A plaster formulation which can be cast into blocks and machined (CNC) to manufacture a mould which can be dried and alloys cast into it directly
A plaster formulation has been developed which can be cast into blocks and machined (CNC) to manufacture a mould which can be dried and alloys cast into it directly (taking out 2 steps in the existing machining process). The production of cast parts usually requires the manufacture of a pattern from which a mould impression is taken. More recent developments have progressed this process to using CAD/CAM techniques to directly machine a mould ready for casting, eliminating the need to manufacture a pattern. This has provided benefits in both time and cost where prototypes or small numbers of castings are required. It provides a fast route to market and offers advantages of increased accuracy of both the mould and finished casting. The application of this process to plaster has been restricted by the problems associated with the current casting plaster relating to the machining characteristics and also the safety limits applied to silica dust. A new plaster formulation has been developed to overcome these problems and is now available for use providing a process for direct mould manufacture from CAD data.
Three formulations of plaster based moulding powder have been developed. One is a lightweight foamed product with high permeability, the second is a non-foamed product for higher strength and the third is a product for high melting-point alloys. - A foamed plaster formulation has been developed which can be used for the casting of low melting point alloys. The moulds are light in weight, which enables to cast bigger pieces and have a high permeability to allow trapped gasses to escape more easily. The setting time of the plaster can be tailored to meet the requirements of the application. Chills can be added to control the freezing of the metal. Compared to the existing plaster formulations we achieved a reduction of drying time of >50%, which is a huge benefit to the end-users because of energy savings and has a positive effect on the environment. - A second formulation has been developed which is a non-foamed plaster and this is used for smaller, more detailed moulds, which need to be stronger, particularly when being removed from the pattern. The product is designed for the casting of light alloys. - The third developed formulation is designed to make moulds which can be used to cast high melting point alloys such as brass or bronze. In this formulation, plaster is used as a binder for refractory materials so that a mould with a high level of detail and smooth surface can be produced but the amount of plaster used is reduced so that the time taken to remove the chemically combined water is minimised and there is less plaster to decompose at high temperatures. - The fourth formulation is developed to mill a plaster block on a CNC machine, directly from the digital design. This leaves out a few steps compared to the traditional way and leads to shorter lead times and the possibility to show customers of the metal casting companies in due time a prototype. - All the formulations are designed to have a high level of fluidity so that a high level of detail and surface finish can be achieved.
The use of convective ovens with the addition of radio frequency heating or microwave heating has been proven to reduce the time for drying plaster moulds by 50% The problems in using plaster as a moulding medium have been hindered by the long drying times, which have to be applied to the mould used for metal casting. The ability to shorten the drying cycle has the possibility of increasing the turnover of foundry partners, increasing the time to market and opening up potential new markets. The current status of the technology is at the initial demonstration stage where the process has been shown to be feasible. Developments under industrial environments need to be carried out.
The method for mould manufacture has been optimised. Plaster has been cast against wood, silicone rubber and resin patterns. The most successful is silicone rubber as it has proved difficult to remove plaster moulds from rigid patterns without damage. The different methods of mould manufacture which can be used in the production of plaster moulds for metal casting has been evaluated. Specific needs are addressed to give an optimised process, which will provide a good quality mould ready for metal casting. The current status is at the industrial trial stage after which more robust procedures will be developed. (Maus machines blocks, which will be cast this summer at CTI, wait for results to put more info in here)