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.
The runner has a significant influence on part formation. Influences include mold filling pressure, packing, melt temperature (frictional heating), cosmetics, shrinkage, warp and residual stresses. Shear induced melt variations developed in all runners (single and multi-cavity as well hot and cold runners) can create temperature variations of over 100F. These are regularly ignored by the mold designer, leaving the molder to deal with the impact.
These shear induced melt variations can be uncontrollably distributed across a given cavity or non-uniformly between cavities in multi-cavity molds. The most commonly used geometrically balanced runners notoriously create cavity-to-cavity rheological variations that cause inconsistencies in shrinkage, warpage as well as filling and pack.
Not until about 15 years ago was this impact on shrinkage and warpage understood. A solution, integrating runner melt management technology and process was introduced about 5 years later (MeltFlipper®) which provided the first method to control both the uniformity of part formation and the control of shrinkage and warpage.
Runners can be cut in a variety of shapes. as full round, parabolic, trapezoid, wide trapezoid, half round, and quarter round. Full round is the most efficient runner shape. It has the lowest pressure drop over the same volume of material.The only disadvantage is that the runner must be cut into both A and B runner plates which requires that the two halves match up when the mold is closed (alignment should be within 0.001inch).
The next most efficient runner cross sectional shape is the “parabolic” ruinner. This design is very common as it is only slightly less efficient than the full round and only needs to be cut into one side of the mold
The accepted ground rule for balancing melt flow in multi-cavity injection molds is to achieve equal flow distance from the injection point to each cavity. The sober truth is that even such “naturally balanced” molds can be prone to non-uniform fills, resulting in good parts from some cavities and short shots, flash, or overpacking in others. This is hardly a new problem for injection molders. But there are some signs that it could become more important as simpler molds and molding jobs go overseas and domestic molders take on proportionally more of the complex, multi-cavity applications.
Not all sources interviewed for this article agree on the scope of the problem. We don’t see a lot of filling imbalances, largely because we avoid varying gate sizes, we always starts with a geometrically balanced mold and frequently adds a Moldflow analysis to forestall potential problems. But, as we find, There may be only so much you can do, even after the Moldflow analysis. Suppose you move a gate to change the weld-line location—that may raise the chances for a shear imbalance to arise. You may have to change gate sizes to compensate. However, we advises against such modifications except as a last resort, because they tend to shrink the process window for the mold.
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. As Equation 1 highlights, 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.
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.