MXPA06005909A - Method and apparatus for performing an in-mold coining operation - Google Patents

Abstract

A hardware configuration and related method for performing a coining-type injection compression operation. The invention is useful in molding lenses since lenses have different thicknesses at various points. The equipment maintains the mold in a closed position during the entire molding cycle. A two part standoff post is designed to provide for slight axial compression during high injection pressure. The standoff post also provides a convenient mounting surface near the parting line to install inserts at various heights.

Description

METHOD AND DEVICE TO PERFORM A PUTTING OPERATION IN THE MOLD FIELD OF THE INVENTION The present invention relates to a method and device for injection molding and compression, which provides the benefits of a punching operation without the need to open the mold. BACKGROUND OF THE INVENTION U.S. Pat. No. 5,417,899 and the article "Simulation of injection molding and compression for optical media", by Fan et al., Describe the punching operation of the prior art, in which the mold is opened in the partition line when the pressure of the internal cavity exceeds the pressing force. The article describes die cutting as a process in which, initially, the thickness of the molding cavity is made to be slightly less than the nominal thickness of the piece. As the screw advances, the pressure of the cavity and the force of the mold cavity exerted on the machine platform increases. When the force exerted by the molten material in the mold is greater than the pressing force fixed in the machine, the mold explodes and opens to decrease the pressure of the cavity. As the screw exceeds a point set in the machine, the process changes from a volumetric flow rate condition to a packing pressure condition applied to the nozzle. During the stages of filling and packing, a balance is maintained between the strength of the molding cavity and the pressing force. When the first is greater than the second, the mold begins to close. This continuous opening and closing of the mold, known as "mold respiration", is different from injection molding and improves mold filling and replication of notches or wells on the surface of the discs, in addition to reducing the packing pressure and the residual stresses in the piece. For this type of molding by injection and compression, the parameter that is set is the press tonnage, more than the displacement, as a function of time. Due to mold respiration for die-cutting, injection and compression molding offers the advantages of lower packing pressure, homogeneous part quality, lower residual stress and greater dimensional accuracy compared to conventional injection molding, and is very suitable for manufacturing extremely fine or complex shaped parts, for which conventional injection molding can not meet quality requirements, or requires extremely high tonnage of pressing. However, despite the advantages of injection and compression molding, the compression stage also adds complexity to the process, and makes it more difficult to control. The article describes that the thickness of the molding cavity during opening and closing is not known a priori. The article proposes an injection algorithm, where repeated cycles of molding are carried out, making adjustments and calculations between each cycle and until the force of the cavity converges with the pressing force, to obtain the correct thickness of the piece for each individual step in the time. Several references, which are described below, provide examples of insertion movement. U.S. Pat. 2,443,826 discloses inserts that are directly attached to the pressing plates 16 and 25. The system relies on the complete displacement of the inserts until reaching stops, or otherwise bottoming out. Once they are fully displaced, there is no way to control the pressure of the internal cavity, so the cavity behaves as a fixed volume, thereby simulating a common compression molding operation. Japanese Patent JP 60009722 shows a spring member after an insert, which is compressed by activating a hydraulic piston located after the opposite insertion. Instead of allowing the cavity to be enlarged, the piston actually reduces the volume of the cavity when closing the doors, and moves the inserts towards each other, against the driving force of the spring. U.S. Pat. 4,900,242 discloses a molding apparatus that utilizes a swinging press assembly or floating die assembly to exert the same compressive force simultaneously in several cavities. Due to the considerable forces involved and the relative movement of multiple molding pieces, it is difficult to maintain a consistent molding volume. In addition, and due to the large number of moving parts, it is correspondingly more difficult to initially configure this equipment when changing inserts. U.S. Pat. 5,015,426 discloses a mold with central entry, for the manufacture of compact discs, ie CD. Due to the uniform flow pattern from the inlet, radially outward and towards the edge of the cavity, the mold is relatively simple in design, with the inserts 10 and 11 lying directly on the pressing plates 21 and 25, respectively. Since the CDs are manufactured to a single uniform thickness, the mold is not configured to receive inserts that have variable-shaped pieces forming surfaces, or to receive inserts that should be set at different heights relative to the mold parting line. By eliminating the height adjustment and the corresponding wedging requirements, the insert can displace a sensor that is housed directly back on the press plate. SUMMARY OF THE INVENTION The present invention relates to a configuration of equipment and related method to perform an operation of die-cutting by injection and compression. The present invention is useful for molding lenses, since they have different thicknesses at various points. Initially, a vertical pole is installed in one half of the mold. The post is axially dimensioned to provide an insert support surface located at a fixed distance from the mold partition line. The vertical post is designed with an internal compression member that allows slight axial compression beyond a predetermined force value. The vertical post can be installed in the fixed half of the mold, the movable half of the mold, or in both. For example, the vertical post can be installed in the mobile half of the mold, with a vertical post of fixed height installed in the fixed half of the mold. An insert is mounted adjustably on the support surface to obtain the desired cavity thickness. Intensified lens material is injected into the cavity until the pressure in the internal cavity exceeds the predetermined force value of the vertical pole. This causes an expansion of the cavity, thereby compressing the insert against the lens material. The expansion, after the axial compression of the vertical post, produces a punching operation without opening the mold. The compression of the insert maintains contact between the insert and the lens material as this material cools and shrinks. The vertical post automatically returns the insert support surface to the initial location, once the mold is opened after each part formation cycle. The initial place corresponds to the desired thickness of the cavity. Axial compression is achieved by a compression member highly resistant to forces disposed within the vertical post. The vertical post includes a stationary lower section bolted to the press plate and a top section capable of axial deflection, with the compression member sandwiched between them. The compression member is subject to compression between 206 to 276 MPa. For example, the compression member is subject to compression at approximately 248 MPa. This corresponds to a predetermined force value that is in the order of, but is slightly less than, the multitone e of the pressing force. The mold press provides a pressing force that exceeds the strength of the internal cavity during the injection step, and that at least the initial phase of solidification of the lens material lasts. The pressing force can be constant during the normally closed phase of the molding cycle.

The adjustable mounting of the insert involves selectively placing wedges between the support surface and the insert, to axially displace the insert relative to the partition line. Note that the distance of the partition line to the support surface is relatively small, since the greater distance to the mold plate is occupied by the vertical post having a known height. Accordingly, different cavity thicknesses can be obtained with the same vertical post configuration. This is significant, since semi-finished lenses are typically manufactured in a variety of thicknesses, between 8 mm to 11 mm, with the same curve finished on one side. In this way, different cavity thicknesses can be obtained with the same vertical post configuration and the same insertion, adjusting the thickness of the wedges arranged between the vertical post and the insert. The adjustable mounting of the insertion passage involves pressing the insert with the vertical post, for example, using a quick release press, also known as a SMED system. The quick release press extends through an opening formed in the receiver and mold. The release press extends radially outward from the axial dimension. The opening is elongated in the axial direction, to provide space for the press to move axially with the insert. The insert is pressed towards the upper section, to produce an axial deflection therewith. BRIEF DESCRIPTION OF THE DRAWINGS The advantages, nature and various additional features of the present invention will be fully appreciated when considering the illustrative modalities, which will be described in detail in relation to the accompanying drawings. In the drawings, similar reference numbers denote similar components in the figures. Figure 1 is a graph illustrating various internal cavity forces and a constant mold pressing force, in a simplified punching operation in accordance with the prior art. Figure 2 is a graph illustrating the internal cavity strength and a mold pressing force that varies stepwise in an advanced punching operation, also in accordance with the prior art. Figure 3 is a graph illustrating the internal cavity strength and the mold pressing force in accordance with one embodiment of the present invention. Figure 4 is a graph illustrating the internal cavity force and the insertion compression force in accordance with an embodiment of the present invention. Figure 5 is a schematic drawing showing the components in accordance with an embodiment of the present invention, designed to perform a die-cutting operation in the mold. DETAILED DESCRIPTION OF THE PREFERRED MODALITIES In general, compression and injection molding is characterized by a compression step, where the inserts are compressed against the molding material as it cools. This compression helps the replication of the molding by keeping the inserts in intimate contact with the molding material as it shrinks. The pieces with different thicknesses along their profiles, like ophthalmological lenses made of polycarbonate, will experience different degrees of shrinkage according to their thickness. Although compression helps to improve the replication of the molding, it creates a problem to control the thickness of the pieces. The compression step inherently changes the distance between inserts, thereby altering the thickness of the cavity. One approach to handling this problem is to secure the inserts with the mold to the desired cavity thickness. Then the entire half of the mold is displaced, or slightly opened during the punching operation, in a process known as the pressing end. Referring now to the drawings, and in particular to Figures 1, 2 and 3, a series of graphs are shown where they all show forces on the vertical axis, with respect to time on the horizontal axis. The horizontal axis shows time units representative of the stages in an injection molding cycle. All the graphs show a similar curve (10) representing the internal force of the cavity, ie the force exerted by the molding material on the inserts. From the Fan article, we know that one method to calculate is to integrate the cavity pressure of the mold to obtain the force that is exerted on the wall of the mold. The curve 10 is plotted together with the curves (20, 30 and 40) that represent the mold pressing force in each example, ie, the force exerted by the movable side of the mold against the stationary side of the mold. Typically, the pressing force of the mold is at least 10 tons, and may be in the range of 100 tons or more. In the graphs, the mold pressing force is expressed in kN. Figure 1 is a simplified example of a punching operation, where Ti represents the time when the cavity is almost filled with the molding material. At x the internal cavity force 10 exceeds the mold pressing force, whereby the mold is opened. As the molding material cools and shrinks, the force curve 10 is reduced and the mold can be reclosed and the inserts kept in intimate contact with the now solidified molding material. There is a problem with this simplified approach, in the sense that the compression force is still very high, which can be detrimental to the portions of the piece, which are cooled at different speeds. Figure 2 is an advanced punching process, where the mold pressing force is reduced to better control the internal cavity strength that decreases. This process is discussed in Fan's article, in addition to the US. 5,417,899. Examples of the pressing force profile set the initial pressing force 30a to 267 kN, the second stage 30b to 196 kN, and the final stage 30c to 133 kN. However, this process requires a convergence of forces between the two curves at a time T2 that is difficult to calculate. There are two important disadvantages in the advanced known die cutting process. First, it is a process of complex configuration and operation. Therefore, changes from one piece or lens to another always requires a whole new initialization procedure. Second, equipment or operator errors that cause an overfill condition can burst the mold. The deburring is a surplus of molding material that spills from the perimeter of insertion inside the mold during the opening of the mold. When the mold is closed again with many tons of force, this excess molding material can damage the mold.

Figure 3 illustrates an embodiment of the present invention, where the pressing force 40 is set above the maximum internal cavity force 10. In this way, the complex profile of pressing force and convergence calculations is eliminated, and it greatly reduces the possibility of deburring, to the point of being practically eliminated. As described above, the punching operation requires that the pressing force be controlled very precisely at each instant of the molding cycle. In contrast to this, the present invention provides a simplified method and device, which allows the cavity to expand without the need to open the mold. Accordingly, we define such operation as Die Cut Operation within the Mold. The operation includes maintaining the pressing force at a value, which exceeds the internal cavity force during periods of the molding cycle when it is typically closed. The pressing force can remain relatively constant as shown in Figure 3, or it can be several, as long as it exceeds the internal cavity force at all times. The Die Cutting Operation inside the mold is achieved by equipping the mold with entries on the edges with a vertical post that supports the insertion, and provides several features that will be described later in greater detail. Functionally, the vertical pole operates as a rigid and fixed platform to receive the insertion. The vertical post is capable of slight axial compression under an extremely high threshold force which, for example, may exceed 69 MPa. Figure 4 adds a force curve 45 for the insert passing through a Die Cut Operation within the Mold. Note that the initial portion of curve 45a is flat and almost zero. In this region, during a condition of partial filling of the cavity, the insertion is passive. As the initial volume of the cavity is filled, and the point where the internal cavity force exceeds the threshold of the vertical post is exceeded, both the insertion and the vertical post are compressed, thereby increasing the cavity. This threshold value is represented by the dotted line 45x. An important aspect of the present invention is to provide a compression member that functions as a variable force compression member. That is, it requires an increasing force to compress the member in each progressive stage. Thus, a first force will compress the member in a first amount, but a second, higher force is necessary to compress the member an additional amount, even if it is equal. Figure 4 illustrates how the force of the compression member is equal and opposite to the internal cavity force. Accordingly, the vertical post of the present invention provides a self-adjusting capability. If the compression member has a geometric response curve, a force that increases geometrically to effect the same degree of displacement would be necessary, as the member is more and more compressed. The punching operation according to the prior art has no response curve, and it is necessary to program the pressing force with a force value for instant time in the molding cycle. The present invention provides an important feedback function, in the sense that each instance of internal cavity force exceeding the threshold is automatically counterposed with an equal opposite force produced by the compression member. This is represented by a portion of curve 45b above line 45x. As the molding material cools and shrinks, the compression member returns to its initial displacement. Once it reaches its initial point, the compression force on the molding material falls to zero, which is shown as portion 45c. Of course, it is also possible to configure the compression member with a pre-load, to start the molding process at a higher point of the geometric response curve. In such circumstances, the flat portions 45a and 45c of the force curve of the compression member would extend horizontally to 100 kN, which corresponds, for example, to 5,000 psi. Notwithstanding the preload, the insertion does not exert any pressure on the partially filled cavity. Referring now to Figure 5, the vertical post according to the present invention is shown, including the upper body 60 and the lower body 70. The lower body 70 is a stationary component that is placed and held in the pressing plate 68. One or more support plates 66 may be placed on the pressing plate 68 to facilitate connection or positioning. The upper body 60 includes a support surface 60a at its upper end for receiving an insert that is subsequently assembled and a SMED connection assembly 60d securing the insertion post to the upper body 60. A compression member 62 is disposed between the body upper 60 and a lower body 70. The upper and lower bodies are connected to each other by a bolt 64 which passes through the compression member 62. The bolt 64 is able to slide downwardly within the lower body 70, due to the absence of strings near the head of the bolt. The entire vertical post, from the pressing plate 68 to the supporting surface 60a, provides a fixed platform which is fixed at a known distance 60b from the partition line, represented by the upper part of the key. In other words, the vertical post is a height adjuster tool that replaces the prior art practice of securing the insert assembly with the press plate by a threaded adjuster body. Typically, the threaded adjusting body is positioned and clamped with the pressing plate. By rotating the lower portion of the adjuster body, the upper portion that loads the insertion can be brought to the receiver to adjust its position relative to the partition line. Since the threaded adjusting body must withstand the pressing force several tons, the thread essentially makes the adjustment unstable. The installation routine is tedious, and inherently requires being tuned by the mere act of removing an insert and installing another. As can be appreciated, the vertical post of Figure 5 provides a support surface 60a very close to the partition line. Insert 50 can be installed on top of one or more wedges 60c to easily adjust its location relative to the partition line. The wedges 60c have openings through which an insertion post 52 extends to be fixed in place relatively easily by the SMED connector 60d. Accordingly, the vertical post provides a structure for reducing the distance from the insert support surface to the partition line. Given the agile installation process of the insert, you can wedge the inserts and join a stump pin in a clean room. In other words, the vertical post allows the calibration of the insert before installing the inserts. In addition, the inserts can be pre-heated and connected via SMED 60d in a manner that essentially reduces machine downtime. SMED 60d is engaged by a hollow shaft to an upper body 60 after passing through an opening 84 formed inside the mold wall 82 and the insertion receiver 80. A roller then slides along the shaft to be connected to the lower end of the mold. post 52. The roller can be extended and retracted by a lever 60e that is accessible from the outside of the mold. The opening 84 is elongated in the downward direction, as indicated by reference numeral 84a. This provides space for the entire SMED connector 60d to move axially downward with the upper body 60. The compression member 62 has the characteristics of requiring an extremely high force to admit a deformation, which results in an axial downward displacement of the upper body 60, SMED and the insert. The compression member can be formed from one or more highly incompressible polymeric materials, highly incompressible rubber or plastic, springs made of metal or other high strength materials. For example, Belleville resorters made of steel for tools can be used. A combination of springs and materials can be used to adjust the force versus displacement characteristics of the compression member. In one embodiment, a Belleville washer is sandwiched between two flat metal washers made of stainless steel or for tools. The flat washers provide a wear surface against which the inner and outer peripheries of the Belleville washer can slide under high compression loads, without damaging the upper and lower bodies 60 and 70. The compression of the member 62, i.e. the reduction height resulting from the compression, is schematically illustrated by dotted line 62a. In a practical embodiment of the present invention, a Belleville washer is installed with a threshold force parameter of more than 138 MPa on the vertical pole, and is clamped by bolt 64 with a pre-load representing a fraction of the threshold force . For example, a round with a compressive force between 206 and 276 MPa can be used. For all intensive purposes, the assembled vertical pole acts as a solid assembly to receive the insertion. This simplifies the installation of the vertical post, and creates an essentially rigid and fixed support surface 60a. In other words, for forces less than 206 MPa, as they would appear in a change of inserts, the post is essentially solid and stationary. As can be seen in Figure 4, only in the unit of time 4 under an almost complete cavity filling, the forces involved tend to the vertical post compressibility threshold. As the pressure of the internal cavity exceeds the threshold, the insert can be retracted against the driving force of the compression member, thereby enlarging the cavity. In case of a pre-load, the pin 64 opposes the force of the compression member. This corresponds to the curve sections 45a and 45c. As soon as the curve exceeds the 45x line, the opposite force is gradually transferred to the internal cavity force. In this way, the cavity is expanded to the high temperature piece volume without further action by the operator, and without the need for further process adjustments, even if the molding conditions change slightly with respect to time. As the piece cools and shrinks, around the unit of time 11, the opposite force is gradually transferred to the bolt 64. For example, with a Belleville washer of 138 MPa, the first millimeter of displacement may require 152 MPa. The next millimeter of displacement may require 180 MPa, and the next millimeter may require 234 MPa. In this way, enormous loads can be absorbed in a self-adjusting manner, while the possibility of the compression member touching bottom is highly unlikely, and can be easily prevented. In other words, the compression member requires a force per unit of compression increase that follows a non-linear curve. For example, the compression member may follow a geometric force curve. In another example, the compression member may follow a geometric force curve. In another example, the compression member may follow an exponential force curve. By selecting an appropriate compression member, the axial displacement in section 45b can be adjusted according to the die-cutting requirements of the particular process. However, the self-adjusting force provided by the compression member that is equal and opposite to the pressure of the internal cavity will remain the same, as illustrated in curve 45d which tracks the force curve of the internal cavity 10. Preferred embodiments have been described for methods and apparatus used for die-cutting operations within the mold (intended to illustrate and not limit), it is done Note that those skilled in the art can make modifications and variations in light of the above disclosures. Accordingly, it will be understood that changes may be made in the particular embodiments of the disclosed invention, which fall within the scope and spirit of the present invention, as described in the appended claims. Having described the present invention with the details and particularities required by the patent laws, what is claimed and desired to be protected by a Patent Letter is described in the appended claims.

Claims (23)

1. CLAIMS 1. A method for configuring a mold for injection molding and compression of a lens, comprising the steps of: installing a vertical post in a mold half, which is axially dimensioned to provide an insert support surface located at a fixed distance of the mold partition line, where the vertical post admits a slight axial compression beyond a predetermined force value; fitting an insert into the support surface to obtain a desired cavity thickness; and injecting lens material into the cavity until the pressure of the internal cavity exceeds the predetermined force value of the vertical post, thereby compressing the insert against the lens material. The method of claim 1, wherein the axial compression of the vertical post produces a punching operation without opening the mold. The method of claim 1, wherein the compression of the insert maintains contact between the insert and the lens material as the lens material cools and shrinks. 4. The method of claim 1, wherein the vertical post automatically returns the insert support surface to the initial location once the mold is opened after each part formation cycle. 5. The method of claim 4, wherein the initial location corresponds to the desired cavity thickness. The method of claim 1, wherein the axial compression is achieved by a compression member highly resistant to forces disposed within the vertical post. The method of claim 6, wherein the vertical post includes a stationary lower section secured to the press plate and an upper axial deflection section, with the compression member sandwiched therebetween. The method of claim 7, wherein the compression member is subject to compressions of between 207 and 276 MPa. The method of claim 6, wherein the amount of force per unit of compression in increments on the compression member follows a curve selected from the group comprised of a non-linear curve, a geometric curve and an exponential curve. The method of claim 6, wherein during compression, the compression member provides a self-adjusting force equal to, and opposite to, the internal cavity pressure. The method of claim 1, wherein the predetermined force value is in the order of, and is less than, the multi-tonnage pressing force of the mold. The method of claim 1, wherein the adjustable mounting step comprises selectively placing wedges between the support surface and the insert, to axially displace the insert relative to the partition line. The method of claim 12, wherein different cavity thicknesses can be obtained with the same vertical post configuration. The method of claim 13, wherein different cavity thicknesses can be obtained with the same vertical post configuration and the same insert, by adjusting the thickness of the wedges arranged between the vertical post and the insert. 15. The method of claim 14, wherein lenses with a central thickness of between 8 and 11 mm are formed. The method of claim 1, wherein the adjustable mounting step comprises pressing the insert against the vertical post. The method of claim 1, wherein the adjustable mounting step comprises pressing the quick release of the insert against the vertical post. The method of claim 17, wherein the quick release press extends through an opening formed in the receiver and the mold. 19. The method of claim 18, wherein the release pressing extends radially outwardly from the axial dimension. The method of claim 19, wherein the opening is elongated in the axial direction, to provide space for the press to move axially with the insert. 21. The method of claim 7, where the insert is pressed with the upper section so that there is axial deflection with it. The method of claim 1, further comprising the step of: providing a pressing force that exceeds the internal cavity force during the injection step, and during at least the initial phase of solidification of the lens material. 23. The method of claim 22, wherein the pressing force is relatively constant.