GB2321461A

GB2321461A - A pattern forming thermoplastic composition

Info

Publication number: GB2321461A
Application number: GB9626528A
Authority: GB
Inventors: William Neill; Michael Stradling
Original assignee: CAMPBELL DUSSEK Ltd
Current assignee: CAMPBELL DUSSEK Ltd
Priority date: 1996-12-20
Filing date: 1996-12-20
Publication date: 1998-07-29
Also published as: FR2757527A1; FR2757527B1; JPH10277700A; DE19754051A1; GB9626528D0

Abstract

A pattern forming thermoplastic composition comprising, as a filler, beta-cyclodextrin. The composition may comprise ethylene-vinyl acetate copolymer, tosin ester and waxes.

Description

1 A Pattern Forming Thermoplastic Composition 2321461 This invention

relates to a pattern forming thermoplastic composition and use of the composition in an investment casting process.

Investment casting processes, which are also known as'lost-wax' processes, have been known for many years. In an investment casting process an investment casting is usually formed by heating and melting a pattern forming thermoplastic composition, introducing the mobile composition into a mold and then cooling the material until it solidifies to form a positive pattern. The positive pattern is then removed from the mold and encased in a mold-forming material, usually a ceramic material, and the mold-forming material is caused or allowed to form a shell or cast around the thermoplastic positive pattern. A major portion of the thermoplastic pattern is then removed from the shell by melting the composition at a moderately elevated temperature. Any thermoplastic material remaining in the shell after this step is removed by a substantially higher temperature which causes vaporization, burning or both, so that, except for any ash residue, the inner surface of the shell is clean. The shell can then be used for forming a final casting. This process allows the surface characteristics of the positive pattern to be transferred to the final casting via the ceramic shell.

The quality of a final casting depends on the quality of the positive pattern, which in turn depends on the quality of the pattern forming thermoplastic composition from which the positive pattern is molded.

In an investment casting process the quality of the final casting is affected if the pattern forming thermoplastic composition shrinks when it solidifies. The amount of shrinkage can be reduced by incorporating filler materials such as crosslinked polystyrene or Bisphenol A into the pattern forming thermoplastic composition. Unfortunately, some filler materials are considered to be environmentally unfriendly.

1 2 Usually the filler makes up to about 40% by volume of the thermoplastic pattern-forming composition. The filler usually has a particle size of from 175 to 250 mesh. Thus, for a typical filler, at least about 90%, preferably 100%, by weight of the particles will pass through a 100 mesh sieve, and at least about 50% by weight of the particles will pass through a 200 mesh sieve.

US-A-5,270,360 discloses the use of crosslinked poly(methyl methacrylate) as a filler for a pattern forming thermoplastic composition.

US-A-5,518,537 discloses the use of thermoplastic resin powders of cellulose acetate esters as a filler for a pattern forming thermoplastic composition.

The aim of the present invention is to provide an alternative filler for a pattern forming thermoplastic composition. It is also the aim of the present invention to provide an environmentally friendly filler for a pattern forming thermoplastic composition.

In accordance with the present invention there is provided a pattern forming thermoplastic composition comprising, as a filler, beta cyclodextrin.

In accordance with the present invention there is also provided use of beta cyclo-dextrin as a filler in a pattern forming thermoplastic composition.

In accordance with the present invention there is also provided an investment casting process comprising the steps of: a) preparing a positive pattern from a pattern forming thermoplastic composition; b) encasing the positive pattern in a mold forming material such as a ceramic, and allowing or causing the mold forming material to form a shell or cast around the positive pattern; and c) removing the positive pattern from the shell or cast; characterised in that the pattern forming thermoplastic composition comprises beta cyclo-dextrin as a filler.

3 Beta cyclo-dextrin is currently used for encapsulating pharmaceuticals. It is manufactured from maize and it is a renewable resource. It has no hazardous thermal decomposition products so it is environmentally friendly.

The inventor has found that beta cyclo-dextrin exhibits all the required characteristics of a good filler for a pattern forming thermoplastic composition: it leaves a low ash residue when it is burnt; it burns cleanly with little soot; and it is inert. Beta cyclo-dextrin is thermally stable to 300 OC.

The pattern forming thermoplastic composition preferably comprises from 5% to 70% by weight, even more preferably from 20% to about 60% by weight, of particulate beta cyclo-dextrin that is uniformly distributed throughout the thermoplastic material. Other filler materials may be added to or combined with the beta cyclo-dextrin.

The invention will now be described by reference to the following example:

EXAMPLE

The following pattern forming thermoplastic composition was prepared:

Fully refined paraffin wax, mpt 60 OC Microcrystalline wax, mpt 70 OC Microcrystalline wax, mpt 78 OG Fully refined paraffin wax, mpt 57 OC Synthetic oxidised microcrystalline wax, mpt 70OC Pentaerythritol rosin ester, mpt 103 T Ethylene vinyl acetate copolymerA Beta cyclo-dextrin ASTM D AI 27 ASTM E-28 10.6% 7.8% 1.7% 4.5% 10.1% 33.0% 2.3% 30.0% 4 A Melt index 150 g/10 min @ 190 OC, vinyl acetate incorporation 19% wlw.

Method of Preparation of a Pattern Forming Thermoplastic Composition The pattern forming thermoplastic composition was prepared by the following method:

The five wax components were melted together at 130 OC. The rosin ester was added to the melted wax and the mixture was stirred at 130 C until the rosin ester had completely dissolved. The ethylene vinyl acetate was then added to the mixture and the mixture was stirred at 130 OC until the ethylene vinyl acetate had completely dissolved. The mixture was allowed to cool to 105 'C, and then the beta cyclo-dextrin was added.

A disposable pattern was prepared from the pattern forming thermoplastic composition. The disposable pattern had equivalent linear and volume contraction when compared to a disposable pattern prepared from a comparable pattern forming thermoplastic composition comprising crosslinked polystyrene as the filler. The disposable pattern prepared above was also found to have a superior surface finish.

Claims

1. A pattern forming thermoplastic composition comprising, as a filler, beta cyclo-dextrin.

2. Use of beta cyclodextrin as a filler in a pattern forming thermoplastic composition.

3. An investment casting process comprising the steps of: a) preparing a positive pattern from a pattern forming thermoplastic composition; b) encasing the positive pattern in a mold forming material such as a ceramic, and allowing or causing the mold forming material to form a shell or cast around the positive pattern; and c) removing the positive pattern from the shell or cast; characterised in that the pattern forming thermoplastic composition comprises beta cyclo-dextrin as a filler.

4. The composition, use or process claimed in any one of the preceding claims, wherein the beta cyclo-dextrin comprises discrete particles.

5. The composition, use or process claimed in claim 5, wherein the beta cyclo-dextrin comprises from 5 to 70%, preferably from 20 to 60%, by weight of discrete particles.

6. The composition, use or process claimed in claims 5 or 6, wherein at least 90%, preferably 100%, by weight of the particles are capable of passing through a 100 mesh, and at least 50% by weight of the particles are capable of passing through a 200 mesh.