Overview: Purpose of a runner system is to transfer melt from injection molding machine to mold cavity. Runner system design can range from very simple to very complex. Investing in a runner system will shorten molding cycle and reduce material waste during mold use. Optimal runner system design, or "best" runner design, depends on product yield, effective injection pressure, and available investment costs.
Runner system design should follow these four steps. First, determine type of runner system (two-plate cold runner, three-plate cold runner, two-plate hot runner, three-plate hot runner, hot runner-to-cold runner); second, determine runner path based on mold layout; third, determine diameter of each section of feeding system to balance pressure drop, shear rate, and material utilization; finally, integrate design details of feeding system into mold design.
Objectives in Feed System Design: Primary function of runner system is to transfer molten plastic from injection molding machine to mold cavity. In most mold applications, melt passes through runner system, traversing most of mold's height and width. In some family molds, melt may even flow through runner into different cavities. As shown in diagram below, one cavity on the left represents cup body, and the other represents cup lid.

 

Left image above shows a two-plate mold cold runner system, while right image shows a schematic diagram of a three-plate mold cold runner system. Additionally, following aspects need to be considered in runner system design:
Minimum Pressure Drop
When melt enters cavity through runner system, melt pressure inside screw of injection molding machine will increase. Runner system design must provide sufficient pressure for melt to flow into cavity. Pressure restriction must be avoided, otherwise, it will affect injection speed. As shown in diagram below, maximum required pressure exceeds injection molding machine pressure.

 

In cold runner design, a runner system that is too large will require more cooling time and more material. Some products can use recycled materials, but many cannot. A common ratio is 30% recycled material. For example, a mold with two cavities and a total volume of 50cc, with a 30% material recycling rate, would have a runner volume of 15cc.
In hot runner design, a runner system that is too large will reduce material turnover rate within hot runner. A low turnover rate is detrimental to material utilization. Firstly, material's prolonged residence time in hot runner system can lead to cracking. This is particularly problematic for transparent or white products, potentially causing them to blacken or experience reduced mechanical properties. Secondly, a large amount of material in hot runner system can hinder color changes during molding, not only because a significant amount of molten plastic needs to be flushed away, but also because polymer melt experiences lower shear stress along feed system walls.
Runner System Type: Two-Plate Mold
Why is it called a two-plate mold? Because it consists of a mother plate (A) and a male plate (B), with runner sandwiched between them like a sandwich. During molding, injection molding machine's nozzle matches radius of sprue bushing. Polymer melt first flows downwards along sprue bushing, passing through top clamping plate and thickness of plate A. Then, material flows through parting plane, through runners and gate, into one or more mold cavities.

 

Once injection melt has been completely injected and cooled to solidify, mold will open from parting plane, i.e., between plates A and B. Typically, front mold, also called female mold or fixed mold, is fixed to one side of injection molding machine, while rear mold, also called male mold or moving mold, moves with injection molding machine. To facilitate demolding, a reverse cone is usually provided on ejector pin below gate to ensure that gate and connected runner remain attached to half B.

 

Type Three Runner System: Three-Plate Mold
A three-plate mold, as name suggests, has one more plate than a two-plate mold. This is where name comes from. Indeed, there is an additional stripper plate between plate A and upper fixed plate. Three-plate molds are also called fine-gate molds. Runner system is usually located between plate A and stripper plate, and then glue enters cavity directly through dispensing.

 

Following diagram shows classic three-plate mold opening sequence. Figure 1: Three-plate mold closed state; Figure 2: Three-plate mold first opening state; Figure 3: Three-plate mold fully opened state.

 

Runner cross-sectional shape: Runner cross-sections generally have following four shapes: circular, trapezoidal, U-shaped (or improved trapezoidal), and semi-circular.