WO1991002607A1

WO1991002607A1 - Mould release agent utilising vulcanisable silicone rubber

Info

Publication number: WO1991002607A1
Application number: PCT/AU1990/000358
Authority: WO
Inventors: John Ridnell
Original assignee: Fentak Pty Ltd
Current assignee: Fentak Pty Ltd
Priority date: 1989-08-18
Filing date: 1990-08-17
Publication date: 1991-03-07
Anticipated expiration: 1992-02-18

Abstract

The casting of metal articles in sand moulds is effected by breaking the moulded sand material away from the article after casting and cooling. The sand moulds are generally manufactured by mixing the sand with an argillaceous or resinous binder, shaping the mould material around a pattern of wood, plastic or metal to form the mould and then separating the pattern from the mould. The bonded sand, however, tends to stick to the pattern, to the detriment of the pattern and the mould, resulting in about 20 % rejects, a problem which has been identified with a wide variety of known binders thus far. In order to overcome the problem, a release agent in the form of a dispersion or solution of room temperature vulcanisable silicone rubber in a volatile organic solvent is prepared for the purposes of applying it by brush, spray or other appropriate means as a coating to the pattern. Various solvents, such as trichlorethane, benzene, toluene, xylene, solvent naphtha, mineral spirits and mineral turpentine may be used, with a preference for toluene, based on a weight of 96/4 to 50/50 for toluene to silicone rubber. The solution or dispersion allows for a coating application of 0.01 to 1.00 mm thick of silicone rubber, firmly bonded to the pattern after a curing period of about 16 hours at 25 °C and a substantial reduction in the rejection rate. Maintenance of the cured coating is relatively simple and requires no more than touching up at wear spots, when and where they occur.

Description

MOULD RELEASE AGENT UTILISING VULCANISABLE SILICONE RUBBER

The present invention relates to release agents and in particular to release agents used in conjunction with sand moulds. For many years the casting of metal articles has been undertaken by use of sand moulds which are broken away from the metal object once it has been cast and cooled sufficiently. The sand moulds are fabricated by mixing the sand with a binder (usually resin)., typically of clay or resinous material, and then forming the binder and sand mix around a "pattern" having the shape of the article which it is eventually desired to cast; the pattern being fabricated from say timber or other appropriate material. As the sand mould is destroyed after casting of each metal product it is important that sand moulds may be quickly and conveniently cast from the pattern without release problems. Unfortunately the bonded sand tends to stick to the pattern and in some extreme cases it has even been found necessary to destroy the pattern in order to obtain one satisfactory mould. Differing clays, resins and other binders have been used in conjunction with sand in order to achieve moulds of suitable strength but all known binders used for this purpose including phenolic polyurethane, furane resins, sodium silicate, clays and other binders suffer similar release difficulties.

It is therefore the object of the present invention to ameliorate the above mentioned difficulty with regard to release of moulds from patterns and at least to provide the industry with a choice.

According to the present invention there is provided a release agent comprising a dispersion or solution of room temperature vulcanisable silicone (RTV) rubber and a suitable organic solvent in proportions such that the rubber may be applied to the pattern as a relatively thin coat with the solvent evaporating relatively soon after application so as to permit curing.

The present invention additionally teaches a method of ensuring ready release of a pattern from a sand mould of the type above described by means of the steps of a) coating the pattern with a release agent as above described

b) allowing the organic solvent to evaporate and the rubber to cure

c) forming the bonded 3and mould around the pattern d) allowing the bonded sand mould to set and removing same from the pattern.

One embodiment of the present invention will now be described by way of example only.

It has been discovered that "Dow Corning 780" (TM) room temperature vulcanising silicone rubber which uses an Oxime as the vulcanising agent acts as an effective release agent which will adhere to timber, plastic and metal patterns so as to form a reasonably durable release agent whereas it will not adhere to the sand mould.

In order that the RTV silicone rubber may be simply coated on to the pattern with a brush spray or other suitable means in a relatively thin coat (so as not to effect substantially the shape of the mould) it is dispersed in dry toluene solvent in a ratio of 15/85 rubber to toluene by weight.

The dispersion process involves introduction of the rubber beneath the toluene surface and mixing in an air free closed container so as to exclude the presence of water. The resulting solution is not so viscous as to present application difficulties and may be readily packaged and transported in liquid form having a long shelf life. These proportions additionally facilitate ready evaporation of the toluene once exposed to air and satisfactory subsequent curing of the RTV silicone rubber due to contact with moisture in the air.

Toluene has been selected as one of the better solvents having regard to its price, toxicity and relative volatility as compared with other solvents which would otherwise perform equally well such as trichloroethane, benzene, xylene, solvent naptha, mineral spirits, mineral terpentine or other organic solvents that are non reactive with the RTV rubber or the mould surface and yet are sufficiently volatile to evaporate rapidly when applied to the mould surface.

At 25 degrees Celsius a cure time of 16 hours for the release agent once coated on the pattern to a thickness of approximately .1 mm has been observed. In addition to facilitating ready release of a bonded sand mould from a pattern a release agent in accordance with the present invention serves to protect the pattern from attack of binder resins which may degrade the pattern. The release agent additionally helps avoid damage which may result from removal of adhered binder. It has been found that with many patterns the release agent will adhere to the pattern during the course of fabrication of 10 or more moulds irrespective of the type of binder used to bond the sand. The release agent will eventually become mechanically eroded depending upon handling methods and the shape of the pattern although the pattern may be readily "touched up" with release agent in accordance with the present invention at the spots where it tends to wear. It should be appreciated that the relative proportions of solvent to rubber may be varied according to the thickness of rubber required as a release agent and the desired cure time. Alternate solvents may additionally be utilised according to the vulcanising agent utilised in the rubber.

It has been found that use of a release agent in accordance with the present invention has avoided the situation whereby in accordance with existing technology up to a 20% reject rate is inherent in production of sand moulds.

Claims

The Claims defining the invention are as follows.

1. A release agent comprising a dispersion or solution of room temperature vulcanisable silicone (RTV) rubber and a suitable organic solvent in proportions such that the rubber may be applied to the pattern as a relatively thin coat with the solvent evaporating relatively soon after application so as to permit curing.

2. A method of enhancing the release characteristics of a pattern from a bonded sand mould by means of the steps of a) coating the pattern with a release agent as above described

b) allowing the organic solvent to evaporate and the rubber to cure

c) forming the bonded sand mould around the pattern d) allowing the bonded sand mould to set and removing same from the pattern.

3. A method in accordance with claim 2 hereof wherein the pattern is coated with room temperature vulcanisable silicone rubber such that the finished thickness of the coating is between .01 mm and 1 mm.

4. A release agent in accordance with claim 1 hereof wherein the organic solvent is toluene added to the room temperature vulcanisable rubber in the ratio of between 96/4 and 50/50 by weight respectively.