Plastic and nonmetallic article shaping or treating: processes – Mechanical shaping or molding to form or reform shaped article – Shaping against forming surface
Reexamination Certificate
2000-02-16
2002-08-06
Heitbrink, Jill L. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Mechanical shaping or molding to form or reform shaped article
Shaping against forming surface
C264S328120, C264S334000, C425S573000, C425SDIG005
Reexamination Certificate
active
06428734
ABSTRACT:
BACKGROUND OF THE INVENTION
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 Theological 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 build-up, 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; and
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 directions) 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.
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 in a direction away from the injection point(s), or objects having a size of more than 60 cm in a direction away from the injection point(s) without using high pressure technology. It is also the object of this invention to provide a possibility for objects having a size of more than 80 cm along each of its three dimensional axes. Furthermore, the present invention provides for even flowing of the molten material from all three axes, rather than singular or two-dimensional flow methods that are currently in use. The present invention allows for uniformity along the flow lines and formation of elongate large sized three dimensional structures that is not problematic with existing methods.
SUMMARY OF THE INVENTION
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 at least two distinct injection points are provided at opposite sides of the mould.
The expressions “opposite sides” of the mould or “opposite sides” of the moulded object, as used herein, should be understood as designating 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 until 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 located at opposite sides of the mould 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 that 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 opposite 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.
Molten material is injected into a mould from plural positions on the mould, allowing for plural flows of molten material through the mould. The flows flow in opposite directions from three dimensional axes of the mould from injection points in one plane along two dimensional axes of the mould, and from at least one further injection point along a third dimensional axis perpendicular to the one plane, thus creating three dimensional injection of molten material into the mould. This allows for even flowing of the molten material from all axes.
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 particularities 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 central point of the mould.
The injection points at opposite sides of the mould may furthermore 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
Forment Bernard
Vandevelde Michel
Creighton Wray James
Heitbrink Jill L.
Innova Packaging Systems N.V.
Narasimhan Meera P.
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