EP0954423A1

EP0954423A1 - Gas injection-moulded hollow plastic component and method and device for its production

Info

Publication number: EP0954423A1
Application number: EP97948077A
Authority: EP
Inventors: Mikael Hildesson; Tor Hesselgren; Lars Odelmark
Original assignee: Lear Corp Sweden AB; Volvo Lastvagnar AB
Current assignee: Volvo Truck Corp; Lear Corp Sweden AB
Priority date: 1996-12-04
Filing date: 1997-12-04
Publication date: 1999-11-10
Also published as: SE9604474D0; WO1998024610A1; JP2001505148A; SE508096C2; AU5421998A; SE9604474L; BR9713669A

Abstract

Gas injection-moulded, hollow plastic article, for example, a radiator grill for a motor vehicle, comprising a rectangular frame (2, 3, 4) with upper and lower hollow frame members (2, 3) and intermediate hollow mouldings (6, 7, 8), all of which are gas injection-moulded in one piece with each other. The frame members (2, 3) and the mouldings (6, 7, 8) have end portions joined to side frame members (4), and have cavities with decreasing cross-sectional area towards the ends.

Description

GAS INJECTION-MOULDED HOLLOW PLASTIC COMPONENT AND METHOD AND DEVICE FOR ITS PRODUCTION

The present invention relates to a gas injection moulded plastic article with at least one cavity enclosed in the same. The invention also relates to a method and an injection moulding device for manufacturing such a plastic article.

Gas injection moulding is used, for example, when making plastic panels with hollow elongated reinforcing ribs, where the cavities in the ribs are made by injecting gas in the not yet hardened plastic in the mould cavities against the surfaces of which the external surfaces of the ribs are to be formed. The longer the ribs are, and therefore the longer the cavities or channels therein are, the more locations with gas injection needles are used in the mould to assure that the excess plastic material will be pressed out of the mould cavity before it hardens, so that the reinforcing ribs will have a uniform thickness along their entire length. US 5 204 051, for example, describes and shows the manufacture of plastic articles with cavities by gas injection moulding. The plastic material and the gas are in this case injected into a cavity of constant cross-section, so that the plastic article in the area forming the cavity will have a constant outer cross-sectional contour. This also means that the cavity will at least theoretically have a constant cross-section, but minor deviations due to the variations in gas pressure and hardening rate are difficult to avoid. US 5 204 051 illustrates the use of either two gas injection needles inserted into the plastic melt in the vicinity of the opposite ends of the mould cavity or a single gas injection needle inserted centrally into the mould cavity. This latter alternative is limited to cavities or relatively small dimensions in relation to their cross-sectional area.

The purpose of the present invention is to achieve a plastic article with at least one cavity enclosed in the same, which can be manufactured using a single gas injection needle for a given length of cavity, which required, with previously employed technology, at least two gas injection needles to achieve a result of comparable quality. This is achieved according to the invention by virtue of the fact that the cavity in areas remote from a gas injection location have a cross-sectional area which decreases with increasing distance from the gas injection location.

The invention is based on the simple fact that a larger volume of fluent plastic melt can flow per unit of time through a larger flowthrough area than through a smaller flowthrough area. If the plastic melt on one side of the gas injection needle tends to flow out more rapidly than the plastic melt on the opposite side, the former will be slowed down when the flowthrough area decreases, so that a balance is achieved in relation to the latter which is still within a section of larger cross-section.

A plastic article which can be injection moulded with advantage in the above mentioned manner is a radiator grill for motor vehicles, comprising a frame and a plurality of hollow mouldings surrounded by the frame and joined thereto. All of these mouldings are gas injection moulded in one piece with the frame and have end portions connected to the frame, the cavities of which have decreasing cross- sections towards the ends. Preferably, the frame also has hollow frame portions with cavities which have end portions with decreasing cross-section towards the ends.

An injection moulding device for manufacturing a plastic article according to the invention is characterized in that the mould cavity has substantially greater length than height and width and has end portions with decreasing cross-sectional area towards the ends, and that means are arranged for supplying fluent plastic under pressure and gas under pressure to the mould cavity midway between its two ends.

The invention will be described in more detail with reference to examples shown in the accompanying drawings, where Figure 1 shows a perspective view of the back of a plastic article in the form of a radiator grill for motor vehicles, Figure 2 shows a section along the line II-II in Fig. 1 , Figure 3 shows a section along the line III-III in Fig. 3, and Figure 4 shows a section through a schematically represented moulding device.

The radiator grill shown in Figs. 1-3 comprises a rectangular frame, generally desig- nated 1 , which consists of a horizontal upper frame member 2, a horizontal lower frame member 3 and two vertical side frame members 4, which should join the upper and lower frame members 2 and 3 to each other. Between the frame members 2 and 3 there also extend a pair of vertical mouldings 5, and between these vertical mouldings and the respective side frame member 4, a pair of horizontal mouldings 6 and 7 extend. Finally, a horizontal moulding 8 extends via the vertical mouldings 5 from one side frame member 4 to the other.

All of the components 2-8 shown and described are made in one piece with each other in a gas injection moulding process in such a manner that cavities are formed in the frame members 2 and 3 and in the mouldings 6, 7 and 8. These cavities, which extend over the entire length of the frame members, are designated 9, 10, 11, 12 and 13 in Fig. 2. All of them have, as is illustrated in Fig. 3, end portions with gradually decreasing cross-section towards their ends which are joined to the side frame members 4. As is particularly evident in Fig. 2, the cavity 9 of the upper frame member has a larger depth "a " than height "b". This provides greater rigidity for the panel as a whole and in the upper frame member 2, so that it can be grasped when removing the panel.

Fig. 4 shows schematically an injection moulding device which can be used for manufacturing the plastic article according to the invention and which comprises upper and lower mould halves 20 and 21, respectively, which define together a mould cavity 22 and which can be moved apart by means not shown in more detail here, from the position shown to open the mould cavity 22. The nozzle 24 of an injection moulding device opens into a channel 23 through the upper mould half 20. The injection moulding device has a cylinder 25 and a piston 26 axially displaceable in the cylinder, by means of which fluent plastic in the cylinder 25 can be pressed into the mould cavity 22.

The mould halves 20 and 21 define, in addition to the mould cavity 22, a pair of so- called spill chambers 27, which corri riunicate with the mould cavity 22 via individual channels 28. Into each channel 28 there opens a bore 29 in which an ejector pin 30 can move. A corresponding ejector pin 31 is disposed in a bore 32, opening into each spill chamber 27. A gas needle 33 extends directly opposite the channel 23 and has an opening 33a through which gas can be injected into the plastic material into the mould cavity 22. The gas needle 33 can be fixed in the position shown or be extractable from the mould cavity 4.

34 in Fig. 4 designates a control unit, 35 designates a compressed gas source, 36 and 37 are drive means for driving the piston 26 and the ejector pins 30, respectively. Fig. 4 is symmetrical relative to a center plane A, and to the left of the plane A there is illustrated the state after the piston 26 has reached the bottom of the cylinder 25, and the mould cavity 22 has been filled completely with fluent plastic b while air is purged and a small amount of plastic material c has entered the spill chamber 27. During the plastic injection phase, the control unit 34 keeps, via the drive means 37, the ejector pins 30 in the position shown to the left in Fig. 4, in which the upper end surface 30a of each ejector pin 30 leaves a narrow passage 28a open, through which first air and then plastic material can pass to the spill chamber 27.

When a predetermined amount of plastic material has been pressed out into the spill chamber 27, the control unit 34 activates the compressed gas source 35 so that gas under pressure (preferably nitrogen) is injected through the gas needle 33 and out through its opening 33a into the still not hardened plastic, which is pressed out through the channels 28 and into the spill chambers 27 until they are entirely filled, as illustrated in the right-hand half of Fig. 4. During the gas injection phase, the control unit 34 keeps the ejector pins 30 in the position shown to the right in Fig. 4, in which their end surfaces 30a are at a lower level to free the entire flowthrough area of the channels 28.

By virtue of the fact that the mould cavity has end portions 22a with gradually decreasing cross-sectional area towards the ends, a more balanced filling of the cavity is achieved than if it had the same cross-sectional area along its entire length. Should the plastic material on one side of the needle 33 tend to flow out more rapidly towards its end of the mould cavity than the plastic flows towards the other end, the counter-pressure against the more rapidly flowing plastic will increase as it reaches the tapering portion of the mould cavity, so that the gas pressure will increase against the less rapidly flowing plastic material which still has not reached the corresponding opposite tapering portion of the mould cavity, until a balanced filling is achieved.

When the plastic has hardened, the mould cavity 22 is opened and the control unit 34 activates the drive means 37 of the ejector pins 30 to move the pins 30 upwards to lift the plastic article from the lower mould half 21. As the pins 30 move up through the channel 29, the communication between the plastic in the mould cavity 22 and the plastic in the spill chamber 27 is cut off, so that the latter can thereafter be ejected by means of the ejector pin 31. In Fig. 1 , 40 designates the holes left by the gas needles and 41 the remains of the plugs formed above the surfaces 30a of the ejector pins 30.

Fig. 4 shows a plastic article 2 with a cavity 9, which means that the section through the mould is taken through the mould cavity in which the upper frame portion 2 of the radiator grill shown in Figs. 1-3 is moulded. It will be understood, of course, that for the sake of illustration, Fig. 4 is very simplified and that parts of the upper frame portion 2, revealed in Figs. 1 and 2 but which are of no interest for explaining the invention, have been left out.

Claims

1. Gas injection moulded plastic article with at least one cavity enclosed in the same, characterized in that the cavity (9-13) in areas remote from a gas injection location (40) has a cross-sectional area which decreases with increasing distance from the gas injection location.

2. Plastic article according to Claim 1, characterized in that it has at least one elongated channel (9-13) enclosed in the same, and which has end portions with decreasing cross-sectional area towards its ends.

3. Plastic article according to Claim 2, characterized in that the channel (10,11 and 9,12, 13, respectively) has a cross-section which is symmetric relative to a plane midway between both ends of the channel and that the gas injection location (40) is located in this plane.

4. Plastic article according one of Claims 1-3, characterized in that it forms a panel for a motor vehicle and comprises a frame (2,3,4) and a plurality of hollow mouldings (6,7,8) joined to and surrounding the frame, all of said mouldings being gas injection moulded in one piece with the frame and having end portions joined to the frame, the cavities (10,11,12) of which have cross-sections decreasing towards the ends, and that the frame has at least one hollow frame member (2,3), the end portions of which, which are joined to two opposite frame members (4), have cavities (9,13) with cross-sections decreasing towards the ends.

5. Method of manufacturing a plastic article with a cavity enclosed in the same, comprising the following steps: injection of plastic material under pressure into a mould cavity, cooling of the plastic material closest to the walls of the mould cavity, injection of gas under pressure into the fluent plastic, which is surrounded by the cooled plastic, pressing out fluent plastic to at least one spill chamber located outside the mould cavity and communicating with the mould cavity, by means of the injected gas to form a cavity in the plastic, and separating the plastic in the spill chamber from the plastic in the mould cavity, characterized in that the gas is injected into a mould cavity which, in areas remote from the gas injection location, has cross-sectional areas which decreases with increasing distance from the gas injection location.

6. Method according to Claim 5 for manufacturing a plastic article with a cavity in the form of an elongated channel, characterized in that the plastic and the gas are injected midway between the two ends of an elongated mould cavity with cross-sectional areas decreasing towards the ends.

7. Injection moulding device with a mould cavity defined by movable walls for manufacturing a plastic article with a cavity enclosed in the same, comprising means for injecting under pressure fluent plastic into the mould cavity, means for injecting gas under pressure into the plastic in the mould cavity, at least one spill chamber located outside the mould cavity and communicating with the mould cavity, said spill chamber being arranged to receive the plastic forced out by the injected gas, and means for opening and closing the communication with the spill chamber, characterized in that mould cavity (22, Fig. 4) has substantially greater length than height and width and has end portions with decreasing cross-sectional area towards the ends, and that means (25,26,33) are arranged for supplying fluent plastic under pressure and gas under pressure to the mould cavity midway between its two ends.