{"id":8734,"date":"2026-08-17T09:00:00","date_gmt":"2026-08-17T08:00:00","guid":{"rendered":"https:\/\/uplastech.com\/thermoforming-vacuum-pump-sizing\/"},"modified":"2026-11-02T02:29:20","modified_gmt":"2026-11-02T02:29:20","slug":"thermoforming-vacuum-pump-sizing","status":"publish","type":"post","link":"https:\/\/uplastech.com\/fr\/thermoforming-vacuum-pump-sizing\/","title":{"rendered":"Vacuum Pump Sizing: Flow, Evacuation Time and the Leak You Cannot See"},"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\/11\/thermoforming-vacuum-pump-sizing.jpg\" alt=\"A vacuum pump and receiver tank beside a thermoforming machine with hoses and a vacuum gauge\" width=\"1536\" height=\"909\"><figcaption>The pump is sized for the leak, not for the mould.<\/figcaption><\/figure>\n<div class=\"toc\"><\/div>\n<h2 id=\"physics\">Evacuation is a race against the leak<\/h2>\n<p>Vacuum forming needs the air under the sheet removed faster than the sheet cools. That sounds like a pump sizing problem and it is really a leak problem, because a leak admits air continuously while the pump removes it, and the pressure the cavity reaches is the point where the two rates balance.<\/p>\n<p>The arithmetic is simple enough to do on a napkin. A cavity of 20 litres at atmospheric pressure contains 20 litres of air. A pump rated 63 cubic metres an hour removes about 17 litres a second at atmospheric pressure, so in principle the cavity is evacuated in a little over a second. In practice it takes longer, because a pump's effective capacity falls as the pressure drops and because the leak is adding air the whole time.<\/p>\n<p>The leak is the term that dominates. A seal that leaks one litre per minute is admitting air at 0.017 litres per second, which sounds negligible against 17 litres a second. But as the cavity pressure falls, the pump's capacity falls with it, and the leak rate stays roughly constant in mass terms. At a low enough pressure the two are equal and the cavity stops evacuating altogether, no matter how long the pump runs.<\/p>\n<p>That is why the practical advice on a vacuum problem is always to find the leak first. A pump that is twice as large moves the equilibrium point only slightly, and it costs twice as much to run.<\/p>\n<h2 id=\"curve\">Two pumps, one leak<\/h2>\n<p>The chart on the left plots time to reach forming vacuum against effective leak rate for a 250 cubic metre per hour pump and a 63 cubic metre per hour one, on the same 20 litre cavity. At a leak of half a litre per minute the large pump takes 0.6 seconds and the small one 1.4. At 20 litres per minute they take 5.4 and 19 seconds.<\/p>\n<figure>\n<img decoding=\"async\" src=\"https:\/\/uplastech.com\/wp-content\/uploads\/2026\/11\/thermoforming-vacuum-pump-sizing-chart.jpg\" alt=\"Time to reach forming vacuum against leak rate for two pump sizes, and a bar chart of where vacuum is lost\" width=\"1536\" height=\"909\"><figcaption>Left: the curves converge as the leak grows, so the bigger pump buys less. Right: two thirds of vacuum problems are leaks and wear.<\/figcaption><\/figure>\n<p>The shape of the two curves is the argument. At low leak rates the pump size matters by a factor of two or more. At high leak rates the curves flatten and converge, because the leak is now the limiting term and the pump is only removing what the leak admits. Above about 20 litres per minute a larger pump buys almost nothing.<\/p>\n<p>The practical consequence is a diagnostic. If a station forms well on one tool and badly on another, the tool is leaking. If it forms badly on every tool, the pump, the pipework or the receiver is the problem. That single comparison separates the two families of cause before any instrument is used.<\/p>\n<p>The second consequence is about receiver sizing. A receiver is a reservoir of vacuum that absorbs the initial rush of air when the valve opens. Without one, the pump sees the full cavity volume at atmospheric pressure and the pressure spike travels back up the line. A receiver of 20 to 50 times the cavity volume turns a spike into a smooth draw, and it is cheaper than a larger pump.<\/p>\n<h2 id=\"losses\">Where vacuum actually goes<\/h2>\n<p>The right hand panel shows the distribution: seal and clamp leaks 34 percent, pump wear 24, pipework and valves 18, porous or wet sheet 14, cycle too short 6. The first three are 76 percent and all three are mechanical rather than process.<\/p>\n<p>Seal and clamp leaks are the largest single cause and they are the easiest to find. A sheet that is not clamped evenly, a seal that has hardened, a clamp frame that is distorted, a tool with a damaged O-ring groove: each admits air at the perimeter. The test is to close the machine on a blank sheet, pull vacuum, and watch the gauge with the pump isolated. A gauge that rises faster than about 0.05 bar per minute indicates a leak worth chasing.<\/p>\n<p>Pump wear at 24 percent is the slow one. A rotary vane pump loses capacity as the vanes wear and the clearances open, and the loss is gradual enough that nobody notices until the cycle time has crept up by 30 percent. Vanes are a consumable, and a maintenance schedule that replaces them on hours rather than on failure is what keeps a station consistent.<\/p>\n<p>Pipework and valves at 18 percent is where installations go wrong. A hose that is too small, a valve with a restricted bore, a long run with several elbows, a filter that has never been changed: each adds a pressure drop between the cavity and the pump, and the pump's capacity at the cavity is what matters. A pump rated 250 cubic metres an hour at its inlet may deliver a fraction of that at the tool.<\/p>\n<h2 id=\"size\">Sizing the pump properly<\/h2>\n<p>The starting point is the cavity volume, which is the volume of air between the sheet and the tool plus the volume of the vacuum lines. For a shallow tool that is small, a few litres. For a deep draw with a large plenum it can be 50 litres or more, and the plenum volume is often forgotten because it is inside the machine.<\/p>\n<p>The second input is the required evacuation time, which comes from the process rather than from the pump catalogue. The sheet has to be pulled into the tool before it cools below its forming range, and for a thin sheet that window can be under a second. For a thick sheet it can be three or four seconds, which is much easier to achieve.<\/p>\n<p>The third input is the leak rate, and it should be measured rather than assumed. Close the tool on a blank sheet, isolate the pump, and time the pressure rise. A rise of 0.05 bar per minute on a 20 litre cavity corresponds to a leak of about one litre per minute at atmospheric pressure, which is a reasonable target for a well-maintained station.<\/p>\n<table>\n<thead>\n<tr>\n<th>Cavity volume<\/th>\n<th>Target time<\/th>\n<th>Pump capacity needed<\/th>\n<th>Receiver<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>5 litres<\/td>\n<td>1 second<\/td>\n<td>About 40 cubic metres per hour<\/td>\n<td>100 to 250 litres<\/td>\n<\/tr>\n<tr>\n<td>20 litres<\/td>\n<td>1 second<\/td>\n<td>About 120 cubic metres per hour<\/td>\n<td>400 to 1000 litres<\/td>\n<\/tr>\n<tr>\n<td>50 litres<\/td>\n<td>2 seconds<\/td>\n<td>About 150 cubic metres per hour<\/td>\n<td>1000 to 2500 litres<\/td>\n<\/tr>\n<tr>\n<td>50 litres<\/td>\n<td>4 seconds<\/td>\n<td>About 80 cubic metres per hour<\/td>\n<td>1000 to 2500 litres<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 1 \u2014 Indicative pump capacity for a tight tool, before allowing for leaks.<\/p>\n<p>Those figures assume a tight tool and short pipework. A station with a measurable leak needs the leak added to the flow requirement, and if the leak is large enough the honest answer is to fix the leak rather than to buy capacity. Adding pump capacity to compensate for a leak is a way of paying twice.<\/p>\n<h2 id=\"symptoms\">Reading the symptoms<\/h2>\n<p>Incomplete forming at the corners with a good vacuum reading usually means the vacuum is arriving too late, not too weakly. The sheet has already cooled by the time the pressure drops, so it stretches less. The fix is a larger valve or a shorter line, not a larger pump.<\/p>\n<p>Good vacuum on a gauge but poor forming can mean the gauge is measuring at the pump rather than at the tool. A gauge upstream of a restricted valve reads a pressure the cavity never sees. Moving the gauge to the tool, or adding a second one, resolves the argument in a minute.<\/p>\n<p>A vacuum that holds on a blank but collapses on a real sheet points at the sheet: porosity, moisture, or a sheet that is too hot and has lost its own integrity. Wet sheet is a common cause in humid climates and it is diagnosed by drying a sample and repeating the test.<\/p>\n<table>\n<thead>\n<tr>\n<th>Symptom<\/th>\n<th>Likely cause<\/th>\n<th>First check<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Poor forming on one tool only<\/td>\n<td>Tool seal or O-ring<\/td>\n<td>Leak test with a blank sheet<\/td>\n<\/tr>\n<tr>\n<td>Poor forming on every tool<\/td>\n<td>Pump, valve or pipework<\/td>\n<td>Gauge at the tool, not at the pump<\/td>\n<\/tr>\n<tr>\n<td>Vacuum holds, corners still thin<\/td>\n<td>Vacuum arriving late<\/td>\n<td>Valve bore and line length<\/td>\n<\/tr>\n<tr>\n<td>Vacuum collapses on real sheet<\/td>\n<td>Porous or wet sheet<\/td>\n<td>Dry a sample and retest<\/td>\n<\/tr>\n<tr>\n<td>Cycle time creeping up over months<\/td>\n<td>Pump vane wear<\/td>\n<td>Service hours against the schedule<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 2 \u2014 Symptoms and the first check for each.<\/p>\n<h2 id=\"maintain\">Keeping the vacuum where it was<\/h2>\n<p>A vacuum system that is commissioned well and never touched will lose a third of its capacity within two years, silently. The losses are vane wear, filter loading, seal hardening and slow leaks at joints, and none of them announce themselves.<\/p>\n<p>A quarterly check of three numbers keeps it honest: the evacuation time on a reference tool, the pressure rise with the pump isolated, and the pump's service hours against its schedule. Any of the three drifting is a warning, and all three are measured in under ten minutes.<\/p>\n<p>The seals deserve a specific mention because they are the largest single cause and the cheapest to replace. A clamp seal that has hardened will still look fine and will still hold most of the vacuum, which is why it is missed. Replacing perimeter seals on a schedule rather than on failure is the single highest-return maintenance item on a forming station.<\/p>\n<div class=\"faq\">\n<h3>How do I size a vacuum pump for thermoforming?<\/h3>\n<p>Start from the cavity plus plenum volume, the evacuation time the process needs, and the measured leak rate. A 20 litre cavity needing one second takes roughly 120 cubic metres per hour on a tight tool. Add the leak to the flow requirement, or fix the leak instead.<\/p>\n<h3>Why does a bigger pump not improve my forming?<\/h3>\n<p>Because the leak is the limiting term. At high leak rates the pump and the leak reach an equilibrium and the cavity stops evacuating regardless of pump size. Find the leak with a blank sheet and a pressure rise test before spending money on capacity.<\/p>\n<h3>How do I test for a vacuum leak?<\/h3>\n<p>Close the machine on a blank sheet, pull vacuum, isolate the pump and watch the gauge. A rise of more than about 0.05 bar per minute indicates a leak worth chasing. A leak of one litre per minute on a 20 litre cavity gives roughly that rate.<\/p>\n<h3>What size receiver do I need?<\/h3>\n<p>Twenty to fifty times the cavity volume is the usual range, which for a 20 litre cavity is 400 to 1000 litres. The receiver absorbs the initial rush of air and smooths the draw, and it is usually cheaper than a larger pump.<\/p>\n<h3>Why does vacuum forming work on a blank but fail on a real sheet?<\/h3>\n<p>Because the sheet itself is leaking or collapsing. Porous sheet, damp sheet and sheet that is too hot all behave differently from a solid blank. Dry a sample and repeat the test, and check the sheet temperature against the forming window.<\/p>\n<h3>How often should vacuum pump vanes be replaced?<\/h3>\n<p>On hours, not on failure. Vane wear is gradual and the symptom is a slowly rising cycle time rather than a sudden loss, so a pump that is serviced only when it fails has usually been degrading the process for months.<\/p>\n<\/div>\n<div class=\"cta\">\n<p class=\"cta-h\">A forming station that holds its vacuum<\/p>\n<p>Tell us the tool volume, the sheet and the cycle you need. We will work back to the pump capacity and receiver size, and tell you whether the problem you are seeing is capacity or a leak.<\/p>\n<p><a class=\"btn\" href=\"https:\/\/uplastech.com\/fr\/formage-sous-vide\/\">Discuss a forming station<\/a>\n<\/div>\n<div class=\"rel\">\n<h4>Continue reading<\/h4>\n<ul>\n<li><a href=\"https:\/\/uplastech.com\/fr\/thermoforming-vacuum-vs-pressure\/\">Thermoforming Vacuum versus Pressure Forming<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/fr\/thermoforming-compressed-air-system\/\">Compressed Air in Pressure Forming: Volume and Dew Point<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/fr\/thermoforming-cooling-cycle\/\">Thermoforming Cooling Cycle: What Sets It<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/fr\/thermoforming-machine-types\/\">Thermoforming Machine Types: Four Families<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/fr\/thermoforming-demolding\/\">Thermoforming Demoulding: Release and Ejection<\/a><\/li>\n<\/ul>\n<\/div>\n<\/div>","protected":false},"excerpt":{"rendered":"<p>A bigger vacuum pump does not fix a leaking tool. Doubling the pump capacity halves the evacuation time on a tight tool and barely moves it on a leaky one, which is why the first measurement in any vacuum problem is the leak rate.<\/p>","protected":false},"author":1,"featured_media":8732,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[29],"tags":[],"class_list":["post-8734","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-vacuum-forming"],"meta_box":{"post-to-quiz_to":[]},"_links":{"self":[{"href":"https:\/\/uplastech.com\/fr\/wp-json\/wp\/v2\/posts\/8734","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/uplastech.com\/fr\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/uplastech.com\/fr\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/uplastech.com\/fr\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/uplastech.com\/fr\/wp-json\/wp\/v2\/comments?post=8734"}],"version-history":[{"count":1,"href":"https:\/\/uplastech.com\/fr\/wp-json\/wp\/v2\/posts\/8734\/revisions"}],"predecessor-version":[{"id":8899,"href":"https:\/\/uplastech.com\/fr\/wp-json\/wp\/v2\/posts\/8734\/revisions\/8899"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/uplastech.com\/fr\/wp-json\/wp\/v2\/media\/8732"}],"wp:attachment":[{"href":"https:\/\/uplastech.com\/fr\/wp-json\/wp\/v2\/media?parent=8734"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/uplastech.com\/fr\/wp-json\/wp\/v2\/categories?post=8734"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/uplastech.com\/fr\/wp-json\/wp\/v2\/tags?post=8734"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}