{"id":7970,"date":"2025-12-20T09:00:00","date_gmt":"2025-12-20T09:00:00","guid":{"rendered":"https:\/\/uplastech.com\/thermoforming-billow-forming\/"},"modified":"2025-12-20T09:00:00","modified_gmt":"2025-12-20T09:00:00","slug":"thermoforming-billow-forming","status":"publish","type":"post","link":"https:\/\/uplastech.com\/id\/thermoforming-billow-forming\/","title":{"rendered":"Billow Forming: Pre-Stretching the Sheet Before It Touches the 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\/thermoforming-billow-forming.jpg\" alt=\"Wall ratio against draw ratio with and without a billow, and cycle time by forming method\" width=\"1536\" height=\"909\"><figcaption>A billow plus plug holds a 0.50 wall ratio at a 1.5 draw against 0.31 for vacuum, for about four seconds of extra cycle time.<\/figcaption><\/figure>\n<div class=\"toc\"><\/div>\n<h2 id=\"idea\">Blow first, then form<\/h2>\n<p>Billow forming, also called bubble assist or pre-blow, blows the heated sheet into a bubble before it touches the mould. The bubble stretches the sheet evenly over its whole area, and the sheet is then drawn down onto the tool while it is already thin and uniformly strained.<\/p>\n<p>The point is where the thinning happens. Without a billow, the sheet thins where it first touches the mould and then in the corners, so the wall distribution is decided by the geometry of the tool. With a billow, the sheet is thinned first, over a controlled bubble, and then the tool forms it with much less additional stretching.<\/p>\n<p>The same idea appears in blow moulding as pre-blow, and in injection blow moulding as the stretch rod. It is the standard way to make a wall distribution more uniform without changing the tool, and it is one of the few process changes that improves the part rather than trading one defect for another.<\/p>\n<h2 id=\"wall\">The gap widens as the draw deepens<\/h2>\n<p>At a 0.3 draw, a billow plus a plug gives a wall ratio of 0.80 against 0.62 for plain vacuum, a difference of 0.18. At 0.6 it is 0.72 against 0.50, at 0.9 it is 0.64 against 0.42, at 1.2 it is 0.56 against 0.36 and at 1.5 it is 0.50 against 0.31.<\/p>\n<p>The difference grows with depth, from 0.18 at a 0.3 draw to 0.22 at 0.6 and 0.9, then eases to 0.20 and 0.19. The absolute advantage peaks around a 0.6 to 0.9 draw, which is exactly the range where a plain vacuum form starts to fail. That is why billow forming is associated with deep parts: it is not that it helps more when the draw is deep, it is that the alternative stops working.<\/p>\n<p>In design terms, a part with a minimum wall of 1.0 mm at a 1.2 draw needs a 2.8 mm sheet with plain vacuum and a 1.8 mm sheet with a billow. That is a 36 percent material saving, and on a large part it is worth more than the tool.<\/p>\n<table>\n<thead>\n<tr>\n<th>Draw ratio<\/th>\n<th>Billow plus plug<\/th>\n<th>Vacuum only<\/th>\n<th>Difference<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>0.3<\/td>\n<td>0.80<\/td>\n<td>0.62<\/td>\n<td>+0.18<\/td>\n<\/tr>\n<tr>\n<td>0.6<\/td>\n<td>0.72<\/td>\n<td>0.50<\/td>\n<td>+0.22<\/td>\n<\/tr>\n<tr>\n<td>0.9<\/td>\n<td>0.64<\/td>\n<td>0.42<\/td>\n<td>+0.22<\/td>\n<\/tr>\n<tr>\n<td>1.2<\/td>\n<td>0.56<\/td>\n<td>0.36<\/td>\n<td>+0.20<\/td>\n<\/tr>\n<tr>\n<td>1.5<\/td>\n<td>0.50<\/td>\n<td>0.31<\/td>\n<td>+0.19<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 1 \u2014 Wall ratio by draw, with and without a billow.<\/p>\n<h2 id=\"setting\">Setting the billow height<\/h2>\n<p>The billow is set by pressure and time, and the useful measure is the height of the bubble relative to the clamp opening. Too small a billow does nothing; too large a one thins the pole so much that the bottom of the part fails.<\/p>\n<p>A practical starting point is a bubble height of 15 to 25 percent of the clamp diameter. That pre-strains the sheet by roughly 10 to 15 percent in each direction, which is enough to even out the distribution without over-thinning the pole. From there, increase until the wall at the base corner stops improving.<\/p>\n<table>\n<thead>\n<tr>\n<th>Billow height<\/th>\n<th>Biaxial pre-strain<\/th>\n<th>Pole thickness<\/th>\n<th>Effect<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>10 percent of clamp<\/td>\n<td>5 percent<\/td>\n<td>90 percent<\/td>\n<td>Barely visible in the wall<\/td>\n<\/tr>\n<tr>\n<td>15 percent<\/td>\n<td>10 percent<\/td>\n<td>84 percent<\/td>\n<td>The usual starting point<\/td>\n<\/tr>\n<tr>\n<td>20 percent<\/td>\n<td>15 percent<\/td>\n<td>78 percent<\/td>\n<td>Good balance for deep draws<\/td>\n<\/tr>\n<tr>\n<td>25 percent<\/td>\n<td>22 percent<\/td>\n<td>70 percent<\/td>\n<td>Pole starts to limit the part<\/td>\n<\/tr>\n<tr>\n<td>30 percent<\/td>\n<td>30 percent<\/td>\n<td>60 percent<\/td>\n<td>Splits at the pole on thin sheet<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 2 \u2014 Billow height against the pre-strain it puts into the sheet.<\/p>\n<p>The timing matters as much as the height. The bubble has to be released before the sheet cools against it, or the pre-strain is locked in and the subsequent forming produces a part with a thin, cold base. The sequence is: billow to height, hold for a fraction of a second, release, plug, vacuum. Total billow time is usually 0.3 to 1.0 seconds.<\/p>\n<p>The plug has to be matched to the billow. A plug that was sized for an unbillowed sheet will be too large once the sheet is pre-stretched, and it will punch through the thinned pole. In practice the plug is reduced in size or retracted slightly, and the vacuum does more of the final forming.<\/p>\n<div class=\"callout\">Billow and plug are a pair. Changing one without the other is the most common reason a billow conversion disappoints: the bubble helps, the plug then ruins it.<\/div>\n<h2 id=\"cost\">What it costs<\/h2>\n<p>The cost is cycle time, and it is small. A plain vacuum form of a 300 mm part runs about 22 seconds; adding a billow plus a plug takes it to 26. A pressure box at 31 seconds and a match mould at 38 are the next steps up, and each buys something different rather than more of the same.<\/p>\n<p>Four seconds on a 22 second cycle is 18 percent less output, which sounds worse than it is. The alternative for a deep part is a thicker sheet, which costs more material than the lost machine time, or a different process, which costs a tool. On most deep-draw jobs the billow is the cheapest of the three.<\/p>\n<p>There is also a quality cost that is easy to miss. A billow adds a control loop: the pressure, the height and the release point all have to be repeatable, or the wall distribution varies from shot to shot. A machine without a proportional valve and a height sensor will produce a billow that is roughly right, and roughly right is not good enough on a part designed to a minimum wall.<\/p>\n<h2 id=\"when\">When not to billow<\/h2>\n<p>A shallow part does not need it. Below a 0.4 draw the wall ratio with plain vacuum is already 0.55 to 0.62, and a billow adds a control variable for a gain of a few percent. The exception is a part with a sharp internal feature that the billow helps the sheet negotiate, which is a different problem.<\/p>\n<p>A part with a printed or laminated surface may not survive it. The pre-stretch strains the print by 10 to 15 percent before the tool touches it, and a registered print will be out of register by the time the part is formed. The same applies to a laminated film that is already close to its elongation limit.<\/p>\n<p>And a part that is already at the material's forming limit will fail. If the sheet is being formed near its burst strain, adding a billow puts it over the edge, and the failure appears as a split at the pole rather than as a thin wall. The formability test described for free forming is the way to find that limit before committing to the process.<\/p>\n<h2 id=\"specify\">What to specify<\/h2>\n<p>Billow forming is usually a process choice rather than a drawing feature, but the drawing has to leave room for it.<\/p>\n<ul>\n<li>Minimum wall at the base corner, which is the number the billow is there to protect.<\/li>\n<li>Draw ratio, so the process can be matched to it.<\/li>\n<li>Whether the sheet is printed or laminated, and the allowable strain on the decoration.<\/li>\n<li>Plug geometry and the clearance to the mould, which change once a billow is used.<\/li>\n<li>Cycle time expectation, so the four seconds is priced rather than discovered.<\/li>\n<li>Sheet thickness, because the billow allows a thinner sheet for the same minimum wall.<\/li>\n<\/ul>\n<p>The sheet thickness is where the money is. A part designed at 2.8 mm because plain vacuum cannot hold the corner can often be made at 1.8 mm with a billow, and the saving on a large part repays the process development in the first production run.<\/p>\n<div class=\"faq\">\n<h3>What is billow forming?<\/h3>\n<p>Blowing the heated sheet into a bubble before it touches the mould, so that it is pre-stretched evenly. The sheet is then formed onto the tool with less additional thinning.<\/p>\n<h3>How much does a billow improve the wall?<\/h3>\n<p>The wall ratio at a 1.5 draw goes from 0.31 with plain vacuum to 0.50 with a billow and plug. At a 0.3 draw the gain is 0.62 to 0.80.<\/p>\n<h3>How high should the billow be?<\/h3>\n<p>About 15 to 25 percent of the clamp diameter as a starting point, which pre-strains the sheet by 10 to 15 percent. Increase until the base corner stops improving.<\/p>\n<h3>Does billow forming cost cycle time?<\/h3>\n<p>About four seconds on a 22 second cycle, so roughly 18 percent of output. On deep parts that is cheaper than a thicker sheet or a different tool.<\/p>\n<h3>Can billow forming be used with printed sheet?<\/h3>\n<p>Only if the print can tolerate 10 to 15 percent strain. A registered print will be out of register after the pre-stretch, and a laminated film may split.<\/p>\n<h3>Does the plug need to change when a billow is added?<\/h3>\n<p>Yes. The sheet is already thinner and larger, so a plug sized for an unbillowed sheet will punch through. Reduce the plug or let the vacuum do more of the final forming.<\/p>\n<\/div>\n<div class=\"cta\">\n<p class=\"cta-h\">Deep part that keeps thinning at the base?<\/p>\n<p>Send the draw ratio and the minimum wall you need. We will tell you whether a billow and a plug will hold it, and how much thinner the sheet can be as a result.<\/p>\n<p><a class=\"btn\" href=\"https:\/\/uplastech.com\/id\/custom-vacuum-forming\/\">Ask about billow forming<\/a>\n<\/div>\n<div class=\"rel\">\n<h4>Continue reading<\/h4>\n<ul>\n<li><a href=\"https:\/\/uplastech.com\/id\/thermoforming-free-forming\/\">Free Forming: Shaping Without a Mould Surface<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/id\/thermoforming-drape-forming\/\">Drape Forming: The Oldest Method and Where It Still Wins<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/id\/plug-assist-thin-gauge-forming\/\">Plug Assist in Thin-Gauge Thermoforming: Prestretch and Wall Profile<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/id\/thermoforming-vacuum-vs-pressure\/\">Vacuum Versus Pressure Forming: Detail, Cost and the Tool Each Needs<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/id\/custom-vacuum-forming\/\">Custom Vacuum Forming: Tooling, Materials and Tolerances<\/a><\/li>\n<\/ul>\n<\/div>\n<\/div>","protected":false},"excerpt":{"rendered":"<p>A billow holds a 0.64 wall ratio at a 0.9 draw where plain vacuum gives 0.42, and the gap widens as the draw deepens. It costs about four seconds of cycle time, which is the entire price.<\/p>","protected":false},"author":1,"featured_media":7969,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_seopress_titles_title":"Billow Forming: Pre-Stretch and Wall Ratio","_seopress_titles_desc":"Billow forming in thermoforming: blowing a bubble to pre-stretch the sheet before it touches the mould, wall ratio against draw compared with vacuum, setting th","_seopress_robots_index":"","_seopress_robots_follow":"","_seopress_robots_imageindex":"","_seopress_robots_snippet":"","_seopress_robots_primary_cat":"29","_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":[29],"tags":[],"class_list":["post-7970","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\/id\/wp-json\/wp\/v2\/posts\/7970","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/uplastech.com\/id\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/uplastech.com\/id\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/uplastech.com\/id\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/uplastech.com\/id\/wp-json\/wp\/v2\/comments?post=7970"}],"version-history":[{"count":0,"href":"https:\/\/uplastech.com\/id\/wp-json\/wp\/v2\/posts\/7970\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/uplastech.com\/id\/wp-json\/wp\/v2\/media\/7969"}],"wp:attachment":[{"href":"https:\/\/uplastech.com\/id\/wp-json\/wp\/v2\/media?parent=7970"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/uplastech.com\/id\/wp-json\/wp\/v2\/categories?post=7970"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/uplastech.com\/id\/wp-json\/wp\/v2\/tags?post=7970"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}