{"id":8543,"date":"2026-06-26T09:00:00","date_gmt":"2026-06-26T08:00:00","guid":{"rendered":"https:\/\/uplastech.com\/thermoforming-clamp-frame\/"},"modified":"2026-11-02T02:28:22","modified_gmt":"2026-11-02T02:28:22","slug":"thermoforming-clamp-frame","status":"publish","type":"post","link":"https:\/\/uplastech.com\/fr\/thermoforming-clamp-frame\/","title":{"rendered":"The Clamp Frame: The Thermoforming Setting Nobody Adjusts"},"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-clamp-frame.jpg\" alt=\"A white plastic sheet gripped along its edge by the steel clamp frame of a thermoforming machine, the frame teeth biting into the sheet\" width=\"1536\" height=\"909\"><figcaption>The teeth hold the sheet. If it moves, the part is short before the vacuum starts.<\/figcaption><\/figure>\n<div class=\"toc\"><\/div>\n<h2 id=\"what\">What the frame actually does<\/h2>\n<p>The clamp frame holds the sheet flat and in position while it is heated and formed. It has to do three things: resist the shrinkage that occurs when the sheet is heated, hold the sheet against the pull of the plug and the vacuum, and release the part without leaving a mark that extends into the finished area.<\/p>\n<p>Those three requirements conflict. More grip holds the sheet better but marks it deeper and reduces the usable area. A wider frame holds better but consumes more material. A frame that is set up for one sheet thickness will be wrong for another, and many machines run three or four thicknesses in a week.<\/p>\n<p>The frame is therefore a setting, not a fixture. It has a clamping force, a tooth profile, a margin width and a position relative to the heater, and all four affect the part. Treating it as part of the machine rather than part of the process is why slip problems persist for years.<\/p>\n<h2 id=\"slip\">Slip is the failure that looks like a forming problem<\/h2>\n<p>The chart on the left plots sheet slip against clamping force. At 20 percent of the frame rating the sheet moves 3.2 mm during forming. At 60 percent it moves 1.2 mm, and at 100 percent it moves 0.3 mm. The curve falls steeply and then flattens, which is the shape of any friction limit.<\/p>\n<p>A slip of 3 mm on a part that is 200 mm across is 1.5 percent of the dimension, which is more than the tolerance on most formed parts. It is also uneven: the sheet slips more on the side where the plug or the vacuum pulls hardest, so the part is not just short, it is asymmetric.<\/p>\n<p>The symptom is a part that is thin at one end and thick at the other, or a part whose features are off centre. It is very often diagnosed as a heating problem, because uneven heating produces a similar result. The way to tell them apart is to mark the sheet at the frame line before forming and measure where the mark ends up.<\/p>\n<p>Slip also happens during heating, not only during forming. A sheet that is heated to 160 C expands and then softens, and the frame has to hold it against its own shrinkage as it relaxes. A frame that is adequate during forming can still let the sheet creep during a long heating cycle.<\/p>\n<figure>\n<img decoding=\"async\" src=\"https:\/\/uplastech.com\/wp-content\/uploads\/2026\/11\/thermoforming-clamp-frame-chart.jpg\" alt=\"Sheet slip falling from 3.2 mm at low clamping force to 0.3 mm at full force, and a bar chart of clamp frame problems\" width=\"1536\" height=\"909\"><figcaption>Left: slip against clamping force. Right: what a badly set frame causes.<\/figcaption><\/figure>\n<h2 id=\"margin\">The margin costs material twice<\/h2>\n<p>The clamp margin is the strip of sheet that the frame covers. It cannot be formed, so it is scrap, and its width is set by the frame design rather than by the part. A 40 mm margin on each side of a 600 mm sheet is 13 percent of the width, which on a part that yields 70 percent overall is a large share of the loss.<\/p>\n<p>The margin also has to be heated, which costs energy, and it has to be cooled, which costs time. On a thick sheet the margin is the slowest part of the sheet to reach temperature and the slowest to cool, so it can set the cycle time for the whole job.<\/p>\n<p>Reducing the margin is a machine decision, not a process one. A frame with a narrower clamping face, or a frame that grips with a serrated edge instead of a flat face, can hold the same sheet with less covered area. That is worth doing when a job runs for years.<\/p>\n<p>The teeth have a cost too. A serrated frame that grips well leaves marks that extend a few millimetres into the sheet, and the trim line has to be outside them. A frame with a smooth face marks less but grips less, and needs a wider margin to compensate.<\/p>\n<h2 id=\"maintain\">Maintenance that is never done<\/h2>\n<p>The frame is a mechanical assembly with springs, pneumatic cylinders and wear surfaces, and it is usually the least maintained part of the machine. A cylinder that is 20 percent down on force, a spring that has taken a set, or a tooth that has worn flat will all reduce grip without any visible sign.<\/p>\n<p>The check is simple: clamp a sheet, mark the frame line, form the part, and measure the slip. Do it on each side of the frame. A frame that slips 0.3 mm on three sides and 2 mm on the fourth has one weak cylinder, and it takes ten minutes to find.<\/p>\n<p>The tooth profile wears as well. A serrated edge that has rounded over grips like a smooth one, and the operator compensates by increasing the force until something else breaks. Replacing or re-cutting the teeth is a maintenance item that is almost never scheduled.<\/p>\n<table>\n<thead>\n<tr>\n<th>Check<\/th>\n<th>Frequency<\/th>\n<th>M\u00e9thode<\/th>\n<th>Accept<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Slip on each side<\/td>\n<td>Monthly<\/td>\n<td>Mark, form, measure<\/td>\n<td>Under 1 mm<\/td>\n<\/tr>\n<tr>\n<td>Clamping force<\/td>\n<td>Quarterly<\/td>\n<td>Pressure gauge on each cylinder<\/td>\n<td>Within 10 % of setting<\/td>\n<\/tr>\n<tr>\n<td>Tooth condition<\/td>\n<td>Annually<\/td>\n<td>Visual and a straight edge<\/td>\n<td>No rounding over<\/td>\n<\/tr>\n<tr>\n<td>Frame alignment<\/td>\n<td>Annually<\/td>\n<td>Diagonal measurement<\/td>\n<td>Within 1 mm<\/td>\n<\/tr>\n<tr>\n<td>Heater to frame distance<\/td>\n<td>Annually<\/td>\n<td>Tape measure<\/td>\n<td>Per the machine drawing<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 1 \u2014 A frame maintenance schedule<\/p>\n<h2 id=\"marks\">Clamp marks and the trim line<\/h2>\n<p>A clamp mark is a witness mark left by the frame teeth. It is acceptable as long as it is outside the trim line, and unacceptable the moment it is inside. That is a layout decision: the trim line has to be placed with the mark in mind, not the other way round.<\/p>\n<p>Deep marks are a sign that the force is too high for the sheet thickness. A thin sheet needs less force than a thick one, because the teeth do not have to bite as deep to hold it. Running a thin sheet at the setting for a thick one produces marks that go halfway through.<\/p>\n<p>On a cosmetic part, the answer is often a smooth frame face with a higher force, which marks less deeply over a wider area. The trade is a wider margin against a shallower mark, and it is worth doing when the part is visible.<\/p>\n<h2 id=\"uneven\">Uneven heating at the frame line<\/h2>\n<p>The frame is a heat sink. It conducts heat out of the sheet at the edge, so the sheet near the clamp is cooler than the sheet in the middle. That is why the corners of a formed part are often the last to form and the first to tear.<\/p>\n<p>The effect is larger on a metal frame than on a composite one, and larger on a thick frame than a thin one. Some machines use an insulating pad between the frame and the sheet for exactly this reason, and it is a cheap retrofit.<\/p>\n<p>The other side of the same problem is that the frame shadows the sheet from the heaters. The area under the frame is not heated from above, so the sheet at the frame line is heated only by conduction along the sheet, which is slow. The result is a cold band that extends further into the part than the frame itself.<\/p>\n<p>That cold band sets the effective margin. If the cold band extends 30 mm inside the frame line, the part has to start 30 mm inside it, and the usable area is smaller than the frame suggests. Measuring the temperature profile across the sheet, from the frame inwards, shows where the part can actually begin.<\/p>\n<table>\n<thead>\n<tr>\n<th>Frame width<\/th>\n<th>Cold band<\/th>\n<th>Usable loss per side<\/th>\n<th>On a 600 mm sheet<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>25 mm<\/td>\n<td>15 mm<\/td>\n<td>40 mm<\/td>\n<td>13 %<\/td>\n<\/tr>\n<tr>\n<td>40 mm<\/td>\n<td>25 mm<\/td>\n<td>65 mm<\/td>\n<td>22 %<\/td>\n<\/tr>\n<tr>\n<td>50 mm<\/td>\n<td>30 mm<\/td>\n<td>80 mm<\/td>\n<td>27 %<\/td>\n<\/tr>\n<tr>\n<td>50 mm, insulated<\/td>\n<td>15 mm<\/td>\n<td>65 mm<\/td>\n<td>22 %<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 2 \u2014 The margin you pay for is the frame plus the cold band<\/p>\n<p>The insulated row is the one to notice. An insulating pad between the frame and the sheet does not change the frame width, but it halves the cold band, and on a 600 mm sheet that recovers 5 percent of the material. It is one of the cheapest improvements available on an older machine.<\/p>\n<div class=\"faq\">\n<h3>Why is my formed part short on one side?<\/h3>\n<p>The sheet slipped in the clamp frame on that side. Mark the frame line before forming and measure where the mark ends up. A slip of 2 to 3 mm is enough to make a part out of tolerance.<\/p>\n<h3>How tight should the clamp frame be?<\/h3>\n<p>Enough to hold the sheet with less than 1 mm of slip, and no more. Above about 60 percent of the frame rating the slip curve flattens, so extra force buys nothing and only deepens the clamp marks.<\/p>\n<h3>How wide should the clamp margin be?<\/h3>\n<p>As narrow as the frame allows, typically 25 to 50 mm per side. The margin is scrap, and on a small part it can be 15 percent of the sheet. A serrated or narrow-face frame reduces it.<\/p>\n<h3>Why are the corners of my part always the last to form?<\/h3>\n<p>The frame conducts heat out of the sheet at the edge, so the corners are cooler. An insulating pad between the frame and the sheet, or a longer heating cycle, brings them up to temperature.<\/p>\n<h3>Can clamp marks be avoided?<\/h3>\n<p>They can be kept outside the trim line, which is a layout decision. Deep marks mean the force is too high for the sheet thickness; a smooth frame face marks less but needs a wider margin.<\/p>\n<h3>Does the frame need maintenance?<\/h3>\n<p>Yes, and it rarely gets it. Cylinders lose force, springs take a set and teeth round over, all without a visible sign. A monthly slip check on each side finds a weak cylinder in ten minutes.<\/p>\n<\/div>\n<div class=\"cta\">\n<p class=\"cta-h\">Parts coming out short or off centre?<\/p>\n<p>Send a photograph of the part and the clamp mark, the sheet size and the frame dimensions. We will tell you whether it is slip, heating or the tool.<\/p>\n<p><a class=\"btn\" href=\"https:\/\/uplastech.com\/fr\/formage-sous-vide\/\">Ask about clamp frames<\/a>\n<\/div>\n<div class=\"rel\">\n<h4>Continue reading<\/h4>\n<ul>\n<li><a href=\"https:\/\/uplastech.com\/fr\/thermoforming-sheet-sag-control\/\">Sheet Sag Control: Keeping the Sheet Where You Put It<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/fr\/thermoforming-heating-element-types\/\">Heating Element Types in Thermoforming<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/fr\/thermoforming-sheet-yield\/\">Sheet Yield in Thermoforming: Nesting, Margins and the Cost of a Bad Layout<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/fr\/thermoforming-webbing\/\">Webbing in Thermoforming: The Fold Between Two Draws<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/fr\/thermoforming-radiant-vs-contact-heating\/\">Radiant Against Contact Heating: Choosing How to Heat the Sheet<\/a><\/li>\n<\/ul>\n<\/div>\n<\/div>","protected":false},"excerpt":{"rendered":"<p>A sheet that slips in the frame during forming comes out short, thin at one end and out of position. The clamp is treated as a fixture, but it is a process setting, and it has an optimum just like temperature and vacuum.<\/p>","protected":false},"author":1,"featured_media":8541,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[29],"tags":[],"class_list":["post-8543","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\/8543","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=8543"}],"version-history":[{"count":1,"href":"https:\/\/uplastech.com\/fr\/wp-json\/wp\/v2\/posts\/8543\/revisions"}],"predecessor-version":[{"id":8878,"href":"https:\/\/uplastech.com\/fr\/wp-json\/wp\/v2\/posts\/8543\/revisions\/8878"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/uplastech.com\/fr\/wp-json\/wp\/v2\/media\/8541"}],"wp:attachment":[{"href":"https:\/\/uplastech.com\/fr\/wp-json\/wp\/v2\/media?parent=8543"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/uplastech.com\/fr\/wp-json\/wp\/v2\/categories?post=8543"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/uplastech.com\/fr\/wp-json\/wp\/v2\/tags?post=8543"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}