GB2137538A - A method of producing cast metal dies patterns and core-boxes - Google Patents

Abstract

A cast metal die is produced by casting a zinc based alloy around a pattern 1. After casting, the alloy is aged by subjecting it to a temperature of up to 276 DEG C for a predetermined time. This results in conversion of metastable phases formed during and shortly after solidification of the alloy as a result of which the casting expands. This expansion is used to compensate for <IMAGE>

Description

SPECIFICATION A method of producing cast metal dies, patterns and core-boxes utilising the ageing properties of certain zinc-based alloys The object of the present invention is to provide a method of producing accurate, cast metal dies as well as patterns and core-boxes utilising the ageing properties of certain zinc-based alloys Such metal dies as well as patterns and coreboxes are in common use in foundry operations for subsequent prodcution of components in a wide range of materials, including ferrous and nonferrous metals, plastics, ceramics and glass, the material of said components being introduced into the metal die, or mould, cavity by any conventional process, including gravity and pressure die-casting, forging, injection-moulding, sqeeze-casting, etc.

Core-boxes are utilized where it is necessary to produce usually sand-cores which when placed in a metal die used to produce a subsequent cast component enable the said cast component to be produced with cavities rather than be solid.

In its simplest form a die consists of two parts, each part having an appropiate cavity relative to the shape of the final cast component which is required to be produced. In addition there is provided means for introducing metal into the said cavity with the two die halves are held together.

Such cavities can be produced by directly machining into solid material or by machining blocks which have the rough shape of the required cavity cast in, usually by conventional foundry casting techniques.

This method can produce very accurate die cavities but can be expensive particularly when the cavity is complex.

Other methods involve casting the die directly, rather than machining. However all such casting methods involve casting around an initial pattern and inherent in all such casting operations contraction allowances have to be made. Such contraction allowances make it difficult for cast dies to match the dimensional accuracy obtained by direct machining.

The use of certain zinc-based alloys having particular ageing properties can assist in improving the accuracy of cast metal dies.

In conventional casting of dies, one method comprises the steps of casting around a pattern (male form) with a suitable material in a fluid state, such as plaster or an epoxy-resin to form a solid two-part, or multi-part, initial die set, the cavity of which will be faithful replication of the said pattern, removing the said pattern, forming a complementary pattern of bonded refractory material against each part of the said initial die set, removing each said complementary pattern of bonded refracted material and placing in a mould, casting in said moulds and against said patterns of bonded refrac tory material corresponding cast metal final die parts.When a suitable material, such as metal, plastic, ceramic, or glass is introduced into the cavity of the said final metal die set the resulting compo nent will have the shape of the initial pattern but will be reduced in size due to the shrinkages inherent in the casting process.

Normally a heat-cure process is employed to form the patterns of bonded refractory material, so although plaster and epoxy-resin have been mentioned for the material of the intitial die set, any material which will withstand such heat-cure process may be used.

The present invention utilises certain zinc-based alloys containing aluminium and/or copper for the material of the intial die set. Such alloys expand on ageing at temperatures up to 276"C. The magnitude of the expansion depends on the composition and the temperature and duration of ageing. Typically, expansion can be completed by ageing at 150 C for 400 hours, 175"C for 100 hours or 2000for 24 hours.

The expansion is due to the conversion of metastable phases formed during and shortly after solidification into the phase stable at low temperature, the overall density of the alloy being reduced as a result of the phase reactions.

On such phase reaction is the reaction of aluminium-rich alpha phase with the zinc- and copper-rich epsilon phase to form zinc and the copper- and aluminium-rich T' phase. The magnitude of the expansion caused by this phase reaction is greatest at a composition of approximately Zn-15wt%Al- 15wt%Cu.

Another phase reaction is the precipitation of copper-rich phases from solid-solution in zinc. The magnitude of the expansion caused by this phase reaction is greatest at a composition of approximately Zn-3wt%Cu.

Useful expansions can be obtained using a variety of alloys with zinc contents > 60wt% and aluminium plus copper contents < 40wt% in total. Outside this range expansions would be insufficient to be useful.

The time or temperature of ageing can be adjusted to give any desired expansion up to the maximum possible for any selected alloy. Typically, expansions similar in magnitude to the 'pattern-makers shrinkage' for common alloys based on aluminium, zinc, magnesium, copper, iron or steel can be induced by ageing.

If an expansion exceeding that required is produced, the process can be reversed by a heattreatment at a temperature in excess of 270"C and typically 300"C for the period necessary to achieve the required dimensions, followed by air cooling.

Thus by utilising such zinc-based alloys containing aluminium and/or copper for the material of the initial die set, compensation for shrinkage on casting can be effected by ageing the said die set before forming the said complementary pattern of bonded refractory sand. Thus the method can be employed to produce a subsequent cast component where the intial pattern used to form the initial die set can be identical to, or near to the size of the subsequent cast component. Any contractions inherent in the casting process can be compensated for by expanding the initial die set made in the zinc-based alloy containing aluminium and/or copper by the appropriate amount by ageing. In addition, dimensions of subsequent cast components can easily be adjusted by altering the size of the initial die set, avoiding expensive alterations to initial patterns.

Another application involves the production of a metal core-box from an existing core-box, where the dimensions of the said replicated core-box can be made identical to those of the existing core-box.

In order that the invention shall be clearly understood, one form of procedure for the production of a cast component is as follows, with reference to the accompanying diagrams in which Figures 1,2,3 and 4 are cross-sectional views of the stages of manufacture of a cast metal die part.

Referring to Figure 1, a pattern or existing component of aluminium alloy, steel, brass, cast-iron or similar relatively high melting point material 1 is bedded to a predetermined joint face line 2 in a plastic refractory medium, such as a suitable foundry sand in a conventional moulding box. The pattern or existing component and joint face are coated with a suitable parting agent and a second moulding box 3 placed on top of that containing the pattern.

Foundry sand or other suitable plastic refractory material is rammed into the box to replicate the shape of the pattern and the joint face 4. The top moulding box is removed, inverted, and the refractory removed to a preselected depth over the replicated surface of the pattern to determine the external shape and dimensions of the cavity 5. A running system is also formed in the refractory so that when the box is replaced in its former position over the box containing the pattern, metal may be cast into the said cavity over the pattern.

Zinc-based alloy containing aluminium and/or copper is melted according to normal foundry practice and heated to a temperature approximately 50"C above the liquidus tenperature of the alloy and is cast into the cavity to form a master die half.

Immediately before casting, the pattern in the lower box is preheated to a temperature of about 300 C to mimimise the temperature difference between the cast master die and the pattern.

Referring to Figure 2, the master die half 1 is fettled and bedded into a plastics refractory medium such as a suitable foundry sand in a moulding box, and the pattern 2 is replaced in the cavity of the master die which it formed on casting. The upper surface of the pattern and surrounding master die are coated with a suitable release agent, and a second die half 3 produced in a similar way to the first using an identical zinc based alloy containing aluminium and/orcopper.

The two master die halves are fettled, cleaned and clamped firmly together with strong clamps, then heated at typically 175 C for 24 hours to allow the alloy to grow to the size necessary to compensate for normal pattern makers shrinkage.

When the required period of heat-treatment is complete master dies are removed from the heating medium and allowed to cool to room temperature in air.

Referring to Figure 3, from each die half 1 a ceramic replica 2 is made by conventional techniques, the material for such replica being typically resin-bonded zircon sand or other refractory material capable of closely reproducing the detail of the master die.

Referring to Figure 4, each replica 1 is mounted to a predetermined joint face line 2 in a plastic refractory medium such as foundry sand 3 in a conventional moulding box 4. Another box 5 containing a cavity shaped to the approximate externai dimensions of the required final die half, such cavity and running system being formed by conventional foundarytechniques in a suitable ceramic material, is placed over the first box 4.

The material of the final die half, in a fluid state, is poured via the running system into the cavity between the ceramic replica and the refractory material in the upper box. On solidification, this forms a final die half 6. The process is repeated using the ceramic replica of the second master die to produce a second matching half, such that when put together with the first final die half the assembly may be used as a mould for gravity or pressure die-casting, squeeze-casting, or other techniques. If the castings produced are slightly under- or oversized, an appropriate further heat-treatment of the master die is carried out to rectify the defect, and the process is repeated from the master die heattreatment stage.

If the required expansion of the master die cannot be achieved by the process as described, the method may be modified by casting zinc based alloy containing aluminium and/or copper against the ceramic replica instead of the material ofthefinal die half.

The intermediate die so produced may be heattreated to produce the required expansion and used in place of the master die in the process as described to produce a final die of the dimensions required.

Where the pattern orcomponentwhich isto be replicated is of a material which has a low melting point or is thermally degraded by contact with molten zinc based alloy the process is modified as follows: The pattern or existing component of say wood, epoxy-resin, zinc-based alloy etc. is bedded to a predetermined joint face line which could be formed by a flat solid surface or in a plastic refractory medium, such as a suitable sand. A frame is placed around the said pattern to form a mould which is then filled with plaster in a fluid state, or fine resin-bonded zircon sand which when set will form an accurate negative replica of the original pattern or component. The accurate negative replica is used as a pattern to produce an accurate positive replica by repeating this replicating process. The plaster or resin-bonded zircon sand or other material used to produce this second, positive replica must be capable of acting as a refractory moulding material for zinc-based alloy containing aluminium and/or copper. Such refractory positive replica is substituted for the pattern or component in the process for production of master die and final die halves as described in the form of procedure above.

Claims (13)

1. A method of producing a cast metal die including the steps of casting zinc based alloys around a pattern, removing the casting so formed from the pattern and ageing the casting to expand it.

2. A method as claimed in claim 1, in which the zinc based alloy has a zinc content of greater than 60% by weight.

3. A method as claimed in claim 1 or 2, in which the zinc based alloy has an aluminium and copper content of less than 40% by weight.

4. A method as claimed in claim 1,2, or 3, in which the zinc based alloy contains approximately 15% by weight of aluminium and 15% by weight of copper.

5. A method as claimed in any of claims 1 to 4, in which the zinc based alloy contains approximately 3% by weight of copper.

6. A method as claimed in any preceding claim, in which ageing is carried out at up to 276"C.

7. A method as claimed in any preceding claim in which ageing is carried out at approximately 150"C for approximately 400 hours.

8. A method as claimed in any of claims 1 to 6, in which ageing is carried out at approximately 175"C for approximately 100 hours.

9. A method as claimed in any of claims 1 to 6, in which ageing is carried out at approximately 200"C for approximately 24 hours.

10. A method of casting as claimed in any preceding claim, in which the zinc based alloy is melted and heated to a temperature of 50"C above the liquidustemperature before being cast.

11. A method of casting as claimed in any preceding claim, in which the pattern is preheated to a temperature of approximately 300"C to minimise the temperature difference between the cast metal and the pattern.

12. A method of producing a cast metal die substantially as herein before described with reference to the accompanying drawings.

13. A cast metal die when made bythe method claimed in any preceding claim.