DE29818994U1 - Device for vacuum injection molding of metal - Google Patents

Description

Applicant: Friedrich Wieser

Our file: 48935 Al/Gr

Device for vacuum injection molding of metal

I. Area of application

The invention relates to an injection molding device with which a liquid, hardenable material is introduced into a defined mold cavity and hardens there, subsequently forming the desired molded part.

II. Technical background

Injection molding is carried out using both plastic, particularly thermoplastics, and metal, such as light metals such as aluminum.

In order to make it easier to remove the finished molded part, the mold usually consists of two or more molded parts that can be placed close to one another and form the mold cavity between them.

The general problem with injection molding is to really fill the mold cavity completely with liquid material, i.e. to prevent the ingress of air or gas in the form of bubbles and their manifestation in the molded part.

This problem occurs particularly in aluminum injection molding, where the liquid aluminum is injected into the mold cavity at a temperature of approximately 650 ^° C and a pressure of approximately 1,000 bar.

An air bubble of, for example, the size of a pea introduced into the molded part under these conditions can - after the molded part has been removed from the mold and the pressure has thus been reduced to ambient pressure - increase in size to a multiple of its original size, provided that it is close enough to the surface of the molded part and the metal bordering the surface is therefore sufficiently elastic and can bend outwards.

Such a molded part then no longer corresponds to the desired shape in the area of the air bubble due to the bulging, and this area of the molded part is also not resilient because the bulging area can be easily destroyed, and thus the desired overall wall thickness of the molded part, for example an aluminum engine block, is no longer present in this area.

This effect becomes more pronounced when the molded part containing the air bubbles is heated up significantly after molding, as happens, for example, with engine blocks during operation, but also as is necessary, for example, during further processing of the molded part by subjecting the aluminum molded part to a thermal hardening process.

However, such hardening processes are desirable because they allow the tensile/compressive strength of the molded part to be almost doubled, for example from 220 N/cm ² to 400 N/cm ² for aluminum. If die-cast aluminum parts, some of which contain air inclusions, are subjected to such a thermal hardening process, high rejection rates occur.

In order to avoid air inclusions, attempts have been made in the past during injection molding, especially of plastics, by

·

·

• · &igr;

Cavity before and during the introduction of the liquid material. This negative pressure must not only be applied before the first liquid material arrives at the sprue, but if possible also while the liquid material is advancing into the mold cavity. For this reason, the negative pressure was applied directly to the mold cavity, for example to a ring channel that was formed around the actual mold cavity between the two mold parts and was connected to the actual mold cavity via connecting channels with such a small cross-section that these connecting channels could no longer be completely penetrated by the quickly hardening plastic.

This procedure is very difficult when injection molding with light metals such as aluminum, since the high initial temperatures of the liquid material cause very small, almost dust-like particles of the liquid material to break away from the total volume of the liquid material and penetrate into the smallest of cavities as so-called flakes, particularly in the previously described arrangement into the connecting channels with a small cross-section to the vacuum channel. This flake therefore quickly clogs these connecting channels with a small cross-section and/or the filters in the vacuum system, with the result that the vacuum applied is significantly reduced with each shot, and this would make it necessary to clean the connecting channels very frequently, replace the filters, etc.

IM. Description of the invention

a) Technical task

It is therefore the object of the present invention to create a device for injection molding, in particular die casting using metal, by means of which the probability of air inclusions in the molded part is much lower than in the prior art.

b) Solution to the task

This object is achieved by the features of claim 1. Advantageous embodiments emerge from the subclaims.

The negative pressure in the mold cavity is not caused by applying negative pressure directly in the mold cavity, but by applying negative pressure in the supply space for the liquid material, and in particular in the areas of the supply space that do not come into contact with liquid material at any stage of the injection molding process. When injection molding metal, it is known in the context of the die casting process to move a defined amount of liquid material along the inside of an acceleration cylinder towards the sprue channel, by moving the filling volume at the front by a pushing filling piston and in the direction of the anvil, which forms the front end of the filling volume in the direction of movement and which lies tightly inside the acceleration cylinder.

First, the filling volume extends from the sprue end of the cylinder to the filling opening on the top of the cylinder lying flat or sloping towards the sprue, through which the liquid material is introduced into the filling volume without pressure. The filling piston is then moved away from the filling opening in the direction of the sprue end of the acceleration cylinder, and the filling volume loses contact with the filling opening, so that the filling volume now contains a mixture of liquid material and ambient air, since the filling volume can never be completely filled with liquid material via the filling opening.

This unit is now increasingly accelerated along the acceleration cylinder, until the filling volume at the sprue end hits the anvil as a stop, usually the front closure of the acceleration cylinder

at its sprue-side end, and is thereby decelerated very quickly. The filling piston, which is usually accelerated to apply the acceleration, is not actively decelerated and presses with its large mass on the filling volume in the direction of the anvil. In this end position, the filling volume is already connected to the sprue of the mold, so that the pressure now exerted by the filling piston and supported by the kinetic energy of the liquid material in the filling volume itself forces the liquid material very quickly and under high pressure through the sprue into the mold.

According to the invention, however, the anvil, which limits the filling volume on the side opposite the filling piston, is also moved along the acceleration cylinder in the direction of the sprue of the mold together with the filling piston and the filling volume. The anvil, which hits a stop in its sprue-side end position, can under certain circumstances also completely leave the acceleration cylinder in this end position and then be located in a corresponding recess in one of the mold parts or another component of the injection system, depending on how far the acceleration cylinder extends in the direction of the sprue position.

Over the majority of the movement distance, the filling piston and anvil will be moved synchronously with each other while maintaining the mutual distance and thus the size of the filling volume enclosed between them, either by actively retracting the anvil synchronously with the movement of the filling piston, or only passively due to the movement of the filling piston and the only low compressibility of the contents of the filling volume, which consists primarily of liquid metal.

In conventional die casting, the air in the mold is displaced by the liquid material penetrating into the mold cavity and pressed outwards through the contact joints between the mold halves or mold parts.

This is already avoided according to the invention by applying a negative pressure before the liquid material arrives in the mold cavity, the air being sucked out of the mold cavity, whereby the vacuum is sucked out in particular inside the acceleration cylinder and in particular at its front end and on the back of the anvil, as at a point that never comes into contact with liquid material.

The vacuum channel can either be located in the anvil rod and open into its outer circumference shortly before the anvil, or in the rear face of the anvil, or the vacuum channel is incorporated in the acceleration cylinder itself and opens into its front end.

In this way, however, the suction from the mold cavity is interrupted as soon as the anvil reaches the sprue and begins to overflow it.

Since the filling volume between the anvil and the filling piston contains not only liquid material but also residual air, there is still a risk that this residual air could still get into the (previously evacuated) mold cavity. This is prevented by the sprue channel opening at such a point in the filling volume in its final position inside the acceleration cylinder that when the anvil is stopped and material is thus transferred from the filling volume via the sprue channel into the mold cavity, there is definitely only liquid material and no residual air.

In the case of a horizontally running acceleration cylinder, the sprue does not open into the upper part of the cylinder cross-section, but rather at the side or bottom of the cylinder cross-section.

In the longitudinal direction, i.e. in the direction of movement along the acceleration cylinder, the mouth of the sprue is in the front area of the filling volume

in its end position, i.e. in the front area of the wall of the acceleration cylinder in the area of the filling volume, or in the front surface of the anvil facing the filling volume, which must then also be penetrated by the sprue channel. The front area is therefore predestined because when the anvil and thus also the contents of the filling volume are slowed down, the liquid metal will collect at the front end in the longitudinal direction, in accordance with the higher specific weight of the liquid metal compared to the residual air, while the residual air will collect in the rear area, and closer to the upper end due to gravity.

This effect can be enhanced by providing an escape space for the residual air in the filling volume at the end position of the filling volume.

Such an escape space can be arranged inside the acceleration cylinder as a cavity that ends in the running surface, i.e. the inner circumference of the acceleration cylinder. However, the escape space can also be formed in the front surface of the rear filling piston, or in one of the molded parts, provided that the filling volume comes into contact with this molded part in its end position.

The escape space should preferably not be arranged longitudinally in the front area of the filling volume in its final position, but rather in its rear area and/or in the upper area of the cross-section of the filling volume, in particular opening out at the highest point of the cross-section.

Furthermore, it is important to ensure that the acceleration begins gently at the beginning of the movement of the filling volume away from the filling opening, i.e. with a linearly constant acceleration or with a linear, but not exponentially increasing acceleration, in order to prevent turbulent mixing of the residual air present in the upper cross-sectional area of the filling opening with the liquid material.

It is also necessary that the amount of liquid material in the filling volume is significantly greater than the amount required to fill the mold inclusions of the sprue under the prevailing pressure, so that it is ensured that at least the residual air, but also a residual amount of liquid material, remains in the filling volume at its end position when the mold cavity is already filled.

To prevent the ingress of foreign air into the mold cavity,

&iacgr;&ogr; additional seals, preferably in the form of channels subjected to negative pressure as negative pressure seals, are arranged, on the one hand, around the mold cavity on all separating surfaces between the mold parts, between the mold parts and the intermediate plates or pressure plates supporting them and at the contact points between the acceleration cylinder and the mold parts or intermediate plates and pressure plates or at the contact point of the anvil rod that can be moved in the longitudinal direction and carries the anvil through the mold parts, between plates and pressure plates.

Preferably, a mold consists of only two mold parts, and these are arranged with a vertically oriented separating surface between the mold parts, whereby the acceleration cylinder and thus the acceleration path of the liquid material is preferably arranged below the mold, so that the liquid material rises from the filling volume into the mold. If the acceleration cylinder is arranged above the mold or in the height area of the mold, the advantage could be exploited that the sprue channel can then run from the filling volume directly to the side or downwards towards the mold, thus further reducing the probability that the residual air, which tends to collect in the upper area of the filling volume, gets into the mold, but this leads to an unfavorable machine design with the parts of the acceleration cylinder subject to strong dynamic loads at a relatively high height.

The acceleration cylinder itself is usually arranged horizontally, but can also slope down or rise towards the end point, i.e. the point of contact with the sprue.

■ .

c) Examples of implementation

An embodiment according to the invention is described in more detail below using the figures as an example. They show:

Fig. 1a: a longitudinal section through a die-casting system in the filling position,

Fig. 1b: a longitudinal section through a die casting machine in the injection position,

Fig. 2a: a modified injection molding system in the injection position,

Fig. 2b: a cross-section through the filling volume in the injection position, and

Fig. 2c: another variant of the die-casting machine according to the previous figures.

Figures 1a and 1b show a first embodiment of an injection molding system in longitudinal section, i.e. cut vertically in the longitudinal direction 10, the direction of displacement of the filling volume along the acceleration cylinder 16.

Fig. 1a shows the situation in the filling position, i.e. when filling the filling volume 6 with liquid aluminum through a filling opening 15 in the highest point of the acceleration cylinder 16. Filling takes place without pressure from a crucible 14 by pouring. In the acceleration cylinder 16, an anvil 13 is located on the inner circumference in the longitudinal direction 10 at the front in the direction of movement and a filling piston 12 at the rear in the direction of movement, each of which has the head of a

&iacgr;&ogr;·

from the filling volume 6 outwardly pointing anvil rod 22 or piston rod 21 and can be moved independently of each other, but also synchronously with the absorbed filling volume between them from the filling position of Fig. 1a to the injection position of Fig. 1b.

In the injection position shown in Fig. 1b, the anvil 13 is at the front end of its movement path, on the left in Fig. 1b. The anvil 13 is located in a cylindrical recess in the mold and thus already outside the acceleration cylinder 16, although the recess is aligned with the inner circumference of the acceleration cylinder. Likewise - as Fig. 2a shows - the anvil 13 can still be in the acceleration cylinder 16 in its end position.

What both solutions have in common, however, is that the anvil 13 in this end position rests against a stop that ends the movement in the forward direction, and that the filling volume adjoining the anvil 13 on the back is connected to the sprue 4, in that it opens into the filling volume 6. The horizontal movement path of the filling volume 6 and thus the acceleration cylinder 16 end in the lower area or below the mold, which consists of the mold parts 1 and 2, which are pressed against each other on a vertical separating surface 11 and leave the mold cavity 3 and the sprue 4 free between them, whereby the actual mold cavity 3 is preferably formed in only one of the mold parts 1 or 2. The sprue 4 can extend through the other mold part 2, or be located in the same mold part as the mold cavity 3.

The mold parts 1 and 2 can be moved relative to one another and removed from one another in order to facilitate the removal of the finished mold part and are supported on the rear sides facing away from the separating surface 11 initially by an intermediate plate 19a, 19b and this in turn by a pressure plate 18a, 18b.

li·

The anvil rod 22 carrying the anvil 13 penetrates the left part of the mold arrangement, i.e. the left mold part 1, the support plate 19a carrying it and the pressure plate 18a. The anvil rod 22 preferably has a round cross-section and is guided and mounted in a corresponding suitable through-opening.

Likewise, the inner cross-section of the acceleration cylinder 16 is usually a round cross-section.

&iacgr;&ogr; The front surface facing the filling volume 6 is bevelled - at least towards the side of the mouth 7 of the sprue 4 - in order to facilitate the flow of the liquid material from the filling volume 6 into the sprue 4.

The die casting process takes place in such a way that when the liquid material is filled in, as shown in Fig. 1a, the anvil 13 and the filling piston 12 occupy a defined distance between themselves and thus a defined filling volume 6 below the filling opening 15, which is largely filled from a crucible 14. However, since the filling opening should not be wetted by the liquid material, a small amount of residual air 20 remains in the filling volume 6.

The anvil 13 and the filling piston 12 are then moved preferably synchronously to one another and thus with the filling volume 6 in between them still remaining the same in the direction of Fig. 1a to the left, i.e. in the direction of the mold 1, 2, and initially slowly but increasingly accelerated until a speed of about 90 m/s is reached shortly before the injection position and the anvil 13 hits the end stop and is suddenly braked. The filling piston 12, which has previously pushed the filling volume 6 and the anvil 13 in front of it, presses either only due to its inert mass or by further active drive in the forward direction towards the anvil 13, whereby the filling volume 6 in between is put under pressure with the result that the residual air 20 contained therein is compressed, while the liquid material can hardly or not at all be compressed.

Due to the inertial forces, a layering is also formed within the filling volume 6 in such a way that the heavier liquid material occupies the lower and front area of the filling volume and in particular covers the opening 7" of the sprue 4, which is arranged in the front area at the highest point of this filling volume 6 in Figures 1a, 1b. The residual air 20, on the other hand, will collect at the rear end and preferably in the upper area due to its lower specific weight compared to the liquid metal. Depending on how strong the negative acceleration is when the filling volume 6 is decelerated, gravity plays such a subordinate role that the horizontal coating shown in Fig. 2c can also occur when the anvil 13 is stopped, with the residual air 20 arranged parallel to the right, rear end face of the filling volume 6 and the remaining area to the left of this separating surface being filled with liquid material.

In order to reliably prevent the residual air 20 from penetrating the sprue 4 and thus the mold cavity 3, an escape space 8 for the residual air, which is highly compressed in this state, is arranged in the solution of Figures 1a, 1b above the highest point of the inner cross section of the acceleration cylinder 16 in the area in the longitudinal direction 10, which represents the rear area of the filling volume 6 in the injection position of Figure 1b. In contrast, the orifice 7" is also arranged at the highest point in the front area of the filling volume 6.

In contrast, Fig. 2a, 2b shows an arrangement in which - in addition to the already mentioned continuation of the acceleration cylinder 16 through the length of the mold - the escape space 8 is arranged at the same location as described above in Fig. 1, but the mouth 7 or T of the sprue in the filling volume 6 is not located at the highest point of the cross-section of the filling volume, but rather at half height or even at the lowest point, but again in the frontmost area in the longitudinal direction 10 of the filling volume 6.

IS·

Fig. 2c, on the other hand, shows the escape space 8 at the highest point and in the frontmost area with respect to the filling volume, while the mouth of the sprue 4 is located significantly below the highest point of the filling volume 6 in its front area. This latter solution is particularly suitable for injection molding processes that work with relatively little deceleration of the filling volume 6 at the injection position.

What all solutions have in common is that before the filling volume 6 comes into contact with the mouth 7 or T or 7" of the sprue 4, the mold cavity 3 is placed under negative pressure, i.e. partially or largely evacuated, namely down to approx. 960 mbar, i.e. 95 - 98% vacuum.

In order to prevent large amounts of metal particles from entering the connection for the mold vacuum, the vacuum is not applied directly in the mold cavity 3 or in the sprue 4, but in the path of movement of the anvil 13, in a region in the longitudinal direction 10, which ensures that the application of the vacuum by the anvil 13 is only terminated when the anvil 13 overflows the mouth 7, by blocking the mouth of the mold vacuum from the sprue.

As Figures 1a, 1b show, the evacuation channel 23 runs centrally in the anvil rod 22 and ends on the outer surface of the anvil rod 22 shortly before the anvil 13.

Fig. 2a, on the other hand, shows a solution in which the evacuation channel 23' opens directly into the inner surface of the acceleration cylinder 16, directly at its front end on the injection mold side. An opening in the front face of the acceleration cylinder or in the end stop for the anvil 13 is also possible.

In order to prevent air from the environment from penetrating into the mold cavity 3 along the parting surface 11, which is no longer actively evacuated once the anvil 13 overflows the mouth 7, at least one, preferably several, vacuum sealing channels 17a, 17'a, 17"a in the form of ring channels are arranged around the mold cavity 3 in one of the mold parts 1 on the parting surface 11, whose vacuum source - which is preferably separate from the vacuum source 24 for the mold vacuum - is supplied. The individual vacuum sealing channels 17 have no connection to the mold interior except for the unavoidable 100% tightness along the parting plane 11. However, there is no intentional connection here. The vacuum sealing channels 17a, 17'a, 17"a are also connected to one another. 17'a, 17"a only connected via the common vacuum source (not shown).

In the same way and for the same purpose, vacuum sealing channels 17b are arranged between the intermediate plate 19a and the anvil rod 22 penetrating it, as well as a channel 17c between the left mold part 1 and the left intermediate plate 19a.

Likewise, a channel 17d seals between the right mold part 2 and the right intermediate plate 19, and another channel 17e seals between the acceleration cylinder 16 and the right mold part 2. These are also preferably ring-shaped closed channels.

LIST OF REFERENCE SYMBOLS

5 1 molded part 13 Anvil 2 molded part 14 melting pot 3 Mold cavity 20 15 Filling opening 4 Sprue channel 16 cylinder 10 5 Feed room 17 Vacuum sealing channel 6 Filling volume 18a, 18b printing plate 7 mouth 19a, 19b Intermediate plate 8th Alternative space 25 20 Residual air 9 Tread 21 Piston rod 15 10 Longitudinal direction 22 Anvil bar 11 Separation surface 23 Evacuation channel 12 Filling piston 24 Vacuum source 30

Claims (12)

PROTECTION CLAIMS 1. Device for producing molded parts from liquid, hardenable material, with a mold cavity (3) with a sprue (4) in a mold (1,2),
a filling volume (6) for providing the liquid material outside Ø of the mold cavity (3), characterized in that the filling volume (6) is movable along an acceleration cylinder (16) from a filling position to a point at which the filling volume (6) comes into contact with the sprue (4) of the mold cavity (3) (the sprue position), and the sprue channel (4) connecting the mold cavity (3) and the filling volume (6) when filling the mold (1, 2) opens at a point in the filling volume (6) (located in the sprue position) at which only liquid material is present when the filling volume (6) is filled with liquid material and air. 2. Device according to one of the preceding claims,
characterized in that the filling volume (6) is accommodated between an anvil (13) and a filling piston (12) as a front-side limitation in the acceleration cylinder (16). 3. Device according to one of the preceding claims,
characterized in that Anvil (13) and filling piston (12) can be moved simultaneously, in particular synchronously, along the acceleration cylinder (16). V9 4. Device according to one of the preceding claims, characterized in that the anvil (13) is actively movable along the acceleration cylinder (16). 5. Device according to one of the preceding claims, characterized in that the mouth (7) of the sprue (4) is arranged below the upper region of the cross section of the filling volume (6). &iacgr;&ogr; 6. Device according to one of the preceding claims, characterized in that the mouth (7') of the sprue (4) is arranged at the lowest point of the cross-section of the filling volume (6). 7. Device according to one of the preceding claims, characterized in that the mouth (7") is arranged at the front area of the filling volume (6) in the longitudinal direction (10). 8. Device according to one of the preceding claims, characterized in that an escape space (8) for the air contained in the filling volume (6) is arranged outside the inner running surface (9) of the acceleration cylinder (16), in particular as a cavity (8) opening into the running surface (9) in the inner circumference of the cylinder (16) and/or the mold (1, 2). 9. Device according to one of the preceding claims, characterized in that the separating surface (11) between the molded parts (1, 2) is arranged essentially vertically and in particular the acceleration cylinder (16) is connected to the sprue channel (4) below the molded parts (1, 2). 10. Device according to claim 9, characterized in that the acceleration cylinder (16) is arranged horizontally. 11. Device according to claim 9, characterized in that the acceleration cylinder (16) is arranged sloping towards the sprue channel (4). &iacgr;&ogr; 12. Device according to one of the preceding claims, characterized in that negative pressure sealing channels (17) are arranged around the mold cavity (3) to prevent the penetration of outside air into the mold cavity (3).