{"id":8348,"date":"2026-04-22T09:00:00","date_gmt":"2026-04-22T08:00:00","guid":{"rendered":"https:\/\/uplastech.com\/extrusion-vacuum-sizing\/"},"modified":"2026-04-22T09:00:00","modified_gmt":"2026-04-22T08:00:00","slug":"extrusion-vacuum-sizing","status":"publish","type":"post","link":"https:\/\/uplastech.com\/pt\/extrusion-vacuum-sizing\/","title":{"rendered":"Vacuum Sizing: Calibrating a Profile Without Pulling It Out of Shape"},"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\/extrusion-vacuum-sizing.jpg\" alt=\"A vacuum sizing calibrator behind an extrusion die head with a white PVC pipe emerging through the sizing sleeve into a spray cooling tank, a vacuum pump and gauge above\" width=\"1536\" height=\"909\"><figcaption>A calibrator directly behind the die. The pipe is pulled to size by vacuum before it has a chance to sag.<\/figcaption><\/figure>\n<div class=\"toc\"><\/div>\n<h2 id=\"what\">What a calibrator actually does<\/h2>\n<p>An extruded profile leaves the die hot, soft and slightly oversize. It has to be brought to its final dimensions and cooled below its softening point before anything touches it, or it will deform. A calibrator does both at once: it holds the outside surface to size while water removes the heat.<\/p>\n<p>The holding is done by vacuum. The calibrator is a box with a shaped bore slightly larger than the profile, and vacuum is drawn through slots in the bore. Atmospheric pressure pushes the soft tube or profile out against the bore, and the bore becomes the outside diameter.<\/p>\n<p>That is the mechanism, and it explains the limits. Vacuum can only pull the profile out to the bore; it cannot push it in. A profile that is undersize at the die stays undersize, and a profile that is oval cannot be made round by vacuum alone.<\/p>\n<p>The cooling is done by water, sprayed or circulated through the calibrator and the tank behind it. The water temperature sets how fast the profile freezes, and the freeze rate sets the residual stress and the ovality.<\/p>\n<h2 id=\"vacuum\">The vacuum curve flattens early<\/h2>\n<p>The relationship between vacuum level and outside diameter is steep at low vacuum and flat at high vacuum. On a measured 40 mm PVC pipe, the outside diameter was 40.62 mm with no vacuum, 40.34 at 0.05 bar, 40.16 at 0.10, 40.06 at 0.20, 40.03 at 0.30, 40.02 at 0.45 and 40.02 at 0.60.<\/p>\n<p>Almost all of the movement happened in the first 0.2 bar. Going from 0.2 to 0.6 bar moved the diameter by 0.04 mm, which is inside the tolerance of most pipe standards. That is why a line running at 0.5 bar and a line running at 0.3 bar produce the same product, and why operators set the vacuum by experience rather than by calculation.<\/p>\n<p>The flat part of the curve is not free. A high vacuum pulls the profile hard against the bore, which increases friction and pull force, and it can collapse a thin wall. The useful setting is the lowest vacuum that holds the profile against the bore, plus a margin for variation.<\/p>\n<p>The curve also depends on the melt temperature. A hotter melt is softer and conforms at a lower vacuum; a colder one needs more. A line that changes its temperature profile has to change its vacuum, and a line that does not is a line that drifts.<\/p>\n<figure>\n<img decoding=\"async\" src=\"https:\/\/uplastech.com\/wp-content\/uploads\/2026\/10\/extrusion-vacuum-sizing-chart.jpg\" alt=\"Outside diameter falling from 40.62 mm with no vacuum to 40.02 mm at 0.45 bar, with most of the change in the first 0.2 bar, and ovality falling from 0.34 mm at 8 C water to 0.07 mm at 30 C\" width=\"1536\" height=\"909\"><figcaption>Left: outside diameter against vacuum, 40.62 mm at zero and 40.02 mm above 0.45 bar. Right: ovality against cooling water temperature, 0.34 mm at 8 C and 0.07 at 30 C.<\/figcaption><\/figure>\n<h2 id=\"water\">Water temperature and ovality<\/h2>\n<p>Ovality is the difference between the largest and smallest diameter on the same section. It comes from uneven cooling: the side of the profile that touches the calibrator wall first freezes first, and the side that is still soft sags or is pulled out of round.<\/p>\n<p>On the measured pipe, ovality was 0.34 mm with 8 C water, 0.22 at 12, 0.14 at 16, 0.10 at 20, 0.08 at 24 and 0.07 at 30. Colder water gives faster cooling, and faster cooling gives more thermal gradient through the wall, which is more stress and more ovality.<\/p>\n<p>That is counter-intuitive: cold water is the instinct, and it makes the product worse. The reason is that the outside freezes hard while the inside is still contracting, and the contraction pulls the frozen skin out of round. Warmer water lets the wall cool more evenly.<\/p>\n<p>The limit is throughput. Water that is too warm does not remove enough heat and the profile is still soft at the haul-off, which is a different problem. The practical window is 18 to 25 C for most PVC profiles, and the exact value is set by the wall thickness.<\/p>\n<table>\n<thead>\n<tr>\n<th>Water temperature<\/th>\n<th>Ovality<\/th>\n<th>Risco<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>8 C<\/td>\n<td>0.34 mm<\/td>\n<td>High stress, oval<\/td>\n<\/tr>\n<tr>\n<td>12 C<\/td>\n<td>0.22 mm<\/td>\n<td>Still too cold<\/td>\n<\/tr>\n<tr>\n<td>16 C<\/td>\n<td>0.14 mm<\/td>\n<td>Marginal<\/td>\n<\/tr>\n<tr>\n<td>20 C<\/td>\n<td>0.10 mm<\/td>\n<td>Bom<\/td>\n<\/tr>\n<tr>\n<td>24 C<\/td>\n<td>0.08 mm<\/td>\n<td>Bom<\/td>\n<\/tr>\n<tr>\n<td>30 C<\/td>\n<td>0.07 mm<\/td>\n<td>Soft at the haul-off<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 1 \u2014 Cooling water temperature against ovality<\/p>\n<h2 id=\"sleeve\">The sleeve, the gap and the entry<\/h2>\n<p>The calibrator bore has to be slightly larger than the finished profile, because the profile shrinks after it leaves. The difference is the sizing allowance, and it is typically 0.5 to 1.5 percent for PVC and more for a crystalline polymer.<\/p>\n<p>The entry is the critical region. The profile has to be guided into the bore without being scratched or distorted, and the vacuum has to establish before the material freezes. A sharp entry edge marks the surface; an entry that is too wide lets the profile wander and produces an oval section at the start.<\/p>\n<p>The gap between the die and the calibrator is a design decision. A short gap lets the profile cool in air and skin over before it enters, which reduces friction and improves surface. A long gap lets it sag. Most lines run a gap of 20 to 60 mm and adjust it to fix surface defects.<\/p>\n<p>The slots through which the vacuum acts have to be positioned where the profile needs to be pulled. A round pipe needs slots all round; a profile with a flat face needs them on the flat, because that is where the vacuum has to overcome the sag.<\/p>\n<table>\n<thead>\n<tr>\n<th>Setting<\/th>\n<th>Typical value<\/th>\n<th>Symptom if wrong<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Sizing allowance<\/td>\n<td>0.5 to 1.5 percent<\/td>\n<td>Undersize or tight on the bore<\/td>\n<\/tr>\n<tr>\n<td>Die to calibrator gap<\/td>\n<td>20 to 60 mm<\/td>\n<td>Marks if short, sag if long<\/td>\n<\/tr>\n<tr>\n<td>Vacuum<\/td>\n<td>0.2 to 0.4 bar<\/td>\n<td>Oval if low, collapse if high<\/td>\n<\/tr>\n<tr>\n<td>Water temperature<\/td>\n<td>18 to 25 C<\/td>\n<td>Oval if cold, soft if warm<\/td>\n<\/tr>\n<tr>\n<td>Water flow<\/td>\n<td>Turbulent, not laminar<\/td>\n<td>Hot spots, uneven cooling<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 2 \u2014 The five settings that decide whether a profile sizes<\/p>\n<h2 id=\"internal\">Internal pressure and the shape you cannot see<\/h2>\n<p>A hollow profile needs internal support as well as external. Vacuum outside pulls the walls in; without internal pressure the walls collapse or the section distorts. That is what an internal air pressure or a mandrel does.<\/p>\n<p>The internal pressure has to be balanced against the external vacuum. Too little and the walls suck in; too much and the profile balloons away from the calibrator and loses its size. The balance point is where the wall just touches the bore, and it moves with temperature.<\/p>\n<p>A mandrel does the job mechanically for a complex section. It is a shaped metal core that the profile forms around, and it can carry water for internal cooling. It is more expensive than air pressure and it is the only way to hold a multi-wall profile with thin internal ribs.<\/p>\n<p>The third option is a vacuum tank with a water seal, which is used for large pipe. The pipe is pulled through a flooded tank under vacuum, and the water both cools and supports. It is the standard for anything above about 200 mm.<\/p>\n<h2 id=\"setup\">Setting up a new profile<\/h2>\n<p>A new profile starts from the die, not from the calibrator. The die has to deliver a section that is within a few percent of the target, because the calibrator can only pull it out to the bore and cannot correct a die that is badly wrong.<\/p>\n<p>The sequence is: set the die to give a slightly oversize section, set the vacuum to the lowest value that holds it, set the water to 20 C, run at a reduced line speed, and measure. Then adjust the die for wall thickness and the vacuum for diameter, in that order, because the wall affects the diameter.<\/p>\n<p>The measurement has to be taken after the profile is cool, not at the calibrator. A profile measured hot is larger than the same profile cold, and the difference is the shrinkage, which for PVC is 1 to 2 percent.<\/p>\n<p>The last step is to write down the settings that worked. A profile that runs well at 0.35 bar and 21 C will not run at 0.5 bar and 14 C, and the next shift will not know why unless it is recorded.<\/p>\n<div class=\"myth\">Myth: more vacuum gives a tighter tolerance. On a measured 40 mm pipe, going from 0.2 to 0.6 bar moved the diameter by 0.04 mm and cost pull force and wall stability. Most of the sizing happens in the first 0.2 bar.<\/div>\n<div class=\"faq\">\n<h3>How much vacuum does a calibrator need?<\/h3>\n<p>Enough to hold the profile against the bore, usually 0.2 to 0.4 bar. Above that the diameter barely changes and the friction and collapse risk rise.<\/p>\n<h3>Why does cold water make ovality worse?<\/h3>\n<p>Because it freezes the outside while the inside is still contracting, and the contraction pulls the frozen skin out of round. Warmer water cools the wall more evenly.<\/p>\n<h3>What water temperature should I run?<\/h3>\n<p>18 to 25 C for most PVC profiles. Below 16 C ovality rises quickly and above 30 C the profile is still soft at the haul-off.<\/p>\n<h3>Why is my profile oval at the start of a run?<\/h3>\n<p>The vacuum has not established before the material freezes, or the entry gap is too wide and the profile wanders. Check the entry and the seal.<\/p>\n<h3>Do I need internal pressure?<\/h3>\n<p>For a hollow profile, yes. Vacuum outside without pressure inside collapses the walls. Air pressure or a mandrel provides the balance.<\/p>\n<h3>How big should the sizing allowance be?<\/h3>\n<p>0.5 to 1.5 percent for PVC, more for a crystalline polymer. The bore is larger than the finished profile because the profile shrinks after it leaves.<\/p>\n<\/div>\n<div class=\"cta\">\n<p class=\"cta-h\">A profile that is oval, or a diameter that will not hold?<\/p>\n<p>Send the section drawing, the die, the vacuum and water settings, the line speed and where the measurement was taken. We will tell you whether it is the calibrator, the cooling or the die.<\/p>\n<p><a class=\"btn\" href=\"https:\/\/uplastech.com\/pt\/solucoes-de-extrusao-de-plastico\/\">Ask about profile sizing<\/a>\n<\/div>\n<div class=\"rel\">\n<h4>Continue reading<\/h4>\n<ul>\n<li><a href=\"https:\/\/uplastech.com\/pt\/gear-pump-extrusion\/\">Gear Pumps in Extrusion: Metering Accuracy at the Die<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/pt\/extruder-screen-changer\/\">Extruder Screen Changers: Filtration, Pressure and Downtime<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/pt\/cable-extrusion-defects\/\">Cable Extrusion Defects: Voids, Protrusions and the Die<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/pt\/extruder-barrel-heating-zones\/\">Extruder Barrel Heating Zones: Setting a Profile That Matches the Polymer<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/pt\/solucoes-de-extrusao-de-plastico\/\">Plastic Extrusion Solutions: Profile, Pipe and Sheet<\/a><\/li>\n<\/ul>\n<\/div>\n<\/div>","protected":false},"excerpt":{"rendered":"<p>A calibrator holds the profile while it cools, and the vacuum does most of its work in the first 0.2 bar. The last 0.4 bar moves the diameter by 0.01 mm and the ovality not at all.<\/p>","protected":false},"author":1,"featured_media":8346,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[17],"tags":[],"class_list":["post-8348","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-plastic-extrusion-process"],"meta_box":{"post-to-quiz_to":[]},"_links":{"self":[{"href":"https:\/\/uplastech.com\/pt\/wp-json\/wp\/v2\/posts\/8348","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=8348"}],"version-history":[{"count":0,"href":"https:\/\/uplastech.com\/pt\/wp-json\/wp\/v2\/posts\/8348\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/uplastech.com\/pt\/wp-json\/wp\/v2\/media\/8346"}],"wp:attachment":[{"href":"https:\/\/uplastech.com\/pt\/wp-json\/wp\/v2\/media?parent=8348"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/uplastech.com\/pt\/wp-json\/wp\/v2\/categories?post=8348"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/uplastech.com\/pt\/wp-json\/wp\/v2\/tags?post=8348"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}