{"id":9013,"date":"2026-11-02T09:00:00","date_gmt":"2026-11-02T09:00:00","guid":{"rendered":"https:\/\/uplastech.com\/hot-runner-leak-diagnosis\/"},"modified":"2026-11-02T09:00:00","modified_gmt":"2026-11-02T09:00:00","slug":"hot-runner-leak-diagnosis","status":"publish","type":"post","link":"https:\/\/uplastech.com\/pt\/hot-runner-leak-diagnosis\/","title":{"rendered":"Hot Runner Leaks: The Temperature Tells You Weeks Early"},"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\/im-hot-runner-manifold.jpg\" alt=\"A hot runner manifold with heaters, thermocouples and nozzle drops on a bench\" width=\"1536\" height=\"909\"><figcaption>Every joint in this block is a potential leak path.<\/figcaption><\/figure>\n<div class=\"toc\"><\/div>\n<h2 id=\"leak-temp\">A leak is a temperature problem first<\/h2>\n<p>A hot runner leak is molten polymer escaping from a flow channel into a gap it was never meant to enter: between a nozzle and the manifold, around a valve pin, or into a thermocouple pocket that was drilled a little too deep.<\/p>\n<p>Once plastic is in the gap it cools, solidifies and insulates. The heater serving that zone now has to work harder to hold setpoint, and the controller shows it as a rising power output or a temperature that sags under load. That happens long before plastic appears on the mould face.<\/p>\n<p>The signal is small at first. A nozzle that used to sit within one degree of setpoint starts wandering by two, then four. The operator compensates by nudging the setpoint up, which accelerates the degradation of the material in the gap and makes the leak worse.<\/p>\n<p>The whole diagnostic window exists in that drift. A plant that logs heater duty cycle and zone deviation will see a leak developing over weeks. A plant that only looks at parts will see it when the mould stops.<\/p>\n<h2 id=\"trace\">Reading the temperature trace<\/h2>\n<p>The chart on the left is an eight week record from a single drop running PA66 at 290 degrees Celsius. The deviation from setpoint goes 0, 1, 2, 4, 7, 11, 16 and 22 degrees across the eight weeks, against an alarm threshold of five.<\/p>\n<p>The curve is exponential, not linear. For the first three weeks the deviation is within normal process noise and nobody would act. By week five it has crossed the threshold. By week eight the drop cannot hold temperature at all and the part shows a cold slug.<\/p>\n<p>That shape is why a fixed alarm at five degrees is worth having but is not enough. The useful number is the trend: a deviation that has doubled over two weeks is a leak even if it is still under the alarm.<\/p>\n<p>Heater duty cycle is the better signal where it is available. A zone whose duty has risen from 40 to 70 percent at steady state is losing heat into something that was not there before, and the most common something is solidified plastic.<\/p>\n<figure>\n<img decoding=\"async\" src=\"https:\/\/uplastech.com\/wp-content\/uploads\/2026\/11\/hot-runner-leak-diagnosis-chart.jpg\" alt=\"Nozzle temperature deviation rising over eight weeks against an alarm threshold, and share of leaks by location\" width=\"1536\" height=\"909\"><figcaption>The deviation doubles every couple of weeks until the drop fails.<\/figcaption><\/figure>\n<h2 id=\"where\">Where leaks start<\/h2>\n<p>Manifold joints account for 34 percent of leaks. The joint between a nozzle and the manifold relies on a metal to metal seal held by thermal expansion, and it only seals when everything is at temperature. A machine started cold and run hard will leak at that joint.<\/p>\n<p>Nozzle seats are 27 percent. The seat is a small conical face that has to be perfectly matched, and it wears every time a nozzle is removed for cleaning. A seat that has been reworked twice is a seat that will leak.<\/p>\n<p>Thermocouple pockets are 18 percent, which surprises people because a pocket is not a flow channel. But a pocket drilled through into the melt channel becomes a dead end that fills with degraded material, and the pressure eventually pushes it out through the thermocouple hole.<\/p>\n<p>Valve pin seals are 13 percent and residue in the drop is 8 percent. Valve pins wear their guides, and a worn guide lets melt past the pin into the actuator area, which is the most expensive version of this failure.<\/p>\n<table>\n<thead>\n<tr>\n<th>Location<\/th>\n<th>Share<\/th>\n<th>Typical cause<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Manifold joint<\/td>\n<td>34%<\/td>\n<td>Cold start, insufficient preheat<\/td>\n<\/tr>\n<tr>\n<td>Nozzle seat<\/td>\n<td>27%<\/td>\n<td>Worn or reworked seat face<\/td>\n<\/tr>\n<tr>\n<td>Thermocouple pocket<\/td>\n<td>18%<\/td>\n<td>Pocket drilled into the melt channel<\/td>\n<\/tr>\n<tr>\n<td>Valve pin seal<\/td>\n<td>13%<\/td>\n<td>Worn guide, polymer past the pin<\/td>\n<\/tr>\n<tr>\n<td>Residue in the drop<\/td>\n<td>8%<\/td>\n<td>Degraded material blocking the tip<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 1 \u2014 Where hot runner leaks actually start.<\/p>\n<h2 id=\"confirm\">Confirming a leak without pulling the mould<\/h2>\n<p>Start with the electrical check. Measure the resistance of each heater and compare against the value recorded at installation. A heater that has dropped in resistance is shorting through carbonised plastic, which is what a leak becomes after a few weeks at temperature.<\/p>\n<p>Then check the thermocouple response. Pull the thermocouple, and if the pocket is full of plastic the leak has already reached it. A clean pocket that reads correctly rules out that path but not the others.<\/p>\n<p>Then run a temperature soak test. Bring the manifold to temperature with no injection, hold for thirty minutes, and watch the deviation on each zone. A leaking zone will drift as the plastic in the gap expands and contracts, and a sound zone will not.<\/p>\n<p>Finally, look at the parts. A leak that has not reached the surface still changes the thermal history of one drop, and that shows as a colour or gloss difference in the cavity fed by that drop. Mapping the difference to a cavity is often the fastest confirmation available.<\/p>\n<table>\n<thead>\n<tr>\n<th>Check<\/th>\n<th>What it shows<\/th>\n<th>Time<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Heater resistance<\/td>\n<td>Carbon tracking in the gap<\/td>\n<td>10 min<\/td>\n<\/tr>\n<tr>\n<td>Thermocouple pocket<\/td>\n<td>Plastic in the pocket<\/td>\n<td>15 min<\/td>\n<\/tr>\n<tr>\n<td>Soak test at temperature<\/td>\n<td>Zone that will not hold<\/td>\n<td>30 min<\/td>\n<\/tr>\n<tr>\n<td>Cavity to cavity comparison<\/td>\n<td>Which drop is affected<\/td>\n<td>One cycle<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 2 \u2014 Confirming a leak in order of cost.<\/p>\n<h2 id=\"part\">What a leak does to the part<\/h2>\n<p>The obvious effect is a cold slug or a short shot on the affected cavity. Less obvious is a slow drift in dimensions, because a leaking drop has less melt available and the packing is reduced without any change in the machine settings.<\/p>\n<p>There is also contamination. Plastic that has sat in a gap at temperature degrades, and when it breaks loose it appears as black specks or brown streaks in a single cavity. A single cavity with contamination is a hot runner problem until proven otherwise.<\/p>\n<p>And there is the cosmetic effect. A drop that is losing heat through a leak runs cooler at the gate, which changes the flow pattern and produces flow marks or a weld line in a different place. That is often the first thing the customer sees.<\/p>\n<p>All three effects get blamed on the resin, the dryer or the operator before anyone looks at the hot runner. The temperature log would have pointed at the drop weeks earlier.<\/p>\n<h2 id=\"prevent\">Preventing the next one<\/h2>\n<p>Preheat properly. A manifold brought to temperature slowly and soaked before the first shot seals at the joints; one that is started cold and injected immediately does not. The preheat time is in the manifold supplier manual and it is almost always longer than the plant allows.<\/p>\n<p>Torque the nozzles to specification and record it. A nozzle that is under-torqued leaks at the seat; one that is over-torqued distorts the seat and leaks later. The torque value matters more than the technique.<\/p>\n<p>Keep the thermocouple pockets out of the melt channel. That is a manifold design decision, and it is worth asking about at the specification stage rather than discovering it during a leak.<\/p>\n<p>And log the zone data. Deviation and duty cycle per zone, once a shift, is enough to catch a leak while it is still a temperature trend. It is the cheapest maintenance tool in a hot runner plant.<\/p>\n<div class=\"faq\">\n<h3>How can I tell if my hot runner is leaking before I see plastic?<\/h3>\n<p>Watch the temperature deviation and the heater duty cycle per zone. A zone that needs more power to hold setpoint is losing heat into solidified plastic in a gap.<\/p>\n<h3>Where do hot runner leaks usually start?<\/h3>\n<p>Manifold joints at 34 percent and nozzle seats at 27 percent. Both depend on a metal to metal seal that only works at temperature, so cold starts are a common cause.<\/p>\n<h3>Why does a thermocouple pocket cause a leak?<\/h3>\n<p>If the pocket is drilled through into the melt channel it becomes a dead end that fills with degraded material, which is then pushed out through the thermocouple hole.<\/p>\n<h3>What does a leaking drop do to the part?<\/h3>\n<p>Cold slugs and short shots on one cavity, dimensional drift from reduced packing, black specks from degraded material, and flow marks from a cooler gate.<\/p>\n<h3>How long should a hot runner be preheated?<\/h3>\n<p>Long enough for the manifold to soak at temperature before the first shot, typically 30 to 45 minutes depending on mass. Starting cold and injecting immediately is the usual cause of a joint leak.<\/p>\n<h3>Can a leak be repaired without replacing the manifold?<\/h3>\n<p>Usually yes for a nozzle seat, which can be reworked or replaced. A leak that has carbonised inside the manifold often requires the manifold to be cleaned and the affected joint remachined.<\/p>\n<\/div>\n<div class=\"cta\">\n<p class=\"cta-h\">Chasing a single cavity defect?<\/p>\n<p>Tell us the cavity, the hot runner layout and the temperature log. We will tell you whether it is a leak or a gate problem.<\/p>\n<p><a class=\"btn\" href=\"https:\/\/uplastech.com\/pt\/injection-molding\/\">Ask about hot runners<\/a>\n<\/div>\n<div class=\"rel\">\n<h4>Continue reading<\/h4>\n<ul>\n<li><a href=\"https:\/\/uplastech.com\/pt\/hot-runner-manifold-design\/\">Hot Runner Manifold Design<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/pt\/gate-design-injection-molding\/\">Gate Design in Injection Molding<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/pt\/melt-temperature-barrel-profile\/\">Melt Temperature and Barrel Profile<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/pt\/colour-streaks-injection-molding\/\">Colour Streaks in Injection Molding<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/pt\/injection-molding-machine-selection\/\">Injection Molding Machine Selection<\/a><\/li>\n<\/ul>\n<\/div>\n<\/div>","protected":false},"excerpt":{"rendered":"<p>A hot runner leak starts as a small drop in heater duty and ends as a mould full of frozen plastic. The temperature trace gives you the weeks in between.<\/p>","protected":false},"author":1,"featured_media":7514,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[35],"tags":[],"class_list":["post-9013","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\/9013","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=9013"}],"version-history":[{"count":0,"href":"https:\/\/uplastech.com\/pt\/wp-json\/wp\/v2\/posts\/9013\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/uplastech.com\/pt\/wp-json\/wp\/v2\/media\/7514"}],"wp:attachment":[{"href":"https:\/\/uplastech.com\/pt\/wp-json\/wp\/v2\/media?parent=9013"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/uplastech.com\/pt\/wp-json\/wp\/v2\/categories?post=9013"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/uplastech.com\/pt\/wp-json\/wp\/v2\/tags?post=9013"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}