Patent Application
20080299244
This invention relates, in general, to an apparatus and method of producing optically molded parts and is particularly, but not exclusively, applicable to a compression molding system for the production of large, optically-transparent plastic sheets, such as windows for cars.
In the production of glazing units (such as windscreens and headlight lenses for cars), optical perfection is a critical success factor for the manufacturer. Any blemish in either the surface or the body of the plastic, e.g. polycarbonate, sheet renders the molded product sub-standard and subject to rejection. Not only does this result in scrap that has a direct cost overhead for the manufacturer, but the return on capital employed (for the machine) is reduced and the overall productivity and profitability of the manufacturer is thus adversely affected.
In the molding of plastic articles, it is known that limited cavity pressure and reduced internal shear within the melt during injection augment the overall properties and finish of the molded sheet. Specifically, with lower shear, lower internal stresses are experienced within the molded part. Conversely, with increasing internal stresses, transparent plastic materials become increasingly more opaque and, more particularly, begin to acquire either a coloured hazing effect or internal stress cracks (“stress whitening”) that both cause distortion in any image viewed through the solidified plastic.
In terms of the production of transparent plastic sheets and the like, compression molding is a prevalent technology. In compression molding, a mold cavity is established through the bring together of mold plates. Molten plastic is then injected into the cavity. At some point during the injection phase, under the controlled application of applied clamp force, the plates of the mold are brought into closer relation to compress the plastic melt and thereby either to cause the melt to fill all the available space in the mold cavity or otherwise generally to compress the melt against the cool molding surfaces.
One particular compression molding technique employs an inclined mold plate whose angle of inclination is set, during melt injection, by operation of a bank of hydraulic cylinders (or “upper counter force cylinders”) that generate force sufficient to resist melt injection pressure. Lower counter force cylinders provide a complementary (balancing/re-orientating) role within the mold, although these lower counter force cylinders do not contribute to the mechanism by which angle of the inclined plate is initially set.
The angle of inclination is permitted by virtue of complementary bushings coupled to and located along upper and lower edges of the inclined plate (e.g. the cavity plate) and is defined relative to a second plate (e.g. the core plate) held substantially perpendicular to the major axis of the machine nozzle and parallel to mounting faces of respective platens of the injection molding machine. In essence, the bushings can be considered to provide a longitudinal swivel bearing that permits the inclinable plate to be rotated (along its top edge) about cooperating bearing surfaces on the inclined plate and second plate. In this configuration, the upper counter force cylinders exert a constant but highly resistive force on the bearings, with this resistive force designed always to surpass the maximum possible cavity force (and thus to resist opening of the inclined plate/mold during injection of melt). The angle of inclination continuously decreases with an increasing degree of clamping applied to the device.
The inclined plate design and operation can be understood with reference to US patent application US2004/0169296 and German patents DE10259076, DE10302102 and DE10202246.
Since the angle of the inclined plates varies during the injection cycle and it is conventional that the hot runner (and therefore the injection nozzle) interfaces into this hot side of the mold, a bendable injection path and/or a flexible manifold is required.
The benefits of the use of such tilting compression (relative to conventional parallel compression) molding are that there is a small applied stroke at commencement of flow, but a large stroke at the flow path end. Consequently, less material must be redistributed and a longer flow path is realizable.
From an injection perspective, the initial cavity pressure/force at the start of injection is zero, whereas this cavity force inevitably builds over time with the delivery of increasing volumes of melt. In practical terms, this means that the bushing is subjected to the entire force developed by the upper counter force cylinders for the entire time, and the force experienced/seen by the bushing (FBushing) varies from high (at the start of injection) to relatively low (at the end of injection). Since the bushing must initially and independently bear and resist the high resistive force generated by the upper counter force cylinders, the bearing is substantial in terms of both its size and material. Such a conventional bearing therefore is both relatively expensive and, more critically, takes up significant real estate in relation to the total available molding surface. The latter point is of particular concern in the glazing market, since glazed units (such as windows) generally have large surface areas and any reduction in available molding surface between a machine's platens limits commercial opportunity. Indeed, to accommodate both the mold and the bearings for the inclined plate, it may in fact be necessary for a manufacturer of glazed parts to purchase a physically larger machine (with increased tie bar separation) to accommodate the overall dimensions of the inclinable mold plate and its related bearings, rather than a smaller-sized machine having platens and a clamp unit sufficiently adequate to resist injection pressures and to mount a mold for a similarly sized plastic part made from opaque plastic material.
According to a first aspect of the present invention there is provided a method of injection compression molding parts in a mold cavity defined between a first plate and a second plate, the first plate being selectively inclinable relative to the second plate, the first plate inclined by operation of force applied substantially along one edge of the first plate by a counter force cylinder, the first plate having a bearing and wherein the second plate has a complementary bushing into which the bearing engages and rotates, the method comprising: determining a magnitude of force acting across at least one of the bearing and its bushing; exercising dynamic control of force (FCFC) generated by the counter force cylinder in response to the determined magnitude of force, whereby the force in the bushing (FBUSHING) is modeled to reflect a force profile and exceed an instantaneous value of a cavity force in the mold cavity.
According to another aspect of the present invention there is provided an injection compression molding system containing: a first plate having a front surface and a rear surface; a second plate having a front surface closable against the front surface of the first plate to define a mold cavity between the first and second plates; a bearing and a bushing that cooperate together, one of the bearing and the bushing being coupled to the front surface of the first plate, while the other one of the bushing and the bearing is coupled to the front surface of the second plate; a counter force cylinder coupled to the first plate and arranged to vary inclination of the first plate relative to the second plate during injection compression, the counter force cylinder generating cylinder force (FCFC) that, in use, is applied to the bushing and bearing; means for assessing a magnitude of the force (FBUSHING) acting across at least one of the bearing and its bushing; and a controller coupled to the counter force cylinder and arranged dynamically to control, in response to the magnitude of force and during injection compression, cylinder force (FCFC).
In yet another aspect of the present invention there is provided an injection compression mold comprising: a first plate having a front surface and a rear surface; a second plate having a front surface closable against the front surface of the first plate to define a mold cavity between the first and second plates; a first bearing and a first bushing that cooperate together, one of the first bearing and the first bushing being coupled to the front surface of the first plate, while the other one of the first bushing and the first bearing is coupled to the front surface of the second plate, the first bearing and the first bushing permitting the first plate to be selectively inclined relative to the second plate; wherein the first bearing is hollow and includes means for determining a magnitude of the force (FBUSHING) acting across at least one of the first bearing and the first bushing.
The present invention therefore advantageously provides a mechanism that reduces the average and (generally also the) instantaneous force acting on a bearing in a tilting mold used in injection compression molding.
Specifically, an aspect of the present invention provides the dynamic control of, typically, hydraulic pressure (from counter force cylinders) acting to angle an inclined plate in an injection compression molding environment.
Beneficially, at least one aspect described herein permits the bearing's overall size to be reduced and/or for the system to benefit from reduced wear and/or improved part quality.
Exemplary embodiments of the present invention will now be described with reference to the accompanying drawings, in which:
To provide a context for the present invention,
In terms of system components, the injection molding system 10 is wholly unremarkable since its parts are all well known to the skilled addressee. For this reason,
The molding system 10 may, however, be adapted to support the improvements to the compression molding process and inclined mold that will be described subsequently. In terms of the general configuration of the mold 12,
Along a bottom edge of the mold 12, a bank of lower counter force cylinders 52 is coupled (and acts) between the second plate 44 and the manifold plate 45. To permit the inclination of the first plate 36, a second bearing 54 includes a generally cylindrical bearing surface coupled to the rear surface 40 of the first plate 36. The second bearing interacts with a complementary bearing surface 58 of a second bushing 59 fixed to (and typically recessed within) the manifold plate 45. Again, the second bearing 54 and its bushing 59 are substantial in size and are of solid construction.
Through the angling of the first plate 36, melt 19 injected into the mold cavity 18 at a gate 60 initially sees a small channel in the mold cavity that progressively widens as the melt moves away from the gate 60. As summarized above and as described in DE 10259076, the angle of inclination (α—see
Clearly, while not expressly shown, the mold 12 includes pairs of first and second bearings to provide a certain degree of balance. The above description and the accompanying drawings concentrate on either a particular quadrant or either the left or right side of the mold (when split downwardly through a centreline of the mold 12), as will be readily understood.
With the bank of upper counter force cylinders acting across the width of the prior art mold 12, it has only now been realized that the hydraulic actuation of those counter force cylinders causes deflection of the inclined (first) plate 36. This deflection results in a non-uniform load being seen across the width of the various bearings and complementary bushings in the prior art mold of
In
Turning to
The new bearing 100 will act against a complementary shaped bearing surface of a bushing on the second (upright) plate 44 in a manner similar to that illustrated in
The strain gauge 102, which is linked to the machine controller 28, measures strain experienced within and across the bearing 100 before and during the injection compression process (see
Since the cavity force is initially zero at the instant injection commences, the upper counter cylinder force is ramped up or pre-set to a limited but finite level above zero prior to commencement of the injection process. At the end of injection, the upper counter cylinder force will only then essentially correspond to the constantly high force experienced in the prior art. In this way, the amount of overall force experienced in the bushing (FBUSHING) is reduced relative to the prior art; this can be seen by comparing
Towards a point in time when the cavity is filled, the mold is fully closed to parallel by actuation of the clamp cylinders. In other words, the machine controller 28 performs parallelism control through the clamp unit 30 and, as necessary, further control of the banks of counter force cylinders.
Since there is decreased force within the newly configured bearing 100 and its bushing 140, the overall effect is that, for identically treated materials having identical properties, there is reduced wear experienced by the various bearings and bushings. With decreased wear and decreased load, any compression molding system (such as that of
Also, in a preferred embodiment, a stress release notch 104, 106 is cut into the side faces of the bearing 100 at a location relatively proximate to an outer (cylindrical) surface defined parallel to an axis of the internal bore 101. The notch 104, 106, which may be on both sides of the bearing 100, acts to relieve peak stresses experienced at each edge 112 by providing some limited flexibility in the bearing surface at the edge 112.
In a preferred embodiment, as shown in
In addressing a total solution for a bearing, another aspect contemplates that the internal bore 101 is conically-shaped with an smaller diameter 110 facing outwardly and away from a central region of the mold 12 and a larger diameter 112 facing inwardly towards the centre of the mold 12. The shaping of the bore 101 of the bearing 100 compensates for deflection of the first (inclined) plate that arises from the application of counter cylinder force across the width of the mold. In this way, the bearing 100 and its complementary bushing 140 are exposed to a substantially constant amount of force which reduces overall wear and provides greater balance; this is seen in the accompanying force diagram in
In overview, dynamic modeling of the cavity force by the cylinder counter force can be achieved through the location of a strain gauge (or the like) in any suitably shaped internal region of a bearing. While the preferred embodiment makes use of a generally cylindrical bearing having a cone-shaped internal bore 101, this is merely preferred and reflects the desire and benefit of combining cavity force modeling with complementary but independent mechanical compensation of forces induced across the width of the bearing and bushing by operation of the counter force cylinders 42. By having the upper cylinder counter force mirror (but exceed by an appropriate safety factor) the cavity force, the average load seen through the bearing can therefore be reduced and the bearing can be reduced in size and/or cheaper materials (having lower strength properties) can be used in its manufacture. Also, the concept of having a force-profiled (substantially) conical internal bore 101 can be implemented independently of the measurement of applied force and controlled variation of counter cylinder force and this mechanical change can again contribute in reducing the overall size of the bearing 100 (and its related bushing 140).
With respect to the second bearing 54 and bushing 59 associated with the lower counter cylinders 52, the issue of space is not as problematic since the second bearing is positioned between the first plate and the manifold plate and its location therefore has no direct impact on mold mounting capabilities. Of course, the second bearing could also benefit from being made smaller by using a suitably modeled internal bore to modify its load characteristics.
It will, of course, be appreciated that the above description has been given by way of example only and that modifications in detail may be made within the scope of the present invention. For example, while a principal aspect of the present invention is the control of upper cylinder counter force through measured force experienced in the hollow bearing, it will be understood that benefit alone can be achieved by using a force-profiled (internal cone-shaped bearing). More particularly, by providing a bearing has a wall-thickness that is force-profiled, the use of that bearing alone could increase the available molding surface because such a force-profiled bearing can be made smaller in size. Furthermore, while a preferred embodiment of the present invention contemplates measurement of force (with a strain gauge in the hollow internal bearing), force measurement could be implied or made elsewhere within the system. For example, cavity melt pressure could be measured to imply load on the bearing, although it will be understood that direct measurement (wherever this is taken along the first plate 36, for example) will provide information that can be used to most accurately model an instantaneous cavity force.
One could also determine the approximate force in the cavity based on either the position of the injection screw (since this correlates with the volume of melt in the cavity) or the injection oil pressure (which correlates with the melt pressure at the gate) and then to control the cylinder force accordingly. However, these mechanisms are of course less accurate than direct measurement undertaken at the bearing.
It is merely design option as to which of the cylindrical bearing or the complementary bushing is located on the front and rear surfaces of the inclined (first) plate 36. Conventionally, the complementary bushings are coupled to the upright (second) plate 44 and the manifold plate 45. Equally, while the preferred embodiment describes the first bearing as being located on a front face of the first plate and the complementary bushing to be recessed on the second plate, it is equally plausible for these locations to be reversed such that the bushing is on the inclinable (first) plate and the bearing on the second (upright plate).
While the preferred embodiment has been described with reference to glazing application, the present invention could of course find application in other molding systems, using non-transparent materials, where an inclined mold is used.
