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January 3 2005

Technical Paper - Improving Shear Induced Imbalance In Hot Runner Systems

January 03, 2005
Technical Paper - Improving Shear Induced Imbalance In Hot Runner Systems

Improving Shear Induced Imbalance
In Hot Runner Systems

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.

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”. Material first enters the flow channel, and 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 molecules in the direction of the flowing material or 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.
Image 1
An annular zone of shear thinned material develops around the central area of flow at a lower 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 as in the case of the transition from the point of injection to the primary flow channel of a hot runner manifold, the annular ring of shear-thinned material does not remain intact but concentrates the low and high sheared material at opposite sides.
Image 3
Upon 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 4
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 up to this point.
Image 5
When the melt is divided at 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. In this example of an 8 cavity single level design the inside 4 cavities will fill before the outside 4 cavities since the shear thinned material is of a lower viscosity and flows at a faster rate than the non-shear thinned material.
Image 6
These same principles manifest in stack molding applications, however in this condition the cavities on the parting line closest to the injection unit have are first to fill. 
Image 9
Therefore, 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. These distinctly different flow groups produced in multiple cavity molds are the route cause of inconsistency between all cavities in the mold, affecting part quality, processing, and ultimately 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. The combination of part defects and processing obstacles has become the limiting factor against choosing higher cavitation tooling, thus eliminating the opportunity for increased productivity.

ADDRESSING IMBALANCE

Prior to the recent awareness of the shear-thinning phenomenon found in both hot and cold runner systems, attempts to balance cavity filling were made by adjusting flow diameters, gate diameters, and temperatures. While improved cavity filling could be achieved, it is generally not continuous. The modifications made would only realize a success for a given 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).

Another trend in hot runner system design for improving balance in multi-cavity molds has been to incorporate full level changes at certain intersections. While balance is somewhat improved compared to prior designs, this solution introduces still another shear distribution 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 are consistent and evenly distribute proportioned amounts of sheared and unsheared material.
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
Molders can perform a simple test procedure developed by Prof. John P. Beaumont referred to as, “The Five Step Process. This was covered in a earlier issue of “Moldmaking Technology” issue (please fill in the appropriate info)

FULL SHOT PRESSURE ANALYSIS – NYLON 6/6:

A molding study was performed on a 16-cavity mold. The purpose of the study was to monitor cavity-to-cavity pressure differences while molding full shots. The test was performed using an industry standard manifold design and with a manifold utilizing melt rotation. Nylon 6/6 was used specifically for this experiment as it requires more aggressive processing conditions and the material’s tendency to freeze at the gates.

Pressure transducers were equipped in every cavity. By measuring the pressure in each cavity independently, the readings would not only indicate the pressure within the cavity while filling, but also the moment when the cavity would begin to fill. This second aspect is important because if the hot runner is not pressure balanced, frozen gates will experience different pressure levels at the beginning of injection, which may cause some gates to open before others.  So, a system that is not designed to correct shear induced imbalance will also in some cases further the extent of the imbalance by causing a pressure imbalance, leading to the non-simultaneous opening of gates and uneven cavity fill.

When molding full shots incorporating the optimized melt rotation manifold, the 16 pressure profiles are visibly consistent over the entire curve, which indicates a balanced pressure condition within the hot runner system. The consistency not only shows balanced cavity filling, it also indicates that the frozen gates are “breaking loose” at the same time (the initial rise of pressure is consistent across all 16 cavities).
Image 2
As part of the experimentation, the nozzle temperatures were reduced from 635ºF to 600ºF. The purpose was to measure the effect that the temperature change had on resulting cavity pressures during the molding process. After a reduction of temperature, the pressure profiles during filling remained consistent. The opening of the frozen gates also appeared to remain consistent, despite the lower temperature. The illustration shows the comparison of the pressure profiles when molding at 635ºF and 600ºF.
Image 3
Nozzle temperatures were reduced further to 580ºF, and the effects recorded. When compared to the pressure profiles attained when the nozzles were set at 635ºF, these pressure readings clearly demonstrate the effects of uneven opening of frozen gates.
Image 5
When molding full shots using an industry standard manifold design with nozzles set at 635ºF, the shear-induced imbalance created within the manifold results in obvious inconsistencies within the pressure profiles recorded with the pressure transducers. The initial rise of the profiles appears consistent, indicating simultaneous opening of gates. However, the transducer readings begin to deviate toward the end of fill, indicating hydrostatic filling conditions (certain cavities filling before others).
Image 6
When comparing the two sets of pressure profiles, (industry standard manifold design, and the optimized melt rotation manifold design, both with nozzles set at 635ºF), cavity-to-cavity pressure consistency can be more readily seen in the hot runner system that was utilizing the technology.
Image 6
To complete the comparison, the system molding with the industry standard manifold design was subject to the same reduction in temperature as the Opti-flo™ system. Nozzle temperatures were reduced from 635ºF to 600ºF. The effects of the temperature decrease were immediately evident, as the pressure profiles show. The sporadic initial rise of pressure between each cavity indicates frozen gates not opening simultaneously. This is the direct result of pressure differentials inside the hot runner system due to shear-induced imbalance.
Image 7
Nozzle temperatures were then reduced further, from 600ºF to 580º, which results in complete loss of the injection molding process. Gates open at various times, and fill rates are inconsistent, and would result in stopping production in normal operations.
Image 8
Compare the image of the pressure profiles for the industry standard manifold design (nozzles set to 580ºF) to the optimized melt rotation system pressure profiles (nozzles set to 580ºF). It is quite clear that the processing window becomes much larger as a result.  It also presents the opportunity in certain applications to reduce hot runner system temperature set points creating the opportunity for reduced cycle time.

Finally, to conclude the full shot molding study, the nozzle temperatures of the hot runner system with the industry standard manifold design were individually manipulated to simulate a pressure balance which is of course the standard practice used. After some time, and as expected an acceptable process was established, and the pressure profiles were recorded. However it is important to note in order to achieve the pressure balance for this system, the highest nozzle temperature was set to 680ºF, and the lowest was set to 560ºF. This large difference in nozzle temperatures could possibly introduce variations of material properties from cavity to cavity. Differences in shrink rates and final part dimensions could also be affected. Of particular importance is the opportunity to reduce the cooling phase of the molding cycle since it is dictated by the part or cavity that requires the most cooling and therefore is significant to the overall cycle time.
Image 9
It is well known in the injection molding community that adjusting nozzle temperatures can simulate balance within a hot runner system. However, the temperature variation needed to produce balance is quite large at times. While allowing the production of parts, a continuous and optimal molding environment is usually not achieved for long, and the goal of balanced cavity filling is compromised by longer cycles, complex and lengthy start-ups, increased scrap, inconsistent material properties, and other molding defects. Utilizing the optimized melt rotation hot runner technology, the melt delivery system is improved, opening the processing window and allowing the opportunity to improve efficiencies, reduced set-up and cycle time, and reduced scrap.

BENEFITS

As with nearly all improvements, the “silver bullet” for injection molding has yet to be invented. Experimentation with melt rotation technology has shown improvements can be achieved particularly in applications such as aggressive cycle, multi-cavity molds, shear sensitive materials, and filled resins. Benefits achieved are part quality, shorter mold qualification and initial process set up time, and increased productivity.
Image 10
The melt rotation technology is patented and owned by Beaumont Runner Technologies who also provide melt rotation technology in cold runner molds under the trademark MeltFlipperTM. INCOE Corporation is the exclusive global supplier incorporating melt rotation technology in hot runner systems under the trademark
Opti-flo TM. 


Acknowledgements:

Professor John Beaumont – Penn State University inventor of MeltFlipper® technology and the “Five Step Process”.
Image 18

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, caps/ closures, electronics, medical disposables, packaging , pharmaceutical 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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