Study on the influence of post-treatment conditions on the internal stress of polystyrene products

The polymer products in daily use generally have internal stress, and the uneven or excessive distribution of internal stress will cause warpage or stress cracking, which will affect the normal use of products. Using the heat treatment method of metal annealing to reduce its internal stress for reference, the polymer products were placed in the drying oven and water bath pot for post-treatment, and the influence of different post-treatment process parameters on the internal stress of polystyrene products represented by the mobile phone lens products was studied by using the self-developed molecular material transparent products internal stress detection device. The results showed that increasing the post-treatment temperature below the hot deformation temperature can effectively reduce the internal stress and improve the internal stress distribution of polystyrene products at the same post-treatment time under different post-treatment temperatures and slow cooling conditions, and there was an optimal post-treatment temperature of 60 Centigrade;the polystyrene products after post-treatment had the secondary internal stress under the rapid cooling conditions, so that the internal stress distribution of products was worse than that of polystyrene products without post-treatment. The experimental results can lay a theoretical and practical foundation for seeking effective measures to reduce the internal stress of polymer products in the future.


Introduction
With the rapid rise of polymer industry and the development of the times, many polymer products have been widely used in daily life, such as mobile phone screen, optical lens, beverage bottle, etc. The polymer products in daily use generally have internal stress, and the uneven or excessive distribution of internal stress will cause warping deformation or stress cracking of products, thus affecting the normal use of products [1][2][3][4] . The reason for the internal stress of products is that the materials after heating and melting are affected by many factors during the processing, resulting in different stress distribution in products [5][6][7][8] . At present, consumers have higher and higher requirements for product quality, so it is of great economic significance to improve the internal stress distribution or effectively reduce the internal stress of products. The research shows that the post-treatment (annealing similar to metal material) can effectively improve the internal structure of the material and reduce the internal stress of the product [9][10][11] , but most of the research focuses on the influence of the post-treatment process on the change of the aggregate structure of the material, while the systematic research on the change of the internal stress distribution of the polymer material under different post- treatment conditions is rarely reported. In this paper, the internal stress distribution of polystyrene (PS) products under different post-treatment conditions was systematically studied by using a self-made internal stress distribution detection device. As we all know, PS is easy to be processed and molded, and has the advantages of transparency, cheapness, rigidity, insulation, good print-ability, etc. It can be widely used in light industry market, daily decoration, lighting indication and packaging, etc. Therefore, PS products under the post-treatment conditions are studied,and the law of internal stress distribution has very good practical and economic significance.

Self-developed internal stress detection device
The device is designed and developed independently, which mainly uses stress birefringence effect and polarized light interference principle, as shown in Figure.1. The main components of the device are composed of two polarizers, incandescent lamp and online imaging system. It has the advantages of fast and convenient application, simple operation and online real-time imaging. Moreover, the detection device has a wide range of applications and can be used in medical products, automobile manufacturing, food and beverage packaging, high-end optical products and other industries. At present, the device has been patented [12] .
(a)Device design drawing.
(b)Device object. Figure 1. A novel internal stress detection device for transparent products of polymer materials

Experimental scheme
Under the same injection molding process conditions as shown in Table 1, the PS lenses are injected with injection molding machine for the post-treatment experiment.The PS lenses are post processed by using drying box and water bath pot with different process conditions, and the corresponding posttreatment process parameter setting range is shown in Table 2.The self-developed internal stress detection device is used to detect the internal stress of PS products under different post-processing conditions. The placement position is shown in Figure 2, and the imaging system provided by the

The internal stress analysis of PS lens under different post-treatment temperature and slow cooling conditions
The PS lens is post treated for 30 min in the electric blast drying oven at different temperatures, the product is taken out and immersed in a constant temperature water bath filled with 40℃ liquid water for 10 min with slow cooling condition, and then the internal stress distribution is characterized by using the self-developed internal stress detection device. The corresponding internal stress distribution pictures are shown in Figure 3.   Figure 3(b)(c)(d),which indicates that the internal stress decreases. When the post-treatment temperature is 60 o C, the stress fringe color of PS lens is lighter than that of 50 o C, which indicates that the stress decreases and the distribution is better; when the post-treatment temperature continues to rise to 70 o C and 80 o C, the changes of the stress fringe color and distribution of PS lens are not obvious, which indicates that the stress distribution and value of PS lens are at its best when post-treatment temperature is 60 o C.

The internal stress analysis of PS lens under different post-treatment temperature and rapid cooling conditions
The PS lens is post treated for 30 min in the electric blast drying oven at different temperatures, the product is taken out and immersed in a constant temperature water bath filled with 0 o C ice water for 10 min with rapid cooling condition, and then the internal stress distribution is characterized by using the self-developed internal stress detection device.The corresponding internal stress distribution pictures are shown in Figure 4.  Figure 4 that the internal stress of PS lens after post-treatment and rapid cooling has not been improved correspondingly, and the color of stress stripe is deepened instead, especially at the corner of PS lens, which indicates that the stress concentration at the corner causes the stress to increase. This is due to the secondary internal stress of polystyrene products under the condition of rapid cooling,which makes the internal stress distribution of PS lenses worse than the PS lens without post-treatment.

Conclusion
In this experiment the influence of different post-treatment conditions on the internal stress of typical PS products represented by mobile phone lenses was studied. Under the same post-treatment time, different post-treatment temperatures and slow cooling conditions, increasing the post-treatment temperature below the thermal deformation temperature can effectively reduce the internal stress and improve the internal stress distribution of polystyrene products, and there is an optimal post-treatment temperature of 60℃. After the post-treatment, the polystyrene products have secondary internal stress under the condition of rapid cooling, so that the distribution of internal stress in the products is worse than that of the polystyrene products without post-treatment.The experimental results can lay a theoretical and practical foundation for seeking effective measures to reduce the internal stress of polymer products in the future.