## 1 Introduction
However, the research on the weathering mechanism and material screening technology in uniform thermal environment is still not sufficient to fully grasp and clearly regulate the inhomogeneous distribution characteristics of external factors of auto-parts weathering in non-uniform thermal environment, as well as the resulting non-equilibrium weathering problem. The weathering of parts results from the combined effects of internal and external factors, among which solar radiation and temperature are the main external factors [21; 22]. The interior trim parts of the car are usually not directly affected by rainwater, and the fluctuation of humidity in the closed cabin is relatively small, thus water is a comparatively secondary external factor. There is the particular case of convertible sports automobiles which can be greatly affected by rainwater. The influence of radiation on the degradation of various plastics, such as the dashboard, was investigated. The experimental results conclude that the color difference and gloss loss rate of plastics increased significantly with the increment of cumulative radiation [6; 23]. [PERSON] et al. [24; 25] verified that the weathering reaction rate of polypropylene and some other materials increased exponentially with temperature through experiments and weathering kinetics studies. Massive experiments and research results prove that the weathering processes of the materials conform to the Arrhenius formulas or the deformed Arrhenius formulas and increase exponentially with the intensity of weathering external factors [6; 22; 26]. Thus, it can be seen that the non-uniform distribution of solar radiation and temperature can result in the problem of inconsistent weathering process among parts. Therefore, it is urgent to study the distribution characteristics of external factors of automobile weathering in non-uniform thermal environment. IP/DP box (abbreviated from Instrument Panel/Door Panel) [27] and several artificial weathering standards [4; 5; 28] have been specially developed for the weathering research of auto-parts, among which SAEJ2412-2015 is the most important weathering standard for interior trim currently referenced to. SAEJ2412-2015 specifies the test method in detail and is able to simulate the extreme values of weathering external factors in the cabin. However, there is less attention paid to some important experimental design parameters, such as the discrepancy of weathering external factors among different parts, the temperature in dark reaction period and the duration of light and dark reaction. The natural weathering test results of the whole vehicle are closest to the actual application, which is propitious to present comprehensive test design parameters for various artificial weathering tests. However, there are still some serious deficiencies, such as long test cycle, high cost, poor repeatability and limited measuring points, which not only lengthen the development cycle of new car models, but also are not conducive to the rapid and in-depth analysis on the external factors distribution of automotive weathering, as well as the mechanism of disequilibrium weathering. To remedy these inadequacies, a numerical analysis method of all-weather weathering external factors based on automobile heat transfer model is proposed. The method can be used to quickly predict the laws of automotive physical parameters, orientations and locations, as well as their influences on the external factors' distribution of automobile weathering at any time, in any area and in any direction. Furthermore, it can also be applied to the formulation of various artificial accelerated weathering test schemes, as well as the disequilibrium regulation on external factors distribution and auto-parts weathering. Experimental Study on Non-Uniform External Factors Distribution and Weathering Inconsistency of Auto-Parts  
The thermal environment inside and outside an automotive is very complicated.