are:
If melt temperature and material temperature is too high, it will cause overheating to decompose and form carbides. Especially for some heat-sensitive materials, temperature range is narrow, and temperature at the end of barrel must be controlled not to be too high.
Accumulated material coking: If molten plastic stays in a place for too long, there will be coking accumulating material, causing black spots, areas that may cause material retention, such as connection between nozzle and barrel, wall of barrel, melt ring, contact part of nozzle and gate, corner of hot runner, dead corner of main runner, etc.
If gap between barrel is too large, gap between screw barrel and screw is too large, material will stay in barrel, retained material will be decomposed after long-term overheating, resulting in black spots.
Additives reduce, decompose and change color. Additives include antistatic agents, violet/infrared absorbers and general dyeing agents. They are generally more active than raw materials. Under action of shearing force of processing temperature, when raw materials are not decomposed, they are already decomposed and become dark, yellow-brown or even black, forming black spots and impurities during molding.

2. Black spots and impurities caused by external factors during production

If mold material is not good, iron powder will fall off parting surface, forming surface or contact surface, causing black spots.
Thimble is rough and easy to burn, dropping iron powder and causing black spots.
Slider grinds iron powder, causing black spots.
Water leaks in slider, rust or other stains, rust and stains fly out by slider movement, and black spots will be formed on the product.

Black spot identification:

If black spots appear on entire surface of product, and there are black spots deep in part, they should belong to black spots before forming; if black spots only appear on the surface, they should be black spots in forming process.
If it is only distributed on a specific area of surface at the same time, it is undoubtedly a black spot in molding; if black spot is larger (usually 0.5~1mm), it should be a carbonized black spot of raw material; if black spot is dull, brittle, bulky and porous at the same time, they can be judged as carbonized black spots: if density of black spots is particularly high, check that raw material has no obvious impurities. Generally, it should generally be a phenomenon that original material is clean after material is changed, otherwise, reason should be investigated.

3. Solution

1. For molding impurities, black spots are formed due to mixing of foreign matter into material. Cleanliness of all links from production, packaging, storage, transportation, reporting, mixing to barrel must be strictly controlled.
2. For carbonized black spots, processing temperature should be strictly controlled. In actual production, there is a difference between processing temperature and actual processing temperature. For same equipment, different back pressure, cycle time, one-time injection volume, and thermocouple insertion positions will cause different carbonization and degradation trends of material at same processing temperature.
Specifically, it means that when temperature is same, back pressure is small, cycle is short, and one-time injection volume is large. When thermocouple is inserted in upper part of barrel, melt drop/decompose 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 screw barrel, nozzle, and runner, so that there is a smooth transition at turning point, and area where material may be retained is eliminated.
3. For black spots on the surface of screw thread and wall of screw barrel that additive is degraded or deposited or carbonized material has been deposited. Generally with production, these black spots will break away from their original attachment surface and enter melt under various strong shearing actions in barrel and be discharged. This "discharge" process is process in which black spots and impurities appear in product.
For example, in production of changing materials and changing colors, this "discharge" of impurities is inevitable, and we must try our best to shorten this process. It is "cleaning". There are two cleaning methods below:
Air-melting glue cleaning screw (air shot after melting glue).
Melt at injection station, shoot in the air, then melt at the injection station, and shoot in the air, and so on repeatedly;
Practice has proved that back pressure plays a great role in cleaning, melt and screw are strongly sheared. Melting speed is slow, so melting time is also long. This effect is good and foreign body is cleaned up quickly. Note: There is a principle of back pressure at irregular times-as high as possible, until barrel just doesn't heat up automatically.
4. For black spots in forming process, black spots in forming process must be attached to surface of cavity first, and then fixed to surface of product by melt. Therefore, this method of removing black spots can prevent it from being in cavity.
For molds with poor materials, determine cause and location first. All cavities with relative movement, mold cores, including sliders, neutrons, and thimble/blocks, may be burned. After confirming burn, repair damaged part, reduce friction of sliding part, and add lubricating oil.
5. To reduce relative movement speed of sliding part, such as pushing forward/retracting, opening and closing mold, moving slider as slowly as possible. There is rust and other stains between slider and mold plate. Remove slider, remove rust and other stains, and tighten water nozzle.
For oil or water splashing on smooth molding surface of mold cavity, causing black spots, you should often wipe the oil and water prone areas to prevent them from appearing on molding surface.

Gud Mould Industry Co., Ltd