GB2205522A

GB2205522A - Moulding an article having an internal helical gear formation thereon

Info

Publication number: GB2205522A
Application number: GB08713442A
Authority: GB
Inventors: Keith John Palmer; Joseph Schinagl
Original assignee: Mono Pumps Ltd
Current assignee: NOV Process and Flow Technologies UK Ltd
Priority date: 1987-06-09
Filing date: 1987-06-09
Publication date: 1988-12-14
Also published as: GB8713442D0

Abstract

An article having an internal helical gear formation thereon of a given pitch, such as a helical gear pump or motor stator, is produced by an injection moulding process in which the mould core (50) is withdrawn from the mould (12, 14) after vulcanising or curing of the mouldable material (16) by simultaneously rotating and axially withdrawing the core from the mould at exactly the said given pitch of the helical gear formation, while the moulded article is retained in the closed mould. <IMAGE>

Description

METHOD AND APPARATUS FOR MOULDING AN ARTICLE The present invention relates to a method and apparatus for moulding an article having internal helical gear formation thereon of a given pitch.

While the invention is not exclusively directed to this particular problem, difficulty has been experienced in satisfactorily moulding the stators of helical gear pumps and motors to a high degree of accuracy and finish and at an economically acceptable production rate. The stator consists of a rubber or rubber-like material tube, usually with at least one radially extending flange and a through-bore having an internal helical gear formation thereon. In order to mould such a product it is necessary to include in the mould cavity a core having an external helical gear formation conforming to the shape of the internal helical gear formation to be moulded. After moulding, and vulcanizing of the rubber, or curing of similar material, the mould is opened and the moulded stator then has to be stripped off the core.This is a difficult, time consuming operation and sometimes involves damage to the interior surface, in particular, of the finished stator.

It is now proposed, according to one aspect of the present invention, to provide a method of moulding an article having an internal helical gear formation thereon of a given pitch, said method comprising the steps of providing a mould having at least two mould parts together forming a mould cavity conforming to the exterior shape of the article to be moulded, positioning a core in said mould, said core having an external helical gear formation conforming to the shape of the internal helical gear formation to be moulded, closing the mould parts, injecting a mouldable material into the mould cavity, allowing the material to vulcanise or cure, simultaneously rotating and axially withdrawing the core from the mould at exactly said given pitch of the helical gear formation, subsequently opening said mould parts and ejecting the moulded article from said mould parts.

It will be appreciated that with the method of the invention, the finished article is retained in the mould cavity until after the core has been itself removed. This removal of the core is achieved by simultaneously rotating and axially withdrawing the core from the mould at exactly the given pitch of the helical gear formation. Thus, the core is, so to speak, screwed out of the moulded article, such as the stator, thereby causing no damage at all to the stator.

Preferably, air is allowed to flow into the mould cavity adjacent the end of the core, during the withdrawal of the core to prevent a partial vacuum being formed in the mould by the withdrawal of the core. The air involved may be simply atmospheric air or, as an alternative, compressed air may be forced into the mould cavity adjacent the end of the stator to assist in core removal.

In order to accellerate the moulding process, the core is preferably provided with an electric heater, for example, a one kilowatt cartridge heater. This will effectively maintain the correct temperature of al of the mould parts. Alternatively, if one wishes to mould a thermoplastic material, the mould parts and/or the case may be provided with cooling means, such as passages for a cooling liquid or gas.

According to a further aspect of the invention, there is provided apparatus for moulding an article having an internal helical gear formation thereon of a given pitch, said apparatus comprising a mould having at least two mould parts, together forming a mould cavity conforming to the exterior shape of the article to be moulded, a core having an external gear formation conforming to the shape of the internal gear formation to be moulded, a core support for mounting said core in said mould cavity and means for simultaneously rotating and axially moving the core support in a direction to withdraw the core from the mould cavity at exactly said given pitch of the helical gear formation.

In one particular construction the core support comprises a helically threaded rod, the helical threads on said rod having the same pitch as the helical threads on said core, said core being mounted coaxially with said threaded rod, a fixed frame carrying a fixed nut cooperating with the helically threaded rod, and means to move the helically threaded rod axially relative to said fixed frame whereby axial movement of this threaded rod through said nut, causes rotation thereof to withdraw and rotate the core from the mould cavity at exactly said given pitch.

Of course, alternative electrical, mechanical or fluid pressure operated means could be provided to ensure that the core support is caused simultaneously to rotate and move axially as it is withdrawn from the mould cavity at exactly the pitch of the helical gear formation.

In order that the invention may more readily be understood, the following description is given, merely by way of example, reference being made to the accompanying drawings, in which: Figure 1 is a side elevation of one embodiment of apparatus according to the invention, partly in section; Figure 2 is a plan view of a portion of the apparatus of Figure 1; and Figure 3 is an end view of a helical gear pump stator manufactured using the apparatus of Figures 1 and 2.

Referring first to Figure 1, there is illustrated a moulding apparatus 10 having a mould formed by first and second mould parts 12,14, together defining a mould cavity 16 which is formed with a radial recess 18. A radially extending sprue feed port 20 passes through the mould part 12 to the mould cavity 16 and an aperture 22 passes axially through an end portion of the mould part 14.

The mould cavity is closed by a third mould part 24 having an annulus 25 which projects into the radial recess 18.

The third mould part 24 in fact spans two identical moulds and is itself mounted on a pair of axially extending guides 26,28 which pass through a fixed frame part 30 and are connected together by a movable frame part 32. The fixed mould part 30 is rigidly connected to a table 34 as is a hydraulic motor 36 provided with a gearbox which is connected to the movable frame part 32, which is axially slidable on guides 26,28.

Extending between and parallel to the guides 26,28 are two elongate threaded rods 40,42 which are connected to the movable frame part 32 and pass through apertures formed in the third mould part 24. The threaded rods are provided with a multistart thread which engages in respective nuts 44,46 mounted in the fixed frame part 30.

Mounted on the forward ends of the threaded rods 40,42 are cores 48,50 each having an external helical gear formation corresponding to the internal gear formation to be formed on the stator to be moulded. The pitch of the threaded rods 40,42 is identical to the pitch of external helical gear formation on the cores.

As shown in Figure 1, in the closed position of the apparatus the third mould part 24 abuts the first and second mould parts 12,14 to complete a sealed mould cavity 16 in which is inserted the core 48 or 50.

In use of the above described apparatus, with the assembly mounted as shown in Figure 1, rubber or other mouldable material is injected through the sprue feed port 20 into the mould cavity 16 and is allowed to vulcanize or cure. As can be seen in phantom #in Figure li the core 50 shown in that Figure is illustrated as having an electrical heater cartridge 52, for example a 1 kw heater cartridge.

This is used, together with similar heaters (not shown) in the mould parts 12,14 to produce the necessary vulcanizing or curing temperature. When curing is completed, the mould parts 12,14 are maintained closed and the hydraulic motors 36,38 are actuated to cause the movable frame part 32, and with it the threaded rods 40,42 to move axially to the left as seen in Figures 1 and 2. Because the threaded rods are engaged with the nuts 44,46 as these rods are moved axially, they are caused to rotate. The pitch of the threads 40,42 is chosen to be identical to the pitch of the helical gear formation on the cores 48,50 so that as these are withdrawn axially they are rotated at exactly the same pitch as their own helical gear formations.

The sprue formed in the port 20 should be sufficient to prevent the core rotating during this withdrawal but the mould parts 12,14 could be provided with other radially extending projections or recesses, so that a groove or rib is formed on the exterior surface of the moulded part, this assisting in preventing rotation. Since the third mould part stays in position, it further restrains the moulded stator against movement. During the withdrawal of the core, air is allowed to flow in through the aperture 22 and, if desired, compressed air can be applied to the aperture 22 to assist in the withdrawal.

Subsequent to withdrawal, the mould parts 12,14 and if necessary, the mould part 24, are opened and the moulded stator can then be ejected.

It will be appreciated that the method and apparatus of the present invention prevent any undue distortion taking place as the moulded stator or other article is separated from its associated core so that a good finish can readily be achieved. The operation can also be carried out very efficiently and quickly by reducing the overall cost of the moulded stators or other articles.

Claims

1. A method of moulding an article having an internal helical gear formation thereon of a given pitch, said method comprising the steps of providing a mould having at least two mould parts together forming a mould cavity conforming to the exterior shape of the article to be moulded, positioning a core in said mould, said core having an external helical gear formation conforming to the shape of the internal helical gear formation to be moulded, closing the mould parts, injecting a mouldable material into the mould cavity, allowing the material to vulcanise or cure, simultaneously rotating and axially withdrawing the core from the mould at exactly said given pitch of the helical gear formation, subsequently opening said mould parts and ejecting the moulded article from said mould parts.

2. A method according to claim 1, wherein air is allowed to flow into the mould cavity adjacent the end of the core, during the withdrawal of the core to prevent a partial vacuum being formed in the mould by the withdrawal of the core.

3. A method according to claim 2, wherein the air is compressed air forced into the mould cavity.

4. A method according to any preceding claim wherein the core is heated at least during the curing of the material.

5. A method according to any preceding claim, wherein the article is a rotor for a helical gear.pump or motor.

6. A method of moulding an article, substantially as hereinbefore described, with reference to and as illustrated in the accompanying drawings.

7. Apparatus for moulding an article having an internal helical gear formation thereon of a given pitch, said apparatus comprising a mould having at least two mould parts, together forming a mould #cavity conforming to the exterior shape of the article to be moulded, a core having an external gear formation conforming to the shape of the internal gear formation to be moulded, a core support for mounting said core in said mould cavity and means for simultaneously rotating and axially moving the core support in a direction to withdraw the core from the mould cavity at exactly said given pitch of the helical gear formation.

8. Apparatus according to claim 7, and including an air inlet in said mould parts at a position adjacent the end of said core remote from said core support, said aperture allowing air to flow into the cavity during the withdrawal of the core, thereby to prevent the formation of a partial cavity in the mould during this withdrawal.

9. Apparatus according to claim 7 or 8, wherein the core is provided with an electrical heater.

10. Apparatus according to claim 7 or 8, wherein the mould parts and/or the core are provided with cooling means.

11. Apparatus according to claim 7, 8, 9 or 10, wherein said core support comprises a helically threaded rod, the helical threads on said rod having the same pitch as the helical threads on said core, said core being mounted coaxially with said threaded rod, a fixed frame carrying a fixed nut cooperating with the helically threaded rod, and means to move the helically threaded rod axially relative to said fixed frame whereby axial movement of this threaded rod through said nut, causes rotation thereof to withdraw and rotate the core from the mould cavity at exactly said given pitch.

12. Apparatus according to claim 11 further comprising at least one elongate guide extending parallel to said threaded support rod and passing through said fixed frame, and carrying one of said mould parts on one end thereof adjacent that end of the helically threaded rod on which is mounted said core, and a fluid pressure operated means connected between said frame and said helically threaded support rod to effect axial displacement of said helically threaded support rod relative to said frame.

13. Apparatus for moulding an article having an internal helical gear formation thereon, said apparatus being substantially as hereinbefore described, with reference to and as illustrated in the accompanying drawings.