In multi-cavity mold balanced filling is critical for making parts identical and holding tight tolerances. The main reasons for unbalanced filling are
(1) non-uniformly melted plastic disrupt the flow in runner, gate and part.
(2) non-uniform cooling of mold
(3) unbalanced flow path
(4) laminar floe of melt.
Balanced filling is essential for multi-cavity molds to maximize the quality and reduce part variation. Balanced filling involves the designing of a runner system so that each cavity fills at same time and same temperature so that the molded part will have uniform density and will shrink to same size when material solidifies. In such a runner system the distance between the runner entry point from where plastic enters the mold and gates of each cavity is same. This method of balancing runner is called artificial balancin
When part is very complex then single point injection is not enough to fill the part completely. In that case multi-point injection is employed. Multi-point filling reduces the required filling pressure and provide more uniform packaging and shrinking throughout part.