New diecasting method is on the runner: a method of diecasting runner design that induces a restricted flow can save process costs and improve quality.

The design criteria involved in a new, restricted-flow method of diecasting runner design directly contradict some rules that many in the diecasting industry have adopted for decades without questioning. The seasoned diecaster will immediately recognize two aspects of the new runner design that do not fit into conventional wisdom.

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First, high pressure diecasting (HPDC) design rules say that the melt should experience the least possible amount of resistance up to the gate so that sufficient machine power will be conserved to inject the melt through the gates, where the smallest cross section is typically located. Second, traditional diecasters believe that an expansion of the runner following the smallest cross section will result in air entrainment.

The new runner design contradicts both of these tenants.

But when compared with traditional diecasting, the restricted-flow method improves yield, reduces porosity, enhances mechanical properties and, in some cases, reduces cycle time. And it does so with no other capital equipment than existing cold- or hot-chamber HPDC machinery, which is used without any modification.

If It's Broke, Fix It

High pressure diecasters are facing significant challenges in keeping process costs down and meeting higher standards of quality. The push to reduce costs has made diecasters more sensitive to the need to increase productivity, reduce rejection rates and improve material yield. The latter is especially significant for those casting magnesium alloys due to the associated higher recycling costs.

To answer those demands, the new runner design technique employs a constriction of the melt flow. This is done by using a much thinner runner (Fig. 1) with a tightened section (Fig. 2) that pre-conditions the melt by both shearing the flow and accelerating it well before it reaches the gate.

[FIGURE 1 OMITTED]

The restriction exhibits the smallest cross section in the runner and is followed by a gentle or nonexistent expansion in the melt flow direction. When the expansion is nonexistent, a parallel port to the melt pre-conditioner opens abruptly into a larger runner.

In most cases, a plunger velocity lower than that used with a typical HPDC runner is employed without risking cold runs, and the melt is allowed to fill the casting cavity through conventional gates at a lower velocity. This allows restricted-flow runners to yield both the higher casting quality associated with greater melt velocities and the more controlled fill front and better venting obtained with lower gate velocities.

Evidence from industry trials demonstrates that almost all machines in use today are capable of injecting melt through very small cross sections while still successfully filling a casting cavity, and a casting's mechanical properties improve with use of the restricted-flow runner, indicating that the degree of porosity is lower.

More Than Commercial Trials

The restricted-flow runner design philosophy is built on several practical scientific principles. The acceleration gained at the restricted flow section, or melt pre-conditioner, causes the metal stream to reach velocities that are up to five times those traditionally encountered at HPDC gates, which shears the metal flow and increases turbulence. Such conditioning allows diecasters to fill the casting cavity at a lower rate than in traditional HPDC by subsequently decelerating the melt and facilitates an improved venting of gases.

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As it enters the runner, the melt consists predominantly of liquid alloy, but it also contains solid and gaseous suspensions. The constriction encountered by the flow at the melt pre-conditioner shears that suspension into smaller entities (Fig. 3). Consequently, the microstructure of a casting filled through the new style of runner is finer and generally without larger alpha-aluminum grains (Fig. 4). Since the grains are finer and more closely packed, the shrinkage-prone eutectic pools are also finer and more isolated. This helps refine the shrinkage pores, which gives rise to improved mechanical properties with greater repeatability. Furthermore, finer pores, whether they are sheared bubbles or refined shrinkage, improve the X-ray quality of a casting.

In addition to refining the suspensions, the melt pre-conditioner also helps disperse the defect-forming suspensions more homogenously through increased turbulence. Higher turbulence also creates greater rates of bombardment, which together with enhanced levels of momentum aid in the further attrition of the suspensions. The increased dispersion of defect-forming suspensions also increases mechanical properties and improves their repeatability by making crack-propagation difficult.

Testing the Process

Several diecasting facilities spanning four continents have used the restricted-flow technique, and some have commenced production under license.

Table 1 shows 12 commercial castings produced in the new process during 2006. The data in the table shows that several castings of very different sizes have benefited from yield improvement when converted from HPDC to restricted-flow.

Three castings (numbers 5, 6 and 11) have benefited specifically from porosity reduction, with two of those recording significant improvements in rejection rates. Another (number 6) has gained a 12% improvement in productivity, while number 11 achieved customer elongation specifications with a restricted-flow runner not previously possible.

The vastly differing sizes, thicknesses and shapes of the die castings (Fig. 5) to which the restricted-flow concept has been applied and the different alloys in which they have been cast are a testament to its technical scope and flexibility. Further evidence of the addressable bandwidth of the new philosophy comes from the fact that parts as light as 0.44 lbs. (200 g) and as heavy as 11.02 lbs. (5 kg) have been cast with the technique. All of the castings have shown improvements in one of several areas.

Improved Yield--The smaller restricted-flow runner is the major contributor to the improvement in yield compared with conventional HPDC. However, in many cases, it was also possible to remove overflows with the new runner. This was presumably due to the associated improved venting of the cavity and the potentially improved flow conditions and filling patterns.

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For the examples presented in Table 1, casting number 11 achieved the highest savings in shot weight at 2.2 lbs. (1 kg), and the average percentage shot weight improvements were 19% for magnesium (casting numbers 1-4) and 15% for aluminum (numbers 5-12). For the production examples numbers 1-7, the metal savings ranged from 2.2 to 42 tons per year. For those castings awaiting production (numbers 8-12), savings in melting requirements will vary from 94 to 1,540 tons per year.

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For castings manufactured in high volume, yield improvements result in considerable savings in the costs of alloy material, melting and handling. Those made with magnesium alloys (numbers 1-4) will also benefit from reduced recycling costs. The smaller shot weights also enable the diecasting operations to leave a smaller footprint on the environment through a reduced requirement for energy and, consequently, greenhouse emissions.

Reduced Porosity--Shearing of gas bubbles at the melt pre-conditioner and the refinement of shrinkage pores improve the X-ray quality of the casting, as pores smaller than the resolution of the X-ray equipment do not appear in the image. Improved venting of the cavity due to the lower gate velocities in the new runner design also aids in the reduction of the total porosity content.

[FIGURE 5 OMITTED]

A reduction in rejection rate facilitated through the adoption of restricted-flow runner design enabled the transformation of an unprofitable part at a 40% rejection rate due to porosity into a profitable product (number 5). The application of the restricted-flow technology in early 2006 to this relatively low volume aluminum part dramatically reduced the rejection rate to less than 1% in production.

Improved Productivity--Since a smaller restricted-flow runner would result in less heat in the die, in some cases it is possible to reduce the time it takes to remove the necessary amount of heat before the part is ejected. This is especially tree for thin-walled parts, which usually solidify much earlier than the runners. The reduced cycle time increases process productivity.

Casting number 6 in Table 1 achieved a 12% improvement in productivity through a conversion to the new, method. The application of the restricted-flow philosophy helped reduce the shot weight from 3.24 to 2.56 lbs. (1.47 to 1.16 kg), a reduction of 2%. This meant that a substantially lower amount of heat had to be removed by the die before the casting was ejected. As a consequence, cycle time was reduced by 12%.

This gain came on top of a reduction in the rejection rate from 5% to 0.2% in production and a yield improvement from 52 to 65% when converted from HPDC.

Expanded Process Capability--As casting soundness improves with the application of restricted-flow runner design, the diecaster may be able to achieve quality specifications that were previously not possible using traditional HPDC.

The best example of this is found in casting number 11. The diecaster of this mass produced special aluminum alloy part was experiencing a significant rejection ,ate because 30% of the castings would not achieve customer elongation specifications. The problem was porosity, and the diecaster was attempting to address it by using larger runners without any success. With the introduction of the new runner, which was 2.2 lbs. leaner, the diecaster achieved the specified elongation with consistency. The rejection rate with the new runner design is expected to be close to zero.

Tackling Higher Melt Velocities

It is important to address the concerns that are traditionally associated with the use of higher melt velocities in the runner. Die erosion, soldering and flashing are three such issues that diecasters would identify, immediately. The following discussion illustrates how the restricted-flow runner design philosophy addresses each of these.

Die Erosion--Concerns in traditional HPDC stem mainly from the fact that the highest melt velocities are encountered at the gate, resulting in erosion of the gate land and the area of first contact on the cavity face. In contrast, in restricted-flow, the gate velocities are the same or lower than in conventional HPDC, so an erosion of the gate area and the casting cavity actually may be reduced.

While it has not yet been a problem during commercial production using the new runner design, some erosion may be expected over time in the melt pre-conditioner region where melt velocities are highest. Consequently, the use of a runner insert which could be replaced as required is recommended for this area. Alternatively, one of the many established coating systems, such as titanium carbide or titanium carbo-nitride, may be used. Another option is to restore the runner through weld repair.

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Soldering--No soldering has been observed at the melt restriction-zone, or melt pre-conditioner, where melt velocities are highest, or elsewhere during any of the trials or production runs. A factor that may be assisting is the very small size of the melt pre-conditioner (Fig. 2), which results in a large surface-to-volume ratio of the runner at the pre-conditioner. Consequently, the die material surrounding the melt pre-conditioner does not overheat and is less inclined to solder to the alloy. It has been shown that soldering tendency decreases in areas where the alloy solidifies at a higher rate.

Flash--Flash is actually lower with restricted-flow runners, possibly because the lower plunger velocity reduces the pressure spike experienced by the melt as the plunger comes to rest at the end of its stroke. This is another strong point of the design philosophy, as reduced trimming operations can sometimes reduce process cost significantly.

For More Information

"Computer Model Diecasting Shot Sleeves to Predict Distortion," J. Brevick, K, Narayan and A. Jain, MODERN CASTING, March 2002, p.34.

Dayalan Gunasegaram, Michel Giverd, Robert O'Donnell and Barrie Finnin, CSIRO Manufacturing and Materials Technology, Clayton, Australia

Dayalan Gunasegaram is senior research scientist, Michel Givord is research engineer, Robert O'Donnell is research manager of metallurgy capability and commercial point of contact for the new runner design, and Baffle Finnin is general manager of alley technologies for the Australian Commonwealth Scientific and Industrial Research Organization, Clayton, Australia,

Table 1. Commercial Activity Undertaken Using Restricted Flow
Diecasting Runners

No.   Alloy    Volume     Cavities   Shot weight      Yield
                          Per Die    (g Change *)   (% Change)

1      Mg      180,000       1        1,060-910       71-83
2      Mg        NA          1        1,000-810       65-81
3      Mg      350,000       2         410-310        30-45
4      Mg      80,000        1         750-680        57-67
5      AI      20,000        1       1,170-1,070      57-63
6      AI      30,000        1       1,471-1,141      52-65
7      AI      500,000       1         595-520        53-61
8      AI     4,000,000      4       1,932-1,630      50-60
9      AI     6,000,000      8       1,492-1,157      35-46
10     AI        NA          1       3,150-2,900      65-70
11     AI     1,400,000      1       5,900-4,900      40-49
12     AI      500,000       2       4,670-4,500      65-75

No.   Reject Rate                       Key benefits
      (% Change)    Yield[tons saved]     Porosity     Other

1          -             ** [30]
2          -             ** [NA]
3          -             ** [20]
4          -             ** [7]
5        40-<1           ** [2]              **
6        5-0.2           ** [11]             **        ** ([dagger])
7          -             ** [42]
8          -            ** [333]
9          -            ** [277]
10         -             ** [NA]
11         -           ** [1,543]            **        ** ([double
                                                          dagger])
12         -             ** [43]

* Changes indicate the improvement from traditional diecasting
runner design to restricted flow.

* Indicates an area in which the casting showed a significant
improvement.

([dagger]) Cycle time reduced by 4 seconds (12%), and process
setting window for the machine widened.

([double dagger]) Elongation in a key area increased from 11 to 17%.