METHOD AND APPARATUS FOR FORMATION OF A MOULDED APERTURE

This invention relates to a method and apparatus for the formation of fine apertures in injection moulded parts.

An example of a field in which fine apertures need to be formed in moulded parts is in the manufacture of artificial infant feeding teats, which are employed to dispense liquid from a container to a consuming infant through one or more fine apertures formed in the suckling end thereof. The dispensing apertures are typically small relative to the dimensions of the teat part and the thickness of the teat material, but the size of the aperture(s) can have a substantial impact on the rate of flow of liquid therethrough. In the case of infant feeding teats manufactured on a mass scale, it is desirable that the dispensing apertures be relatively uniform from one moulding cycle to the next over a large number of cycles.

In the known art, apertures such as those in a teat for infant feeding can be made by a number of different methods.

A first method of the art involves a protrusion mounted to one mould half of an injection moulding tool. When the tool is closed with the two mould halves brought together during the injection cycle, the protrusion is arranged to be sufficiently close to the opposing mould half to effectively create a closed mould section and leave an aperture in the moulding.

In the practice of the first method, a gap can form between the two mould halves at the tip of the protrusion due to inaccuracy of mould manufacture and closing, lash between parts and general wear through extended use. This can result in flashing (the formation of a thin membrane across the aperture) which can reduce or obstruct the flow of liquid through the aperture, in use.

A second method of the known art, intended to ameliorate the deficiencies of the first described method, involves mounting a movable blade within the injection mould that, on

activation, actually impacts the other half of the mould mechanically, creating a definite aperture. However, due to tool tolerances, lash and wear, the aperture size is again poorly controlled.

A third method of the known art, further intended to ameliorate the deficiencies inherent in the first two methods, is to perform a secondary cutting operation on a teat moulded without an aperture.

In the practice of the third method described, tool wear again makes the results highly variable, and desirous of improvement. Further, the third method described, being a separate operation, increases cost and complexity of the total manufacturing process.

A fourth method of the known art, as an alternative to the third method described, is to perform a secondary operation to create an aperture using a laser. The intent is to create the aperture with a non-contact system that avoids wear.

In the practice of the fourth method described, the limitations of laser technology for the creation of fine perforations become evident. Firstly, laser perforation at small sizes requires the location of the part to be controlled very accurately (typically to less than 0.5mm, and ideally to within 0.1mm). The size of a perforation is directly dependent on how ideally placed in the focal plane the part is held. It is known in the art of part manufacture from flexible materials that the laser input tolerance requirement is actually comparable to the manufactured tolerance of the moulded part. Secondly, it is known in the art of injection moulding that the thickness tolerance of a part will be of the order of greater than 100 microns. The effectiveness of a laser perforating system is also highly dependent on part thickness at the critical area. Thirdly, it is known in laser technology that occasional laser misfire occurs, which will randomly yield finished parts without an aperture at all.

Bearing in mind the above discussed issues with the prior art, embodiments of the present invention aim to enable provision of a close tolerance aperture within an injection moulded part, taking account of the lash, wear, and general tolerances of the injection

moulding process as currently practiced.

In accordance with the present invention, there is provided an injection moulding tool having first and second tool parts which are relatively movable between an open configuration and a closed configuration, the first and second tool parts having respective first and second mould parts which together define a mould cavity when the tool parts are in the closed configuration, wherein the first tool part carries an aperture formation pin which projects into the mould cavity and is urged into contact with an opposing surface of the second mould part by a spring means such that the aperture formation pin in the mould cavity during material injection effects formation of an aperture in the resulting moulded product.

The invention also provides a method of forming an injection moulded product containing an aperture by providing and operating an injection moulding tool as defined.

In general terms the invention provides for an injection moulding tool having first and second interfitting parts which together define a moulding cavity for forming a moulded item. First and second inserted pins are preferably provided in the respective mould parts, opposed to one another and restrained in the moulding tool by respective flanges or the like. The exposed surfaces of the pins are preferably formed as part of the same plane, curve, or compound shape as the surrounding part of the mould forming surface. In other words, preferably there is no substantial surface discontinuity at the interface of the pin surfaces in the mold cavity.

One of the pins carries a protrusion at its end of a section size and shape to provide the desired fine aperture, and of a length sufficient to bridge the gap across the moulding cavity to the end surface of the opposing pin and cause contact pressure thereon. At least one of the pins is able to displace axially in the moulding tool under the influence of said contact pressure, to maintain a total force transmission to the protrusion less than the break force of the protrusion, and is urged in the direction of the other pin by a suitable resilient spring means. The use of elastomeric block inserts has been found to be particularly useful, although not essential to the practice of the invention wherein the resilient spring

means may be provided by any convenient mechanism.

The strength of the spring means should be sufficient to in all cases resist movement of the pin due to injection pressure forces acting on the exposed pin surfaces. To this end, the size of the end of the movable pin is preferably kept to a practical minimum, and the strength of the resilient bias provided by the spring means is preferably selected according to the intended injection moulding pressure and the size of the exposed end.

The protrusion may be on either pin. The spring means may be installed on either pin. It is most preferred for convenience of mould service, that the spring means and the protrusion be on and behind the one pin preferably located on the fixed side of the injection tool.

The selection of suitable materials is within the known art, giving appropriate weight to the forces calculated to arise within the moulding operation.

In one form of the invention the injection moulding tool is adapted for producing a liquid dispensing means, such as an infant feeding teat or the like, having a dispensing aperture formed by the aperture formation pin.

In accordance with the present invention, there is also provided a method of forming an injection moulded product containing an aperture, comprising providing first and second mould parts which are relatively moveable between an open configuration and a closed configuration in which the first and second mould parts interfit to define a mould cavity, providing an aperture formation pin in the first mould part having a protrusion that extends into the mould cavity in the closed configuration, providing a spring means arranged to urge the aperture formation pin protrusion into contact with an opposing surface of the second mould part when the mould cavity is in the closed configuration, and injecting polymeric material into the closed mould cavity so as to form a moulded product with an aperture void in the location of the aperture formation pin protrusion.

The invention is described in greater detail hereinafter, by way of example only, and with reference to the accompanying drawings in which:

Figure 1 is a longitudinal central cross-sectional view of an infant feeding teat having a fine aperture;

Figure 2 is an enlarged view of the aperture containing end of the teat section from Figure 1; Figure 3 is a sectional view of an injection moulding tool;

Figures 4(a), 4(b) and 4(c) are sectional diagrams of a portion of an injection moulding tool for forming and aperture in a part, illustrating some dimensional tolerance issues with a mould of the known art;

Figure 5 is a sectional view of an injection moulding tool, constructed in accordance with an embodiment of the present invention, for manufacturing an infant feeding teat;

Figure 6 is a more detailed view of a portion of the moulding tool of Figure 5;

Figures 7(a) and 7(b) are diagrammatic sectional views similar to Figure 6, illustrating different aperture forming mechanism variations; and Figures 8(a) and 8(b) each show a movable pin from the preferred embodiment, illustrating different aperture forming mechanism variations.

An artificial infant feeding teat 10 is shown in central longitudinal cross-section in Figure 1, comprising generally a base flange 11, a body 12, and a bulbous end portion 13. The end of the bulbous end 13 is provided with a fine aperture 14. In use the infant feeding teat is secured over the mouth of a liquid container by the flange 11 to enable an infant to suckle on the end portion 13 and body 12 and receive liquid from the container through the aperture 14 using suction and mouth action on the flexible teat. The size of the aperture 14 in the end of the teat is instrumental in determining the rate of flow of liquid from the container to the infant, and it is desirable that the dimensions of this aperture be accurately reproduced during the manufacturing process. The end portion of the teat shown in Figure 1 is shown in more detail in Figure 2.

Figure 2 shows in more detail the aperture 14, characterised by a diameter 'a' in the case of a round aperture, although the aperture may be of any shape required to achieve a particular technical outcome. For example, a fine aperture tool is readily made with a rectangular or square aperture. The bulbous teat end 13 is characterised in the aperture

area by a thickness 't'.

Figure 3 is a sectional view of a tool for the manufacture of injection moulded parts. The tool, in use, is mounted within an injection moulding machine (not shown), which facilitates relative movement of the tool parts 6 and 7 to open the mould for removal of a moulded product. In the drawing, tool part 6 would typically be considered the fixed part with tool part 7 movable with respect thereto to open the mould for product removal.

Mould parts 8 and 9 form the mould cavity, and a number of non-identical plates 30-46 form the remaining structure of the tool and provide air and fluid conduit systems for operation of the mould.

It will be understood by those skilled in the art that the tolerance in the thickness (exemplified by dimension 'd') for each of the plates 30-46 is additive through the stacked plates, which can have a significant impact on the separation of mould parts 8 and 9 in the region of the aperture formation, as discussed further below.

Referring to Figure 4, a moulding tool according to prior art construction is illustrated in which the mould parts 8 and 9 at the end of the mould cavity are separated by a gap 'g' which may be unduly influenced by the cumulative tolerances mentioned above. The mould part 8 is shown with a protrusion 11 having an extent 'h' intended to bridge the gap across the mould cavity to mould part 9 during the injection cycle to leave an aperture in the end of the moulded product. Ideally the dimension 'h' of the protrusion 11 is identical to the dimension 'g' in the mould cavity to prevent injected material from reaching the region between the end of protrusion 11 and the abutting surface of mould part 9, as shown in Figure 4(a).

As mentioned, however, tolerancing issues can affect the actual size of mould gap dimension 'g ¹ . If the mould gap 'g' becomes larger than the extent 'h' of protrusion 11 (as shown in Figure 4(b)), injected material will flow between parts 8 and 9 over the tip of pin 11 and no aperture will be formed. In the case where mould gap 'g' is less than protrusion length 'h', as shown in Figure 4(c), damage to the protrusion 11 may occur, causing at best distorted apertures, and at worst breakage of the mould part.

Figure 5 shows in section the cavity portion of an injection moulding tool constructed in accordance with an embodiment of the present invention. A mould cavity 20 is defined by forming features of the a cavity block 18 and mould core 19 which are part of the fixed and movable portions of the tool, respectively. The mould core 19 comprises several coaxial core portions, including a central core pin 24 fixed within the core portion of the mould. Under action of an injection moulding machine, during an injection moulding cycle the separated cavity block 18 and mould core 19 are brought together when the tool is closed in a direction parallel to the axis of the core and pin 24. Whilst the mould is closed fluid polymer material is injected into the mould cavity through an injector (not shown). After the polymer has solidified the mould core portion is separated again from the cavity portion and the moulded part is ejected from the tool.

The pin 24 is fixed in the centre of core portion 19 of the tool, and in this example is coaxial and coterminal with the mould core at the end of the teat formation cavity. A movable pin 25 is supported in the cavity portion 18 of the moulding tool, coaxial with the fixed pin 24. An end of the movable pin 25 carries a protrusion 21 that extends into the mould cavity 20 to contact the end surface of the fixed pin 24 when the tool is closed.

The movable pin 25 is supported in the cavity block 18 by a carrier sleeve 27 which allows the pin 25 a range of axial movement. A spring means 26 is arranged at the end of the movable pin 25 opposite to the protrusion 21, to urge the movable pin axially in the direction of the fixed pin 24 and ensure contact between the protrusion 21 and fixed pin end surface during material injection. The movable pin 25 and spring means 26 are both supported in the cavity portion 18 of the tool by the carrier sleeve 27, which facilitates service replacement as a sub-assembly. The fixed pin 24 may also be readily replaced if necessary, for example where excessive wear takes place over time.

Referring to Figure 6, detail of the movable pin assembly, fixed pin end and surrounding moulding tool is shown in cross-section. As shown in this drawing, the movable pin 25 preferably has at least one flange interfitting with the carrier sleeve 27 and providing a clearance dimension V, 'kl' between the pin and carrier to limit the extent to which

protrusion 21 projects into the mould cavity. The spring means 26 as shown comprises a block of an elastomeric material within a cavity formed within the carrier sleeve 27 between the end of the movable pin 25 and the plate 30 supporting the cavity block 18. The elastomeric block 26 is resiliently compressible within its cavity to allow axial movement of the pin 25 against the spring compression force. Other forms of spring may alternatively be employed, such as suitable forms of leaf spring or coil spring.

As an example, the elastomeric material may comprise a polyurethane material or other synthetic rubber such as Neoprene. The spring force provided on the moveable pin is a function of the hardness of the material. For example, in one application a force of 25Nf was required to ensure good hole formation quality, which was provided by a spring measuring 5.2mm in axial height with an operating tolerance to 4.8mm in compression utilising a Neoprene material of 80 Shore 'A' hardness.

Figures 7(a) shows a cross-sectional diagrammatic view of the aperture formation mechanism of the preferred embodiment similar to Figure 6. Figure 7(b) also shows the aperture formation mechanism with variation in the detail of the movable pin 25 and carrying sleeve 27.

As seen in Figure 7(a), the carrier sleeve 27 is mounted in the cavity block 18 and has an end surface 28 that substantially aligns with the cavity forming surface 16 of the cavity block 18. Similarly the core pin 24 is mounted in the core 19 and has an end surface substantially aligned with the core forming surface 17. The pin 25 is constructed with a pin end surface 29 from which extends the aperture forming protrusion 21. The diameter of the protrusion 21 is selected to provide the desired aperture diameter in the moulded part. The length of the protrusion 21 from the pin end surface 29 is selected to substantially correspond to the distance across the mould cavity 20 between the carrier sleeve end 29 and the end of core pin 24. With these dimensions the pin end surface 29 substanially aligns with the sleeve end surface 28 when the mould is closed and the pin protrusion 21 bears against the end of the core pin 24, as shown.

The mechanism shown in Figure 7(b) differs from that of Figure 7(a) in the end

configuration of the movable pin 25 and carrier sleeve 27. In this case the end surface 29 of the movable pin 25 does not impinge on the mould cavity but remains within the confines of the carrier sleeve. Only the protrusion 21 projects into the mould cavity 20 through a correspondingly sized hole in the carrier sleeve end surface 28.

In order to form a fine aperture in the moulded product the diameter of the protrusion 21 will necessarily be small. For example, for some applications the protrusion diameter may be of the order of 1.0 to 0.1 millimeters. To minimise the likelihood of damage to the thin protrusion during operation of the moulding tool it is preferable that the protrusion 21 be kept as short as possible whilst still being capable of bridging the gap across the mould cavity. Also, it is preferable for the axis of the movable pin 25 to be substantially perpendicular to the contact surface of the fixed pin 24 so that forces on the protrusion during tool opening and closing are substantially axial.

Figure 8(a) shows the movable pin 25 in isolation, illustrating the pin end surface 29 and the protrusion 21 extending therefrom. In addition to maintaining the protrusion length as short as practicable, another way in which the strength of the pin is improved is by designing the protrusion 21 with a conical profile, as illustrated in Figure 8(b). In this case the diameter of the protrusion 21 adjacent the pin end surface 29 is larger than at the tip of the protrusion. The diameter of the protrusion at its tip defines the minimum dimension of the aperture in the moulded product. The base of the protrusion may be up to several times larger than at the tip, allowing for greater structural strength an rendering the movable pin less prone to damage during use.

Flow dynamics and injection pressure of the fluid polymer material in the mould cavity during the injection moulding process may also be a consideration in the aperture forming mechanism design. Referring to Figure 7(b), the fluid material injected under pressure into the mould cavity will flow horizontally to the protrusion 21 but the injection pressure could force material up the side of the pin to vent into the tool. This would create a small stand-up of material around the moulded aperture in the product, which is generally undesireable. Such an occurrence is less likely in the mechanism configuration illustrated in Figure 7(b) where the material would have already cured at the vertical split between

movable pin and sleeve before final injection hold pressure is achieved. The pins shown in Figures 8(a) and 8(b) each include a small flat section inducated at 28 to provide a small gap for venting gases from the mould cavity during injection.

The injection pressure in the mould cavity should also be considered in selection of the spring means 26. In particular, the pressure inside the mould cavity will act upon the end surface 29 of the movable pin 25. Accordingly, the resilient force provided by the spring means 26 should be sufficient to overcome such injection pressures on the movable pin and maintain contact between the protrusion 21 and fixed pin 24 whilst the mould is closed.

The superiority of the method of the present invention over the most advanced of the methods of the known art can be demonstrated by measuring the variation in aperture. As many of the apertures of the known art are somewhat irregular, a flow rate measurement was chosen as most representative of the aperture size, and relevant to the intended application.

TEST METHOD A closure suited to each teat was mounted into a liquid capture apparatus. The sample teat was inserted into the closure, and a bottle screwed into the closure. The base of the bottle was vented to avoid the creation of vacuum inside the bottle. With the bottle inverted (teat down) and vertical, a vacuum of -5kPa gauge was applied to the exterior of the teat. Flow rate per minute was measured as liquid passage through the teat aperture.

SAMPLES

Prior art:- Commercially available silicone teat with laser perforated aperture.

Present invention:- Injection moulded thermoplastic elastomer teat with 0.8 x 0.4mm nominal aperture formed according to an embodiment of the present invention.

RESULTS

The foregoing description of the invention has been presented by way of example only, and many modifications may be made to the embodiments without departing from the spirit and scope of the present invention as defined in the claims.