{"id":7974,"date":"2025-12-22T09:00:00","date_gmt":"2025-12-22T09:00:00","guid":{"rendered":"https:\/\/uplastech.com\/black-spots-contamination\/"},"modified":"2025-12-22T09:00:00","modified_gmt":"2025-12-22T09:00:00","slug":"black-spots-contamination","status":"publish","type":"post","link":"https:\/\/uplastech.com\/ko\/black-spots-contamination\/","title":{"rendered":"Black Spots and Contamination: Tracing Carbon Through the Barrel"},"content":{"rendered":"<style>\n.upl-blog{font-size:17px;line-height:1.78;color:#1f2937}\n.upl-blog p{margin:0 0 18px}\n.upl-blog h2{font-size:26px;line-height:1.3;color:#0b2b33;margin:46px 0 14px;font-weight:700;letter-spacing:-.015em}\n.upl-blog h3{font-size:19.5px;color:#0d3b47;margin:28px 0 10px;font-weight:700}\n.upl-blog ul,.upl-blog ol{margin:0 0 20px;padding-left:22px}\n.upl-blog li{margin:0 0 9px}\n.upl-blog figure{margin:0 0 34px}\n.upl-blog figure img{width:100%;height:auto;display:block;border-radius:10px}\n.upl-blog figcaption{font-size:13px;color:#6b7280;margin-top:10px;line-height:1.5}\n.upl-blog .toc{background:none;border:0;border-radius:0;padding:0;margin:0 0 34px}\n.upl-blog .toc .lwptoc{margin:0}\n.upl-blog .toc h4{margin:0 0 10px;font-size:13px;letter-spacing:.1em;text-transform:uppercase;color:#0d5563}\n.upl-blog .toc ol{margin:0;padding-left:18px;font-size:14.5px}\n.upl-blog .toc li{margin-bottom:6px}\n.upl-blog table{width:100%;border-collapse:collapse;font-size:14.5px;margin:0 0 10px}\n.upl-blog th{background:#0d3b47;color:#fff;text-align:left;padding:11px 13px;font-weight:600;font-size:13.5px;border:0}\n.upl-blog td{padding:11px 13px;border:0;border-bottom:1px solid #e8ebed;vertical-align:top}\n.upl-blog tbody tr:nth-child(even){background:#fafbfc}\n.upl-blog .tablecap{font-size:12.5px;color:#6b7280;margin:0 0 30px}\n.upl-blog .myth{background:#fff8ef;border:1px solid #f6ddbd;border-radius:8px;padding:16px 20px;margin:0 0 26px;font-size:15.5px}\n.upl-blog .myth b{color:#a85c10}\n.upl-blog .callout{background:#f2f8f9;border-radius:8px;padding:18px 22px;margin:0 0 26px;font-size:15.5px;border-left:3px solid #14707f}\n.upl-blog .faq h3{border-top:1px solid #eceff1;padding-top:22px;margin-top:0;font-size:18px}\n.upl-blog .faq h3:first-of-type{border-top:0;padding-top:0}\n.upl-blog .cta{background:linear-gradient(135deg,#0b2b33,#0d5563);color:#fff;border-radius:12px;padding:34px;margin:48px 0 30px}\n.upl-blog .cta h2,.upl-blog .cta .cta-h{color:#fff;margin:0 0 10px;font-size:24px;font-weight:700;line-height:1.3}\n.upl-blog .cta p{color:#c9dee3;margin:0 0 20px;font-size:16px}\n.upl-blog .btn{display:inline-block;background:#e07a1f;color:#fff;text-decoration:none;font-weight:700;padding:13px 26px;border-radius:8px;font-size:15.5px}\n.upl-blog .rel{border-top:1px solid #e8ebed;margin-top:40px;padding-top:26px}\n.upl-blog .rel h4{font-size:13px;letter-spacing:.1em;text-transform:uppercase;color:#6b7280;margin:0 0 14px}\n.upl-blog .rel ul{list-style:none;padding:0;margin:0}\n.upl-blog .rel li{padding:11px 0;border-bottom:1px dashed #e8ebed}\n.upl-blog .rel a{text-decoration:none;font-weight:600;font-size:15.5px}\n@media(max-width:640px){.upl-blog h2{font-size:22px}.upl-blog table{font-size:13px}.upl-blog .cta{padding:24px}}\n\/* \u5355\u7bc7\u9875\u9690\u85cf WhatsApp \u95ee\u5019\u5361\uff0c\u53ea\u4fdd\u7559\u5706\u5f62\u6c14\u6ce1\uff0c\u907f\u514d\u906e\u6321\u4fa7\u680f\u8be2\u76d8\u63d0\u4ea4\u6309\u94ae\u3002\n   \u6ce8\u610f\uff1aClick to Chat \u7684\u70b9\u51fb\u903b\u8f91\u662f\u300c\u95ee\u5019\u5361\u5143\u7d20\u5b58\u5728\u5c31\u4e0d\u8df3\u8f6c\u300d\uff0c\u53ea\u9760 CSS \u9690\u85cf\u4f1a\u8ba9\u6c14\u6ce1\u53d8\u6210\u6b7b\u94fe\u63a5\u3002\n   Code Snippets #9\u300cUPL single post: WhatsApp bubble jumps straight to chat\u300d\u8d1f\u8d23\u628a\u8be5\u8282\u70b9\u4ece DOM \u79fb\u9664\uff0c\n   \u4e0b\u9762\u8fd9\u6761 CSS \u53ea\u662f\u79fb\u9664\u524d\u7684\u89c6\u89c9\u515c\u5e95\u3002\u5220 #9 \u524d\u5fc5\u987b\u5148\u5220\u8fd9\u6761\u89c4\u5219\u3002 *\/\nbody.single-post .ht_ctc_chat_greetings_box{display:none !important}\n<\/style>\n<div class=\"upl-blog\">\n<figure>\n<img fetchpriority=\"high\" decoding=\"async\" src=\"https:\/\/uplastech.com\/wp-content\/uploads\/2026\/10\/black-spots-contamination.jpg\" alt=\"Black spots per thousand parts against melt residence time, and the share of contamination sources\" width=\"1536\" height=\"909\"><figcaption>Spot count rises from 3 per 1000 parts at a two minute residence to 60 at ten, and 34 percent of cases trace to barrel dead spots.<\/figcaption><\/figure>\n<div class=\"toc\"><\/div>\n<h2 id=\"what\">A black spot is degraded polymer<\/h2>\n<p>A black spot is a particle of material that has been hot for too long. It may be the same polymer as the part, oxidised and carbonised, or it may be a foreign material that has picked up carbon from the barrel. Either way it is a thermal history problem before it is a cleanliness problem.<\/p>\n<p>The particle forms in the places where material sits still: the clearance between the screw flight and the barrel, the dead spot behind the non-return valve, the corners of the nozzle, and the space under a worn check ring. Material in those places is replenished slowly, so it sees dozens of cycles at temperature while the rest of the melt passes through in a few minutes.<\/p>\n<p>Once formed, the particle is carried into the part at random. That is why black spots come in bursts: a particle dislodges, travels for a while, and then another one lets go. A part with one spot and a part with twenty can come from the same shot sequence.<\/p>\n<h2 id=\"residence\">Residence time is the multiplier<\/h2>\n<p>The relationship between residence time and spot count is steep. At a two minute melt residence a clean barrel produces about 3 spots per 1000 parts; at four minutes 8, at six 18, at eight 35 and at ten 60. Ten minutes gives twenty times the spots of two.<\/p>\n<p>Residence time is set by the shot size relative to the barrel capacity, the screw recovery time and the cycle. A small shot in a large barrel is the classic cause: a 20 percent shot in a 200 gram barrel means most of the material sits through five cycles before it is used.<\/p>\n<p>This is why the standard advice is to keep the shot between 25 and 75 percent of the barrel capacity, and to prefer a smaller barrel on a dedicated machine. It is also why a job moved to a larger press to get more clamp often starts producing spots: the clamp was fine, the barrel was not.<\/p>\n<table>\n<thead>\n<tr>\n<th>Residence time<\/th>\n<th>Spots per 1000 parts<\/th>\n<th>Relative<\/th>\n<th>Verdict<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>2 min<\/td>\n<td>3<\/td>\n<td>1.0<\/td>\n<td>Clean<\/td>\n<\/tr>\n<tr>\n<td>4 min<\/td>\n<td>8<\/td>\n<td>2.7<\/td>\n<td>Acceptable<\/td>\n<\/tr>\n<tr>\n<td>6 min<\/td>\n<td>18<\/td>\n<td>6.0<\/td>\n<td>Cosmetic risk on light colours<\/td>\n<\/tr>\n<tr>\n<td>8 min<\/td>\n<td>35<\/td>\n<td>12<\/td>\n<td>Rejects<\/td>\n<\/tr>\n<tr>\n<td>10 min<\/td>\n<td>60<\/td>\n<td>20<\/td>\n<td>Stop and fix the process<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 1 \u2014 Spot count against melt residence time on a clean barrel.<\/p>\n<h2 id=\"source\">Where it actually comes from<\/h2>\n<p>When a shop traces its contamination cases, the distribution is consistent and it is not what people expect. Barrel dead spots account for about 34 percent, colour and additive masterbatches 22 percent, regrind 18 percent, hopper and airborne contamination 14 percent and screw wear 12 percent.<\/p>\n<p>The barrel is first because it is where the heat is. A non-return valve with a worn seat, a screw with a rounded flight, or a nozzle with a step in the bore will hold material indefinitely, and no amount of purging removes a dead spot that is refilled every cycle.<\/p>\n<p>Masterbatch is second because a carrier that is incompatible with the base resin will not disperse, and the undispersed particles carbonise at the screw tip. Regrind is third because it carries dust, and because it has already had a heat history. Neither is a fault of the machine.<\/p>\n<p>The practical tracing method is elimination, in order of cost. Run virgin material only; if the spots stop, the problem is regrind or masterbatch. Then purge with a commercial purging compound and inspect the purge slug for black specks; if it comes out dirty, the barrel is the source. Then pull the screw and look at the flights and the check ring.<\/p>\n<h2 id=\"purge\">Purge, and purge properly<\/h2>\n<p>Purging is not the same as running through. Running material through a barrel displaces the melt in the flow path but leaves the dead spots untouched, because the material in a dead spot is not in the flow path. A purge compound works by being more viscous and more abrasive than the process material, so it scours the surfaces it passes.<\/p>\n<p>The method matters as much as the compound. The screw should be run at a moderate back pressure with the barrel hot, the purge should be allowed to sit for a minute or two so it can soften the deposit, and the process should be repeated until the slug is clean. A single pass at production settings does very little.<\/p>\n<p>For a barrel with a real dead spot, purging is a holding action. The permanent fix is to replace the non-return valve, or to have the screw and barrel measured and reconditioned. A shop that purges every week and still sees spots has a mechanical problem, not a housekeeping one.<\/p>\n<table>\n<thead>\n<tr>\n<th>What the purge slug shows<\/th>\n<th>Likely cause<\/th>\n<th>Next step<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Clean, no specks<\/td>\n<td>Barrel is not the source<\/td>\n<td>Check regrind and masterbatch<\/td>\n<\/tr>\n<tr>\n<td>Fine black specks throughout<\/td>\n<td>General degradation, long residence<\/td>\n<td>Reduce shot size or melt temperature<\/td>\n<\/tr>\n<tr>\n<td>Specks only at the start<\/td>\n<td>Material sitting in the nozzle<\/td>\n<td>Check the nozzle bore and heater band<\/td>\n<\/tr>\n<tr>\n<td>Large flakes<\/td>\n<td>Cured skin from the hopper or the feed throat<\/td>\n<td>Inspect the feed zone and the hopper<\/td>\n<\/tr>\n<tr>\n<td>Streaks of a different colour<\/td>\n<td>Residue from a previous job<\/td>\n<td>Longer purge with a scouring compound<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 2 \u2014 Reading the purge slug, which is the cheapest diagnostic available.<\/p>\n<div class=\"callout\">If the spots come back within a shift of purging, stop purging and pull the screw. Repeated purging is a cost with no endpoint.<\/div>\n<h2 id=\"wear\">Screw and barrel wear<\/h2>\n<p>A screw and barrel are a wear pair, and the clearance between the flight and the barrel grows with every tonne of glass-filled or mineral-filled material that passes through. When the clearance exceeds about 0.15 mm on a 40 mm screw, the melt starts to slip back over the flights, the recovery time rises, and the residence time with it.<\/p>\n<p>Wear also creates the dead spot that produces the spots. A worn flight has a rounded edge, and the material that sits under the round is never exchanged. That is why a machine that has run glass-filled nylon for years will produce spots on a natural polypropylene job even after a thorough purge.<\/p>\n<p>The measurement is straightforward: pull the screw, measure the flight diameter at several points, and compare with the barrel bore. A shop that does this annually knows when to plan a replacement, and a shop that does not finds out when a cosmetic job fails.<\/p>\n<h2 id=\"check\">What to check, in order<\/h2>\n<p>A contamination investigation should follow cost order, cheapest first, and stop as soon as the source is found.<\/p>\n<ol>\n<li>Check the shot size as a percentage of barrel capacity. Below 25 percent, residence time is the likely cause.<\/li>\n<li>Run virgin material with no regrind and no masterbatch for 50 shots and inspect.<\/li>\n<li>Purge with a commercial compound and examine the slug for specks.<\/li>\n<li>Inspect the hopper, the dryer and the area around the machine for dust and spillage.<\/li>\n<li>Pull the nozzle and look for a step or a carbon ring in the bore.<\/li>\n<li>Pull the screw, measure the flights, and inspect the check ring and seat.<\/li>\n<\/ol>\n<p>The first item costs nothing and finds a surprising share of the cases. A job that was moved to a bigger press for clamp force, or a mould that was reworked to a smaller shot, will both change the residence time without anyone noticing.<\/p>\n<div class=\"faq\">\n<h3>What causes black spots in injection moulding?<\/h3>\n<p>Degraded polymer that has been hot for too long, usually in a barrel dead spot, behind a worn check ring, or in an undispersed masterbatch particle. Residence time is the multiplier.<\/p>\n<h3>How does residence time affect black spots?<\/h3>\n<p>Steeply. Three spots per 1000 parts at two minutes becomes 60 at ten minutes. Keep the shot between 25 and 75 percent of barrel capacity.<\/p>\n<h3>Does purging remove black spots?<\/h3>\n<p>It removes loose contamination but not a dead spot, because material in a dead spot is not in the flow path. If spots return within a shift, the cause is mechanical.<\/p>\n<h3>Why did black spots start after moving to a bigger machine?<\/h3>\n<p>Because the shot became a smaller fraction of the barrel, so the residence time rose. The clamp was the reason for the move, but the barrel is what changed the defect rate.<\/p>\n<h3>Can regrind cause black spots?<\/h3>\n<p>Yes. Regrind carries dust and has already had a heat history. Running virgin only for 50 shots is the quickest way to find out whether it is the cause.<\/p>\n<h3>How much screw wear is too much?<\/h3>\n<p>About 0.15 mm of flight clearance on a 40 mm screw. Beyond that the melt slips back, recovery time rises and the dead spot behind the rounded flight grows.<\/p>\n<\/div>\n<div class=\"cta\">\n<p class=\"cta-h\">Chasing black spots on a cosmetic part?<\/p>\n<p>Tell us the material, the shot size and the barrel capacity. We will work out the residence time first, because that is free to check and it explains a third of the cases.<\/p>\n<p><a class=\"btn\" href=\"https:\/\/uplastech.com\/ko\/custom-injection-molding\/\">Talk to an injection molding engineer<\/a>\n<\/div>\n<div class=\"rel\">\n<h4>Continue reading<\/h4>\n<ul>\n<li><a href=\"https:\/\/uplastech.com\/ko\/silver-streak-moisture\/\">Silver Streak and Moisture: Drying Before You Blame the Tool<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/ko\/burn-marks-diesel-effect\/\">Burn Marks and the Diesel Effect: Trapped Gas in a Cavity<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/ko\/flow-marks-injection-molding\/\">Flow Marks in Injection Molding: Why the Front Stalls<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/ko\/flash-injection-molding\/\">Flash in Injection Molding: Clamp, Vent and Tool Wear<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/ko\/custom-injection-molding\/\">Custom Injection Molding: Tooling, Materials and Volumes<\/a><\/li>\n<\/ul>\n<\/div>\n<\/div>","protected":false},"excerpt":{"rendered":"<p>Spot count goes from 3 per 1000 parts at two minutes of residence to 60 at ten. Two thirds of contamination starts before the melt, in the barrel, the additive or the regrind, not at the nozzle.<\/p>","protected":false},"author":1,"featured_media":7973,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_seopress_titles_title":"Black Spots in Injection Molding: Causes","_seopress_titles_desc":"Black spots and contamination in injection moulding: degradation against melt residence time, where contamination starts, purging compounds, screw and barrel we","_seopress_robots_index":"","_seopress_robots_follow":"","_seopress_robots_imageindex":"","_seopress_robots_snippet":"","_seopress_robots_primary_cat":"35","_seopress_robots_breadcrumbs":"","_seopress_robots_freeze_modified_date":"","_seopress_robots_custom_modified_date":"","_seopress_robots_canonical":"","_seopress_social_fb_title":"","_seopress_social_fb_desc":"","_seopress_social_fb_img":"","_seopress_social_fb_img_attachment_id":0,"_seopress_social_fb_img_width":0,"_seopress_social_fb_img_height":0,"_seopress_social_twitter_title":"","_seopress_social_twitter_desc":"","_seopress_social_twitter_img":"","_seopress_social_twitter_img_attachment_id":0,"_seopress_social_twitter_img_width":0,"_seopress_social_twitter_img_height":0,"_seopress_redirections_value":"","_seopress_redirections_enabled":"","_seopress_redirections_enabled_regex":"","_seopress_redirections_logged_status":"","_seopress_redirections_param":"","_seopress_redirections_type":0,"_seopress_analysis_target_kw":"","footnotes":""},"categories":[35],"tags":[],"class_list":["post-7974","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-injection-molding"],"meta_box":{"post-to-quiz_to":[]},"_links":{"self":[{"href":"https:\/\/uplastech.com\/ko\/wp-json\/wp\/v2\/posts\/7974","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/uplastech.com\/ko\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/uplastech.com\/ko\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/uplastech.com\/ko\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/uplastech.com\/ko\/wp-json\/wp\/v2\/comments?post=7974"}],"version-history":[{"count":0,"href":"https:\/\/uplastech.com\/ko\/wp-json\/wp\/v2\/posts\/7974\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/uplastech.com\/ko\/wp-json\/wp\/v2\/media\/7973"}],"wp:attachment":[{"href":"https:\/\/uplastech.com\/ko\/wp-json\/wp\/v2\/media?parent=7974"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/uplastech.com\/ko\/wp-json\/wp\/v2\/categories?post=7974"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/uplastech.com\/ko\/wp-json\/wp\/v2\/tags?post=7974"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}