Object of the Invention
The present invention is applicable to the plastic material part injection moulding industry, and more specifically to the sector of injecting moulds and injection moulding processes.
More specifically, the object of the present invention is a method which allows extracting the air from inside the plastic injection mould during the injection process.
Background of the Invention
A thermoplastic injection mould is formed, among other components, by two shaping plates, one referred to as cavity plate and another referred to as die plate, as well as a set of ejector plates where several ejector pins are housed, which will serve for ejecting the injected part.
An injection mould can have one or several shaping cavities, wherein at the time of injection the duly melted plastic material is introduced under pressure through an injection nozzle or point, until completely filling of each of the cavities.
Each mould incorporates a refrigeration circuit made up of machined boreholes in the shaping plates which serve to cool the injected plastic and set the part.
The ejector pins located in through housings structurally configured in the die plate are activated through the ejector plates, being responsible for extracting the part from the cavity once the plastic has set.
This housing does not have a uniform diameter along its entire length, instead the ejector pins are really tightly fitted, with a very narrow clearance marked by standard regulation, in the portion nearest the cavity, whereas at rest the housing has a greater diameter to prevent unnecessary friction with the ejector pins.
During the injection process the mould is closed and therefore the cavity is full of air which is gradually compressed as the plastic material is injected, reducing its volume and increasing its pressure, which complicates filling it with the plastic material.
This entire process requires, in many cases, an increase in temperature of the plastic mass as well as a greater injection pressure so that the entire mould cavity can be filled, mainly in areas with a difficult configuration, which entails a longing cooling time before being able to demould the duly set part.
In many cases, due to the temperature and pressure excess applied to the plastic material to fill the cavity, internal stresses are produced in the part itself such that the dimensional stability is lost, negatively affecting the final quality of the product.
To solve these problems some solutions are known which allow the exit of air but at the same time prevent the exit of the injected material.
These solutions include the creation of air removal conduits located in the perimeter of the cavity, usually in the area furthest from the injection point. The air mass also escapes the cavity through the small clearance between the ejector pins and their housings.
The arrangement of a plugging element in the access opening of the ejector conduit, enabling the exit of air, preventing the exit of the injected material, is also known.
This plugging element is in some cases made up of an air permeable filter and in other cases by a cap or closing element which tends to remain in an open position by action of a spring. This closing element allows removing air and closes automatically when the pressure of the injected material overcomes the pushing of the spring, thus preventing the exit of said injected material through the removal conduit, although this solution is not applicable with all materials.
These moulds have several drawbacks in relation to the air removal means, the most noteworthy being that the injected material acts as a plunger pushing the air through the removal conduit, which requires greater injection pressure; and that the injected material cools more readily since it is in contact with the air, and therefore it is necessary to inject said material at a greater temperature with the subsequent energy cost. In order to achieve a greater effectiveness in removing the air it is necessary for the plugging element be located at the end opposite the injection nozzle, the molten material reaching this area with less fluidity. It is necessary to invest more energy and production costs to obtain the part during the process.
Furthermore, these problems also affect the quality of the obtained part since air bubbles are trapped in the moulded parts and clearances are formed in certain configurations. The inadequate extraction of air produces grooves in the outer faces of the injected parts and during the filling of the mould the injected material is subjected to stresses which cause imperfections and deformations in the obtained parts.
 US 2006093702 A1
discloses a retractable pin injection molding assembly including a cartridge comprising a mold cavity, a cavity sleeve to accept water and vacuum lines at the back of the mold, an ejector assembly, and a pin connection block with a groove for a locking slide, so that the cartridge may be changed rapidly without disassembling the injection molding assembly. Also provided is a mechanism for continuously adjusting the length of the ejector pins within the mold cavity.
 US 2004022885 A1
discloses an ejector pin of an injection mold includes a sleeve, a valve structure, and a biasing member. The sleeve has a passage hole and an enlarged hollow portion, which communicate with each other via a stepped portion engaged therebetween. The valve structure has a valve portion and a collar portion, which are disposed in the sleeve. The collar portion of the valve structure is biased by the biasing member such as a spring and stopped by the stepped portion. Gaps interposed between the valve structure and the sleeve serve as a gas passageway through which gas in a molding cavity is exhausted by a vacuum pump.
 US 2006269651 A1
discloses a metal mold apparatus including a pair of metal molds defining a molding space into which a melted resin material is filled; and an ejector pin provided in one of the metal molds and capable of protruding toward the molding space; wherein the ejector pin has a vent passage which connects the molding space and the exterior of the one metal mold.
 EP 0 385 762 A2
discloses an injection molding apparatus which comprises a stationary mold having an abutment surface and a movable mold having an abutment surface detachably mounted to the stationary mold such that the abutment surface of the stationary mold abuts the abutment surface of the removable mold.
 US 7158854 B1
discloses a controller for a mold vacuum unit uses a programmable logic controller to provide flexible translation of pre-existing signals present on the injection-molding machine avoiding the need for modification of that machine by the addition of limit switches or the like.
 US 2005126740 A1
discloses a vacuum die casting apparatus including a casting cavity, which is evacuatable via a vacuum valve. A liquid casting material is pressable into the casting cavity by a piston actuated by an actuator.
Description of the Invention
The method of the present invention, which is defined by the features of claim 1, solves the problems pointed out above while at the same time constituting an efficient alternative for the complete extraction of air from the moulds.
To that end, the plastic injection mould with inner air extraction is structured based on traditional elements of this type of moulds, i.e., comprising closing means, one or more injection cavities provided with at least one injection nozzle for introducing the hot material in a liquid state and at least one ejector device formed by a set of ejector pins which with their movement through their respective housings are responsible for pushing the moulded part out.
To make extracting the air from inside the cavity easier, the mould comprises an internal circuit communicated with a vacuum pump or suction element, which will be the responsible for extracting the air from the mould cavity before and during the injection process.
This circuit is made up of a conduit, specifically drilled in the structure of the mould itself, transversely intercepting the housings of the ejector pins creating a communication of the air of the cavity through the space between the ejector pin and its housing, thus causing its suction.
Since the time available for that air extraction during the injection process is limited, so as not to increase the injection time, it is necessary to close any space so that the vacuum pump or suction element works with the greatest efficiency and performance possible. Thus, the mould provides for the arrangement of bushing seals placed in each of the ejection elements, duly fixed in the die plate and located in the opposite portion of the shaping area or injection area to close the existing space between the ejector pin and its housing, which space in this portion has a greater clearance than the one found in the area of contact with the cavity.
Since the air which can be suctioned through the ejectors will not be sufficient in most injection methods, the mould of the present invention provides, to speed up the removal of the air from the mould cavity as much as possible, the use of a cut-off valve connected in the ejector circuit and housed in the shaping cavity itself, which is activated by means of a piston. Said piston and valve elements are also controlled by a timer responsible for indicating the time which it must be open or closed depending on the type of injection process to be performed.
Thus, from using the mould described above a method of action in the manufacture of parts by injection could be derived such that the presence of air inside the mould is prevented, which method would comprise the steps of:
- a) closing the mould;
- b) activating the air duct connected to a vacuum pump or a suction device transversely intercepting the housings of the ejector pins of the ejector devices producing the suction of the air from the mould
- c) injecting the plastic into the mould
- d) filling the mould (1); and
- e) cooling and ejecting the part
wherein the air duct and the suction device are activated throughout the entire injection of material into the mould.
Description of the Drawings
To complement the description which is being made and for the purpose of aiding to better understand the features of the invention, a set of drawings is attached to the present specification in which the following has been depicted with an illustrative and non-limiting character:
Figure 1 shows respective schematic plan and elevational section views of an injection mould.
Figure 2 shows an elevational view of one of the ejectors used in the mould of the previous figure.
Figure 3 shows a schematic elevational section view of an injection mould and a detail thereof.
As can be seen in the figures, the injection mould (1) with inner air extraction is structured, based on closing means (not depicted), one or more injection cavities (2) provided with at least one injection nozzle (not depicted) for introducing the hot material in a liquid state and at least one ejector device for ejecting the part which is structured based on a housing (3) for an ejector pin (4) which with its movement is responsible for pushing the moulded part out.
The mould (1) comprises an air duct (6) connected to a vacuum pump or a suction device (7) transversely intercepting the housing (3) of the ejector pins (4) causing a suction of the air from inside the mould (1) through the existing space between the ejector pin (4) and said housing (3).
Also according to an example and as can be seen in the figures, especially in Figures 2 and 3, the ejector devices have a bushing seal (5) sealing the existing space between the ejector pin (4) and the housing (3) such that said space does not connect with the lower one of greater volume, causing loss of suction effectiveness.
Therefore, for sealing that space between the ejector pin (4) and the wall of the housing (3) said bushing seal (5) is located in the lower portion of the so-called die plate (8).
The space comprised between the housing (3) and the ejector pin (4) must be sufficient for extracting the air from inside but however insufficient for the molten material to be able to escape through it.
To that end, and according to an example which can be seen in Figure 2, said housing (3) has two different diameters. It has a smaller diameter located in the proximity of the injection cavity (2) such that the molten material cannot escape through it but does allow the passing of the air, and which is governed by tolerances according to the regulation, in the order of 0.002 mm greater than the ejector pin (4). On the other hand, and as can be seen in said
Figure 2, below the area with said smaller diameter there is another area with a larger diameter producing a widening for the purpose of reducing the friction between said ejector pin (4) and its housing (3), said widening constituting the space transversely intercepting the air duct (6) and through which the air will be extracted from inside the mould (1).
According to another example, the air duct (6) which intersects with the housing (3) containing the ejector pin has a sector with a section for causing an increase in circulation rate of the air therethrough and the suction of the air contained in the chamber or injection cavity due to the Venturi effect.
Because extracting the air from inside the mould (1) is not sufficient with that performed through the ejector devices or because it is necessary to do so more quickly, the mould (1) has an additional air removal conduit, as can be seen in Figure 3, said additional conduit comprising a piston (9) responsible for activating a valve (10) which allow extracting the air also as a result of the intersection with the air duct (6) connected to the suction pump (7).
Said piston (9) and valve (10) elements are furthermore controlled by a timer responsible for indicating the time which this must be open or closed depending on the type of injection process to be performed. The injection process is thus synchronised with the air extraction such that the aforementioned valve (10) closes before the plastic is injected into the injection cavity (2).
Thus, a method of action in the injection manufacture of parts could be derived from the use of the mould (1) described above such that the presence of air inside the mould (1) is prevented, which method would comprise the steps of:
- a) closing the mould (1);
- b) activating the air duct (6) connected to a vacuum pump or a suction device (7) transversely intercepting the housings (3) of the ejector pins (4) of the ejector devices producing the suction of the air from the mould (1);
- c) injecting the plastic into the mould (1);
- d) filling the mould (1); and
- e) cooling and ejecting the part
wherein the air duct (6) and the suction device (7) are activated throughout the entire injection of material into the mould (1).
And wherein, the mould (1) has an additional conduit for air extraction, once the mould (1) is closed and simultaneously with the air extraction through the ejector devices but before injecting the plastic into the mould (1), it comprises the steps of:
a1) activating the additional conduit and the valve (10);
a2) extracting the air from inside the mould (1) through the intersection of the air duct (6) with the additional conduit;
a3) closing the valve (10).
Absaugungsverfahren zum Absaugen der Luft in einer Spritzgussform (1), wobei die Form (1) aufweist:
- einen oder mehrere Einspritzhohlräume (2), die mit mindestens einer Einspritzdüse zum Einführen des heißen Werkstoffs in einem flüssigen Zustand versehen sind;
- mindestens eine durch ein Gehäuse (3) gebildete Auswerfervorrichtung für einen Auswerferstift (4), verantwortlich zum Herauslösen des bereits geformten Teils;
- einen mit einer Vakuumpumpe oder einer Saugvorrichtung (7) verbundenen Luftkanal (6), der mindestens ein Gehäuse (3) eines Auswerferstifts (4) quer unterbricht, um eine Absaugung der Luft aus dem Innern der Form (1) durch den vorhandenen Raum zwischen dem Auswerferstift (4) und dem Gehäuse (3) zu bewirken;
- eine zusätzliche Luftentnahmeleitung, die den mit der Vakuumpumpe oder der Saugvorrichtung (7) verbundenen Luftkanal (6) kreuzt, wobei das Verfahren die folgenden Schritte umfasst:
a) Schließen der Form (1);
b) Betätigen des mit der Vakuumpumpe oder der Saugvorrichtung (7) verbundenen Luftkanals (6), der die Gehäuse (3) der Auswerferstifte (4) der Auswerfervorrichtungen unterbricht, die die Absaugung der Luft aus der Form (1) erzeugen;
c) Einspritzen des Kunststoffs in die Form (1);
d) Füllen der Form (1); und
e) Kühlen und Auswerfen des Teils, wobei der Luftkanal (6) und die Vakuumpumpe oder die Saugvorrichtung (7) über die gesamte Einspritzung von Werkstoff in die Form (1) betätigt werden,
dadurch gekennzeichnet, dass die zusätzliche Luftentnahmeleitung einen Kolben (9) aufweist, der zur Betätigung eines Ventils (10) verantwortlich ist, das in dem Einspritzhohlraum (2) selbst aufgenommen ist, und dadurch, dass, sobald die Form (1) geschlossen ist, gleichzeitig mit der Entziehung von Luft durch die Auswerfervorrichtungen, jedoch vor dem Einspritzen des Kunststoffs in die Form (1), es außerdem die Schritte umfasst:
a1) Betätigen der zusätzlichen Leitung und des Ventils (10);
a2) Entziehen der Luft aus dem Innern der Form (1) durch die Kreuzungsstelle des Luftkanals (6) mit der zusätzlichen Leitung; und
a3) Schließen des Ventils (10).