{"id":3780,"date":"2026-11-10T11:57:21","date_gmt":"2026-11-10T03:57:21","guid":{"rendered":"http:\/\/www.crystalpalace-art.com\/blog\/?p=3780"},"modified":"2026-11-10T11:57:21","modified_gmt":"2026-11-10T03:57:21","slug":"how-to-design-a-proper-venting-system-for-a-plastic-mould-46eb-1ce805","status":"publish","type":"post","link":"http:\/\/www.crystalpalace-art.com\/blog\/2026\/11\/10\/how-to-design-a-proper-venting-system-for-a-plastic-mould-46eb-1ce805\/","title":{"rendered":"How to design a proper venting system for a plastic mould?"},"content":{"rendered":"

Alright, let\u2019s cut to the chase\u2014if you\u2019re running a plastic mould shop (or working with one), venting\u2019s probably the last thing you obsess over\u2026 until it\u2019s the reason your parts come out with burns, short shots, or that weird \u201csilver streak\u201d that makes your client send them right back. I\u2019ve seen it a hundred times: moulds that fit like a glove, tonneages dialed in just right, material sorted, and still, the quality\u2019s garbage because of bad vents. As a plastic mould supplier, we live and breathe this stuff\u2014so let\u2019s break down how to nail a proper venting system, no jargon, no fluff, just what actually works in the shop. Plastic Mould<\/a><\/p>\n

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First, let\u2019s get the basics straight: vents are tiny, narrow channels cut into the mould\u2019s cavity (or core) that let the air trapped inside the mould cavity escape when you inject molten plastic. If that air can\u2019t get out fast enough, two things happen: either the plastic hits a wall of compressed air, which burns the part (that\u2019s the brown, charred line right at the parting line, ugh) or the air pressure pushes back so hard the plastic can\u2019t fill the cavity entirely\u2014short shot. Simple, right? But sizing and placing those vents? That\u2019s where 90% of people mess up.<\/p>\n

Let\u2019s start with sizing, because too shallow is useless, too deep means flash. Here\u2019s the hard rule I live by: vent depth has to be thinner than the material\u2019s \u201cflow front thickness.\u201d For most common plastics\u2014PP, PE, ABS, PS\u2014that number is around 0.025 mm (0.001 inch). If you go deeper than that, the molten plastic will squirt into the vent and leave a thin, unwanted flash on the part. Too shallow? Air can\u2019t escape through it\u2014test done. Now, vent width? That\u2019s tied to the parting line, but usually, we make vents 3-10 mm wide. For small, intricate parts (like phone components), narrow vents (3-5 mm) work; for larger parts (like crates or buckets), we go wider (8-10 mm) to speed up air flow. Wait, what about engineering plastics? Like PA or PC? Those need even shallower vents\u20140.02 mm max\u2014because they\u2019re stiffer and flash way easier. Trust me, we had a job once with a PA6 part that kept flashing until we dropped the vent depth from 0.03 mm to 0.018 mm. Game. Changer.<\/p>\n

Next, placement. This is where most DIY moulders go wrong\u2014they just stick vents at the parting line wherever, but that\u2019s not enough. Air gets trapped in the last place the plastic fills, right? So vents have to be at the \u201cend of fill\u201d points. Let\u2019s use a simple example: if you\u2019re making a rectangular container, the plastic injects from one side (the gate), so it flows across the length, and the last spot it fills is the opposite end. That\u2019s where you need a vent. But wait\u2014sometimes even in that long rectangle, there are tight corners or bosses that trap air. Those need mini vents, too. We call those \u201ccore vents\u201d sometimes\u2014tiny holes in the core insert, not just the parting line. For example, a part with a 5 mm tall boss that\u2019s hard to fill? We drill a 0.5 mm diameter hole in the core, then make a shallow vent groove leading to the backside of the mould to let air out there. No more short shots on bosses. Also, never put vents near the gate. The first thing plastic does is fill the gate area\u2014if you have a vent there, the plastic will bleed out into the vent and cause flash right at the gate. That\u2019s a quick fix rule: vents only at the fill ends, not the start.<\/p>\n

Now, vent length. Short vents are better, generally. The longer the vent, the more friction the air has to push through, so it\u2019s slower to escape. We try to keep vents under 10 mm long, but if you have a part where the end of fill is far from the parting line (like a deep cup), we add a \u201cvent slot\u201d that runs along the side of the mould insert, not just a straight hole. That\u2019s called a \u201cside vent\u201d and it works way better for deep cavities. Another pro tip: add a vent \u201cchoke\u201d at the end where the air exits. Wait, no\u2014let me rephrase: when air comes out of the vent, it should escape to the atmosphere, not get trapped in the mould\u2019s plates. So we often make a \u201cvent path\u201d that leads the air away from the cavity, like a small groove that connects multiple vents together, then a big exit hole at the edge of the mould. That way, all the trapped air flows to that exit, not builds up between the vents.<\/p>\n

Wait, what about multi-cavity moulds? Those are trickier. Each cavity has its own fill rate, so you can\u2019t just use the same vent size for all. If one cavity is farther from the gate, it\u2019ll fill slower, so its vent needs to be a bit wider (like 8 mm vs 5 mm for the closer one). We always test multi-cavity moulds with a \u201cflow test\u201d first\u2014run a shot with no plastic, just air, then check where the air is stuck. That tells us exactly where to add extra vents. Once, we had a 8-cavity mould for disposable cups, and 2 of the cavities kept having short shots. We added a 0.03 mm deep vent to those two cavities, and bam\u2014no more issues. It was the same design, just those ends were deeper, so air trapped there.<\/p>\n

Now, common mistakes we see all the time that waste everyone\u2019s time. First, people using too many vents. You don\u2019t need vents every 5 mm\u2014over-venting can cause flash, and it\u2019s a waste of time machining the mould. Stick to 1 vent every 25-50 mm along the fill path, except for tight spots (bosses, ribs) where you need 1 vent per rib. Second, not polishing the vent channels. If the vent has rough edges, air gets caught in the scratches, so it can\u2019t escape. We always lap our vent grooves to a smooth finish, same as the mould cavity. Third, forgetting to check vent depth after machining. Sometimes when we machine a vent, the tool wears a little, so the depth ends up being deeper than we planned. We use a feeler gauge every time to confirm the depth is within 0.02-0.03 mm for PP\/ABS. That\u2019s a cheap check that avoids a huge rework bill.<\/p>\n

Wait, what about over-moulding inserts? Oh, big one. If you\u2019re over-moulding a metal insert inside plastic, air gets trapped between the insert and the plastic. You need tiny vents right next to the insert, like a 0.02 mm deep groove right at the insert\u2019s edge. We once had a job where automotive sensors had metal inserts, and they kept having air bubbles under the plastic. We added those tiny vents, and the bubble issue vanished. No more rejections from the auto client\u2014priceless.<\/p>\n

Now, testing. You can\u2019t just design vents and cross your fingers. We do two quick tests before sending a mould out for production. First, a \u201cmoisture test\u201d: inject a small shot of plastic and check the vents for any flash. If there\u2019s even a tiny flash, the vent is too deep\u2014grind it down a little. Second, a \u201cshort shot test\u201d: inject 80-90% of the normal shot volume, so the plastic doesn\u2019t fill the cavity. The last spot where the plastic stops is exactly where you need more vents. If it\u2019s a tight corner, add a core vent. If it\u2019s the end of the parting line, make the existing vent a little wider. We do this test every single time\u2014saves us so many headaches later.<\/p>\n

As a plastic mould supplier, I\u2019ll tell you this: venting is not a \u201cset it and forget it\u201d thing. It has to be tailored to the plastic, the part geometry, the mould design. A vent that works for PP won\u2019t work for a high-temperature engineering plastic like PEEK, because PEEK flows slower and compresses air more easily. For PEEK, we go a tiny bit wider (10-12 mm) but keep the depth still under 0.02 mm. Also, cycle time matters. If you\u2019re running a fast cycle (like disposable parts, 10 seconds a shot), you need faster venting\u2014so smaller, closer vents. For slower cycles (like large parts, 1 minute a shot), you can get away with slightly bigger vents, but not too big.<\/p>\n

Wait, another trick: use \u201cvent pins\u201d for areas where you can\u2019t machine a groove, like around ejector pins. If an ejector pin is in a spot where air can trap, we make the ejector pin with a tiny groove along its edge (0.02 mm deep) so air can escape past it. That\u2019s a clever hack we use all the time\u2014no extra machining, just modifying standard parts.<\/p>\n

Now, let\u2019s wrap this up with a quick checklist you can use next time you\u2019re designing a vent system:<\/p>\n

    \n
  1. Vent depth: 0.015-0.03 mm (adjust for plastic type\u2014thinner for engineering plastics, thicker for low-cost ones)<\/li>\n
  2. Vent width: 3-10 mm (small for small parts, big for large parts)<\/li>\n
  3. Placement: At the end of fill, tight corners, near bosses\/ribs, away from gates<\/li>\n
  4. Length: Keep under 10 mm; use side vents for deep cavities<\/li>\n
  5. Test: Do a short shot and flash check before production<\/li>\n
  6. Smooth finish: Lap vent channels to avoid air friction<\/li>\n<\/ol>\n

    Look, I\u2019ve been doing this for 15 years, and venting is still the number one cause of mould issues. It\u2019s not rocket science, but it\u2019s detail work. If you skip this step, you\u2019re going to get parts that don\u2019t pass quality checks, have to rework the mould, and lose time and money. That\u2019s why we make venting a top priority when we build a mould for our clients\u2014we don\u2019t just build a mould that makes a part, we build a mould that makes a good part every time.<\/p>\n

    <\/p>\n

    If you\u2019re tired of dealing with mould issues like short shots, flash, or burnt parts, hit us up to talk mould design and venting solutions. We\u2019ve worked with everything from small consumer parts to large industrial components, and we\u2019ll make sure your venting system is set up for success, not failure.<\/p>\n

    Mould Components<\/a> References:<\/p>\n

      \n
    1. Rosato, D. V., & Rosato, M. G. (2004). Injection Molding Handbook (5th ed.). Hanser Publishers.<\/li>\n
    2. Manufacturing Processes for Plastics (2018). Society of Manufacturing Engineers (SME).<\/li>\n
    3. Tool and Manufacturing Engineers Handbook (2020). Vol. 3: Materials, Finishing, and Coating. SME.<\/li>\n<\/ol>\n
      \n

      Wuxi Santaizi Metal Products Co., Ltd.<\/a>
      As one of the most professional plastic mould manufacturers and suppliers in China, we warmly welcome you to buy high-grade plastic mould made in China here from our factory. All customized metal products are with high quality and competitive price. Contact us for pricelist.
      Address: No. 23 Xufeng Road, Xushe Town, Yixing City, Wuxi City, Jiangsu Province, China
      E-mail: Ksstzmj@163.com
      WebSite:       https:\/\/www.wuxisantaizi.com\/<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"

      Alright, let\u2019s cut to the chase\u2014if you\u2019re running a plastic mould shop (or working with one), … How to design a proper venting system for a plastic mould?<\/span>Read more<\/a><\/p>\n","protected":false},"author":325,"featured_media":3780,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[3743],"class_list":["post-3780","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-industry","tag-plastic-mould-4f66-1d1614"],"_links":{"self":[{"href":"http:\/\/www.crystalpalace-art.com\/blog\/wp-json\/wp\/v2\/posts\/3780","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/www.crystalpalace-art.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/www.crystalpalace-art.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/www.crystalpalace-art.com\/blog\/wp-json\/wp\/v2\/users\/325"}],"replies":[{"embeddable":true,"href":"http:\/\/www.crystalpalace-art.com\/blog\/wp-json\/wp\/v2\/comments?post=3780"}],"version-history":[{"count":0,"href":"http:\/\/www.crystalpalace-art.com\/blog\/wp-json\/wp\/v2\/posts\/3780\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"http:\/\/www.crystalpalace-art.com\/blog\/wp-json\/wp\/v2\/posts\/3780"}],"wp:attachment":[{"href":"http:\/\/www.crystalpalace-art.com\/blog\/wp-json\/wp\/v2\/media?parent=3780"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/www.crystalpalace-art.com\/blog\/wp-json\/wp\/v2\/categories?post=3780"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/www.crystalpalace-art.com\/blog\/wp-json\/wp\/v2\/tags?post=3780"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}