After injection molding production, plastic parts randomly suffer from abnormal surface defects such as black spots of varying sizes, linear black streaks and burnt black wire drawing. These defects seriously damage the surface appearance of products, lead to failure in dimensional and visual inspection, and directly reduce the product yield rate.

The main causes are summarized as follows:

Impurities and dust are mixed into raw materials during storage and conveying; random mixing of different types of raw materials causes material contamination, which precipitates into black spot impurities after molding.

The plastic raw materials used have insufficient heat resistance and poor thermal stability. They are prone to decomposition and carbonization under high-temperature plasticization, forming black spot and linear defects on the part surface.

Excessively high proportion of runner material and recycled crushed material, together with incomplete screening and filtration, result in impurities and carbonized scraps inside. Irregular management of crushing and recycling mixes defective waste materials and triggers black spot issues.

Unreasonable setting of injection molding process parameters: excessively high barrel and nozzle temperatures far exceed the temperature tolerance of raw materials, causing materials to stagnate at high temperature for a long time and get burnt and carbonized.

The tonnage of the injection molding machine does not match product molding requirements. Insufficient clamping force and plasticization load of the equipment cause abnormal plasticization shearing, aggravating overheating, aging and burning of materials.

Long-term continuous production without regular equipment maintenance leads to carbon deposits and aged rubber scale adhered to the inner barrel wall and screw surface. Burnt residual materials keep mixing into molten material and bring out black streaks and spots.

During long-term continuous mold operation, oil stains, mold release agent residue, dust and iron scraps accumulate on moving parts such as ejector pins and sliders. These contaminants fall off during mold opening and closing and adhere to part surfaces to form black spots.

Insufficient vent slots, blocked vents or unreasonable vent positions on the mold fail to exhaust air inside the cavity in a timely manner. The compressed air generates high temperature and causes burning, resulting in local carbonization, blackening and wire drawing defects.

Dead corners and blind turning areas exist in the mold runner design, where molten material tends to stagnate and accumulate for a long time. Repeated heating causes aging and carbonization, continuously producing black material that forms streaks with injection flow.

Inadequate daily cleaning of the mold cavity surface leaves residual oil stains, mold release agent residue, dust impurities and previous burnt marks. These defects are replicated on product surfaces during molding and cause poor appearance quality.

Corresponding Improvement Measures

Unify and standardize raw material management; replace with new qualified pure raw materials, store different materials in separate zones to avoid mixing. Clean the barrel and hopper thoroughly before production to completely isolate impurity contamination.

For raw materials with poor heat resistance, add an appropriate amount of special heat stabilizers and antioxidant additives to enhance high-temperature oxidation resistance and thermal stability, and prevent material decomposition and carbonization under high temperature.

Strictly control the usage proportion of runner material and recycled material; conduct thorough screening, filtration and dust removal for recycled materials, promptly eliminate carbonized and burnt defective scraps, and standardize crushing and feeding procedures.

Reasonably lower process temperatures according to raw material physical properties, appropriately reduce temperatures of each barrel section and nozzle to cut down thermal load on materials, and avoid burning and carbonization during high-temperature melting.

Recalculate and match machine models based on product structure, glue volume and molding resistance; select injection molding machines with appropriate tonnage to ensure uniform plasticization and stable shearing, and reduce material aging.

Formulate a regular equipment maintenance plan; conduct regular complete disassembly, cleaning and high-temperature purging for barrels and screws to fully remove inner wall carbon deposits, aged rubber scale and burnt residual material.

Establish a daily mold maintenance system; regularly disassemble and clean moving accessories including ejector pins, sliders and guide pillars, wipe off oil stains, dust and scale deposits to keep mold moving parts clean.

Comprehensively inspect and repair the mold vent system; widen and deepen vent slots, add vent positions at key locations, and regularly clear blockages in vent slots. Ensure rapid and smooth air exhaust from the cavity to avoid burning.

Optimize and improve the mold runner structure; modify right-angle and dead-corner designs with smooth transition structures to eliminate material stagnation areas. Regularly polish, grind and clean runners to remove scale deposits.

Fully clean and wipe the mold cavity before and after production; perform regular mirror polishing maintenance to remove residual oil stains, mold release agent and burnt marks on the mold surface, and keep the cavity clean and dust-free.