Low pressure casting method, low pressure casting machine and use of low pressure casting machine

Abstract

The inventio relates to a low pressure die casting method, comprising, preferably in the given time sequence, the steps of: holding a molten metal (6) in a holding furnace (8) at a certain holding furnace level (10), optionally pre-cooling a casting mold (4), pre-pressuring the molten metal (6) in the holding furnace (8) starting from a certain pre-pressure application starting time at a certain pre-pressure application speed to force the molten metal (6) from the holding furnace (8) through a riser tube (12) to the casting mold (4), and injection of the molten metal into the casting mold (4) under low pressure, wherein a certain overall pre-pressure time is preset by varying the pre-pressure application speed depending on the holding furnace level (10) and / or the pre-pressure application starting time depending on the holding furnace level (10). The invention also relates to a low pressure die casting machine and a use of a low pressure die casting machine.

Description

[0001] Low pressure die casting (LPDC), or low pressure casting, is a metal casting process that falls between high-pressure die casting (HPDC) and gravity die casting in terms of pressure used. LPDC relies upon pressurization of the molten metal from which a component is to be cast before feeding the molten metal into the casting mold, also called die. Commonly, pressurization levels of up to 0.8 bar are used to feed the molten metal into the casting mold. The relatively low pressure required to lift the metal into the casting mold is sometimes followed by the application of a higher pressure stage in an effort to reduce any porosity in the casting. During this stage, the pressure might be raised to 1 bar.

[0002] Usually, the casting mold or mold is placed at, or above, the level of the metal being poured. In sealed systems for LPDC, a furnace, also named holding furnace, that can be sealed and pressurized is used to maintain the molten metal at a constant temperature and composition until it is ready to be injected into the mold. Before filling the molten metal into the holding furnace, the desired metal or metal alloy from which a cast component will be produced is melted in a separate furnace, i.e., the melting furnace, ensuring it has the correct temperature and composition for casting.

[0003] In the holding furnace, usually one or more hollow tubes connect the top (outlet) of the furnace and extend down into the molten metal. The mold is placed on top of the furnace, the melt in the furnace is pressured and the molten metal is forced up through hollow tubes into the mold. The one or more hollow tubes are also referred to as stalks or riser tubes and make up, optionally among further system parts, the system guiding the molten metal from the holding furnace to the casting mold.

[0004] Low-pressure (LP) casting is mainly used for the manufacture of complex, high-quality components such as aluminum alloy engine blocks and suspension components for cars, cylinder heads, aluminum wheels, heat sinks for electronic components, pump housings, impellers, and even golf club heads. LP casting is a favored manufacturing technique across various industries, notably automotive and aerospace, and widely used for the casting of components which require good integrity and good cosmetic appearance when finely machined or polished. This is because the slow, substantially turbulence-free entry of liquid metal into the mold at the relatively low pressures used in LPDC reduces porosity due to trapped gasses and solidification shrinkage. This controlled process also allows for the production of complex geometries, including relatively thin walls and intricate details. With reduced machining requirements, improved metallurgical properties, and compatibility with automation, low-pressure casting emerges as a versatile and valuable manufacturing technique.

[0005] In addition to the benefits of pressurized mold filling, low-pressure casting dies usually contain integral cooling passages that aid in controlled cooling, further enhancing mechanical properties. Commonly used materials for LPDC are, among others, aluminum (Al), magnesium (Mg), copper (Cu) or Zinc (Zn) or alloys of the mentioned metals.

[0006] Producing a casting part by LPDC typically starts with melting the metal alloy from which the component is to be cast in a furnace. Heating continues until the metal alloy reaches the appropriate temperature for casting. The appropriate temperature for casting depends on various factors. One key factor is the melting point of the specific alloy composition being used.

[0007] Once the molten metal reaches the appropriate casting temperature, it is transferred to a holding furnace, which is usually positioned beneath the mold. This holding furnace helps maintain the molten metal at the required temperature and ensures a continuous supply of material to the mold for the casting process.

[0008] Pre-cooling or pre-chilling the casting mold represents another possible step in the LPDC process before injection of the molten metal into the mold and ensures that the casting mold is at the desired temperature when the metal is introduced. Pre-cooling contributes to controlling the solidification of the molten metal and improving the overall quality of the casting. Typically, the casting mold is cooled using a cooling medium, such as water or a water-based coolant. The cooling medium usually circulates through channels within the casting mold to absorb heat and lower the mold's temperature. The duration of pre-cooling and the temperature to which the casting mold is cooled are optimized based on various factors.

[0009] For casting the component in the mold, the molten metal held in the holding furnace must first be transported to the exit or outlet of the holding furnace. Typically, the holding furnace is connected to the casting mold via at least one riser tube, which in turn is connected to a gating system, which connects the at least one riser tube to the entry of the casting mold or mold cavity. Further, a sprue, which is a passage or channel through which the molten metal flows into the casting mold and typically is a part of the gating system, may be positioned at the end of the gating system and at the entry to the casting mold. During the first pressure stage or pre-pressurization stage of the LPDC process, the molten metal in the holding furnace is forced through the riser tube until reaching the exit of the holding furnace by applying low pressure, typically a pressure below 0.8 bar.

[0010] The distance the molten metal must travel depends on the holding furnace level, i.e., the filling level of the molten metal in the holding furnace and the riser tube and further on the length of the riser tube. Thus, for different holding furnace levels, a different distance must be overcome by the molten metal to be injected into the casting mold. At the end of the riser tube, which represents the exit of the holding furnace, a valve or similar element can be arranged that can be opened to discharge the molten metal from the holding furnace and riser tube into the gating system.

[0011] Next, in the following second pressure stage, the molten metal is then forced through a system of tubes called gating system guiding the molten metal to the entry of the casting mold using again low pressure, typically again a pressure below 0.8 bar. In this first two pressure stages, the pre-pressurization stage or pre-pressure stage until reaching the exit of the holding furnace and the second pressure stage for forcing the molten metal through the gating system, the pressure is increased at a relatively fast rate.

[0012] When the molten metal reaches the casting mold or die cavity, the next pressure stage is entered and the rate of pressure increase is reduced to avoid excess free-surface turbulence, thereby limiting air and oxide film entrainment. In this next or third pressure stage, the casting mold is filled with the molten metal. Once the casting mold is fully filled with the molten metal, the fourth pressure stage is entered, and the pressure is quickly increased to an over-pressure. In the following stage, the fifth pressure stage, the over-pressure is held for a certain time until the component to be cast is solidified.

[0013] The molten metal begins to cool as it enters the casting mold and starts to solidify once it gets down to the liquidus temperature. The solidification process takes place under constant pressure in the fifth pressure stage, which ensures complete filling of the mold as the metal cools and contracts or shrinks. This controlled solidification reduces the formation of defects and ensures a uniform structure within the cast component. After the metal has solidified within the casting mold, the pressure is reduced in the sixth and last pressure stage. The casting mold, now containing the solidified cast component, is allowed to cool down. Once sufficiently cooled, the mold is opened, and the finished casting is removed.

[0014] In LPDC, pre-pressure application or pre-pressurization in the first pressure stage and the pressure application in the second pressure stage is crucial to overcome resistance within the casting system, including the riser system and the gating system, and helps establishing a consistent and stable flow of molten metal to the casting mold.

[0015] Resistance can be caused by factors such as the length and complexity of channels through which the molten metal flows. As a key aspect of the optimization and control of the low-pressure die casting process, pre-pressurization allows for the establishment of conditions that lead to a smooth, stable, and controlled injection of molten metal into the casting mold, contributing to the production of high-quality castings.

[0016] Here, pre-pressurization or the first pressure stage is used as a term to designate the pre-pressure application for forcing the molten metal of the holding furnace at least through the riser tube to the exit of the holding furnace level at the end of the riser tube and before entering the gating system leading the molten metal further to the entry of the casting mold. Optionally, pre-pressurization can include also the pre-pressure application to force the molten metal all the way until reaching the entry of the casting mold.

[0017] In order to achieve the highest possible quality of the part to be cast, it is necessary to determine and optimize the parameters of a low pressure die casting process in advance. In particular, predetermining the parameters of the pre-pressurization helps in specifying various further parameters of the LPDC process. For example, knowing the pre-pressurization parameters in advance enables programming of pre-cooling of the casting mold.

[0018] However, unlike the pressure application time for the second and the following pressure stages, the necessary pre-pressurization application time differs depending on different holding furnace levels, i.e., the amount of molten metal contained in the holding furnace. This is because for example for a full holding furnace, less time is needed to transport the molten metal to the end of the riser tube or exit of the holding furnace at the same pre-pressure application speed as the distance to travel through the riser tube is already shorter for the molten metal. Consequently, this does not allow to program pre-cooling of the casting mold in advance as the time needed for pre-cooling would then also differ based on the holding furnace level. This in turn causes the LPDC processes for different holding furnace levels to be unstable with respect to productivity and / or reliability.

[0019] Known LPDC processes or methods to control a LPDC process are based on regulating the pressure applied during pre-pressurization based on measuring the actual applied pressure and comparing it to the desired pressure pattern or curve. On the other hand, control methods for a LPDC process are also known in which the holding furnace level is kept constant so that the moving distance of the molten metal from the holding furnace to the entry of the casting mold is kept constant. The known approaches, however, do not help in predetermining the pre-pressurization process in advance depending on the holding furnace level.

[0020] Therefore, the present invention is faced with the problem of providing an improved low pressure die casting method and a corresponding low pressure die casting machine, which enable increasing the stability of the LPDC process, in particular with regard to the pre-pressure application process.

[0021] According to a first aspect of the present invention, said problem is solved by a low pressure die casting method, comprising, preferably in the given time sequence / order, the steps of: holding a molten metal in a holding furnace at a (certain) holding furnace level, optionally pre-cooling a casting mold, pre-pressuring the molten metal in the holding furnace starting from a certain pre-pressure application starting time at a certain pre-pressure application speed to force the molten metal from the holding furnace through a riser tube to the casting mold, and injection of the molten metal into the casting mold under (low) pressure, wherein a substantially constant overall pre-pressure time is preset, wherein, by varying the pre-pressure application speed and / or the pre-pressure application starting time, the preset substantially constant overall pre-pressure time is substantially met.

[0022] In this way, by presetting the overall pre-pressure time, it is possible to keep the pre-pressure time substantially constant for different holding furnace levels. Further, this way, it is possible to calculate the overall pre-pressure time in the first place. Additionally, further steps of the low pressure die casting method, such as for example a pre-cooling step of the mold, can be better coordinated based on the calculated or preset overall pre-pressure time. For instance, pre-cooling can be programmed in advance depending on a holding furnace level or a pressure curve of a casting process.

[0023] Further, the pre-pressure applications for different holding furnace levels can be coordinated and predetermined, so that, in relation to the start of the overall pre-pressure process, injection of molten metal starts at the same point in time for different holding levels. This leads to a more stable and predictable low pressure die casting process.

[0024] In particular, when casting two or more components consecutively, thanks to the present invention, the overall pre-pressure time can be kept constant for the casting processes of the two or more components by varying the pre-pressure application speed and / or the pre-pressure application starting time depending on the respective holding furnace level, whereby the holding furnace levels of the two or more casting processes can be different.

[0025] The pre-cooling or pre-chilling of the casting mold may occur shortly before or during injection of the molten metal into the casting mold. Typically, the molten metal may injected into the casting mold under low pressure in the range of 0.1-1.0 bar, in particular, around 0.7 bar. The optional step of pre-cooling the casting mold before the injection of molten metal may help to control the solidification process and improve the casting quality.

[0026] The overall pre-pressure process or application and thus the overall pre-pressure time may include a waiting time before pre-pressure application starts at the pre-pressure application starting time. During the waiting time, the molten metal is held inside the holding furnace without applying pressure to it.

[0027] Preferably, if the overall pre-pressure time comprises a waiting time, the overall pre-pressure time is the sum of the waiting time and the time the pre-pressure is applied, i.e. the time difference between the pre-pressure application starting time and the pre-pressure application end time.

[0028] Preferably, if the overall pre-pressure time does not comprise a waiting time, the overall pre-pressure time corresponds to the time the pre-pressure is applied, i.e. the time difference between the pre-pressure application starting time and the pre-pressure application end time.

[0029] The overall pre-pressure time can be calculated and predetermined for different holding furnace levels by varying the pre-pressure application speed and / or the pre-pressure application starting time depending on the holding furnace level.

[0030] Preferably, during pre-pressure application, the pre-pressure application speed is kept constant or stable, i.e., there preferably is no acceleration of the pressure application speed.

[0031] The pre-pressure application speed may in particular be a pre-pressure application rate, e.g. the rate by which the pressure on the molten metal in the holding furnace is increased. For example, the pre-pressure application speed may comprise the unit pressure per time, e.g. mbar per second.

[0032] The molten metal is preferably a molten metal alloy. For example, the molten metal alloy can be an aluminum (Al) alloy or a magnesium (Mg) alloy. The step on injection of the molten metal into the casting mold under low pressure may include using a plunger or piston. The controlled injection ensures proper mold filling.

[0033] It is possible, that the holding furnace level is known or given in advance of the low pressure die casting process or can be determined for example by knowing the amount of molten metal and the holding furnace geometry. Further, the holding furnace level can be sensed using a sensor, such as a fill-level-sensor in or at the holding furnace, set up accordingly.

[0034] Preferably, the riser tube fluidly connects the holding furnace and the casting mold. In between the riser tube and the casting mold a gating system may be positioned, through which the molten metal must be forced in order to be injected into the casting mold. The gating system may consist of channels, runners, gates, and other parts that direct and regulate the flow of molten metal during the casting operation. Further, the gating system may be designed in a way to ensure that the molten metal is delivered to the casting mold in a way that minimizes turbulence and defects while achieving complete and uniform filling.

[0035] The step of pre-pressure application to the molten metal in the holding furnace may lead to overcoming any resistance in the system, ensuring a controlled and stable filling of the mold.

[0036] The above-described low pressure die casting method may further include the following steps: Creation of a casting mold or die, preferably out of metal, typically steel, which is a pattern or mold that defines the final shape of the casting.

[0037] Preparation of the casting mold for the casting process by coating it with a refractory material to withstand the high temperatures and pressures during casting.

[0038] Preparation of the molten metal to be used for casting by melting the desired metal or metal alloy in a separate furnace, i.e., a melting furnace, ensuring it reaches the correct temperature and composition for casting.

[0039] Transferring the molten metal to the holding furnace, where it is maintained at a consistent temperature and composition until it is ready to be injected into the mold.

[0040] Solidification of the molten metal in the mold cavity so that it begins to take the shape of the die or casting mold. Once solidification is complete, the casting mold may be allowed to cool, and the casting solidifies further.

[0041] Opening of the mold, revealing the solidified cast component inside.

[0042] Trimming off excess material of the cast component and application of finishing processes, such as machining or surface treatments, to the component to meet the required specifications.

[0043] According to a second aspect of the present invention, said problem is further solved by a low pressure die casting machine, comprising a casting mold for casting components out of a molten metal, a holding furnace for holding the molten metal at a certain holding furnace level, a riser tube fluidly connecting the holding furnace to the casting mold, and a pressure control system set up to apply pre-pressure to the molten metal in the holding furnace to force the molten metal through the riser tube, whereby the pressure control system is further set up to meet a substantially constant overall preset pre-pressure time by varying the pre-pressure application speed and / or the pre-pressure application starting time depending on the holding furnace level.

[0044] According to a third aspect of the present invention, said problem is further solved by the use of the afore-described low pressure die casting machine or an embodiment thereof in the afore-described low pressure die casting method or an embodiment thereof.

[0045] The advantages and technical effects described above for the low pressure die casting method according to the first aspect of the present invention apply correspondingly also to the low pressure die casting machine according to the second aspect of the invention or an embodiment thereof and to the use of the afore-described low pressure die casting machine or an embodiment thereof in the afore-described low pressure die casting method or an embodiment thereof according to the third aspect of the invention.

[0046] The pressure control system of the low pressure die casting machine may be implemented in a system control controlling the whole low pressure die casting machine and its application. Further, the pressure control system may act as a system control for the whole low pressure die casting machine.

[0047] The pressure control system may be set up to keep the overall pre-pressure time constant for different holding furnace levels by varying the pre-pressure application speed and / or the pre-pressure application starting time depending on the holding furnace level.

[0048] Further various preferred embodiments of the low pressure die casting method, the low pressure die casting machine and of the use of the low pressure die casting machine are described below, wherein the various embodiments can be combined with one another and apply accordingly to embodiments of the low pressure die casting method, the low pressure die casting machine and the use of the low pressure die casting machine.

[0049] According to a first exemplary embodiment, the pre-pressure application starting time is set depending on the holding furnace level of the molten metal in the holding furnace, and the pre-pressure application speed is set to be substantially the same for all possible holding furnace levels.

[0050] Preferably, the overall pre-pressure time includes a waiting time before pre-pressure application starts at the pre-pressure application starting time depending on the holding furnace level. For example, for the holding furnace level of the molten metal in the holding furnace being greater than a minimum holding furnace level corresponding to an empty furnace or a minimum possible holding furnace level, there may be a waiting time included whereas for the minimum holding furnace level no waiting time may be included.

[0051] This way, the pre-pressure application starting time may be set to an earlier point in time during the overall pre-pressure application or time, e.g., directly at the beginning of the overall pre-pressure application with no waiting time or shortly after with only a short waiting time, for a minimum holding furnace level or a holding furnace level slightly above the minimum holding furnace level. Further, the pre-pressure application starting time may be set to a later point in time the during the overall pre-pressure application or time, e.g. substantially after the beginning of the overall pre-pressure application with a substantial waiting time for a substantially higher holding furnace level. Thus, the overall pre-pressure time can be kept constant for different holding furnace levels.

[0052] Further, a stable or constant pre-pressure application speed by setting the pre-pressure application speed to be the same for all possible holding furnace levels leads to more stable and comparable conditions during the process of forcing the molten metal through the riser tube. The pre-pressure application speed may affect, for instance, the uniformity of the molten metal distribution. Further, a higher pre-pressure application speed may lead to increased mechanical stress of the components of the low pressure casting machine through which the molten metal is forced to flow or to thermal shocks.

[0053] Additionally, by setting the pre-pressure application speed to be substantially the same for all possible holding furnace levels, the riser tube residence time can be kept constant for all holding furnace levels. The riser tube residence time may be the time, the molten metal needs to travel through the riser tube and is proportional to the length or height of the riser tube, thus the distance the molten metal needs to travel, and anti-proportional to the pre-pressure application speed. Additionally, the riser tube residence time may also depend on the holding furnace level as for a higher holding furnace level the distance the molten metal needs to travel in the riser tube may be shorter. Thus, this way, due to the more stable conditions in the riser tube, the quality for castings starting from different holding furnace levels can be kept constant as cold-runs or similar quality issues can be avoided or at least reduced. Moreover, the melt temperature when injecting the molten metal into the casting mold is not influenced differently for different holding furnace levels.

[0054] According to another exemplary embodiment of the low pressure die casting method, a first pre-pressure application starting time is set for a first holding furnace level and a second pre-pressure application starting time is set for a second holding furnace level, and the pre-pressure application starting times depending on the holding furnace level of the molten metal in the holding furnace are set by interpolation based on the first and second pre-pressure application starting times of the first and second holding furnace levels.

[0055] This way, different pre-pressure application starting times can be easily set for different holding furnace levels. The first holding furnace level may correspond to a minimum holding furnace level, e.g., a minimum possible holding furnace level to start from for casting a component (empty furnace), and the second holding furnace level may correspond to a maximum holding furnace level, e.g., a maximum holding furnace level as regards the maximum capacity of the holding furnace (full furnace).

[0056] According to another exemplary embodiment of the low pressure die casting method, the pre-pressure application starting time is set to be the same for all possible holding furnace levels, and the pre-pressure application speed is set depending on the holding furnace level of the molten metal in the holding furnace.

[0057] This way, a constant overall pre-pressure application time may be achieved for all holding furnace levels without having to apply a waiting time before the pre-pressure application starting time for higher holding furnace levels compared to lower holding furnace levels. Thus, starting from the same pre-pressure application starting time, the respective pre-pressure application speed for a specific holding furnace level may be adapted depending on the holding furnace level so that for different holding furnace levels a constant overall pre-pressure time can be preset without applying a waiting time before the pre-pressure application starting time for a LPDC process.

[0058] According to another exemplary embodiment of the low pressure die casting method, a first pre-pressure application speed is set for a first holding furnace level and a second pre-pressure application speed is set for a second holding furnace level, and the pre-pressure application speeds depending on the holding furnace level of the molten metal in the holding furnace are set by interpolation based on the first and second pre-pressure application speeds of the first and second holding furnace levels.

[0059] This way, different pre-pressure application speeds can be easily set for different holding furnace levels. The first holding furnace level may correspond to a minimum holding furnace level, e.g., a minimum possible holding furnace level to start from for casting a component (empty furnace), and the second holding furnace level may correspond to a maximum holding furnace level, e.g., a maximum holding furnace level as regards the maximum capacity of the holding furnace (full furnace).

[0060] According to another exemplary embodiment of the low pressure die casting method, the pre-pressure application starting time is set depending on the holding furnace level of the molten metal in the holding furnace, and the pre-pressure application speed is set depending on the holding furnace level of the molten metal in the holding furnace.

[0061] This way, a greater flexibility in presetting the overall pre-pressure time is achieved. Thus, a waiting time may be applied depending on the holding furnace level and at the same time the pre-pressure application speed may be varied for different holding furnace levels.

[0062] According to another exemplary embodiment of the low pressure die casting method, the overall pre-pressure time includes a waiting time before pre-pressure application starts at the pre-pressure application starting time if the holding furnace level of the molten metal in the holding furnace is greater than a minimum holding furnace level.

[0063] This way, the same overall pre-pressure time can be preset for different holding furnace levels, or the overall pre-pressure time can be kept constant at a constant pre-pressure application speed.

[0064] According to another exemplary embodiment of the low pressure die casting method, the method further comprises the step of pre-cooling the casting mold, and the pre-cooling time is calculated based on the preset overall pre-pressure time. Preferably, the pre-cooling time in particular is the pre-cooling starting time.

[0065] This way, the pre-cooling of the casting mold can be programmed in advance of the low pressure die casting process, in particular, in advance of the pre-pressure application.

[0066] According to another exemplary embodiment of the low pressure die casting method, the method further comprises the step of sensing the holding furnace level. Thus, the holding furnace level can be directly inferred through a sensor, e.g., a fill-level sensor positioned in or at the holding furnace and set up accordingly.

[0067] According to an exemplary embodiment of the low pressure die casting machine, a sensor for sensing the holding furnace level is provided, and the pressure control system is set up to preset a certain overall pre-pressure time by varying the pre-pressure application speed and / or the pre-pressure application starting time depending on the holding furnace level sensed by the sensor. Thus, the pressure control system must not rely on other, possibly external, sources or on estimates for information on the holding furnace level. The sensor can be a fill-level sensor for sensing the fill level of the molten metal and being positioned in or at the holding furnace and set up accordingly.

[0068] According to another exemplary embodiment of the low pressure die casting method, at least two components are successively cast, with each casting process starting at a different holding furnace level, and among the at least two successive casting processes, the overall pre-pressure time is kept constant by, for each casting process, varying the pre-pressure application speed depending on the holding furnace level and / or the pre-pressure application starting time depending on the holding furnace level.

[0069] According to another exemplary embodiment of the low pressure die casting method, the pre-pressure application speed is kept constant during pre-pressure application. This way more stable flow conditions of the molten metal through the riser tube and possibly through further components of the low pressure die casting machine used for the low pressure die casting process can be achieved.

[0070] Further advantageous exemplary embodiments of the invention are indicated by the following detailed description of a number of practical examples of the present invention, in particular in connection with the figures.

[0071] The figures attached to the application, however, are only intended to be used for the purpose of clarification, and not to determine the scope of protection of the invention. The attached drawings are intended only as examples reflecting the general concept of the present invention. In particular, features shown in the figures should not in any way be considered an essential component part of the invention.

[0072] In the following, the invention will be described in more detail with reference to the figures. Fig. 1shows an exemplary embodiment of a low pressure die casting machine according to an embodiment of the invention in a schematic view, Fig. 2shows a diagram with exemplary pressure curves for a low pressure die casting method according to the state of the art, Fig. 3shows a diagram with exemplary pressure curves for a low pressure die casting method according to an embodiment of the invention, and Fig. 4shows a diagram with further exemplary pressure curves for a low pressure die casting method according to an embodiment of the invention.

[0073] Fig. 1 depicts an exemplary embodiment of a low pressure die casting machine 2 (LPDC machine 2) in a schematic view. The LPDC machine 2 comprises a casting mold 4 for casting components out of a molten metal 6. The molten metal 6 is contained in a holding furnace 8 for holding the molten metal 6 at a certain holding furnace level 10 and at a specific temperature. A riser tube 12 fluidly connects the holding furnace 8 to the casting mold 4. The molten metal 8 can be filled into the holding furnace 8 through an inlet 14, e.g., after having been molten and brought to the desired temperature in a melting furnace (not shown).

[0074] In the shown example, the holding furnace 8 of the LPDC machine 2 is contained within a pressure chamber 16. In the pressure chamber 16, pressure is applied to push the molten metal 6 into the casting mold 4. For instance, during pre-pressure application or pre-pressurization, pressure is applied to the molten metal 6 at a specific pre-pressure application speed to force the molten metal 6 through the riser tube 12 towards the casting mold 4.

[0075] For the purpose of exerting pressure onto the molten metal 6 in the holding furnace 8, the pressure chamber 16 contains an inlet 18. The inlet 18 is connected to a schematically shown pressure control system 20, which can also serve as the general control system for monitoring and regulating various parameters of the LPDC process carried out by the LPDC machine 2, such as temperature, pressure, injection speed and timing to ensure consistent and high-quality castings.

[0076] The pressure system 20 is set up to apply pre-pressure to the molten metal 6 in the holding furnace 8 to force the molten metal 6 through the riser tube 12. For this purpose, gas, such as e.g. atmosphere gas, is introduced to the pressure chamber 16 via the inlet 18 to increase the pressure applied to the molten metal 6 in the holding furnace 8 and thus force the molten metal 6 to rise upwards through the riser tube 12. Moreover, during consecutive pressure stages after the pre-pressure application, the molten metal 6 can be forced further up through a gating system (now shown) and the sprue 22, which is a channel or passage through which the molten metal 6 is introduced into the casting mold 4 and may be a part of the gating system (not shown). A gating system, i.e., the further components of the gating system besides the sprue 22, could for example be positioned in the fluid way of the molten metal 6 between the riser tube 12 and the sprue 22.

[0077] Moreover, the pressure control system 20 is further set up to preset a certain overall pre-pressure time by varying the pre-pressure application speed and / or the pre-pressure application starting time depending on the holding furnace level 10. This way, stable conditions during a LPDC process can be achieved with the LPDC machine 2, as the overall pre-pressure time can be calculated and thus controlled in advance of the LPDC process.

[0078] A sensor (not shown) for sensing the holding furnace level 10 may be provided and may be connected to the pressure control system 20. That way the pressure control system 20 may be set up to preset a certain overall pre-pressure time by varying the pre-pressure application speed and / or the pre-pressure application starting time depending on the holding furnace level 10 sensed by the sensor.

[0079] The LPDC machine 2 may include further components such as a vacuum system for removing air and gases from the casting mold or mold cavity before injection of the molten metal or a cooling system used to rapidly cool the casting after the molten metal has been injected into the casting mold and solidifies.

[0080] Fig. 2 shows a diagram 30 with exemplary pressure curves 32, 34 for a low pressure die casting method according to the state of the art. The pressure (p) is shown in the unit mbar on the ordinate or y-axis and the time (t) is displayed in the unit seconds (s) on the abscissa or x-axis of the diagram 30.

[0081] The diagram 30 displays two graphs 32, 34 corresponding to two different pressure curves depicting the pressure evolution during two different LPDC processes or methods starting from two different holding furnace levels ff (full furnace - corresponding to a maximum holding furnace level) and ef (empty furnace - corresponding to a minimum holding furnace level).

[0082] It can be seen, that in both graphs 32, 34 the pre-pressure application starting time is set to be t = 0 s (bottom left corner of diagram 30), i.e., no waiting time is applied. Hence at t = 0 s, pressure is starting to be applied to the molten metal 6 in the holding furnace 8. The pre-pressure application stage corresponds for the two graphs 32, 34 to the first section 32a, 34a of each graph 32, 34, respectively. The first section 32a thus extends from t = 0 s to approximately t = 7 s for graph 32 and from t = 0 s to t = 17 s for graph 34. During the pre-pressure application 32a, 34a, the pre-pressure application speed (Δp / Δs in the unit mbar / s) is nearly the same for the two graphs 32, 34.

[0083] Hence, for the two graphs 32, 34 the overall pre-pressure times 33, 35 corresponding to the holding furnace levels ff and ef differ in a time amount of Δt = 17 s - 7 s = 10s (see reference sign 36 in diagram 30). Hence the overall pre-pressure time differs correlating to the holding furnace level 10 leading to unstable LPDC conditions for different holding furnace levels.

[0084] Pressure stages 32a and 34a correspond to the pre-pressure application or pre-pressurization stage, where pressure is applied to the molten metal 6 in the holding furnace 8 to force the molten metal up the riser tube 12 and thereby starting the actual casting process. At the end of pressure stage 32a, 34a, the molten metal has reached the end or exit of the riser tube, thus leaving the area of the holding furnace 8 and entering the area in the vicinity of the casting mold 4.

[0085] The further pressure stages 32b-d of graph 32 and 34b-d of graph 34 correspond to further pressure steps during the LPDC process. Pressure stages 32b and 34b correspond to a pressure stage where pressure is applied to force the molten metal 6 further in the direction to the casting mold 4, e.g., through a gating system and / or the sprue 22. Pressure stages 32c, 34c correspond to the fill stage, where the molten metal 6 is introduced or injected into the casting mold 4. Here, the pressure application speed is slightly reduced to allow the molten metal 6 to flow smoothly into the casting mold 4 and fill the mold 4 without causing turbulence or air entrapment. This stage ensures that the casting mold 4 is completely filled with molten metal 6.

[0086] Once the die casting mold 4 is filled, the next pressure stage 32d, 34d is entered and the pressure is quickly increased to an over-pressure to condense the molten metal 6 in the casting mold 4 so that defects such as pores in the cast component are counteracted. After this lastly shown pressure stage, the pressure is held constant during the solidification stage (not shown) as the molten metal 6 solidifies as it cools down. The pressure is maintained in this stage, to counteract any shrinkage or void formation that may occur as the metal solidifies. Once the casting has solidified, the pressure is released, and the cooling process begins (not shown).

[0087] The further pressure stages 32b-d and 34b-d do not depend on the initial furnace holding level 10 anymore, since after the pre-pressure application stage 32a, 34a, the molten metal 6 has reached the end of the riser tube 12 and the remaining way which the molten metal 6 has to take in order to fill the casting mold 4 completely is the same irrespective of the holding furnace level 10. Thus, the pressure evolution of graphs 32, 34 is the same in sections 32b-d and 34b-d.

[0088] Fig. 3 now shows a diagram 40 with exemplary pressure curves 42, 44 for a low pressure die casting method according to an embodiment the invention. Again, the pressure (p) is shown in the unit mbar on the ordinate or y-axis and the time (t) is displayed in the unit seconds (s) on the abscissa or x-axis of the diagram 40.

[0089] The diagram 40 displays two graphs 42, 44 corresponding to two different pressure curves depicting the pressure evolution during two different LPDC processes or methods starting from two different holding furnace levels ff (full furnace - corresponding to a maximum holding furnace level) and ef (empty furnace - corresponding to a minimum holding furnace level).

[0090] It can be seen, that in both graphs 42, 44 the pre-pressure application starting time is set to be t = 0 s (bottom left corner of diagram 40), i.e., no waiting time is applied. Hence at t = 0 s, pressure is starting to be applied to the molten metal 6 in the holding furnace 8. Whereas the pressure evolution is again the same for the pressure stages 42b-d and 44 b-d for graphs 42 and 44, i.e., the same pressure application speed Δp / Δs (in the unit mbar / s) is applied, the pre-pressure application speed Δp / Δs is different for the pre-pressure application stages 42a, 44a for the two graphs 42, 44.

[0091] Here, in the pre-pressure application stage 42a, 44a, the pre-pressure application speed is higher for graph 44, starting from a lower holding furnace level ef (empty furnace) compared to the pre-pressure application speed for graph 42, starting from a higher holding furnace level ff (full furnace). The durations 43, 45 of the pre-pressure application stages 42a, 44a, however, are the same (Δt = 9 s) for both graphs 42, 44.

[0092] By adapting the pre-pressure application speeds depending on the respective holding furnace level 10, a constant overall pre-pressure time can be preset, so that the durations 43, 45 of the pre-pressure application stages 42a, 44a are the same (see reference sign 46). The pre-pressure application speed can be determined and set for different holding furnace levels 10 for example due to interpolation, e.g., by presetting certain pre-pressure application speed values for a minimum holding furnace level (empty furnace) and for a maximum holding furnace level (full furnace) and interpolating for holding furnace levels in between.

[0093] Starting from the same pre-pressure application starting time, in the shown example in Fig. 3, the respective pre-pressure application speed for a specific holding furnace level 10 is adapted depending on the holding furnace level 10 so that for different holding furnace levels 10 a constant overall pre-pressure time can be preset without applying a waiting time before the pre-pressure application starting time for a LPDC process.

[0094] Fig. 4 eventually shows a diagram 50 with further exemplary pressure curves 52, 54 for a low pressure die casting method according to an embodiment of the invention. In the diagram 50, the pressure (p) is shown in the unit mbar on the left-side ordinate or left-side y-axis, the time (t) is displayed in the unit seconds (s) on the abscissa or x-axis and the pressure application speed (Δp / Δs) is is shown in the unit mbar / s on the right-side ordinate or right-side y-axis of the diagram 50.

[0095] The diagram 50 displays two graphs 52, 54 corresponding to two different pressure curves depicting the pressure evolution during two different LPDC processes or methods starting from two different holding furnace levels ff (full furnace - corresponding to a maximum holding furnace level) and ef (empty furnace - corresponding to a minimum holding furnace level). Further, a third graph 58 displays the velocity of the applied pressure (v p ), i.e., the derivative of the applied pressure, for the different pressure stages 52a-d and 54a-d.

[0096] In this diagram 50, again, the pressure evolution of pressure application speed in the further pressure stages 52b-d and 54b-d is the same, which is why only one graph 58 is displayed corresponding to the respective pressure application speeds or pressure velocities in the different pressure stages 52a-d and 54a-d. The points in the graph 58a-58d correspond to the velocity or pressure application speed in the sections 52a-d and 54a-d, wherein the point 58a corresponds to the velocity in the sections 52a and 54a and the point 58b corresponds to the velocity in the sections 52b and 54d and so forth.

[0097] It can be seen that in both graphs 52, 54 the pre-pressure application speed is the same, i.e., the gradient of the two graphs 52, 54 is the same in sections 52a, 54a. Thus, here, by setting the pre-pressure application speed to be the same for all possible holding furnace levels, the riser tube residence time can be kept constant for all holding furnace levels.

[0098] To account for the different holding furnace levels ef (graph 54) and ff (graph 52), the pre-pressure application starting time is different for the two graphs 52, 54. For graph 52 corresponding to a higher holding furnace level (ff), a waiting time 53a of Δt = 5s is applied, after which pre-pressure application starts. Thus, the time span 53b during which pre-pressure is applied in graph 52, is less than the time span 55 during which pre-pressure is applied in graph 54. For graph 54, no waiting time is applied before starting pre-pressure application at t = 0s.

[0099] However, due to the waiting time 53a included, the overall pre-pressure time can be preset to be the same for both graphs 52, 54 corresponding to different holding furnace levels 10. Additionally, the pre-pressure application speed can set to be the same for both graphs 52, 54, so that a constant residence time in the riser tube 12 is accomplished.

Claims

1. Low pressure die casting method, comprising, preferably in the given order, the steps of: - holding a molten metal (6) in a holding furnace (8) at a holding furnace level (10), - optionally pre-cooling a casting mold (4), - pre-pressuring the molten metal (6) in the holding furnace (8) starting from a certain pre-pressure application starting time at a certain pre-pressure application speed to force the molten metal (6) from the holding furnace (8) through a riser tube (12) to the casting mold (4), and - injection of the molten metal into the casting mold (4) under pressure, characterized in that, - a substantially constant overall pre-pressure time is preset, wherein, by varying the pre-pressure application speed and / or the pre-pressure application starting time, the preset substantially constant overall pre-pressure time is substantially met.

2. Low pressure die casting method according to claim 1, characterized in that, - the pre-pressure application starting time is set depending on the holding furnace level (10) of the molten metal (6) in the holding furnace (8), and - the pre-pressure application speed is set to be substantially the same for all possible holding furnace levels (10).

3. Low pressure die casting method according to claim 1 or 2, characterized in that, - a first pre-pressure application starting time is set for a first holding furnace level (10) and a second pre-pressure application starting time is set for a second holding furnace level (10), and - the pre-pressure application starting times depending on the holding furnace level (10) of the molten metal (6) in the holding furnace (8) are set by interpolation based on the first and second pre-pressure application starting times of the first and second holding furnace levels.

4. Low pressure die casting method according to claim 1, characterized in that, - the pre-pressure application starting time is set to be the same for all possible holding furnace levels, and - the pre-pressure application speed is set depending on the holding furnace level (10) of the molten metal (6) in the holding furnace (8).

5. Low pressure die casting method according to claim 1 or 4, characterized in that, - a first pre-pressure application speed is set for a first holding furnace level and a second pre-pressure application speed is set for a second holding furnace level, and - the pre-pressure application speeds depending on the holding furnace level (10) of the molten metal (6) in the holding furnace (8) are set by interpolation based on the first and second pre-pressure application speeds of the first and second holding furnace levels.

6. Low pressure die casting method according to claim 1, characterized in that, - the pre-pressure application starting time is set depending on the holding furnace level (10) of the molten metal (6) in the holding furnace (8), and - the pre-pressure application speed is set depending on the holding furnace level (10) of the molten metal (6) in the holding furnace (8).

7. Low pressure die casting method according to any of claims 1 to 6, characterized in that, - the overall pre-pressure time includes a waiting time before pre-pressure application starts at the pre-pressure application starting time if the holding furnace level (10) of the molten metal (6) in the holding furnace (8) is greater than a minimum holding furnace level.

8. Low pressure die casting method according to any of claims 1 to 7, characterized in that, - the method further comprises the step of pre-cooling the casting mold (4), and - the pre-cooling time is calculated based on the preset overall pre-pressure time.

9. Low pressure die casting method according to any of claims 1 to 8, characterized in that, - the method further comprises the step of sensing the holding furnace level (10).

10. Low pressure die casting method according to any of claims 1 to 9, characterized in that, - at least two components are successively cast, with each casting process starting at a different holding furnace level (10), and - among the at least two successive casting processes, the overall pre-pressure time is kept constant by, for each casting process, varying the pre-pressure application speed depending on the holding furnace level (10) and / or the pre-pressure application starting time depending on the holding furnace level (10).

11. Low pressure die casting method according to any of claims 1 to 10, characterized in that, - the pre-pressure application speed is kept constant during pre-pressure application.

12. Low pressure die casting machine (2), comprising - a casting mold (4) for casting components out of a molten metal (6), - a holding furnace (8) for holding the molten metal (6) at a certain holding furnace level (10), - a riser tube (12) fluidly connecting the holding furnace (8) to the casting mold (4), and - a pressure control system (20) set up to apply pre-pressure to the molten metal (6) in the holding furnace (8) to force the molten metal (6) through the riser tube (12), characterized in that, - the pressure control system (20) is further set up to meet a substantially constant overall preset pre-pressure time by varying the pre-pressure application speed and / or the pre-pressure application starting time depending on the holding furnace level (10).

13. Low pressure die casting machine (2) according to claim 12, characterized in that, - a sensor for sensing the holding furnace level (10) is provided, and - the pressure control system (20) is set up to preset a certain overall pre-pressure time by varying the pre-pressure application speed and / or the pre-pressure application starting time depending on the holding furnace level (10) sensed by the sensor.

14. Use of the low pressure die casting machine (2) of claim 12 or 13 in a low pressure die casting method according to any of claims 1 to 11.

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