{"id":7798,"date":"2025-10-08T09:00:00","date_gmt":"2025-10-08T08:00:00","guid":{"rendered":"https:\/\/uplastech.com\/double-wall-corrugated-pipe\/"},"modified":"2026-10-28T09:22:27","modified_gmt":"2026-10-28T09:22:27","slug":"double-wall-corrugated-pipe","status":"publish","type":"post","link":"https:\/\/uplastech.com\/it\/double-wall-corrugated-pipe\/","title":{"rendered":"Double-Wall Corrugated Pipe: Two Extruders, One Wall Structure"},"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\/double-wall-corrugated-pipe.jpg\" alt=\"Ring stiffness against corrugation height and wall thickness comparison for double-wall pipe\" width=\"1536\" height=\"909\"><figcaption>A solid pipe with the same stiffness would need a 6.2 mm wall. The corrugated one uses 1.6 and 2.4.<\/figcaption><\/figure>\n<div class=\"toc\"><\/div>\n<h2 id=\"product\">What the product is<\/h2>\n<p>A double-wall corrugated pipe has a smooth inner wall that carries the flow and a corrugated outer wall that provides the stiffness. The two are extruded separately and fused at the crest of each corrugation, so the finished wall is a series of closed rings bonded to a liner.<\/p>\n<p>The structure is efficient because bending stiffness depends on the distance of material from the neutral axis. Moving material out to a corrugation crest multiplies its contribution by the square of that distance. A 400 mm pipe with 1.6 mm outer and 2.4 mm inner walls has the ring stiffness of a solid pipe with a 6.2 mm wall, at about 40 percent of the material.<\/p>\n<p>The product is used for stormwater, sewer and drainage, where the load is soil pressure and the requirement is stiffness rather than pressure containment. It is not a pressure pipe and must never be specified as one.<\/p>\n<table>\n<thead>\n<tr>\n<th>Nominal stiffness<\/th>\n<th>Typical application<\/th>\n<th>Burial depth<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>SN4<\/td>\n<td>Garden drainage, shallow burial<\/td>\n<td>Up to 2 m<\/td>\n<\/tr>\n<tr>\n<td>SN8<\/td>\n<td>Stormwater, standard sewer<\/td>\n<td>2 \u2013 4 m<\/td>\n<\/tr>\n<tr>\n<td>SN16<\/td>\n<td>Deep burial, traffic loading<\/td>\n<td>4 \u2013 6 m<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 1 \u2014 Ring stiffness classes for corrugated HDPE pipe. The class is determined by the corrugation geometry and wall thickness, not by the resin alone.<\/p>\n<h2 id=\"process\">Two extruders and one corrugator<\/h2>\n<p>The process uses two extruders feeding a common die head. The inner extruder produces the smooth liner through a mandrel; the outer extruder produces a tube that is blown or drawn into the corrugation moulds. The two meet at the die exit and are pressed together at the crest by the mould blocks.<\/p>\n<p>The corrugator is a pair of moving mould-block trains that close around the tube, form the corrugation under vacuum, hold it while it cools, and open at the end of the run. The blocks determine the corrugation shape, the pitch and the pipe diameter, and changing diameter means changing the block set.<\/p>\n<p>The critical control is the timing of the two layers. If the outer layer reaches the blocks before the inner layer has formed, the liner is distorted. If the inner layer is too cold, it will not fuse to the crest and the wall delaminates. The temperature of both melts and the vacuum at the blocks are the variables that control it.<\/p>\n<div class=\"callout\">A delaminated wall passes a stiffness test and fails in service. The bond at the crest is the only thing holding the structure together, and it is invisible from outside.<\/div>\n<h2 id=\"stiffness\">Ring stiffness: where it comes from<\/h2>\n<p>Ring stiffness is measured by loading a pipe section between two plates and recording deflection. The value, in kN\/m\u00b2, is the load per unit area at 3 percent deflection. It is a structural property of the wall section, not of the material.<\/p>\n<p>Corrugation height is the dominant variable. Stiffness rises roughly with the 2.5 to 3 power of height, which is why a small increase in corrugation depth gives a large increase in stiffness. Pitch has a smaller effect, and wall thickness a smaller one still.<\/p>\n<p>This is why the design of a corrugated pipe is a geometry exercise. Adding material to the walls is the least efficient way to gain stiffness; increasing corrugation height is the most efficient, limited by the diameter and by the block design.<\/p>\n<table>\n<thead>\n<tr>\n<th>Parametro<\/th>\n<th>Effect on stiffness<\/th>\n<th>Practical limit<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Corrugation height<\/td>\n<td>Strong, roughly cubic<\/td>\n<td>Block geometry and diameter<\/td>\n<\/tr>\n<tr>\n<td>Corrugation pitch<\/td>\n<td>Moderato<\/td>\n<td>Block count and line speed<\/td>\n<\/tr>\n<tr>\n<td>Outer wall thickness<\/td>\n<td>Moderato<\/td>\n<td>Material cost<\/td>\n<\/tr>\n<tr>\n<td>Inner wall thickness<\/td>\n<td>Small<\/td>\n<td>Must carry flow, not load<\/td>\n<\/tr>\n<tr>\n<td>Resin modulus<\/td>\n<td>Linear<\/td>\n<td>Fixed by grade<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 2 \u2014 What drives ring stiffness. The order matters when a specification is being met.<\/p>\n<h2 id=\"joint\">Socket, spigot and the joint that has to be watertight<\/h2>\n<p>Corrugated pipe is joined by an integral socket and spigot with an elastomeric seal, or by a separate coupler. The socket is formed in line by a belling operation, or moulded as a separate fitting for large diameters.<\/p>\n<p>The seal sits in a groove in the spigot and compresses against the socket bore. The compression must be right: too little and the joint leaks under settlement, too much and the pipe cannot be pushed home. Most standards specify 30 to 50 percent compression of the seal cross-section.<\/p>\n<p>The joint is also the weak point structurally. A socket interrupts the corrugation, so the pipe at the socket is less stiff than the pipe body. For deep burial, the socket is often reinforced or the pipe is specified with a thicker wall at the joint end.<\/p>\n<h2 id=\"defects\">Defects at line speed<\/h2>\n<table>\n<thead>\n<tr>\n<th>Difetto<\/th>\n<th>Causa<\/th>\n<th>First check<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Inner wall corrugated or wavy<\/td>\n<td>Inner melt too hot or vacuum too high<\/td>\n<td>Inner melt temperature, block vacuum<\/td>\n<\/tr>\n<tr>\n<td>Delamination at the crest<\/td>\n<td>Insufficient fusion<\/td>\n<td>Melt temperatures at the die, contact pressure<\/td>\n<\/tr>\n<tr>\n<td>Corrugation collapse<\/td>\n<td>Cooling too slow before block opening<\/td>\n<td>Block cooling water, line speed<\/td>\n<\/tr>\n<tr>\n<td>Uneven corrugation pitch<\/td>\n<td>Block synchronisation<\/td>\n<td>Corrugator drive and chain tension<\/td>\n<\/tr>\n<tr>\n<td>Ovality<\/td>\n<td>Cooling imbalance around the pipe<\/td>\n<td>Water distribution on the blocks<\/td>\n<\/tr>\n<tr>\n<td>Thin crest<\/td>\n<td>Outer melt too hot, blowing too hard<\/td>\n<td>Outer melt temperature, blow pressure<\/td>\n<\/tr>\n<tr>\n<td>Rough inner surface<\/td>\n<td>Moisture or cold die<\/td>\n<td>Resin drying, die temperature<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 3 \u2014 Double-wall corrugated pipe defects. Delamination is the one that passes inspection and fails in the ground.<\/p>\n<p>The defect that is hardest to catch is a partial delamination. The crest is bonded over most of its width but not all, and the pipe passes a ring stiffness test because the unbonded area is small. Under soil load and traffic, the unbonded section propagates and the wall separates. A peel test on a section cut from the start of each run is the only reliable check.<\/p>\n<h2 id=\"qc\">Quality checks that matter<\/h2>\n<p>Ring stiffness is the headline test and it is done on a short section in a compression frame. It is destructive and it is slow, so it is done on a sample per run rather than continuously.<\/p>\n<p>The continuous checks are dimensional: outside diameter over the corrugation, inside diameter, wall thickness at crest and valley, and pitch. A laser gauge or a shadowgraph at the haul-off gives a record and catches drift.<\/p>\n<p>The joint is checked separately: seal compression, socket ovality and the force needed to assemble. A joint that requires a excavator to push home will be damaged during installation, and the damage will not be visible.<\/p>\n<div class=\"faq\">\n<h3>What is ring stiffness SN8?<\/h3>\n<p>A ring stiffness of 8 kN\/m\u00b2 measured at 3 percent deflection. It is the most common class for stormwater and sewer pipe, suitable for burial depths of 2 to 4 metres.<\/p>\n<h3>How is double-wall corrugated pipe made?<\/h3>\n<p>Two extruders feed a common die. The inner extruder forms a smooth liner on a mandrel; the outer extruder forms a tube that is drawn into moving corrugation mould blocks under vacuum. The two layers fuse at the crest.<\/p>\n<h3>Can corrugated pipe be used for pressure?<\/h3>\n<p>No. It is a gravity drainage product with no pressure rating. The corrugated wall is stiff in bending but has no hoop strength for internal pressure.<\/p>\n<h3>Why does my corrugated pipe delaminate?<\/h3>\n<p>Insufficient fusion at the crest, usually from one melt being too cold or the contact pressure at the blocks being too low. A partial delamination passes a stiffness test and fails in the ground, so a peel test is the reliable check.<\/p>\n<h3>What is the difference between SN4 and SN8 pipe?<\/h3>\n<p>SN4 is 4 kN\/m\u00b2 and is used for shallow garden drainage; SN8 is 8 kN\/m\u00b2 for standard sewer burial. The difference is in corrugation height and wall thickness, not in the resin.<\/p>\n<h3>How deep can SN8 corrugated pipe be buried?<\/h3>\n<p>Typically 2 to 4 metres, depending on the soil, the loading and the installation method. Below that, SN16 or a engineered bedding design is required.<\/p>\n<\/div>\n<div class=\"cta\">\n<p class=\"cta-h\">Producing or specifying corrugated drainage pipe?<\/p>\n<p>Tell us the diameter, the stiffness class and the burial depth. We will tell you the corrugation geometry and the wall combination that meets it at the lowest material cost.<\/p>\n<p><a class=\"btn\" href=\"https:\/\/uplastech.com\/it\/estrusioni-di-tubi\/\">Talk to a pipe extrusion engineer<\/a>\n<\/div>\n<div class=\"rel\">\n<h4>Continue reading<\/h4>\n<ul>\n<li><a href=\"https:\/\/uplastech.com\/it\/hdpe-pipe-extrusion\/\">HDPE Pipe Extrusion: Sag, Ovality and Cooling<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/it\/spiral-wound-pipe\/\">Spiral Wound Pipe: Large Diameter From a Small Machine<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/it\/ottenere-dimensioni-coerenti-del-prodotto-nellestrusione\/\">Consistent Product Dimensions in Extrusion<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/it\/ruolo-del-raffreddamento-nellestrusione-della-plastica\/\">The Role of Cooling in Plastic Extrusion<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/it\/pe-rt-pipe-extrusion\/\">PE-RT Pipe Extrusion<\/a><\/li>\n<\/ul>\n<\/div>\n<\/div>","protected":false},"excerpt":{"rendered":"<p>A corrugated pipe gets SN8 stiffness from geometry rather than material. Two extruders, a corrugator and a set of moving mould blocks make a wall that is 60 percent air.<\/p>","protected":false},"author":1,"featured_media":7797,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_seopress_titles_title":"Double-Wall Corrugated Pipe: Extrusion, Corrugation and Ring","_seopress_titles_desc":"Double-wall corrugated HDPE pipe production: twin extrusion, corrugator mould blocks, ring stiffness, socket joints and the defects that appear at high line spe","_seopress_robots_index":"","_seopress_robots_follow":"","_seopress_robots_imageindex":"","_seopress_robots_snippet":"","_seopress_robots_primary_cat":"17","_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":[17],"tags":[],"class_list":["post-7798","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\/it\/wp-json\/wp\/v2\/posts\/7798","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/uplastech.com\/it\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/uplastech.com\/it\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/uplastech.com\/it\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/uplastech.com\/it\/wp-json\/wp\/v2\/comments?post=7798"}],"version-history":[{"count":1,"href":"https:\/\/uplastech.com\/it\/wp-json\/wp\/v2\/posts\/7798\/revisions"}],"predecessor-version":[{"id":7863,"href":"https:\/\/uplastech.com\/it\/wp-json\/wp\/v2\/posts\/7798\/revisions\/7863"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/uplastech.com\/it\/wp-json\/wp\/v2\/media\/7797"}],"wp:attachment":[{"href":"https:\/\/uplastech.com\/it\/wp-json\/wp\/v2\/media?parent=7798"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/uplastech.com\/it\/wp-json\/wp\/v2\/categories?post=7798"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/uplastech.com\/it\/wp-json\/wp\/v2\/tags?post=7798"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}