The main problems in the processing of heat pipes
The heat pipe is a kind of heat transfer element, which makes full use of the heat conduction principle and the fast heat transfer property of the cooling medium. The heat of the hot object is quickly transferred to the outside of the heat source through the heat pipe, and its thermal conductivity has far exceeded that of any known metal. Heat pipes are often used in current heat dissipation design, including our common notebook computers, mobile phones, etc. The following factors should be considered in the design of heat pipe: heat load or heat to be transferred; Operating temperature; Pipe; Working fluid; Capillary structure; Length and diameter of heat pipe; Contact length of evaporation zone; Contact length of compensation area; Direction; The effect of heat pipe bending and flattening, etc.
According to the specific usage scenario, after the straight pipe is completed, the heat pipe needs to undergo a series of post-processing, such as bending, flattening, etc. The main problems in the post-processing process are as follows.
1. Bending wrinkling:
Heat pipe bending is a machining process of heat pipes to fit the spatial structure of electronic products. Due to the thinning of the outer side of the heat pipe under tensile stress during bending, the inner side of the pipe near the bending die becomes unstable and wrinkled due to compressive stress. Severe inward buckling and wrinkling of sintered heat pipes can lead to a reduction in the area of the internal airflow channels, resulting in a significant decrease in heat transfer efficiency. When the sintering heat pipe is bent, it can also cause the suction core to fall off, causing the heat pipe to fail. When the pipe is bent, the inner wall thickness increases and the outer wall thickness decreases. After undergoing primary and secondary degassing, the heat pipe is in a negative pressure state internally, and the thinning part may also collapse inward due to the influence of atmospheric pressure.
2. Flattening collapse:
When the heat pipe is flattened, the moving die moves downwards, and the flattened surface of the heat pipe continuously widens, ultimately becoming a flat heat pipe with a certain thickness. After cold flattening, the flattening plane shows a collapsed state along the axial direction of the heat pipe, which seriously affects the performance of the heat pipe. Collapse can lead to a decrease in the steam flow area, and even cause the upper and lower flattened planes to come into contact, seriously affecting the void structure of the heat pipe suction core. The literature analyzes the stress during the flattening process of circular tubes and proposes to change the concentrated stress to distributed stress, from the middle stress to the two sides stress, which can effectively solve the problem of flattening collapse.
3. Surface concavity:
After flattening treatment, there will be local pits on the surface of the heat pipe, which cause the heat pipe to not fit tightly with the heat source, leaving a layer of air between the heat pipe and the heat source, increasing the interface thermal resistance and reducing the heat transfer efficiency of the heat pipe. The local pits on the flattened plane of the sintered heat pipe are caused by the uneven plastic deformation of the microstructure. During the deformation process, the difficulty of opening slip systems between grains with different orientations varies, and large-sized grains that are prone to slip undergo deformation, resulting in macroscopic pit morphology.
In order to adapt to the development trend of miniaturization and lightness of electronic products, heat pipes need to adjust the shape of the product according to the internal spatial structure. The flattened heat pipe can adapt well to the internal spatial structure of ultra-thin and portable products such as mobile phones. Compared to before flattening, the sintered liquid absorbing core structure inside the heat pipe has been partially damaged, and the thermal conductivity efficiency of the sintered heat pipe has decreased. At the same time, the flat heat pipe structure can increase the heat exchange area with the heat source. But it is also very important to overcome the mainly problem of heatpipes during bending and flatten process.
