设计原理：
Design principle:
U型槽口的五金连接方式采用自攻螺钉螺接的方式安装五金件。自攻钉的硬度远远大于铝合金型材，自攻钉的螺牙对铝合金型材产生磨损，造成安装孔扩大，时间一长五金件的螺钉连接就会脱扣，而产生危险。而塑钢门窗的内衬为钢材，材料硬度与自攻钉相当而不会发生安装孔扩大和五金件脱扣现象，所以塑钢门窗可以用U型槽。
The hardware connection mode of U notch is fitted with self tapping screws to install hardware. The hardness of self-tapping screws is much higher than that of aluminum alloy profiles. The screw teeth of self-tapping screws cause wear and tear on aluminum alloy profiles, resulting in enlarged mounting holes, and the screw connection of hardware will break off over a long time, thus causing danger. The plastic steel doors and windows are lined with steel, and the hardness of the material is similar to that of self-tapping nails, so the plastic steel doors and windows can use U-groove.