In-mold coating (IMC) of plastic parts is a promising coating process in automotive and consumer products industry. An IMC material, such as carbon black suspension, can potentially be used as a top coat or paint. IMC is performed by injecting a reactive liquid polymer onto a part’s surface in the end of the forming process while the part is still in the mold. The coating material flows under high pressure by compressing the part’s surface, which causes a microchannel of 75-125 micron thickness. The IMC flow, particularly in a microchannel, tends to cause the slip boundary at the channel wall. This research work aims to (i) study rheological properties of carbon black suspended in acrylic resin in both macro- and micro-scale, (ii) study slipping at the wall in the microchannel, and (iii) determine significant factors affecting the slip flow of the carbon black suspension. A custom-built microslit rheometer was developed to conduct experiments to serve these purposes. By analyzing the viscosity data, a slip parameter called slip length could be determined. Statistical design of experiments was then applied to determine the effect of three factors including channel gap, operating temperature, and carbon black content on the slip boundary conditions of the microflow. The results showed that these three factors significantly affected the slip length of the flow of carbon black suspension.