what is an unbalanced runner? Unlike a balanced runner design, an unbalanced runner design is that the runner lengths from the spure to each cavity may be different. And the gate has different sizes, the length of the gate near the spure should be greater than the one away from the spure, or the sectional area near the spure’s gate should be smaller than the one away from the spure.
The Principle of the Runner Design
The runner is usually designed on either half of the plastic injection mold.
When considering the layout of the cavity and the branch runner, it is better to ensure that when projected on the parting surface, the geometric center of the total projection area of the cavity and the runner overlap with the center of the clamping force.
The internal surface roughness of the branch runner is 1.6. In this case, the outer layer melt flows faster than the inner layer melt inside the branch runner, which tends to cool down and thus form a heat insulation layer.
The sectional size of the branch runner is dependent on the plastic part size/type, injection speed, as well as the length of the branch runner.
Generally, when the diameter of a branch runner is smaller than 5 – 6mm, runner size will have a greater influence on fluidity; when the diameter is greater than 8mm, there will be little influence on fluidity.
If the branch runner is very long, it might be better to further extend the branch runner along the flow direction to form a cold slug well, so that the cold materials will not enter the cavity.
The branch runner cannot be too thin, or temperature/pressure loss will increase, which makes it hard to fill up the cavities far from the main runner.
The Runner Design Layout
In a multi-cavity layout, it needs to be guaranteed that the molten plastic can concurrently fill up each cavity in a uniform way. There are 2 layouts, i.e. balanced and unbalanced:
Balanced: uniform filling, with each cavity concurrently filled.
Unbalanced: The runner is designed to be short to save raw materials.
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, The 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.
Injection molding is a widely used method for producing custom plastic parts in large quantities. At the core of this process is the runner system, an essential component that guides molten plastic into molds. The injection mold runner design can have an impact on product quality, cycle time, and costs.
The runner system’s design directly contributes to part consistency. It ensures that each part in the mold is filled uniformly, leading to consistent dimensions, structural integrity, and aesthetic characteristics. A well-optimized runner system minimizes variations in part quality and reduces the need for extensive post-production adjustments.
The runner design significantly affects how molten plastic flows from the injection molding machine to the mold cavities. A well-structured runner system ensures a smooth, controlled flow of material. This prevents issues like inadequate filling or excessive pressure, which can lead to defects such as voids, warping, or incomplete parts.