Method for producing pressure die-cast or injection moulded articles using salt cores

BACKGROUND OF THE INVENTION

The present invention relates to a plant for producing pressure die-cast or injection moulded articles using salt cores.

As is known, pressure die-cast or injection moulded articles are produced by injecting a material in liquid form—either a metal (for example aluminium) or a plastic (for example polyamides)—into a mould bearing a negative impression of the desired article. It is common practice, moreover, to place one or more filler cores inside the mould so as to produce articles with cavities and/or apertures of a particular shape.

For some time now, the problems connected with the need to extract the filler cores from within the article once the material injected into the mould has solidified have led those working in the field to examine the possibility of making the filler cores by compressing salt grains (for example sodium chloride grains) inside moulds bearing a negative impression of the core shape. Indeed, such a solution would enable the filler cores to be removed from within the article simply by washing the said article with a liquid solvent (for example water) capable of dissolving the salt grains, and would also make it possible to adopt design solutions which could not otherwise be used with conventional techniques.

At present, however, the use of salt filler cores is very much limited by the difficulty in obtaining sufficient quantities of salt grains having suitable morphological characteristics to satisfy—at an industrially acceptable cost—the requirements of a plant for producing pressure die-cast or injection moulded articles.

Through experimentation, it has been observed that, in order to withstand the stresses generated during injection of the material in a liquid state into the mould, the salt filler cores need to be extremely compact and, consequently, the shape and particle size of the salt grains used to produce these cores have to be such as to ensure that the maximum possible density is achieved once the salt grains have been compressed inside the moulds for producing the cores.

At present, salt grains that are suitable for producing salt filler cores are only obtained after a lengthy and expensive process involving, firstly, a grinding stage in which generic salt agglomerates are fed into grinders from which salt grains of varying particle size are obtained and, subsequently, a grading stage in which the salt grains produced by the grinders are conveyed into screening machines capable of separating from the stream of salt grains those salt grains that are of a suitable size for producing the salt cores referred to above.

The production process described above has the great disadvantage of giving a low specific yield; consequently, in order to ensure that enough salt grains are produced to satisfy the needs of a plant for producing pressure die-cast or injection moulded articles, the number of grinders and screening machines needed would be such as to make construction of the plant unfeasible on account of the excessive costs involved. Furthermore, the salt grains obtained using the production process described above have many sharp edges which do not particularly lend themselves to achieving high levels of compaction.

SUMMARY OF THE INVENTION

The object of the present invention is, therefore, to provide a plant for producing pressure die-cast or injection moulded articles using salt filler cores which does not have the disadvantages listed above.

According to the present invention, a plant is provided for producing pressure die-cast or injection moulded articles using salt cores, characterized in that it comprises a unit for producing salt grains having a shape and particle size suitable for producing salt filler cores; a press designed to make at least one said salt filler core by compacting a given quantity of salt grains; and a pressure die-casting machine designed to produce the said pressure die-cast or injection moulded articles by injecting a material in the liquid state into a mould containing at least one salt filler core produced by the said press; the said unit for producing salt grains being designed to atomize a saline solution into a plurality of drops and to cause the salt contained in each drop to solidify into a corresponding salt grain.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described with reference to the appended drawings which illustrate a non-limiting embodiment thereof, in which:

FIG. 1

diagrammatically illustrates a plant for producing pressure die-cast or injection moulded articles constructed according to the teachings of the present invention;

FIG. 2

diagrammatically illustrates a first detail of the plant illustrated in

FIG. 1

; and

FIG. 3

illustrates a diagrammatic perspective view of a second detail of the plant illustrated in FIG.

1

.

DETAILED DESCRIPTION OF THE INVENTION

With reference to

FIG. 1

, the reference numeral

1

denotes, overall, a plant that uses salt filler cores for producing pressure die-cast or injection moulded articles.

The plant

1

comprises a unit

2

for processing generic salt agglomerates (for example sodium chloride agglomerates) into salt grains having a shape and particle size suitable for producing salt filler cores; a press

3

designed to compress the salt grains inside moulds bearing a negative impression of the shape of the filler cores needed to produce the abovementioned pressure die-cast or injection moulded articles; a conventional pressure die-casting machine

4

designed to produce the abovementioned articles by injecting a material in the liquid state—either a metal (for example aluminium) or a plastic (for example polyamides)—into a mould containing one or more salt filler cores produced by the press

3

; and, lastly, a salt removal and washing unit

5

designed to remove the salt filler cores that have been trapped within the articles from the said articles as they leave the pressure die-casting machine

4

.

Preferably, but not necessarily, the plant

1

also comprises a container

6

for storing the salt agglomerates which are converted into salt grains by the unit

2

; a container

7

for collecting and storing the salt grains output by the unit

2

; and, optionally, a vibrating screen

8

of known type located upstream of the press

3

and designed to grade the salt grains coming out of the unit

2

and/or the container

7

so as to convey to the press

3

only those salt grains that are suitable for producing the salt filler cores, conveying the remaining salt grains back to the container

6

or to the unit

2

.

In the example illustrated, the press

3

for making the salt filler cores is a conventional press for producing components by compacting metallic powders such as, for example, the MANNESMAN DEMAG E

2

series press, model HPM 150, while the salt removal and washing unit

5

is designed to convey the salt removed from the pressure die-cast or injection moulded articles to the container

6

and/or to the unit

2

so that it can be used again to produce salt grains.

The unit

2

produces the abovementioned salt grains having a shape and particle size suitable for producing salt filler cores by dissolving the salt from the container

6

, the salt removal and washing unit

5

and/or the vibrating screen

8

in a liquid solvent in order to obtain a saline solution which is preferably, but not necessarily, supersaturated with salt; by subsequently atomizing the abovementioned saline solution into minute drops which are all of approximately the same size; and, lastly, by evaporating the liquid solvent present in each drop of saline solution so as to cause the salt in solution/suspension in the drop of saline solution to solidify into a corresponding salt grain of essentially spherical shape.

Given that the dimensions of the salt grains depend essentially on the dimensions of the drops of saline solution, the unit

2

produces a stream of salt grains which are all of approximately the same size. Consequently, by suitably selecting the size of the drops of saline solution, it is possible to obtain a stream of salt grains having a shape and particle size suitable for producing salt filler cores, that is having an essentially spherical shape with a diameter of preferably, but not necessarily, between 100 and 300 &mgr;m.

With reference to

FIG. 2

, the unit

2

for producing the abovementioned salt grains, hereinafter denoted by the reference

10

, comprises a drying tower

11

with an outlet mouth

12

at the bottom from which the salt grains

10

having a given particle size come out; a hot air feed unit

13

designed to convey a stream of air at a given temperature into the drying tower

11

; a salt solution feed unit

14

designed to atomize a liquid solvent containing salt in solution inside the drying tower

11

; and, lastly, a hot air discharge unit

15

designed to convey the hot air generated by the hot air feed unit

13

out of the drying tower

11

.

In the example illustrated, the hot air feed unit

13

in turn comprises a blower unit

16

of known type which is designed to convey a stream of air of given value through a feed pipe

17

and into the drying tower

11

, and a burner

18

of known type which is located along the feed pipe

17

, downstream of the blower unit

16

, and is designed to bring the stream of passing air to a temperature of preferably, but not necessarily, between 400° C. and 700° C. The hot air feed unit

13

lastly comprises an air dispenser

19

which is located inside the drying tower

11

and is designed to impart a preferably, but not necessarily, spiral-like trajectory to the stream of hot air coming from the feed pipe

17

.

The salt solution feed unit

14

, on the other hand, comprises a mixing tank

20

, inside which the salt is dissolved in the abovementioned liquid solvent, and a pump unit

21

designed to convey the liquid solvent containing salt in solution from the mixing tank

20

to the drying tower

11

at a pressure of preferably, but not necessarily, between 2 and 4 bar.

In the example illustrated, the mixing tank

20

is fitted with a mixing device

22

of known type designed to stop the salt suspended in the liquid solvent precipitating at the bottom of the said mixing tank

20

.

The salt solution feed unit

14

, on the other hand, comprises an atomizing device

23

designed to atomize the liquid solvent coming from the pump unit

21

inside the drying tower

11

in such a way that the atomized liquid solvent comes into contact with the stream of hot air produced by the hot air feed unit

13

. In the example illustrated, the atomizing device

23

comprises one or more atomizing nozzles which are positioned inside the drying tower

11

, upstream of the outlet mouth

12

, and point towards the dispenser

19

so that the liquid solvent containing the salt in solution is atomized counter to the stream of hot air, which is travelling in the opposite direction. Needless to say, alternative configurations are possible in which the liquid solvent is atomized in the same direction as the direction of travel of the stream of hot air, in other words in co-current.

With reference to

FIG. 2

, the hot air discharge unit

15

in turn comprises a suction unit

24

designed to suck the hot air from inside the drying tower

11

through a suction pipe

25

, the inlet of which is located inside the drying tower

11

, directly upstream of the outlet mouth

12

, and a chimney

26

designed to convey the hot air sucked up by the suction unit

24

to the outside. In the example illustrated, filtering devices

27

of known type (for example electrostatic precipitators or wet-type filters) are found both upstream and downstream of the suction unit

24

, their function being to extract and recover the particles suspended in the hot air drawn from the drying tower

11

.

Lastly, the unit

2

has a conveyor

28

for conveying the salt grains

10

coming out of the outlet mouth

12

to the container

7

, to the vibrating screen

8

or directly to the press

3

.

With reference to

FIG. 3

, the salt removal and washing unit

5

comprises a cleaning machine

30

designed to remove most of the salt that makes up the filler cores from the articles output by the pressure die-casting machine

4

, a washing machine

31

designed to remove any salt residues left within the articles once the latter have passed through the cleaning machine

30

and, lastly, a conveyor

32

designed to convey the articles from the cleaning machine

30

into the washing machine

31

.

In the example illustrated, the cleaning machine

30

comprises a washing tunnel

33

, inside which there are a plurality of washing stations

34

, and a conveyor

35

designed to advance the articles produced by the pressure die-casting machine

4

inside the washing tunnel

33

along a path P that passes successively through all the washing stations

34

.

Each washing station

34

in the cleaning machine

30

has a plurality of nozzles

36

, each of which is designed to spray one or more jets of a saline solution—preferably, but not necessarily, containing the same salt as that used to make the abovementioned filler cores—on to the articles carried by the conveyor

35

.

In the example illustrated, the saline solution is a saline solution which is saturated or supersaturated with salt so as to make it easier to recover the salt that makes up the filler cores. If a saline solution saturated with salt is used, the filler cores are broken up by the mechanical action of the jets alone. If a saline solution supersaturated with salt is used, the filler cores are broken up by the mechanical action of the jets and by the abrasive action of the salt suspended in the supersaturated saline solution.

The cleaning machine

30

also comprises a recovery tank

37

located underneath the conveyor

35

to recover the saline solution which has been sprayed on to the articles and which thus carries in suspension the salt from which the filler cores are made, a pump unit

38

designed to draw the saline solution from the recovery tank

37

so as to convey it to the nozzles

36

at a given pressure and, lastly, a filtering device

39

designed to extract the excess salt from the saline solution collected in the recovery tank

37

and convey it to the container

6

and/or to the mixing tank

20

to be reused.

A centrifugal filter of the type currently used in the optics industry for recovering cerium oxide from processing waters can effectively be used as a filtering device

39

.

The washing machine

31

, on the other hand, is a conventional machine for washing parts, such as for example the SYNCRO 02/1000 washing machine produced by PA-DA S.N.C., optionally combined with a desalinating device

40

of known type which is designed to recover from the washing water of the washing machine

31

any salt that is left on the pressure die-cast or injection moulded articles after they have passed through the cleaning machine

30

.

The way in which the plant

1

works will now be described. It has been assumed that the press

3

produces only one filler core at a time, that the pressure die-casting machine

4

produces only one pressure die-cast or injection moulded article at a time and that this article is obtained using only one salt filler core.

During operation, the salt and the liquid solvent are poured into the mixing tank

20

in order to obtain the supersaturated saline solution. The supersaturated saline solution is then atomized into minute drops inside the drying tower

11

, where the stream of hot air causes the liquid solvent contained in each drop of saline solution to evaporate, thereby forcing the salt in solution/suspension in each drop of saline solution to solidify into a corresponding salt grain

10

of essentially spherical shape. Once solidification has taken place, the salt grains

10

fall under the effect of gravity to the bottom of the drying tower

11

where they pass through the outlet mouth

12

and on to the conveyor

28

which conveys the salt grains

10

into the container

7

.

The amount of salt needed to produce a single salt core is then taken from the container

7

and conveyed to the press

3

, optionally first passing through the vibrating screen

8

. Once the press has compacted the salt grains

10

to make the salt filler core, a robot arm (of known type and not illustrated) takes the filler core from the press and positions it inside the mould of the pressure die-casting machine

4

.

After the filler core has been placed inside the mould of the pressure die-casting machine

4

, the material in the liquid state is injected into the mould so as to produce the pressure die-cast or injection moulded article with the desired shape.

Once the material injected into the mould has solidified, the article—which contains the filler core trapped inside it—is then conveyed to the salt removal and washing unit

5

where the filler core is broken up and the salt from which it was made is recovered. During its dwell time inside the cleaning machine

30

, the article is sprayed with the jets of saturated or supersaturated saline solution, thereby causing the filler core trapped within the article to break up, whereas during its dwell time inside the washing machine

31

, the article coming out of the cleaning machine

30

is subjected to conventional washing, involving repeated immersion in tanks containing liquid solvent that contains essentially no salt in solution, so as to remove the last residues of salt.

The main advantage of the plant

1

described above and illustrated is that it is extremely economical to build, therefore making the use of salt filler cores economically advantageous compared with conventional manufacturing methods.

In conclusion, it should be clear that modifications and variations can be made to the plant

1

described and illustrated herein without thereby departing from the scope of the present invention.