Fastening in Plastics塑料件的紧固

Options for joining plastic parts塑料件的几种紧固方式

Why ordinary tapping screws crack bosses普通自攻螺钉为什么容易撑裂螺柱

A standard 60° thread pushes the plastic outwards as well as along the axis, producing high hoop stress in the boss. Screws designed for plastics use a narrower flank angle (around 30°), a larger pitch and a deeper thread. They displace less material, put less radial stress on the boss, and engage more plastic for better pull-out strength.标准 60° 牙型在拧入时除了轴向力，还会把塑料向外撑开，在螺柱中产生很大的环向应力。塑料专用螺钉采用较小的牙型角（约 30°）、较大的螺距和较深的牙，挤开的材料少、对螺柱的径向应力小，同时咬合更多塑料，抗拔出能力更好。

Boss design guidelines螺柱（凸台）设计参考

• Hole diameter: about 0.8 × screw diameter for many thermoplastics; glass-filled grades usually need a slightly larger hole底孔直径：多数热塑性塑料约为螺钉直径的 0.8 倍；玻纤增强料通常需要略大一些
• Boss outside diameter: about 2 to 2.5 × screw diameter螺柱外径：约为螺钉直径的 2～2.5 倍
• Engagement length: about 2 to 2.5 × screw diameter旋合长度：约为螺钉直径的 2～2.5 倍
• Add a small counterbore at the hole entry to stop the top of the boss cracking孔口加一段小沉孔，防止螺柱顶部开裂

Setting the tightening torque拧紧扭矩的设定

Two torques are measured in an assembly test: the drive torque needed to form the thread, and the strip torque at which the thread fails. The seating torque must sit between them, and the wider the gap the more robust the assembly — ideally the strip torque is at least two to three times the drive torque.装配试验中要测两个扭矩：成型螺纹所需的拧入扭矩，以及螺纹被破坏时的失效扭矩。拧紧扭矩必须设在两者之间，两者差距越大，装配越稳健；理想情况下失效扭矩至少是拧入扭矩的 2～3 倍。

Plastics creep and relax over time, so clamp load drops after assembly. A larger bearing surface (flange head or washer) and avoiding over-tightening help keep the joint tight.塑料会随时间蠕变、松弛，装配后夹紧力会下降。采用更大的支承面（法兰头或垫圈）、避免过度拧紧，有助于保持连接紧固。

When to use a threaded insert什么时候用螺纹嵌件

If the joint will be opened many times, needs a metric machine thread, or must carry higher loads, a brass threaded insert is usually the better choice. We supply heat-staking, ultrasonic and press-in brass inserts — see Fastener Solutions紧固件解决方案.如果连接需要反复拆装、需要标准公制螺纹，或承受较大载荷，黄铜螺纹嵌件通常是更好的选择。我们供应热熔、超声和压入式黄铜嵌件，详见Fastener Solutions紧固件解决方案。