{"id":38388,"date":"2026-03-06T09:20:00","date_gmt":"2026-03-06T01:20:00","guid":{"rendered":"https:\/\/zetarmold.com\/?p=38388"},"modified":"2026-04-09T08:04:52","modified_gmt":"2026-04-09T00:04:52","slug":"koyu-lekeleri-cozun","status":"publish","type":"post","link":"https:\/\/zetarmold.com\/tr\/koyu-lekeleri-cozun\/","title":{"rendered":"How to Solve Dark Spots on Injection Molded Products: Causes &amp; Fixes"},"content":{"rendered":"<div class=\"callout-key\" style=\"background:#f0f7ff; border-left:4px solid #2563eb; padding:1em 1.2em; border-radius:6px; margin:1.5em 0;\">\n<strong>\u00d6nemli \u00c7\u0131kar\u0131mlar<\/strong><\/p>\n<ul>\n<li>Termal bozunma mekanizmas\u0131: <a href=\"https:\/\/zetarmold.com\/tr\/injection-mold-complete-guide\/\">enjeksiyon kal\u0131b\u0131<\/a>ed parts are caused by three primary mechanisms: material thermal degradation (burn marks), carbonized material from dead zones, and contamination from colorants, release agents, or foreign matter.<\/li>\n<li>The location of dark spots\u2014at the gate, end-of-fill, along flow lines, or at ejector pins\u2014is the fastest diagnostic indicator of the root cause.<\/li>\n<li>Excessive melt temperature, excessive residence time in the barrel, and high injection speed are the three most common process-related causes of dark spots.<\/li>\n<li>Regular purging of the barrel and screw is the single most effective preventive measure for dark spot defects.<\/li>\n<li>Dead zones in the runner system, gates, or barrel are the primary source of carbonized material that causes recurring dark spot patterns.<\/li>\n<\/ul>\n<\/div>\n<h2>What Are Dark Spots on Injection Molded Products and Why Do They Matter?<\/h2>\n<p>Dark spots are among the most visible and quality-critical defects in injection-molded plastic products. They manifest as localized areas of discoloration\u2014ranging from yellowish-brown degradation halos to hard black carbonized particles\u2014embedded in or on the part surface. Unlike flow marks or sink marks that affect surface texture, dark spots represent material composition changes that cannot be removed through post-processing.<\/p>\n<p>Understanding dark spots matters for three reasons:<\/p>\n<ul>\n<li><strong>Aesthetic failure:<\/strong> Dark spots are immediately visible on consumer products, medical devices, and automotive interior components, leading to customer rejections and scrap.<\/li>\n<li><strong>Structural compromise:<\/strong> Carbonized particles embedded in a part create stress concentrators. Parts with dark spots from degraded material may have 15\u201330% reduced impact strength at the defect location.<\/li>\n<li><strong>Process signal:<\/strong> Dark spots indicate that something fundamental is wrong\u2014with temperature control, material handling, machine maintenance, or mold design\u2014that will worsen over time\u2014identifiable through systematic <a href=\"https:\/\/zetarmold.com\/tr\/injection-molding-defects\/\">injection molding defect diagnosis<\/a><sup id=\"fnref1:1\"><a href=\"#fn:1\">1<\/a><\/sup><\/li>\n<\/ul>\n<table>\n<thead>\n<tr>\n<th>Dark Spot Type<\/th>\n<th>G\u00f6r\u00fcn\u00fc\u015f<\/th>\n<th>Typical Location<\/th>\n<th>Primary Cause<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Burn marks<\/td>\n<td>Brown\/black scorched area<\/td>\n<td>End-of-fill, trapped air zones<\/td>\n<td>Diesel effect (compressed air ignition)<\/td>\n<\/tr>\n<tr>\n<td>Thermal degradation spots<\/td>\n<td>Yellow-to-brown discoloration<\/td>\n<td>Gate area, thin sections<\/td>\n<td>Excessive melt temperature or residence time<\/td>\n<\/tr>\n<tr>\n<td>Carbonized black particles<\/td>\n<td>Distinct black specks<\/td>\n<td>Scattered or near gate<\/td>\n<td>Dead zones in barrel, screw, or hot runner<\/td>\n<\/tr>\n<tr>\n<td>Colorant contamination<\/td>\n<td>Random color specks<\/td>\n<td>Distributed throughout part<\/td>\n<td>Incompatible masterbatch or cross-contamination<\/td>\n<\/tr>\n<tr>\n<td>Metal contamination<\/td>\n<td>Metallic dark spots<\/td>\n<td>Near gate or random<\/td>\n<td>Screw\/barrel wear debris<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<div class=\"claim claim-false\" style=\"background-color: #f7efef; border-color: #f7efef; color: #db6f85;\">\n<p><svg xmlns=\"http:\/\/www.w3.org\/2000\/svg\" width=\"24\" height=\"24\" viewbox=\"0 0 24 24\" fill=\"none\" stroke=\"#db6f85\" stroke-width=\"2\" stroke-linecap=\"round\" stroke-linejoin=\"round\"><circle cx=\"12\" cy=\"12\" r=\"10\"\/><line x1=\"15\" y1=\"9\" x2=\"9\" y2=\"15\"\/><line x1=\"9\" y1=\"9\" x2=\"15\" y2=\"15\"\/><\/svg> <b>Dark spots are always caused by excessive melt temperature alone.<\/b><span class='claim-true-or-false'>Yanl\u0131\u015f<\/span><\/p>\n<p class='claim-explanation'>While high melt temperature is a common contributor, dark spots can also result from long residence time at normal temperatures, the diesel effect from trapped air, colorant incompatibility, screw wear, or dead zones in the hot runner system. Correctly diagnosing the type and location of the dark spot before adjusting temperature is essential\u2014reducing temperature alone often fails to solve spots caused by other mechanisms.<\/p>\n<\/div>\n<h2>How Does Thermal Degradation Cause Dark Spots in Injection Molded Parts?<\/h2>\n<figure class=\"wp-block-image size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"800\" height=\"457\" class=\"wp-image-53105\" src=\"https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/hdpe-plastic-resin-pellets.webp\" alt=\"Plastic resin pellets for injection molding\" srcset=\"https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/hdpe-plastic-resin-pellets.webp 1200w, https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/hdpe-plastic-resin-pellets-300x171.webp 300w, https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/hdpe-plastic-resin-pellets-1024x585.webp 1024w, https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/hdpe-plastic-resin-pellets-768x438.webp 768w, https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/hdpe-plastic-resin-pellets-18x10.webp 18w, https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/hdpe-plastic-resin-pellets-600x343.webp 600w\" sizes=\"(max-width: 800px) 100vw, 800px\" \/><figcaption class=\"wp-element-caption\">Plastic resin pellets used in injection molding<\/figcaption><\/figure>\n<p>Termal bozunma, polimer zincirlerinin malzemenin stabilite e\u015fi\u011fini a\u015fan \u0131s\u0131ya maruz kald\u0131\u011f\u0131nda kimyasal olarak ayr\u0131\u015fmas\u0131d\u0131r. Enjeksiyon kal\u0131plama i\u00e7in ilgili \u0131s\u0131 kaynaklar\u0131 \u015funlard\u0131r: namlu \u0131s\u0131t\u0131c\u0131 b\u00f6lgeleri, vidadan kaynaklanan kesme \u0131s\u0131s\u0131 ve s\u0131cak namludaki kalma s\u00fcresi.<\/p>\n<p><strong>Mechanism of thermal degradation:<\/strong> Y\u00fcksek s\u0131cakl\u0131klarda, polimerin ana zincir ba\u011flar\u0131 kopar (zincir k\u0131r\u0131lmas\u0131). Par\u00e7alar, farkl\u0131 \u0131\u015f\u0131k dalga boylar\u0131n\u0131 absorbe eden daha d\u00fc\u015f\u00fck molek\u00fcler a\u011f\u0131rl\u0131kl\u0131 t\u00fcrlere yeniden birle\u015fir ve sar\u0131, kahverengi veya siyah g\u00f6r\u00fcn\u00fcr. ABS i\u00e7in bu s\u00fcre\u00e7, HCN ile ili\u015fkili renk de\u011fi\u015fimine neden olur. PVC i\u00e7in, dehidroklorinasyon HCl salar ve g\u00f6r\u00fcn\u00fcr \u0131\u015f\u0131\u011f\u0131 absorbe eden konjuge polien dizileri \u00fcreterek par\u00e7alar\u0131 giderek daha koyu hale getirir.<\/p>\n<p><strong>Critical temperature thresholds:<\/strong><\/p>\n<ul>\n<li>ABS: Degradation begins above 270\u00b0C; parts typically molded at 210\u2013250\u00b0C<\/li>\n<li>PP: Degradation above 280\u00b0C; process temperature 220\u2013250\u00b0C<\/li>\n<li>PC: Degradation above 320\u00b0C; process temperature 280\u2013310\u00b0C<\/li>\n<li>PVC: Degradation above 200\u00b0C; requires heat stabilizers to extend processing window<\/li>\n<li>Nylon PA66: Degradation above 290\u00b0C; process temperature 260\u2013280\u00b0C<\/li>\n<\/ul>\n<p><strong>Residence time effect:<\/strong> Even at temperatures within the safe processing range, extended residence time degrades the polymer. This occurs when: the machine is too large for the shot size (partial barrel fill), production is paused without purging, or the screw speed is set too low. The standard guideline is to ensure shot utilization of 40\u201380% of barrel capacity\u2014shots smaller than 40% of barrel capacity require optimized <a href=\"https:\/\/zetarmold.com\/tr\/enjeksi%cc%87yon-kaliplama-parametreleri%cc%87\/\">enjeksi\u0307yon kaliplama parametreleri\u0307<\/a><sup id=\"fnref1:2\"><a href=\"#fn:2\">2<\/a><\/sup><\/p>\n<p><strong>Shear-induced degradation:<\/strong> Excessive screw rotation speed or back pressure generates frictional heat that adds to barrel heater temperatures. In thin-walled sections or restricted gates, shear rates can locally spike to levels that degrade even heat-stabilized materials. This produces dark spots specifically at the gate entry or in the thinnest sections of the part.<\/p>\n<h2>What Is the Diesel Effect and How Does It Cause Burn Marks?<\/h2>\n<figure class=\"wp-block-image size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"800\" height=\"457\" class=\"wp-image-53140\" src=\"https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/hdpe-injection-molding-process-v2.webp\" alt=\"Injection molding production process\" srcset=\"https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/hdpe-injection-molding-process-v2.webp 1200w, https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/hdpe-injection-molding-process-v2-300x171.webp 300w, https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/hdpe-injection-molding-process-v2-1024x585.webp 1024w, https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/hdpe-injection-molding-process-v2-768x438.webp 768w, https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/hdpe-injection-molding-process-v2-18x10.webp 18w, https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/hdpe-injection-molding-process-v2-600x343.webp 600w\" sizes=\"(max-width: 800px) 100vw, 800px\" \/><figcaption class=\"wp-element-caption\">Injection molding machine in production<\/figcaption><\/figure>\n<p>\"Dizel etkisi\" (ayr\u0131ca \"hapsolmu\u015f havadan kaynaklanan yan\u0131k izleri\" olarak da adland\u0131r\u0131l\u0131r), kal\u0131p bo\u015flu\u011funda hapsolmu\u015f havan\u0131n ilerleyen eriyik taraf\u0131ndan s\u0131k\u0131\u015ft\u0131r\u0131l\u0131p tutu\u015ftu\u011fu ve hava cebi konumunda polimeri yakt\u0131\u011f\u0131 bir fenomendir. Ad\u0131n\u0131, yak\u0131t\u0131n yaln\u0131zca s\u0131k\u0131\u015ft\u0131rma \u0131s\u0131s\u0131ndan kendili\u011finden tutu\u015ftu\u011fu dizel motor yanmas\u0131ndan al\u0131r.<\/p>\n<p><strong>How it happens:<\/strong> As melt fills the cavity, displaced air must escape through vents. If vents are absent, blocked, or undersized at the end-of-fill locations, the air becomes trapped and is progressively compressed. Air temperature under compression can reach 300\u2013500\u00b0C\u2014well above the ignition temperature of most polymers. The result is a charred black area precisely at the location where air was last trapped.<\/p>\n<p><strong>Identifying diesel effect burn marks:<\/strong> These marks appear at consistent, predictable locations on every part\u2014typically at the end of fill, at corners, in blind pockets, or at weld line meeting points. The scorching is concentrated at the surface in contact with the trapped air, often with a slightly pitted texture from the combustion event.<\/p>\n<p><strong>Solutions for diesel effect:<\/strong><\/p>\n<ul>\n<li>Add or deepen vents at the end-of-fill locations identified by flow simulation or part inspection<\/li>\n<li>Use vacuum-assisted venting for deep pockets or microstructured surfaces where conventional vents are impractical<\/li>\n<li>Reduce injection speed in the last 20\u201330% of fill to allow air more time to escape<\/li>\n<li>Redesign part geometry to eliminate blind pockets that trap air<\/li>\n<li>Check and clean existing vents\u2014they clog over time from material outgassing<\/li>\n<\/ul>\n<h2>How Do Dead Zones in the Machine and Hot Runner Cause Dark Spots?<\/h2>\n<figure class=\"wp-block-image size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"800\" height=\"457\" class=\"wp-image-53133\" src=\"https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/low-volume-mold-tooling-inspection-1.webp\" alt=\"Mold tooling inspection with depth gauge\" srcset=\"https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/low-volume-mold-tooling-inspection-1.webp 1200w, https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/low-volume-mold-tooling-inspection-1-300x171.webp 300w, https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/low-volume-mold-tooling-inspection-1-1024x585.webp 1024w, https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/low-volume-mold-tooling-inspection-1-768x438.webp 768w, https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/low-volume-mold-tooling-inspection-1-18x10.webp 18w, https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/low-volume-mold-tooling-inspection-1-600x343.webp 600w\" sizes=\"(max-width: 800px) 100vw, 800px\" \/><figcaption class=\"wp-element-caption\">Precision mold tooling inspection and measurement<\/figcaption><\/figure>\n<p>Dead zones are areas in the molding system where polymer flow is stagnant\u2014material collects and remains at elevated temperature without being refreshed by new shots. Over hours or days, this stagnant material degrades and carbonizes into hard black particles that are eventually purged forward into the melt stream and deposited in subsequent parts as dark specks.<\/p>\n<p><strong>Common dead zone locations:<\/strong><\/p>\n<ul>\n<li><strong>Screw check ring:<\/strong> Worn or damaged check rings (non-return valves) allow backflow into the screw flights, where material lodges and carbonizes. This produces irregular black speck patterns that worsen as screw wear progresses.<\/li>\n<li><strong>Barrel transition zones:<\/strong> Sudden diameter changes or improper screw flight geometry create local stagnation zones. Proper screw design for each resin type minimizes this risk.<\/li>\n<li><strong>Hot runner manifold bends:<\/strong> Sharp 90\u00b0 bends in hot runner manifolds are classic dead zones. Polymer moving through the main channel creates a stagnant eddy at the bend that degrades over time. Well-designed hot runners use streamlined flow paths with radiused bends.<\/li>\n<li><strong>Hot runner gate tips:<\/strong> Open-gate hot runner nozzle tips can accumulate degraded material around the orifice perimeter if the gate temperature is too high or the thermal profile is not optimized.<\/li>\n<li><strong>Adapter connections:<\/strong> Joints between the nozzle, sprue bushing, and manifold are prone to leakage and dead zones if machining tolerances are not maintained.<\/li>\n<\/ul>\n<p><strong>Addressing dead zones:<\/strong> Systematic purging with a purging compound at the start and end of production runs removes accumulated degraded material before it reaches critical concentration levels. When black speck counts increase over a production run, this is a strong indicator of accumulating dead zone deposits that require a planned purge\u2014an essential aspect of <a href=\"https:\/\/zetarmold.com\/tr\/injection-mold-cooling-system\/\">injection mold cooling system<\/a><sup id=\"fnref1:3\"><a href=\"#fn:3\">3<\/a><\/sup><\/p>\n<h2>What Process Parameter Adjustments Eliminate Dark Spots?<\/h2>\n<figure class=\"wp-block-image size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"800\" height=\"457\" class=\"wp-image-53134\" src=\"https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/prototype-plastic-parts-batch-1.webp\" alt=\"Prototype plastic parts batch\" srcset=\"https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/prototype-plastic-parts-batch-1.webp 1200w, https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/prototype-plastic-parts-batch-1-300x171.webp 300w, https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/prototype-plastic-parts-batch-1-1024x585.webp 1024w, https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/prototype-plastic-parts-batch-1-768x438.webp 768w, https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/prototype-plastic-parts-batch-1-18x10.webp 18w, https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/prototype-plastic-parts-batch-1-600x343.webp 600w\" sizes=\"(max-width: 800px) 100vw, 800px\" \/><figcaption class=\"wp-element-caption\">Batch of injection molded plastic parts<\/figcaption><\/figure>\n<p>Process parameters are the first intervention point when dark spots appear, because they can be adjusted without tooling changes. The following adjustments address the most common parameter-related causes:<\/p>\n<table>\n<thead>\n<tr>\n<th>Parametre<\/th>\n<th>Dark Spot Mechanism<\/th>\n<th>Corrective Adjustment<\/th>\n<th>Expected Result<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Erime s\u0131cakl\u0131\u011f\u0131<\/td>\n<td>Thermal degradation<\/td>\n<td>Reduce by 5\u201310\u00b0C increments<\/td>\n<td>Reduce discoloration at gate\/thin sections<\/td>\n<\/tr>\n<tr>\n<td>Barrel residence time<\/td>\n<td>Prolonged thermal exposure<\/td>\n<td>Reduce shot size or increase cycle rate<\/td>\n<td>Reduce yellow-brown degradation<\/td>\n<\/tr>\n<tr>\n<td>Enjeksiyon h\u0131z\u0131<\/td>\n<td>Shear degradation \/ diesel effect<\/td>\n<td>Reduce fill speed in last 20\u201330% of stroke<\/td>\n<td>Parametre de\u011fi\u015fikliklerinden \u00f6nce temiz bir temel olu\u015fturmak i\u00e7in bir namlu temizli\u011fi \u00e7al\u0131\u015ft\u0131r\u0131n. Bu, kar\u0131\u015ft\u0131r\u0131c\u0131 bir fakt\u00f6r olarak \u00f6l\u00fc b\u00f6lge kontaminasyonunu ortadan kald\u0131r\u0131r.<\/td>\n<\/tr>\n<tr>\n<td>Geri bas\u0131n\u00e7<\/td>\n<td>Shear heat generation<\/td>\n<td>Reduce back pressure setting<\/td>\n<td>Lower overall melt temperature from shear<\/td>\n<\/tr>\n<tr>\n<td>Vida d\u00f6n\u00fc\u015f h\u0131z\u0131<\/td>\n<td>Frictional heat excess<\/td>\n<td>Reduce to minimum adequate level<\/td>\n<td>Reduce shear-generated heat input<\/td>\n<\/tr>\n<tr>\n<td>Cushion (residual)<\/td>\n<td>Dead zone formation in barrel front<\/td>\n<td>Minimize cushion, purge if excessive<\/td>\n<td>Reduce stagnant material at nozzle<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p><strong>Step-by-step diagnostic process:<\/strong><\/p>\n<ol>\n<li>Identify the dark spot type (burn mark, degradation, black speck, contamination)<\/li>\n<li>Note the location pattern (consistent end-of-fill = diesel effect; gate area = shear\/temperature; random = dead zone\/contamination)<\/li>\n<li>Run a purge cycle with commercial purging compound before adjusting parameters<\/li>\n<li>Adjust melt temperature downward in 5\u00b0C increments; assess each batch<\/li>\n<li>If no improvement after temperature reduction, investigate dead zones and vent condition<\/li>\n<\/ol>\n<div class=\"claim claim-true\" style=\"background-color: #eff2ef; border-color: #eff2ef; color: #5b8c70;\">\n<p><svg xmlns=\"http:\/\/www.w3.org\/2000\/svg\" width=\"24\" height=\"24\" viewbox=\"0 0 24 24\" fill=\"none\" stroke=\"#5b8c70\" stroke-width=\"2\" stroke-linecap=\"round\" stroke-linejoin=\"round\"><circle cx=\"12\" cy=\"12\" r=\"10\"\/><polyline points=\"9 12 11 14 15 10\"\/><\/svg> <b>Purging the barrel and screw before a production run reduces dark spot defects significantly.<\/b><span class='claim-true-or-false'>Do\u011fru<\/span><\/p>\n<p class='claim-explanation'>Commercial purging compounds displace degraded and contaminated material from dead zones in the screw, check ring, and barrel. Starting a production run with a thorough purge\u2014especially when changing materials or colors\u2014removes accumulated carbonized deposits that would otherwise contaminate initial shots. This single practice eliminates the majority of black speck and random dark spot defects in well-maintained machines.<\/p>\n<\/div>\n<h2>How Do Mold Design and Maintenance Affect Dark Spot Frequency?<\/h2>\n<figure class=\"wp-block-image size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"800\" height=\"457\" class=\"wp-image-53108\" src=\"https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/injection-molding-quality-testing.webp\" alt=\"Quality inspection of injection molded parts\" srcset=\"https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/injection-molding-quality-testing.webp 1200w, https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/injection-molding-quality-testing-300x171.webp 300w, https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/injection-molding-quality-testing-1024x585.webp 1024w, https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/injection-molding-quality-testing-768x438.webp 768w, https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/injection-molding-quality-testing-18x10.webp 18w, https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/injection-molding-quality-testing-600x343.webp 600w\" sizes=\"(max-width: 800px) 100vw, 800px\" \/><figcaption class=\"wp-element-caption\">Quality inspection of injection molded plastic parts<\/figcaption><\/figure>\n<div class=\"claim claim-false\" style=\"background-color: #f7efef; border-color: #f7efef; color: #db6f85;\">\n<p><svg xmlns=\"http:\/\/www.w3.org\/2000\/svg\" width=\"24\" height=\"24\" viewbox=\"0 0 24 24\" fill=\"none\" stroke=\"#db6f85\" stroke-width=\"2\" stroke-linecap=\"round\" stroke-linejoin=\"round\"><circle cx=\"12\" cy=\"12\" r=\"10\"\/><line x1=\"15\" y1=\"9\" x2=\"9\" y2=\"15\"\/><line x1=\"9\" y1=\"9\" x2=\"15\" y2=\"15\"\/><\/svg> <b>Dark spots always indicate contaminated resin and require changing the entire material batch.<\/b><span class='claim-true-or-false'>Yanl\u0131\u015f<\/span><\/p>\n<p class='claim-explanation'>Dark spots have multiple causes including degraded regrind, overheating at the nozzle, dead zones in the barrel, and air traps\u2014contaminated resin is only one of many possible root causes.<\/p>\n<\/div>\n<div class=\"claim claim-true\" style=\"background-color: #eff2ef; border-color: #eff2ef; color: #5b8c70;\">\n<p><svg xmlns=\"http:\/\/www.w3.org\/2000\/svg\" width=\"24\" height=\"24\" viewbox=\"0 0 24 24\" fill=\"none\" stroke=\"#5b8c70\" stroke-width=\"2\" stroke-linecap=\"round\" stroke-linejoin=\"round\"><circle cx=\"12\" cy=\"12\" r=\"10\"\/><polyline points=\"9 12 11 14 15 10\"\/><\/svg> <b>Purging the barrel and nozzle assembly is a critical first diagnostic step when dark spots appear.<\/b><span class='claim-true-or-false'>Do\u011fru<\/span><\/p>\n<p class='claim-explanation'>Residual degraded material accumulates in dead zones of the barrel and nozzle; thorough purging with a commercial purging compound removes carbonized deposits that cause recurring dark spots.<\/p>\n<\/div>\n<p>While processing parameters address many dark spot causes, mold design and maintenance are critical for preventing defects that cannot be resolved through parameter tuning alone.<\/p>\n<p><strong>Venting design:<\/strong> Adequate venting is the primary mold design factor for preventing diesel-effect burn marks. Vent depth should be 0.02\u20130.05 mm for most thermoplastics (deep enough to allow air escape but shallow enough to prevent flash). Vent land length should be 1.5\u20132 mm, with relief behind to atmosphere. In complex parts, vacuum-assisted venting may be required for features that cannot be conventionally vented.<\/p>\n<p><strong>Runner system geometry:<\/strong> Cold runner systems with dead-end branches or inconsistent flow path lengths promote material stagnation and localized overheating. Balanced runner systems with consistent L\/D ratios and smooth transitions minimize stagnation. For hot runner systems, specifying streamlined flow paths with radiused corners (radius \u2265 tube diameter) at all direction changes eliminates the primary source of dead zone formation.<\/p>\n<p><strong>Gate design:<\/strong> Gates that are too restrictive create high shear rates that locally degrade material. Increasing gate cross-section area by 20\u201330% often resolves gate-area dark spots without compromising cosmetic appearance at the gate mark.<\/p>\n<p><strong>Mold cleaning and maintenance:<\/strong> Mold vents must be cleaned regularly\u2014outgassed material, mold release residue, and colorant particles accumulate and eventually block vents, triggering diesel effect where previously there was none. A standard schedule of vent cleaning every 50,000\u2013100,000 shots (or after each color change) prevents this progressive deterioration through consistent <a href=\"https:\/\/zetarmold.com\/tr\/enjeksiyon-kalibi-sicaklik-kontrolu\/\">enjeksiyon kal\u0131b\u0131 s\u0131cakl\u0131k kontrol\u00fc<\/a><sup id=\"fnref1:4\"><a href=\"#fn:4\">4<\/a><\/sup><\/p>\n<h2>Frequently Asked Questions About Dark Spots on Injection Molded Products<\/h2>\n<p><strong>Q:<\/strong> How do I tell the difference between a diesel burn mark and a thermal degradation dark spot?<br \/>A: Diesel burn marks appear at precisely consistent locations on every part\u2014specifically where air is trapped (end-of-fill, corners, weld line junctions). They have a charred, often pitted texture at the surface. Thermal degradation spots appear in flow-dependent positions (gate area, thin sections) with a more diffuse yellowish-brown gradation rather than a sharp black boundary. The consistency of location is the primary diagnostic indicator.<\/p>\n<p><strong>Q:<\/strong> Why do dark spots appear only after the machine has been running for several hours?<br \/>A: This is a classic indicator of dead zone accumulation. Carbonized material builds up slowly in stagnant zones and is released intermittently in pulses. Early in a production run, the dead zone deposits are minimal; after hours of operation, they reach critical mass and begin contaminating shots. The progressive worsening of dark spot frequency over a production shift is pathognomonic of this mechanism.<\/p>\n<p><strong>Q:<\/strong> Can dark spots come from the masterbatch colorant?<br \/>A: Evet. Uyumsuz masterbatch ta\u015f\u0131y\u0131c\u0131lar\u0131 (renklendirici konsantrenin polimer baz\u0131), temel re\u00e7inenin i\u015fleme s\u0131cakl\u0131\u011f\u0131nda bozunarak renklendirici da\u011f\u0131l\u0131m deseninde koyu lekeler olu\u015fturabilir. Bu, \u00f6zellikle bir re\u00e7ine ailesi i\u00e7in tasarlanm\u0131\u015f bir renklendirici farkl\u0131 bir temel polimer ile kullan\u0131ld\u0131\u011f\u0131nda yayg\u0131nd\u0131r. Masterbatch ta\u015f\u0131y\u0131c\u0131 re\u00e7inesinin, temel malzemenin i\u015fleme s\u0131cakl\u0131k aral\u0131\u011f\u0131 ile uyumlu oldu\u011funu her zaman do\u011frulay\u0131n.<\/p>\n<p><strong>Q:<\/strong> How often should I purge the machine to prevent dark spots?<br \/>A: The frequency depends on the material and production schedule. For heat-sensitive materials (ABS, PVC, PC), purging is recommended at every startup, every planned shutdown, after any production interruption exceeding 15 minutes, and whenever black speck count increases. For more thermally stable materials (PE, PP), purging at startup and shutdown is typically sufficient unless dark spots appear.<\/p>\n<p><strong>Q:<\/strong> Can dark spots be sanded or polished off the part surface?<br \/>A: Only in rare cases where the dark spot is purely a surface deposit (e.g., mold contamination). If the discoloration originates from within the melt (degraded polymer or black specks), it is embedded in or throughout the wall section and cannot be removed by surface treatment. These parts must be rejected.<\/p>\n<p><strong>Q:<\/strong> What is the most effective single action to reduce dark spot frequency in an ongoing production run?<br \/>A: A thorough barrel purge with a reactive or mechanical purging compound followed by a temperature setpoint review. This two-step action removes accumulated dead zone deposits and corrects the primary thermal cause simultaneously\u2014addressing the two most common dark spot mechanisms through optimized <a href=\"https:\/\/zetarmold.com\/tr\/abs-enjeksiyon-kaliplama\/\">ABS injection molding<\/a><sup id=\"fnref1:5\"><a href=\"#fn:5\">5<\/a><\/sup><\/p>\n<h2>Summary: A Systematic Approach to Eliminating Dark Spots in Injection Molding<\/h2>\n<figure class=\"wp-block-image size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"800\" height=\"457\" class=\"wp-image-53145\" src=\"https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/injection-molding-australia-plastic-parts-v2-1.webp\" alt=\"Injection molded plastic parts variety\" srcset=\"https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/injection-molding-australia-plastic-parts-v2-1.webp 1200w, https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/injection-molding-australia-plastic-parts-v2-1-300x171.webp 300w, https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/injection-molding-australia-plastic-parts-v2-1-1024x585.webp 1024w, https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/injection-molding-australia-plastic-parts-v2-1-768x438.webp 768w, https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/injection-molding-australia-plastic-parts-v2-1-18x10.webp 18w, https:\/\/zetarmold.com\/wp-content\/uploads\/2026\/03\/injection-molding-australia-plastic-parts-v2-1-600x343.webp 600w\" sizes=\"(max-width: 800px) 100vw, 800px\" \/><figcaption class=\"wp-element-caption\">Various injection molded plastic parts<\/figcaption><\/figure>\n<p>Dark spots on injection molded products are caused by four distinct mechanisms\u2014thermal degradation, diesel effect, dead zone carbonization, and contamination\u2014each requiring a different corrective action. Attempting to fix all dark spots by simply reducing melt temperature is the most common error: it addresses only one mechanism while leaving the others untouched.<\/p>\n<p>The systematic approach:<\/p>\n<ol>\n<li><strong>Classify by appearance and location:<\/strong> Sharp, pitted black marks at end-of-fill = diesel effect. Diffuse brown discoloration at gate = thermal degradation. Random black specks worsening over a shift = dead zone accumulation. Color-matched specks = masterbatch issue.<\/li>\n<li><strong>Purge first:<\/strong> Run a barrel purge before any parameter changes to establish a clean baseline. This eliminates dead zone contamination as a confounding factor.<\/li>\n<li><strong>Vida Tasar\u0131m\u0131<\/strong> Reduce melt temperature in 5\u00b0C steps, check residence time, review back pressure and screw speed.<\/li>\n<li><strong>Inspect and clean mold vents:<\/strong> Blocked vents are the most common undiagnosed cause of diesel-effect burn marks in mature production tooling.<\/li>\n<li><strong>Evaluate hot runner system:<\/strong> If dark spots persist despite process and vent corrections, a hot runner flow path inspection and temperature profiling is warranted.<\/li>\n<li><strong>Review material handling:<\/strong> Check masterbatch compatibility, drying protocol, and regrind contamination as final-layer causes.<\/li>\n<\/ol>\n<ol class=\"footnotes\">\n<li id=\"fn:1\">\n<p>Systematic diagnosis of injection molding defects using location and appearance classification is the most effective first step before applying any corrective action. <a href=\"#fnref1:1\" rev=\"footnote\">\u21a9<\/a><\/p>\n<\/li>\n<li id=\"fn:2\">\n<p>Residence time and its contribution to dark spot formation are governed by shot utilization and cycle frequency\u2014key injection molding parameters for heat-sensitive materials. <a href=\"#fnref1:2\" rev=\"footnote\">\u21a9<\/a><\/p>\n<\/li>\n<li id=\"fn:3\">\n<p>Dead zone formation in the barrel and hot runner system is a primary driver of progressive black speck defects; injection mold cooling system design also helps minimize thermal gradient dead zones. <a href=\"#fnref1:3\" rev=\"footnote\">\u21a9<\/a><\/p>\n<\/li>\n<li id=\"fn:4\">\n<p>Mold vent cleaning and maintenance schedules are integral to preventing diesel-effect burn marks; precise mold temperature control further reduces outgassing that clogs vents. <a href=\"#fnref1:4\" rev=\"footnote\">\u21a9<\/a><\/p>\n<\/li>\n<li id=\"fn:5\">\n<p>Purging protocols and process optimization for dark spot prevention align with broader principles of ABS injection molding quality control, where heat sensitivity makes systematic process management especially critical. <a href=\"#fnref1:5\" rev=\"footnote\">\u21a9<\/a><\/p>\n<\/li>\n<\/ol>\n<div style=\"background:#f0f4f8;padding:20px;border-radius:8px;margin-top:30px;\">\n<p style=\"margin:0 0 10px;font-size:18px;\"><strong>Need a Quote for Your Injection Molding Project?<\/strong><\/p>\n<p style=\"margin:0 0 10px;\">Get competitive pricing, DFM feedback, and production timeline from ZetarMold\u2019s engineering team.<\/p>\n<p style=\"margin:0;\"><a href=\"https:\/\/zetarmold.com\/tr\/bize-ulasin\/\" style=\"background:#2563eb;color:white;padding:12px 24px;border-radius:6px;text-decoration:none;font-weight:bold;\">Request a Free Quote \u2192<\/a> See our <strong>Injection Molding Complete Guide<\/strong> for a comprehensive overview. See our <a href=\"https:\/\/zetarmold.com\/tr\/injection-molding-complete-guide\/\">Injection Molding Complete Guide<\/a> for a comprehensive overview.<\/p>\n<\/div>","protected":false},"excerpt":{"rendered":"<p>Anahtar \u00c7\u0131kar\u0131mlar Enjeksiyonla kal\u0131planm\u0131\u015f par\u00e7alardaki koyu lekeler \u00fc\u00e7 ana mekanizma nedeniyle olu\u015fur: malzemenin termal bozunmas\u0131 (yan\u0131k izleri), \u00f6l\u00fc b\u00f6lgelerden gelen karbonla\u015fm\u0131\u015f malzeme ve renklendiriciler, ay\u0131r\u0131c\u0131 maddeler veya yabanc\u0131 maddelerden kaynaklanan kirlenme. Koyu lekelerin konumu\u2014kal\u0131p giri\u015finde, dolum sonunda, ak\u0131\u015f \u00e7izgileri boyunca veya iticilerde\u2014k\u00f6k nedenin en h\u0131zl\u0131 te\u015fhis g\u00f6stergesidir [\u2026]<\/p>","protected":false},"author":1,"featured_media":52038,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_seopress_robots_primary_cat":"none","_seopress_titles_title":"How to Solve Dark Spots on Injection Molded Products","_seopress_titles_desc":"Learn what causes dark spots, burn marks, and black specks in injection molded parts\u2014thermal degradation, diesel effect,","_seopress_robots_index":"","_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"footnotes":""},"categories":[46],"tags":[166,165,159,160,187],"meta_box":{"post-to-quiz_to":[]},"_links":{"self":[{"href":"https:\/\/zetarmold.com\/tr\/wp-json\/wp\/v2\/posts\/38388"}],"collection":[{"href":"https:\/\/zetarmold.com\/tr\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/zetarmold.com\/tr\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/zetarmold.com\/tr\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/zetarmold.com\/tr\/wp-json\/wp\/v2\/comments?post=38388"}],"version-history":[{"count":0,"href":"https:\/\/zetarmold.com\/tr\/wp-json\/wp\/v2\/posts\/38388\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/zetarmold.com\/tr\/wp-json\/wp\/v2\/media\/52038"}],"wp:attachment":[{"href":"https:\/\/zetarmold.com\/tr\/wp-json\/wp\/v2\/media?parent=38388"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/zetarmold.com\/tr\/wp-json\/wp\/v2\/categories?post=38388"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/zetarmold.com\/tr\/wp-json\/wp\/v2\/tags?post=38388"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}