## 1 Introduction  
Cotton, flax and hemp plants are the main textile materials in the world [1]. While cotton is not only an important material, it is also one of the most important economic crops in Xinjiang, China, where the main planting of cotton is upland cotton [2, 3]. Its planting area and output have ranked first in China for many years [4]. With the development of agricultural mechanization, cotton's mechanized harvesting has become a trend [5]. Cotton harvested by cotton pickers is collectively referred to as machine-harvested seed cotton, which has a small bulk density and belongs to a loose agricultural fiber material [6]. However, because the integration rate of cotton pickers in China is low, machine-harvested seed cotton requires secondary processing, resulting in its expensive costs of storage and transportation [7, 8]. Therefore, machine-harvested seed cotton is compressed into modules of certain shapes, sizes, and densities before storing and transporting them [9]. Advanced cotton pickers have integrated compression molding equipment with cotton pickers, which can make mechanical harvest and compression molding continuous, solving the problems effectively [10, 11].  
At present, the compression and stress relaxation characteristics of fiber materials have been more widely examined [12]. The compression characteristics of cotton fibers and lint have been studied, while the relationship between compression density and compression force and compression modulus during compression has been derived, and the characteristics of each process has been described [13; 14; 15; 16; 17; 18]. A finite element model of the cotton fiber was developed and the stress changes during compression of the cotton fiber were simulated [19]. However, the research on seed cotton compression and stress relaxation is at the initial stage. [PERSON] studied the compression and creep of seed cotton at small compression densities and established a creep model for seed cotton compression [20; 21; 22]; [PERSON] studied the effect of moisture content and feeding quality on the compression characteristics of seed cotton and established a stress relaxation model for seed cotton [23; 24]. [PERSON] concluded that the cottonseeds breaking rate has increased significantly when the compression density was greater than 400 kg/m\({}^{3}\)[25]. And now research on the stress relaxation characteristics of common agricultural fiber materials has matured. The compression, stress relaxation and creep of the material have been investigated, and rheological models have been developed to analyze the effects of moisture content and compression force on their rheological properties [26; 27; 28; 29; 30]. Rheological properties of the agricultural material have also been studied in terms of material type, initial density, compression speed and cross-sectional dimensions, which could provide implications for the successful development of this study [31; 32; 33; 34; 35; 36; 37]. The lack of understanding on stress relaxation characteristics of seed cotton restricted us from revealing the compression molding mechanism.  
In previous studies on the factors influencing the compression and stress relaxation characteristics of seed cotton, the effects of seed cotton as a mixture and the physical properties of seed cotton on compression and stress relaxation parameters were ignored. In this paper, we investigated the effect of moisture content, trash content, compression density and cross-sectional dimensions on the stress relaxation characteristics of machine-harvested seed cotton through stress relaxation tests during the harvest period.