Art of fine bronze casting

When a pristine door handle is viewed it may be difficult to grasp just how much time and skill is devoted to cleaning and fine tuning the details of the casting before it is patinaed. This process takes hours and sometimes days and some of the steps are shown below.

In the lost wax method of casting wax replicas are “gated” and wax runners or rods are attached to the wax patterns at various points so that the molten bronze can flow through these connections and pass through the gates and  fill the cavity of the shelled piece. Once cast, the runners that connect the individual castings are cut and the bronze can be recycled but a small gate end is left, and this is removed using a cut-off wheel on a hand held grinder.

The casting may contain tiny silica fragments  that were left after the wax piece was shelled that need to be removed. A handheld metal burnisher with a flap wheel made of scotch brite and sand paper is used to remove shell fragments as well as scratches, blemishes and bubbles buried beneath the surface of the casting. When bronze is poured tiny pockets of air underneath the surface are  sometimes created and are only seen once the casting is buffed when they show themselves as tiny dimples or voids. To fill the void a silicon bronze rod is melted with a TIG welder and drops of melted rod fill these voids.

Below left a grinder with coarse grade cut-off wheel. Below right a handheld burnisher with a medium coarse wheel flap and a green very coarse wheel flap.

While the burnisher is an extremely effective tool it takes a fine dremel to get into the veins and undercuts of a casting. The image here shows the tip of the dremel burr as it removes residual white shell from the leaf vein. A  dremel is an invaluable tool for fine detail and a good one will come with several burrs, rasps and files. The dremel bits are made from tungsten carbide steel and fit into the dremel socket which when rotating at a high speed can cut and grind silicon bronze and even 316 stainless steel.

A fixed buffing machine with coarse and fine scotch brite disks is used to clean remaining marks or scratches. The casting is pushed into the path of the rotating disk which removes a small amount of the surface. A consistent and even pressure needs to be applied with the buffing disk to avoid unwanted grooves and directional marks.

Why research and development for custom hardware is expensive

The old and new schools for creating patterns and molds - part 2

Your design skill set and your preference for digital versus free hand designing may influence your choice when it comes to deciding how to create the pattern or file for casting.
Whichever road you take the research and development (R&D) costs will be significant. The R&D costs are often misunderstood, and many consumers have a hard time accepting that the pattern development can run into the thousands depending on the size and intricacy of the design. This cost is on top of the cost incurred in the design process (part 1), so perhaps we are stating the obvious when we state that custom, one of a kind door hardware, is inherently expensive.

Choice of medium for pattern making.

Regardless of whether the design is digital or old school and made with pencil and paper you should be able to use the same mediums when creating your pattern for use in investment casting or sand castings. If you have the budget and are developing a piece for high volume production, then die casting or 3D printing may be good options and may not require a pattern to be made. While die casting and 3D printing do not require a physical pattern, they depend on the design to be made with CAD/CAM software or comparable software that can capture the exact measurements and topography details of your design so that a virtual pattern can be created.


Mediums for Physical Patterns

The pattern for investment casting can be made from any medium that is both strong and non-permeable or that can be made strong by firing in a kiln. Your choice of medium will depend on your manual skills, can you carve or create a design in modelling clay? The choice will also depend on how dense the detail and undercuts are in your design, does the design have a lot of detail or is the design for a smooth piece with the detail confined to the edges? Our patterns are always made in a reductive way by carving or machining rather than by the additive clay modelling method.

Left cast trumpet flower Right hard blue wax pattern with gate in red wax

Mediums for carving or machining

Rigged foam –
High density rigid polyurethane foam comes in a variety of sheet sizes, thickness, and density. We typically use a medium 15lb per ft density. It can be machined on a bandsaw, turned on a lathe or used with cutting routers. It is ideal for moderately detailed patterns but it’s lack of tensile strength and brittle nature make it less useful for very detailed or large patterns.

Basswood & other woods
Basswood is stronger than rigged foam and finer details such as leaf ends or fingers tips that would be to fragile and brittle if rendered in foam are easy to create in this close-grained soft wood. Patterns created in basswood and foam need to be sealed with a paint primer or other non-permeable layer that act as a barrier to protect the pattern from liquid damage that may occur during mold making.

Blue jewelers’ wax
There are many types of wax available in block and sheet form that can be machined or carved to create a pattern, but we typically use a blue hard wax and pattern making.

Creating custom cabinet pulls and knobs

We have recently been commissioned by a home owner and a designer to create custom cabinet knobs and pulls for their respective bedroom and kitchen. The designer approached us with the task of creating 3 styles and sizes of cabinet knob to replace and supplement the ones in the home owners period property. In this case the existing original pieces were in excellent condition and could be used as patterns for a sectional mold that was constructed in silicone rubber.

The second commission was to create a completely new cabinet pull that would use the client’s distinctive family cattle brand as a model for the cabinet doors and drawers. The client did not have a physical piece that could directly be used to form a mold, but he did have a pdf file of the family font and from this we were able to create a 3D CAD model.

CAD File Created Using Client’s Font File

Both projects will be poured in stainless steel using the lost wax method rather than using 3D Stereolithographic printing. As the molds for both projects are relatively simple one-part molds the upfront costs for design adaptation and mold creation were relatively low and added little to the per unit cost of the pulls. The number of units ordered was also modest ranging from 50 to 75 pieces, but it will be was enough to cover the cost of pouring a smaller crucible of steel.

First Waxes Created from New Mold Ready to be Shelled