Influence of Gating System Parameters of Die-Cast Molds on Properties of Al-Si Castings

Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ( https://creativecommons.org/licenses/by/4.0/ ).

The resulting quality of castings indicates the correlation of the design of the mold inlet system and the setting of technological parameters of casting. In this study, the influence of design solutions of the inlet system in a pressure mold on the properties of Al-Si castings was analyzed by computer modelling and subsequently verified experimentally. In the process of computer simulation, the design solutions of the inlet system, the mode of filling the mold depending on the formation of the casting and the homogeneity of the casting represented by the formation of shrinkages were assessed. In the experimental part, homogeneity was monitored by X-ray analysis by evaluating the integrity of the casting and the presence of pores. Mechanical properties such as permanent deformation and surface hardness of castings were determined experimentally, depending on the height of the inlet notch. The height of the inlet notch has been shown to be a key factor, significantly influencing the properties of the die-cast parts and influencing the speed and filling mode of the mold cavity. At the same time, a significant correlation between porosity and mechanical properties of castings is demonstrated. With the increasing share of porosity, the values of permanent deformation of castings increased. It is shown that the surface hardness of castings does not depend on the integrity of the castings but on the degree of subcooling of the melt in contact with the mold and the formation of a fine-grained structure in the peripheral zones of the casting.

Die casting is a casting technology where molten metal is fed out of a mold loading cavity under high pressure and at high speed to a shaping cavity of a permanent mold. There it solidifies and the final cast is thus produced. The rate of a plunger acting upon the melt ranges within units of meters per second. By means of its action, the melt is fed out of the loading chamber through the gating system into the mold cavity. The transition passage between the gating system and the mold cavity is represented by a gate [ 1 ]. In the ingate, the flowing speed of a melt increases and reaches tenths of meters per second. The high flowing speed of the melt causes a rather short period of the mold cavity loading, which equals units and tenths of milliseconds. The method of the mold cavity loading allows the production of thin-walled casts in the correct shape and with high dimensional accuracy and exact copying of the surface relief of the mold cavity [ 2 ]. The final result in designing and structuring the gating systems for the casts produced by die casting technology is a cast showing adequate mechanical and qualitative properties. The gating system must assure rapid and continual loading of the shaping cavity of the mold. The correct structure of the gating systems can shorten the casting cycle duration, reduce the rejection rate, and positively influence the cast macrostructure that directly affects mechanical properties. Porosity and homogeneity correlate with strength characteristics. Final homogeneity is influenced mostly by the structure of the gate. Inside the gate, modulation of the melting occurs along with increase of flowing speed of the melt by which the shaping cavity of the mold is loaded. The mode of the mold cavity loading and the flowing speed in the gate determines the character of the final properties of the cast [ 3 ]. This article addresses the design of parts of the inlet system and the impact of modifications of individual factors on the mechanical properties of castings. The introduction presents the methodology for the design and calculation of inlet systems for castings under pressure. Using the NovaFlow&Solid program, simulations are performed to verify a suitable design solution of the inlet system to cast the electric motor flange and its shaping. The determination of the optimal geometry of the inlet notch is performed based on the assessment of the influence of the height, inlet notch on the selected mechanical properties and the porosity. At the same time, this article describes the mutual correlation of mechanical properties and porosity. It is shown that with the increasing proportion of porosity, the values of the monitored mechanical properties decrease.

2. Design Methodology of Gating System of Die Casting Mold for Die Casting Metals

When designing the inlet system, it is necessary to consider the technology as interconnected components of one complex. Therefore, one must focus primarily on the flow analysis of the melt in the gate. It is important to select the most suitable position for placement of the entrance and the venting system. Subsequently, it is possible to proceed to the solution and calculation of the maximum time of mold cavity filling, analysis of flowing speed of the melt in the gate, determination of the flow volume, determination of the gate dimensions, calculation of the cross section of the gating channel and determination of its shape [4,5].

2.1. Analysis of Flowing of the Melt in Ingate and Selection of the Most Suitable Position for Placement of the Ingate and of the Venting System

The ideal shape of the cast allows flowing of the melt inside the mold cavity along the distinct and direct paths. However, such an ideal shape can only be rarely designed, especially in gating channels and ingate. When creating, both the technical and foundry perspectives must be taken into consideration. A designer is consequently forced to search for an adequate compromise between the required and the ideal shape and suggestions and thus find a better way for the molten metal to flow. All well-known alloys utilized in the foundry industry tend to shrink during solidification and cooling. Unless this property is considered when the mold is designed, the final die-cast parts shall be defective due to shrinking occurring in the course of solidification. The defects shall have cavities in the die-cast part volume (higher porosity) and sinks of diverse sizes [6]. As the shaping mold cavity lacks risers, the high-pressure die casting represents an exception among foundry technologies. Shrinking is eliminated by resistance pressure. Therefore, the gating system must be designed so that the molten metal can transfer the force as long as possible and with minimal losses. The designer must consider pressure gradient and processes occurring inside the mold cavity, from the gate up to vents. It is convenient and applicable when the gating system is designed with the ingate placed in the dividing plane of the mold opposite the venting system. A suitable solution is to identify the pouring gate and vents so that the molten metal flowing inside the shaping mold cavity goes along the shortest trajectories. If possible, in designing the gating channels, the case of two different jets of injected metal encountering in front of the gate should be avoided. This situation is undesirable and cannot be eliminated at all times. In the case of such a situation, the ingate should be placed from inside of the die-cast part. The drawback of the structure of the central gating system is the fact that multiple cavities are not allowed and the extremely long form of the gating system causes a decrease of the speed of the molten metal flow before its entrance into the mold cavity [7,8].

2.2. Calculation of Maximal Time Mold Cavity Filling

The die-cast part should be designed to assure sufficient area for placement of the ingate and the venting channels [9]. The width of the ingate shall be achieved when the ingate area is divided by its height. The ingate area depends on selecting the period of the mold cavity loading and the melt flowing speed in the ingate. The mold cavity loading period is determined based on the following: (1) Thinnest walls of the die-cast part, i.e., thick walls allow longer loading periods, contrary to thin walls as those tend to get solidified prematurely. For that reason, die casting of the thin-walled die-cast parts require a shorter period of mold cavity loading. At the same time, the flowing length must be taken into consideration. If the die-cast part contains thin walls with large areas or the thin walls are placed in a considerable distance from the ingate, the period of the mold cavity loading must be shorter [10]. (2) Thermal properties of alloys and materials, i.e., temperature of liquid, range of solidification and thermal conductivity of mold material. These materials influence the period of solidification. (3) Combination of a die-cast part volume and fins, i.e., thin-walled die-cast parts, die-cast parts with a long trajectory of the melt flowing through the mold cavity and die-cast parts with special requirements regarding quality need larger fins. The condition is justified by the fact that higher metal volume can preserve the required temperature for a longer time. (4) Permitted percentage ratio of metal solidification during loading, i.e., in the case of higher surface quality requirement, it is necessary to preserve the melt with lower ratio of solidification and shorter period of the mold cavity loading.

Maximal time of mold cavity filling t [11]:

 t=K.{Ti−Tf+S.ZTf−Td}.T (s)

K—empirically derived constant related to mold conductivity,

T—the lowest characteristic average thickness of the die-cast part wall (mm),

Tf—liquid temperature (K),

Ti—melt temperature in the ingate (K),

Td—temperature of the mold cavity surface prior to pressing (K),

S—solidification percentage at the end of loading,

Z—conversion factors of stable units connected with the range of solidification.

The K constant gains the following values:

• 0.0312 s/mm between steal AISI P-20 (nitrated steal) and zinc alloys,

• 0.0252 s/mm between steal AISI H-13 (alloys of steal and chromium) and AISI H-21 (alloys of steal, chromium and wolfram) and alloys of magnesium,

• 0.0346 s/mm between steal AISI H-13 and AISI H-21 and alloys of aluminum and brass,

• 0.0124 s/mm between alloys of wolfram and magnesium, zinc, aluminum and brass.

presents permitted values of material solidification depending on the wall thickness.

Table 1

Wall Thickness [mm]Permitted Values of Material S (%)AluminiumMagnesiumZinc<0.85105–150.8–1.255–255–1510–201.25–215–3510–2515–303–220–5020–3520–35Open in a separate window

The Z constant gains the following values:

• 4.8 °C/% for alloys of aluminum ASTM 360, 380 a 384, all sub-eutectic alloys

• AlSi (Cu/Mg) containing less than 12% silicium,

• 5.9 °C/% for alloys of aluminum ASTM 390, supra-eutectic alloys AlSi (Cu/Mg),

• 3.7 °C/% for magnesium alloys,

• 3.2 °C/% for zinc alloys,

• 4.7 °C/% for brass [ 12 ].

2.3. Calculation of Flowing Speed of the Melt in the Ingate

The flowing speed of the molten metal in the ingate influences the mechanical properties of the die-cast part and the quality of its surface. New high-pressure die casting machines can generate a speed of up to 100 m·s−1, yet degradation of the mold commences at approximately 40 m·s−1. Thus, choosing the speed within the range from 40 up to 100 m·s−1 is rather impractical. Porosity caused by the bonding of gas in the die-cast part volume can be decreased without extreme increase of speed by designing the gating system and ingate so that avoidance of shocks and consequent reversible flowing and mixing of the melt is assured. Flowing of the melt through the gating system must be continuous. The reversible flowing effort can be made when the trajectory of melt flowing contains lugs, sharp direction changes or incorrectly reduced diameters [13]. presents recommended values of the flowing speed of the melt in the ingate.

Table 2

Type of AlloyRecommended Values of the Flowing Speed of the Melt in the Ingate (m·s−1)Standard CastingVacuum CastingAluminium20–6015–30Zinc30–50 Magnesium40–60 Copper20–50 Open in a separate window

The flowing speed of the melt in the ingate v1 can be determined according to the following formula:

v1=mcp.t.dch.0.785(m.s−1)

mc—the weight of cast (kg),

p—density of alloy (kg·m−3),

dch—diameter of filling chamber (m).

2.4. Determination of the Flow Volume

The flow serves as a heat accumulator and as a tank of low-quality oxidized metal. The flows are necessary for thin walls of the die-cast part or if the die-cast part must get solidified at a higher temperature [14]. An example is the circumfusing of the cores placed at a considerable distance from the ingate. The melt flows around the core through narrow walls from both directions, and adequate temperature must be assured to achieve a unified and firmly joined bond. presents recommended flow volumes for conventional die casting machines depending on the lowest thickness of the wall.

Table 3

Characteristic Wall Thickness of a Gate Segment (mm)Flow Volume, Percentage Ratio Out of Segment VolumeFor High Surface QualityFor Lower Surface Quality0.90150%75%1.30100%50%1.8050%25%2.5025%25%3.20--Open in a separate window

2.5. Determination of the Ingate Dimensions

Dimensions of the ingate depend on the method of connection of the ingate to the cast. shows a scheme of the method of projection of the ingate connection to the cast with cylindrical area.

AI=Gρ.t.vI (m2)

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G—sum of weight of die-cast part and of the weight of flows (kg).

Ingate length a:

a=2.ρ.R.α360=2.π.R.60360=π.R3 (m)

R—die-cast part radius (m).

Ingate height b:

b=AIa (m)

2.6. Calculation of the Cross Section of the Gating Channel and Determination of Its Shape

The section of the gating channel is trapezoidal, with the slope of the walls ranging between 10°–15°. The channel height to width ratio should vary within the scale from 1:1 up to 1:3. The standard ratio to be selected is 1:2. Calculation of the channel cross section depends on the diversity of the mold. The areas of the cross sections of the channels are mainly influenced by the branching of the media [15]. When the channel is divided into a branching, its total cross section should be increased by 5–30% after each dividing in the direction from the ingate towards the tablet. The procedure in calculation begins with the design proposal of diameters of the channels in the direction from the ingate. Area of the gating channel A:

A=n.2.AI (m2)

Cross section of the gating channel is show in .

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Height of the gating channel CT:

A=CB.CT−CT2.tg(90°−α)=2.CT2−CT2.tg(90°−α)

CT=A2−tg(90°−α) (m)

α—angle of wall inclination of the gating channel [°].

Width of the gating channel CB:

CB=2.CT (m)