It depends on the type of plastic you use. Some plastics are quite viscous once melted, in particular, high strength glass-filled plastics. If it doesn't flow well it won't completely fill a cavity and you'll get what is known as a short fill. More runny plastics flow better (obviously) but they have more of a tendency to flash (get forced out between the joins of the moulding). Its swings and roundabouts, really. The world of injection moulding can be complicated.