The importance of runner analysis on design and molding seems to have been long overlooked. When many flow analyses are carried out, the focus tends to be mainly on the part and cavity.
However, the runners play a much more significant role than many realize. The polymer melt always fills the runner system first after the machine nozzle, and then proceeds into the cavity. The flow and thermal history of the melt in the runner system will have a direct impact on how the cavity is filled and part quality. Also, hot runners add additional residence times to the melt. For low shot weight and temperature-sensitive materials, the longer residence time could impose the risk of thermal degradation to the resin.
In many situations, the pressure drop in the runner system could be excessive and accountable for more than 50 percent – even more than 80 percent – of the total injection pressure. Apparently, if there is too much pressure loss in the runner system, there still would be enough pressure available at the gate to fill/pack the cavity. Poorly designed runner systems are often the root cause for many molding issues that could be resolved by only re-designing/modifying the runners without making any changes to other parts of the mold.
The very first step in designing any hot runner molds is to specify the most suitable gate type, nozzle size and manifold layout. Hot runner nozzles and manifolds usually are manufactured to different product lines according to their molding capability requirements and the inner runner bore sizes. There are strict relations between the runner sizes and the nozzle/manifold outside dimensions.
In many cases, the changes to different runner sizes would mean that a different hot runner product line would be required, which inevitably will lead to many modifications to the mold. Therefore, it’s very important to determine the correct hot runner system right from the beginning in the job quotation and tool design stage to avoid any undesirable costly changes later. The worst case could be that the mistake of putting an incorrect hot runner system on a production mold is not discovered until the mold is in the press.
One way to avoid these potential problems is through flow analysis, which can perform a molding test on the computer, check various design options, reduce design mistakes, highlight critical application areas and enhance the quality of runner system and mold.
The Shape Of The Mold Runner Cross-Section
Rectangular cross-section:
Rectangular-shaped runners are common. They offer advantages such as ease of manufacturing, simple tooling design, and uniform flow distribution. The dimensions of the rectangular cross-section can be adjusted based on the specific molding requirements of the part.
Trapezoidal cross-section:
Trapezoidal-shaped runners are another option in mold runner design. This shape helps promote better flow and reduces pressure drop, resulting in improved filling of the mold cavities. The wider end of the trapezoid is typically connected to the sprue, while the narrower end is connected to the gate.
Circular cross-section:
In some cases, circular-shaped runners may be the first choice. These runners offer excellent flow characteristics and are particularly suitable for parts with complex geometries or when a balanced flow is required. The diameter of the circular cross-section should be carefully determined to ensure optimal flow and minimize pressure loss.
Semi-circular cross-section:
A semi-circular-shaped runner features a half-circle profile. This shape promotes smooth material flow and helps minimize pressure drop. It is often used when balanced flow and reduced pressure loss are critical. The diameter of the semi-circular cross-section should be appropriately sized to accommodate the flow requirements of the specific injection molding process.
U-shaped cross-section:
A U-shaped runner has a curved bottom and two vertical walls that form the shape of a “U.” This design facilitates efficient material flow and allows for easier separation of the runner system from the molded part. The U-shaped cross-section is commonly employed when easy removal of the runner system is desired or when gating is located at the bottom of the part.
The selection of the cross-sectional shape depends on factors such as material properties, part design, mold layout, and production requirements. Each shape has its advantages and is chosen based on the specific needs of the molding process.
The Size Of The Mold Runners And Sub-Runners
The shape and size of the mold runners depend on various factors, including the product design, mold construction, and the specific requirements of the injection molding process. While the product size and wall thickness may influence the runner design, it is not accurate to say that larger cross-sectional runners are always more effective in facilitating the filling process. Material flow behavior, part geometry, gate location, and process parameters determine the optimal runner design.
Additionally, the length of the runner does not directly affect the viscosity of the plastic. The material properties and processing conditions primarily determine viscosity.
A well-designed runner can significantly impact the overall performance and efficiency of the injection molding process:
Mold Runners & Sub-Runners Arrangement
There are two types of mold runner arrangements: balanced and unbalanced. In a balanced runner system, the runners’ length, shape, and cross-sectional dimensions from the sprue to each cavity are designed to be equal. This helps achieve thermal balance and plastic flow balance in each cavity, resulting in consistent part quality. On the other hand, an unbalanced runner system allows the plastic to enter each cavity at different times, leading to variations in the filling process and potentially producing different parts. However, unbalanced runner systems can offer advantages such as more compact cavity arrangements, reduced template size, and shorter overall runner length.
Whether the runner system is balanced or unbalanced, it is important to ensure that the cavities are symmetrical with the center of the mold base. This ensures that the projected center of the cavities and runners align with the center of the clamping force of the injection machine. By doing so, we can avoid additional tilting moments during an injection.
A balanced runner system is advantageous as it allows for consistent injection and holding pressure across all cavities. This is particularly beneficial for multi-cavity molds where maintaining uniformity in producing all products is desired.
Design Principles Of Runners & Sub-Runners
Ensure the molten plastic enters the cavity swiftly with the shortest distance and minimal heat pressure loss.
Enable the melt to feed into the cavity simultaneously from various gates under identical temperature and pressure conditions.
Although larger cross-sectional areas facilitate molding and ensure adequate packing pressure, considering material saving, aim for smaller cross-sectional areas to minimize plastic consumption, which also reduces cooling time.
To conserve material and aid cooling, strive for a minimal surface area to volume ratio in runners.
The surface roughness of runners shouldn’t be too low to prevent dragging cold material into the cavity; typically, an Ra value of 1.6 μm is sufficient.
Runners and gates are usually connected with slopes and arcs, promoting the flow and filling of molten plastic while reducing flow resistance.
with family molds we want to consider the path the molten material needs to take as it is injected from the injection unit into the mold cavities. For molds that utilize a cold runner this control of the flow to the different cavities is often achieved by either varying the size of the runner channel, or by varying the flow length the polymer needs to take to reach the cavity. Since injection molding is a pressure driven process, we can either reduce the size of our flow channel (Runner) or increase the length of the runner to the smaller cavities so the material will preferentially fill the larger cavities first. a change in runner radius (diameter), r, has a much more significant effect on our flow resistance as compared to a change in the runner length, L. Because of this sensitivity, it is preferred to balance the filling pattern of the runners rather than the gates. By selecting to use the runners for balancing the filling pattern of the mold, we can minimize the influence of fill irregularities from inconsistent machining or burning of the runner system. If we attempt to use the gate size alone to address any fill imbalance, we minimize the amount of time we influence the molten material and increase our sensitivity to any dimensional inconsistencies in the gate. Therefore, we are more likely to have a narrow process window and a less robust solution over time.
Significant investment and energy is placed on optimizing plastic part design for manufacturing and assembly. Injection molding simulation packages, such as Autodesk Moldflow, are often used to help drive decisions regarding nominal wall thickness, rib and boss design, and snap fit optimization. However, the analysis is often limited to only looking at the filling pattern in the cavity alone, without consideration of the runner design and how it might influence the mold filling pattern in the mold and the quality of the parts. When we look at why we select injection molding as a technology it is often so we can mass produce dimensionally stable components at an economical price. Therefore, when we move to mold design and looking to scale up manufacturing, we often look at ways to increase capacity without significant investment. This objective is typically achieved by either increasing the number of cavities we include in our mold design or by designing a family mold that can manufacture several different components during the same molding cycle, Figure 1. This second method is often referred to as a family mold in the injection molding industry. Either way it is important to optimize the runner system to help ensure we are producing quality parts. Simulation software like Autodesk Moldflow can help us quickly optimize our runner and gate sizing and design so we can get good parts and a wider processing window.