Preparing a lost foam pattern.
Lost foam patterns can be produced by several means. In volume production, multiple copies of patterns are prepared by moulding. It is also possible to machine foam patterns on a high speed CNC router. Both of these methods involve considerable capital investment, so most hobby casters fabricate their patterns by gluing together simple shapes to build up a complex pattern. This is cheap but time consuming though much less so than preparing a wooden pattern for green sand casting.
On this page the steps to produce a pattern for a simple spoked handwheel will be outlined.
On this page the steps to produce a pattern for a simple spoked handwheel will be outlined.
The first step is to cut the outer ring of the handwheel. The hot wire cutter is set up with the circle cutting attachment, as shown here, and a circular slab is cut.
This shows a close up of the cutting taking place. It is important to set the wire temperature correctly. If the wire is to hot then the cut wire be too wide and any hesitation in turning the blank will cause defects (sinks) in the cut edge. The blank must be turned at a consistent speed throughout the cut.
Once the circular slab is cut it is advanced into the wire to the required inner radius. To do this the wire temperature should be set so that it will only just cut the material. Advance the ring into the wire in small stages and keep checking the radius. Once the inner radius is reached clamp the circle cutter and increase the temperature of the wire a little and then cut the inner circle by slowly rotating the workpiece.
When the cut is complete turn off the wire immediately and lift the completed ring off of the hot wire cutter.
Now advance the circle cutter into the wire to the radius of the inner hub of the hand wheel and cut another circle.
When the cut is complete turn off the wire immediately and lift the completed ring off of the hot wire cutter.
Now advance the circle cutter into the wire to the radius of the inner hub of the hand wheel and cut another circle.
This photo shows the outer ring, which has been reduced in thickness and the central hub. The outer ring thickness was reduced using the fence attachment. Notice that the ring is split where the circle cutter was advanced from the outer radius to the inner radius of the ring.
The outside of the ring is wrapped in masking tape. This serves two purposes. Firstly it gives a better surface finish to the final casting and secondly it closes and secures the split in the ring. Four small spokes are then cut using the fence and carefully shaped with a file to fit between the hub and the ring, as shown here. The spokes are then removed and the ends coated with PVA adhesive (white woodworking adhesive) and the assembly left to set. Small gaps and blemishes can be filled with PVA adhesive.
Once the assembly has set then sprues are attached using PVA adhesive to the hub and the ring as shown in the header photo. The assembly is then ready for casting.
Complex shapes can be built up from cylinders, slabs, strips etc. that are glued together. Spheres are difficult to make but there are a good range of spheres available from craft shops and on e-bay. Fillets, where one part meets another can be formed using wax furniture polish if required.
Polystyrene foam is available in different types. The type shown here is white foam that is made up from expanded beads of polystyrene. This is the type commonly used for packaging. It is suitable for most purposes but it does leave a surface texture on the finished casting. Large panels are available from builders merchants in various thicknesses for use as insulation. There is another type of foam with the trade name Styrofoam. This is extruded polystyrene foam. It is usually a pale blue colour. This is denser and more homogenous than the normal white packaging foam and it gives a much finer surface finish to the finished casting. It is mainly used as an insulation material. It is more difficult to find but there is a UK supplier on e-bay that sells it in thicknesses of up to 100 mm.
Complex shapes can be built up from cylinders, slabs, strips etc. that are glued together. Spheres are difficult to make but there are a good range of spheres available from craft shops and on e-bay. Fillets, where one part meets another can be formed using wax furniture polish if required.
Polystyrene foam is available in different types. The type shown here is white foam that is made up from expanded beads of polystyrene. This is the type commonly used for packaging. It is suitable for most purposes but it does leave a surface texture on the finished casting. Large panels are available from builders merchants in various thicknesses for use as insulation. There is another type of foam with the trade name Styrofoam. This is extruded polystyrene foam. It is usually a pale blue colour. This is denser and more homogenous than the normal white packaging foam and it gives a much finer surface finish to the finished casting. It is mainly used as an insulation material. It is more difficult to find but there is a UK supplier on e-bay that sells it in thicknesses of up to 100 mm.