There are three main runner shapes that are used: (1) full round (2) half round and (3) trapezoidal with each one having certain qualities and performances that may be needed to support delivery of the plastic to each cavity.
The full round is probably the most difficult to machine, as it must be machined into both halves of the mold, which must match perfectly. If they do not match the potential for affecting shear rates, the addition of square corners and sharp edges will adversely affect the plastic flow. A properly machined round runner has the best theoretical flow of the three runner shapes. Half round is the easiest to machine, but it does not have the most robust flow characteristics. The trapezoid will have better flow than the half round and is relatively easy to machine.
Adding a radius to the bottom, as shown, makes it an optimum solution. One other benefit is that when a trapezoidal runner cools it typically sinks on the flat sides, allowing for superior release. This is a good choice for materials that dictate a large runner, such as PVC. Remember, you must get runner release in order to achieve those great cycle times. Many times poor runner design limits the cycle time.
Now that we have established a cavity layout, type of runner and shape, we need to select the gate type. Always consult the design guide for the gate type and size. Any gates that are left attached to the parts may require a second operation to trim the parts, which add cost to the final product. Gates such as sub gates detach themselves during ejection, eliminating that secondary operation.
Next we have to determine the size of the gates and runners. In an optimized system we want to have the smallest runner volume and still be able to fill and pack each cavity the same. Gate size should be as small as possible without introducing shear rates above that known to cause degradation. Filled materials or those that are shear sensitive require larger gates. Contrary to popular opinion, there are normally not large pressure losses across small gates. Our instrumented molds have proven this time after time. Small gates cause shear, which untangles long molecules and lowers viscosity. A rule of thumb is that gates are 40 to 60 percent of part thickness. Gate sizes and types are many times determined by part aesthetics.
In order to minimize pressure losses we would like to maintain the same shear rates in all runners. Working from the part to the sprue bushing we will now establish the first runner size, which will be the diameter of the wall thickness, in this case .100″. When we come to a junction where plastic branches we must increase the runner size by 25 percent to .125″. Each time we branch to another runner level we again increase the runner size by 25 percent (to .156” in this case) until we reach the sprue bushing . The 25 percent increase, as described, will achieve the same shear rates above and below the runner branches. The theoretical value is 26 percent, but 25 percent is close enough.
A word of caution—if the runner has long flow lengths we may need to increase its diameter as much as another 25 percent. This can be validated during the rigorous mold tryout by doing a pressure loss study. Remember that it is easier to open a runner up rather than make it smaller. Runner and gate optimization also can be achieved using flow analysis software.
Doing your homework before the runner has been cut may save problems with trying to get a mold to fill correctly without hindering the machine’s capabilities. This gives you the best chance of delivering the plastic to each cavity at the same time with the same shear rates and the same packability.
Sprues，runners, and gates fulfill the function of conveying the plastics melt from the nozzle of the injection unit to the individual cavities.While it is true that those plastic materials may be reused in the form of regrind, their presence nevertheless means a reduction in the performance of the injection molding machine since they must be plasticized in the barrel. With smaller parts，they may account for 50 % or more of the actual shot weight.
Sprue
The sprue may be considered the continuation of the mold to the nozzle of the injection machine.Single-cavity molds where the sprue links directly to the molded part are said to have direct sprue gating.
Very often, the performance of a single-cavity injection mold is determined by the cooling time of this sprue. In addition to providing adequate cooling of the sprue bushing, the diameter of the smallest opening in the sprue bushing should be kept as small as possible and permitted by proper filling of the cavity. No universally applicable rules can be given here，since filling the cavity depends on many factors. The sprue should have a 1.5° draft.A greater draft may simplify removal from the sprue bushing, but a function of the length of the runner，since it may be assumed that the pressure loss in a runner increases at least proportionally with the length.In all likelihood, it will probably increase more than proportionally since the cross-section is reduced by solidification of the melt along the walls，and the more so, the greater the distance from the sprue. Because the sprue and runner system represent lost material and lost plasticating capacity, the runners should be designed to be as short as possible and with the smallest possible cross-section.The length of the runners is determined by the number of cavities in the mold and the geometrical arrangement of the individual cavities.
Runner
Runners follow channels cut into the parting line instead of sprues, which deliver material into the center of the mold plate. Their design affects part quality and molding efficiency. Thicker runners can make filling pressures too high and cause unwanted long cycle times. Conversely, thick runners can cause unnecessary lengthening of cycle time and increases in costs associated with regrinding. The optimal runner design must strike a balance between mould feasibility and filling pressures. During mold filling, freezing occurs as the heat of the melt is transferred to the mold. The result is less melting material flowing through the runner and a large pressure drop. The round runners create the smallest cross-sectional area of the frozen wall.A trapezoid runner can be an alternative more effective than round runners because it only requires machining in one mold half. Round runners require the matching and low flow restriction of two mold halves.
Runner Cross Sections
More than 35% of the pressure required to fill the mold is often attributed to the runner system. To eliminate this additional pressure drop, optimize the route to each gate. For example, substitute diagonal paths or reorient the cavity to shorten the runner.Factors such as runner thickness, packing density, and runner volume directly impact filling pressure and cycle time. To determine the optimum runner diameter, many factors have to be considered. These involve parts volume, filling velocity, filling pressure, runner length, and material viscosity.Make runners that are at least the same thickness as the nominal wall thickness of the part to ensure adequate packing.
For runners that are long and subject to high volumetric flow rates, increase the runner thickness.
For semicrystalline resins, runners must be smaller than amorphous resins.