Causes and solutions for black spots/streaks and impurities in plastic product production

I. Characteristics and classification of black spots and impurities

(a) Appearance and distribution characteristics

Black spots and impurities are mostly small dark brown particles, usually non-reflective. When the particles are large, the impurities are layered, fragile and porous after crushing. Their distribution methods are diverse, some are irregularly scattered on the whole, some are randomly distributed locally, and some only appear occasionally in a local area.

(b) Two major categories

Impurities are mainly divided into two categories: those formed before molding and those formed during molding. Impurities formed before molding will be mixed in the raw material processing and other links, while those generated during the molding process are related to factors such as processing conditions.

II. Causes of black spots and impurities in plastic products before molding

(a) Raw material processing link

• The raw materials are not cleaned up for various reasons during processing, resulting in black spots on the raw materials.

• Impure granulation can also cause black spot problems.

• Masterbatch or spotted fragments and waste are mixed into the raw materials.

• Impure materials, such as low melting point materials mixed with high melting point materials.

(b) Storage and transportation links

• Impurities may be mixed in during packaging, transportation and storage. 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 material particles.

• Impurities may also be introduced during feeding.

(c) Carbonization of raw materials

The black spots formed by carbonization of raw materials are usually large in volume, with a maximum diameter of 1-2 mm. Most “black spots” are thicker, and there are also thinner ones consisting of one or two layers. This is usually due to long-term storage of raw materials, or local overheating leading to decomposition, carbonization and carbonization to form agglomerates, and the materials are sheared and broken when flowing through the screw or nozzle. Cleaning the raw materials with chlorine bleach can help prevent impurities.

III. Reasons for carbonization of materials during molding

(a) Excessive melt temperature

If the material temperature is too high, it will cause decomposition to form carbides. Especially for some heat-sensitive materials, the temperature range is very narrow, and the barrel temperature must be kept moderate.

(b) Material coking

Molten plastic will coke if it stays in a certain place for too long, thus producing black spots. Areas that may cause material retention include the nozzle-barrel connection, barrel wall, melt ring, nozzle-gate connection, hot runner corners, and main runner dead corners.

(c) Too large barrel gap

If the gap between the barrel and the screw is too large, the material will be retained in the barrel. The retained material will be overheated and decomposed for a long time, resulting in black spots. Washing the barrel and screw with hot water can help prevent carbonization.

(d) Additive degradation and discoloration

Additives such as antistatic agents, ultraviolet/infrared absorbers and general dyes are usually more active than raw materials. Under the shear force at processing temperature, they will decompose into dark, brown or even black colors, which will appear as black spots and impurities after injection molding.

IV. Black spots and impurities caused by external factors

(a) Mold-related problems

• The mold material is not good, and iron powder falls off on the parting surface, molding surface or fitting surface, resulting in black spots. Cleaning the mold and other parts with a wet cloth can prevent impurities.

• The ejector pin is rough and easy to burn, causing iron powder to fall off and form black spots.

• The slider will produce iron powder, causing black spot problems.

• Water leakage inside the slider will cause the slider to rust or produce other stains, which will be thrown out when the slider moves and fall on the product to form black spots.

V. How to identify black spots

• If black spots appear on the entire surface of the product and deep in the parts, they should be formed before injection molding; if they only appear on the surface, they are formed during the molding process. If they are only distributed in specific areas of the surface, they are undoubtedly black spots formed during the injection molding process.

• If the black spots are large (generally 0.5-1 mm), dark in color, fragile, and porous, it can be confirmed that they are caused by carbonization.

• If the black spots are particularly dense and there are no obvious impurities in the raw materials, it is usually caused by not cleaning the previous material in time before changing the material. Otherwise, the raw materials should be checked to find out the cause.

VI. Solutions for stubborn stains

(a) Control the mixing of impurities

For impurities in molded products, black spots are caused by foreign matter mixed in the raw materials. The cleanliness of all links such as production, packaging, storage, transportation, unpacking, mixing and barrel must be strictly controlled. Baking soda paste can effectively clean plastic containers and remove stains, ensuring that all surfaces are free of stubborn residues.

(b) Control carbonization black spots

For black spots caused by carbonization, the injection temperature should be strictly controlled.

Usually, black spots caused by additive degradation or carbonized materials deposited on the screw surface and barrel wall will be mixed into the molten material from the original position under various strong shearing effects in the barrel during the production process. This “precipitation” is the process by which black spots and impurities appear in the product. When changing materials or colors, if this impurity “precipitation” process is unavoidable, it must be minimized, that is, “cleaning” must be performed. For custom injection mold making please contact sales@yuanfang.com.