A runner is a channel cut into the mold that allows plastic material to flow from the nozzle to the cavity.

A change in pressure is expected whenever the viscosity changes or the flow channel thickness (or diameter) changes. Pressure will increase when you have a reduced thickness. As the material hesitates, it will slow down and possibly prematurely freeze,  thereby producing a short shot. If the part hesitates but is able to make a full part, then there may be more internal stresses.

In order to avoid hesitation, keep a constant wall thickness or position gates as far away from the thin region as possible. This ensures that the flow does not have an alternative flow path that would create the hesitation. When the thinnest sections are at the end of fill, the melt can not hesitate and therefore has a better chance of filling.

In the photo here the part has two thin tabs on either end which have the same mold steel dimensions. Note that the thin tab on the opposite side of the part from the gate is able to fill as the melt has no alternate flow path when it reaches this location (i.e. the melt cannot hesitate). Meanwhile the melt entering the thin tab next to the gate hesitated and froze off.


The runner has a significant influence on part formation. Influences include mold filling pressure, packing, melt temperature (frictional heating), cosmetics, shrinkage, warp and residual stresses. Shear induced melt variations developed in all runners (single and multi-cavity as well hot and cold runners) can create temperature variations of over 100F. These are regularly ignored by the mold designer, leaving the molder to deal with the impact.

These shear induced melt variations can be uncontrollably distributed across a given cavity or non-uniformly between cavities in multi-cavity molds.  The most commonly used geometrically balanced runners notoriously create cavity-to-cavity rheological variations that cause inconsistencies in shrinkage,  warpage as well as filling and pack.

Not until about 15 years ago was this impact on shrinkage and warpage understood.  A solution, integrating runner melt management technology and process was introduced about 5 years later (MeltFlipper®) which provided the first method to control both the uniformity of part formation and the control of shrinkage and warpage.


Runners can be cut in a variety of shapes. These are shown below (figure 2) as full round, parabolic, trapezoid, wide trapezoid, half round, and quarter round. Full round is the most efficient runner shape. It has the lowest pressure drop over the same volume of material.The only disadvantage is that the runner must be cut into both A and B runner plates which requires that the two halves match up when the mold is closed (alignment should be within 0.001inch).

The next most efficient runner cross sectional shape is the “parabolic” ruinner.  This design is very common as it is only slightly less efficient than the full round and only needs to be cut into one side of the mold

Other runner shapes such as the trapezoid, half round, and quarter round provide no advantage and are not recommended. For more information on runner shapes, click here.

Runner Shapes

There are two primary types of runner systems – cold runners and hot runners.  Cold runners are cut along the parting line of a mold, are cooled by the mold, solidify and are ejected from the mold during each molding cycle. The advantages of cold runners are their simplicity and consistency. The downside is the solidified runner must be dealt with. In many cases the material can be granulated and fed back (recycled) into the process.

Hot runners, or non-solidifying runners, are specialized heated channels that keep the polymer in a molten state.  The primary advantage of hot runners is that they eliminate the need to manage a frozen/solidified runner.  They also have the advantage of running faster cycles when molding thin walled parts, where the cooling time for the cold runner could dictate cooling time. The downside of hot runners are that they are more complex, higher cost, require more maintenance, create more variation and are difficult/costly to make changes to after initial manufacturing.


Hot Runner


Cold Runner