CAD Design Guidelines

Sharp Edges

A common cause of investment wash is the CAD model containing sharp edges, points or corners. Edges, points and corners should all be curved with a radius of at least 0.125 mm (also known as rounding, filleting or edge softening).

Popular design software such as Matrix has plugins and functions available to perform this.

The most common sources of sharp edges are components with edges that meet to a point and straight, sharp angles.

Holes

Another cause of investment wash is when models contain aspects such as lettering, stone holes, fine gaps or spaces that have a depth that is greater than the width of the opening. The depth of the void should not exceed the width of the opening of the void.

The maximum depth of a hole should not exceed 0.30 mm

If the design requires holes, then a small pilot divot is better, allowing for the entire hole to be drilled out later.

Gaps and Voids

Adequate spacing between parts or components that are not meant to be touching is important. Small gaps or voids can be sources of investment wash or poor formation. Always maintain a gap of at least 0.20 mm between components such as claws, setting posts and walls.

If this is not possible, fill the gaps and they will have to be drilled out at a later stage.

Ensure pilot holes for stones are greater that 0.2mm in diameter, with a depth no larger than the diameter of the hole.

If the design requires a pilot hole, then a small pilot divot is better, allowing for the entire hole to be drilled out later.

Material Thickness

The wall and component thickness should be considered when creating designs. Cast material that is too thin may cause shrinkage, porosity, distortion, or breakages. Factors such as the surface area and complexity of the design may require greater design thicknesses.

The minimum design thickness should be at least 0.4 mm.

Broad and flat areas should be at least 0.50 mm. If the final piece is required to be moulded for multiple copies, then should be at least 0.90 mm thick.

Text Designs and Lettering

CAD designs often include text, hallmarks, logos and stamps and are usually recessed into a section of the design. While these designs look great and can be useful (particularly for showing a makers mark or caratage) stylised fonts and impressions are not always conducive to successful investment and casting.

If the design includes these aspects, consider using square angles or simple fonts such as Sans Serif. Avoid curved designs that taper to a fine point.

Raised text: designs must have a minimum width and spacing between letters of at least of 0.30 mm. Raised text should only have a maximum height of 0.50 mm.

Recessed text: designs must have a minimum width and spacing between letters of at least of 0.30 mm. Recessed designs should be recessed to a maximum of 0.30 mm deep and no deeper than the width of the opening.

If possible, draft or taper by 5-10 degrees so that the bottom of the impression is narrower than the opening

Avoid sharp corners ensuring that edges and corners are rounded to at least 0.125 mm

If a complex or highly stylised design is required, laser engraving may be a more suitable and more successful option.

Naked Edges and Non-Watertight Models

Naked edges are open edges which are not connected to any neighbouring edges, producing a model that is not “watertight”. Curved surfaces are a typical source of naked edges, particularly with text designs on 3D files. Most text fonts are designed with graphic design in mind, not 3D printing.

Ensure to utilise the analysis and check functions in you CAD design program to determine if your model is "watertight" and free of any naked edges

Unsupported Components and Protrusions

Designs that contain aspects that protrude or are poorly supported can break off during the casting process, particularly at the wax handling (cleaning, treeing, etc) and investment stage.

Please ensure that protruding and thin parts are thick enough to support themselves or contain additional supporting sprues or strips to minimise the risk of breakage. The thickness required is specific to the design. Alternatively, a supporting sprue or bridge of at least 0.80 mm in diameter can be inserted on the drawing to add support.

This also applys to components that may have large gaps and openings or unsupported protrursions (i.e. shanks that have an open gap to place the setting in, or claw settings that may require a supporting halo sprue).

Multiple Parts and Components

Designs with multiple parts or components (i.e. setting and shank) need to be exported as separate STL files. This is espeically true of multiple components that are to be cast in different alloys.

Some designs can be printed and cast as one piece but quality and feasibility are not guaranteed.

Finishing and Shrinkage

Castings are typically supplied in a semi-raw form with further finishing required. Finishing can remove up to 0.10 - 0.20 mm of the surface material, so this needs to be factored into the design.

Shrinkage may also occur over the various stages of printing and casting. Allow for at least 1% shrinkage in gold and silver alloys and 2% for platinum alloys. This is especially true for components where accurate dimensions are required (i.e. stone setting spaces and finger holes in shanks).

Identifying Marks

Multiple pieces with similar designs can be difficult to tell the difference between. Examples include; multiple settings with slightly different stone sizes, rings/shanks with slightly different finger sizes.

Please include an identifying mark to each piece. This can be a finger size on the inside of a shank or a spure added to a setting that includes a number or another unique marking.

Designs That Restrict Metal Flow

Models that have design aspects with varying thicknessess and angles can restrict metal flow during the metal injection process.

Main sprues are generally attached to the thickest part of the model to allow optimal metal flow. 

Design aspects that are thinner or encourage a longer or more complex metal flow path may require additional feeder sprues to reduce the metal flow path length.

This can reduce the likelihood of components not forming properly or metal solidifying before it can successfully fill all the required spaces.