Manufacturing method of gravitational metal casting with after-pressure

& Casting mould for such technology

Area of technology

This invention applies to manufacturing method of metal castings suitable for demanding applications, in particular as casting technique of metal tyre curing moulds.

Up to date state-of-the-art

Existing manufacturing technology of metal castings is typically based on gravitational casting in sand moulds. Sand mould is conservatively risered with insulating material and creates outer profile of cast piece Pressing of molten metal from riser part during crystallization does not proceed at higher pressure conditions and is pushed by atmospheric pressure only.

Due to pure atmospheric pressure effect the partial pressure in hot metal will not increase and thereby solubility of hydrogen during crystallization decreases This causes origination of gas pockets in cast piece with attracted oxide inclusions From the same reason ideal copy of cast piece's inner profile, created by liquid metal in pattern on plaster cores, will not be achieved ant this results in shape inaccuracy.

That's why there are efforts to improve the mould's hot-metal filling property and so minimize possible inaccuracies and casting defects.

The solution according to JP 200153353 is designed for pressure casting with cold chamber. It is advantageous for extremely accurate casting without need of any special very hard metal mould and without severe casting device requirements. Upper part of metal mould creating cavity for casting is made of material having 30%-porosity and so it is able to absorb released gasses during mould filling with hot metal. Applied pressure is imposed mechanically by piston through gating system and its amount is limited by porosity of mould's upper part.

Technology based on hot metal's pressing through riser part with help of mechanical piston (the so-called squeeze casting) is also used at solution according to JP 7290227, which is particularly oriented to restrain flashes on the cast piece. With more complex cast pieces, the casting mould must be segmented for the sake of cast piece removal from cavity At mould segment boundaries miniature gaps arise that widen with mould wearing and therewith

also flashes on cast piece gain. This solution speeds the hot metal solidification in gaps and so inhibits creation of larger flashes.

Disadvantages of mechanical hot-metal pressing by piston are related to the fact that piston geometry is always determined for one type of cast piece only. This provokes considerable purchasing costs related to castings' modifications and maintenance expenses for machinery equipment. Volume of hot metal for every piston must be precisely defined before casting. Such requirement signifies a serious problem as it demands to work with liquid metal and measure out exact material amount for particular cavities. At any failure to comply with required volume the casting defects arise, i.e. flashes or incomplete casting, in opposite case. Unfavourable, especially in regard to manufacturing application for tyre curing moulds, is also casting ability limited to pieces with low volume.

Effort to improve metal castings quality can be also seen in manufacturing method depicted in JP 63010056. It is a matter of low pressure casting technology, however in special form. Only one half of mould is cast with cavity filled from bellow and the hot metal is then gradually added at slightly higher pressure as cast piece solidifies and its volume decreases. In the same way casting of the second mould half, horizontally segmented, is made. This technique is time as well as energy-consuming and hence also inflexible from the product-line point of view of. Frequent changes of product line mean more than double time consumption in comparison with other casting techniques. In addition to that this technology application is limited by mass of cast piece. At manufacture of cast pieces weighing more than 200 kg considerable casting defects would arise, especially shrinks and shrinkage porosities with negative effects to casting quality.

The method according to SU 999340 (CS 247104) represents non-traditional technology based on mould cavity filling up with hot metal and subsequent pushing it to mould walls by pressure air supplied into central area of molten mass. Mould walls here are copied very well. However, inside hot metal craters caused by pressure medium remain and will need following work down. Most of all, the main disadvantage of this technology is its ability to produce hollow cast pieces only. Also the supply of compressed air through hollow bar can be a problem during the closing phase when hot metal solidifies and bar must get separated from cast piece.

Another improvement tendency at after-pressure of hot metal in mould cavity during crystallization is represented by process according to EP 1375033. This is technique of hot metal pouring in mould that contains impermeable insulating barrier against penetrating gas. Such barrier causes uniform pressure distribution in area between outer and inner walls of this mould. This technique uses hot metal supplied in such mould at primary pressure lower than surrounding pressure (i.e. with underpressure) and following a gas pressure higher than the primary one is applied to material in mould. Mentioned barrier makes mould walls virtually

gas impermeable. Gas pressure is applied to material in mould immediately after the material has filled up the mould cavity while mould rests in casting furnace. Barrier is made of glaze inhibiting the gas penetration. Mould walls are made up of surfaces having the height-to- width ration 1.0 or greater. Patent further describes an alternative technique of protected molten metal pouring in casting chamber (pressure box) at primary pressure and following pressure increase to higher level in mould. Mould is again positioned in casting furnace having walls coated with glaze against gas penetration.

This technology is disadvantageous as it is designed just for hollow thin-walled cast pieces weighing only several kilograms. And primarily, it offers no solution of gas off-take from hot metal in mould, which is extremely important when mould walls provide barrier against gas penetration. When the mould is not vented then gas pockets arise in cast piece and repairs represent laborious and expensive task.

Resistance melting furnace and resistance malleableizing furnace for casting mould are positioned in large pressure box. When such assembly should be applied to heavy weight cast pieces its purchase costs would be very high.

Above mentioned disadvantages and limitations, primarily as regards the cast-piece weight and geometry, forestall this technique to be applied for large-volume cast pieces such as tyre curing moulds.

That is why in production of large-volume cast pieces, serving as tyre curing moulds, the method of classic gravitational casting at atmospheric pressure with sand casting moulds and inset plaster complete-ring all the time has been used. The plaster complete-ring, which provides inner tyre-tread-pattern part of casting in case of tyre curing mould, is made of plaster mixture having parameters dimensioned according to existing concept of casting technology. Plaster complete-ring providing such parameters functions as insulating material. Such feature extends the period of casting's solidification and supports growing of crystals in cast-piece volume. Its structure then becomes coarse-grained, which supports inter-crystal corrosion and worsens mechanical properties.

Nature of the present invention

Mentioned disadvantages and drawbacks of hitherto known manufacturing methods of metal casting production, especially of tyre curing moulds, are to a large extent removed by manufacturing ¹ technology of gravitational casting with after-pressure and by casting mould for this technology.

Nature of the present invention consists in following technology: Casting mould's cavity is filled up during 60 to 180 seconds with hot metal at atmospheric pressure. After sealing up the gate, hot metal is subjected to an after-pressure when pressure medium is applied into casting mould's cavity from the top and within 60 to 120 seconds it builds up pressure of 40 to 70 kPa. This pressure at constant level further affects hot metal for 10 to 15 minutes. Then the pressure is slowly released during 60 to 90 seconds and resulting cast piece cools of itself to temperature at which it can be released from mould.

Nature of casting mould's design to realize the casting technology according to mentioned invention is as follows: The mould consists of base plate on which a vertically and possibly also horizontally segmented cladding is well fixed. From the top, there is riser plate provided with pressure-medium inlet well fixed to the cladding. All these mentioned components together limit the casting mould's cavity that is connected with gating system consisting of gating orifice, sprue, runner and filters. The whole gating system is put in container.

Technology of gravitational casting with after-pressure (GCAP) according to this invention is advantageous as it reduces amount of input casting raw materials as well as eliminates casting defects in resulting pieces (geometrical inaccuracies, hydrogen pores, oxide inclusions and blebs, overflows, shrinkage porosities etc.) As consequence of fewer casting defects also the volume of manual finishing work necessary for cast pieces decreases. And the lesser amount of scrap is economically significant as well.

Figure overview in drawings

Actual method of gravitational casting with after-pressure according to the invention as well as equipment for its realization is graphically represented in enclosed drawings as follows:

Figure 1 - metal cast manufacturing principle of gravitational casting with after-pressure (GCAP)

Rg. 1

Figure 2 - GCAP application principle of casting: tyre curing moulds

Realization examples

For better understanding of the invention merit, following examples are presented They illustrate particular metal cast manufacturing technology of gravitational casting with after-pressure as well as casting mould designs for this technology realization

Example 1

Metal cast procedure by gravitational casting with after-pressure according to Fig 1 starts with gravitational filling up the casting mould cavity 5 through gating system 6 The casting mould cavity 5 is filled by liquid metal so that this metal partially fills also cavity in riser plate 3 Such filling up takes 70 seconds Then after-pressure of still liquid metal into casting mould's cavity 5 follows This is done by pressure gas medium at 60 kPa level acting for 60 seconds and is further kept at constant level during the crystallization period of 10 minutes

During the mould filling period the inlet 4 of pressure medium is open It serves for degasification of casting mould cavity 5 When the casting mould cavity 5 filling up with liquid metal is finished the inlet orifice 7 is hermetically sealed and fixed Also individual parts of casting mould are fixedly connected and sealed When these conditions are met the after-pressure of liquid metal begins by pressure medium supply The pressure medium is shut down no sooner than the liquid metal gets crystallized Pressure release phase takes here 70 seconds

The realization equipment of this technology, i e casting mould according to Fig 1 , consists of base plate / on which a vertically and also horizontally segmented steel cladding 2a, 2b is well fixed and to this from the top the riser plate 3 provided with pressure-medium inlet 4 is well fixed All these mentioned components together limit the casting mould's cavity 5 Casting mould cavity 5 is connected with gating system 6 consisting of gating orifice 7, sprue 8, runner 9 and filters 10 The whole gating system 6 is put in container 11

Example 2

Metal cast manufacturing procedure of tyre curing mould according to Fig 2 starts with gravitational pouring of aluminium melt where the casting mould cavity 5 is filled by liquid metal through gating system 6 Hot metal fills up the casting mould's cavity 5 (i e the room of future cast piece) up to the height of riser insulating filler 15 As soon as this height is

achieved the hot metal pouring is stopped. Filling process takes 120 seconds. At pouring the inlet 4 of pressure medium (air) is open to draw off gasses arising at filling.

In after-pressing phase first the gating orifice 7 is sealed. The pressure medium mentioned above is supplied to inlet 4. These two steps are made in time interval of 30 seconds.

System' starts getting pressurized and during 90 seconds the pressure increases to the level of 40 kPa. After this value has been achieved the pressure is hold at constant level for 15 minutes when crystallization of hot metal proceeds. As soon as crystallization is finished the supply of pressure medium is shut down and casting mould is slowly depressurized. Pressure releasing takes 90 seconds. After depressurizing the GCAP process is completed and mould with a cast piece can be transported to finishing.

Equipment, i.e. casting mould visible in Fig. 2, similarly to previous example consists of base plate 1 on which a vertically segmented steel cladding 2 is well fixed and to this from the top riser plate 3 provided with pressure-medium inlet 4 is well fixed. These components limit the casting mould's cavity 5. Casting mould cavity 5 is connected with gating system 6 consisting of gating orifice 7, sprue S, runner 9 and filters 10. The assembly is put in container 11. Base plate 1 is in its central part provided with circle ring 12 around which the plaster complete-ring 13 is centred and fixed. This is from inside fixed to supporting circle ring 12 by hold-down filler 14. Riser plate 3 is from below in area of casting mould's cavity J provided with pressure-medium distributing board 16 on which further the inlet orifice 7 as well as pressure-medium inlet 4 is made. Described casting mould is made in following manner:

On steel base plate 1 provided with supporting circle ring 12 the plaster complete-ring 13 is positioned. Emerged cavity between plaster complete-ring 13 and supporting circle ring 12 is packed with mixture of foundry sand and binding as well as hardening agents. After hardening, such mixture will create the hold-down filler 14.

Into vertically segmented steel cladding 2 the gating system 6 is fixed that is put in container 11 consisting of sprue 8, runner 9 and grooves with inset ceramic foam filters 10.

At riser plate 3 the inner circle ring is provided with insulating filler 15. Distributing board 16 of pressure medium is centred and positioned on riser plate 3.

These casting mould's base elements are malleablized in drying furnace where air circulation must be assured. All metal parts of individual mould components that come in contact with liquid metal are treated with protective blacking.

After malleablizing the mould is assembled in casting position in front of melting furnace. On insulating mould bottom plate at first the base plate / is positioned together with plaster complete-ring 13 fixed at supporting circle ring 12 by hold-down filler 14. On such base plate 1 the cladding 2 is well fixed together with embedded container 11 compassing the gating system 6. Then on cladding 2 the riser plate 3 is positioned already provided with distributing board 16 of pressure medium. Placed assembly is centred in position where on distributing board 16 of pressure medium the inlet orifice 7 is axially adjusted according to sprue 5. All connections and gaps are sealed wherewith impermeable pressure system is obtained. Pressure-medium inlet 4 stays open. The mould is ready to casting process and after-pressure.

Industrial usability

Manufacturing method of gravitational metal casting with after-pressure according to the invention can be utilized mostly in all places with higher requirements for shape and dimensional accuracy of cast pieces together with their quality. Typical example of such application is production of moulds, foremost demanding manufacture of curing moulds for tyre industry. In consideration of economical advantage due to material as well as manual work savings this manufacturing method of metal castings can be valuable also in some other less demanding applications.