{"id":9063,"date":"2026-11-06T01:00:00","date_gmt":"2026-11-06T01:00:00","guid":{"rendered":"https:\/\/uplastech.com\/?p=9063"},"modified":"2026-11-05T01:54:15","modified_gmt":"2026-11-05T01:54:15","slug":"silicone-parting-line-design","status":"publish","type":"post","link":"https:\/\/uplastech.com\/nl\/silicone-parting-line-design\/","title":{"rendered":"Parting Line Design for Silicone Molds: Where the Flash Comes From"},"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\/sr-01-lsr-molding.jpg\" alt=\"Liquid silicone rubber molded parts, clear and white, showing clean parting lines on the edges\" width=\"1536\" height=\"909\"><figcaption>Molded silicone parts. A good parting line is thin enough to flash-free, thick enough to survive demolding.<\/figcaption><\/figure>\n<div class=\"toc\"><\/div>\n<h2 id=\"what\">What a parting line does in silicone<\/h2>\n<p>The parting line is the line where the two halves of the mold come together. In metal molds it is a hardened steel edge that holds the flash back. In silicone it is something much softer and more forgiving.<\/p>\n<p>A silicone mold is closed by pressing it against a solid backing board. There is no clamping force in the injection molding sense. The seal is made by the silicone itself deforming at the perimeter.<\/p>\n<p>That is why silicone flash behaves differently. It is not a pressure that escapes through a machined gap; it is an elastomer being squeezed and finding the path of least resistance. Where the mold closes first, the material is trapped. Where it closes last, the silicone has already been squeezed out.<\/p>\n<p>So flash in silicone is a record of the closing sequence. The thickest flash is not where the mold is worst; it is where the mold closed last.<\/p>\n<h2 id=\"where\">Where flash actually appears<\/h2>\n<p>Three locations account for almost all silicone flash.<\/p>\n<p>The first is the perimeter of the part itself, where the two halves meet around the shape. This is the flash you trim off, and it is the normal case.<\/p>\n<p>The second is at the extremities of the mold, usually the far corners or the ends of a long cavity. The mold closes unevenly along a long parting line because the silicone flexes, and the last point to touch is where the squeeze concentrates.<\/p>\n<p>The third is at any geometric feature that interrupts the parting line, such as a hole, a boss or a step. These create a small amount of extra travel in the mold halves, and the silicone escapes there preferentially.<\/p>\n<p>A useful diagnostic is to look at the flash pattern around the perimeter as a map. A uniform thin line means the mold is closing well. Flash that is thick at one point and absent at another means the closing sequence is off, and the fix is mechanical rather than in the compound.<\/p>\n<h2 id=\"too-much\">How much closure is too much<\/h2>\n<p>Silicone molds are not designed to close on a hard stop. The design intent is usually that the mold closes until the flash is thin enough to trim, and the mold's own stiffness plus the backing board sets that limit.<\/p>\n<p>Closing the mold past that point does two things. It squeezes the flash thinner, which is good, and it squeezes the part harder, which loads the mold walls and can close the cavity.<\/p>\n<p>The second effect is the one that causes trouble. A heavily over-closed mold bulges at the walls, and when the part is demolded the cavity springs back, so the part is slightly undersized. Over time this can also cause the mold to take a permanent set.<\/p>\n<p>On a manual press this is a matter of technique. On a vacuum-assisted or semi-automatic setup, the vacuum itself supplies much of the closing force, and the operator only adds light hand pressure on the backing board.<\/p>\n<figure>\n<img decoding=\"async\" src=\"https:\/\/uplastech.com\/wp-content\/uploads\/2026\/11\/silicone-parting-line-design-chart2.jpg\" alt=\"Parting line flash thickness against mold closure pressure for three silicone hardnesses, and the point where cavity depth is lost\" width=\"1536\" height=\"909\"><figcaption>Flash thins with closure but the cavity starts to close past 0.35 MPa on 50A silicone.<\/figcaption><\/figure>\n<h2 id=\"design\">Designing the line<\/h2>\n<p>A parting line that runs in a straight line across the part is the easiest to trim and the easiest to inspect. A line that follows a curve, and especially one that changes direction, is harder.<\/p>\n<p>Put the line where the part is least visible and least stressed. A line across a flange is better than one across a sealing surface, because a flash ridge across a sealing surface is both visible and functionally wrong.<\/p>\n<p>Avoid the line crossing a hole. The flash that forms around a hole is thicker than the flash around the perimeter, because the mold has to travel further to close around the hole feature.<\/p>\n<p>Give the line a physical backing on the perimeter. A silicone mold without a backing board on the parting edge will deflect and flash more, and the deflection is not uniform along the length.<\/p>\n<p>And if the part has a thin section, keep the line away from it. A thin section that is also a flash location is where flash causes the failure, because the flash ridge is a stress raiser on an already stressed section.<\/p>\n<h2 id=\"trim\">Trimming flash without tearing the part<\/h2>\n<p>Silicone flash is soft and it should shear cleanly. If it tears instead, something about the material or the flash geometry is wrong.<\/p>\n<p>The first cause is undercure. Partially cured silicone tears like gum. Full post-cure, in the mould or after demolding, makes flash brittle enough to snap off cleanly.<\/p>\n<p>The second is too much flash. Flash that is thick relative to the part will drag the edge as it is removed. This is where reducing the closure pressure helps, and where a sharper trim tool helps more.<\/p>\n<p>The third is the wrong tool. A sharp blade and a rigid edge support give a clean shear. A dull knife against a compliant part tears.<\/p>\n<p>For high volume, a two-step trim works best: a first pass that removes the bulk of the flash leaving a thin ring, then a second pass at a different angle that removes the ring. Trying to cut the full thickness in one pass is what causes tearing.<\/p>\n<table>\n<thead>\n<tr>\n<th>Flash thickness<\/th>\n<th>Trim method<\/th>\n<th>Risico<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Under 0.3 mm<\/td>\n<td>Single knife pass, sharp edge<\/td>\n<td>Laag<\/td>\n<\/tr>\n<tr>\n<td>0.3 to 0.8 mm<\/td>\n<td>Two-step trim<\/td>\n<td>Matig<\/td>\n<\/tr>\n<tr>\n<td>0.8 to 1.5 mm<\/td>\n<td>Two-step trim plus deburr<\/td>\n<td>High if one pass<\/td>\n<\/tr>\n<tr>\n<td>Over 1.5 mm<\/td>\n<td>Reduce closure, change tool<\/td>\n<td>Part damage likely<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 1 \u2014 Flash thickness and how to trim it.<\/p>\n<h2 id=\"fix\">Fixing flash that will not go away<\/h2>\n<p>If flash is consistent around the whole perimeter, the mould is over-closed or the compound is too soft for the geometry. Reduce the closure, or step up the hardness of the compound.<\/p>\n<p>If flash is heavy at the extremities of a long line, the mold is flexing. Add backing along the parting edge, or split the parting line into two shorter segments with a change of direction, which halves the unsupported length.<\/p>\n<p>If flash is heavy only around a hole or a feature, the design is at fault. Move the parting line away from the feature, or redesign the feature so it does not interrupt the line.<\/p>\n<p>If flash is heavy only in the first few shots after a mould is made, that is post-cure shrinkage. Silicone shrinks during cure, so a freshly made mould closes differently from one that has been cured for a week. Condition the mould before judging it.<\/p>\n<p>And if flash appears after a change of batch, check the compound viscosity and the catalyst ratio. A slightly under-catalysed mix is softer, flows more, and escapes further when the mould closes.<\/p>\n<table>\n<thead>\n<tr>\n<th>Pattern<\/th>\n<th>Oorzaak<\/th>\n<th>Correction<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Even around whole perimeter<\/td>\n<td>Over-closed mould<\/td>\n<td>Reduce closure pressure<\/td>\n<\/tr>\n<tr>\n<td>Heavy at line extremities<\/td>\n<td>Mould flexing<\/td>\n<td>Add edge backing<\/td>\n<\/tr>\n<tr>\n<td>Heavy around one feature<\/td>\n<td>Feature interrupts line<\/td>\n<td>Relocate parting line<\/td>\n<\/tr>\n<tr>\n<td>Heavy only on first shots<\/td>\n<td>Post-cure shrinkage<\/td>\n<td>Condition the mould<\/td>\n<\/tr>\n<tr>\n<td>Changed with new batch<\/td>\n<td>Compound or catalyst ratio<\/td>\n<td>Check mix and cure<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 2 \u2014 Flash patterns and what they point to.<\/p>\n<div class=\"faq\">\n<h3>Why does silicone flash at some points and not others?<\/h3>\n<p>Because flash records the closing sequence. Where the mould closes last, the silicone has been squeezed out. A thick flash point tells you where the mould halves arrive last.<\/p>\n<h3>How do I reduce flash on a silicone mould?<\/h3>\n<p>Reduce the closure force so the mould closes only to the point where flash is thin, add backing along the parting edge to stop the mould flexing, and step up compound hardness if the material is too soft.<\/p>\n<h3>Is flash in silicone a sign of a bad compound?<\/h3>\n<p>Usually not. It is a mould geometry and closure issue. Compound hardness, viscosity and cure ratio affect how much flash appears, but the pattern of where it appears is set by the mould.<\/p>\n<h3>Why does my flash tear instead of cutting cleanly?<\/h3>\n<p>Almost always undercure. Partially cured silicone tears like gum. A full post-cure makes flash brittle enough to shear off with a sharp blade.<\/p>\n<h3>How thick can flash get before it damages the part?<\/h3>\n<p>Over 1.5 mm the flash ridge is thick enough to drag the part edge as it is removed, and tearing or distortion becomes likely. Above that, reduce the closure rather than trimming harder.<\/p>\n<h3>Does silicone shrink during cure and affect flash?<\/h3>\n<p>Yes. Silicone shrinks as it cures, so a freshly made mould closes differently from one that has conditioned for a week. Condition new moulds before judging whether the flash is acceptable.<\/p>\n<\/div>\n<div class=\"cta\">\n<p class=\"cta-h\">Flash that will not trim clean?<\/p>\n<p>Send the part geometry, the compound hardness and a photo of the flash pattern. We will tell you whether it is a closure problem, a mould geometry problem or a cure problem.<\/p>\n<p><a class=\"btn\" href=\"https:\/\/uplastech.com\/nl\/silicone-rubber\/\">Talk to our silicone team<\/a>\n<\/div>\n<div class=\"rel\">\n<h4>Continue reading<\/h4>\n<ul>\n<li><a href=\"https:\/\/uplastech.com\/nl\/silicone-flash-trimming\/\">Silicone Flash Trimming<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/nl\/silicone-compression-molding\/\">Silicone Compression Molding<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/nl\/silicone-mold-sticking\/\">Silicone Mold Sticking<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/nl\/silicone-hardness-shore-a\/\">Silicone Hardness and Shore A<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/nl\/silicone-rubber-types-guide\/\">Silicone Rubber Types Guide<\/a><\/li>\n<\/ul>\n<\/div>\n<\/div>","protected":false},"excerpt":{"rendered":"<p>Silicone does not flash the way metal does. Understanding why the flash is thin at some points and thick at others is about the closing force and the elastomer&#8217;s own behaviour.<\/p>","protected":false},"author":1,"featured_media":7266,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[39],"tags":[],"class_list":["post-9063","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-silicone-rubber-products"],"meta_box":{"post-to-quiz_to":[]},"_links":{"self":[{"href":"https:\/\/uplastech.com\/nl\/wp-json\/wp\/v2\/posts\/9063","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/uplastech.com\/nl\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/uplastech.com\/nl\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/uplastech.com\/nl\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/uplastech.com\/nl\/wp-json\/wp\/v2\/comments?post=9063"}],"version-history":[{"count":1,"href":"https:\/\/uplastech.com\/nl\/wp-json\/wp\/v2\/posts\/9063\/revisions"}],"predecessor-version":[{"id":9075,"href":"https:\/\/uplastech.com\/nl\/wp-json\/wp\/v2\/posts\/9063\/revisions\/9075"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/uplastech.com\/nl\/wp-json\/wp\/v2\/media\/7266"}],"wp:attachment":[{"href":"https:\/\/uplastech.com\/nl\/wp-json\/wp\/v2\/media?parent=9063"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/uplastech.com\/nl\/wp-json\/wp\/v2\/categories?post=9063"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/uplastech.com\/nl\/wp-json\/wp\/v2\/tags?post=9063"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}