GB2339163A - Mould vents - Google Patents

Description

2339163 MOULD VENTS The present invention relates to vents f or moulds

used in f orming vulcanisable rubber or other such mouldable materials and particularly but not exclusively to vents for tyre moulds..

In the moulding of rubber articles such as tyres, the initial heating of the tyre surface in contact with the hot mould reduces the viscosity of the surface rubber so that under the influence of internal moulding pressure local rubber flow occurs. venting is required to allow pockets of air which may become trapped between the green tyre carcass and the hot mould surf ace to escape so that every part of the curing tyre surf ace contacts the mould and the tyre is thus vulcanised with a perfect impression of the mould detail.

Tyre mould vents commonly take the form of small diameter holes drilled through the mould wall normal to the interior surface. Most commonly modern venting utilises so-called 'insert vents' which are small bore tubes introduced through the mould wall. once any trapped air has vented through the hole rubber begins to flow through the vent. However the 2 small diameter of the hole ensures that the rubber cures rapidly thus plugging the vent hole and sealing the mould. After completion of the tyre curing process these plugs of rubber which are still attached to the tyre surface are pulled out of the vent holes when the tyre is demoulded, Such plugs of rubber or, -spue-pips' as they are commonly known, detract from the visual appeal of the moulded tyre and are usually removed by trimming. This trimming operation is time consuming and thus adds to the cost of producing the tyre.

Also a problem may occur when the cured spue-pip breaks off when the tyre is being demoulded and thus remains blocking the vent hole. Such a blocked vent may not be immediately apparent and can cause subsequent poor quality mouldings.

To solve the above-mentioned problems so-called I'spueless vents" have been proposed, which allow trapped air to escape but which close to prevent rubber flow.

US 4 492 554 and US 4 347 212 disclose examples of such -spueless' vents. These known vents comprise a 3 valve held normally open by a coil spring to allow the passage of air. The valve is closed by flowing rubber which moves a valve head portion against the spring tension into seated engagement with a valve seat.

However a problem with known vents occurs when moulds containing them are cleaned. Periodic cleaning of moulds is necessary to remove material deposits which become baked onto the moulding surfaces thus ensuring good surface appearance of the moulded article. Cleaning is commonly carried out by blasting the moulding surface with a mildly abrasive material such as a plastic grit. Such grit may be used dry or wet in a stream of water. Because conventional vents are open during the cleaning operation it is possible for the cleaning medium and removed debris to enter and block the vent. Such blockages may only become evident when the mould is returned to service when imperfect mouldings begin to appear.

Accordingly it is an object of the present invention to provide a spueless vent which avoids the above described problem of blockage during mould cleaning.

4 According to the present invention a vent for a mould having a mould cavity comprises a vent main body, a vent closure member moved by first means to cause movement with respect to the vent main body to open the vent and having an upper end face arranged to be positioned at or near the surface of the mould cavity when the vent is closed, and second means arranged to close the vent when it is cold so that in use, when the mould and vent are hot, the vent may be opened by the second means and be closed by material in the mould cavity contacting the upper end face of the valve closure member and when cold it is closed to allow mould cleaning.

Thus in accordance with the invention the valve is opened when the mould is heated and subsequently closed by the action of the moulded article impinging on the upper end f ace but the valve is closed when the mould is cooled as is the condition for cleaning.

By the vent being hot is meant that the vent is substantially at the moulding temperature of the article, and by cold is meant substantially lower than the moulding temperature. For example in the case of a tyre the moulding temperature may be 1800C.

The first means may comprise a compound spring operating to both open and close the vent dependent on its temperature or it may comprise separate spring means, one to open the vent and another to close the vent.

one such spring means comprises a metallic so-called "shape memory alloy" or IISMAII in which the spring means is provided by changing shape in response to temperature. Typically such memory shape alloys may undergo shape change in the temperature range 900- 1200 and thus are well suited to switching or toggling between one shape at ambient temperature and another shape at the tyre moulding temperature of 1800C.

The vent is preferably formed of a mouldable plastics material.

Further aspects of the present invention will become apparent from the following description of

6 embodiments of the invention in conjunction with the following schematic drawings in which:

Figure 1 shows a part-sectional view of an insert vent according to an embodiment of the invention; Figures 2 shows another sectional view of the insert vent of Figure I taken along plane A-A'; Figure 3 shows yet another sectional view of the insert vent of Figure 1; Figures 4A-4B show other embodiments of the invention; Figure 5 shows an insert vent according to the invention set in place in the sidewall region of a tyre mould; Figures 6A-6C show other embodiments of the invention; and Figure 7 shows another embodiment of the invention.

Shown in Figures 1-3 is a mould vent according to one embodiment of the present invention. In use the vent is disposed in the mould wall with one end face 3 at the moulding surface and the opposite end face 6 open to atmosphere and provides a closeable air escape passage between the two end faces.

I.A 7 The main elements of the vent comprise a vent main body 1, a vent closure member 2 disposed slidable within the vent main body and first and second spring means S1 and S2 respectively for moving the vent closure member 2 with respect to the vent main body in order to open and close the vent.

The vent main body I is generally a tubular structure open at both ends to provide an air escape passage through the interior of the main body. Internally in one part there is a constricted portion 6 which coaxially locates the valve closure member 2 and guides its movements. At the upper end 3 of the vent main body 1, which is the end disposed at the moulding surface, there is an internal conical vent seat 4. A complementary external conical-shaped vent closure face 5 is provided on the upper end of the valve closure member 2. Closure of the valve occurs when the complementary conical-shaped surfaces of the closure face 5 and vent seat 4 are brought together by downward movement of the vent closure member 2 within the main body 1.

In an open state shown in Figure 1, an air escape passage from the upper end face 3 is provided 8 between the complementary conical surf aces 4 and 5, through the internally constricted portion 6 via slots 7s shown in section in Figure 2, and thence via the remaining internal tube portion 7.

In a closed state shown in Figure 3. the air escape passage is closed by the coming together of complementary conical faces 4 and 5 in seated engagement by the movement of the valve closure member 2 within the main body 1 resulting from the material being moulded impinging on and pressing against the closure member end face S.

The role of the spring means S1 and S2 in operation of the vent will now be explained.

The first spring means S1 is interposed between an interior f ace 9 of the valve closure member 2 and the adjacent face 10 of the internally constricted portion 6 of the main body 1 and serves to bias the closure member 2 outward form the main body 1 in the direction indicated by arrow 0 to open the vent.

The second spring means S2 is interposed between a circlip 11 fixed to the lower part of closure member 2 and the adjacent other face 12 of the internally 9 constricted portion 6, and serves to bias with only minimal spring force the closure member 2 inward into the main body 2 in the direction indicated by arrow C to close the vent. The provision of the circlip 11 is necessary in order that the closure member 2 may be initially assembled. into the main body 1 af ter which the second spring means S2 and the circlip 11 are assembled onto the lower part of the closure member 2 as shown. The circlip 11 is a loose sliding fit inside the tubular lower portion to allow the passage of escaping air.

The first spring means S1 comprises a shape-memory alloy. Such materials have the property that they may be set in a particular shape and subsequently plastically deformed from that set shape. However when heated beyond a critical or switching temperature they will return to the set shape.

In this particular embodiment the spring means S1 comprises such a shapememory alloy having a critical temperature in the range 800-1200C. This material in the f orm of a wire is f ormed and set into a helical open coiled spring having a first length ti corresponding to the distance between the faces 9 and 10 when the valve is in the open position as shown in Figure 1.

i In operation of the vent in the moulding of a rubber tyre, trapped air escapes through the open vent until the rubber impinges onto the end face 8 of the closure member and pushes the closure member 2 into the main body 1 to bring the conical faces 4 and 5 into seated engagement so to close the air escape passage between them. The action of the flowing rubber closing the vent compresses and deforms the f irst spring means S1 to a shorter second length f2 as shown in Figure 3.

After the moulding and vulcanisation is complete, the assembly is cooled to below the shape memory alloy critical temperature and the tyre is demoulded. After the tyre is removed the vent remains in the closed state by virtue of plastic deformation of the first spring means S1 and the biasing action of the second spring means 52 which provides minimal spring force sufficient only to keep the vent closed.

However during reheating in preparation for the next moulding cycle once the mould temperature reaches the shape memory alloy critical temperature the first spring means S1 switches again to its original set state and thus the compressed helical coil expands from length t2 to its original set length ti moving the closure member 2 against the minimal spring f orce of S2 outward of the main body 1 so to open the vent in readiness for the new moulding cycle.

Thus in operation of this new vent, once closed the vent will remain closed until heated to the critical or switching temperature of the shape memory alloy of the first spring means. Accordingly after completing the desired number of mouldings, the mould may be taken out of service and cleaned whilst the vents remain closed.

In a variant of the above embodiment the first spring means S1 takes a different form. In this variant the shape-memory alloy spring switches or changes between a longer first length and a shorter second length in response only to temperature. Thus in the hot state the spring has a f irst length fi 12 and in the cold state the spring contracts to a shorter length t3 which is equal to or preferably slightly less than the length t2 representative of the vent closed state. Thus the function of the second spring means S1 is merely to take up the slack or f ree-play of the closure member when the vent is closed. Thus in this variant the vent is both opened and closed in response to temperature and therefore does not require to be cooled with the moulded article in-situ in order to render the vent closed in the cold state.

In a further variant of the above variant the respective ends of the first spring means S1 may be fixedly attached to the main body and the closure member respectively so the spring means S1 moves the closure member not only to open but also to close the vent, thus obviating the need for the second spring means S2.

Whilst in the above-described embodiments use has been made of two forms of shape memory alloy in the spring means, other designs of shape memory alloy spring such as diaphragms or spring fingers or forms of temperature operable spring means may be used.

13 For example it is possible to use bi-metallic spring means or other such spring means.

Further embodiments shown in Figures 4-6 show examples of alternative spring means.

Shown in Figure 4 is another embodiment of the present invention in which the spring means for operating the valve closure member 21 takes the form of integral spring legs 23. As shown in sectional diagram 4A of the vent in the open position these legs 23 are located in an internal groove 25 or grooves provided in the wall of the interior hole 27 in the vent main body 22. Figure 4B is a sectional view taken in the plane A-A' of Figure 4A and shows three such spring legs 23 equi-spaced within the interior of the main body 22 and spaced apart to provide passages 26 for the flow of air from the mould. These spring legs each comprise a conventional bimetallic assembly having differential temperature expansion characteristics. In the cold or cooler state the spring legs are configured to bias the vent closure member 21 inwardly into the main body 22 to close the mould as shown in Figure 4B. When the vent is heated the differential expansion of the spring legs 23 acts to 14 bias the closure member 21 outward to open the vent as shown in Figure 4A. When rubber presses on the valve closure member 21 the spring legs are deformed against the biasing force of the bi-metallic expansion to close the vent again as in Figure 4B. on cooling and removal of the cure4 tyre from the mould the spring legs 23 remain set to keep the vent closed until it is reheated.

Whilst in the second embodiment the spring legs are shown and described as locating the internal grooves 25 on the internal wall of the vent main body they may also be formed integrally with the vent main body 22. Such an arrangement is shown in Figures 6A-6C where temperature operable spring means 63 functioning as previously described are formed integrally with the vent main body and locate into an annular groove 69 on the lower portion of the vent closure member.

In alternative embodiments the spring legs may be replaced by a complete spring disc.

In the embodiments described above the upper end 3 of the vent main body and the end f ace 8 of the closure member 2 f orm, when the vent is closed, a plane surface.

In an alternative embodiment shown in Figure 7 the upper end 3 of the main body is provided with an annular raised portion 7o in this particular embodiment forming a semi-toroid. This raised portion is useful for providing a surface for pressing or striking when fitting the vent into the mould and which afterwards may be removed to leave the vent upper surface flush to the moulding surface.

16

Claims (1)

1. A vent for a mould having a mould cavity comprising a vent main body, a vent closure member moved by first means to cause movement with 'respect to the vent main body to open the vent and having an upper end face arranged to be positioned at or near the surf ace of the mould cavity when the vent is closed, and second means arranged to close the vent when it is cold so that in use, when the mould and vent are hot, the vent may be opened by the second means and be closed by material in the mould cavity contacting the upper end face of the valve closure member and when cold it is closed to allow mould cleaning.

2. A vent according to claim I wherein the first means to open the valve when hot comprises a bias spring of shape-memory metal alloy (SMA).

3. A vent according to claim 2 wherein the bias spring is longer when hot than when cold.

4. A vent according to claim 3 comprising a closure spring to close the vent when the bias spring is at its shorter length.

6. A vent according to any of claim 1 characterised in that the first means comprises temperature operable bi-metallic spring means.

17 7 A vent according to any of claims 1 to 6 characterised in that the vent main body comprises a tubular body and the valve closure member is disposed within the tubular body.

8 A vent according to any of claims 1 to 7 characterised in that the valve se.at between the vent main body and the valve closure member comprises complementary shaped surfaces which are seatingly engaged when the vent is closed.

9 A vent according to claim 8, characterised in that the complementary shaped surfaces of the vent main body and the valve closure member are conical surfaces.