SI24287A - Piston for cold chamber die casting - Google Patents

Abstract

A casting piston is introduced, which consists of a body and a head and is designed for more efficient cooling. The bat is a technically perfected cost-effective alternative for standard copper alloy pistons, commonly used in the foundry industry.

Description

BAT FOR COLD-CHAMBER PRESSURE-MOLDING SYSTEMS

Inventors:

Boštjan Taljat, Matjaz Meglic, Gregor Mali, Ales Brili

The object of the invention is a foundry piston important because of its simplicity of construction, which allows to reduce the total cost of the piston compared to the standard pistons, maintains or improves the possibilities of restoration and provides full flexibility to choose the desired cooling efficiency, also significantly higher than the standard pistons.

DESCRIPTION OF THE INVENTION

BACKGROUND OF THE INVENTION

The presented die-casting piston is useful for cold-chamber die-casting of aluminum and magnesium alloys, as well as for other industrial applications. The piston of the present invention is useful for many applications, but the present invention primarily deals with use in the process of cold chamber injection molding.

In cold chamber casting, the chamber is filled with molten metal, which is pushed into the tool by a piston mounted on the piston rod. The melt hardens in the tool and forms a mold with the required geometry. In this case it is mainly molten aluminum and magnesium, which are processed at relatively high temperatures. This section does not provide more detailed descriptions of the injection molding process or the specific technical characteristics of the casting system components found elsewhere.

The components of the foundry system which are in direct contact with the molten metal are exposed to high thermal loads. The plunger must push the melt into the tool and pressurize high enough for the melt to fill the tool and solidify without porosity. The piston, especially its front surface, is subjected to high thermal and mechanical stresses. The thermal stresses are due to the direct contact of the piston with the molten metal, and the mechanical stresses are due to the high pressure at the front surface of the piston during the final stages of the casting cycle. Optimal piston cooling is therefore crucial to ensure the best casting quality and improve production efficiency.

For optimum piston cooling, it is necessary to determine the cooling intensity of the individual piston parts as a function of the cycle time, in order to achieve the maximum casting quality and the shortest possible production cycle time. Cooling power depends on the type of independent variables that can be controlled. It is easily determined as the product of specific heat, mass flow rate and difference in coolant (HT) temperatures at the inlet and outlet. The specific heat depends on the HT selection, while the mass flow and supply temperature determine the setting of the HT preparation unit. The HT outlet temperature is directly influenced by the cooling efficiency of the piston, which is a parameter of the piston design and material. The cooling power of the piston therefore depends on the cooling efficiency of the piston, the specific heat and mass flow rate of the HT and the supply temperature of the HT. This cooling power is equal to the thermal energy transferred from the melt (TA) to HT in a time unit.

The cooling efficiency of a piston is therefore its ability to transfer thermal energy from TA to HT and is directly dependent on the piston construction and the materials used. It can be expressed as the ratio between the actual and maximum heat transfer from TA to HT at a given HT supply temperature and flow.

The heat transfer from TA to HT is generally determined by: (i) the temperature difference between the piston face surface and the TA, the piston face surface and the associated heat transfer coefficient, (ii) the temperature difference between the piston front surface and the piston inner surface, the corresponding distance, and the thermal conductivity coefficient (KTP) of the piston material, • · · and (iii) the temperature difference between the inner surface of the piston and the HT, the inner surface of the piston and its associated heat transfer coefficient (KPT).

The piston cooling efficiency can be improved by finding a new piston design defined by the following independent parameters: (i) internal piston surface, (ii) piston head thickness, (iii) KTP and (iv) KPT. KTP is directly dependent on the choice of piston head material, and KPT is determined by the quality of the inner surface of the piston. The inner surface of the piston and the thickness of the piston head are derived from the piston construction. Thus, the piston cooling efficiency can be improved by selecting the maximum possible inner surface and the minimum possible thickness of the piston head made from a material with high thermal conductivity.

Intensive cooling of the pistons is advantageous for achieving shorter production cycles and improving productivity, but can also lead to premature hardening of the melt before entering the tool and adversely affect casting performance and casting quality. The piston cooling must therefore be optimized over the entire casting cycle.

The foundry industry uses predominantly standard one-piece, copper-cooled copper alloy pistons (Figure 2b). Copper alloys are used because of their high thermal conductivity, which allows good heat transfer from the melt to the coolant, good sliding properties and satisfactory sealing between the piston and the chamber wall without additional sealing rings. However, such pistons are relatively expensive because of the high price of copper alloys. Simple and inexpensive rebuilding of pistons is usually used to reduce costs. When the surface of the piston in contact with the chamber is worn, the piston is either scaled to a smaller diameter or recovered by welding and turning to the initial diameter. Depending on the construction, the pistons can also be restored several times. Standard pistons are used in small-scale production, smaller piston diameters and generally for less demanding foundry applications.

Various technical solutions are used to improve the casting pistons, which are aimed at improving the cooling or thermal regulation of the pistons, sealing and extending the life by using replaceable wear parts. Such pistons are complex multi-part designs that are used for larger piston diameters for more demanding casting applications in large-scale production.

The standard pistons, however, will still retain their place in simpler and small-scale production. The present invention relates to improving the cooling efficiency of standard pistons and their cost-effectiveness by:

implementation of innovative technical solutions that significantly improve piston head cooling efficiency, optimize piston construction to allow the use of cheaper materials, while improving cooling efficiency and recoverability over standard pistons.

The solution described below fulfills all the above requirements. Figures 1 to 4 show the design of the new piston and illustrate the invention.

Description of the pictures

Figure 1: Drawing of a new injection molding piston with improved cooling efficiency and cost efficiency.

Figure 2: Comparison between (a) the new piston of the present invention and (b) the standard piston.

Figure 3: Tt diagram shows the improved cooling performance of the new piston: (a) standard steel piston, (b) standard copper alloy piston, (c) new casting piston with an Ac / A ratio _f equal to the standard piston (in both cases Ac / A _f = 0.3), (d) a new injection molding piston with an Ac / A ratio of _f = 1.4.

Figure 4: Piston dimensions important for restoration.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 shows a new piston and piston assembly with a specially marked piston body (1), a piston head (2) and an additional guide or piston. sealing ring (3). The surface of the piston head in contact with the melt is denoted by A _f and the surface of the inner wall of the plunger head in contact with the coolant is denoted by A _c . The piston body and head may be assembled in such a way that the piston head can be disassembled and replaced, also with a threaded connection (4), or be indivisible. The invention does not limit the connection between the body and the piston head. Body and piston head assembly

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it must be fitted with adequate safety features and the contact between the body and the piston head (5) must be absolutely tight. In the foundry industry, the piston and piston rod assembly (6) is expanded, as shown in Figure 1.

The piston rod (7) has an internal passage or a central opening (8) through which the piston is supplied with HT. HT delivery can be done in different ways. Two separate tubes can be installed in the center bore of the piston rod, one for the HT inlet to the piston and the other for the HT outlet. Figure 1 shows an established single pipe system (9) that is integrated into the center opening. The pipe is smaller than the center opening and is used to supply coolant (10), and return is made between the inner wall of the piston rod and the middle pipe (11). HT enters the piston through the inlet pipe, flows through the inner surface of the piston forehead and drains through the center opening.

Comparison of the invention and the standard piston shows that the new piston has improved cost and cooling efficiency while maintaining the potential for renewal. Figure 2 shows a comparison of the new casting piston (Figure 2a) and the standard piston (Figure 2b). The plunger head (2) is made individually and then attached to the body (1), the connection being easily disassembled or disassembled. The surface A ', which is crucial for the transfer of thermal energy between TA, or piston, and HT, can be significantly larger than that of the standard piston, Δ?, Which improves the cooling efficiency of the piston. In this case, the piston head has two functions: (i) care for sliding and sealing between the piston and the chamber as for a standard piston, and (ii) transfer heat between TA and HT.

The proposed implementation has the following advantages:

The piston cooling efficiency is significantly improved compared to the standard piston as it increases the surface area of the piston in contact with HT (A _c > A _c ^s ) and can be easily fabricated with the proper geometry of the inside of the piston head. The piston head is mechanically machined before assembly with the body.

The choice of different materials for the piston body and head. The head can be made of copper alloy as with standard pistons, and the body is made of low alloy steel. This allows savings to be made for pistons, especially for larger diameter pistons.

The piston can be renewed by replacing the head, or renewed by the same procedure as for standard pistons, ie by turning to a smaller diameter or by welding and turning to an initial diameter.

The piston cooling efficiency depends on the materials used and the internal surface of the piston, A _c , which is in contact with the coolant. The larger surface A _c of the piston of the invention is achieved by a special geometry on the inside of the piston head. Figure 2 (a) shows ribs in the form of concentric circles. The ribs can be of any shape and section geometry. The height of the rib h _r is greatest at h _{r max} = l _x - hi, and the piston head is in this case made of a disk of diameter D and thickness Ij. The fins also provide mechanical strength to the piston heads, since they are in contact with the coolant and are therefore cooler than the rest of the piston head (their temperature is approximately equal to the HT temperature). Therefore, it is possible to either: (i) reduce h! _min at unchanged d _c , or (ii) increase d _c at unchanged h ₂ min ·

With the piston head material selected, the cooling efficiency can also be expressed as an Ac / A _{f function} . The coefficient is useful for directly comparing the cooling performance of pistons of different design solutions and dimensions. This ratio is 0.2 oz for small and large pistons. 0.4. The ratio of the piston of the present invention is increased by a factor of five, and Ac / A _f thus reaches a value of up to 1.0 for smaller diameter pistons and 1.5 for larger diameter pistons.

Figure 3 shows the temperature T _MM at the center below the piston face, normalized by the temperature TA, as a function of the time tv of one casting cycle t _c for: (a) standard steel piston, (b) standard copper cast piston, (c) new piston for die-casting with an Ac / Af ratio equal to that of the standard piston (Ac / A _f = 0.3); and (d) a new die-casting piston with a ratio A _c / A _f = 1.4. The new piston was made of low alloy steel (body) and copper alloy (head) for the purposes of this study. The results show that the standard steel piston is the least efficient in transferring heat from TA to HT (curve a). The temperature below the front surface of such a piston is higher throughout the cycle than that of other pistons. The standard copper alloy piston has a lower temperature than the new piston with the same geometry (see curves b and c). The reason is that the heat transfer from the surface A _f to the cylindrical part of the piston body and from the body to HT is better with a standard copper alloy piston than with a new steel body piston. The latter is due to the fact that the inner surface of the piston A _c , where heat is transferred to HT, consists of the inner surface of the piston forehead and part of the inner surface of the cylindrical lateral side wall of the piston, together A _c ^s . The construction of the new piston enables the production of a substantially larger surface A _c than the total piston surface in contact with HT at the standard piston (a /), so that in this case the described heat transfer to HT via the inner cylindrical wall becomes negligible. The piston can therefore be designed for significantly better cooling efficiency and shortening of the casting cycle compared to the standard piston (curve d). In the area between curves c and d in Figure 3, the cooling efficiency of the new piston can be arbitrarily chosen at virtually no additional manufacturing cost.

Figure 4 shows the piston dimensions that are important for restoration. This piston can be restored in three different ways: (i) by replacing the piston head as a critical part subject to wear, erosion, thermal fatigue and other fracturing mechanisms, (ii) by turning the piston to a smaller diameter, and (iii) s turning and welding the piston to its initial diameter. The thickness of the front wall of the hi _min piston is determined by the material's resistance to thermal and mechanical stress during the pressure phase of the casting cycle. The front wall of the piston may be thicker, hi to allow more piston restoration, but this will reduce the cooling efficiency. If the pistons are cut to a smaller diameter, the thickness of the guide ring and the distance from the piston surface to the connection between the head and the piston body h _{2max is} selected on the basis of the intended piston diameter reduction. In this case, the piston diameter can be reduced from the initial diameter D to the smallest possible diameter D _min .

It should be noted that the reduction of the piston diameter upon restoration affects both the change in the parameters of the foundry machine and the cooling efficiency of the piston, since the ratio Α, / Af changes in the restoration. The preferred method of restoration is therefore by turning and welding to the initial diameter.

The piston head in contact with the chamber wall at length l _{1 (} provides sliding and sealing. For a proper straight motion of the piston in the chamber, a guide ring (3) is installed at a distance of l ₂ from the piston head, or several guide rings are installed. the ring reduces wear and extends the life of the piston head.The width of the guide ring l ₃ must be large enough to guide the piston.The copper alloy ring can be welded into the groove on the piston body.The invention does not limit the technology of making guide rings or the materials used.

The foundry piston presented is important because of the simplicity of its construction, which allows the total cost of the piston to be reduced compared to standard pistons, maintains or improves restoration possibilities and provides full flexibility to choose the desired cooling efficiency, also significantly higher than the standard pistons.

Claims (10)

PATENT CLAIMS (1) A piston pushing a melt from a casting chamber into a tool or mold using a straight line movement through the central bore of a casting chamber of a die-casting machine and including: an external cylindrical surface facing the inner surface of the casting chamber, with the face (A _f ) in contact with the metal melt (TA), and an opening or other geometric shape at the rear of the piston for its attachment; and is characterized by: that the piston consists of a piston body, a piston head and a guide ring, that the piston body (1) with the outer cylindrical surface is in contact with the casting chamber, the cylindrical central bore with the surface in contact with the coolant and with the piston, the mechanisms for securing the piston body to the piston rod and to attach the piston head to the piston body, which may be threaded but not exclusively limited to the thread; that the piston head (2) with the outer cylindrical surface is in contact with its height (IJ with the casting chamber, the front surface (A _f ) in contact with the metal melt, the rear surface (5) is in contact with the piston body, the inner surface (A _c ) in contact with coolant, and a mechanism for securing the piston head to the piston body, including but not limited to the thread; that the inner surface of the piston head (A _c ) is a plane with the addition of special geometric elements that contain, but are not exclusively limited to, ribs of rectangular cross section in the form of one or more concentric circles or in the form of spirals; that the guide ring (3) with the outer cylindrical surface is in contact with the casting chamber and is positioned on the opposite side of the piston body relative to the position of the piston head; that the guide ring (3) of the cylindrical element is attached to the outer cylindrical surface of the piston body, or is manufactured by a process that may include but is not limited to welding technology; (2) The piston according to claim 1, characterized in that the inner surface of the piston head is: plane limited by a circle with a diameter d _c which is smaller, equal to, or greater than the diameter of which is made a mechanism for attaching the body of the piston to the piston rod; any three-dimensional surface bounded by the height of the piston head l _x , and constructed above a plane bounded by a circle of diameter d _c ; any three-dimensional surface without height restriction, and constructed above a plane bounded by a circle of diameter d _c . (3) A piston according to claim 2, characterized in that the plane diameter (d _c ) and / or the shape of the three-dimensional surface is determined by the required cooling efficiency of the piston, which depends on the ratio of the inner surface of the piston head (A _c ) to the surface of the piston forehead (A _f ). (4) A piston according to any one of the preceding claims, characterized in that: it is attached to a piston rod; that coolant flowing from the inlet pipe or passage in the piston rod comes into direct contact with the inner surface of the piston head (A _e ); that in determining the shape of the inner surface of the piston head, which determines the required cooling efficiency of the piston, the influence of the shape and orientation of the coolant mass flow rate is also taken into account; that the coolant, after being in contact with the inner surface of the piston head, drains back into the drain pipe or passage in the piston rod. (5) The piston according to any one of the preceding claims, characterized in that the piston head is attached to the piston body by a mechanism which, according to its mechanical characteristics, is suitable for this application, allows sealing between the piston head and the piston body, permits disassembly of the piston head from piston bodies, and includes a threaded connection or bayonet joint, but is not limited to the specified union types. (6) A piston according to claims 1 to 4, characterized in that the piston head is permanently, without disassembly, attached to the piston body. (7) The piston according to claim 5 and 6, characterized in that the mechanism for fastening the piston head to the piston body is designed so that reducing the diameter of the piston (D) to a smaller diameter (D _min ) does not affect the integrity of the mechanism or reduce its sealing capacity . (8) Piston according to any one of the preceding claims, characterized in that the piston head and / or guide ring are made of a different material from the piston body. (9) Piston according to any one of the preceding claims, characterized in that more than one guide ring is attached to the piston body. (10) The piston according to claims 1 and 8, characterized in that the piston does not have a guide ring.