[0001] The present invention relates to a mould for the injection moulding of metals such
as magnesium and its alloys, and to an injection moulding process using such a mould.
[0002] More specifically, the latter includes a body in which at least one cavity is formed,
reproducing the shape of the piece to be moulded, and at least one duct for feeding
the molten metal into the cavity.
[0003] In the prior art, once the molten metal is injected, it solidifies in the feeding
ducts as well as in the mould cavity. The ratio of the mass of the moulded piece to
that of the total mass of solidified metal is typically around 0.5. This means that
around half the material is wasted in each moulding cycle and must be recycled in
special processes which are fairly complex and of not insignificant cost.
[0004] The object of the present invention is to counter the aforesaid disadvantages of
the prior art.
[0005] This object is achieved according to the invention by providing a mould, and an associated
moulding process having the characteristics claimed specifically in the subsequent
Claims.
[0006] Advantages and characteristics of the present invention will become apparent from
the detailed description which follows, with reference to the appended drawings, provided
purely by way of non-limitative example, in which:
Figure 1 is a sectioned side elevation view of a mould according to the invention;
Figure 2 is a sectioned front elevation view of the mould of Figure 1;
Figure 3 is a plan view of the mould of the preceding drawings;
Figure 4 is an enlarged-scale view of a detail of the mould of the preceding drawings;
and
Figure 5 shows a section taken on the line V-V of Figure 4.
[0007] A mould for the injection moulding of metals, such as magnesium and its alloys, has
(see Figures 1-3) a body 10 which includes a first die 12 and a second die 14, arranged
facing each other so as to form a cavity 16 reproducing the shape of a piece to be
moulded. The body 10 of the mould also includes, in association with the die 12, a
plurality of plates 18, 20, 22 which directly or indirectly define a plurality of
ducts for introducing the molten metal into the cavity 16 and have a common initial
portion 24 and separate end portions 26.
[0008] An external plate, indicated 18, supports an injection feed socket 28 having a first
duct 30 formed therein which forms part of the common initial portion 24 of the feeding
ducts. A distribution plate, indicated 20, has a second duct 32 formed in it, arranged
in series with and substantially orthogonal to the first duct 30. An injector-support
plate, indicated 22, carries a plurality of injectors 34 which define respective ducts
running substantially orthogonally from the second duct 32 and constituting the separate
portion 26 of each duct.
[0009] The distribution plate 20 is arranged in a cavity 36 defined by the external plate
18, by the injector-support plate 22 and by spacer bars 38, the height of which is
greater than that of the distributor plate 20 and which extend between facing peripheral
portions of the external plate 18 and of the injector-support plate 22. The plates
18, 20 and 22 and the bars 38 are fixed together by screws 40, while additional screws
42 enable the injection-support plate 22 to be fixed to the die 12. Ventilation holes
41 are formed in the bars 38 in order to prevent condensation from forming in the
cavity 36. Spacer elements 44 and positioning elements 46 are arranged between the
distribution plate 20 and the external plate 18 and the injector-support plate 22.
[0010] The feed socket 28, the distribution plate 20 and the injectors 34 have heating means,
in particular electrical resistors 48, operable to keep the metal liquid in the respective
portions 24, 26 of the feeding channels. Electrical resistors 48 are also arranged
in some of the spacer elements 44.
[0011] The separate end portions 26 of the feed ducts also have respective selective interception
means (see Figures 4 and 5) which include a stopper 50 able to slide transverse the
flow direction of the metal. Each stopper 50 is mounted (see Figure 2) on the free
end of a rod 52 of a piston 54 slidable in a cylinder 56 of a respective actuator
member 57, which may be hydraulic or pneumatic, for example.
[0012] The cross sections of the portions 58 of the feed ducts downstream of the stoppers
50 become steadily larger (see figure 4) towards their ends opening into the mould
cavity 16.
[0013] The mould also has means for monitoring and adjusting the temperature, in particular
thermocouples 60 which are associated with the injector-support plate 22, the die
12 and the resistors 48, and are connected to a central control unit 62 which is able
to conveniently adjust the heat generated by the various resistors 48 on the basis
of information on the temperature from the thermocouples 60, in such a way that an
operating temperature corresponding to the design value is achieved across all points
of the mould. In order to achieve better temperature regulation, it has proved to
be advantageous to fit each injector 34 (see Figure 2) with two separate resistors
48a, 48b, the first 48a being positioned between the plates 20, 22 and the second
48b being positioned between the plate 22 and the die 12.
[0014] In order to carry out an injection cycle in the mould described above, a molten metal-feeding
nozzle 66, known per se, is applied (see figure 1) to the opening 64 of the feed socket
28, with the shape of the opening 64 preventing any penetration of air, loss of molten
metal or stoppage of the latter.
[0015] The molten metal, which is at a temperature of around 620-700°C, thus flows into
the first duct 30, enters into the second duct 32 and from there separates into the
ducts 26 formed in the various injectors 34, thereby being delivered into the cavity
16 until it is entirely full. This procedure is completed extremely quickly (it takes,
for example, around 20-40 ms to inject 1,500 g of material) and under extremely high
pressure, which can go as high as 800 bar. Thanks to the structure of the mould, the
extremely high reaction forces generated by such pressure are mainly discharged onto
the bars 38 and onto the external plate 18 and the injector-support plate 22, thus
protecting the components most closely involved in the injection flow, which are undoubtedly
more delicate.
[0016] The central control unit 62 controls the power output of the various resistors 48
in order to ensure that the metal is in a molten state at each point of the ducts,
that is at a temperature over 600°C. At the same time, the average temperature of
the mould, and that of the dies 12, 14 in particular, must be considerably lower (around
250°C) and that of the more sensitive parts, such as the control unit 62 and the actuator
members 57, must be even lower (around 40°C).
[0017] In order to be able to obtain such a differentiated temperature pattern, various
arrangements are used. In particular, thanks to the air in the cavity 36, the distributor
plate 20, where the most heat is produced, is substantially thermally-insulated from
the other plates, to which it is connected only by small thermal bridges constituted
by the spacer elements 44 and the positioning elements 46, as well as by the injectors
34 and the feed socket 28.
[0018] The cylinders 56 of the actuator members 57 are mounted externally of the body 10
of the mould (see figure 3), thereby increasing the possibility of external heat dissipation,
and have internal circuits (not shown in the drawings) for the circulation of a cooling
fluid. Similar circuits could also be arranged in other components, such as the external
plate 18 and the injection-support plate 22.
[0019] The wiring box 62 is also arranged to project from the body 10 of the mould in order
to increase the possibility of heat dissipating outwards, and has insulating panels
68 interposed between it and the body and a perforated bar 70, which both reduce the
heat flowing from the mould.
[0020] In addition, appropriate coupling means, such as the positioning elements 46 (see
Figure 1) of the distribution plate 20 in respect of the injector-support plate 22,
and the interstices 72 (see Figure 4) left in the area where the stopper 50 is mounted
on the rod 52, enable the various elements to be kept correctly aligned, despite the
effect of thermal expansion.
[0021] To return to the description of the injection moulding process, once the molten metal
has filled the cavity 16, solidifying substantially instantly, the stoppers 50 are
pushed by their respective actuator members 57 into positions whereby they intercept
the ducts 26. The liquid metal which is in the portion of the ducts 26 upstream of
the stoppers 50 is thus separated from the solidified metal in the downstream portions
58. It is advantageous if the stoppers 50 are force-fitted to the surfaces forming
their seat, in order to avoid any air seeping into the metal.
[0022] Once the ducts have been blocked, the dies 12, 14 are opened and the moulded piece
is extracted by a conventional method. When it is removed, the piece takes with it
the fragments of metal which solidified in the downstream portions 58 of the ducts
26. These fragments - which do not hinder removal of the piece, thanks to their tapered
shape - constitute the only portion of material which is wasted or must be recycled.
The metal which remained in a liquid state in the upstream portion of the ducts will
form the head of the flow of material to be injected for the next operating cycle.
[0023] In any case, the waste fragments make up only a tiny fraction - less than 5% - of
the material injected into the cavity 16, while with prior art techniques the mass
of the material wasted or to be recycled is about equal to that of the material injected
into the cavity 16.
[0024] Naturally, the principle of the invention remaining the same, manufacturing details
and embodiments may vary widely from those described purely by way of non-limitative
example, without thereby departing from the scope of the invention. In particular,
a mould structure of the type described above could be provided not just for one but
for both the dies forming the mould cavity.
1. A mould for the injection moulding of metals, such as magnesium and its alloys, having
a body (10) which includes at least one cavity (16) reproducing the shape of the piece
to be moulded and at least one duct for feeding the molten metal into the cavity (16),
the said mould being characterised in that the said at least one feed duct is fitted
with heating means.
2. A mould according to Claim 1, characterised in that the said at least one feed duct
has selectively operable interception means.
3. A mould according to Claim 2, characterised in that the said interception means include
a stopper (50) which is slidable transverse the direction of flow of the metal in
said duct.
4. A mould according to Claim 3, characterised in that the said stopper (50) is mounted
on the free end of the rod (52) of a piston (54) slidable in the cylinder (56) of
an actuator member (57).
5. A mould according to any of Claims 2 to 4, characterised in that the cross section
of the portion (58) of feed duct downstream of the said interception means increases
gradually towards the end opening into the mould cavity (16).
6. A mould according to any preceding Claim, characterised in that it includes a plurality
of feed ducts with a common initial portion (24).
7. A mould according to Claim 6, characterised in that the said initial portion (24)
is formed by a first duct (30) and a second duct (32) arranged in series with and
substantially orthogonal to the first duct (30), separate end portions (26) of the
said feed ducts extending substantially orthogonally from the said second duct (32).
8. A mould according to any preceding Claim, characterised in that the said heating means
are electrical resistors (48).
9. A mould according to any preceding Claim, characterised in that the said body (10)
includes at least first and second dies (12, 14) arranged facing each other so as
to define at least one mould cavity (16) which reproduces the shape of the piece to
be moulded, the said body (10) also including a plurality of plates (18, 20, 22),
associated with at least one of the said dies (12), through which the said feed ducts
are formed.
10. A mould according to Claim 9, characterised in that it includes an external plate
(18) bearing an injection feed socket (28) in which the said first duct (30) is formed,
a distribution plate (20) in which the said second duct (32) is formed, and an injector-support
plate (22) carrying a plurality of injectors (34) each defining a respective duct
constituting the separate portion (26) of each duct.
11. A mould according to Claim 10, characterised in that the said feed socket (28) and/or
the said distributor plate (20) and/or the said injectors (34) have heating means,
in particular electrical resistors (48), operable to keep the metal liquid in the
respective portions of feeding duct.
12. A mould according to Claim 10 or 11, characterised in that the said distribution plate
(20) is arranged in a cavity (36) formed by the said external plate (18) and the injector-support
plate (22) and by spacer bars (38) which are higher than the distribution plate (20)
and extend between facing peripheral portions of the external plate (18) and the injector-support
plate (22).
13. A mould according to Claim 12, characterised in that spacer elements (44) and positioning
elements (46) are interposed between the said distributor plate (20) and the external
plate (18) and the injector-support plate (22).
14. A mould according to any preceding Claim, characterised in that it has means for monitoring
and adjusting the temperature.
15. A process for the injection moulding of metals, such as magnesium and its alloys,
which requires the use of a mould according to any preceding Claim.