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.
易腐蚀
Easy to corrode
U型槽口的五金导致铝合金门窗腐蚀渗水，钢和铝是不同的金属，遇到水时会产生电化学反应，（俗话说的结碱现象）影响门窗的使用，破坏门窗的防水构造。
U-notch metal causes aluminum alloy doors and windows to corrode and seep water, steel and aluminum are different metals, when encountered with water will produce electrochemical reactions, (as the saying goes alkaline phenomenon) affect the use of doors and windows, destroy the waterproof structure of doors and windows.