Microfluidics and rheology of dilute carbon black suspensions for in-mould coating (IMC) applications

Abstract

In-mould coating (IMC) is carried out by injecting a thermosetting dilute carbon black suspension onto the surface of an injection-moulded part after the part has solidified, but while it is still in the mould. Due to the microscopic length scale of the IMC flow (10–25 µm), a study of the slip flow and rheological properties at high shear rates of the coating liquid is critical in modelling the IMC process. A customized microslit rheometer using micrometer-sized channel gaps between 25 and 100 µm was developed and used for that purpose. A reduced viscosity of the suspension due to slip (or apparent slip) at the fluid–wall interface was found in the 25 µm gap channel. Rheological equations for slit rheometers using no-slip and slip boundary conditions were used to determine the viscosity of the suspension at the microscale level. By analysing the viscosity data using the rheological equations developed, we can determine the values of the slip parameter, known as slip length. The slip boundary condition and high-shear-rate-plateau viscosity model are then applied to model the slip flow of the suspension. By including both the wall slip and high-shear-rate-plateau viscosity in the flow model, we can better predict the pressure distribution of the suspension.