GB2198384A

GB2198384A - Injection mould

Info

Publication number: GB2198384A
Application number: GB08622443A
Authority: GB
Inventors: Edward Smethurst
Original assignee: Individual
Current assignee: Individual
Priority date: 1986-09-18
Filing date: 1986-09-18
Publication date: 1988-06-15
Anticipated expiration: 2006-09-18
Also published as: GB2198384B; GB8622443D0

Abstract

A method of injection moulding in which two or more mould parts (10) are assembled to form a moulding cavity (11) comprises injecting moulding material into the cavity through at least one submarine gate (12) formed in a part of the mould made of a two-phase quaternary alloy having superplastic properties and having a fine grain micro structure stable at the temperature at which superplastic deformation can take place. <IMAGE>

Description

METHOD OF INJECTION MOULDING This invention relates to a method of injection moulding, and more particularly to a method of injection moulding which utilises submarine gating as herein defined.

By "submarine gating" there is meant a gating technique in which material to be moulded (for example a synthetic plastics or rubber material) is injected into a cavity between two or more mould parts through a gate which is not located at any join between the mould parts, but at a position spaced therefrom.

Previously, moulds for injection moulding methods which utilise submarine gating have been made from special die steel. This has been necessary because when the mould parts are separated, shearing of the sprue formed in the gate, (especially where the injected material is a hard plastics material) exerts a substantial force on the mould in the region of the gate. Obviously, after repeated moulding operations, this would result in damage to the mould if the mould were not sufficiently hard.

Until the present invention, it has not been found possible to manufacture these moulds of other materials, for example light alloys, because of this problem.

According to the invention, a method of injection moulding includes the steps of assembling two or more mould parts so that a moulding cavity is formed therebetween, injecting material to be moulded into the cavity through at least one submarine gate, said method being characterised in that at least that part of the mould in which the submarine gate is provided is made of a superplastic two-phase quaternary alloy having a fine grain microstructure stable at the temperature of superplastic deformation.

One alloy providing the desired characteristics consists of zinc within the range 70%-82% by weight and aluminium 30t-18 by weight to which is added magnesium in proportions up to 0.6% by weight and up to 2E by weight of one of the elements, copper, nickel, silver and calcium.

It has been found that a mould made of said alloy does not become damaged after repeated moulding operations.

The mould part in which the submarine gate is provided is itself much easier to produce than moulds of special steel and provides significant advantages over these, i.e. the mould part is less susceptible to moisture attack and other corrosion and is more easily machined than steel, whilst being sufficiently hard to withstand the forces imposed in repeated injection moulding operations.

Other suitable alloys from which the said part of the mould may be made and their preparation are described in detail in patent specification GB 1 259 782 and patent no. US 3676115.

The alloy may be wrought or cast if required, to enhance the fineness of the grain microstructure.

Preferably, all parts of the mould are made frzm said alloy.

The material to be moulded is preferably material in the lower melting range, i.e. 14000 to 35000.

The invention will now be described with the aid of the accompanying drawings in which: Figure 1 is a side view of a mould part for use in a method according to the invention; and Figure 2 is an end view of the mould part of Figure 1.

Referring to the drawings, a mould part 10 is made of a wrought superplastic two-phase quaternary zinc-aluminium based binary alloy comprising 7890 zinc, 22E aluminium, to which was added 0.5% magnesium and 0.5% copper to form the quatenary alloy. The alloy was pretreated for two hours at 3600C and isothermally for two hours at 2600C.

Alternatively, the mould part could be made of any wrought superplastic two-phase quaternary alloy having a fine grain microstructure stable at the temperature of superplastic deformation, wherein there is between 70% and 82% zinc by weight, and between 30 and 1894 aluminium by weight, and up to 0.6 magnesium and up to 2z of either copper, nickel, silver or calcium, together with a grain refining agent.

The mould part 10 has therein a moulding cavity 11 into which moulding material is arranged to be injected.

The moulding cavity 11 is submarine gated, i.e. gates 12, two of which are shown in the drawing, extend from a central recess 13 to which the moulding material is fed, into the cavity 11 at positions spaced from the surface 14 of the mould part which, in use, joins or engages another mould part, with the cavity 11 formed between the two mould parts.

The recess 13 extends from the mould to a moulding material feed means (not shown) as is well known in the art.

To perform an injection moulding operation, a second mould part is placed in contact with surface 14, so that the top 15 of cavity 11 is closed by the second mould part.

Moulding material comprising, in the present case polypropylene, or other synthetic plastics material is then injected into the cavity 11 from recess 13, through the submarine gates 12 until the cavity 11 is filled with moulding material.

The moulding material is permitted to set and then the mould parts are separated and the moulded workpiece is removed from the cavity 11, preferably by ejection although no ejection means are shown in the Figures.

It will be appreciated that as the moulded workpiece is thus removed from the cavity 11, sprues formed in the submarine gates 12 will be sheared from the workpiece at the connections 16 between the submarine gates 12 and the cavity 11.

Thus the resultant workpiece will have no sprue which would need subsequently to be removed, as in edge moulding methods which do not use submarine gating.

Separately the material which would have solidified in the gate 12 and cavity 11, is also ejected to clear the mould. This can be achieved because the gates 12 incline downwardly from cavity 11. The thus removed material can be recycled.

The mould parts are then again re-assembled and further moulding material is injected into the cavity 11 from the recess 13.

It has been found that after 5000 repeated injection moulding operations, there was no appreciable damage to the mould resulting from shearing of the sprues as the workpieces were removed. The indications are that 250,000 repeated operations will not damage the mould sufficiently to render it unserviceable.

Claims

1. A method of injection moulding includes the steps of assembling two or more mould parts so that a moulding cavity is formed therebetween, injecting material to be moulded into the cavity through at least one submarine gate, said method being characterized in that at least that part of the mould in which the submarine gate is provided is made of a two-phase quaternary alloy having superplastic properties and having a fine grain microstructure stable at the temperature at which superplastic deformation can take place.

2. A method of injection moulding according to claim 1 characterized in that the alloy comprises zinc within the range 70%-82% by weight and aluminium 30%-18% by weight to which is added magnesium in proportions up to 0.6% by weight and up to 2% by weight of an element chosen from the group of elements comprising copper, nickel, silver and calcium.

3. A method of injection moulding according to claim 1 characterized in that all parts of the said mould are made from/two phase quaternary alloy having superplastic properties and having a fine grain microstructure stable at the temperature at which superplastic deformation can take place.

4. A method of injection moulding according to claim 2 characterized in that the alloy is pretreated for two hours at 3600C and isothermally for two hours at 2600C.

5. A mould for use in performing the method according to claim 1 comprises two or more mould parts at least one of which is made from a two-phase quaternary alloy having superplastic properties and a fine grain microstructure stable at the temperature at which superplastic deformation can take place and is formed with at least one submarine gate.