Black Spots and Impurities in Molded Parts

Introduction: Understanding the Defect

Black spots and impurities are among the most critical factors contributing to scrap rates in 플라스틱 사출 성형[^1]. These defects primarily ruin the aesthetic quality of the product, leading to rejection.

Typically, these imperfections appear as small, dark brown, non-reflective particles. Larger impurities may appear layered, brittle, and porous when crushed. To solve the problem, we must first understand their distribution:

Random Distribution: Some spots are scattered throughout the entire part (indicating pre-molding contamination[^2]).
Localized Distribution: Some appear only in specific areas or on the surface (indicating issues during the molding process).

> Key Insight: Impurities found deep inside the part existed before molding. Impurities found only on the surface were likely introduced during molding.

Part 1: Root Cause Analysis

We can categorize the causes of black spots[^3] into two main phases: Pre-Molding 그리고 During Molding.

1. Pre-Molding Causes (Material & Handling)

These defects originate before the plastic even enters the mold cavity.

Raw Material Issues: Impurities can stem from the manufacturing process, such as unclean processing, impure pelletizing, or mixing with incompatible materials (e.g., high melting point particles in low melting point resin).
오염: During packaging, transportation, or storage, foreign objects can mix with the resin. If you observe foreign matter on the surface of the pellets immediately after opening the bag, this is the source.
Recycled Material: Using crushed scrap (regrind) that contains dust, metal shavings, or degraded particles is a common culprit.
Carbonization: Large black spots (1-2mm) are often chunks of carbonized material. This happens when plastic is trapped in a "dead spot" (like the screw tip or nozzle) for too long, overheats, degrades, and eventually breaks off into the melt stream.
Additive Degradation: Certain additives (like antistatic agents or colorants) can be more chemically active than the base resin. They may decompose and turn black at temperatures where the base resin is still stable.

2. During Molding Causes (Machine & Mold)

These defects are generated by the equipment or the environment during the production cycle.

Mold Wear: Poor quality mold steel can shed iron powder from parting lines or sliding surfaces. Rough ejector pins or sliding blocks can also generate metal dust.
Lubrication & Rust: Rust or dirty grease from inside sliding blocks can be flung onto the product. Similarly, oil or water splashing onto the cavity surface can cause surface defects.
Environmental Dust: Airborne dust and suspended particles in the factory can settle into the open mold or the hopper if not properly covered.

Part 2: Diagnostic & Solutions

Identifying the type of black spot helps determine the fix.

Defect Characteristic	Likely Cause	Recommended Solution
Scattered throughout part (deep & surface)	Pre-molding contamination or Carbonization	Check raw material source; Clean screw and barrel; Check for dead spots.
Surface only (localized)	Mold issues or Environment	Clean mold surface; Check for rust/oil; Cover hopper; Improve workshop cleanliness.
Large spots (0.5 - 1mm)	Carbonized Material	Lower melt temperature; Reduce residence time; Clean nozzle and check for burnt material.
High density of spots after changeover	Incomplete purging	Increase back pressure during cleaning; Perform multiple purging cycles.

Solutions for Carbonization (The "Burnt" Material)

Carbonization is often due to excessive heat or material stagnation.

온도 제어: Ensure the displayed temperature matches the actual melt temperature. High back pressure and long cycle times can increase actual heat even if the setpoint is normal.
Eliminate Dead Spots: Inspect the nozzle, screw tip, and flow channels. Any area where plastic can stagnate will eventually burn and create black specks.
Check Clearances: Excessive clearance between the screw and the barrel allows material to get trapped and degrade over time.

Solutions for Mold & Mechanical Issues

Maintenance: Regularly inspect sliding blocks, ejector pins, and cores. If they show signs of galling or burning, repair them immediately.
Lubrication: Reduce friction by applying appropriate lubrication to sliding parts, but ensure no excess grease enters the cavity.
Speed Control: Reduce the speed of mold opening/closing and slider movement to minimize friction and wear.
Cleaning: Remove rust and debris from water channels and slider interiors. Tighten water nozzles to prevent leaks.

Part 3: The Cleaning Process

When switching materials or colors, or when black spots appear, a thorough cleaning (purging) of the injection unit is required.

Method: Perform "Air Shooting" (purging without a mold). Melt the plastic, inject it into the air, and repeat.
Technique: For stubborn carbonized deposits, use a high back pressure. This increases the shear force between the melt and the screw, effectively scrubbing the barrel walls.
Goal: Continue until the purged material is completely clean and free of black specks.

> 전문가 팁: If you suspect the raw material itself is the issue, perform a "virgin material test." Run a cycle with brand new, clean resin. If the black spots disappear, the issue was your material or regrind. If they persist, the issue is likely the machine or mold.

[^1]: Explore this resource to discover essential practices that enhance quality and reduce defects in plastic injection molding. [^2]: Learning about pre-molding contamination can help identify and eliminate sources of defects. [^3]: Understanding the causes of black spots can help improve product quality and reduce scrap rates.

저자 소개

Alex Yu

아이디얼 프로 매뉴팩처링

알렉스는 Ideal Pro의 공동 창립자이지만, 주요 프로젝트를 총괄하거나 기술 팀을 이끌지 않을 때는 장거리 자전거 타기를 즐기거나, 동네에서 가장 맛있는 커피를 찾아다니거나, 다음 여행 계획을 세우는 모습을 볼 수 있습니다. 국제 무역 영업과 프로젝트 관리 분야에서 20년 이상의 경력을 쌓아온 그는 촉박한 마감 기한부터 까다로운 엔지니어링 문제까지, 그 사이의 모든 상황을 다 겪어보았습니다. 그는 훌륭한 성과는 충분한 휴식을 취한 마음과 맛있는 커피에서 나온다고 믿으며, 맡은 모든 프로젝트에 그와 같은 호기심과 세심한 주의를 기울입니다. 사무실에 있지 않을 때는 대개 야외 어딘가에서 새로운 장소를 탐험하며, 나중에 나눌 이야기를 수집하곤 합니다.