{"id":7792,"date":"2025-10-05T09:00:00","date_gmt":"2025-10-05T08:00:00","guid":{"rendered":"https:\/\/uplastech.com\/pmma-acrylic-thermoforming\/"},"modified":"2026-10-28T09:22:21","modified_gmt":"2026-10-28T09:22:21","slug":"pmma-acrylic-thermoforming","status":"publish","type":"post","link":"https:\/\/uplastech.com\/pl\/pmma-acrylic-thermoforming\/","title":{"rendered":"PMMA Thermoforming: Forming Acrylic Without Losing the Clarity You Paid For"},"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\/pmma-acrylic-thermoforming.jpg\" alt=\"PMMA thermoforming window by thickness and wall retention against draw ratio\" width=\"1536\" height=\"909\"><figcaption>A white band at a radius is not a surface defect. It is a line of reduced toughness.<\/figcaption><\/figure>\n<div class=\"toc\"><\/div>\n<h2 id=\"why\">Why form acrylic rather than mould it<\/h2>\n<p>Injection moulding gives a more precise acrylic part with a better surface and no trimming. Thermoforming gives a cheaper part at low volume, in sizes that no mould could economically produce, from sheet that is already optically perfect.<\/p>\n<p>The economics decide it. A 1,200 mm display window cannot be injection moulded at any reasonable cost. A run of 200 covers for a machine needs a tool in two weeks, not twelve. And a formed part starts from sheet that has already been polished or textured by the extruder, so the former inherits an optical quality it does not have to create.<\/p>\n<p>The trade is that the former has to preserve that quality through a heating cycle, and acrylic is unforgiving about it. The failure modes are specific: stress whitening at radii, bubbles from moisture, and crazing that appears weeks later.<\/p>\n<h2 id=\"drying\">Drying: the defect you cannot see at first<\/h2>\n<p>PMMA absorbs about 0.3 percent moisture. At forming temperature that moisture becomes steam, and steam in a softening sheet produces bubbles that are visible through the thickness \u2014 a defect that is fatal in a transparent part.<\/p>\n<p>Cast acrylic absorbs less than extruded and dries faster. Extruded sheet can hold enough moisture to bubble after a week in a humid warehouse, and it gives no warning. The target is 0.05 percent, reached at 80 \u00b0C in four to six hours for a 3 mm sheet.<\/p>\n<p>Cast sheet should be dried slowly and evenly. A rapid heat-up on a thick cast sheet can produce internal stress that shows as a faint haze after forming, and it cannot be removed.<\/p>\n<div class=\"callout\">If the sheet has been stored in an unheated warehouse through a humid season, assume it is wet regardless of what the label says. Four hours of drying costs less than one rejected sheet.<\/div>\n<h2 id=\"window\">The forming window and the colour of the failure<\/h2>\n<p>PMMA forms between 155 and 190 \u00b0C at the surface. Below 155 the sheet is stiff and the material whitens at every bend. Above 190 it sags badly, the surface takes on a fine orange-peel texture, and bubbles appear more readily.<\/p>\n<p>The failure at the cold end is distinctive and worth learning to read. Stress whitening appears as a white, slightly opaque band on the outside of a radius, and it is permanent. It is not a surface mark that can be polished out; the polymer has yielded and craze lines have formed through the thickness.<\/p>\n<p>The failure at the hot end is sag. A 3 mm acrylic sheet at 190 \u00b0C will sag 30 to 50 mm across a 600 mm span, which means the sheet is no longer flat when the mould arrives. The part that results has uneven wall and often a visible flow pattern where the sagging sheet touched the mould first.<\/p>\n<table>\n<thead>\n<tr>\n<th>Grubo\u015b\u0107<\/th>\n<th>Surface temperature<\/th>\n<th>Czas nagrzewania<\/th>\n<th>Mould temperature<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>1.5 mm<\/td>\n<td>160 \u2013 180 \u00b0C<\/td>\n<td>30 \u2013 45 s<\/td>\n<td>70 \u2013 85 \u00b0C<\/td>\n<\/tr>\n<tr>\n<td>3.0 mm<\/td>\n<td>165 \u2013 185 \u00b0C<\/td>\n<td>70 \u2013 100 s<\/td>\n<td>70 \u2013 85 \u00b0C<\/td>\n<\/tr>\n<tr>\n<td>5.0 mm<\/td>\n<td>170 \u2013 188 \u00b0C<\/td>\n<td>130 \u2013 180 s<\/td>\n<td>75 \u2013 90 \u00b0C<\/td>\n<\/tr>\n<tr>\n<td>8.0 mm<\/td>\n<td>175 \u2013 190 \u00b0C<\/td>\n<td>230 \u2013 310 s<\/td>\n<td>75 \u2013 90 \u00b0C<\/td>\n<\/tr>\n<tr>\n<td>12.0 mm<\/td>\n<td>178 \u2013 190 \u00b0C<\/td>\n<td>380 \u2013 500 s<\/td>\n<td>80 \u2013 95 \u00b0C<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 1 \u2014 PMMA forming parameters. The window is 25 to 30 \u00b0C wide, and the cold end is the one that produces permanent damage.<\/p>\n<h2 id=\"draw\">Draw ratio and the corners that fail first<\/h2>\n<p>Acrylic loses wall faster than most thermoforming materials because it strain-hardens less at forming temperature. At a draw ratio of 0.6 the base retains about 55 percent; at 1.0 it retains 38; at 1.2 it is 33 and the part has lost the rigidity that justified the material.<\/p>\n<p>The corner is worse. A square or rectangular cavity has four corners where the sheet stretches in two directions at once, and the corner wall retains about 70 percent of what the base does. That is where acrylic tears during demoulding, and it is where a stress crack starts in service.<\/p>\n<p>The practical limit for acrylic is a draw ratio of 0.9 for a part that must stay clear, and 1.1 for a part where some thinning is acceptable. Beyond that, plug assist is required, and a plug on a transparent part leaves a mark that may be visible through the finished component.<\/p>\n<table>\n<thead>\n<tr>\n<th>Draw ratio<\/th>\n<th>Base centre<\/th>\n<th>Base corner<\/th>\n<th>Verdict<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>0.3<\/td>\n<td>80%<\/td>\n<td>56%<\/td>\n<td>Comfortable<\/td>\n<\/tr>\n<tr>\n<td>0.5<\/td>\n<td>65%<\/td>\n<td>46%<\/td>\n<td>Normal production<\/td>\n<\/tr>\n<tr>\n<td>0.7<\/td>\n<td>50%<\/td>\n<td>35%<\/td>\n<td>Plug assist advised<\/td>\n<\/tr>\n<tr>\n<td>0.9<\/td>\n<td>42%<\/td>\n<td>29%<\/td>\n<td>Limit for clear parts<\/td>\n<\/tr>\n<tr>\n<td>1.2<\/td>\n<td>33%<\/td>\n<td>23%<\/td>\n<td>Not recommended<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 2 \u2014 Wall retained in PMMA against draw ratio, vacuum only. Percentages are of original sheet thickness.<\/p>\n<h2 id=\"mould\">Moulds: polished aluminium and a hot surface<\/h2>\n<p>Acrylic moulds are aluminium, machined and polished. The cavity surface finish transfers directly to the part, and on a transparent component any tool mark is magnified. A mould for a clear part should be polished to at least 800 grit and preferably to a mirror.<\/p>\n<p>Mould temperature is 70 to 90 \u00b0C, which is hot by thermoforming standards and requires cartridge heaters or a hot oil circuit. A cold mould chills the sheet on contact and produces stress whitening at the point of first contact, which on a clear part is a visible white spot in the middle of the product.<\/p>\n<p>Draft must be generous \u2014 3 degrees minimum on a polished surface, more on a textured one. Acrylic grips a polished tool, and a part that is dragged out will be scratched on a surface that cannot be repaired.<\/p>\n<div class=\"myth\"><b>\"A cold mould gives a glossier part.\"<\/b> On acrylic a cold mould gives a whiter part. The gloss comes from the tool polish and from forming above the softening point, not from quenching.<\/div>\n<h2 id=\"stress\">Stress, crazing and annealing<\/h2>\n<p>A formed acrylic part carries two kinds of stress: the frozen orientation from stretching, and the thermal gradient from cooling. Both raise the likelihood of crazing, and crazing in acrylic is the beginning of a crack.<\/p>\n<p>Crazing appears as a fine network of silvery lines, usually at an edge, a hole or a radius, and it appears after the part has been in service for days or weeks. It is accelerated by solvents, by assembly stress and by ultraviolet light.<\/p>\n<p>Annealing removes most of the frozen stress. Two hours at 80 \u00b0C for a 3 mm part, four hours for 6 mm, then cooling at 10 \u00b0C per hour. The part comes out slightly smaller \u2014 about 0.3 percent \u2014 and markedly more resistant to crazing.<\/p>\n<p>For a display window or a machine guard, annealing is optional and cheap insurance. For a part that will be drilled, bonded with solvent or mounted under stress, it is mandatory.<\/p>\n<h2 id=\"trim\">Trimming and bonding<\/h2>\n<p>Acrylic trims cleanly with a router or a saw. A steel rule die works on thin sheet and leaves a slightly ragged edge on thick sheet. The cut edge should be sanded and, for a stressed part, polished, because a rough edge is a crack initiator.<\/p>\n<p>Solvent bonding with methylene chloride or a proprietary acrylic cement gives a joint that is optically clear and as strong as the sheet, if the edges are machined flat and the parts are held without movement for the full cure time. A joint that is moved while the solvent is still working will craze.<\/p>\n<p>Adhesive bonding with a two-part acrylic or a UV-curing adhesive works for dissimilar materials. The surface must be cleaned with isopropanol, never with a ketone, which attacks the acrylic.<\/p>\n<div class=\"faq\">\n<h3>At what temperature does acrylic sheet form?<\/h3>\n<p>155 to 190 \u00b0C at the surface, with 165 to 185 \u00b0C the practical band for most work. Below 155 the sheet stress-whitens at radii; above 190 it sags and takes on surface texture.<\/p>\n<h3>Why does my formed acrylic part have white marks at the corners?<\/h3>\n<p>Stress whitening from forming too cold. The material has yielded and craze lines have formed. It cannot be polished out. Raise the sheet temperature by 10 \u00b0C and increase the corner radius.<\/p>\n<h3>Does acrylic sheet need drying before thermoforming?<\/h3>\n<p>Yes, to 0.05 percent. Four to six hours at 80 \u00b0C for a 3 mm sheet. Moisture produces bubbles visible through the thickness, which is fatal in a transparent part.<\/p>\n<h3>Can formed acrylic parts be annealed?<\/h3>\n<p>Yes. Two hours at 80 \u00b0C for 3 mm, longer for thicker, then slow cooling. Annealing reduces frozen stress and makes the part much more resistant to crazing after drilling or solvent bonding.<\/p>\n<h3>What draw ratio can acrylic take?<\/h3>\n<p>0.9 depth over width for a clear part, 1.1 if some thinning is acceptable. Beyond that the base drops below 35 percent and the part loses rigidity, and the corners drop below 25 percent and tear.<\/p>\n<h3>What mould temperature for acrylic thermoforming?<\/h3>\n<p>70 to 90 \u00b0C. A cold mould produces stress whitening wherever it touches the sheet first, which on a clear part is a visible white spot.<\/p>\n<\/div>\n<div class=\"cta\">\n<p class=\"cta-h\">Forming acrylic parts and losing them to whitening or crazing?<\/p>\n<p>Send us the drawing, the sheet grade and the radius. We will tell you the draw ratio you can hold, whether the tool needs a plug, and what the annealing schedule should be.<\/p>\n<p><a class=\"btn\" href=\"https:\/\/uplastech.com\/pl\/custom-vacuum-forming\/\">Talk to a thermoforming engineer<\/a>\n<\/div>\n<div class=\"rel\">\n<h4>Continue reading<\/h4>\n<ul>\n<li><a href=\"https:\/\/uplastech.com\/pl\/pmma-acrylic-injection-molding\/\">PMMA Acrylic Injection Molding<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/pl\/pc-thermoforming-guide\/\">PC Thermoforming Guide<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/pl\/znaczenie-katow-pochylenia-w-termoformowaniu-kompleksowy-przewodnik\/\">Draft Angles in Thermoforming<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/pl\/acrylic-heat-bending\/\">Acrylic Heat Bending<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/pl\/pvc-thermoforming-guide\/\">PVC Thermoforming Guide<\/a><\/li>\n<\/ul>\n<\/div>\n<\/div>","protected":false},"excerpt":{"rendered":"<p>Acrylic forms at 155 to 190 \u00b0C and turns white at every radius if you go in cold. The clarity survives only if the sheet is dry, the mould is hot and the radius is generous.<\/p>","protected":false},"author":1,"featured_media":7791,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_seopress_titles_title":"PMMA Thermoforming: Forming Window, Radius and Clarity","_seopress_titles_desc":"Acrylic thermoforming guide: 155-190 C window, drying, stress whitening at radii, draw ratio, mould temperature, annealing, trimming and bonding acrylic.","_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-7792","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\/pl\/wp-json\/wp\/v2\/posts\/7792","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/uplastech.com\/pl\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/uplastech.com\/pl\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/uplastech.com\/pl\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/uplastech.com\/pl\/wp-json\/wp\/v2\/comments?post=7792"}],"version-history":[{"count":1,"href":"https:\/\/uplastech.com\/pl\/wp-json\/wp\/v2\/posts\/7792\/revisions"}],"predecessor-version":[{"id":7860,"href":"https:\/\/uplastech.com\/pl\/wp-json\/wp\/v2\/posts\/7792\/revisions\/7860"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/uplastech.com\/pl\/wp-json\/wp\/v2\/media\/7791"}],"wp:attachment":[{"href":"https:\/\/uplastech.com\/pl\/wp-json\/wp\/v2\/media?parent=7792"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/uplastech.com\/pl\/wp-json\/wp\/v2\/categories?post=7792"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/uplastech.com\/pl\/wp-json\/wp\/v2\/tags?post=7792"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}