Runner Balancung

7/31/2019 Runner Balancung

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The Real Cause ofMulti-Cavity Mold

Imbalances


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A lesson on shear induced melt variations endssome confusion regarding balancing runnersystems for multi-cavity molds.

FIGURE 1 Typical multi-cavity defect despite naturally

balanced runner system, due to the Beaumont Effect.


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The Beaumont Effect is recognized as the sourceof mold filling imbalances and product variations inmolds with four or more cavities.

The worst failures caused by the phenomena can bemost easily (visually) recognized in molds with 8 or

more cavities.However, shear induced melt variations impactevery injection molded part, with a greater influencethan most realize.

Even though the phenomenon has been understood,even taught at plastics engineering technologyprograms there is still a lot of confusion in regards tohow it is developed and predicted with simulationsoftware.


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FIGURE 2 Moldflow temperature plot shows shearheated material near runner walls.Notice the temperature distribution pattern as themelt flows around the corners.


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FIGURE 3 Multiple branches leads to significant temperaturevariation in material reaching each cavity.This material variation is the fundamental cause of the defectseen in Figure 1.


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FIGURE 4More mesh layers within runners improve accuracy.


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FIGURE 5Radiused runner systems fail to avoid the Beaumont Effect,despite misleading analysis results.


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FIGURE 6Often left out of mold models, the machine nozzle as shown onthe right should be included when simulating for the BeaumontEffect.


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FIGURE 7Turning inertia effects on in Moldflow can improve flow front

accuracy.


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Understanding the Beaumont Effect

Figure 1 is a short shot showing the characteristic failure

developed in an eight cavity mold using the industry standardgeometrically balanced (often referred to as naturallybalanced) runner system. Many confuse the root cause of thisproblem as being mold deflection, higher mold tooltemperatures in the center, poor venting or high shear caused

by sharp corners.Its an interesting debate, especially for moldmakers usingsimulation software to predict problems like this beforecommitting to a mold design. Sometimes excessive molddeflection, poor venting or higher mold temperature in the

center region can in fact be a problem. However even in thesecases, the Beaumont Effect is generally the most significantcontributor and ignoring it is unwise.


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Injection molding simulation software1 can predict each ofthese troublesome phenomenons, and even help determinean optimal solution.

To understand the development of the Beaumont Effect, wemust focus on the relationship between shear, viscosity andtemperature occurring within the flowing material.Plastic flowing through a runner exhibits laminar flow with

maximum shear occurring near the runner walls, and nearzero shear conditions existing in the middle of the runnerchannel.Molten polymers are also pseudo plastic non-Newtonianfluids, which means the materials viscosity is shear thinned.

This shear thinning (where viscosity decreases with the rateof shear) causes molten material flowing near the outermostperimeter of the channel to actually have a much lowerviscosity than in the center of the channel.


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We also know that frictional heating of the melt occurs as itflows along the length of a channel. Where the shear isgreatest, so is the frictional heating. The development of these

shear induced melt variations actually begins in the nozzle ofthe molding machines and builds through the sprue andrunner. The longer the channel, the more the heat builds up.Shear exposure at a corner is extremely brief and thereby does

not have the ability to raise the heat in the material by anysignificant amount. The effect of a sharp corner has negligibleeffects on a materials viscosity through either temperature ornon-Newtonian shear thinning. Note in Figure 2 that thefrictional heating builds near the outermost perimeter of the

channel as it travels the length of the primary runner andcontinues around the corner. The corner itself has no influencecompared to the shear heating near the contiguous wall flowlength.


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Understanding the Beaumont EffectWhat happens at branching corners is important tounderstand. Figure 3 clearly shows the hotter melt developedprior to the first branch. Follow its outer high sheared laminatesthrough the first and second branches and into the two cavitieson the left. The two right hand cavities receive the material that

was originally flowing through the center, near zero shear,laminates from the primary runner. This will inevitably causedensity, shrinkage and warpage variation in fully packed parts.Though the negative effect of this variation is most recognized inmolds with two or more branches, the phenomena can influence

even single cavity molds.2 Proper use of injection moldingsimulation can predict the development of these melt conditionsand help designers understand their distribution within a moldand their potential negative effects.


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Recognizing the Problem

While modern mold simulation tools can predict the Beaumont Effect, itrequires significant computational resources. Particular attention must be

paid to the mesh through the runner system and gateseven the machinenozzle should be modeled, as the effect starts building there. No matterwhich injection molding simulation software you use, the guidelines belowwill improve your prediction of shear induced melt variations. 3-D mesh, preferably with tetrahedral elements 12-20 elements through thickness of runners and gates Include machine nozzle in model Optional: Higher level flow solutions (such as the inertia solver optionwithin some injection molding simulation software1)Following these guidelines will effectively turn on the ability to predict theBeaumont Effect in multi-cavity simulations. The high element count through

the thickness of the runners is necessary to capture the material variation.Experience shows that between 12 and 20 elements are typically required(see Figure 4).

Fi 5 h th lt f i l ti i V l t l t I thi


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Figure 5 shows the results of a simulation using Voxel type elements. In thiscase, an ill-advised attempt at avoiding the Beaumont Effect was made byadding sweeping arches to the runners. Although the radiused runners appearto provide balanced material in the simulation, the physical tests disagree. Thesimulation in this case is misleading and can be seen to be flat out wrong. Asimulation technology vendor noted that "although the structured mesh usedhere should be sufficiently dense, the interpolation error common to brickmeshes in curved areas can actually be seen in the temperature distributionafter the first branch. As the flow becomes diagonal to the elements in the voxelmesh, error is induced and both the filling and thermal variations are completelymissed. A tetrahedral mesh will avoid this."Since material shear starts building in the machine nozzle before even entering

a mold, it should be included in the CAE model. Otherwise, some of the shearinduced temperature variation will be missed. In some injection moldingsimulation software1 beam elements can replace the 3-D elements for straightsections before any branching occurs. This can help to optimize solve time,since beam elements can accurately predict the shear variation across a meltstream before it branches. However, 3-D elements will still be necessary at the

first branch, and all sections following (see Figure 6).With some injection molding simulation software for use on digital prototypes3,a higher level flow solver may be activated by turning on the programs inertiaeffects. Though the Beaumont Effect is not a result of inertia, the associatedhigher level solver will improve accuracy at the flow front. This same solver alsoprovides the opportunity to predict the potential for jetting (see Figure 7).


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SummaryFollowing these guidelines will help predict the BeaumontEffect, but will also cost more in terms of solution time versus

using the programs default settings. In the case of multi-cavity molds, properly capturing the Beaumont Effect isabsolutely essential for predicting thermal variations thatcontinue from the gates into the part forming cavities. Oncethe effect can be seen, and recognized that it is unavoidable,

getting creative with runner layouts can improve partuniformity.

References1Autodesk Simulation Moldflow.

2 As presented at Autodesk University MoldflowPowertracks.3 Autodesk Moldflow Insight.


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