AbstractFiber reinforced thermoplastics (FRTP) injection molded articles have many advantages. However, moldings with a complex geometry often encounter weldline problems. Mechanical properties of the injection moldings are often reduced by weldlines, because the fibers at weldline regions orient perpendicularly to the flow directions. We have reported that weldline strength is improved by “back‐flow” during the holding stage. In this article, we have attempted to adapt the SCI (Simultaneous Composite Injection) molding method to improve weld strength of FRTP injection moldings. The back‐flow is expected to occur when there is a difference of holding pressure between two cylinders of the SCI molding machine. The effect of pressure difference between two injection cylinders on the back‐flow phenomena is discussed for fiber reinforced polycarbonate. It is clear that the strength reduction by the weldline can be suppressed down to negligibly small levels in SCI molding. © 1995 John Wiley