{"id":8005,"date":"2026-01-06T09:00:00","date_gmt":"2026-01-06T09:00:00","guid":{"rendered":"https:\/\/uplastech.com\/silicone-mold-design\/"},"modified":"2026-01-06T09:00:00","modified_gmt":"2026-01-06T09:00:00","slug":"silicone-mold-design","status":"publish","type":"post","link":"https:\/\/uplastech.com\/es\/silicone-mold-design\/","title":{"rendered":"Silicone Mould Design: Shrink Is Filler, Flash Is Pressure, Parting Line Is a Decision"},"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\/silicone-mold-design.jpg\" alt=\"Linear shrinkage against filler loading, and flash thickness against clamp pressure\" width=\"1536\" height=\"909\"><figcaption>Shrinkage falls from 3.8 to 2.2 percent as filler goes from 20 to 60 phr, and flash from 0.35 to 0.03 mm as clamp pressure goes from 0 to 20 MPa.<\/figcaption><\/figure>\n<div class=\"toc\"><\/div>\n<h2 id=\"shrink\">Shrink is set by the filler, not the cure<\/h2>\n<p>Silicone shrinks 2 to 4 percent, which is more than most thermoplastics and far more than most people allow for. The shrink is mostly thermal: the part is moulded at 170 C and used at 20, and the coefficient of expansion does the rest. But the amount is set by how much of the volume is filler.<\/p>\n<p>Filler does not expand the way the polymer does, so a compound with more filler shrinks less. Across a 20 to 60 phr range, the linear shrinkage falls from 3.8 to 2.2 percent. That is a 42 percent reduction in the shrink allowance for a change that a compounder makes routinely.<\/p>\n<p>The practical problem is that the shrink allowance in a mould is cut once, and the compound changes over the life of the tool. A mould cut for a 40 phr compound at 3.0 percent will produce parts 0.4 percent oversize if the compound moves to 60 phr, which on a 100 mm part is 0.4 mm and on a 300 mm part is 1.2 mm.<\/p>\n<p>The answer is to cut the tool for a shrink range rather than a value, and to specify the compound on the drawing. A tool cut at the middle of a 2.6 to 3.4 percent band produces parts that are within tolerance across the band, and a tool cut at 3.0 with no band does not.<\/p>\n<table>\n<thead>\n<tr>\n<th>Filler loading<\/th>\n<th>Shrink<\/th>\n<th>Typical hardness<\/th>\n<th>Where it is used<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>20 phr<\/td>\n<td>3.8 percent<\/td>\n<td>30 to 40 Shore A<\/td>\n<td>Soft seals, membranes<\/td>\n<\/tr>\n<tr>\n<td>30 phr<\/td>\n<td>3.4 percent<\/td>\n<td>45 to 55 Shore A<\/td>\n<td>General gaskets<\/td>\n<\/tr>\n<tr>\n<td>40 phr<\/td>\n<td>3.0 percent<\/td>\n<td>55 to 65 Shore A<\/td>\n<td>The common middle<\/td>\n<\/tr>\n<tr>\n<td>50 phr<\/td>\n<td>2.6 percent<\/td>\n<td>65 to 75 Shore A<\/td>\n<td>Industrial seals<\/td>\n<\/tr>\n<tr>\n<td>60 phr<\/td>\n<td>2.2 percent<\/td>\n<td>75 to 85 Shore A<\/td>\n<td>High-modulus parts<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 1 \u2014 Filler loading against shrink, hardness and use.<\/p>\n<h2 id=\"flash\">Flash is a pressure problem<\/h2>\n<p>Silicone flashes more than any thermoplastic, and the reason is that it has no frozen skin. A thermoplastic entering a cavity freezes against the steel within milliseconds and that frozen layer seals the parting line. Silicone stays fluid until it cures, so it flows into every gap the tool has, however small.<\/p>\n<p>The result is that flash thickness is set by clamp pressure and by the tool's flatness, not by the tool's clearance. At zero clamp pressure the flash is 0.35 mm, at 5 MPa it is 0.18, at 10 MPa 0.09, at 15 MPa 0.05 and at 20 MPa 0.03.<\/p>\n<p>That is why a silicone tool is a press tool. The platens have to be flat and parallel, the tool has to be supported so it does not bow, and the clamp force has to be enough for the projected area. A 200 by 200 mm part at 10 MPa needs 40 tonnes, and that is the number that decides which press the tool goes on.<\/p>\n<p>Deflashing is possible, and it is what most shops do, but it is a cost that scales with the flash area rather than the flash thickness. A tool that flashes 0.05 mm still needs the same deflashing operation as one that flashes 0.3, and the only way to remove the operation is to get the flash below the visible threshold, which on silicone is about 0.02 mm.<\/p>\n<div class=\"callout\">Calculate the clamp force from the projected area at 10 MPa before quoting a silicone tool. A tool that needs 80 tonnes will not run on the press the shop already has, and that changes the answer more than the tool price does.<\/div>\n<h2 id=\"parting\">The parting line is a decision, not a default<\/h2>\n<p>The parting line is where the two halves of the tool meet, and on a silicone part it is visible, because silicone flashes there and because the line interrupts the surface. Where it lands is a design decision that is usually left to the toolmaker, and it is the one thing that cannot be changed afterwards without a new tool.<\/p>\n<p>Three rules cover most cases. Put the line on an edge rather than a face, because a line on an edge reads as a corner and a line on a face reads as a defect. Put it where the flash can be trimmed without cutting into a sealing surface. And put it so that the part stays on the half that ejects it, which means on the side with more surface area or more shrinkage grip.<\/p>\n<p>The corner is the worst place for a parting line. A corner carries the highest stress in service, the flash there is hardest to remove without nicking the radius, and a nick in a corner is where a tear starts. A line 2 mm up the wall from the corner is invisible and trims cleanly.<\/p>\n<p>For a part that has to seal on both faces, the parting line has to be outside both sealing bands. That is a geometry constraint that may force a different split, and it is worth drawing the sealing bands on the tool layout before the split is chosen rather than after.<\/p>\n<h2 id=\"vents\">Venting a material that traps air<\/h2>\n<p>Silicone traps air because it is viscous and because it cures before the air can escape. The air comes from the compound, from the cavity, and from the space between the insert and the tool if there is one. If it cannot leave, it becomes a void, a short, or a burnt patch where the compressed air reached curing temperature.<\/p>\n<p>The vents are shallow. A silicone vent is 0.01 to 0.02 mm deep and 3 to 5 mm wide, which is shallower than a thermoplastic vent because silicone will flash into anything deeper. That shallow depth also means the vent blocks easily, so it needs cleaning on a schedule rather than on failure.<\/p>\n<p>Vacuum assist is common on silicone tools for exactly this reason. Pulling a vacuum on the cavity before the tool closes removes the air rather than relying on the vents, and it allows a tighter tool with less flash because the air does not have to be pushed out through the parting line.<\/p>\n<p>Inserts and bonded parts need their own venting. A metal insert in a silicone part traps air between the insert and the tool, and the air has nowhere to go. A small groove or a vent hole under the insert solves it, and it is a detail that is almost always missed on the first tool.<\/p>\n<h2 id=\"material\">Tool material and life<\/h2>\n<p>Silicone tools are usually steel, sometimes aluminium for prototypes, and they last a long time because silicone is soft and does not abrade the tool. A steel tool for a compression moulding job runs hundreds of thousands of parts, and its life is limited by damage and by the parting line wearing rather than by erosion.<\/p>\n<p>The surface finish matters more than the material. A polished cavity gives a glossy part and releases easily; a roughened one grips and needs more release agent. That is why a silicone tool that has been in service for years and has been cleaned with abrasive pads starts to stick, and why the fix is a repolish rather than more release agent.<\/p>\n<p>The heating is part of the tool design. Silicone tools are heated by platens, by cartridge heaters or by oil, and the temperature uniformity across the cavity is what decides whether the cure is even. A 10 C gradient across a 300 mm tool gives a 10 C gradient in cure state, which shows up as a hardness variation across the part.<\/p>\n<table>\n<thead>\n<tr>\n<th>Tool element<\/th>\n<th>Silicone requirement<\/th>\n<th>Por qu\u00e9<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Parting line<\/td>\n<td>Flat to 0.01 mm<\/td>\n<td>Flash is set by flatness at pressure<\/td>\n<\/tr>\n<tr>\n<td>Vents<\/td>\n<td>0.01 to 0.02 mm deep<\/td>\n<td>Deeper and it flashes<\/td>\n<\/tr>\n<tr>\n<td>Draft<\/td>\n<td>2 to 3 degrees<\/td>\n<td>Rubber grips more than thermoplastic<\/td>\n<\/tr>\n<tr>\n<td>Cavity finish<\/td>\n<td>SPI B-1 or better<\/td>\n<td>Release and gloss<\/td>\n<\/tr>\n<tr>\n<td>Calefacci\u00f3n<\/td>\n<td>Within 5 C across the cavity<\/td>\n<td>Cure state uniformity<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 2 \u2014 What a silicone tool needs that a thermoplastic tool does not.<\/p>\n<h2 id=\"specify\">What to specify<\/h2>\n<p>A silicone mould drawing has to carry the things that the toolmaker cannot infer from the part.<\/p>\n<ul>\n<li>Shrink allowance with the range it covers, and the compound it was based on.<\/li>\n<li>Parting line position, drawn on the part, not left to the toolmaker.<\/li>\n<li>Clamp pressure and the projected area, so the press can be checked.<\/li>\n<li>Vent positions and depth, and whether the tool runs under vacuum.<\/li>\n<li>Draft on every vertical face, and the finish required on each.<\/li>\n<li>Insert positions and how the air under each insert is vented.<\/li>\n<\/ul>\n<p>The parting line and the shrink range are the two that cannot be fixed later. Everything else can be reworked, polished, vented or re-clamped; a parting line in the wrong place is a new tool.<\/p>\n<div class=\"myth\">Myth: silicone moulds do not need draft because the material is flexible. They need 2 to 3 degrees, because flexible rubber grips a core harder than rigid plastic does, and a part torn off a core has a stretched edge that will not seal.<\/div>\n<div class=\"faq\">\n<h3>How much does silicone shrink?<\/h3>\n<p>Two to four percent, and the amount is set by filler loading. It falls from 3.8 percent at 20 phr to 2.2 at 60 phr.<\/p>\n<h3>Why does silicone flash so much?<\/h3>\n<p>Because it has no frozen skin. It stays fluid until it cures, so it flows into every gap in the tool. Flash thickness is set by clamp pressure and tool flatness, not by clearance.<\/p>\n<h3>What clamp pressure does a silicone tool need?<\/h3>\n<p>About 10 MPa on the projected area. A 200 by 200 mm part needs 40 tonnes, and that decides which press it runs on.<\/p>\n<h3>Where should the parting line go?<\/h3>\n<p>On an edge rather than a face, away from sealing surfaces, and on the half that will eject the part. A corner is the worst place because the flash is hardest to remove there.<\/p>\n<h3>How deep should a silicone vent be?<\/h3>\n<p>0.01 to 0.02 mm deep and 3 to 5 mm wide. Deeper than that and the silicone flashes into the vent instead of venting.<\/p>\n<h3>Does a silicone tool need draft?<\/h3>\n<p>Yes, 2 to 3 degrees. Rubber grips a core harder than rigid plastic, and a part pulled off without draft has a stretched edge that will not seal.<\/p>\n<\/div>\n<div class=\"cta\">\n<p class=\"cta-h\">Designing a silicone part and its tool?<\/p>\n<p>Send the part drawing with the sealing surfaces marked. We will come back on shrink range, parting line, clamp force and venting, and flag what cannot be changed after the tool is cut.<\/p>\n<p><a class=\"btn\" href=\"https:\/\/uplastech.com\/es\/silicone-rubber\/\">Ask about silicone tooling<\/a>\n<\/div>\n<div class=\"rel\">\n<h4>Continue reading<\/h4>\n<ul>\n<li><a href=\"https:\/\/uplastech.com\/es\/silicone-mold-release\/\">Mould Release for Silicone: Internal, External and the Transfer That Follows<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/es\/silicone-vulcanization-temperature\/\">Vulcanisation Temperature and Time: The Cure Curve Behind Every Part<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/es\/silicone-compression-molding\/\">Silicone Compression Molding: Pressure, Venting and Cure<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/es\/silicone-compound-formulation\/\">Silicone Compound Formulation: Filler, Catalyst and Inhibitor<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/es\/silicone-transfer-molding\/\">Silicone Transfer Molding: Pot Life, Runner and Scrap<\/a><\/li>\n<\/ul>\n<\/div>\n<\/div>","protected":false},"excerpt":{"rendered":"<p>Shrink falls from 3.8 to 2.2 percent across a 40 phr change in filler, and flash from 0.35 to 0.03 mm across the clamp pressure range. The parting line is the one thing the tool cannot fix afterwards.<\/p>","protected":false},"author":1,"featured_media":8004,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_seopress_titles_title":"Silicone Mould Design: Shrink, Flash, Parting","_seopress_titles_desc":"Silicone mould design: shrinkage against filler loading, flash against clamp pressure, parting line placement, venting for a material that traps air, tool mater","_seopress_robots_index":"","_seopress_robots_follow":"","_seopress_robots_imageindex":"","_seopress_robots_snippet":"","_seopress_robots_primary_cat":"39","_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":[39],"tags":[],"class_list":["post-8005","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\/es\/wp-json\/wp\/v2\/posts\/8005","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/uplastech.com\/es\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/uplastech.com\/es\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/uplastech.com\/es\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/uplastech.com\/es\/wp-json\/wp\/v2\/comments?post=8005"}],"version-history":[{"count":0,"href":"https:\/\/uplastech.com\/es\/wp-json\/wp\/v2\/posts\/8005\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/uplastech.com\/es\/wp-json\/wp\/v2\/media\/8004"}],"wp:attachment":[{"href":"https:\/\/uplastech.com\/es\/wp-json\/wp\/v2\/media?parent=8005"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/uplastech.com\/es\/wp-json\/wp\/v2\/categories?post=8005"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/uplastech.com\/es\/wp-json\/wp\/v2\/tags?post=8005"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}