掌握注塑成型：基础知识、应用和设计技巧

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在本指南中，您将找到有关注塑成型所需的所有信息。掌握技术的基本原理并快速学习可行的设计技巧，这将节省您的时间并降低成本。

第 1 部分

注塑成型基础知识

什么是注塑成型？它是如何工作的以及它的用途是什么？

在本节中，我们将回答这些问题并向您展示注塑零件的常见示例，以帮助您熟悉该技术的基本原理和应用。

什么是注塑成型？

注塑成型是一种批量生产的制造技术 相同的塑料部件 具有良好的公差。在注塑成型中，聚合物颗粒首先熔化，然后在压力下注入模具，液体塑料在模具中冷却并固化。注塑成型使用的材料是热塑性聚合物，可以着色或填充其他添加剂。

几乎你周围的每个塑料部件 使用注塑成型制造：从汽车零部件到电子外壳，再到厨房用具。

注塑如此受欢迎，是因为单位成本极低 当制造大批量时 。注塑成型提供高重复性 以及良好的设计灵活性 。注塑成型的主要限制通常归结为经济因素，因为初始投资较高 对于模具是必需的。此外，周转时间 从设计到生产很慢（至少4周）。

注塑工艺

如今，注塑成型广泛应用于消费品和工程应用。您周围的几乎所有塑料制品都是通过注塑成型制造的。这是因为该技术可以非常大量地生产相同的零件 （通常为 1,000 至 100,000 多个单位）每个零件的成本非常低 （通常每单位 1-5 美元）。

但与其他技术相比，启动成本 注塑成型率相对较高，主要是因为需要定制模具。一个模具的成本可能在 3,000 美元到 100,000 美元以上之间，具体取决于其复杂性、材料（铝或钢）和精度（原型、试运行或大规模生产模具）。

所有热塑性材料都可以注塑成型。某些类型的有机硅和其他热固性树脂也与注塑工艺兼容。注塑中最常用的材料有：

• 聚丙烯 (PP)： 约占全球产量的 38%
• ABS： 约占全球产量的 27%
• 聚乙烯 (PE)： 约占全球产量的 15%
• 聚苯乙烯 (PS)： 约占全球产量的 8%

即使我们考虑到所有其他可能的制造技术，仅用这四种材料进行注塑成型就占超过 40% 每年全球生产的所有塑料部件！

注塑简史

塑料取代象牙

1869 年，约翰·韦斯利·海厄特 (John Wesley Hyatt) 发明了赛璐珞，这是第一种实用的人造塑料，旨在取代象牙来生产……台球！早期的注塑机使用料筒来加热塑料，并使用柱塞将其注射到模具中。

革命性的发明

20 世纪 50 年代中期，往复螺杆的发明一手革新了塑料行业。往复螺杆解决了以往系统面临的塑料加热不均匀的关键问题，为塑料件的大规模生产开辟了新的前景。

今天的注塑

如今，注塑成型市场规模达 3000 亿美元。全球每年通过注塑成型生产 5 多万吨塑料零件。近年来，出于环境原因，对可生物降解材料的需求不断增加。

注塑机：它们如何工作？

注塑机由 3 个主要部分组成：注射装置 ,模具 - 整个过程的核心 - 以及夹紧/顶出装置 .

在本节中，我们将研究每个系统的用途以及它们的基本操作机制如何影响注塑过程的最终结果。

在视频中观看大型注塑机每 3 秒生产 72 个瓶盖的运行情况：

注射装置

注射装置的目的是熔化原料塑料并将其引导到模具中。它由料斗组成 ，桶 ，以及往复螺杆 .

注射成型工艺的工作原理如下：

1. 首先将聚合物颗粒干燥并放入料斗中，与着色颜料或其他增强添加剂混合。
2. 颗粒被送入机筒，同时被加热、混合并通过变螺距螺杆移向模具。螺杆和机筒的几何形状经过优化，有助于将压力建立到正确的水平并熔化材料。
3. 然后，冲头向前移动，熔化的塑料通过流道系统注入模具中，填充整个型腔。当材料冷却时，它会重新凝固并形成模具的形状。
4. 最后，模具打开，现在的固体部件被顶针推出。然后模具闭合并重复该过程。

   整个过程可以非常快地重复：周期大约需要30到90秒 取决于零件的尺寸。

   零件弹出后，它被分配到传送带上或保存容器中。通常，注塑零件可以立即使用，几乎不需要任何后处理。

制造模具

模具就像照片的底片：其几何形状和表面纹理直接转移到注塑件上。

它通常占注塑成型启动成本的最大部分：对于简单的几何形状和相对较小的生产运行（1,000 至 10,000 件），典型模具的成本约为 2,000-5,000 美元，对于针对全面生产订单（100,000 件或更多）优化的模具，成本可能高达 100,000 美元。

这是因为设计和制造高质量模具需要高水平的专业知识，才能精确生产数千（或数十万）个零件。

模具通常采用 CNC 加工 由铝或工具钢制成，然后按要求的标准进行加工。除了零件的反面之外，它们还具有其他功能，例如促进材料流入模具的流道系统，以及帮助和加速零件冷却的内部水冷却通道。

在制造和设计指南中了解有关 CNC 加工的更多信息 →

3D 打印材料的最新进展使得能够以极低的成本制造适合小批量注塑成型（100 个零件或更少）的模具。由于传统模具制造的成本非常高，如此小的产量在过去在经济上是不可行的。

*用于生产数万件塑料零件的工业模具设计。左侧显示型腔，右侧显示型芯。*

模具的解剖

最简单的模具是直拉模具。它由两半组成：腔体 （正面）和核心 （背面）。

在大多数情况下，直拉模具 是首选，因为它们设计和制造简单，总成本相对较低。但存在一些设计限制：零件的每一侧都必须具有 2.D 几何形状，并且没有悬垂（即不受下方支持的区域）。

如果需要更复杂的几何形状，则可伸缩侧作用芯 或需要其他插入。

侧动型芯是从顶部或底部进入模具的移动元件，用于制造具有悬伸的零件（例如型腔或孔）。不过，应谨慎使用副作用，因为成本会迅速增加。

有趣的事实： 大约 50% 的典型注塑周期专用于冷却和固化。最小化设计厚度是加快这一步骤并降低成本的关键。

模具的两个面：A面和B面

注塑零件有两个面：A 面，面向型腔（模具的前半部分）；B 面，面向型芯（模具的后半部分）。这两个方面通常有不同的目的：

• A面 通常具有更好的视觉外观，通常被称为化妆品面 。根据您的设计规范，A 侧的面将是光滑的或具有纹理。

• B 面 通常包含零件的隐藏（但非常重要）的结构元素（凸台、肋、卡扣配合等）。因此，它被称为功能端 。 B 面的表面通常比较粗糙，并且有明显的顶针痕迹。

将材料注入模具：流道系统

跑步者系统 是引导熔化的塑料进入模具型腔的通道。它控制流量和压力 将液态塑料注入型腔，并在弹出后将其移除（折断）。流道系统通常由3个主要部分组成：

• 浇口 是所有熔化塑料进入模具时最初流经的主要通道。
• 跑步者 将熔化的塑料沿着模具两半相交的表面展开，并将支线连接到浇口。可能有一个或多个流道，将材料引导至一个或多个零件。脱模后，流道系统与零件分离。这是注塑过程中唯一的材料浪费，其中 15-30% 可以回收再利用。
• 大门 （是材料进入模具型腔的入口点。它的几何形状和位置很重要，因为它决定了塑料的流动。

不同的闸门类型适合不同的应用。注塑成型中使用的浇口有4种类型：

• 边缘门 在模具两半的分型线处注射材料，是最常见的浇口类型。稍后必须手动移除流道系统，从而在注射点留下一个小缺陷。
• 隧道闸门 在分型线下方注入材料。当零件从模具中弹出时，流道系统会折断，无需手动拆卸。这使得这种类型的浇口非常适合超大体积。
• 哨门 从型腔背面注入材料，隐藏因破坏其他浇口类型而留下的小缺陷。这些浇口用于需要出色视觉外观的零件。
• 热门提示 直接连接到支刺并从零件的顶部注入塑料。这样在流道系统上不会浪费任何材料，使其成为大规模生产的理想选择，但在注射点会看到凹痕。

遗迹

在流道系统与零件连接的地方，通常可以看到一个小缺陷，称为痕迹。

如果出于美观目的不希望存在痕迹，那么也可以将其“隐藏”在零件的功能 B 面中。

夹紧和顶出系统

注塑机的远端是锁模系统。锁模系统具有双重用途：它在注射过程中保持模具的两个部件紧密闭合，并在打开后将部件推出模具。

零件弹出后，落到传送带或桶上进行存储，然后循环重新开始。

然而，模具不同移动部件的对齐从来都不是完美的。这会导致产生 2 种常见缺陷，这些缺陷在几乎每个注塑零件上都可见：

• 分型线 它们在模具两半相交的零件一侧可见。它们是由模具的微小错位和略微圆滑的边缘引起的。

• 弹出（或见证）标记 它们在零件隐藏的 B 面上可见。它们的产生是因为顶出销稍微突出于模具表面之上或凹陷于模具表面之下。

 下图显示了用于制造遥控器外壳两侧的模具。小测验：尝试找到*核心*（A 侧）、*型腔*（B 侧）、流道系统 ，顶针 ，副作用核心 和通风口 在这个模具上。

注塑成型的优点和局限性

注塑成型是一项历史悠久的成熟制造技术，但随着新的技术进步，它不断得到完善和改进。

以下是注塑成型的主要优点和缺点的快速概述，可帮助您了解它是否是适合您的应用的解决方案。

注塑成型的优点

塑料的大批量生产

注塑成型是制造大批量相同塑料零件最具成本竞争力的技术。一旦创建了模具并设置了机器，就可以非常快速且成本非常低地制造附加零件。

注塑成型的建议最小生产量为 500 件。此时，规模经济开始发挥作用，相对较高的模具初始成本对单价的影响不太明显。

材料范围广

几乎所有热塑性材料（以及一些热固性材料和有机硅）都可以注塑成型。这为设计提供了非常广泛的具有不同物理特性的可用材料。

通过注塑成型生产的零件具有非常好的物理性能。可以通过使用添加剂（例如玻璃纤维）或将不同颗粒（例如 PC/ABS 共混物）混合在一起来定制它们的性能，以达到所需的强度、刚度或抗冲击性水平。

非常高的生产率

典型的注塑周期持续 15 至 60 秒，具体取决于零件的尺寸和模具的复杂程度。相比之下，CNC 加工或 3D 打印可能需要几分钟到几小时才能生产出相同的几何形状。此外，单个模具可以容纳多个零件，进一步提高了该制造工艺的生产能力。

这意味着每小时可以生产数百（甚至数千）个相同的零件。

出色的重复性和公差

注塑工艺具有高度可重复性，生产的部件基本相同。当然，随着时间的推移，模具会出现一些磨损，但典型的试运行铝模具可以承受 5,000 到 10,000 次循环，而工具钢制成的大规模生产模具可以承受 100,000 次以上的循环。

通常，注塑成型生产的零件公差为 ± 0.500 毫米（0.020 英寸）。在某些情况下，更严格的公差也可以达到 ± 0.125 mm (0.005’’)。这种精度水平足以满足大多数应用，可与 CNC 加工和 3D 打印相媲美。

出色的视觉外观

注塑成型的一个关键优势是它可以生产几乎不需要额外精加工的成品。模具表面可以进行高度抛光，以制造出镜面般的零件。或者可以对它们进行喷砂处理以形成纹理表面。 SPI 标准规定了可以达到的精加工水平。

获取表面处理/材料兼容性建议→

注塑成型的局限性

模具启动成本高

注射成型的主要经济限制是模具的高成本。由于必须为每种几何形状制作定制模具，因此启动成本非常高。这些主要与模具的设计和制造有关，通常成本在 5,000 美元到 100,000 美元之间。因此，注塑成型仅在生产量超过 500 件时才具有经济可行性。

设计变更成本高昂

模具制造出来后，修改成本非常高。设计变更通常需要从头开始创建新模具。因此，正确设计注塑成型零件非常重要。

在第 2 部分中，我们列出了注塑成型设计时需要牢记的最重要的设计注意事项。在第 5 部分中，我们还将了解如何通过创建零件的物理原型来降低风险。

比其他技术更长的交货时间

注塑成型的典型周转时间为 6 至 10 周。制造模具需要 4-6 周，再加上生产和运输需要 2-4 周。如果需要更改设计（这很常见），周转时间就会相应增加。

相比之下，桌面 3D 打印机制造的零件可以在一夜之间准备好交付，而工业 3D 打印系统的交货时间通常为 3-5 天。 CNC 加工零件通常在 10 天内交付，最快 5 天内交付。

注塑成型产品示例

如果您现在环顾四周，您至少会看到一些通过注塑成型制造的产品。您现在可能实际上正在查看一个：您用来阅读本指南的设备的外壳。

要识别它们，请注意以下 3 件事：分界线 , 见证标记 位于隐藏面且壁厚相对均匀 贯穿整个部分。

我们收集了一些通常通过注塑成型制造的产品示例，以帮助您更好地了解通过这种制造工艺可以实现什么目标。

玩具

包装

微缩模型

汽车

电气

医疗保健

乐高积木

乐高积木是注塑零件最知名的例子之一。它们是使用模具制造的，如图中的模具，在退役之前生产了 1.2 亿块乐高积木（即 1500 万个循环）。

乐高积木使用的材料是 ABS，因为它具有高抗冲击性和优异的成型性。每一块砖都经过完美设计，公差低至 10 微米（或人类头发丝的十分之一）。

这在一定程度上是通过使用最佳设计实践来实现的，我们将在下一节中研究这些实践（均匀的壁厚、拔模角度、肋、浮雕文本等）。

退役的乐高积木模具

瓶盖

许多塑料包装产品都是注塑成型的。事实上，包装是注塑成型的最大市场。

例如，瓶盖由聚丙烯注塑而成。聚丙烯（PP）具有优异的耐化学性，适合与食品接触。

在瓶盖上，您还可以看到所有常见的不可避免的注塑缺陷（分型线、顶出痕迹等）和常见的设计特征（肋条、脱模底切等）。

模型飞机

模型飞机是注塑零件的另一个常见例子。这里使用的材料主要是聚苯乙烯（PS），因为它成本低且易于成型。

模型飞机套件的有趣之处在于它们仍然附有滑道系统。因此，您可以看到熔化的塑料填充空模具的路径。

汽车零件

汽车内部几乎所有塑料部件都是注塑成型的。汽车行业最常用的 3 种注塑材料是用于非关键部件的聚丙烯 (PP)、具有良好耐候性的 PVC 和具有高冲击强度的 ABS。

汽车一半以上的塑料部件都是由其中一种材料制成的，包括保险杠、车身内部部件和仪表板。

消费电子产品

几乎所有批量生产的消费电子设备的外壳都是注塑成型的。 ABS 和聚苯乙烯 (PS) 因其优异的抗冲击性和良好的电绝缘性而成为首选。

医疗设备

许多可灭菌和生物相容性材料可用于注塑成型。

医用级硅胶是医疗行业中比较流行的材料之一。但硅胶是热固性材料，因此需要特殊的机械和工艺控制，从而增加了成本。

对于要求不太严格的应用，其他材料更常见，例如 ABS、聚丙烯 (PP) 和聚乙烯 (PE)。

了解有关医疗器械制造的更多信息 →

第 2 部分

有几个因素可能会影响质量 最终产品和可重复性 的过程。为了充分发挥该过程的优势，设计者必须遵循一定的设计准则。

在本节中，我们概述了注塑成型的常见缺陷以及基本和高级指南 设计零件时要遵循的要求，包括将成本降至最低的建议。

常见注塑缺陷

注塑成型中的大多数缺陷与熔化材料的流动或凝固过程中冷却速率不均匀有关。

以下是设计注塑零件时需要记住的缺陷列表。在下一节中，我们将了解如何通过遵循良好的设计实践来避免这些问题。

变形

当某些部分比其他部分冷却（并因此收缩）得更快时，该零件可能会因内应力而永久弯曲。

壁厚不恒定的零件最容易发生翘曲。

缩痕

当零件内部在其表面之前凝固时，原本平坦的表面上可能会出现一个小凹槽，称为缩痕。

壁厚或筋设计不良的零件最容易下沉。

拖动标记

当塑料收缩时，它会对模具施加压力。顶出过程中，零件壁会滑动并刮擦模具，从而导致拖痕。

具有垂直壁（且无拔模角）的零件最容易出现拖痕。

编织线

当两股水流相遇时，可能会出现细小的毛发状变色。这些熔接线会影响零件的美观，但通常也会降低零件的强度。

几何形状突变或有孔的零件更容易出现熔合线。

短镜头

模具中的滞留空气会抑制注射过程中材料的流动，从而导致零件不完整。良好的设计可以提高熔融塑料的流动性。

壁很薄或筋设计不良的零件更容易出现短射。

处理底切

最简单的模具（直拉模具）由两半组成。不过，带有底切的特征（例如螺纹的齿或卡扣接头的钩）可能无法使用直拉模具制造。这要么是因为模具无法进行 CNC 加工，要么是因为材料阻碍了零件的顶出。

注塑成型中的底切是无法使用简单的两部分模具制造的零件特征，因为在模具打开或顶出过程中材料会妨碍。

螺纹的齿或卡扣接头的钩子都是底切的例子。

这里有一些可以帮助您处理底切的想法：

使用关闭装置避免底切

完全避免底切可能是最佳选择 。底切总是会增加模具的成本、复杂性和维护要求。巧妙的重新设计通常可以消除底切。

关闭是处理零件内部区域（用于卡扣配合）或零件侧面（用于孔或手柄）的底切的有用技巧。

以下是一些如何重新设计注塑零件以避免底切的示例：本质上，去除底切下方区域的材料，从而完全消除该问题。

移动分型线

处理倒扣最简单的方法是移动模具的分型线与其相交。

该解决方案适用于许多外表面具有底切的设计。不要忘记相应地调整拔模角度。

使用脱模底切（凹凸）

当特征足够灵活在顶出过程中在模具上变形时，可以使用脱模底切（也称为凹凸） 。脱模底切用于制造瓶盖中的螺纹。

倒扣只能在以下条件下使用：

• 脱模底切必须远离加劲特征 ，例如角和肋。
• 底切必须有导程角 30 度到 45 度。
• 注塑件必须有空间 并且必须灵活 足以膨胀和变形。

建议避免纤维增强塑料制成的零件出现剥离底切。通常，柔性塑料 PP、HDPE 或尼龙 (PA) 等材料可承受高达其直径 5% 的倒扣。

*带有脱模底切的示例零件。零件在从模具中推出时会变形。*

滑动副动作和核心

当无法重新设计注塑零件以避免底切时，可使用滑动侧作用和型芯。

侧作用核心是插入 当模具关闭时滑入并在模具打开前滑出。请记住，这些机制会增加成本和复杂性 到模具中。

设计辅助操作时请遵循以下准则：

• 需要有空间供核心移入和移出 。这意味着该特征必须位于零件的另一侧。
• 侧面动作必须垂直移动 。以 90° 以外的角度移动会更加复杂，会增加成本和交货时间。
• 不要忘记添加拔模角度 像往常一样按照您的设计，考虑侧面动作核心的运动。

常见设计特征

了解如何利用这些实用指南来设计注塑零件中最常见的功能。使用它们来改进设计的功能，同时仍然遵守基本设计规则。

螺纹紧固件（凸台和嵌件）

有 3 种方法可以向注塑零件添加紧固件：直接在零件上设计螺纹、添加可连接螺钉的凸台或包含螺纹嵌件。

直接在零件上建模螺纹 是可能的，但不推荐，因为螺纹的齿本质上是底切，大大增加了模具的复杂性和成本（我们将在后面的部分中更多地讨论底切）。带螺纹的注塑零件的一个示例是瓶盖。

老板

凸台在注塑成型零件中非常常见，用作连接或组装点 。它们由圆柱形突出物组成，带有设计用于容纳螺钉、螺纹嵌件或其他类型的紧固和组装硬件的孔。将老板视为一根自行闭合的肋骨 围成一圈。

凸台用作连接点或紧固点（与自攻螺钉或螺纹嵌件结合使用）。

*老板推荐设计*

当凸台用作__紧固点__时，凸台的外径应为螺钉或嵌件标称直径的2倍，其内径应等于螺钉芯部的直径。的洞 即使组装不需要全部深度，凸台也应延伸到底壁水平，以在整个特征中保持__均匀的壁厚__。添加倒角以便于插入螺钉或嵌件。

__为了获得最佳结果：__

避免设计融入主墙的凸台

用肋支撑凸台或将它们连接到主墙上

对于带刀片的凸台，使用的外径等于 2× 刀片公称尺寸

线程

金属螺纹嵌件 可添加到塑料注塑零件中，为机器螺钉等紧固件提供耐用的螺纹孔。使用嵌件的优点是它们允许多次组装和拆卸循环 .

嵌件通过热、超声波或模内插入安装在注塑成型零件中。要设计将容纳螺纹嵌件的凸台，请使用与上述类似的指南，使用嵌件的直径作为引导尺寸。

*放置在凸台中的螺纹嵌件*

__为了获得最佳结果：__

避免直接在注塑零件上添加螺纹

设计凸台的外径等于螺钉或嵌件标称直径的 2 倍

在螺纹边缘添加 0.8 毫米的浮雕

使用螺距大于 0.8 毫米的螺纹（每英寸 32 个螺纹）

使用梯形或偏梯形螺纹

处理所产生的底切的最佳方法：

使用螺距大于 0.8 毫米的螺纹（每英寸 32 个螺纹）

对于外螺纹，将它们沿着分型线放置

排骨

当推荐的最大壁厚仍不足以满足零件的功能要求时，可以使用加强筋来提高其刚度。

设计加强筋时：

● 使用厚度等于0.5×主壁厚

● 定义小于3×筋厚度的高度

● 使用半径大于 1/4 × 筋厚度的基础圆角

● 添加至少 0.25° - 0.5° 的拔模角

● 添加分钟。肋与壁之间的距离为4×肋厚度

卡扣接头

卡扣接头是一种非常简单、经济且快速的方式无需紧固件或工具即可连接两个零件 。卡扣接头存在多种设计可能性。

根据经验，偏转 卡扣接头的性能主要取决于其长度和许用力 可以应用于其宽度（因为其厚度或多或少由零件的壁厚决定）。此外，卡扣接头是底切的另一个例子。

*带有卡扣接头的组件示例*

在上面的示例中，显示了最常见的卡扣接头设计（称为__悬臂卡扣接头__）。与加强筋一样，为卡扣接头添加拔模角度并使用 0.5 倍壁厚的最小厚度。

设计卡扣接头的具体准则是一个大主题，超出了本文的范围。 For more detailed information, please refer to this article from MIT.

For best results:

Add a draft angle to the vertical walls of your snap-fit joints

Design snap-fits with thickness greater than 0.5x the wall thickness

Adjust their width and length to control their deflection and permissible force

Living hinges

Living hinges are thin sections of plastic that connect 2 segments of a part and allow it to flex and bend 。 Typically these hinges are incorporated in mass-produced containers, such as plastic bottles. A well-designed living hinge can last for up to a million cycles without failure.

The material used to injection mold a living hinge must be flexible. Polypropylene (PP) and Polyethylene (PE) are good choices for consumer application and Nylon (PA) for engineering uses.

A well-designed hinge is shown below. The recommended minimum thickness  of the hinge ranges between 0.20 and 0.35 mm, with higher thicknesses resulting in more durable, but stiffer, parts.

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*Example of a living hinge (left) and recommended design dimensions for PP or PE (right)*

Before going to full-scale production, prototype your living hinges using CNC machining or 3D printing to determine the geometry and stiffness that best fits your application. Add generous fillets and design shoulders with a uniform wall thickness as the main body of the part to improve the material flow in the mold and minimize the stresses. Divide hinges longer than 150 mm in two (or more) to improve lifetime.

For detailed guidelines, please refer to this MIT guide.

For best results ：

• Design hinges with a thickness between 0.20 and 0.35 mm

• Select a flexible material (PP, PE or PA) for parts with living hinges

• Use shoulders with a thickness equal the thickness of the main wall

• Add fillets as large as possible

Crush ribs

Crush Ribs are small protruding features that deform to create friction when different components are pushed together, securing their possition.

Crush ribs can be an economical alternative for manufacturing high tolerance holes for tight fits 。 They are commonly used to house bearings or shafts and other press fit applications.

An example of a part with crush ribs is shown below. Using three crush ribs is recommended to ensure good alignment. The recommended height/radius for each rib is 2 mm 。 Add a minimum interference of 0.25 mm between the crush rib and the fitted part. Because of the small surface contact with the mold, crush ribs can be designed without a draft angle.

*Example of an crush rib (left) and recommended design dimensions (right)*

__For best results:__

Add a minimum interference of 0.25 mm between crush rib and the component

Do not add a draft angle on the vertical walls of a crush rib

Lettering and symbols

Text is a very common feature that can be useful for logos, labels, warnings, diagrams and instructions, saving the expense of stick-on or painted labels.

When adding text, choose embossed text over engraved text, as it’s easier to CNC machine on the mold and thus more economical.

Also raising the text 0.5 mm above the part surface will ensure that the letters are easy to read. We recommend selecting a bold, rounded font style with uniform line thickness, with a size of 20 points or larger. Some font examples include:Century Gothic Bold, Arial and Verdana.

Use embossed text (0.5 mm height) instead of engraved texted

Use a font with uniform thickness and a minimum font size of 20 points

Align the text perpendicular to the parting line

Use a height (or depth) greater than 0.5 mm

Tolerances

Injection molding typically produces parts with tolerances of ± 0.250 mm (0.010").

Tighter tolerances are feasible in certain circumstances (down to ± 0.125 mm - and even ± 0.025 mm), but they increase the cost drastically.

For small production runs (<10,000 units), consider using a secondary operation (such as drilling) to improve accuracy. This ensures the correct interference of the part with other components or inserts (for example, when using press fits).

Design rules for injection molding

One of the biggest benefits of injection molding is how easily complex geometries can be formed, allowing a single part to serve multiple functions.

Once the mold is manufactured, these complex parts can be reproduced at a very low cost. But changes to the mold design at later stages of development can be very expensive, so achieving the best results on the first time 是必不可少的。 Follow the guidelines below to avoid the most common defects in injection molding.

Use a constant wall thickness

Use a uniform wall thickness throughout the part (if possible) and avoid thick sections 。 This is essential as non-uniform walls can lead to warping or the part as the melted material cools down.

If sections of different thickness are required, make the transition as smooth as possible using a chamfer or fillet. This way the material will flow more evenly inside the cavity, ensuring that the whole mold will be fully filled.

Industry Application Best material Why it’s a fit Aerospace Airframes, turbines, fasteners Titanium High strength-to-weight ratio, heat and corrosion resistance Medical Implants, surgical tools Titanium / copper Titanium is biocompatible; copper is antimicrobial (used externally only) Electronics Wiring, PCBs, motors Copper Excellent conductivity and ease of forming HVAC Heat exchangers, radiators Copper Superior thermal conductivity Marine Underwater fasteners, piping Titanium Outstanding corrosion resistance Automotive Exhausts, wiring harnesses Titanium / copper Lightweight strength or conductivity needs Construction Plumbing, cladding Copper Durable, corrosion-resistant, aesthetic

For best results:

Use a uniform wall thickness within the recommended values

When different thickness are required, smoothen the transition using a chamfer or fillet with length that is 3x the difference in thickness

Hollow out thick sections

Thick sections can lead to various defects, including warping and sinking. Limiting the maximum thickness of any section of your design to the recommended values by making them hollow is essential.

To improve the strength of hollow section, use ribs to design structures of equal strength and stiffness but reduced wall thickness. A well-designed part with hollow sections is shown below:

*Hollow thick sections and add ribs to improve stiffness*

Ribs can also be used to improve the stiffness of __horizontal sections__ without increasing their thickness. Remember though that the wall thickness limitations still apply. Exceeding the recommended rib thickness (see below) can result in sink marks.

*The wall thickness limitations still apply for ribs*

For best results:

Hollow out thick sections and use ribs to improve the strength and stiffness of the part

Design ribs with max. thickness equal to 0.5x the wall thickness

Design ribs with max. height equal to 3x the wall thickness

Add smooth transitions

Recommended: 3 × wall thickness difference

Sometimes sections with different wall thicknesses cannot be avoided. In these cases, use a chamfer or fillet to make the transition as smooth as possible.

Similarly, the base of vertical features (like ribs, bosses, snap-fits) must also always be rounded.

Round all edges

The uniform wall thickness limitation also applies to edges and corners:the transition must be as smooth as possible to ensure good material flow.

For interior edges , use a radius of at least 0.5 x the wall thickness 。 For exterior edges , add a radius equal to the interior radius plus the wall thickness 。 This way you ensure that the thickness of the walls is constant everywhere (even at the corners).

Adding to this, sharp corners result in stress concentrations which can result in weaker parts.

*Add wide radii to all edges to maintain uniform wall thickness and avoid defects*

For best results:

Add a fillet equal to 0.5x the wall thickness to internal corners

Add a fillet equal to 1.5x the wall thickness to external corners

Add draft angles

To make the ejection of the part from the mold easier, a draft angle must be added to all vertical walls. Walls without a draft angle will have drag marks on their surface, due to the high friction with the mold during ejection.

A minimum draft angle of 2° is recommended. Larger draft angles (up to 5o °) should be used on taller features.

Learn more about the importance of draft angles in this article →

A good rule of thumb is to increase the draft angle by one degree for every 25 mm 。 For example, add a draft angle of 3o degrees to a feature that is 75 mm tall. Larger draft angle should be used if the part has a textured surface finish 。 As a rule of thumb, add 1o to 2o extra degrees to the results of the above calculations.

Remember that draft angles are also necessary for ribs. Be aware though that adding an angle will reduce the thickness of the top of the rib, so make sure that your design complies with the recommended minimum wall thickness.

*Add a draft angle (minimum 2o)to all vertical walls*

__For best results:__

Add a minimum draft angle of 2o degrees to all vertical walls

For features taller than 50 mm, increase the draft angle by one degree every 25 mm

For parts with textured surface finish, increase the the draft angle by 1-2o extra degrees

Part 3

Injection molding materials

Injection molding is compatible with a wide range of plastics. In this section, you’ll learn more about the key characteristics of the most popular materials. We’ll also discuss the standard surface finishes that can be applied to injection molded parts.

Materials used for injection molding

All thermoplastics can be injection molded. Some thermosets and liquid silicones are also compatible with the injection molding process.

They can be also reinforced with fibers, rubber particles, minerals or flame retardant agents to modify their physical properties. For example fiberglass can be mixed with the pellets at ratios of 10%, 15% or 30% resulting in parts with higher stiffness.

Polypropylene (PP)

The most common Injection molding plastic. Excellent chemical resistance. Food-safe grades available. Not suitable for mechanical applications.

ABS

Common thermoplastic with high impact resistance, low-cost &low density. Vulnerable to solvents.

Polyethylene (PE)

Lightweight thermoplastic with good impact strength &weather resistance. Suitable for outdoor applications.

Polystyrene (PS)

The Injection molding plastic with the lowest cost. Food-safe grades available. Not suitable for mechanical applications.

Polyurethane (PU)

Thermoplastic with high impact strength and good mechanical properties &hardness. Suitable for molding parts with thick walls.

Nylon (PA 6)

Engineering thermoplastic with excellent mechanical properties and high chemical &abrasion resistance. Susceptible to moisture.

Polycarbonate (PC)

The plastic with the highest impact strength. High thermal resistance, weather resistance &toughness. Can be colored or transparent.

PC/ABS

Blend of two thermoplastics resulting in high impact strength, excellent thermal stability, and high stiffness. Vulnerable to solvents.

POM (Acetal/Delrin)

Engineering thermoplastic with high strength, stiffness &moisture resistance and self-lubricating properties. Relatively prone to warping.

PEEK

High-performance engineering thermoplastic with excellent strength and thermal &chemical resistance. Used to replace metal parts.

Silicone rubber

Thermoset with excellent heat &chemical resistance and customizable shore hardness. Food-safe and medical grade available.

An additive that is commonly used to improve the stiffness of the injection molded parts is fiberglass. The glass fibers can be mixed with the pellets at ratios of 10%, 15% or 30%, resulting in different mechanical properties.

Colorant can be added to the mixture (at a ratio of about 3%) to create a great variety of colored parts. Standard colors include red, green, yellow, blue, black and white and they can be mixed to create different shades.

Surface finishes and SPI standards

Surface finishes can be used to give an injection molded part a certain look or feel. Besides cosmetic purposes surface finishes can also serve technical needs 。 For example, the average surface roughness (Ra) can dramatically influence the lifetime of sliding parts such as plain bearings.

Injection molded parts are not usually post-processed, but the mold itself can be finished to various degrees.

Keep in mind that rough surfaces increase the friction between the part and the mold during ejection, therefore a larger draft angle is required.

The Society of Plastics Industry (SPI) explains several standard finishing procedures that result in different part surface finishes.

Finish 描述 SPI standards* Applications Glossy finish The mold is first smoothed and then polished with a diamond buff, resulting in a mirror-like finish. A-1
A-2
A-3 Suitable for parts that require the smoothest surface finish for cosmetic or functional purposes (Ra less than 0.10 μm). The A-1 finish is suitable for parts with mirror-like finish and lenses. Semi-gloss finish The mold is smoothed with fine grit sandpaper, resulting in a fine surface finish. B-1
B-2
B-3 Suitable for parts that require a good visual appearance , but not a high glossy look. Matte finish The mold is smoothed using fine stone powder, removing all machining marks. C-1
C-2
C-3 Suitable for parts with low visual appearance requirements , but machining marks are not acceptable. Textured finish The mold is first smoothed with fine stone powder and then sandblasted, resulting in a textured surface. D-1
D-2
D-3 Suitable for parts that require a satin or dull textured surface finish. As-machined finish The mold is finished to the machinist's discretion. Tool marks will be visible. - Suitable for non-cosmetic parts , such industrial or hidden components.

When selecting a glossy surface finish, remember these useful tips:

• A high glossy mold finish is not equivalent to a high glossy finished product. It is significantly subject to other factors such as plastic resin used, molding condition and mold design. For example, ABS will produce parts with a higher glossy surface finish than PP. To find the recommended material and surface finish combination visit the appendix.
• Finer surface finishes require a higher grade material for the mold. To achieve a very fine polish, tool steels with the highest hardness are required. This has an impact on the overall cost (material cost, machining time and post-processing time).

Part 4

Cost reduction tips

Learn more about the main cost drivers in injection molding and actionable design tips that will help you reduce the costs of your project.

Cost drivers in injection molding

The biggest costs in injection molding are:

• Tooling costs determined by the total cost of designing and machining the mold
• Material costs determined by the volume of the material used and its price per kilogram
• Production costs determined by the total time the Injection molding machine is used

Tooling costs are constant (starting at $3,000 and up to $5,000). This cost is independent of the total number of manufactured parts, while the material and production costs are dependent on the production volume.

For smaller productions (1,000 to 10,000 units), the cost of tooling has the greatest impact on the overall cost (approximately 50-70%). So, it’s worthwhile altering your design accordingly to simplify the process of manufacturing of the mold (and its cost).

For larger volumes to full-scale production (10,000 to 100,000+ units), the contribution of the tooling costs to the overall cost is overshadowed by the material and production costs. So, your main design efforts should focus on minimizing both the volume part and the time of the molding cycle.

Here we collected some tips to help you minimize the cost of your Injection molded project.

Tip #1:Stick to the straight-pull mold

Side-action cores and the other in-mold mechanisms can increase the cost of tooling by 15% to 30%. This translates to a minimum additional cost for tooling of approximately $1,000 to $1,500.

In a previous section, we examined ways to deal with undercuts. To keep your production on-budget, avoid using side-action cores and other mechanisms unless absolutely necessary.

Tip #2:Redesign the injection molded part to avoid undercuts

Undercuts always add cost and complexity, as well as maintenance to the mold. A clever redesign can often eliminate undercuts.

Tip #3:Make the injection molded part smaller

Smaller parts can be molded faster resulting in a higher production output, making the cost per part lower. Smaller parts also result in lower material costs and reduce the price of the mold.

Tip #4:Fit multiple parts in one mold

As we saw in a previous section, fitting multiple parts in the same mold is common practice. Usually, 6 to 8 small identical parts can fit in the same mold, essentially reducing the total production time by about 80%.

Parts with different geometries can also fit in the same mold (remember, the model airplane example). This is a great solution for reducing the overall cost of assembly.

Here’s an advanced technique:

In some cases, the main body of 2 parts of an assembly is the same. With some creative design, you can create interlocks points or hinges at symmetrical locations, essentially mirroring the part. This way the same mold can be used to manufacture both halves, cutting the tooling costs in half.

Tip #5:Avoid small details

To manufacture a mold with small details require longer machining and finishing times. Text is an example of this and might even require specialized machining techniques such as electrical discharge machining (EDM) resulting in higher costs.

Tip #6:Use lower grade finishes

Finishes are usually applied to the mold by hand, which can be an expensive process, especially for high-grade finishes. If your part is not for cosmetic use, don’t apply a costly high-grade finish.

Tip #7:Minimize the part volume by reducing wall thickness

Reducing the wall thickness of your part is the best way to minimize the part volume. Not only does it mean less material is used, but also the injection molding cycle is greatly accelerated.

For example, reducing the wall thickness from 3 mm to 2 mm can reduce the cycle time by 50% to 75%.

Thinner walls mean that the mold can be filled quicker. More importantly, parts thinner parts cool and solidify much faster. Remember that about half the injection molding cycle is spent on the solidification of the part while the machine is kept idle.

Care must be taken through to not overly reduce the stiffness of the part which would downgrade its mechanical performance. Ribs in key locations can be used to increase stiffness.

Tip #8:Consider secondary operations

For lower volume productions (less than 1000 parts), it may be more cost effective to use a secondary operation to complete your injection molded parts. For example, you could drill a hole after molding rather than using an expensive mold with side-action cores.

Part 5

Start Injection molding

Once your design ready and optimized for injection molding, what’s next? In this section we’ll take you through the steps needed to start manufacturing with injection molding.

Step 1:Start small and prototype fast

Before you commit to any expensive injection molding tooling, first create and test a functional prototype of your design.

This step is essential for launching a successful product. This way design errors can be identified early, while the cost of change is still low.

There are 3 solutions for prototyping:

1. 3D printing (with SLS, SLA or Material Jetting)
2. CNC machining in plastic
3. Low-run injection molding with 3D printed moldsThese processes can create realistic prototypes for form and function that closely resemble the appearance of the final injection molding product.

Use the information below as a quick comparison guide to decide which solution is best for your application.

Prototyping with 3D printing

Designs optimized for injection molding can be easily 3D printed

The prototyping solution with the lowest cost and fastest turnaround

Not every injection molding material is available for 3D printing

3D printed parts are 30-50% weaker than injection molded parts

Prototyping with CNC machining

Material properties identical to the injection molded parts

Excellent accuracy and finishing

Design modifications may be need, as different design restrictions apply

More expensive than 3D printing with longer lead time

Prototyping with low-run injection molding

The most realistic prototypes with accurate material properties

The actual process and mold design is simulated

The prototyping solution with the highest cost

Smaller availability than CNC or 3D printing

Step 2 :Make a “pilot run” (500 - 10,000 parts)

With the design finalized, it time to get started with Injection molding with a small pilot run.

The minimum order volume for injection molding is 500 units. For these quantities, the molds are usually CNC machined from aluminum. Aluminum molds are relatively easy to manufacture and low in cost (starting at about $3,000 to $5,000) but can withstand up to 5,000 - 10,000 injection cycles.

At this stage, the typical cost per part varies between $1 and $5, depending on the geometry of your design and the selected material. The typical lead time for such orders is 6-8 weeks.

Don’t get confused by the term “pilot run”. If you only require a few thousand parts, then this would be your final production step.

The parts manufactured with “pilot” aluminum molds have physical properties and accuracy identical to parts manufactured with “full-scale production” tool steel molds.

Step 3 :Scale up production (100,000+ parts)

When producing parts massive quantities of identical parts (from 10,000 to 100,000+ units) then special Injection molding tooling is required.

For these volumes, the molds are CNC machined from tool steel and can withstand millions of Injection molding cycles. They are also equipped with advanced features to maximize production speeds, such as hot-tip gates and intricate cooling channels.

The typical unit cost at this stage varies between a few cents to $1 and the typical lead time is 4 to 6 months, due to the complexity of designing and manufacturing the mold.

Part 6

Useful resources

In this guide we touched on everything you need to get started with injection molding - but there’s plenty more to learn.

Here are the most useful resources on injection molding and other digital manufacturing technologies if you want to delve deeper.

Other guides

Want to learn more about digital manufacturing? There are more technologies to explore: