{"id":7539,"date":"2026-10-26T06:30:26","date_gmt":"2026-10-26T06:30:26","guid":{"rendered":"https:\/\/uplastech.com\/vacuum-forming-defects-troubleshooting\/"},"modified":"2026-10-26T07:41:25","modified_gmt":"2026-10-26T07:41:25","slug":"vacuum-forming-defects-troubleshooting","status":"publish","type":"post","link":"https:\/\/uplastech.com\/de\/vacuum-forming-defects-troubleshooting\/","title":{"rendered":"Vacuum Forming Defects: A Troubleshooting Guide for 14 Faults"},"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 \u81ea\u52a8\u5c55\u5f00\u7684\u95ee\u5019\u5361\uff0c\u53ea\u4fdd\u7559\u5706\u5f62\u6c14\u6ce1\uff0c\u907f\u514d\u906e\u6321\u4fa7\u680f\u8be2\u76d8\u63d0\u4ea4\u6309\u94ae *\/\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\/vf-webbing-defect.jpg\" alt=\"Webbing defect between two raised features on a vacuum formed ABS part, with a thin membrane of plastic stretched across the gap\" width=\"1536\" height=\"909\"><figcaption>Webbing between two raised features. The sheet bridged the gap instead of drawing into both. It is one of the most common vacuum forming defects and one of the few that is almost always a heating or draw-rate problem rather than a mold problem.<\/figcaption><\/figure>\n<div class=\"toc\"><\/div>\n<h2 id=\"method\">How to troubleshoot a forming defect properly<\/h2>\n<p>Most troubleshooting fails for the same reason: someone changes three variables at once, gets a better part, and never learns which change mattered. The defect returns a month later and the whole cycle starts again.<\/p>\n<p>The method that works is boring and fast:<\/p>\n<ol>\n<li><strong>Identify the defect precisely.<\/strong> \"Bad part\" is not a defect. \"Webbing between the two bosses on the left side, 15 mm long, appears on parts 3 onwards but not part 1\" is a defect. Location and pattern carry most of the diagnostic information.<\/li>\n<li><strong>Decide which of the four systems it belongs to<\/strong> \u2014 material, heating, mold, or machine. The defect's location usually tells you: a defect that repeats with the element pitch is heating; a defect that follows the geometry is mold; a defect that appears only after the mold warms up is thermal.<\/li>\n<li><strong>Change one thing.<\/strong> Record the before and after.<\/li>\n<li><strong>Verify on at least ten parts.<\/strong> Forming defects are frequently intermittent, and one good part proves nothing.<\/li>\n<\/ol>\n<div class=\"callout\">\n<p><strong>The single most useful diagnostic:<\/strong> form three parts and photograph them side by side \u2014 the first off the mold, one from mid-run, and one after the mold has reached temperature. Roughly half of all \"random\" defects are actually a warm-up effect that only appears in a stable process.<\/p>\n<\/div>\n<h2 id=\"webbing\">Webbing and bridging<\/h2>\n<p>Webbing is a thin membrane of plastic stretched across a gap between two features, where the sheet bridged instead of drawing into both. Bridging is the same failure with more material \u2014 a thick band that spans the gap entirely.<\/p>\n<p>The mechanism: the sheet between two features has to stretch to reach both. If it reaches its limit before either feature is filled, it spans the gap as a straight membrane. It happens when the sheet is too hot (low strength, stretches easily but also flows), when the gap is narrow relative to its depth, or when the draw is too fast for the material to redistribute.<\/p>\n<table>\n<thead>\n<tr>\n<th>Ursache<\/th>\n<th>How to recognise it<\/th>\n<th>Fix<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Sheet too hot in the web zone<\/td>\n<td>Webbing is localised and follows a hot band<\/td>\n<td>Profile the heat away from the web; thermal image the sheet<\/td>\n<\/tr>\n<tr>\n<td>Sheet too hot overall<\/td>\n<td>Webbing on every gap, plus sag<\/td>\n<td>Reduce forming temperature 5 \u2013 10 \u00b0C<\/td>\n<\/tr>\n<tr>\n<td>Draw too fast<\/td>\n<td>Webbing worse at high vacuum rate<\/td>\n<td>Slow the vacuum application; add a dwell before full vacuum<\/td>\n<\/tr>\n<tr>\n<td>Gap too narrow for its depth<\/td>\n<td>Webbing always in the same geometry, regardless of settings<\/td>\n<td>Widen the gap, reduce the depth, or add a radius<\/td>\n<\/tr>\n<tr>\n<td>Insufficient material between features<\/td>\n<td>Webbing plus a thin web<\/td>\n<td>Increase the web width in the design; add plug assist<\/td>\n<\/tr>\n<tr>\n<td>Too much material between features<\/td>\n<td>Bridging \u2014 a thick band, not a thin membrane<\/td>\n<td>Reduce web width, or reduce local heat so the web stays stiff<\/td>\n<\/tr>\n<tr>\n<td>Sheet gauge too thin for the draw<\/td>\n<td>Webbing on a new, thinner sheet<\/td>\n<td>Thicker sheet, or reduce draw ratio<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Webbing that appears only on one side of a symmetric part is almost always a heating problem, not a mold problem. Check the heater bank on that side before touching the tool.<\/p>\n<h3>What actually works<\/h3>\n<p>In order of effectiveness on a real job:<\/p>\n<ol>\n<li><strong>Cool the web zone.<\/strong> A mask, a shadow plate, or a zone setting that leaves the web 10 \u2013 15 \u00b0C cooler than the pockets. The web stays stiff and resists bridging while the pockets stay soft and fill.<\/li>\n<li><strong>Slow the draw.<\/strong> A controlled vacuum rate \u2014 often with a bleed valve or a two-stage vacuum \u2014 lets the sheet stretch progressively rather than snapping across the gap.<\/li>\n<li><strong>Add plug assist.<\/strong> A plug that pushes into the gap first mechanically forces material into both features.<\/li>\n<li><strong>Change the geometry.<\/strong> Widen the gap, reduce its depth, or add a generous radius at the base of each feature. This is a design change, but it is the permanent fix.<\/li>\n<\/ol>\n<h2 id=\"thin\">Excessive thinning and tearing<\/h2>\n<p>Thinning becomes a defect when it reaches a hole. Tearing usually starts at a corner or at the point of maximum draw, and it is preceded by stress whitening \u2014 a visible lightening of the material where it is about to fail.<\/p>\n<table>\n<thead>\n<tr>\n<th>Ursache<\/th>\n<th>How to recognise it<\/th>\n<th>Fix<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Draw ratio too high for the material<\/td>\n<td>Tears at the same place on every part<\/td>\n<td>Design change: shallower draw, bigger radius, or split the part<\/td>\n<\/tr>\n<tr>\n<td>Sheet too cold<\/td>\n<td>Sharp-edged tear, no whitening, part feels stiff<\/td>\n<td>Increase forming temperature 5 \u2013 10 \u00b0C<\/td>\n<\/tr>\n<tr>\n<td>Sheet too hot<\/td>\n<td>Thin, stringy tear with heavy thinning around it<\/td>\n<td>Reduce temperature; the material has lost strength<\/td>\n<\/tr>\n<tr>\n<td>Corner radius too small<\/td>\n<td>Tear exactly at a sharp corner<\/td>\n<td>Increase radius to \u2265 1 \u00d7 sheet thickness<\/td>\n<\/tr>\n<tr>\n<td>Vacuum applied too fast<\/td>\n<td>Tears on the first few parts after a change<\/td>\n<td>Slow the vacuum, or add a pre-stretch<\/td>\n<\/tr>\n<tr>\n<td>Local hot spot<\/td>\n<td>Tear at a location that matches a heater element<\/td>\n<td>Mask or re-zone the heater<\/td>\n<\/tr>\n<tr>\n<td>Material too thin at that point to begin with<\/td>\n<td>Tears on one side only<\/td>\n<td>Check sheet gauge across the sheet \u2014 gauge variation is a real cause<\/td>\n<\/tr>\n<tr>\n<td>Degraded material<\/td>\n<td>Tears easily, surface looks dull or yellowed<\/td>\n<td>Reduce temperature; check for a runaway heater<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">A tear with visible stress whitening around it failed slowly \u2014 the material was stretching beyond its limit. A tear with no whitening failed suddenly, which points to a mechanical cause (a sharp edge on the mold, a pinch point, or the sheet catching on the clamp frame).<\/p>\n<h2 id=\"marks\">Chill marks, flow lines and drag marks<\/h2>\n<p>Chill marks are dull, matte, sometimes slightly raised areas where the sheet touched a cold mold surface and froze before it had finished stretching. They are cosmetic but they are also a reliable indicator of a thermal problem, so they should not be dismissed.<\/p>\n<table>\n<thead>\n<tr>\n<th>Defekt<\/th>\n<th>Erscheinungsbild<\/th>\n<th>Ursache<\/th>\n<th>Fix<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Chill mark<\/td>\n<td>Dull matte patch, often at a corner or a deep draw<\/td>\n<td>Mold too cold, or sheet too cold at touchdown<\/td>\n<td>Raise mold temperature; raise sheet temperature; slow the draw<\/td>\n<\/tr>\n<tr>\n<td>Flow line<\/td>\n<td>Faint wavy line where the sheet first contacted<\/td>\n<td>Sheet touching the mold before vacuum, then sliding<\/td>\n<td>Reduce sag; apply vacuum earlier; reduce draw speed<\/td>\n<\/tr>\n<tr>\n<td>Drag mark<\/td>\n<td>Straight scuff along the draw direction<\/td>\n<td>Sheet dragging over a mold feature \u2014 insufficient draft<\/td>\n<td>Add draft; polish the mold; add a radius<\/td>\n<\/tr>\n<tr>\n<td>Mold mark \/ witness<\/td>\n<td>Exact copy of a mold defect, screw head or scratch<\/td>\n<td>Mold surface damage or a proud fastener<\/td>\n<td>Repair the mold; countersink fasteners<\/td>\n<\/tr>\n<tr>\n<td>Plug mark<\/td>\n<td>Circular dull patch where the plug contacted<\/td>\n<td>Plug too cold, too fast, or too hard a material<\/td>\n<td>Heat the plug; slow it; use a softer plug material<\/td>\n<\/tr>\n<tr>\n<td>Vacuum dimple<\/td>\n<td>Small circular depression on the show face<\/td>\n<td>Vacuum hole too large for the local sheet thickness<\/td>\n<td>Reduce hole diameter; switch to slots; move holes off the cosmetic face<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Chill marks and flow lines are the two defects most often accepted as \"normal\". On a cosmetic part they are not \u2014 both are fixable with mold temperature and draw control.<\/p>\n<h2 id=\"dimples\">Dimples, pinholes and vacuum marks<\/h2>\n<p>These three are grouped because they all come from the vacuum system rather than from the sheet.<\/p>\n<ul>\n<li><strong>Dimples<\/strong> are the sheet being drawn into a vacuum hole. The hole is too large for the local sheet thickness, or the sheet is too hot and too soft when it arrives.<\/li>\n<li><strong>Pinholes<\/strong> are the sheet being drawn into a hole far enough to rupture. This happens on thin sheet, at high vacuum, with large holes \u2014 or where a hole sits at the deepest point of a draw and the sheet is already thin there.<\/li>\n<li><strong>Vacuum marks<\/strong> are a broader term for the visible witness left by any vacuum feature, including slot marks and manifold marks where a channel is too close to the surface.<\/li>\n<\/ul>\n<table>\n<thead>\n<tr>\n<th>Ursache<\/th>\n<th>Fix<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Hole diameter &gt; \u2153 of local sheet thickness<\/td>\n<td>Reduce hole size, or drill smaller holes and increase their number<\/td>\n<\/tr>\n<tr>\n<td>Round holes on a cosmetic face<\/td>\n<td>Replace with narrow slots; the sheet bridges a slot instead of entering it<\/td>\n<\/tr>\n<tr>\n<td>Excess vacuum (too high, held too long)<\/td>\n<td>Reduce vacuum level; vent the mold after forming<\/td>\n<\/tr>\n<tr>\n<td>Sheet too hot at touchdown<\/td>\n<td>Reduce temperature; increase draw speed so the sheet arrives cooler<\/td>\n<\/tr>\n<tr>\n<td>Hole placed at the thinnest point of the draw<\/td>\n<td>Relocate the hole; add a hole nearby in a thicker area<\/td>\n<\/tr>\n<tr>\n<td>Manifold channel too close to the mold surface<\/td>\n<td>Increase wall thickness between channel and surface, or re-route<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Dimples on the inside of a part are usually acceptable; on the outside they are a reject. If the vacuum holes must be on the cosmetic side, use slots and polish the slot edges.<\/p>\n<h2 id=\"warp\">Warpage and distortion<\/h2>\n<p>Warpage is the most expensive defect in the list because it often appears after the part has been shipped. A part that measures flat at the machine can be visibly twisted 24 hours later, when the internal stresses have finished relaxing.<\/p>\n<p>The mechanism is differential shrinkage. Material that cooled while stretched wants to recover; material that cooled while constrained cannot. If the two are in the same part, the part distorts to accommodate the difference.<\/p>\n<table>\n<thead>\n<tr>\n<th>Ursache<\/th>\n<th>How to recognise it<\/th>\n<th>Fix<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Uneven sheet heating<\/td>\n<td>Warpage direction matches a hot or cold band<\/td>\n<td>Thermal image the sheet; balance the heater zones<\/td>\n<\/tr>\n<tr>\n<td>Uneven mold temperature<\/td>\n<td>Warpage follows the cooling channel layout<\/td>\n<td>Re-route cooling; check flow rate through each circuit<\/td>\n<\/tr>\n<tr>\n<td>Part ejected too hot<\/td>\n<td>Warpage worse on fast cycles<\/td>\n<td>Extend cooling; add a cooling jig<\/td>\n<\/tr>\n<tr>\n<td>Thickness variation across the part<\/td>\n<td>Thin areas warp more<\/td>\n<td>Improve thickness distribution; see the wall thickness guide<\/td>\n<\/tr>\n<tr>\n<td>Asymmetric geometry<\/td>\n<td>Warpage is inherent to the shape<\/td>\n<td>Design symmetry, or add ribs; plan a cooling fixture<\/td>\n<\/tr>\n<tr>\n<td>Large flat areas with no ribbing<\/td>\n<td>Dishing or oil-canning<\/td>\n<td>Add ribs or a crown; cool on a flat fixture<\/td>\n<\/tr>\n<tr>\n<td>Trim released stress<\/td>\n<td>Part was flat before trimming, warped after<\/td>\n<td>Trim after cooling; trim in a fixture<\/td>\n<\/tr>\n<tr>\n<td>Material with high shrinkage (PP, HDPE)<\/td>\n<td>Warpage on every part<\/td>\n<td>Accept and design for it; or change material<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">For a part with a tight flatness requirement, the answer is usually a cooling fixture \u2014 a jig that holds the part in its correct shape while it cools to room temperature. It is a real cost, and it should be in the quote rather than discovered at first article.<\/p>\n<h2 id=\"bubbles\">Bubbles, blisters and silver streaking<\/h2>\n<p>These are all moisture or degradation. The distinction matters because the fixes are different.<\/p>\n<table>\n<thead>\n<tr>\n<th>Defekt<\/th>\n<th>Erscheinungsbild<\/th>\n<th>Ursache<\/th>\n<th>Fix<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Bubbles<\/td>\n<td>Rounded voids inside the wall, often clustered<\/td>\n<td>Moisture turning to steam<\/td>\n<td>Dry the sheet properly before forming<\/td>\n<\/tr>\n<tr>\n<td>Blasen<\/td>\n<td>Surface bubbles that have burst or are about to<\/td>\n<td>Severe moisture, or surface over-heating<\/td>\n<td>Dry; reduce peak surface temperature<\/td>\n<\/tr>\n<tr>\n<td>Silver streaking<\/td>\n<td>Fine silvery lines along the flow direction<\/td>\n<td>Moisture, or slight degradation<\/td>\n<td>Dry; reduce temperature; check for a hot element<\/td>\n<\/tr>\n<tr>\n<td>Cloudiness \/ haze<\/td>\n<td>Loss of clarity in a clear material<\/td>\n<td>Moisture (PC, PMMA, PETG)<\/td>\n<td>Dry; check storage conditions<\/td>\n<\/tr>\n<tr>\n<td>Yellowing \/ browning<\/td>\n<td>Colour shift, especially PVC and PC<\/td>\n<td>Thermal degradation<\/td>\n<td>Reduce temperature immediately; purge; check element control<\/td>\n<\/tr>\n<tr>\n<td>Fish eyes<\/td>\n<td>Small circular unmelts or gels<\/td>\n<td>Contamination or cross-linked material<\/td>\n<td>Check regrind content; check for mixed material<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">If bubbles appear only in the thickest section of the part, it is moisture concentrating there. If they appear uniformly across a whole sheet, the sheet was wet when it went in.<\/p>\n<h2 id=\"surface\">Surface defects: orange peel, gloss bands, texture loss<\/h2>\n<p>These are cosmetic but they are the ones buyers notice first, and they are almost always a heating or a mold-surface issue.<\/p>\n<ul>\n<li><strong>Orange peel<\/strong> \u2014 a fine, dimpled surface texture that was not intended. Caused by the sheet surface relaxing against a mold surface it cannot fully conform to, usually because the sheet is too cold at touchdown or the vacuum is insufficient to pull it into the mold's micro-texture.<\/li>\n<li><strong>Gloss bands<\/strong> \u2014 stripes of different gloss. Caused by uneven heating (the hotter band flows and finishes glossier) or by uneven mold temperature.<\/li>\n<li><strong>Texture loss<\/strong> \u2014 the mold's texture does not reproduce. Caused by excessive stretch (texture is stretched away), insufficient vacuum, or sheet too cold to conform.<\/li>\n<li><strong>Scuffing<\/strong> \u2014 a rubbed, polished-looking area where the sheet dragged across a textured mold wall. Caused by insufficient draft for the texture depth.<\/li>\n<\/ul>\n<div class=\"callout\">\n<p><strong>The texture draft rule:<\/strong> add about 1\u00b0 of draft for every 0.025 mm of texture depth, on top of the base draft for the geometry. A leather grain 0.15 mm deep needs 6\u00b0 of extra draft. Under-drafted textured molds produce scuffing that looks like a material fault and cannot be polished out.<\/p>\n<\/div>\n<h2 id=\"incomplete\">Incomplete forming and short draws<\/h2>\n<p>The sheet does not reach the mold surface \u2014 a corner stays rounded, a detail stays soft, or a deep pocket does not fill. This is a vacuum or a temperature problem, in that order.<\/p>\n<table>\n<thead>\n<tr>\n<th>Ursache<\/th>\n<th>Diagnostic<\/th>\n<th>Fix<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Insufficient vacuum at that point<\/td>\n<td>The affected area is far from the vacuum connection<\/td>\n<td>Add holes; enlarge the manifold; add a second vacuum connection<\/td>\n<\/tr>\n<tr>\n<td>Vacuum holes blocked<\/td>\n<td>Affects one area, appeared gradually<\/td>\n<td>Clean the holes; add a filter; check for sheet debris<\/td>\n<\/tr>\n<tr>\n<td>Sheet too cold<\/td>\n<td>Whole part under-formed<\/td>\n<td>Increase temperature or dwell<\/td>\n<\/tr>\n<tr>\n<td>Sheet cooled during transfer<\/td>\n<td>Under-forming worse on slow machines or long transfer<\/td>\n<td>Reduce transfer time; raise sheet temperature to compensate<\/td>\n<\/tr>\n<tr>\n<td>Air trapped with no escape<\/td>\n<td>Local void in a closed feature<\/td>\n<td>Add a vent at the deepest point of the trap<\/td>\n<\/tr>\n<tr>\n<td>Draw ratio beyond the material<\/td>\n<td>Always the same feature<\/td>\n<td>Design change or plug assist<\/td>\n<\/tr>\n<tr>\n<td>Vacuum applied too late<\/td>\n<td>Part forms but detail is soft<\/td>\n<td>Trigger vacuum earlier in the sequence<\/td>\n<\/tr>\n<tr>\n<td>Mold too cold<\/td>\n<td>Detail soft, chill marks present<\/td>\n<td>Raise mold temperature<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">A vacuum gauge on the mold, not on the pump, is the single most useful instrument for this class of defect. The pressure at the mold is often half the pressure at the tank.<\/p>\n<h2 id=\"cracking\">Cracking, crazing and stress whitening<\/h2>\n<p>These appear either immediately (at forming) or later (in service). The distinction determines whether you have a process problem or a material problem.<\/p>\n<table>\n<thead>\n<tr>\n<th>Symptom<\/th>\n<th>Timing<\/th>\n<th>Ursache<\/th>\n<th>Fix<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Stress whitening<\/td>\n<td>At forming<\/td>\n<td>Material stretched beyond its limit<\/td>\n<td>Reduce draw ratio; raise temperature; bigger radii<\/td>\n<\/tr>\n<tr>\n<td>Crack at a corner<\/td>\n<td>At forming or on ejection<\/td>\n<td>Sharp corner plus residual stress<\/td>\n<td>Radius; slower ejection; warmer mold<\/td>\n<\/tr>\n<tr>\n<td>Crazing (fine surface cracks)<\/td>\n<td>Days to weeks<\/td>\n<td>Residual stress plus a chemical or UV exposure<\/td>\n<td>Reduce forming stress; check chemical contact; check material grade<\/td>\n<\/tr>\n<tr>\n<td>Brittle failure in service<\/td>\n<td>Weeks to months<\/td>\n<td>Degraded material from over-heating<\/td>\n<td>Reduce forming temperature; verify material identity<\/td>\n<\/tr>\n<tr>\n<td>Impact crack from a drop<\/td>\n<td>In service<\/td>\n<td>Wall too thin at the impact point<\/td>\n<td>Thickness at that feature; ribbing<\/td>\n<\/tr>\n<tr>\n<td>Crack along a trim edge<\/td>\n<td>In service<\/td>\n<td>Micro-cracks from a poor trim<\/td>\n<td>Sharper trim tool; trim hot; anneal<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Annealing \u2014 reheating a formed part to just below its forming temperature and cooling slowly \u2014 relieves residual stress and is worth considering on any part that will see chemical exposure or a long service life.<\/p>\n<h2 id=\"trim\">Trim and flange defects<\/h2>\n<p>Trimming is where a good formed part often becomes a reject, and the defects are mechanical rather than thermal.<\/p>\n<ul>\n<li><strong>Ragged or serrated edge<\/strong> \u2014 dull trim tool, wrong clearance, or trimming cold material. On heavy gauge, trimming warm (within a few minutes of forming) gives a much cleaner edge than trimming cold.<\/li>\n<li><strong>Stress whitening along the trim<\/strong> \u2014 the trim tool is crushing rather than shearing. Check clearance: typically 5 \u2013 10 % of sheet thickness per side.<\/li>\n<li><strong>Flange distortion<\/strong> \u2014 the part relaxes when the flange is released. Trim in a fixture, or leave a margin and trim after the part has cooled.<\/li>\n<li><strong>Cracks propagating from the trim<\/strong> \u2014 a nick left by the trim tool becomes a crack initiator. Deburr, or use a trim tool that leaves a radius.<\/li>\n<li><strong>Dimensional error on the trim<\/strong> \u2014 the part has shrunk between forming and trimming, and the fixture was made for the formed dimension rather than the cooled dimension.<\/li>\n<\/ul>\n<h2 id=\"matrix\">A one-page diagnostic matrix<\/h2>\n<p>The table we keep on the wall. Find the defect, look across to the three most likely causes, and check them in that order.<\/p>\n<table>\n<thead>\n<tr>\n<th>Defekt<\/th>\n<th>Cause 1<\/th>\n<th>Cause 2<\/th>\n<th>Cause 3<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Webbing<\/td>\n<td>Sheet too hot in web zone<\/td>\n<td>Draw too fast<\/td>\n<td>Gap too narrow for depth<\/td>\n<\/tr>\n<tr>\n<td>Bridging<\/td>\n<td>Excess material in web<\/td>\n<td>Sheet too hot overall<\/td>\n<td>Web too wide<\/td>\n<\/tr>\n<tr>\n<td>Tearing<\/td>\n<td>Draw ratio too high<\/td>\n<td>Sheet too cold<\/td>\n<td>Sharp corner<\/td>\n<\/tr>\n<tr>\n<td>Chill marks<\/td>\n<td>Mold too cold<\/td>\n<td>Sheet too cold at touchdown<\/td>\n<td>Draw too slow<\/td>\n<\/tr>\n<tr>\n<td>Dimples<\/td>\n<td>Hole too large<\/td>\n<td>Sheet too hot<\/td>\n<td>Vacuum too high<\/td>\n<\/tr>\n<tr>\n<td>Verzug<\/td>\n<td>Uneven heating<\/td>\n<td>Uneven mold temperature<\/td>\n<td>Ejected too hot<\/td>\n<\/tr>\n<tr>\n<td>Bubbles<\/td>\n<td>Moisture<\/td>\n<td>Surface over-heated<\/td>\n<td>Contaminated material<\/td>\n<\/tr>\n<tr>\n<td>Orange peel<\/td>\n<td>Sheet too cold<\/td>\n<td>Insufficient vacuum<\/td>\n<td>Mold surface too rough<\/td>\n<\/tr>\n<tr>\n<td>Texture loss<\/td>\n<td>Excessive stretch<\/td>\n<td>Sheet too cold<\/td>\n<td>Insufficient vacuum<\/td>\n<\/tr>\n<tr>\n<td>Incomplete form<\/td>\n<td>Insufficient vacuum at that point<\/td>\n<td>Blocked holes<\/td>\n<td>Sheet too cold<\/td>\n<\/tr>\n<tr>\n<td>Gloss bands<\/td>\n<td>Uneven heating<\/td>\n<td>Uneven mold temperature<\/td>\n<td>Gauge variation<\/td>\n<\/tr>\n<tr>\n<td>Scuffing<\/td>\n<td>Insufficient draft for texture<\/td>\n<td>Mold surface rough<\/td>\n<td>Sheet too hot<\/td>\n<\/tr>\n<tr>\n<td>Stress whitening<\/td>\n<td>Over-stretch<\/td>\n<td>Sharp corner<\/td>\n<td>Ejection too fast<\/td>\n<\/tr>\n<tr>\n<td>Ragged trim<\/td>\n<td>Dull trim tool<\/td>\n<td>Wrong clearance<\/td>\n<td>Trimming too cold<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">When two of the three causes point at the same system \u2014 for example two heating causes \u2014 start there. The odds are that it is the third one too.<\/p>\n<h2 id=\"prevent\">Prevention: the five process records that stop repeat defects<\/h2>\n<p>Defects recur because the process that produced good parts was never written down. Five records prevent almost all of it:<\/p>\n<ol>\n<li><strong>A setup sheet per mold per material<\/strong>, with oven zone settings, dwell time, top\/bottom balance, mold temperature, vacuum level and cooling time. Not the machine's memory \u2014 a sheet on the machine that an operator can read.<\/li>\n<li><strong>Sheet temperature at touchdown<\/strong>, measured with a pyrometer at a marked point on the sheet. One number that captures the whole heating system.<\/li>\n<li><strong>Mold temperature at start and at equilibrium<\/strong>, with the time to reach equilibrium. This is what makes first-article approval meaningful.<\/li>\n<li><strong>A first-article section<\/strong> \u2014 a formed part cut through the worst corner, measured and kept. It is the reference for every subsequent argument about thickness.<\/li>\n<li><strong>A defect photo log<\/strong>, with the cause and the fix that worked. After two years this is the most valuable document in the shop, because it converts a week of diagnosis into a five-minute lookup.<\/li>\n<\/ol>\n<figure>\n<img decoding=\"async\" src=\"https:\/\/uplastech.com\/wp-content\/uploads\/2026\/10\/vf-trimmed-parts.jpg\" alt=\"Stack of trimmed white HIPS vacuum formed covers with cleanly routed edges on a workbench\" width=\"1536\" height=\"909\"><figcaption>The end state these records are protecting: covers with even walls, no webbing, no chill marks, and a routed edge with a consistent radius. Every defect in this guide is a step away from this.<\/figcaption><\/figure>\n<h2 id=\"faq\">Frequently asked questions<\/h2>\n<div class=\"faq\">\n<h3>What causes webbing in vacuum forming?<\/h3>\n<p>The sheet bridging a gap between two features instead of drawing into both. The usual causes are the sheet being too hot in the web area, the draw being too fast, or the gap being too narrow for its depth. Cooling the web zone by 10 \u2013 15 \u00b0C, slowing the vacuum, or adding plug assist fixes most cases.<\/p>\n<h3>Why does my formed part warp after a day?<\/h3>\n<p>Residual stress from uneven heating or uneven cooling is relaxing. The part was not flat when it was made \u2014 it was held flat by the mold and the trim. Check sheet heating uniformity with a thermal camera, check mold temperature uniformity across the part, and cool the part in a fixture if flatness is critical.<\/p>\n<h3>What are chill marks and are they a problem?<\/h3>\n<p>Dull matte patches where the sheet touched a cold mold and froze before it finished stretching. On a non-cosmetic part they are harmless. On a visible surface they are a reject, and they indicate the mold or the sheet is too cold \u2014 which also means the part has higher residual stress than it should.<\/p>\n<h3>Why do I get small dimples on the outside of the part?<\/h3>\n<p>The vacuum holes are too large relative to the sheet thickness at that point, so the sheet is drawn into the hole. Reduce the hole diameter to under a third of the local sheet thickness, or replace round holes with narrow slots, which the sheet bridges rather than entering.<\/p>\n<h3>Why do bubbles appear in polycarbonate parts?<\/h3>\n<p>Moisture. PC absorbs water and releases it as steam at forming temperature. Dry at 120 \u00b0C for 2 \u2013 4 hours immediately before forming and store the sheet dry. Once bubbles form, the sheet cannot be recovered.<\/p>\n<h3>What causes orange peel on a formed surface?<\/h3>\n<p>The sheet is too cold to conform to the mold's micro-surface, or the vacuum is insufficient to pull it in. It is more common on textured molds and on thick sheet, where the surface cools fast. Raise the sheet temperature and check that the vacuum reaches the full mold area.<\/p>\n<h3>Why does the texture disappear in some areas?<\/h3>\n<p>The sheet stretched more in those areas, and stretching flattens the mold texture. It is a normal consequence of deep draws. If uniform texture matters, keep the draw shallow in textured areas or accept and specify a texture fade.<\/p>\n<h3>Why does my part tear at the same corner every time?<\/h3>\n<p>That corner is the point of maximum draw. Check the corner radius \u2014 it should be at least one sheet thickness, and preferably 6 % of the draw depth. If the radius is already adequate, the draw ratio is too high for the material and the fix is a design change or plug assist.<\/p>\n<h3>How do I stop a part cracking in service?<\/h3>\n<p>Three things, in order: make sure the wall is thick enough at the crack location, make sure the material was not degraded by over-heating, and relieve residual stress by annealing. A crack that starts at a trim edge usually needs a sharper trim tool or a deburr operation.<\/p>\n<h3>Why does the same mold produce good parts on one machine and bad on another?<\/h3>\n<p>Oven power, heater type, platen size, vacuum volume and transfer speed all differ. A mold is only half the process \u2014 the oven and the vacuum system are the other half. When moving a mold between machines, re-establish the sheet temperature at touchdown and the vacuum level before changing anything on the mold.<\/p>\n<\/div>\n<div class=\"cta\">\n<p class=\"cta-h\">Send a photo of the defect and we will tell you what to change<\/p>\n<p>Most forming defects are diagnosed from one clear photograph and one sheet temperature reading. If it is a geometry problem, we will say so \u2014 and tell you the design change that fixes it permanently.<\/p>\n<p><a class=\"btn\" href=\"https:\/\/uplastech.com\/de\/vakuumverformung\/\">Get a defect diagnosis<\/a>\n<\/div>\n<div class=\"rel\">\n<h4>Continue reading<\/h4>\n<ul>\n<li><a href=\"https:\/\/uplastech.com\/de\/vacuum-forming-mold-design\/\">Vacuum Forming Mold Design: Materials, Draft, Vents and Tool Life<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/de\/thermoforming-sheet-heating-temperature-window\/\">Sheet Heating: The Temperature Window That Decides Everything<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/de\/vacuum-forming-wall-thickness\/\">Wall Thickness in Vacuum Forming: Why Corners Thin<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/de\/custom-vacuum-forming-guide\/\">Vacuum Forming Guide: Tooling Cost &amp; Design Rules<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/de\/thick-gauge-vs-thin-gauge-thermoforming\/\">Thick Gauge vs Thin Gauge Thermoforming<\/a><\/li>\n<\/ul>\n<\/div>\n<\/div>","protected":false},"excerpt":{"rendered":"<p>Webbing, tearing, chill marks, dimples, warpage, bubbles, orange peel, short draws, cracking and trim faults \u2014 causes, how to recognise each, and the fix that actually works.<\/p>","protected":false},"author":1,"featured_media":7532,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_seopress_titles_title":"Vacuum Forming Defects: A Troubleshooting Guide","_seopress_titles_desc":"Webbing, tearing, chill marks, dimples, warpage, bubbles, orange peel, short draws, cracking and trim faults: causes, how to recognise each one, and the fix that actually works.","_seopress_robots_index":"1","_seopress_robots_follow":"1","_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":"https:\/\/uplastech.com\/vacuum-forming-defects-troubleshooting\/","_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":"vacuum forming defects","footnotes":""},"categories":[29],"tags":[],"class_list":["post-7539","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\/de\/wp-json\/wp\/v2\/posts\/7539","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/uplastech.com\/de\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/uplastech.com\/de\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/uplastech.com\/de\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/uplastech.com\/de\/wp-json\/wp\/v2\/comments?post=7539"}],"version-history":[{"count":1,"href":"https:\/\/uplastech.com\/de\/wp-json\/wp\/v2\/posts\/7539\/revisions"}],"predecessor-version":[{"id":7544,"href":"https:\/\/uplastech.com\/de\/wp-json\/wp\/v2\/posts\/7539\/revisions\/7544"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/uplastech.com\/de\/wp-json\/wp\/v2\/media\/7532"}],"wp:attachment":[{"href":"https:\/\/uplastech.com\/de\/wp-json\/wp\/v2\/media?parent=7539"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/uplastech.com\/de\/wp-json\/wp\/v2\/categories?post=7539"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/uplastech.com\/de\/wp-json\/wp\/v2\/tags?post=7539"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}