{"id":7689,"date":"2025-08-20T09:00:00","date_gmt":"2025-08-20T08:00:00","guid":{"rendered":"https:\/\/uplastech.com\/colour-changeover-in-extrusion\/"},"modified":"2025-08-20T09:00:00","modified_gmt":"2025-08-20T08:00:00","slug":"colour-changeover-in-extrusion","status":"publish","type":"post","link":"https:\/\/uplastech.com\/it\/colour-changeover-in-extrusion\/","title":{"rendered":"Colour Changeover in Extrusion: Purge Compounds, Sequence and Downtime"},"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 \u81ea\u52a8\u5c55\u5f00\u7684\u95ee\u5019\u5361\uff0c\u53ea\u4fdd\u7559\u5706\u5f62\u6c14\u6ce1\uff0c\u907f\u514d\u906e\u6321\u4fa7\u680f\u8be2\u76d8\u63d0\u4ea4\u6309\u94ae *\/\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\/colour-changeover-in-extrusion.jpg\" alt=\"Chart of colour transition during extrusion changeover showing purge cycles and the time to reach acceptable colour match\" width=\"1536\" height=\"909\"><figcaption>Colour changeover is a curve, not an event. The first metres are unusable, the middle is debatable, and only the last stretch meets the colour standard \u2014 the job is to shorten the middle.<\/figcaption><\/figure>\n<div class=\"toc\"><\/div>\n<h2 id=\"cost\">What a colour change actually costs<\/h2>\n<p>Ask a production manager what a colour change costs and the answer is usually \"two hours of downtime\". That is the small part. The real cost is the material that goes through the line before the product is saleable, plus the labour, plus the risk of a colour that is close but not close enough.<\/p>\n<p>On a profile line running 150 kg\/h with a two-hour changeover, the material consumed is 300 kg. If half of that is scrap, the changeover costs 150 kg of material plus two hours of machine time plus the labour. On a coloured engineering compound that can be more than the value of the order.<\/p>\n<div class=\"callout\">\n<p><strong>The number that matters:<\/strong> the transition length \u2014 how many metres of product pass before the colour is within tolerance. On a well-run line changing from dark to light, that is 30\u201380 m. On a badly run line it can be 300 m, and the difference is almost entirely method rather than equipment.<\/p>\n<\/div>\n<h2 id=\"sequence\">The sequence that works<\/h2>\n<p>Colour changeover is a mechanical cleaning problem disguised as a material problem. No purge compound removes degraded material from a dead spot in the die; only dismantling and cleaning does. The sequence below assumes both are needed.<\/p>\n<table>\n<thead>\n<tr>\n<th>Passo<\/th>\n<th>Azione<\/th>\n<th>Time<\/th>\n<th>Scopo<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>1. Stop feed<\/td>\n<td>Empty the hopper, run the screw dry<\/td>\n<td>2 \u2013 5 min<\/td>\n<td>Removes the old colour from the feed zone<\/td>\n<\/tr>\n<tr>\n<td>2. Purge (mechanical)<\/td>\n<td>Run a purging compound through<\/td>\n<td>5 \u2013 15 min<\/td>\n<td>Displaces old melt from barrel and screw<\/td>\n<\/tr>\n<tr>\n<td>3. Purge (colour)<\/td>\n<td>Run the new colour through<\/td>\n<td>5 \u2013 10 min<\/td>\n<td>Displaces the purge compound<\/td>\n<\/tr>\n<tr>\n<td>4. Die clean<\/td>\n<td>Open and clean the die, adapters, screen pack<\/td>\n<td>10 \u2013 30 min<\/td>\n<td>Removes the material no purge can reach<\/td>\n<\/tr>\n<tr>\n<td>5. Reassemble and stabilise<\/td>\n<td>Run to temperature, check output<\/td>\n<td>10 \u2013 20 min<\/td>\n<td>Returns to steady state<\/td>\n<\/tr>\n<tr>\n<td>6. Transition run<\/td>\n<td>Run until colour matches<\/td>\n<td>5 \u2013 20 min<\/td>\n<td>The unavoidable tail<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 1 \u2014 The six steps of a colour changeover. Step 4 is the one that gets skipped under time pressure, and it is the one that causes the 300 m transition.<\/p>\n<p>The order matters. Purging before dismantling pushes most of the old material out while the line is still assembled; purging after dismantling wastes compound because the barrel is already partly empty. Running the new colour through before cleaning the die contaminates the die with the old colour and extends the transition.<\/p>\n<h2 id=\"purge\">Purge compounds: what they do and do not do<\/h2>\n<table>\n<thead>\n<tr>\n<th>Purge type<\/th>\n<th>Mechanism<\/th>\n<th>Il migliore per<\/th>\n<th>Limitation<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Mechanical (abrasive filled)<\/td>\n<td>Scours the barrel and screw with filler<\/td>\n<td>Carbon deposit, degraded material<\/td>\n<td>Wears screws; not for all resins<\/td>\n<\/tr>\n<tr>\n<td>Chemical (foaming)<\/td>\n<td>Releases gas that lifts deposits off surfaces<\/td>\n<td>Hot runner, dead spots<\/td>\n<td>Needs dwell time; can leave residue<\/td>\n<\/tr>\n<tr>\n<td>Displacing (high viscosity)<\/td>\n<td>Pushes old melt out by plug flow<\/td>\n<td>Simple colour changes<\/td>\n<td>Poor at cleaning, good at displacing<\/td>\n<\/tr>\n<tr>\n<td>Combination<\/td>\n<td>Mechanical plus chemical<\/td>\n<td>Difficult changes<\/td>\n<td>Most expensive<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 2 \u2014 Purge compound types. Most colour changes only need a displacing purge; the abrasive types are for cleaning, not for colour.<\/p>\n<p>The common mistake is using an abrasive purge for a colour change. It scours the barrel, which is not the problem, and it leaves its own residue, which is. A simple displacing purge \u2014 often the new colour's own material, run generously \u2014 is usually faster and cleaner.<\/p>\n<p>How much purge is enough? A practical rule is two to three times the barrel volume. A 60 mm extruder with a 30:1 screw holds roughly 4\u20135 kg of melt, so 10\u201315 kg of purge. Running less than that simply mixes old and new rather than displacing.<\/p>\n<h2 id=\"direction\">Dark to light versus light to dark<\/h2>\n<p>The direction of the change matters enormously, and it is not symmetrical.<\/p>\n<table>\n<thead>\n<tr>\n<th>Direction<\/th>\n<th>Difficulty<\/th>\n<th>Perch\u00e9<\/th>\n<th>Strategia<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Light to dark<\/td>\n<td>Easy<\/td>\n<td>A small amount of dark pigment hides light residue<\/td>\n<td>Short purge, minimal cleaning<\/td>\n<\/tr>\n<tr>\n<td>Dark to light<\/td>\n<td>Hard<\/td>\n<td>Even traces of dark pigment show<\/td>\n<td>Long purge, full die clean, sometimes a mid-tone<\/td>\n<\/tr>\n<tr>\n<td>Colour to colour (similar)<\/td>\n<td>Moderato<\/td>\n<td>Hue shift shows before lightness shift<\/td>\n<td>Standard purge<\/td>\n<\/tr>\n<tr>\n<td>Colour to natural<\/td>\n<td>Hardest<\/td>\n<td>No pigment to hide anything<\/td>\n<td>Full strip-down; consider a dedicated screw<\/td>\n<\/tr>\n<tr>\n<td>Natural to colour<\/td>\n<td>Easy<\/td>\n<td>Pigment covers residue<\/td>\n<td>Short purge<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 3 \u2014 Changeover difficulty by direction. Dark to natural is the case where a plant will genuinely consider a second screw.<\/p>\n<p>The mid-tone trick is worth knowing for dark-to-light changes: run a grey or beige transition colour between the black and the white. The mid-tone absorbs the dark residue quickly, and the white then has less to displace. It costs an extra purge step but often shortens the total transition.<\/p>\n<h2 id=\"die\">The die is where the time goes<\/h2>\n<p>Most of a long transition happens in the die, not in the barrel. The die has dead spots: the land, the corners of the flow channel, the space behind the lip, the interface with the adapter. Material sits there and bleeds out slowly over hundreds of metres.<\/p>\n<table>\n<thead>\n<tr>\n<th>Die feature<\/th>\n<th>Dead spot<\/th>\n<th>Effect on transition<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Land section<\/td>\n<td>Long parallel walls hold a film of old melt<\/td>\n<td>Continuous thin streaks<\/td>\n<\/tr>\n<tr>\n<td>Sharp internal corners<\/td>\n<td>Material recirculates in the corner<\/td>\n<td>Streaks that persist for hundreds of metres<\/td>\n<\/tr>\n<tr>\n<td>Die lip adjustment screws<\/td>\n<td>Threaded holes hold old material<\/td>\n<td>Occasional dark specks<\/td>\n<\/tr>\n<tr>\n<td>Adapter and screen changer<\/td>\n<td>Large volume, slow turnover<\/td>\n<td>Long initial transition<\/td>\n<\/tr>\n<tr>\n<td>Thermocouple wells<\/td>\n<td>Blind holes<\/td>\n<td>Specks that appear intermittently<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 4 \u2014 Where old colour hides. The sharp internal corner is the worst offender and it is also the easiest to fix at the design stage.<\/p>\n<p>This is the argument for streamlined die design. A die with radiused internal corners and no blind holes will change colour in a fraction of the time of one with sharp corners and thermocouple wells drilled into the flow path. It is worth asking about when buying a die, and it is rarely mentioned in quotations.<\/p>\n<h2 id=\"planning\">Scheduling to minimise changeovers<\/h2>\n<table>\n<thead>\n<tr>\n<th>Practice<\/th>\n<th>Effect<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Sequence light \u2192 dark within a day<\/td>\n<td>Every change after the first is easy<\/td>\n<\/tr>\n<tr>\n<td>Group all natural\/natural-colour runs<\/td>\n<td>One hard change instead of several<\/td>\n<\/tr>\n<tr>\n<td>Run the darkest colour last before a shutdown<\/td>\n<td>Cleaning happens during planned downtime<\/td>\n<\/tr>\n<tr>\n<td>Keep a dedicated screw for natural<\/td>\n<td>Eliminates the hardest change entirely<\/td>\n<\/tr>\n<tr>\n<td>Batch by die, not just by colour<\/td>\n<td>Die cleaning is the long step<\/td>\n<\/tr>\n<tr>\n<td>Avoid same-day colour changes on the same die<\/td>\n<td>Two transitions instead of one<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 5 \u2014 Scheduling rules. Sequencing light to dark can turn a day of hard changeovers into one hard change and several easy ones.<\/p>\n<p>The dedicated-screw option is more common than people expect in plants that run a lot of natural product. A spare screw costs a fraction of a new extruder and it removes the single hardest changeover from the schedule. Swapping a screw takes 30\u201360 minutes, which is less than a bad dark-to-natural transition.<\/p>\n<h2 id=\"measuring\">Measuring the transition<\/h2>\n<table>\n<thead>\n<tr>\n<th>Metodo<\/th>\n<th>What it gives you<\/th>\n<th>Utilizzo<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Visual match to a standard<\/td>\n<td>A yes\/no at the machine<\/td>\n<td>Production acceptance<\/td>\n<\/tr>\n<tr>\n<td>Spectrophotometer, \u0394E<\/td>\n<td>Numeric difference<\/td>\n<td>Setting the acceptance criterion<\/td>\n<\/tr>\n<tr>\n<td>Transition length in metres<\/td>\n<td>How much product was scrapped<\/td>\n<td>Tracking improvement<\/td>\n<\/tr>\n<tr>\n<td>Purge consumption per change<\/td>\n<td>Material cost of changeover<\/td>\n<td>Costing and benchmarking<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 6 \u2014 Measuring changeover performance. Transition length is the number to put on the board \u2014 it captures both the purge and the die cleaning in one figure.<\/p>\n<p>\u0394E of 1.0 is generally the limit of what the eye can detect side by side; \u0394E of 2\u20133 is acceptable for most industrial products if the parts are not adjacent. Setting the acceptance criterion in \u0394E rather than \"looks right\" removes the argument at the machine and makes the transition length measurable.<\/p>\n<div class=\"faq\">\n<h3>How long does a colour changeover take?<\/h3>\n<p>From 45 minutes for a light-to-dark change on a simple die to 3\u20134 hours for dark-to-natural with a full die strip-down. The transition length \u2014 metres before the colour is in tolerance \u2014 is the more useful number, typically 30\u201380 m on a well-run line.<\/p>\n<h3>Why does my colour take so long to come through?<\/h3>\n<p>Almost always the die. Dead spots in the land, sharp internal corners and blind holes hold old material and release it slowly. The barrel clears in a few minutes; the die can take hundreds of metres. Cleaning the die properly is faster than waiting for it to flush.<\/p>\n<h3>Is a purge compound necessary?<\/h3>\n<p>For simple changes, no \u2014 running the new colour generously does the same job. For cleaning (carbon, degraded material, gel) a proper purge compound is essential, because no amount of new material removes deposits. Match the compound to the problem.<\/p>\n<h3>How much purge do I need?<\/h3>\n<p>Two to three times the barrel volume. A 60 mm extruder holds about 4\u20135 kg of melt, so 10\u201315 kg of purge. Less than that mixes rather than displaces, which is why short purges produce a long transition.<\/p>\n<h3>Why is dark to light so much harder?<\/h3>\n<p>Because a small amount of dark pigment is visible against a light background, while a small amount of light pigment is invisible against dark. The physics is not symmetrical, so the process should not be either. Use a longer purge, clean the die, and consider a mid-tone transition.<\/p>\n<h3>Can I avoid die cleaning?<\/h3>\n<p>Not for a dark-to-light change. You can reduce the need with a streamlined die design \u2014 radiused corners, no blind holes, no thermocouple wells in the flow path \u2014 but the land section always holds a film that has to be removed mechanically.<\/p>\n<h3>Does the screen pack need changing?<\/h3>\n<p>Yes, if it is loaded. A contaminated screen holds old colour and releases it slowly, and it also raises head pressure which changes the flow pattern. Change the screen as part of the colour changeover, not separately.<\/p>\n<h3>What is a reasonable \u0394E for colour acceptance?<\/h3>\n<p>1.0 is the visual detection limit for adjacent samples; 2.0\u20133.0 is typical for industrial products. Set the criterion with the customer before the first run, because arguing about colour at the machine is expensive for everyone.<\/p>\n<\/div>\n<div class=\"cta\">\n<p class=\"cta-h\">Losing too much material on colour changes?<\/p>\n<p>Tell us the resin, the colours you change between, the die and your current transition length. We will tell you whether the limit is purge method, die design or scheduling \u2014 and what a realistic target is.<\/p>\n<p><a class=\"btn\" href=\"https:\/\/uplastech.com\/it\/soluzioni-per-lestrusione-di-plastica\/\">Talk to an extrusion engineer<\/a>\n<\/div>\n<div class=\"rel\">\n<h4>Continue reading<\/h4>\n<ul>\n<li><a href=\"https:\/\/uplastech.com\/it\/extrusion-die-design\/\">Extrusion Die Design for Profiles: Flow Balance and Die Swell<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/it\/regrind-recycled-content-extrusion\/\">Regrind and Recycled Content in Extrusion: Ratios and Property Loss<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/it\/plastic-drying-before-extrusion\/\">Plastic Drying Before Extrusion: Hygroscopic vs Non-Hygroscopic Resins<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/it\/difetti-di-estrusione-della-plastica\/\">Plastic Extrusion Defects: 12 Problems, Causes and Fixes<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/it\/extrusion-quality-control\/\">Extrusion Quality Control: Inspection, SPC and Acceptance<\/a><\/li>\n<\/ul>\n<\/div>\n<\/div>","protected":false},"excerpt":{"rendered":"<p>A colour change costs more in scrap than in labour. The purge sequence, the mechanical cleaning no compound replaces, and how to get from black to natural in one pass.<\/p>","protected":false},"author":1,"featured_media":7683,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_seopress_titles_title":"Colour Changeover in Extrusion: Purge Compounds, Sequence an","_seopress_titles_desc":"A colour change costs more in scrap than in labour. 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