Apparatus and method for co-injection moulding of plastic materials

0001The present invention relates to an apparatus and method for moulding plastic materials by co-injection. This designation indicates the moulding of objects having layered thickness: two outer layers of a more valuable plastic material, and a core of a different, normally cheaper, plastic material.

0002The object of the present invention is to provide an apparatus and a method for co-injection, whereby it is possible to obtain the perfect confinement of the less valuable plastic material in the core of the moulded object, in order to maintain unaltered the aesthetic characteristics of the pieces with respect to similar pieces obtained by single-component moulding.

0003According to the invention, said object is achieved by means of a moulding apparatus whose primary characteristic is defined in claim 1, and by means of a moulding process whose primary characteristic is defined in claim 5.

0004The invention shall now be described in detail with reference to the accompanying drawings, provided purely by way of non limiting example, in which: <ul id="ul0001" list-style="dash" compact="compact"><li>Figure 1 is a schematic, longitudinal section view of a part of an apparatus for moulding plastic materials by means of co-injection, according to the invention,</li><li>Figure 2 shows a detail of Figure 1 in enlarged scale, and</li><li>Figure 3 shows, in reduced scale, the detail of Figure 2 during the subsequent steps of the co-injection moulding method.</li></ul>

0005Referring initially to Figure 1, the number 1 globally designates an injector nozzle which constitutes the fundamental element of the apparatus and of the method for co-injection moulding according to the invention. The injector nozzle 1, which shall now be described in detail, is operatively associated to an injection conduit, summarily shown with the reference number 2, of a mould 3. Normally, the injection equipment may include a plurality of identical injectors 1.

0006The injector nozzle 1 comprises a body, globally designated by the reference number 4, which includes an external tubular part 5 with associated heating electrical resistors 6 and an internal tubular part 7 which defines a central axis passage 8. At the side oriented towards the injection conduit 2, the body 4 of the injector nozzle 1 is provided with a nozzle terminal generically designated as G and facing, at its free end, the injection conduit 2. Typically, the nozzle terminal G is formed by a hollow internal element 9a of a material with high thermal conducibility, and by a hollow external element 9b of a material with lower thermal conducibility.

0007The nozzle terminal G is coaxially aligned with the central axial passage 8, through which extend a tubular shutter H and, within it, a pin shutter K.

0008From the side of the body 4 of the injector nozzle 1 that is opposite to the injection conduit 2 is positioned a hot chamber body 10 through which are formed a first channel 11 for the flow of a first fluid plastic material A, and a second flow channel 12 of a second fluid plastic material B. The plastic materials A and B are different from each other, and typically the plastic material A is more valuable whilst the plastic material B is less valuable.

0009The first flow channel 11 is separated from the central passage 8 and is formed, for example, according to a substantially helical configuration between the internal element 7 and the external element 5 of the body 4. Said first flow channel 11 joins the central passage 8 at the nozzle terminal G, through an annular chamber 13 formed between the internal element 7 of the body 4 and in part the external element 5 and in part the nozzle terminal G.

0010The second flow passage 12 extends axially along the central passage 8, i.e. it consists of the axial annular inter-space between the internal element 7 of the body 4 and the tubular shutter H. Said second flow channel 12 ends upstream of the nozzle terminal G and, as shall be seen, it is connected through it with the injection conduit 2 by means of one or more substantially radial or oblique ports 14 formed in proximity to the end of said tubular shutter H that is oriented towards the nozzle terminal G.

0011The tubular shutter H is axially movable along the central passage 8 between a retracted position, in which as shall be seen the communication between the flow channel 11 and the nozzle terminal G is open, and an advanced position in which said communication is closed. The movements of the tubular shutter H between the retracted and advanced positions are commanded by means of a first fluid linear actuator 15.

0012The pin shutter K can slide within the tubular shutter H and is movable, by means of a second fluid linear actuator 16 coaxial to the first linear actuator 15, between an advanced position in which it shuts off communication between the flow channel 11 and the injection conduit 2, and a retracted position in which said communication is open.

0013The linear actuators 15 and 16 are operatively connected to an electro-hydraulic control group, not shown in the drawings, whereby they are operated according to a sequential cycle of co-injection of the two plastic materials A, B into the cavity of the mould 3, in the manners described below with reference to Figure 3.

0014The co-injection cycle, as schematically shown in Figure 3, may comprise eight successive steps, number 1 through 7, which follow each other in the direction of the arrow visible in Figure 3, and whereof the first one coincides with the eighth one.

0015In the initial step, designated "1", the pin shutter is in the advanced position and the tubular shutter H is in the retracted position. In this way, both the communication between the injection conduit 2 and the first flow channel 11, and the communication between said injection conduit 2 and the second flow channel 12 are shut off. The passage of both materials A and B from the injector nozzle 1 to the cavity of the mould 3 is thus prevented.

0016In the second step, designated by the number "2", the pin shutter K is moved to a first retracted position, in order to open communication between the flow channel 11 and the injection conduit 2 to inject the fluid plastic material A into the mould 3, whilst the ports 14 remain closed. The plastic material A will form the skin of the artefact.

0017In the third step, designated "3", the pin shutter K is moved to the advanced position, in order to close communication between the flow channel 11 and the injection conduit 2. In this way, the flow of the material A towards the cavity of the mould 3 is stopped.

0018In the subsequent step, designated "4", the tubular shutter H is placed in the advanced position, in order to shut off communication between the flow channel 11 and the nozzle terminal G. A preferential passage is thereby created for the material B which will form the core of the artefact.

0019Then, in the subsequent step designated "5", the pin shutter K is brought to a second retracted position in which it opens the ports 14, placing the second flow channel 12 in communication with the nozzle terminal G as well as with the injection conduit 2 to inject the second fluid plastic material B into the cavity of the mould.

0020Then, in the following step, designated "6", the pin shutter K is moved back to the advanced position, in order to close communication between the second flow channel 12 and the injection conduit 2. In this way, the injection conduit 2 and the point of injection of the second material B are also cleaned.

0021In the subsequent step, designated "7", the pin shutter K is returned to the first retracted position, in which the ports 14 are closed, and the tubular shutter H is placed in the retracted position, in order to complete the injection of the first material A. At the end, the material B is totally confined in the core of the artefact, inside the material A.

0022In the final step, designated "1", the initial condition is restored, so the pin shutter K is placed in the advanced position and the tubular shutter H is in the retracted position, thereby shutting off communication with the injection conduit to both flow channels 11 and 12.

0023It shall be readily apparent from the above description that the independent and combined operation of the two shutters K and J allows to obtain a perfect confinement of the less valuable material B in the core of the moulded object, within the more valuable material A. This feature enables to safeguard the aesthetic characteristics of the moulded pieces, whose final appearance will be comparable to that of pieces obtained with single-component moulding, hence at considerably higher cost. Naturally, the construction details and the embodiments of the apparatus according to the invention may vary widely from what is described and illustrated herein, without thereby departing from the scope of the present invention as defined in the appended claims. Thus, for example, the injection steps of the first and the second plastic materials could be reversed with respect to the above description: in such a case the co-injection sequential cycle shall provide initially injection of the second material B and subsequently injection of the first material A. It also to be pointed out that the steps disclosed in the above as "3", "4" and "6" could be suppressed. Accordingly, shutting off the flow of the first plastic material A (or of the second plastic material B, respectively) and opening the flow of the second plastic material B (or of the first plastic material A, respectively)into the mould 3 would be carried out in a more abrupt fashion, i.e. without an intermediate step of interruption of the flow of both first and second plastic materials A and B. Moreover, even the step "7" could be made optional, and in such a case step "1" will immediately follow step "6". Lastly, the actuators 15 and 16 could consist, instead of fluid jacks, of electrical or different actuators. The method according to the invention can be advantageously employed in a variety of applications, among which: <ul id="ul0002" list-style="none" compact="compact"><li>1. injection at the core of a recycled or quite cheap material, provided that such material be injectable;</li><li>2. injection at the core of a foamed and/or gaseous meterial (i.e. Mu-cell)</li><li>3. injection at the skin of an aesthetically good materal and at the core of a structural material (i.e. having high resistance performance but poor aesthetical features)</li><li>4. insertion at the core of a sound-proof material</li><li>5. differently coloured materials even with an outer transparent layer to achieve decorative effects</li><li>6. mixed articles</li></ul> The only aspect to be taken into consideration is the mutual compatibility between the two different materials. Adhesion therebetween is not fundamental: the article will retain its desired performances even if adhesion between the two materials is low.