{"id":7786,"date":"2025-10-02T09:00:00","date_gmt":"2025-10-02T08:00:00","guid":{"rendered":"https:\/\/uplastech.com\/liquid-silicone-rubber-lsr\/"},"modified":"2026-10-28T09:22:14","modified_gmt":"2026-10-28T09:22:14","slug":"liquid-silicone-rubber-lsr","status":"publish","type":"post","link":"https:\/\/uplastech.com\/pt\/liquid-silicone-rubber-lsr\/","title":{"rendered":"Liquid Silicone Rubber Injection Moulding: Metering, Cold Runners and the Hot Mould"},"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\/liquid-silicone-rubber-lsr.jpg\" alt=\"LSR cure completeness against mix ratio error and cycle time against wall thickness\" width=\"1536\" height=\"909\"><figcaption>The mould is at 190 \u00b0C and the compound in the runner is at 25. Everything depends on that boundary.<\/figcaption><\/figure>\n<div class=\"toc\"><\/div>\n<h2 id=\"what\">What LSR is and why the process looks like plastic<\/h2>\n<p>Liquid silicone rubber is a two-component addition-cure system with a viscosity of 100 to 1,000 Pa\u00b7s \u2014 thin enough to pump through a hose and inject through a small gate. Part A contains the vinyl-functional polymer and the platinum catalyst; part B contains the polymer and a hydride crosslinker. Mixed, they cure.<\/p>\n<p>That single fact explains the whole process. Because the material is pumpable, it can be metered, mixed and injected by a machine that looks like a plastic injection moulding machine. Because it cures on contact with heat, the mould can be at 180 to 200 \u00b0C while the runner stays at 25 \u00b0C. Nothing else in rubber processing works this way.<\/p>\n<p>The result is a cycle measured in seconds rather than minutes, no preform, no flash to trim in most tools, and a part with tolerances closer to plastic than to rubber. The price is a machine that costs several times a compression press and a tool that must be built to a much higher standard.<\/p>\n<table>\n<thead>\n<tr>\n<th>Fator<\/th>\n<th>LSR<\/th>\n<th>HCR compression<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Dura\u00e7\u00e3o do ciclo<\/td>\n<td>20 \u2013 60 s<\/td>\n<td>3 \u2013 10 min<\/td>\n<\/tr>\n<tr>\n<td>Runner waste<\/td>\n<td>None with cold runner<\/td>\n<td>Flash only<\/td>\n<\/tr>\n<tr>\n<td>Tolerance<\/td>\n<td>\u00b10.05 mm typical<\/td>\n<td>\u00b10.15 mm typical<\/td>\n<\/tr>\n<tr>\n<td>Automatiza\u00e7\u00e3o<\/td>\n<td>Fully automated<\/td>\n<td>Manual loading<\/td>\n<\/tr>\n<tr>\n<td>Tool cost<\/td>\n<td>Highest<\/td>\n<td>Lowest<\/td>\n<\/tr>\n<tr>\n<td>Machine cost<\/td>\n<td>Highest<\/td>\n<td>Lowest<\/td>\n<\/tr>\n<tr>\n<td>Economic volume<\/td>\n<td>10,000+<\/td>\n<td>100 \u2013 100,000<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 1 \u2014 LSR against HCR compression moulding. The crossover is usually around 10,000 parts per year per tool.<\/p>\n<h2 id=\"metering\">Metering: the part of the process that fails quietly<\/h2>\n<p>LSR is mixed at 1:1 by volume or by weight, and the ratio must hold within about one percent. A pump that is worn, a check valve that leaks back, a drum that is nearly empty and drawing air \u2014 all produce a ratio error, and a ratio error produces a part that is partly under-cured.<\/p>\n<p>The failure is quiet because the part still looks like silicone. It is soft, slightly tacky, and it will not hold compression set. On a production line it passes a visual check; in the field the seal takes a set and leaks after a month.<\/p>\n<p>The cure curve is steep against ratio error. One percent of error costs about four percent of cure completeness; three percent costs nearly thirty. This is why LSR machines have pressure and flow monitoring on both drums and why a good shop checks the ratio at the start of every shift, not every month.<\/p>\n<p>Mixing itself is a separate problem. A static mixer works if the flow rate is within its design range and the element count is right; below that range the mixing is incomplete and the part has streaks. A dynamic mixer is more forgiving but adds a seal that wears. Either way, the mixer is a consumable and should be on a replacement schedule.<\/p>\n<div class=\"callout\">Ask for the mix ratio log. A supplier who cannot show one is not measuring the variable that decides whether your seal works in year three.<\/div>\n<h2 id=\"coldrunner\">Cold runner and the needle seal<\/h2>\n<p>The cold runner keeps the mixed compound at 20 to 30 \u00b0C right up to the gate. It is not a heated manifold like a plastic hot runner; it is a cooled one, and the gate is opened by a needle that retracts at the moment of injection.<\/p>\n<p>The needle seal is the most critical component in the tool. It must close cleanly against the gate seat, hold the compound without curing it, and open without stringing. A worn needle leaves a small amount of compound at the gate that cures on the next cycle, and the cured plug is injected into the next part as a hard inclusion.<\/p>\n<p>Temperature control of the cold runner is equally critical. Above about 40 \u00b0C the compound starts to cure slowly in the runner; below 15 \u00b0C it becomes too viscous to inject. The window is narrower than it looks, and it is the reason LSR tools use chilled water rather than tower water.<\/p>\n<div class=\"myth\"><b>\"Cold runner means no waste.\"<\/b> Cold runner means no cured runner waste. The compound in the runner still degrades over time and must be purged at the start of a run and after any stop longer than a few minutes.<\/div>\n<h2 id=\"mould\">The hot mould: 180 to 200 \u00b0C and very even<\/h2>\n<p>LSR cures by addition reaction, and the rate doubles roughly every 10 \u00b0C. At 170 \u00b0C a 2 mm section needs about 25 seconds; at 190 \u00b0C it needs 12. Most production tools run at 185 to 195 \u00b0C to keep the cycle short.<\/p>\n<p>Temperature uniformity matters more than the absolute value. A tool with a 15 \u00b0C spread across the cavities produces parts that vary in hardness by two or three points, and the variation follows the position in the tool. Cartridge heaters in a steel tool give better uniformity than a hot oil circuit in an aluminium one, which is why LSR tools are usually steel.<\/p>\n<p>The mould must also be vented properly. LSR is low viscosity and fills fast, and air that cannot escape is compressed into a void. Vents of 0.005 to 0.01 mm depth at the parting line are normal, and they must be cleaned regularly because cured silicone builds up in them.<\/p>\n<table>\n<thead>\n<tr>\n<th>Par\u00e2metro<\/th>\n<th>Typical value<\/th>\n<th>Note<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Mould temperature<\/td>\n<td>185 \u2013 195 \u00b0C<\/td>\n<td>Steel tool, cartridge heated<\/td>\n<\/tr>\n<tr>\n<td>Cold runner temperature<\/td>\n<td>20 \u2013 28 \u00b0C<\/td>\n<td>Chilled water<\/td>\n<\/tr>\n<tr>\n<td>Injection pressure<\/td>\n<td>80 \u2013 150 bar<\/td>\n<td>Low compared with plastic<\/td>\n<\/tr>\n<tr>\n<td>Cure time, 2 mm<\/td>\n<td>10 \u2013 15 s<\/td>\n<td>Doubles roughly every 10 \u00b0C down<\/td>\n<\/tr>\n<tr>\n<td>Cure time, 5 mm<\/td>\n<td>28 \u2013 40 s<\/td>\n<td>Section controls, not volume<\/td>\n<\/tr>\n<tr>\n<td>Mix ratio<\/td>\n<td>1:1 \u00b1 1%<\/td>\n<td>The variable that decides everything<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 2 \u2014 LSR process parameters. Injection pressure is low because the viscosity is low; the difficulty is in metering and temperature, not pressure.<\/p>\n<h2 id=\"tooling\">Tooling: steel, precision and a needle<\/h2>\n<p>An LSR tool is a precision tool. Cavities are usually hardened steel, often with a coating, because silicone releases poorly from bare steel and the parts are small enough that a damaged cavity is expensive. The parting line must be lapped to a fit that holds a 0.005 mm vent.<\/p>\n<p>Ejection is often by stripper plate rather than pins, because a pin mark on a soft silicone part is permanent and because the parts are frequently too small for a reliable pin. Air-assisted ejection is common on thin-walled parts.<\/p>\n<p>Multi-cavity tools are the norm \u2014 8, 16, 32, even 64 cavities \u2014 because the cycle is short and the machine time is the cost. A balanced runner is essential: an unbalanced tool produces cavities that fill at different rates, and the last cavity gets material that has been in the runner longer and is partly cured.<\/p>\n<h2 id=\"defects\">Defects and what each one means<\/h2>\n<table>\n<thead>\n<tr>\n<th>Defeito<\/th>\n<th>Causa<\/th>\n<th>First check<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Tacky or soft parts<\/td>\n<td>Mix ratio error or under-cure<\/td>\n<td>Ratio log, then cure time<\/td>\n<\/tr>\n<tr>\n<td>Hard inclusions in the part<\/td>\n<td>Cured plug from a worn needle seal<\/td>\n<td>Inspect the gate seat<\/td>\n<\/tr>\n<tr>\n<td>Streaks or cloudy areas<\/td>\n<td>Incomplete mixing<\/td>\n<td>Mixer element condition<\/td>\n<\/tr>\n<tr>\n<td>Voids at the last point to fill<\/td>\n<td>Venting<\/td>\n<td>Vent depth and cleanliness<\/td>\n<\/tr>\n<tr>\n<td>Flash<\/td>\n<td>Parting line wear or too much injection pressure<\/td>\n<td>Parting line, then pressure<\/td>\n<\/tr>\n<tr>\n<td>Parts stick in the cavity<\/td>\n<td>Too hot, no release, or an undercut<\/td>\n<td>Mould temperature, draft<\/td>\n<\/tr>\n<tr>\n<td>Hardness varies by cavity<\/td>\n<td>Temperature imbalance across the tool<\/td>\n<td>Thermal map of the tool<\/td>\n<\/tr>\n<tr>\n<td>Bubbles in thick sections<\/td>\n<td>Air entrained at the gate<\/td>\n<td>Injection speed profile<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 3 \u2014 LSR defects. The first three are process and metering; the rest are tooling.<\/p>\n<p>The one that is easiest to miss is hardness variation by cavity. It looks like a material problem and is almost always a thermal one: the cavities at the edge of the tool run cooler than the ones in the centre, and cooler means less complete cure. A thermal image of the open tool at operating temperature will show it in thirty seconds.<\/p>\n<div class=\"faq\">\n<h3>What volume justifies LSR over compression moulded silicone?<\/h3>\n<p>Roughly 10,000 parts per year per tool. Below that the tooling and machine time dominate; above it, the cycle time advantage and the absence of flash make LSR cheaper per part.<\/p>\n<h3>Why does LSR need a cold runner?<\/h3>\n<p>Because the compound cures at mould temperature. A conventional runner would cure with the part and be scrap. The cold runner holds the compound at 20 to 28 \u00b0C until the moment of injection.<\/p>\n<h3>How accurate must the 1:1 mix ratio be?<\/h3>\n<p>Within one percent. One percent of error costs about four percent of cure completeness, which shows as softness and poor compression set rather than as a visible defect.<\/p>\n<h3>What is the mould temperature for LSR?<\/h3>\n<p>185 to 195 \u00b0C for most production tools. Cure time roughly doubles for every 10 \u00b0C below that, so a cooler tool is a slower tool.<\/p>\n<h3>Can LSR be overmoulded onto plastic?<\/h3>\n<p>Yes, and it is one of its main advantages. LSR bonds to many thermoplastics, particularly polycarbonate, PBT and nylon, without a primer, provided the substrate surface is clean and the mould temperature is high enough.<\/p>\n<h3>Why do my LSR parts have hard specks in them?<\/h3>\n<p>A worn needle seal at the gate. A small amount of compound cures on the gate seat between shots and is injected into the next part. Inspect and replace the needle on schedule.<\/p>\n<\/div>\n<div class=\"cta\">\n<p class=\"cta-h\">Sourcing LSR parts and need a tool that holds tolerance?<\/p>\n<p>Send us the part drawing, the annual volume and the hardness. We will tell you the cavity count, the cold runner design and what the tool costs at each volume.<\/p>\n<p><a class=\"btn\" href=\"https:\/\/uplastech.com\/pt\/custom-injection-molding\/\">Talk to a silicone engineer<\/a>\n<\/div>\n<div class=\"rel\">\n<h4>Continue reading<\/h4>\n<ul>\n<li><a href=\"https:\/\/uplastech.com\/pt\/silicone-rubber-types-guide\/\">Silicone Rubber Types: HCR, LSR and RTV<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/pt\/hcr-solid-silicone-rubber\/\">HCR Solid Silicone Rubber<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/pt\/silicone-compound-formulation\/\">Silicone Compound Formulation: Base, Filler and Cure<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/pt\/medical-grade-silicone\/\">Medical-Grade Silicone: What the Standard Actually Requires<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/pt\/elastomer-overmolding-tpe\/\">Overmolding TPE onto Rigid Plastic<\/a><\/li>\n<\/ul>\n<\/div>\n<\/div>","protected":false},"excerpt":{"rendered":"<p>LSR is not rubber moulding with a thinner material. It is a metering process, and one percent of mix ratio error costs you four percent of cure \u2014 which you will find in the field, not on the line.<\/p>","protected":false},"author":1,"featured_media":7785,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_seopress_titles_title":"LSR Injection Moulding: Metering, Cold Runner and Cycle","_seopress_titles_desc":"Liquid silicone rubber injection moulding: 1:1 metering accuracy, static and dynamic mixing, cold runner needle seals, hot mould temperature and the defects eac","_seopress_robots_index":"","_seopress_robots_follow":"","_seopress_robots_imageindex":"","_seopress_robots_snippet":"","_seopress_robots_primary_cat":"39","_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":[39],"tags":[],"class_list":["post-7786","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-silicone-rubber-products"],"meta_box":{"post-to-quiz_to":[]},"_links":{"self":[{"href":"https:\/\/uplastech.com\/pt\/wp-json\/wp\/v2\/posts\/7786","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=7786"}],"version-history":[{"count":1,"href":"https:\/\/uplastech.com\/pt\/wp-json\/wp\/v2\/posts\/7786\/revisions"}],"predecessor-version":[{"id":7857,"href":"https:\/\/uplastech.com\/pt\/wp-json\/wp\/v2\/posts\/7786\/revisions\/7857"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/uplastech.com\/pt\/wp-json\/wp\/v2\/media\/7785"}],"wp:attachment":[{"href":"https:\/\/uplastech.com\/pt\/wp-json\/wp\/v2\/media?parent=7786"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/uplastech.com\/pt\/wp-json\/wp\/v2\/categories?post=7786"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/uplastech.com\/pt\/wp-json\/wp\/v2\/tags?post=7786"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}