Generation mechanism of gloss defect for high-glossy injection-molded surface
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Generation mechanism of gloss defect for high-glossy injection-molded surface Jinsu Gim1 and Byungohk Rhee2,* Center for Coating Materials and Processing, Engineering Research Center, Seoul National University, Seoul 08826, Republic of Korea 2 Department of Mechanical Engineering, Ajou University, Suwon 16499, Republic of Korea (Received March 13, 2020; final revision received May 12, 2020; accepted June 6, 2020)
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The gloss defect on a glossy surface is one of the surface defects to be found on injection-molded products. In this paper, the effects of the filling and packing stages on surface gloss are investigated. Based on observations, a generation mechanism of gloss difference is proposed. The difference of the surface gloss can be explained by the replication of the shrinking polymer surface represented by a replication factor. The replication factor is the ratio of melt pressure to surface stiffness, which is influenced by the filling condition and the material properties. The melt pressure as a driving factor to the replication reflects the effect of the flow front speed and viscosity. The surface stiffness as a resisting factor reflects the effect of the flow front speed, mold temperature in the filling stage, and storage modulus. The replication factor shows a high correlation to the surface gloss over a wide range of filling conditions. The proposed mechanism recommends a uniform and high flow front speed and mold temperature to suppress gloss defects. Keywords: gloss defect, shrinkage, melt pressure, surface stiffness, injection molding
1. Introduction The gloss defect on a glossy surface is one of the surface defects to be found on injection-molded products. It is characterized by a gloss difference perpendicular to the direction of the flow front movement, as shown in Fig. 1a. The gloss defect is commonly observed near the locations of the gate operated sequentially in a hot-runner mold, as shown in Fig. 1b. The gloss defect can easily be seen on a highly glossy surface (Gim et al., 2018), where it appears dusty and cloudy due to the blurred reflected image on the surface. It may also damage sequential post-processing such as coating and painting. There has been no clear understanding of the physical generation mechanism of gloss defects, though there are various possible causes and ambiguous names attributed to them. Yuan et al. (2015), Jeon et al. (2017), and Bott (2012) suggested that the hesitation of the flow pattern is the cause of the gloss defect and described it as a flow hesitation mark or halo mark. Suhartono et al. (2017) considered the cause of the gloss defect to be the excessive stress applied by the movement of a gate valve pin and called it the stress mark. Yuan et al. (2014) suggested a change in melt pressure to be the cause and described it as a pressure transition mark. Goodship (2004a; 2004b) pointed out that cooling and shrinkage differences caused poor replication at some locations. To control the gloss defect, several methods have been suggested. Typi
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