Process for manufacturing injection moulded objects, device for carrying out such process and new moulded objects

Abstract: The invention relates to a process for manufacturing injection moulded objects of thermoplastic material by injecting molten thermoplastic material into a mould from at least two distinct injection points whereas the flows of molten material from each injection point each fill part of the mould and flow together to fill the entire mould while still in molten state, wherein objects having a size of more than 80 cm along each of its three dimensional axes are produced by providing at least two distinct injection points at opposite sides of the mould. The invention also relates to injection moulding equipment for the manufacture of moulded objects of thermoplastic material, comprising at least two distinct injection points provided at opposite sides of a single mould assembly. The invention further also relates to threedimensional moulded objects of thermoplastic material having a size of more than 80 cm along each of its threedimensional axes.

Description: The present invention relates to an injection moulding process, more particularly to a process for manufacturing injection moulded objects of thermoplastic material, to a device for carrying out such processes, and to a new type of moulded objects obtainable with such process. Injection moulding of thermoplastic materials to produce objects of all sorts of shapes and sizes, is well known in the art. It is also known however that there are certain limitations to the size of the objects to be moulded (in function also of the complexity of the moulded shapes) as a consequence of the rheologic properties of the molten thermoplastic in the moulds. As the flow of molten plastic progresses into the mould cavity the material will cool down, which together with the rheological resistances in the mould will cause the pressure to build up at the point of injection; these pressures will of course be related to the length of the flowlines in the mould and therefore to the size of the object to be moulded. To overcome these problems several solutions have been proposed in the art, such as : high pressure equipment to overcome the pressure buildup, but this approach is quite expensive due to the material costs involved; use of heated moulds to avoid the cooling down of the molten plastic in the mould, but this approach is also complex and costly due to the structural complexity of the different parts of such a heated mould; . use of a plurality of injection points into the mould cavity to increase the speed of injection and the volume of the molten material flowing into the mould so as to reach the other side of the mould and fill the mould before the pressure builds up too much, but this approach has only shown satisfactory where the size of the object to be moulded in the direction(s) away from the injection points (the "second dimension" or base area, and the "third dimension" or height of the three dimensional object) is still relatively limited, i.e. at most about 50 - 60 cm, and at the absolute most about 80 cm when also using high pressure technology and/or heated mould technology and/or very special thermoplastics. In this respect reference is made for instance, to EP-A-0019336. The object of the present invention is to provide a method for manufacturing injection moulded objects of thermoplastic material which overcomes the drawbacks of the known art and which in particular allows the injection moulding of objects having a size of more than 80 cm along each of its three dimensional axes. The invention therefor provides a process for manufacturing injection moulded objects of thermoplastic material by injecting molten thermoplastic material into a mould from at least two distinct injection points whereas the flows of molten material from each injection point fill part of the mould and flow together to fill the entire mould while still in molten state, wherein said at least two distinct injection points are provided at positions on the mould. or on the moulded object from which two flows of molten plastic flowing in essentially opposite directions are able to fill the volume of said mould or object. Whereas according to the state of the art of injection moulding of thermoplastics multiple injection points into one single mould are known, these multiple injection points have been located on essentially one side of the mould whereas the distance between these injection points has been kept relatively close to achieve as quickly as possible the merger of the individual progressing flows of molten material and form one relatively uniform flow of molten material progressing in essentially one direction. It has now been found surprisingly that by using more than one flow of molten material flowing in essentially opposite directions, in accordance with this invention, whereby the merging of the progressing flows is postponed untill the actual filling of the mould at the points of merger of these flows, moulded objects of excellent quality could be obtained, contrary to the generally admitted prejudice that the individual flows should merge together as soon as possible to avoid quality loss at the merging line of the individual flows of molten material. The injection of the molten material at the separate injection points is preferably carried out substantially simultaneously, but under specific circumstances depending on the shape and structure of the mould and the internal flowlines in the mould it may be preferable to start with the injection at one of the opposite injection points and only subsequently start injection at the other injection point(s), in a pre-established timing sequence which may readily be determined experimentally by the skilled art worker in function of the particularities of the object to be moulded. Thus also, the actual position of the injection points to the mould are essentially determined by the length of the overall flowlines of the molten material in the mould structure whereby the injection points are preferably positioned in such way that the length of the flowlines or the flowtime of the molten material from the opposite injection points on the mould to the point of merger of the molten material from the opposite injection points is essentially evenly distributed. This also will be easily established by the skilled art worker in function of the particularities of the objects to be moulded. The number of injection points in accordance with the invention is at least two, but this number may be more as required by the particularites of the objects to be moulded. Thus there may be used three or more injection points from different angles and extremities of the mould to provide more than two flows of molten plastic material flowing in essentially opposite directions. In this context it should be observed that for instance four injection points essentially located according to the four summits of a tetraeder are considered to provide flows of molten material globally flowing in essentially opposite directions before merging together at a more or less cetral point of the mould. The injection points at of the mould may futhermore also each consist of a plurality of injection points on each respective opposite side of the mould. According to another embodiment of the invention "low pressure" technology is preferably used to produce objects having a size of more than 60 cm along each of its three dimensional axes. The expression "low pressure" technology as used herein refers to the use of injection pumps allowing a maximum cavity pressure of 100 bar , as opposed to "high pressure" technology which uses injection pumps with maximum cavity pressures well above 250 bar. It has also been found surprisingly that with the present invention it is easy to avoid any problems of air entrapment between flows of molten material merging from opposite directions, contrary to what would have been expected in view of the state of the art prejudices. According to a specific feature of the invention this is achieved by allowing the the air displaced in the mould by the flows of molten thermoplastic material to escape through slits which allow through the air but not the molten thermoplastic material. In practice it has been found that the slits and interstices anyway existing in standard good quality multy-piece moulds provide the necessary properties for this preferred feature of the invention. According to still another feature of the invention, the thermoplastics used are preferably polyolefines such as in particular polyethylenes (low density polyethylenes, hich density polyethylenes, linear low density polyethylenes, etc.) and polypropylene; the process is however also suitable for any other type of thermoplastic resins. The invention also relates to devices for carrying out the injection moulding process according to the invention. In particular the invention thus relates to an injection mouldig equipment for the manufacture of moulded objects of thermoplastic material, comprising at least two distinct injection points positioned in such a way that the flow of molten material from said injection points flows in essentially opposite directions According to the invention, the equipment is specifically designed for the injection moulding of polyolefine objects having a size of more than 80 cm along each of its three dimensional axes or, when the equipment is of the low pressure technology type, for the injection moulding of polyolefine objects having a size of more than 60 cm along each of its three dimensional axes. According to preferred feature of the invention, the mould assembly of the injection moulding device comprises slits of a size allowing through air but not the molten thermoplastic material. As already stated above standard good quality multy-piece moulds generally provide the necessary properties for this preferred feature of the invention. The invention finally also relates to threedimensional injection moulded objects of thermoplastic material with features unacheaveable and therefore unknown before in the art, i.e. having a size of more than 80 cm along each of its threedimensional axes. More in particular the invention relates to injection moulded thermoplastic cases with a bottom and a peripheral wall or peripheral walls, with length and width axes or diameter of the bottom and a height of the wallls of at least 80 cm, preferably of at least 100 cm. According to a preferred feature of the invention these threedimensional injection moulded objects are made of a thermoplastic polyolefine, preferably polyethylene, polypropylene or mixtures thereof. The invention will be further explained herebelow with reference to the attached drawings representing specific embodiments of the invention. It should be observed that the specific features of these embodiments are only described as preferred examples of what is intended within the above general disclosure of the invention and should by no means be interpretted as limiting the scope of the invention as such and as set forth in the claims. In the attached drawings : Figure 1 is a top view of part of a moulding device - in accordance with the invention - for carrying out the process in accordance with the invention, showing the female mould part, Figure 2 is a bottom view of the male mould part fitting in the female mould part of the moulding device of figure 1, Figure 3 is a sectionnal view of the moulding device according to the invention, according to plane III - III of figures 1 and 2, Figure 4 is a sectionnal view of the moulding device according to the invention, according to plane IV - IV of figures 1 and 2, and Figure 5 is a perspective view of a threedimensional moulded object in accordance with the invention. In figure 1 the female mould part of the moulding device is designated as a whole with reference numeral (1). In figure 2 the male mould part of the moulding device is designated as a whole with reference numeral (2). The male (2) and female (1) parts of the mould define together the shape of a threedimensional volume corresponding to a threedimensionnal object to be moulded in said mould. In the present example this threedimensional object corresponds to a moulded plastic case as represented in figure 5 and designated there as a whole with reference numeral (3). The bottom of the threeedimensionnal object is defined by the volume between the surfaces (4) of the female mould part (1) and the surface (5) of the male mould part (2). The injection point on one side of the mould (in this example on the female part side of the mould) is designated by refernce numeral (6), near the centre of surface (4), whereas the molten material accesses into the mould through a nozzle (7) as represented schematically on figure 4. The injection point on the opposite side of the mould consists of a plurality (eight in total) of injection nozzles (8) through the closing cover part (9) of the male mould part (1), as schematically shown more particularly on figure 3. Whereas the surface of the male mould part (2) defining the inner surface of the moulded plastic case (3) is smooth, the surface of the female mould part (1) defining the outer surface of the moulded plastic case (3), is provided with linear grooves (10) on surface (4), defining longitudinal and transversal ribs (11) on the bottom of the moulded plastic case (3), and with linear grooves (12) on its lateral surfaces (13), defining horizotal reinforcing ribs (14) on the outer surface of the moulded plastic case (3). The female mould (1) further comprises insert parts (15) and (16) respectively defining mould cavities (17) and (18) corresponding to respective corner-reinforcing beams (19) and wall-reinforcing beams (20) of the moulded plastic case (3). The mould device is furthermore equiped with cooperating guiding means (respecticely cilindrical beams (21) and cavities (22), and rods (23) and bores (24)) for the perfect alignment of the various mould parts during operation. In addition to the linear grooves (10) on the bottom surface (4) of the mould, defining the longitudinal and transversal ribs (11) on the bottom of the moulded plastic case (3), the female mould also comprises in the area of the insert parts (15) and (16), defining the respective corner-reinforcing beams (19) and wall-reinforcing beams (20) of the moulded plastic case (3), deeper grooves or cavities (26) which define the supporting "legs" (27) of the moulmded plastic case (3). The female and male mould are made up of separate metal pieces adequately assembled and fitted together in such way that upon opening the mould some parts may move outwardly to free the moulded object. These separate metal pieces of the mould parts are represented very schematically on the sectionnal views of figures 3 and 4. Figure 4 also schematically shows a pneumatic jack (28) used for opening the mould and moving outwardly some of the mould parts. On the schematic views of the invention, represented on the enclosed drawings the various piping necessary for leading the molten polyolefine to the injection nozzles has not been represented. The required piping has to be rigid in view of the considerable pressures involved, even in so called "low pressure" equipment. The connections of this rigid piping to the nozzles on the mould are preferably of a quick assembly type (known per se) to allow the quick andefficient opening of the mould upon each injection cycle. In carrying out the process of the invention with the equipment described hereabove one stream of molten polyolefine is injected into the mould through injection nozzle (7) at injection point (6), and another stream is injected evenly through the plurality of injection points (8). The flow of molten polyolefine from injection point (6) flows between surfaces (4) and (5) of the female and male mould, thus filling the cavity corresponding to the bottom of the threedimensional moulded object (3), and subsequently flows between walls (13) and (25) of the female and male mould. More or less simultaneously the flows of molten polyolefine from injection points (8) flow between walls (13) and (25), merging with each other while progressing. The flows of molten polyolefine from injection point (6) and from injection points (8) merge at the level of a plane corresponding to approximately half of the height of the threedimensional object (3). Other details of the embodiment of the invention given here by way of example will be apparent to the skilled art worker from the accompanying drawings. Such further details also constitute prefered specific features of the invention. The specific example described hereabove in particular consists of a moulded plastic case which can very suitably be used as a case for handling vegetable and fruit crops for wholesale purposes. The particular case refered to has a prefered length of 130 cm, a prefered width of 115 cm and a prefered heigth of 125 cm (external sizes). Using the moulding process according to the invention such cases can very suitably be manufactured from polyolefines or polyolefine mixtures obtained from plastic recycling sources.

Claims: Process for manufacturing injection moulded objects (3) of thermoplastic material by injecting molten thermoplastic material into a mould (4) (2) from at least two distinct injection points (6) (8) whereas the flows of molten material from each injection point each fill part of the mould and flow together to fill the entire mould while still in molten state, characterised in that said at least two distinct injection points (6) (8) are provided at positions of the mould from which at least two flows of molten material, flowing in essentially opposite directions are able to fill the volume of said mould, to produce objects (3) having a size of more than 80 cm along each of its three dimensional axes or, when using injection moulding with a maximum cavity pressure of 100 bar, objects having a size of more than 60 cm along each of its three dimensional axes. Processs according to claim 1, characterised in that it is used to produce objects having a size of more than 80 cm along each of its three dimensional axes or, when using low pressure technology, objects having a size of more than 60 cm along each of its three dimensional axes. Process according to claim 2, characterised by an injection point at a position along the third dimensional axis of the mould. Process according to any one of the preceding claims, characterised in that the air in the mould displaced by the flows of molten thermoplastic material escapes through slits which allow through the air but not the molten thermoplastic material. Process according to any one of the preceding claims, characterised in that moulded objects of polyolefine are manufactured. Injection moulding equipment for the manufacture of moulded objects (3) of thermoplastic material, comprising a mould (1), (2) with at least two distinct injection points (6), (8) at opposite sides of the mould, characterised in that at least two injection points are positionned in such a way that the flow of molten material from said injection points flows in essentially opposite directions, and in that the equipment is designed for the injection moulding of polyolefine objects having a size of more than 80 cm along each of its three dimensional axes, or when the equipment allows a maximum cavity pressure of 100 bar, of polyolefine objects having a size of more than 60 cm along each of its three dimensional axes. Injection moulding equipment according to claim 6 characterised by an injection point at a position along the third dimensional axis of the mould. Injection moulding equipment according to any one of claims 6 and 7, characterised in that the mould assembly comprises slits of a size allowing through air but not the molten thermoplastic material. Threedimensional injection moulded object of thermoplastic material, characterised by a size of more than 80 cm along each of its threedimensional axes. Threedimensional injection moulded object according to claim 9 characterised in that the thermoplastic material is polyethylene, polypropylene or a mixture thereof. Injection Moulded thermoplastic case according to anyone of claims 9 and 10 characterised by a length, a width and a height of at least 1 m.

Inventors: VANDEVELDE MICHEL, VANDEVELDE, MICHEL
Applicants: INNOVA PACKAGING SYSTEMS N V, INNOVA PACKAGING SYSTEMS N.V.
Application year: 1996
Main classification: B29C45
IPC codes: B65D1, B29C45