GB2124526A

GB2124526A - Casting hollow articles

Info

Publication number: GB2124526A
Application number: GB08320460A
Authority: GB
Inventors: Anthony Charles Worswick
Original assignee: Individual
Current assignee: Individual
Priority date: 1982-07-30
Filing date: 1983-07-29
Publication date: 1984-02-22
Also published as: GB2124526B; GB8320460D0

Abstract

In the casting of closed hollow articles, air or other gas is deliberately introduced into the interior of the cast shell in order to facilitate drainage of the residual molten metal by gravity after mould-filling pressure is released. The air is introduced through an aperture formed by puncturing or locally melting the shell or (Fig. 2, not shown) by way of the conduit provided for feeding the molten metal to the mould. <IMAGE>

Description

SPECIFICATION Casting hollow articles This invention relates to the production of hollow articles by the process known as slush casting.

Slush casting permits the production of hollow articles without the use of a core. The liquid material is fed into a die shape or form and allowed to cool partly so that a solid wall or shell is formed, whereupon the still liquid material remaining in the centre of the cast article is removed.

To remove the molten material it is convenient to provide an exit at the bottom of the moulding cavity so that the liquid material can run out under gravity. The same path is conveniently used for feeding the material into the mould cavity initially.

However a serious disadvantage of this arrangement is that in practice air is often unable to reach the centre of the moulded article to replace the liquid material, and the resulting vacuum tends to hold the liquid material within the article, resulting in very uneven outflow or in retention of the material in the centre of the article.

We have found that this problem can be overcome if, after the solid shell has formed, air or other gas is deliberately introduced into the shell e.g. via the metal inlet or through the shell wall, the liquid remaining within the shell thus being enabled to flow out.

In one embodiment of the invention, after the solid shell has formed the shell is deliberately punctured preferably in an upper region thereby allowing air or other gas to enter or replace the metal which flows out from the shell. The shell can be punctured for example by a punch or by local melting or maintaining a small part of the die of mould cavity wall hot enough to prevent the formation of a shell locally.

Alternatively, air can be admitted into the shell by way of the molten metal inlet/outlet.

If the puncture method is used the puncture (punctures) is preferably in a region of the cast article where it will not impair its performance or appearance.

The invention is particularly applicable to the production of substantially closed hollow bodies, for example hollow fusible cores for injection moulding.

The invention is illustrated by the accompanying schematic drawing of slush casting equipment, in which: Figure 1 shows a first form of slush casting equipment, and Figure 2 shows a second form of slush casting equipment.

Figure 1 shows a split mould or die 1 containing a mould cavity 2.

Associated with the mould is a molten metal bath 3 in which is immersed a metal pump unit 4 connected to the bottom of the mould cavity by a heated tube 5. In operation, air pressure is applied through a pipe 6 to the surface of the metal in the pump unit and is thereby forced up the pipe 5 and into the mould cavity. The air originally in the mould cavity is forced out through slight gaps in the die or mould or through air escape passages deliberately provided, and the metal quickly fills the mould cavity. The metal solidifies in the mould cavity, beginning at the walls of the cavity, thereby forming a solid metal shell containing still liquid metal. After a time sufficient for the solid metal shell to form, the air pressure is removed from the metal in the pump. As the pump is below the mould, the liquid metal in the mould will try to flow back into the pump and bath.

To enable this return flow to happen reliably and quickly, so that the entire central region of the cast article is emptied of metal, the solid cast shell is deliberately punctured at or near its highest point, in such a way as to allow ambient air to enter the central region of the cast article, where the metal is still liquid.

The puncturing of the cast shell can be done in any convenient way, for example by a punch 7 in the mould or die. The punch is normally retracted and its tip may for example lie flush with the wall of the mould cavity. The punch has an air duct such that after the punch has been advanced through the cast shell into the interior of the casting, air can flow from the surroundings into the casting, to replace the molten metal which flows back into the metal bath. Preferably, the air duct comprises one or more air slots on the outside of the punch, terminating short of the tip of the punch so as not to become clogged by the molten metal when the metal enters the mould cavity. However any other convenient form of vacuum-breaking device for allowing air to enter the interior of the casting can be used.

The size of the puncture required to allow entry of air will depend on the size of the casting.

Instead of air, other gases can be used to replace the molten metal. The air or other gas can be allowed to flow in naturally as the molten metal runs out of the casting, or can be supplied under pressure for quicker expulsion of the molten metal.

Alternatively part of the cast component may be maintained in a liquid state by means of a heated section of the die cavity wall, this section being fitted with a small valve mechanism which allows air to be admitted into the casting centre when the valve is opened. The operation of this valve would normally be sequenced to take place immediately after the pressure had been removed from the filling pump.

Figure 2 illustrates an alternative embodiment of the invention in which the air or gas is introduced through the aperture used for introduction and drainage of molten metal, instead of through an additional aperture in the shell.

Accordingly there is no punch 7, but a valve 8 is fitted into the die feeding pipe 5 near to the point where it meets the die members. Since the die feeder pipe is always maintained above the melting point of the metal alloy, any alloy present in the pipe is therefore molten. This valve is opened at the same time as the pressure is removed from the metal feeding pump 4, the vacuum formed by the metal attempting to drain from the die cavity sucking air in through this valve and thereby forming a hollow cavity.

However any other convenient form of vacuumbreaking device for allowing air to enter the interior of the casting can be used. Instead of air, other gases can be used to replace the molten metal. The air or other gas can be allowed to flow in naturally as the molten metal runs out of the casting, or can be supplied under pressure for quicker expulsion of the molten metal.

The deliberate introduction of air into the cast shell completely eliminates problems encountered hitherto in slush casting when the residual molten metal was unable to run out of the cast shell owing to inadequate access for air to replace the metal.

The above methods of allowing air into the components each have their specific applications.

For instance, admitting air through the wall of the casting into the centre of the casting is more applicable to thin castings, whereas allowing air into the shell via a valve in the feed pipe is particularly useful for the manufacture of hollow articles with a thick wall section.

The invention is particularly applicable to the production of hollow metal cores for use in the injection moulding of plastics articles, for example plastics inlet manifolds. Metal cores for injection moulding are made from a low melting point alloy (for example 1 500C). The cores are placed in an injection mould, the plastics material is injection moulded around the metal cores, and subsequently the cores are heated, by induction heating or by increased ambient temperature, so that the cores are melted out of the injection moulded components. Induction heating is the preferred method and it is very advantageous if the metal core is hollow as this greatly facilitates melting by induction heating. Hollow cores are also light, use relatively little material, and require relatively little heat both form manufacture of the core and for melting the core out of the moulded article, and are greatly preferable to solid metal cores. The present invention, which enables hollow fusible metal cores to be cast in a way that reliably ensures removal of surplus metal from the interior of the core, is therefore a great advance both technically and economically.

Claims

1. A method of slush casting to form a substantially closed hollow article in which molten metal is charged from below into a substantially closed mould and cools in contact with the wall of the mould to form a solid shell, residual molten metal remaining within the shell is drained from the shell and a gas is introduced into the shell as the metal is drained out.

2. A method as claimed in claim 1 in which the gas is introduced through an aperture formed in the wall of the shell.

3. A method as claimed in claim 2 in which the aperture is formed by mechanically puncturing the shell.

4. A method as claimed in claim 2 in which the aperture is formed by localised melting of the shell.

5. A method as claimed in claim 2 in which the aperture is formed by locally preventing solidification of the metal in the mould.

6. A method as claimed in claim 1 in which the gas is introduced into by way of a conduit provided for charging and/or draining the molten metal.