TechArchive

Original article date: May 1999

Alan Quinn explains how MCP Equipment’s advanced computerised vacuum casting machines can save time and money alongside rapid prototyping facilities.

Those who use rapid prototyping techniques such as stereolithography face a cost dilemma when they need to produce the original prototype in numbers for technical evaluation and market testing.

It seldom makes economic sense to use the rapid prototyping system itself for replication. Most are far too expensive to run for anything other than a master prototype. Some systems also produce prototypes that are too brittle to withstand practical trials.

One way around the problem is to operate a vacuum casting machine alongside the rapid prototyper. Costing about a tenth of the CAD system the vac-caster employs a mould made from the original to cast several facsimiles faithfully replicating the details in a strong resin with a finish as good as that of injection moulding.

MCP Equipment supplies vacuum casting systems for shots up to 14 0 The company says that current generation of machines are much more suitable for this prototyping role than were their predecessors. One serious drawback with earlier system was inconsistent control of resin mixing processes and mixing times. This resulted in unacceptable reject rates.

The new systems overcome such problems by automatic sequencing and monitoring of all operations under microprocessor control. Apart from supervision there is no significant manual element in the process. They are also self-contained and fully integrated as systems . Meaning that the casting resins and mouldmaking materials are formulated for compatibility. In MCP’s view this contributes significantly to the quality and economy of the finished casting.

There are also important advantages in the use of silicone rubber as the mouldmaking medium. The material is inexpensive and reproduces detail faithfully. Moulds can also be made very quickly much faster than hand tooling for an injection moulding.

How vacuum casting works

A vac-caster of medium capacity will typically have two chambers. The two-part polyurethane resin is mixed in the upper chamber and the mix is cast into the mould in the lower.

A split two-part mould is made by casting silicon rubber around the master prototype in stages. This is then placed in the casting chamber which can be provided with an electric jack for fine-tuning the mould position.

A robotic unit controls the mixing of the resin and hardener and the pouring of the mix into the mould.

Both operations are carried out with both chambers evacuated. This ensures that all air – the usual cause of imperfections – is extracted during mixing. Evacuation also draws the mix into every part of the mould during casting no matter how intricate the mould may be.

MCP says that with this technique there is nothing to prevent the casting being an exact facsimile of the master clear of both internal and external defects and with a finish identical to that of the mould.

The vacuum in the machine is continuously maintained at a pre-set value by a semiconductor sensor. A digital display of the operational parameters is updated by a PLC throughout the process.

The casting will attain initial hardness in the casting chamber. Some applications require rather more than this and employ a hot-air drying chamber to speed hardening and improve molecular bonding and heat resistance.

The two-part mould is then split and the casting removed. Several castings can be produced from the same mould before wear and tear necessitates a new set. One telephone R&D centre has used the same mould for over 60 complex castings.

• MCP Equipment
• Tel: 01785 815651

May 1999