Runners should convey the molten material to the tool with minimum loss of pressure. On the one hand, we want runners to have the minimum diameter, as this material needs to be either thrown away or re-processed, and larger diameters can mean longer cycle times. On the other hand, diameters need to be big enough to avoid pressure loss. To achieve this, a round cross section of the runner would be ideal, as it has the best surface to volume ratio. However, this is expensive to manufacture. A good compromise between pressure loss, manufacturing cost and de-moulding characteristics is usually found by using trapezoidal shape. Depth is about equal to base, and the taper is about 3-5 ° per side.

The runner lay out should be such that the pressure loss is from the sprue bushing to the gate is equal for each cavity. This can be achieved by ensuring the flow path is of equal length, or by varying the diameter of the runner. If the different cavities fill at different pressure, this will result in different dimensions, different levels of moulded in stresses and different mechanical performance.

It is recommended that at each runner intersection, the primary runner overruns the secondary runner by one diameter. This provides an area where colder, higher viscosity material can be trapped, allowing the hotter, lower viscosity material to flow into the tool.

For high volume series, it is often attractive to use hot runners. For polyamide 66, it is recommended to use externally heated hot runners. Needle shut off valves can be used with these materials.