EP0499420A1

EP0499420A1 - Mould forming

Info

Publication number: EP0499420A1
Application number: EP92301055A
Authority: EP
Inventors: Robert Norman c/o British Ceramic Res.Ltd Deakin; Ian Stanley c/o British Ceramic Res.Ltd Wallbank
Original assignee: British Ceramic Research Association Ltd; British Ceramic Research Ltd
Current assignee: British Ceramic Research Association Ltd; British Ceramic Research Ltd
Priority date: 1991-02-12
Filing date: 1992-02-07
Publication date: 1992-08-19
Also published as: GB9102954D0

Abstract

A method of forming a case mould usable to produce working moulds in the production of ceramic articles. The method comprising heating a piece of thermoplastics material sheet to a softening point, placing said piece on a suitably shaped former and forming to a required shape by thermo-forming. The former is preferably a block mould and the piece is desirably shaped by differential pressure thermo-forming.

Description

This invention concerns improvements in or relating to a method of making moulds, and particularly but not exclusively moulds for use in the ceramic industry, and especially 'case moulds' .
Most of the shaping methods employed in the ceramic tableware and sanitaryware manufacturing industries depend heavily on plaster of Paris working moulds. Because of their short lives these are required in large numbers. The traditional method for producing working moulds against which clay articles are formed is as follows.
Firstly a "block" mould is produced in one or more parts by casting plaster of Paris against a model of the required shape. A "case" mould is then produced by casting either hard plaster, silicone rubber or epoxy resin against each of the block mould parts. Resin and silicone case moulds are generally more durable. They are, however more expensive to produce. (It is usual practice also to produce master block and case moulds at this stage). "Working" moulds are in turn cast against the case moulds and it is from these working moulds that the required shapes are produced by slip casting or plastic making. The working moulds are usually fabricated from plaster of Paris although some porous plastic alternatives have been developed for roller making and casting at elevated pressure.
A number of disadvantages are encountered with this method and particularly when resin and silicone case moulds are used, for example:

high labour input.
expensive raw materials.
hazardous materials requiring personal protection equipment.
time-consuming finishing operations.
the need for release agents which may affect the properties of the working moulds.
multi-component systems.

According to the present invention there is provided a method of forming a case mould of the type hereinbefore described, the method comprising heating a piece of thermo plastics material sheet to a softening point, placing said piece on a suitably shaped former and forming said piece to a required shape by thermo-forming.
The piece is preferably shaped by differential pressure thermo-forming, and desirably by vacuum forming.
The former is preferably a block mould of the type hereinbefore described.
The former may approximate the required case mould profile. In this instance the thermo-formed sheet may subsequently be machined to give an acceptable working profile.
The thermoplastics material may be any of those conventionally used in thermo-forming methods, including acrylic plastic, ABS, polystyrene, PVC, polyolefins e.g. polypropylene, high density polyethylene.
A backing material may be attached to an innerside of the shaped piece. The backing material may comprise any of a polymer concrete, hydraulic concrete or plaster of Paris. The polymer concrete may be made from sand or a light weight filler such as Fillite, bonded with an epoxy or polyester based resin. Glass fibre filler may be added to the polymer to further increase strength and toughness.
The thermoplastics material is preferably heated to its Vicat softening point.
The present invention also provides a method of making a mould for producing ceramic articles, the method comprising forming a case mould by a method according to any of the preceding seven paragraphs, and casting a curable material against said case mould to produce a working mould against which a clay article may be cast.
The block mould is preferably formed by casting a curable material against a slightly over sized model of the required shape,
The curable material is desirably plaster of Paris or a porous plastic body.
An embodiment of the present invention will now be described by way of example only;
A double cup handle shape case mould was formed using a plaster of Paris block mould with dimensions of approximately 160 x 100 mm. The permeability of the plaster was measured as 3.8 x 10⁻³ cm²/s per cm of water pressure. A 2 mm thick sheet of polypropylene was heated to between 140 and 160°C to its Vicat softening point, placed on the block mould, and formed into the required shape by producing a vacuum of 40-60 mbar on the opposite side of the block mould to the plastic sheet. The entire operation was completed in under 12 minutes, with 7 minutes heating time, 2 minutes forming time, and 2-3 minutes cooling time. The reproduction of surface features was at least as good as usually achieved using traditional techniques. The conventional plaster block mould used as the forming tool allows the air to escape from between the thermoplastics sheet and the tool during forming.
A backing material is subsequently applied to the shaped plastic sheet. The backing material comprises a polymer concrete of polyester based resin with a light weight filler such as Fillite and up to 2 percent chopped glass fibre as an additional filler to provide strength and toughness.
This method of case mould manufacture has a number of advantages over the traditional method. For example there is a reduced labour requirement and no extra tooling is necessary. The process time involved is greatly reduced, as opposed to the time for the traditional method which in some cases can be 5-7 days. Cheaper raw materials are generally used. This simplified case mould production method allows new shapes to be developed more readily. There is a reduction in the toxic hazard present when silicone rubber or epoxy resin are used. Natural release of working moulds from the case mould will generally be obtained alleviating the need for release agents. There is no requirement for any finishing operations on the case moulds once formed. The case moulds formed in this manner may be used to make either plaster of Paris or synthetic plastic working moulds.
Various modifications may be made without departing from the scope of the invention. For example different thermoplastics materials may be used such as acrylic plastic, ABS, polystyrene, PVC, or polyolefins e.g. high density polyethylene. A different backing material could be provided which may comprise a polyester based resin and sand could be used as a filler. Alternatively hydraulic concrete or plaster of Paris could be used as a backing material. In certain cases a backing layer may not be required.
An an alternative to vacuum forming, increased pressure could be provided on the inner side of the plastic sheet to form same to the required shape. Additionally, final detail may be imparted to the case mould or the working mould produced by the method described above, by a machining operation.
Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to whether or not particular emphasis has been placed thereon.

Claims

A method of forming a case mould of the type hereinbefore described, characterised in that the method comprises heating a piece of thermo plastics material sheet to a softening point, placing said piece on a suitably shaped former and forming said piece to a required shape by thermo-forming.
A method according to claim 1, characterised in that the piece is shaped by differential pressure thermo-forming.
A method according to claim 2, characterised in that the piece is shaped by vacuum forming.
A method according to any of the preceding claims, characterised in that the former is a block mould of the type hereinbefore described.
A method according to any of the preceding claims, characterised in that the former approximates the required case mould profile.
A method according to claim 5, characterised in that the thermo-formed sheet is subsequently machined to give an acceptable working profile.
A method according to any of the preceding claims, characterised in that the thermoplastics material is any of acrylic plastic, ABS, polystyrene, PVC, a Polyolefin.
A method according to any of the preceding claims, characterised in that a backing material is attached to an innerside of the shaped piece.
A method according to claim 8, characterised in that the backing material comprises hydraulic concrete or plaster of paris.
A method according to claim 8, characterised in that the backing material comprises a polymer concrete.
A method according to claim 10, characterised in that the polymer concrete is made from sand or a light weight filler, bonded with an epoxy or polyester based resin.
A method according to claims 10 or 11, characterised in that glass fibre filler is added to the polymer to further increase strength and toughness.
A method according to any of the preceding claims, characterised in that the thermoplastics material is heated to its Vicat softening point.
A method of making a mould for producing ceramic articles, characterised in that the method comprises forming a case mould by a method according to any of the preceding claims and casting a curable material against said case mould to produce a working mould against which a clay article may be cast.
A method according to claim 14, characterised in that the former is a block mould which is formed by casting a curable material against a slightly over sized model of the required shape.
A method according to claims 14 or 15, characterised in that the curable material is plaster of paris or a porous plastic body.