{"id":7942,"date":"2025-12-06T09:00:00","date_gmt":"2025-12-06T09:00:00","guid":{"rendered":"https:\/\/uplastech.com\/flow-marks-injection-molding\/"},"modified":"2025-12-06T09:00:00","modified_gmt":"2025-12-06T09:00:00","slug":"flow-marks-injection-molding","status":"publish","type":"post","link":"https:\/\/uplastech.com\/pt\/flow-marks-injection-molding\/","title":{"rendered":"Flow Marks and Record Grooves: Reading the Surface of a Filling Problem"},"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\/flow-marks-injection-molding.jpg\" alt=\"Flow mark depth against injection speed, and severity against melt temperature\" width=\"1536\" height=\"909\"><figcaption>Flow mark depth falls from 18 microns at 10 mm\/s to 1.5 at 200, and a cold melt records every ripple of the front.<\/figcaption><\/figure>\n<div class=\"toc\"><\/div>\n<h2 id=\"what\">What a flow mark is<\/h2>\n<p>A flow mark, sometimes called a record groove, is a series of faint concentric lines or a wavy pattern on the surface of a moulded part, usually radiating from the gate. It is the shape of the advancing flow front, frozen into the surface as the polymer cooled against the mould.<\/p>\n<p>The mechanism is a hesitation effect. The flow front advances in a series of surges: the centre moves ahead, the material near the cold mould wall hesitates, then the centre slows and the wall catches up. Each surge leaves a slight ridge, and if the melt is viscous enough and the wall cold enough, those ridges do not relax before the part solidifies.<\/p>\n<p>That is why flow marks are a filling problem and not a surface problem. They are not caused by a rough tool, and polishing the tool will not remove them. They are caused by the flow front advancing unevenly and the material freezing before it can smooth itself out.<\/p>\n<h2 id=\"speed\">Speed is the first lever<\/h2>\n<p>The depth of the mark falls steeply with injection speed. At 10 mm per second the marks are about 18 microns deep, at 30 they are 11, at 60 they are 6, at 100 they are 3 and at 200 they are 1.5. Below about 3 microns they are invisible on a textured surface and barely detectable on a polished one.<\/p>\n<p>The reason is that a faster front spends less time hesitating. The material reaches the wall and is dragged along by the shear rather than stopping and restarting, and the front advances as a steady wave rather than a series of surges. Faster filling also generates more shear heat, which raises the local temperature and lets the surface relax.<\/p>\n<p>The limit is jetting. Above a certain speed, and below a certain gate size, the melt enters the cavity as a rope rather than a wave, and that produces a much worse defect than a flow mark. The practical window is fast enough to avoid record grooves and slow enough to avoid jetting, and it is usually wider than people think.<\/p>\n<h2 id=\"temperature\">Temperature is the second<\/h2>\n<p>A cold melt is viscous, and a viscous melt does not relax the ridges before it freezes. The severity index runs 9 at a 200 \u00b0C melt, 7 at 220, 5 at 240, 3 at 260 and 1.5 at 280. The relationship is close to linear over that range, which makes it easy to predict: a 20 \u00b0C rise removes about two points of severity.<\/p>\n<p>Mould temperature matters as much and is often neglected. A cold mould chills the surface instantly, which freezes the ridges in place. Raising the mould temperature by 20 \u00b0C is often more effective than raising the melt temperature by 20 \u00b0C, because it keeps the surface mobile for longer rather than making the bulk hotter.<\/p>\n<table>\n<thead>\n<tr>\n<th>Lever<\/th>\n<th>Range<\/th>\n<th>Effect on flow mark depth<\/th>\n<th>Custo<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Injection speed<\/td>\n<td>10 to 200 mm\/s<\/td>\n<td>18 to 1.5 microns<\/td>\n<td>Higher pressure, jetting risk<\/td>\n<\/tr>\n<tr>\n<td>Temperatura de fus\u00e3o<\/td>\n<td>200 to 280 \u00b0C<\/td>\n<td>Severity 9 to 1.5<\/td>\n<td>Longer cooling, degradation risk<\/td>\n<\/tr>\n<tr>\n<td>Mould temperature<\/td>\n<td>30 to 80 \u00b0C<\/td>\n<td>Often the largest effect per degree<\/td>\n<td>Longer cycle<\/td>\n<\/tr>\n<tr>\n<td>Gate size<\/td>\n<td>Larger is better<\/td>\n<td>A larger gate raises the local shear heat<\/td>\n<td>A tool change<\/td>\n<\/tr>\n<tr>\n<td>Material<\/td>\n<td>Higher melt index<\/td>\n<td>Less viscous, so fewer marks<\/td>\n<td>A material change and re-validation<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 1 \u2014 The five levers and what each one buys.<\/p>\n<h2 id=\"gate\">The gate and the runner<\/h2>\n<p>A small gate is a flow mark generator. It forces the melt through a restriction at high shear, which produces a cold, oriented, uneven front that enters the cavity and leaves a record of itself. A larger gate, or a fan gate that spreads the flow, gives a much smoother entry.<\/p>\n<p>The runner matters too. A long, thin runner loses heat and pressure, so the material that arrives at the gate is cooler and slower than the material that left the nozzle. A short, generously sized runner delivers a better front. This is one of the cases where a slightly larger runner costs almost nothing and fixes a surface defect.<\/p>\n<p>Gate position is the third factor. A gate that directs the flow straight at a wall produces a hesitation on the far side and a strong flow mark there. A gate that directs the flow along the wall, or into an open area, produces a much more even front.<\/p>\n<div class=\"callout\">If the flow marks start at the gate and fade along the flow, the gate is the problem. If they cover the whole part, the melt temperature or the fill speed is the problem.<\/div>\n<h2 id=\"material\">Materials that mark easily<\/h2>\n<p>Flow marks are worst in materials with a high viscosity and a sharp melting point, because both prevent the surface from relaxing. Semi-crystalline polymers with a narrow processing window, and filled grades with a high filler loading, are the usual offenders.<\/p>\n<table>\n<thead>\n<tr>\n<th>Material<\/th>\n<th>Tendency<\/th>\n<th>Porqu\u00ea<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>PP, unfilled<\/td>\n<td>Moderado<\/td>\n<td>Crystalline, but low viscosity at normal melt temperatures<\/td>\n<\/tr>\n<tr>\n<td>PP, 30% talc or glass<\/td>\n<td>Elevado<\/td>\n<td>Filler raises viscosity and reduces relaxation<\/td>\n<\/tr>\n<tr>\n<td>PA66<\/td>\n<td>Moderado a elevado<\/td>\n<td>Sharp melting point and fast crystallisation<\/td>\n<\/tr>\n<tr>\n<td>POM<\/td>\n<td>Elevado<\/td>\n<td>Very fast crystallisation, so the surface sets almost immediately<\/td>\n<\/tr>\n<tr>\n<td>ABS<\/td>\n<td>Baixa a moderada<\/td>\n<td>Amorphous, so the surface relaxes more readily<\/td>\n<\/tr>\n<tr>\n<td>PC<\/td>\n<td>Baixa<\/td>\n<td>High viscosity but slow to freeze, so ridges relax<\/td>\n<\/tr>\n<tr>\n<td>PMMA<\/td>\n<td>Baixa<\/td>\n<td>Amorphous and slow to set<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 2 \u2014 How prone each material is to flow marks, and why.<\/p>\n<p>The practical consequence is that a tool that runs a clean part in ABS may produce visible marks in a talc-filled PP with the same settings, and the settings have to change with the material rather than being carried over. That is obvious in theory and frequently ignored in a shop that runs both.<\/p>\n<h2 id=\"differential\">Telling it from the neighbours<\/h2>\n<p>Three surface defects are routinely confused with flow marks, and they have different causes.<\/p>\n<ul>\n<li>A weld line is a single line where two fronts meet, not a series of ripples. It is a strength problem as well as a cosmetic one.<\/li>\n<li>A colour streak is a line of different colour, not a ridge in the surface. It comes from dispersion, not from flow.<\/li>\n<li>A drag mark or a scratch is a single line with a direction, and it is caused by ejection or handling, not by filling.<\/li>\n<li>A gate blush is a dull patch right at the gate, caused by shear, and it does not extend along the flow.<\/li>\n<\/ul>\n<p>The distinction is worth making with a fingertip as well as an eye. A flow mark is a ridge you can feel with a fingernail across the flow direction; a colour streak is not. That test takes a second and it settles the diagnosis before anyone touches a machine setting.<\/p>\n<h2 id=\"fix\">A sequence that works<\/h2>\n<ol>\n<li>Raise the injection speed in steps and watch the marks. Most flow marks disappear between 60 and 120 mm\/s.<\/li>\n<li>If speed alone does not do it, raise the mould temperature by 10 to 20 \u00b0C. This is often the single most effective change.<\/li>\n<li>Raise the melt temperature within the material's window, in 10 \u00b0C steps, and stop at the first sign of degradation.<\/li>\n<li>Check the gate size and the runner. If the marks start sharply at the gate, this is where the answer is.<\/li>\n<li>Consider a higher melt index grade only after the process and the gate have been exhausted.<\/li>\n<li>If the part is textured, check whether the marks are actually visible in the texture before spending money on a fix.<\/li>\n<\/ol>\n<p>The last item is the one that saves the most money. A 6 micron flow mark on a matte texture is invisible, and a part that is cosmetically acceptable should not be re-engineered because a polished sample showed a defect that the production texture hides.<\/p>\n<div class=\"faq\">\n<h3>What causes flow marks in injection molding?<\/h3>\n<p>The advancing flow front hesitating and leaving a ridge in the surface, which then freezes before it can relax. It is a filling problem, not a tool surface problem.<\/p>\n<h3>Does faster injection remove flow marks?<\/h3>\n<p>Usually yes. Depth falls from about 18 microns at 10 mm\/s to 1.5 at 200. The limit is jetting, which appears if the gate is too small for the speed.<\/p>\n<h3>Is mould temperature or melt temperature more effective?<\/h3>\n<p>Often the mould temperature, because it keeps the surface mobile for longer instead of making the bulk hotter. A 20 \u00b0C rise in mould temperature can remove marks that a melt temperature rise will not.<\/p>\n<h3>How do I tell a flow mark from a weld line?<\/h3>\n<p>A flow mark is a series of ripples across the whole flow path; a weld line is a single line where two fronts meet. Run a fingernail across the flow direction: a flow mark is a ridge, a weld line is a groove.<\/p>\n<h3>Can polishing the tool fix flow marks?<\/h3>\n<p>No. The marks are in the polymer, not the steel. Polishing changes how visible they are but does not remove them.<\/p>\n<h3>Why does my talc-filled PP mark when ABS did not?<\/h3>\n<p>Filler raises the viscosity and reduces relaxation, so the ridges freeze in. The settings have to change with the material rather than being carried over.<\/p>\n<\/div>\n<div class=\"cta\">\n<p class=\"cta-h\">Surface marks that will not polish out?<\/p>\n<p>Send the material, the gate detail and a photo of the mark. We will tell you whether it is a flow mark, a weld line or a colour issue, and which setting or tool change fixes it.<\/p>\n<p><a class=\"btn\" href=\"https:\/\/uplastech.com\/pt\/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\/pt\/injection-molding-defects\/\">Injection Molding Defects: 12 Problems, Their Causes and Fixes<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/pt\/weld-line-strength\/\">Weld Lines in Injection Molding: Why the Part Breaks There<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/pt\/injection-molding-material-selection\/\">Injection Molding Material Selection: 14 Plastics Compared<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/pt\/colour-matching-injection-molding\/\">Colour Matching in Injection Molding: Masterbatch and Drift<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/pt\/scientific-molding-five-tests\/\">Scientific Molding: The Five Studies and Why the Order Matters<\/a><\/li>\n<\/ul>\n<\/div>\n<\/div>","protected":false},"excerpt":{"rendered":"<p>A flow mark is the flow front frozen into the surface, 18 microns deep at 10 mm\/s and 1.5 at 200. It is a filling problem, not a polishing problem, and the fix is almost always speed and temperature.<\/p>","protected":false},"author":1,"featured_media":7941,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_seopress_titles_title":"Flow Marks and Record Grooves in Molding","_seopress_titles_desc":"Flow marks and record grooves explained: how the flow front leaves a record in the surface, depth against injection speed and melt temperature, gate and runner","_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-7942","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\/pt\/wp-json\/wp\/v2\/posts\/7942","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/uplastech.com\/pt\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/uplastech.com\/pt\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/uplastech.com\/pt\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/uplastech.com\/pt\/wp-json\/wp\/v2\/comments?post=7942"}],"version-history":[{"count":0,"href":"https:\/\/uplastech.com\/pt\/wp-json\/wp\/v2\/posts\/7942\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/uplastech.com\/pt\/wp-json\/wp\/v2\/media\/7941"}],"wp:attachment":[{"href":"https:\/\/uplastech.com\/pt\/wp-json\/wp\/v2\/media?parent=7942"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/uplastech.com\/pt\/wp-json\/wp\/v2\/categories?post=7942"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/uplastech.com\/pt\/wp-json\/wp\/v2\/tags?post=7942"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}