Injection Velocity is a machine control you set that determines how fast the screw moves forward during the filling phase of the moulding cycle. It describes how far the screw travels in a fixed period of time and is usually reported in inches per second of screw travel. Knit Lines are locations in the moulded part where two or more melt fronts meet. Knit lines are often described by breaking them down into two subcategories: meld lines or weld lines.