WO 2003103923 A1
A molding method including the steps of: providing at least one mold (1, 2) having at least one inner cavity (4); placing inside the cavity (4) a body (20) defined by a sheet of deformable metal material; injecting plastic material (33) under pressure inside the cavity (4) to deform the sheet towards the walls of the cavity, so that the sheet rests on the walls (5, 7, 8) and assumes the shape of at least part of the cavity (4); allowing the injected material to set, and extracting a body of plastic material formed in one piece and having one portion covered with a shaped sheet of metal.
METHOD OF INJECTION MOLDING A PART OF PLASTIC MATERIAL
TECHNICAL FIELD The present invention relates to a method of molding a part of plastic material.
Methods of molding parts of plastic material are known in which liquid plastic material is injected under pressure into a mold cavity; and the molds comprise a number of component parts, which can be disassembled to remove the part from the mold once the plastic material has set and cooled.
In many applications, a portion of the molding is coated with metal for aesthetic and/or structural reasons .
The metal coating is normally deposited electrochemically, e.g. by chromium or nickel plating.
Known methods, however, fail to provide for obtaining a metal coating of appropriate thickness and consistency, so that the coating is easily removed, e.g. by wear.
DISCLOSURE OF INVENTION It is an object of the present invention to provide a method of molding parts of plastic material, designed to eliminate the drawbacks of known methods.
According to the present invention, there is provided a method of molding a part of plastic material, characterized by comprising the steps of : providing at least one mold having at least one inner cavity; placing inside said cavity a body defined by a sheet of deformable metal material; injecting plastic material under pressure inside said cavity to deform said sheet of metal material towards the walls of said cavity, so that said sheet of metal material rests on said walls and assumes the shape of at least part of said cavity; allowing said plastic material to set, and extracting a body of plastic material formed in one piece and having at least one portion covered with a shaped sheet of metal .
BRIEF DESCRIPTION OF THE DRAWINGS
A preferred, non-limiting embodiment of the invention will be described by way of example with reference to the accompanying drawings, in which:
Figure 1 shows a longitudinal section of a mold which may be used to implement the method according to the present invention; Figure 2 shows a first step in the method according to the present invention;
Figure 3 shows a second step in the method according to the present invention; Figure 4 shows a third step in the method according to the present invention;
Figure 5 shows, by way of example, a view in perspective of a part molded from plastic material and which can be produced using the method according to the present invention.
BEST MODE FOR CARRYING OUT THE INVENTION Figure 1 shows a mold which can be used to implement the method according to the present invention. More specifically, the mold comprises a first top half-mold 1 defined by a number of parts (detailed later on) ; and a second bottom half-mold 2 also defined by a number of parts (not detailed for the sake of simplicity) . Top half-mold 1 defines a first cavity 4 defined by a cylindrical wall 5 coaxial with an axis 6, by an annular, substantially C-shaped wall 7 radially larger than cylindrical wall 5, and by a substantially flat wall 8 connected to annular wall 7 and crosswise to axis 6. More specifically, half-mold 1 comprises a first and a second portion 10 and 11 facing each other and defining opposite portions of walls 5 and 7; and a third portion 13 defining wall 8 and the adjacent edge of annular wall 7. Half-mold 2 defines a substantially cylindrical second cavity 16 coaxial with axis 6, communicating with cavity 4, and in turn defined by a wall portion 17 having a thread or similar (e.g. a profile forming a bayonet connection) .
In a first step in the method according to the present invention, a cup-shaped body 20 defined by a roughly 0.4-0.6 mm thick sheet of deformable metal material (e.g. aluminium) is inserted inside first cavity
In the example shown, cup-shaped body 20 comprises a tubular portion 21 closed on a first side by an integral dome-shaped portion 22, and having a peripheral annular edge 24 extending radially outwards from a second side of tubular portion 21.
Cup-shaped body 20 is positioned coaxially with axis 6, with tubular portion 21 facing cylindrical wall 5 and annular wall 7, and with dome-shaped portions 22 facing wall 8; and half-molds 1 and 2 are positioned so that peripheral annular edge 24 is gripped in fluidtight manner between half-molds 1 and 2 in the closed position.
In a second step in the method according to the present invention, a male cylindrical member 30 is inserted inside second cavity 16 and a large portion of first cavity 4. More specifically, member 30 is positioned coaxially with axis 6, with an end portion 30a facing cup-shaped body 20, which is thus interposed between the walls defining cavity 4 and member 30. In a third step in the method according to the present invention (Figure 4) , thermoplastic material 33 is injected under pressure inside the gap 34 (Figure 3) defined by cup-shaped body 20 and member 30. The injection pressure may conveniently be roughly 1200-2500 bars, and the injection temperature roughly 180-350 °C.
The injected thermoplastic material 33 exerts pressure (Pascal's law) on the sheet of metal material forming cup-shaped body 20, and pushes the sheet of metal material towards the walls of cavity 4 so that it rests on cylindrical wall 5, annular wall 7, and wall 8, thus assuming the shape of cavity 4. The thermoplastic material 33 injected into cavity 4 thus fills all the space between the deformed sheet of metal material and member 30.
The thermoplastic material 33 also penetrates second cavity 16 having no sheet of metal material, e.g. penetrates a cylindrical tubular gap between wall 17 and member 30.
Preferably, though not exclusively, member 30 has a nozzle 37 on end portion 30a, by which the liquid thermoplastic material 33 is expelled.
In a fourth step in the method according to the present invention, the injected thermoplastic material is allowed to set.
In a fifth step in the method according to the present invention, the mold is opened to extract the finished part. In the example shown, third portion 13 is detached and removed from first and second portion 10,
First and second portion 10, 11 are then withdrawn in opposite directions to extract the part of plastic material, complete with the cup-shaped body shaped by the operations described above, from half-mold 1.
Bottom half-mold 2 is then opened to extract the whole part . Figure 5 shows, by way of a non-limiting example, a view in perspective of a part which can be produced using the method according to the present invention. The part shown is a fuel tank cap of a prestige vehicle (e.g. a limited-number sports car) , which comprises a threaded tubular portion made of plastic material and closed at one end by an integral portion covered with a shaped sheet of metal .
Obviously, the method described and illustrated herein also provided for producing parts other than the one shown in Figure 5. For example, the method according to the present invention may be used to advantage for producing:
- a vehicle gear lever knob;
- a vehicle door handle body; - an electric torch body;
- an umbrella handle;
- a handbag handle.
The finished part has a shaped metal layer of considerable thickness, which is not deposited, and which is firmly retained on the part of plastic material; and the resulting mechanical consistency and look are far superior to those achievable using known methods .
The molding method also provides for so shaping the sheet of metal material that, in section, it comprises at least a first portion of transverse dimensions Dl (Figure 4) greater than the transverse dimensions D2 of a second portion, so that the shaped sheet firmly envelopes the underlying portion of plastic material .
Clearly, changes may be made to the method as described herein without, however, departing from the scope of the present invention.
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