One of the basic requirements for successfully molded parts is that we must have cavity balance in order to achieve process robustness and fast cycle times that are necessary to be profitable in today’s competitive, global market. In this article we will take a basic look at good and bad cavity layouts and runner designs.

Good cavity and runner design is crucial in order to achieve a balanced fill where all cavities fill at the same time with the same shear rate. If some fill prior to others, pack and hold will not have the same influence on all cavities and dimensions will be different. If this is the case, band-aids will need to be applied to the process in order to achieve the level of quality required. This can be done by trying to manipulate the injection speeds, over packing most cavities as well as many other adjustments. This will result in a very small process window that will not encompass all the normal variations that are expected. Needless to say, the process will never be optimized or as profitable as it could have been.

We will start with a traditional eight-cavity layout . With this design the inner four cavities will generally fill first due to orientation stress from the inside square corners. Related to plastic flow, tee intersections cause shear imbalance in the runner. This phenomenon will cause the plastic to continue to turn in the same direction when the next junction is reached, thus filling the inside cavities first. When the inside cavities are full, flow stops while the outside cavities continue filling. During this secondary filling of the outside cavities, the inner cavities’ plastic is cooling and unorienting, raising the viscosity in the inner cavities. This allows more influence in the outside cavities during pack and hold, which provides the potential for the outside cavities’ parts to be larger. The more the fill imbalance the more problematic the mold will become.

A better choice would be to use a runner. The cavities have the opportunity to fill at nearly the same rate and the orientation stress developed will be nearly the same.
It is important to use large radii in the runner layout. This will reduce the shear imbalance and allows even filling.

An area that should not be overlooked is the cold slug well. It must be slightly larger in diameter than the largest sprue diameter and 1.5 to two times that diameter in depth .

This will give us the best opportunity to catch partially cooled material that develops at the end of the nozzle between cycles. This is a lot like playing baseball; you want to catch the ball in the catcher’s mitt every time or the game could be a loss. So, we want to catch the one and only cold slug in the well every time so as not to affect the plastic flow in the runners gates. Not catching it would cause a roaming balance issue from cavity to cavity, creating another variable.