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October 7 2004

Technical Paper- Improving Shear Induced Imbalance In Hot Runner Systems

October 07, 2004
Technical Paper - Improving Shear Induced Imbalance In Hot Runner Systems

 

Improving Shear Induced Imbalance
In Hot Runner Systems


Opti-Flo(R)
Hot Runner Manifold Systems
with MeltFlipper® Technology

INCOE® Corporation
INTRODUCTION

Shear induced flow imbalance is recognized as contributing to a number of molding problems in multi-cavity molds. Uneven cavity filling is responsible for voids, core shift, inconsistent dimensional properties, and other part defects. It is obvious that if the process starts out incorrectly, obtaining a repeatable, optimum condition is nearly, if not completely, impossible.

A patented system of melt manipulation (MeltFlipper®) was introduced to the market several years ago that corrects the imbalance condition in cold runner molds. Recently, this technology has been made available to hot runner systems. The hot runner manifold construction incorporates the patented MeltFlipper™ melt rotation design.  This technology provides a scientifically designed hot runner system, ensuring an even melt distribution and balanced filling to and across all cavities while avoiding restrictive mixers.
Image 1
The Opti-Flo(R) Hot Runner design provides an alternative to the common practice of creating melt temperature variations in the nozzles, altering the diameter of certain flow channels, or incorporating different gate orifice sizes for each cavity in an attempt to artificially balance cavity filling. The result with Opti-Flo(R) technology is improved cavity-to-cavity consistency and product quality.

SHEAR AND VISCOSITY

In both cold and hot runner systems, shear rates are already known to be highest near the flow channel wall. As the molten plastic flows easily through the center of the channel, the material along the perimeter is dragged against the channel wall, causing a high rate of shear.

Flow imbalance is created by the very nature of how all plastic flows within a channel during injection molding, known as “fountain flow”. As the material first enters the flow channel it is deposited on the channel wall, with the following material flowing through this layer and advancing to the front. At the transition of the layers, the moving plastic drags along the inside creating laminates of plastic flow resulting in the center laminate to flow faster.  The dragging of the material orients the polymer to accommodate the direction of the flowing material allowing it to move freely in the direction of the flow front. This condition, known as shear thinning, reduces the viscosity of the material. Since all plastic is non-Newtonian, the viscosity is directly affected by shear and temperature. The lower the viscosity, the easier the melt flows. Hot runners behave in a similar fashion as cold runners in that a layer of material forms where the melt meets the steel in the runner plate (in hot runners, it is the manifold channel). The viscosity is lowered due to the shear thinning effect created between laminates.
Image 2
An annular zone of low viscosity, shear thinned material develops around the central area of flow at a higher viscosity. The viscosity differential between the two zones will vary depending on material type, temperature, flow rate, and other processing parameters, but regardless, the fluid properties are no longer homogeneous within the flow.

VISCOSITY AND IMBALANCE

When molten resin splits at an intersection, the annular ring of shear thinned material does not remain intact but concentrates the low and high viscosity material at opposite sides of the secondary channel.
Image 3
When the material is eventually split again at the next intersection in the manifold, the low and high viscosity regions of the flow channel become separated. Once separated, the ratio of sheared material to non-sheared material differs from channel to channel, resulting in cavity-to-cavity imbalance.
Image 4
In typical 8 cavity layouts (without level change), the inside 4 cavities will fill first. This commonly observed imbalance is due to the disproportionate amount of sheared material directed to these inside cavities by the configuration of the flow intersections and the natural tendency of the material to shear.
Image 5
Once entering the hot runner system, the material will develop a shear thinned ring of material on the outside diameter of the flow channel. After the first intersection under the system inlet, the shear profile will lose its bull’s-eye shape. The sheared material will be found more prevalent on the top side of the flow diameter on each side of the intersection. The resulting shear profiles are identical in shape at this point.
Image 6
As the material continues down the primary runner, it will continue to develop shear thinned material. After the material has been split a second time at the intersection of the primary and secondary runner, the resulting shear profile will be prominent in the upper left corner of the flow channel. Again, both resulting flows have identical shear profiles.
Image 7
When the melt is divided for the last time at the intersection of the secondary and tertiary runners, the sheared and non-sheared material is disproportionately divided. The result is unbalanced filling characteristics between the inside and outside cavities.
Image 8
These same principles manifest in stack molding applications, however are slightly different in that the cavities on the parting line closest to the injection unit have a tendency to fill first. 
Image 9
In typical 8 cavity layouts, 2 distinct groups of parts will develop. The inside 4 cavities which fill first are considered flow group 1, while the remaining 4 outside cavities which fill last are considered flow group 2. All cavities within a flow group will exhibit similar characteristics; part weight, material properties, etc. As the cavitation increases, the number of distinct flow groups that develop also increases. For example, a typical 16 cavity system will create 4 distinct flow groups, 32 cavities will create 8, and so on. While useful for analysis and troubleshooting, the distinct flow groups found in multiple cavity molds still present the problem of inconsistency between all cavities in the mold, which affects part quality, processing, and productivity.

IMPERFECTIONS FROM IMBALANCE

Many part defects can be attributed to flow imbalance. Cavities that fill first are typically heavier and larger than the remaining parts, potentially sticking in the mold and causing damage. Flash, sink, and short shots are typical abnormalities that can also arise from uneven cavity filling. Dimensional variations between cavities also result due to uneven shrink rates and part weights. Additionally, uneven cavity filling contributes to core shift in certain applications.

Processing obstacles also become apparent from unbalanced filling. Cycle times and injection pressures are increased since cavities are not filling simultaneously. The processing window also becomes narrower as a “sweet spot” needs to be attained for each production run to find the right balance between flashing certain cavities and under packing others.

Ultimately, productivity is reduced from longer set-up times and higher scrap rates. Combined with part defects and processing obstacles, these become the limiting factor against choosing higher cavitation tooling, eliminating the opportunity for increased productivity.

ADDRESSING IMBALANCE

Until recently recognizing the shear thinning phenomenon found in hot and cold runner systems, attempts to balance cavity filling were made by adjusting flow diameters, gate diameters, and temperatures. While even cavity filling could be achieved, it was not always continuous. The modifications made would be for that production run. As the processing variables changed during later production runs, the modifications made would not be sufficient to accommodate the new processing parameters. Temperature variation can be adjusted easily. Runner and gate diameters cannot, so balance may not be regained.

Even after making modifications to the runner system, other problems can still potentially persist. Disproportionate amounts of sheared material are still directed to the cavities, continuing to create mechanical differences as the material properties of the resin are different (i.e. shrink, warpage, temperature, uneven distribution of filled material).

The trend in hot runner system design for multi-cavity molds has been to incorporate full level changes at certain intersections. While improving filling balance compared to prior designs, this solution introduces still another condition.
Image 10
Manifold designs incorporating full level changes direct the shear section of the flow profile to the outer cavities, which now fill first.
Image 11
MELT ROTATION AND SYMMETRY

By manipulating the orientation of the melt at flow intersections (called melt rotation), proportioned amounts of sheared and unsheared material can be distributed to all cavities resulting in balanced filling. Melt rotation helps achieve symmetry at intersections, a necessary property of the shear profile so that when split, the shear profiles of the two resulting flows consistently and evenly distribute proportioned amounts of sheared and unsheared material. Melt rotation in the hot runner design is required to achieve symmetry at intersections.
Image 12
As shown, systems without melt manipulation do not fully compensate for the effects of shear thinning, and cannot maintain symmetrical flow throughout the system. Once symmetry is lost and the melt is divided, unbalanced flow results.

Image 13

Image 14

As shown, simple elevation change or not, symmetry within the melt is lost. However, applying melt rotation results in even distribution of the sheared and non-sheared material at each flow intersection. This is neither achieved with a level change that can be considered a full rotation, nor no level change at all. Rather, it can be considered to be somewhere in between, and is achieved with a melt rotation designed specifically for the intended application.
Image 15
THE FIVE STEP PROCESS

Any molder can identify the imbalances in their own molding applications by applying the “5 Step Process”. This simple yet effective tool will identify those molding applications where the benefits of Opti-Flo(R) technology can improve your molding operation and reduce your part costs.

Step 1: Mold Samples:  Identify each flow group for the particular layout and number of cavities. Number each cavity by combining the flow group number and a cavity-identifying letter, such as 1A through 1D for flow group 1 that contains four individual cavities. Once you have established a reasonable process for the mold, reduce screw feed and set the hold pressure and hold time to the minimum value possible. Reduce the screw feed until the best-filling cavity in the mold is about 80% full to avoid masking any imbalances due to unvented air, thin regions, or other hesitation effects. The original injection rate should remain constant
Image 16
Step 2: Weigh Parts:  Collect all the molded parts from a single shot and weigh them individually, being sure to label each part as indicated in Step 1. This can be done immediately since the samples do not need to be conditioned.

Step 3: Determine Mold Steel Variation in Flow 1: Identify the parts molded from Flow 1 (two parts in a four cavity mold, four parts in molds with eight or more cavities). Contrast the weight of these parts to each other using a graph. These differences are variations resulting primarily from dimensional discrepancies in the mold steel.

Step 4: Determine Mold Steel Variation in Other Flows: Identify each of the other flows in the mold and repeat Step 3. This will isolate the effect of the dimensional variations in the mold steel on each of the other flow groups.

Step 5: Determine Shear-Induced Variation: Identify the parts molded from Flow 1 and determine their average weight. Contrast this to the average weight of the groups of parts molded from each of other flows. The result is the shear-induced variation created within the runner. This shear-induced variation is independent of dimensional differences in the mold steel.

BENEFITS

As with nearly all improvements, the “silver bullet” for injection molding has yet to be invented. Preliminary tests have shown improvements can be achieved with melt rotation, particularly in multi-cavity aggressive cycle applications, high shear and filled materials.
Image 17
Acknowledgements:

Professor John Beaumont – Penn State University inventor of MeltFlipper® technology and the “Five Step Process”.
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About INCOE® Corporation
Since 1958, INCOE® has engineered productivity built hot runner systems starting with their original patented design of the first commercial hot runner nozzle. Today, a wide range of nozzles and manifolds, pre-wired unitized systems, complete hot halves and advanced control technologies provide optimized systems suitable for appliances, automotive, electronics, medical disposables, packaging and technical markets. A network of representatives in over 35 countries are supported by INCOE® facilities located in the United States, Germany, Brazil, China, Hong Kong and Singapore. Wherever your molding operation is, INCOE® can support your business with complete hot runner systems engineered for your application. That's
INCOE® Hot Runner Performance.


For more information:

INCOE® CORPORATION
1740 E. Maple Road
Troy, MI 48083
T (248) 616 0220
F (248) 616 0225
e-mail: info@incoe.com

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