WO2001000394A2

WO2001000394A2 - Method of filling a mold for manufacturing an ophthalmic lens

Info

Publication number: WO2001000394A2
Application number: PCT/US2000/017562
Authority: WO
Inventors: Kai C. Su
Original assignee: Technology Resource International Corporation
Priority date: 1999-06-30
Filing date: 2000-06-26
Publication date: 2001-01-04
Also published as: TW454096B; WO2001000394A3; AU5639500A

Abstract

An apparatus and method for filling a mold for casting optical lens. Two molding shells and a closure member (14) form a mold with a molding cavity. To fill the mold a fluid lens-forming material is introduced into the molding cavity through a casting opening (20A, 20B) formed in the closure member. Air is venting out during the filling process through a venting opening (18) also formed in the closure member. Both the casting opening and the venting opening are located in the upper portion of the molding cavity but spaced apart from each other at an acute angle.

Description

METHOD OF FILLING A MOLD FOR MANUFACTURING AN

OPHTHALMIC LENS

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention comprises an apparatus and method of filling a mold for manufacturing an ophthalmic lens.

Background Art

Most lenses today are made by molding plastic because of its light weight, density, refractive index, and impact resistance. To form a lens, two molding shells, often referred as a front mold and a back mold in the art of lens making, are used. Each molding shell has a facing inside surface. When these two molding shells are properly positioned at a desired distance and rotational orientation to each other, their facing inside surfaces are a negative image of the surfaces of the lens to be formed. A closure member is used to cooperate with the molding shells to form a mold with a molding cavity. Then a fluid lens-forming material, normally a liquid monomer, is inserted into and contained in the molding cavity defined by the two molding shells and the closure member. The closure member normally has a casting opening through which the fluid lens-forming material is introduced into the cavity. Once the fluid lens-forming material is in the cavity, it is cured to form a hardened polymeric lens taking the shape of the molds.

Currently, there are two ways to fill a mold in molding the lenses in the art. The first, conventional way is a "top" filling method as shown in Fig. 1 of the present invention. In Fig. 1, a mold M with a molding cavity C is vertically positioned so that an opening V is at the top of the mold M. A filling device (not shown) in connection with a reservoir of a fluid lens-forming material delivers the fluid lens-forming material into the molding cavity C through a filling needle via the opening V. The opening V also functions as an air venting channel during the process. Inevitably, some air bubbles B generated during the filling process may be easily trapped at the opening V and may form an air block that slows down the filling process. Moreover, the "syrup"effect well known to glass makers is also evident with this "top" filling method. That is, given the viscosity of the lens-forming material, the fluid flows slowly, in "runs," into the molding cavity C and the air block formed at the opening V further frustrates the filling process. Because of the uneven, slow flow of the fluid lens- forming material into the molding cavity C, the likelihood of optical defects in the resulting lens is increased.

The second filling method is a "bottom" filling method as shown in Fig. 2 of the present invention. In Fig. 2, a casting opening O is separated from the venting opening V in about 180°. The mold M is positioned so that the casting opening O is located at the bottom of the mold M. The lens-forming material is introduced into the molding cavity C from the bottom through the casting opening O. To overcome gravity, pumping means (not shown) may be used to cause the lens-forming material to flow into the cavity C without significantly pressurizing it. Air venting is provided through the venting opening V located at the top of the molding cavity C. Because this filling method separates the venting opening from the casting opening, the interference of air bubbles with the flow of the fluid lens-forming material is eliminated and the "syrup" effect is reduced greatly. Thus, it is able to shorten the filling process significantly.

However, the bottom filling method introduces a potential new quality problem. As known to the people skilled in the art, the center region of a formed optical lens is the most optically active area. Therefore, any defect in this region will compromise the quality of the formed lens. Because air bubbles inevitably produce optical defects, it is preferred not to allow the air bubbles to pass through this optically active region. However, as shown in Fig. 2, the air bubbles B have to travel through the most optically active region to reach the venting opening V. In fact, the air bubbles B cross the entire molding cavity C, which significantly increases the likelihood of optical defects in the finished lens. Thus, the bottom filling method may seriously compromise the optical quality of the formed optical lens. SUMMARY OF THE INVENTION

The present invention overcomes the disadvantages of the prior art and discloses an entirely new method of filling a mold for molding lens. In essence, the present invention discloses a "side" filling method. One embodiment of the present invention comprises a method of filling a mold for molding an optical lens, wherein the mold is formed of two molding shells disposed on edge and substantially vertically oriented, with an annular closure member being disposed at the periphery of the molding shells and cooperating with the two molding shells to define a molding cavity. The molding cavity has a center, a vertical axis and a horizontal axis perpendicular thereto. Both axes pass through the center of the molding cavity and thereby divide the mold into an upper portion and a lower portion. At least one venting opening and at least one casting opening are disposed at the upper portion of the mold and spaced apart from each other at an angle. The angle is an acute angle and both venting opening and casting opening are communicating with the molding cavity.

In practicing the present invention, a fluid lens-forming material is introduced into the molding cavity through the casting opening. The fluid lens-forming material is then cured so that the fluid lens-forming material is hardened to form the optical lens. And the formed optical lens is removed from the molding cavity.

Because of the separation and the special geometric arrangement of the venting opening and the casting opening, the fluid lens-forming material is filled into the molding cavity without interference from the air venting process. Any trapped air bubbles travel to the venting opening and out of the mold from there due to gravity without passing through the optically active area of the optical lens formed. Moreover, the filling of the fluid lens-forming material is further facilitated by gravity and a pumping device is no longer necessary. Thus, the present invention avoids the "syrup" effect, improves the quality of the lens formed and further facilitates the filling process.

According to another embodiment of the present invention, molding an optical lens can be achieved by the steps of providing a mold having two molding shells disposed on edge and substantially vertically oriented, and an annular closure member disposed at the periphery cooperating with the two molding shells defining a molding cavity with a center and having a vertical axis and a horizontal axis perpendicular to each other, both axes passing through the center to thereby divide the mold into an upper portion and a lower portion. A venting opening and a plurality of casting openings are disposed at the upper portion of the mold and are spaced apart from each other. The venting opening and each of the casting openings form an acute angle between them. The venting opening and the plurality of casting openings are communicating with the molding cavity. After a fluid lens-forming material is introduced into the molding cavity through at least one of the plurality of the casting openings, the fluid lens-forming material is cured so that the fluid lens-forming material is hardened to form the optical lens, and then the optical lens is removed from the molding cavity. Because this embodiment utilizes a plurality of casting openings, in addition to the advantages provided by the embodiment of the present invention with a single casting opening, the filling process may be further shortened and thus the production time for molding an optical lens may be further reduced.

In still another embodiment of the present invention, an optical lens can be formed by using a mold having two molding shells disposed on edge and substantially vertically oriented, and an annular closure member disposed at the periphery cooperating with the two molding shells to define a molding cavity having a center, a vertical axis and a horizontal axis perpendicular to each other, both axes passing through the center and thereby dividing the mold into an upper portion and a lower portion, and a venting opening disposed at an upper portion of the mold and communicating with the molding cavity. The steps of forming the optical lens include injecting a fluid lens-forming material into the molding cavity at a position at the upper portion of the mold but apart from the location of the venting opening, curing the fluid lens-forming material so that the fluid lens-forming material is hardened to form the optical lens, and removing the optical lens from the molding cavity. In this embodiment, the venting opening is located at the highest or upper most point of the molding cavity and the position of injecting the fluid lens-forming material into the molding cavity is lower than the position of the venting opening. In all of the embodiments, the closure member can be in the form of a gasket or a sleeve. For the latter, the sleeve can be formed from an elastomeric material. Thus, a filling needle can be used to pierce the elastomeric sleeve and then inject the fluid lens- forming mixture into the molding cavity.

The venting opening and casting opening can be pre-manufactured, or they may be formed during the filling process by, say, a filling needle. The sizes of the venting opening and the casting opening may be same or may be different.

Another embodiment of the present invention represents improvement in an apparatus for molding an optical lens of the type having a pair of opposed molding shells disposed on edge and substantially vertically oriented and which form a molding cavity therebetween with a closure member, the molding cavity having a center, a vertical axis and a horizontal axis perpendicular to each other, a casting opening communicating with the cavity for the introduction therethrough into the cavity a fluid lens-forming material and a venting opening communicating into the cavity, the improvement is achieved by the venting opening being disposed at the upper most point of the molding cavity, where the vertical axis intersects the closure member, and the venting opening and the casting opening forming an acute angle therebetween.

Other objects, advantages and uses for the present invention will be more clearly understood by reference to the remainder of this document.

BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWINGS

Fig. 1 is a vertical cross-sectional view of a prior art filling a mold from the top of the mold.

Fig. 2 is a vertical cross-sectional view of a prior art filling a mold from the bottom of the mold. Fig. 3 is a perspective view of the first embodiment mold of the present invention with a portion of the closure member broken away for clarity.

Fig. 4 is a vertical cross-sectional view of the mold of Fig. 3.

Fig. 5 is a vertical cross-sectional view of the second embodiment of the mold used in the present invention.

Fig. 6 is a vertical cross-sectional view of the third alternative embodiment of the mold used in the present invention.

Fig. 7 is a vertical cross-sectional view of the fourth embodiment of the mold used in the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is more particularly described in the following examples that are intended as illustrative only since numerous modifications and variations therein will be apparent to those skilled in the art. As used in the specification and in the claims, "a" can mean one or more, depending upon the context in which it is used. The embodiments are now described with reference to the figures, in which like numbers indicate like parts throughout the figures.

Referring generally to Figs. 3-7, the first embodiment of the present invention comprises an apparatus and method of filling a mold 10 for manufacturing an optical lens. The mold 10 is formed by two molding shells 12 A, 12B disposed on edge and an annular closure member 14 at the periphery of the molding shells 12 A, 12B, defining between them a molding cavity 16.

In the first embodiment, the molding shells 12 A, 12B are rotationally aligned relative to each other and axially positioned apart from each other at the desired axial separation distance (e.g., the appropriate separation to produce a lens of a desired thickness). When properly positioned, the facing inside surfaces of the molding shells 12 A, 12B are a negative image of the required opposing surfaces of the optical lens to be formed. These surfaces are coaxial in the embodiments shown with a common axis A. Cross-sectionally, they have a common center 13. Thus, if the molding shells 12A, 12B are disposed on edge, the axis A is substantially horizontal and the molding shells 11 A, 1 IB are substantially vertical. Moreover, as shown in Figs. 4-7, the molding cavity 16, and thus the mold 10, can be divided into an upper portion 11 A and a lower portion 1 IB by a horizontal axis X which passes through the center 13. The mold 10 has a vertical axis Y, which is peφendicular to the axis X and passes through the center 13.

As shown in Figs. 3-7, the molding shells 12 A, 12B have a generally circular outside contour. Obviously, the contour may take other suitable geometric shapes such as oval, rectangular, or square. The intersection of the axis Y and the outside contour of the upper portion of 11A defines a highest or upper most point 17 of the molding cavity 16.

The closure member 14 in the embodiment shown in Fig. 3 is a tubular sleeve. The tubular sleeve engages the edges of the molding shells 12 A, 12B to form the molding cavity 16. The sleeve can be formed from a strip which is wound around the edges of the molding shells 12 A, 12B. This strip may be an adhesive synthetic material or a heat-shrink material so that the formed sleeve adheres to the molding shells. More preferably, however, the strip can be an elastomeric material so that the resulting sleeve allows the molding shells 12 A, 12B to slide along the axis A during the curing process to compensate for the shrinkage of the lens-forming material. Alternatively, the closure member 14 may be in the form of a gasket (not shown).

The closure member 14 has a venting opening 18 and a casting opening 20. The venting opening 18 is in communication with the molding cavity 16 so that the air may be vented out from the molding cavity 16. In the embodiments shown, the venting opening 18 is in the upper portion 11A of the molding cavity 16, preferably at or adjacent the upper most point 17 of the molding cavity 16.

The casting opening 20 is also in communication with the molding cavity 16 so that a fluid lens-forming material may be introduced through it into the molding cavity 16. The casting opening 20 is also located in the upper 11 A portion of the molding cavity 16 as well, but apart from the venting opening 18. In accordance with the present invention, as shown in Figs. 3-7, the position of the venting opening 18 is always higher than the position of the casting opening 20 so that air bubbles can readily be vented outward there through. Moreover, as shown in Fig. 4, the venting opening 18 and the casting opening 20 form an angle, , therebetween. The angle α can be viewed as the angle between the axis L,, which is a radial axis connecting the center 13 and the center of the casting opening 20, and the axis L₀, which is a radial axis connecting the center 13 and the center of the venting opening 18. For the embodiments shown in Figs. 3-7, the axis L₀ merges with the axis Y, although it does not have to.

The present invention requires that the angle α be an acute angle. This ensures that during the filling process, the air bubbles B will not travel through the most optically active area of the lens to reach the venting opening 18. Instead, the bubbles likely will travel along the inside surface of the closure member 14 to reach the venting opening 18 as shown in Fig. 4. Indeed, because of gravity, the air bubbles B will always be above the surface of the lens-forming fluid filled in the molding cavity during the filling process. Thus, the present invention makes it possible that the most optically active area of the lens may not be disturbed at all. In the present invention, while the angle α may take any nonzero value between 0 to 90°, the preferred range for the angle α is 5° to 85°, and the optimal range for the angle α is 5° to 50°. It is obvious that although the casting opening 20 shown in Figs. 3 and 4 is on the right of the venting opening 18, the casting opening 20 may be located at the left as well as shown in Fig. 5.

The casting opening 20 may be a hole as shown in Figs. 3-5. Cross-sectionally, the casting opening 20 may be in circular, oval, or other proper geometric shapes. Similarly, the venting opening 18 may be a hole in practice as shown in Figs. 3-5. Cross-sectionally, the venting opening 18 may be in circular, oval, or other proper geometric shapes. The venting opening 18 and the casting opening 20 may be identical, or different, size-wise and shape-wise.

Alternatively, as shown in Fig. 7, the venting opening 18 may be in association with a venting port 22. Similarly, the casting opening 20 may be associated with a casting port 24. The venting port 22 may be used to support a conventional air pump (not shown) for creating a suction to facilitate air venting. The casting port 24 may be used to support or position a filling needle (not shown) for better filling.

In the embodiments shown in Figs. 3-7, the venting opening 18 and the casting opening 20 can be pre-manufactured. Or alternatively, for the embodiment where the closure member 14 is formed of elastomeric material or other synthetic material, the venting opening 18 and the casting opening 20 may be formed by a filling needle directly piercing through the closure member 14 before or during the filling process.

The fluids lens-forming material is introduced into the molding cavity 16 through the casting opening 20. The filling can be done at a filling station (not shown) where the mold 10 is disposed substantially vertically so that the venting opening 18 is located at or adjacent the upper most point 17 of the molding cavity 16. During the filling process, a filling needle (not shown) is used for injecting a desired amount of the fluid lens-forming material into the molding cavity 16. The filling needle is connected to a reservoir (not shown) of the fluid lens-forming material. The filling needle enters the casting opening 20, if one has been pre-manufactured, or pierces the closure member 14 to create one if not. Either way, the filling needle establishes fluid communication between the reservoir and the molding cavity 16 through the casting opening 20. It is preferable that the filling needle injects the desired amount of fluid lens-forming material into the molding cavity 16 in a continuous manner.

Since the casting opening 20 is located at the upper portion 11 A of the molding cavity 16, gravity helps the flow of the lens-forming material into the molding cavity 16. Therefore, it is not necessary to provide pumping means to fill the molding cavity. Moreover, because of gravity, the lens-forming material naturally fills the molding cavity 16 from the bottom up while the air bubbles B travel to the venting opening 18 without passing through the center region of the molding cavity 16. Consequently, the likelihood of having optical defects in the most optically active region of the formed lens is greatly reduced and better lenses can be made.

Once a proper amount of the fluid lens-forming material is introduced into the molding cavity 16, the filling needle is withdrawn from the cavity 16 and the closure member 14. The fluid lens-forming material is then cured and hardened. As one skilled in the art will appreciate, curing can be accomplished in a number of ways. For example, the curing method of the present invention involves exposing the fluid lens- forming material to an ultraviolet ("UV") light for a desired time. Alternatively, after exposing the fluid lens-forming material to UV light, the fluid lens-forming material is then heated for a predetermined time, such as in an infra-red ("IR") oven. The second heating step solidifies the fluid lens-forming material to form the hardened lens if the material is not sufficiently cured in the UV step.

Fig. 6 shows another or third embodiment of the present invention, wherein the closure member 14 has two casting openings 20A, 20B. The casting opening 20A is located at an angle , apart from the venting opening 18 while the casting opening 20B is located at an angle ₂ apart from the venting opening 18. Angles , and α₂ can be different acute angles. However, preferably, they have same value. In other words, the casting openings 20A, 20B are preferably symmetric about the venting opening 18, which is positioned at or adjacent to the highest or upper most point 17 on the molding cavity 16. Casting openings 20A, 20B allow two filling needles to introduce the fluid lens-forming material into the molding cavity 16 simultaneously and thus shorten the filling process significantly. Furthermore, the closure member 14 may have more than two casting openings. Again, each of the multiple casting openings can be located at a same or different angle apart from the venting opening than the rest of the casting openings. The casting openings may be symmetrically distributed about the venting opening. Or they may be asymmetrically distributed about the venting opening. Obviously, it is not necessary to use every casting opening available to fill the mold. Nevertheless, incorporating a mold with multiple casting openings of the present invention with automation technology available in the art may greatly reduce the production time of molding an optical lens.

The fluid lens-forming material is preferably a monomer. One monomer that can be used to practice the present invention is manufactured by P.P.G. located at Pittsburgh, Pennsylvania, and sold under the trademark CR 424. As one skilled in the art will appreciate, other lens-forming materials known in the art can be used with the present invention.

In practicing the present invention, as discussed above, the closure member 14 may be a sleeve made from elastomeric material so that a filling needle may pierce through the sleeve to establish a fluid communication channel with the molding cavity 16. In these embodiments, a suitable sleeve material is thermoplastic rubber that contains KRATON^®G, a styrene-ethylene-propylene (butylene) block copolymer, sold by Shell Oil Company of Houston, Texas. Such rubber includes those sold under the trademarks DYNAFLEX^®G2703, 2711, and 2712 by GLS Corporation of Cary, Illinois. These rubbers have a Shore A hardness ranging from about 43 to 62, a specific gravity of about 0.9g/cc, a tensile modulus at 300% elongation ranging from about 355 to 470, tensile strength at break of about 680 to 1000 psi, and a tear strength of about 113 to 127. These rubbers have smooth surfaces and have a thickness in a range of 1 mm - 8 mm. The sleeve material of the present invention, however, is not limited to a single material. In fact, the desired sleeve material can vary depending on the specific monomer compound used to form the lens. That is, a certain sleeve material may be prefeπed with a particular lens-forming fluid and different type of sleeve material with another lens-forming material. Among many available materials, it is found that the present invention can be satisfactorily practiced with polyurethane, polyvinyl chloride, organosilicon elastomer as well as KRATON^®G.

Although the present invention has been described with reference to specific details of certain embodiments thereof, it is not intended that such details should be regarded as limitations upon the scope of the invention except as and to the extent that they are included in the accompanying claims. Many modifications and variations are possible in light of the above disclosure.

For example, the invention has been described with reference to use a venting opening to vent out air from the molding cavity. An air pump may be used to cooperate with the venting opening to outlet air from the molding cavity, which, of course, is made possible by the separation of the venting opening and the casting opening provided by the present invention. Moreover, just like plurality of casting openings may be utilized to facilitate placing a lens-forming fluid into a molding cavity, a plurality of venting openings may be used to outlet air from the molding cavity. For example, a plurality of venting openings can be aπanged at the top of the upper portion of the mold, but spaced apart from each other.

Claims

What is claimed is:

1. A method for molding an optical lens, comprising the steps of: a. providing a mold having two molding shells disposed on edge and substantially vertically oriented, and an annular closure member disposed at the periphery of the molding shells cooperating with the two molding shells to define a molding cavity having a center, a vertical axis and a horizontal axis peφendicular thereto, both axes passing through the center to thereby divide the mold into an upper portion and a lower portion, a venting opening and a casting opening disposed at the upper portion of the mold and spaced apart from each other at an angle, both venting opening and casting opening communicating with the molding cavity; b. introducing a fluid lens-forming material into the molding cavity through the casting opening; c. curing the fluid lens-forming material so that the fluid lens-forming material is hardened to form the optical lens; and d. removing the optical lens from the molding cavity.

2. The method of Claim 1, wherein the venting opening is located at or adjacent the upper most point of the molding cavity.

3. The method of Claim 1 , wherein the position of the casting opening is lower than the position of the venting opening at the molding cavity.

4. The method of Claim 1 , wherein the angle between the casting opening and the venting opening is an acute angle.

5. The method of Claim 1 , wherein the closure member is in the form of a sleeve disposed around the molding shells.

6. The method of Claim 1, wherein the fluid lens-forming material is a monomer.

7. A method for molding an optical lens, comprising the steps of: a. providing a mold having two molding shells disposed on edge and substantially vertically oriented, and an annular closure member disposed at the periphery cooperating with the two molding shells to define a molding cavity which has a center, a vertical axis and a horizontal axis peφendicular to each other, both axes passing through the center to thereby divide the mold into an upper portion and a lower portion, a venting opening disposed at the upper portion of the mold and communicating with the molding cavity; b. injecting a fluid lens-forming material into the molding cavity at a position at the upper portion of the mold but apart from the location of the venting opening; c. curing the fluid lens-forming material so that the fluid lens-forming material is hardened to form the optical lens; and d. removing the optical lens from the molding cavity.

8. The method of Claim 7, wherein the venting opening is located at or adjacent the upper most point of the molding cavity.

9. The method of Claim 7, wherein the fluid lens-forming material is a monomer.

10. The method of Claim 7, wherein the closure member is in the form of a sleeve disposed around the molding shells.

11. The method of Claim 10, wherein the sleeve is formed from an elastomeric material.

12. The method of Claim 11, wherein the step of injecting the fluid lens-forming material further comprises the steps of: a. piercing the elastomeric sleeve by a filling needle; and b. injecting the fluid lens-forming material into the cavity through the filling needle.

13. A method for molding an optical lens, comprising the steps of: a. providing a mold having two molding shells disposed on edge and substantially vertically oriented, and an annular closure member disposed at the periphery cooperating with the two molding shells defining a molding cavity with a center and having a vertical axis and a horizontal axis peφendicular to each other, both axes passing through the center thereby dividing the mold into an upper portion and a lower portion, a venting opening and a plurality of casting openings disposed at the upper portion of the mold and spaced apart from each other, the venting opening and each of the casting openings forming an angle between them, the venting opening and the plurality of casting openings communicating with the molding cavity; b. introducing a fluid lens-forming material into the molding cavity through the casting opening; c. curing the fluid lens-forming material so that the fluid lens-forming material is hardened to form the optical lens; and d. removing the optical lens from the molding cavity.

14. The method of Claim 13, wherein the venting opening is located at or adjacent the upper most point of the molding cavity.

15. The method of Claim 13, wherein the position of each casting opening is lower than the position of the venting opening at the molding cavity.

16. The method of Claim 13, wherein each angle is an acute angle.

17. The method of Claim 13, wherein the closure member is in the form of a sleeve disposed around the molding shells.

18. The method of Claim 17, wherein the sleeve is made from an elastomeric material.

19. The method of Claim 13, wherein the casting openings are substantially identical to each other.

20. The method of Claim 13, wherein the casting openings are substantially symmetrical about the venting opening.

21. An apparatus for molding an optical lens, comprising: a. a front mold and a back mold; and b. a closure member cooperating with the front mold and the back mold to form a molding cavity therebetween for molding the optical lens, wherein the closure member has at least one venting opening and at least one casting opening spaced apart from each other at an acute angle.

22. The apparatus of Claim 21 , further comprising means for substantially vertically positioning the molds so that the at least one venting opening is located at or adjacent the upper most point of the molding cavity.

23. The apparatus of Claim 21 , wherein the closure member is a gasket.

24. The apparatus of Claim 21 , wherein the closure member is a sleeve.

25. The apparatus of Claim 24, wherein the sleeve is made of an elastomeric material selected from the group of polyurethane, polyvinyl chloride, organosilicon elastomer and KRATON^®G.

26. An improvement in an apparatus for molding an optical lens of the type having a pair of opposed molding shells disposed on edge and substantially vertically oriented and which form a molding cavity therebetween with a closure member, the molding cavity having a center, a vertical axis and a horizontal axis peφendicular to each other, a casting opening communicating with the cavity for the introduction therethrough into the cavity a fluid lens-forming material and a venting opening communicating into the cavity, the improvement comprising: the venting opening being disposed at the upper most point of the molding cavity, where the vertical axis intersects the closure member, the venting opening and the casting opening forming an acute angle therebetween.