Integrally Cast Lettering & Marking

One of the primary benefits provided by the Investment Casting process is that part detail can very often be furnished at negligible additional cost.  Engineers frequently take advantage of this “free” capability by incorporating cast lettering and marking into their part designs.  Marking can be almost anything from letters, numbers, logos, trademarks or be simply decorative.

There are basically two types of cast marking, “Integral” or “Applied”.  Integral-marking is embedded into a pattern mold so that the identical marking appears on each part cast.  Applied-marking such as that made by laser, vibro-etch or ink stamping is applied to the finished product and used most often to identify castings with information that changes part-to-part or lot-to-lot such as serial numbers or heat treat lot data.  We will devote our attention here to integral-marking.

Integral-marking, being permanently cut into the pattern mold, is somewhat difficult to alter.  Although there are methods by which to integrally cast variable information this does add an element of cost to the casting and so is better suited for one of the applied-marking methods.

Integral-marking can be either raised or depressed on the casting surface but given a choice foundries tend to prefer that marking be raised.  The reason for this preference is that positive features on a part are the inverse in the pattern mold.  Thus a depressed part marking will be raised on the surface of the mold so will be very susceptible to damage from handling of the mold.  A designer desiring a depressed integral-marking (thus raised in the pattern mold) should consider designing the depressed lettering to be on a “raised pad” with the pad height being taller than the marking is deep.  In this manner, in the pattern mold, the “pad” will be deeper in the mold than the lettering is high so the lettering be somewhat more protected from damage when the mold is being handled.

Though commonplace, whether integral-marking be raised or depressed, there are other challenges that can be mitigated with good design.  With investment casting it is important that integral-marking be sufficiently substantial in height and stroke-width that the process will robustly reproduce them.  If marking is too shallow it will not be legible on the cast part or if too deep the integral-marking may suffer from metal penetration so, for example, the loops of a letter “B” fill with metal to then more resemble a letter “D”.

O’Fallon Casting suggests that the raised height or depth of the integral-marking be .010 - .015” from the casting surface depending on the choice of characters or fonts.  Characters and forts will cast optimally without any sharp edges or corners.

Incorporating Integral-marking into designs is one way to take advantage of the investment casting’s ability to provide part detail at minimal expense.  If you have any question if an integral-marking should be included in your casting design, please consult your O’Fallon Casting Sales Engineer.

Surface Texture of Investment Castings

The Investment Casting Institute’s “The Investment Casting Handbook” states that a 125 RMS surface texture is typical (for steel castings) and that no other casting process produces a finer surface finish than does investment casting.  Although “The Investment Casting Handbook” says Typical, a 125 RMS is commonly cited by the industry and a 125 RMS Max has become a common callout on many investment casting drawings.  As the surface texture provided by O’Fallon Casting is generally much better than a 125 RMS Max requirement it is not considered to be an issue at Contract Review.  However, every few years a profilometer inspection of an as-cast surface texture will find and reject some area of a part for being in excess of a 125 RMS Max drawing requirement.

Because of these occasional incidents I argue against placing a surface texture requirement on a casting drawing unless the texture is important to the function of the part.  My thoughts are echoed in ANSI/ASME B46.1 Appendix B on the Control & Production of Surface Texture.  Appendix B1 states that:  “Surface characteristics should not be controlled on a drawing…” as “Unnecessary restrictions may increase production costs…”.  Since the surface texture of a casting is a function of a given foundry process, the options available to improve the native surface texture will entail some nature of secondary operation that will add expense to the cost of the product. 

ANSI/ASME B46.1 Appendix B reinforces this notion by further stating that “Surface texture is the result of the processing method…”  In effect a Sand Casting will have a sand cast surface texture while a Die Casting will have a die cast surface texture and an Investment Casting an investment casting surface texture.
An accompanying Figure (B1) in ANSI/ASME B46.1 shows the normal ranges of surface texture by processing method which for Investment Castings is 60 – 200 RMS.  Designers should consider 60 – 200RMS as an appropriate range of variation when specifying a surface texture requirement and also deciding if a given surface should be machined to a finer finish or remain as-cast.

The occasional rejection of castings to a 125 RMS Max requirement is also in part explained by ANSI/ASME B46.1 where it states that “Castings are characterized by random distribution of nondirectional deviations from the nominal surface.” 
 ANSI/ASME B46.1 further states that “Surface characteristics of castings should never be considered on the same basis as machined surfaces.”  An inspection of an as-cast surface texture with a profilometer is in itself an inappropriate method for a foundry product.  The inspection of Surface texture as-cast surfaces should instead be performed by visual comparison to a standard.  GAR Electoforming (http://garelectroforming.com) manufactures a “C-9 Cast Microfinish Comparator” that can be used as a standard for visual and tactual comparison to a cast surface consistent with ASNI/ASME B46.1.  The use of this type of Comparator is highly recommended for the evaluation of an as-cast surface texture. 

Overall it is recommended that a Surface Texture should only be specified on a drawing when it is important to the function of the end product.  The surface texture of a casting will be representative of its manufacturing process and it should be inspected visually to an appropriate surface texture gage.

Edge & Corner Radii

Edges & Corners are external features such as you would see on any cube such as a child’s Alphabet Block.  Structurally Edges & Corners have little impact on the strength of a casting.  However, Edges and Corners are still an important consideration of casting design.

Investment Castings will naturally exhibit a .008” - .012” radius along an edge even when tooled “sharp”.  I often say that foundries “don’t cast razor blades” (but then even razor blades are not truly “sharp” either).  The freezing of the wave front and back pressure from air in the cavity prevent the liquid metal from achieving a truly “sharp” edge.  Casting Designers should provide for this condition on their designs with an allowance of at least a .015” R Max along cast edges & corners.

Foundries, however, generally prefer not to have “sharp” edges and corners on cast parts.  In Investment Casting a sharp edge can create a weak point in the ceramic shell mold.  (If you’ve ever tried to paint a sharp corner you’ll understand that it can be very difficult to build thickness along a sharp edge.)  A vulnerable “thin” area in the ceramic shell that may cause the shell to crack from the stress of the De-Wax operation or fail as the mass of the hot metal is being cast.  O’Fallon Casting recommends that a radius roughly equivalent to the wall thickness be allowed on all external Edges & Corners.

OFC also recommends that dimensions for Corner & Edge radii have an R-Max tolerance applied.  An R-Max tolerance provides flexibility to the pattern mold designer to strategically omit the Edge Radii along parting planes in the mold.  Omitting an edge Radii allows the parting plane in the pattern mold be located at the top of features which simplifies the design of the mold and makes it less expensive to design and build.  Cosmetically, in this manner, the parting lines on the casting will blend with the “Sharp” edge.  (Edges along non-parting surfaces in the pattern mold will be cut with the full radii.)  An R-Max allows for a Sharp Edge at parting planes that will reduce the cost of the Pattern Mold and also improve the cosmetic appearance of a casting.

If for the function of the casting it is necessary for edges and corners have a radius this can certainly be accommodated.  A design includes a ± tolerance (rather than an R-Max) for the Edge Radii the parting planes then need to be placed adjacent to the root of the Edge Radius rather than at the top of features.  This requirement will increase both the design and construction time of a pattern mold and so also its expense.  Also a casting produced from a tool cut with an edge radius at the parting surfaces will exhibit a residual parting line adjacent the root of the edge radius and that will likely grow heavier as the pattern mold wears.

Edge and Corner radii are an important consideration for casting design.  Even when tooled sharp, castings will display a slight radius that should be noted on a drawing as .015 R Max.  Foundries prefer that casting designs permit an Edge Radius equivalent to the wall thickness and recommend that they be designated with an R-Max tolerance so that the radius may be omitted from parting surfaces in the pattern mold.

Why Castings need a Fillet Radii

By definition a Fillet Radii is a rounding of an interior corner and are employed on castings to increase their load bearing strength and to improve both manufacturability and quality.  For those reasons a fillet radius should be a standard allowance on every casting design.

A Fillet Radius makes a structure stronger because it redirects stresses from being concentrated at a sharp interior corner and distributes them over the broader volume of the fillet.  The effect of the Fillet Radius is to mitigate a potential weak point of cast structure making transitions within a structure, especially between right angled walls, stronger.

Because of this faculty to mitigate stress concentrations a Fillet Radius also improves the manufacturability of a casting.  Castings often require a mechanical “Straightening” operation to restore flatness, perpendicularity and parallelism to the part and a filleted corner is less prone to cracking during this operation.  Better casting manufacturability provides a foundry with control of its costs and promotes more consistent deliveries.

The foundry industry works diligently to avoid turbulence in the flow of hot metal as their products are being cast.  Turbulence can cause a mixing of the metal with air to trap gas or lead to the formation of oxides that may become embedded in the casting.  Fillet radii help to minimize the turbulence in the metal as it flows through the cavities of the mold thus inhibiting the formation defects from oxides and trapped gas and so contributes to the better quality of a cast product.

In the Investment Casting process a thickness of ceramic shell is formed about a pattern assembly by repeated dips of the assembly into slurry.  Sharp internal corners in a part configuration can provide nucleation sites onto which bubbles of air form and adhere to the pattern while the slurry is being applied.  Air bubbles on the pattern will become small voids in the ceramic shell that will subsequently fill with metal and become positives on the surface of the casting.  A fillet radius improves the ability for the pattern to shed air bubbles as the slurry is applied thus reducing the propensity of casting to exhibit positive metal.

A general “Rule of Thumb” for an aluminum investment casting is to apply a fillet radius equal to 1x to 1.5x of the wall thickness with a recommended minimum of .06”R.  It is also advisable to specify fillet radii with a Max tolerance.

Customers of O’Fallon Casting are provided a free Concurrent Engineering service to assist in their design and improve their use of castings.  If you have questions regarding Fillet Radii or other investment casting design considerations call your O’Fallon Casting Sales Engineer for assistance.

How an Investment Casting is Manufactured

Investment Casting is a cost effective method for the production of precise, near-net-shape and complex metal parts.  Although Investment Casting is relatively ancient in origin it is today a highly sophisticated manufacturing process.

By definition Investment Casting is a foundry process by which a metal casting is produced from a ceramic mold that was formed by a disposable pattern.  Much as Sand Castings are produced from sand molds and Die Castings from metal dies, the Investment Casting process name originates from the ceramic “Investment” in which parts are cast.

There are eight basic steps for the manufacture of an Investment Casting:

1. The first step of the process is the production of a Disposable Pattern.  Generally Investment Casting patterns are made of Wax and produced by an aluminum injection mold.  However, Investment Casting patterns might alternatively be produced from a rubber or soft metal molds or formed by an Additive Manufacturing process such as SLA, SLS, or Voxeljet.


2. Once the wax pattern has been formed it is attached to a Runner System.  The runner system is also comprised of wax that is cut to size from injected or extruded stock and is “wax-welded” together.  A complete and “Assembled” part & runner system is referred to as a “Sprue” or “Tree” and may be comprised of many multiple Wax Patterns.


3. The Assembled wax Sprue is then ready to be “Invested” or coated with ceramic.  The most prevalent method to “Invest” a wax sprue is the Ceramic Shell Process.  In the Ceramic Shell Process a sprue will dipped multiple times into vats of specialized slurry and refractory sand to form a Shell.  The shell is allowed dry time after each layer is applied and once a sufficient layers of ceramic have been applied to the sprue the completed shell is allowed to dry fully.


4. Once the shell is dry a DeWax operation is performed, generally with a steam autoclave, to melt and eliminate the wax from the sprue.


5. With the wax removed the shell is Fired in an oven to create a crystalline ceramic structure that will withstand the weight and thermal shock of the metal to be cast into it.


6. Once the shell has been fired molten metal is then Cast to fill the cavities in the sprue that had been formed by the wax pattern & runner system.


7. After the metal has solidified the ceramic is removed from the Sprue with high pressure water or some other form of knockout system.


8. With the ceramic removed the metal parts are cut–off from their runner system with a saw or other cutting method such as a laser.

Any number of additional operations might be performed, such has heat treatment, straightening, Non-Destructive Testing, etc., once a part is in metal.  Although the manner in which they are performed will vary between manufacturers, the Investment Casting process always requires these same eight basic steps.

The Five Part Solution

My introduction to the subject of Part Count Reduction came about as an observation of the results of a Design Project in my sophomore year of college.  I’ve come to refer to the anecdote as “The Five Part Solution” which might have been long forgotten had the saga not come with an accompanying lesson in humility.

The Design Project was a first instance of being challenged for an original engineering solution and I recall “enthusiastically” devoting several weekends to research and then to developing a concept for my proposal.  With some degree of pride I completed my design, an electro-mechanical masterpiece replete with springs, hinges, micro-switches, transformers, etc.  Accompanying my submission was a full set of Assembly & Detail drawings of each part and included a 40 (or perhaps 50) piece Parts List.

On entering the classroom to submit my proposal I noted that a group of students were chatting excitedly in a corner of the room.  Their attention had been drawn by another student who was busily demonstrating the operation of his design proposal on a mock-up he’d constructed of wood in his garage.  His design was a stunning contrast to my own elaborate proposal as it contained only five mechanical members and required no external power to operate; the Five Part Solution.

 I still remember being struck by the elegant simplicity of the Five Part Solution which has garnered even greater appreciation as I’ve grown to understand the full impact of engineering decisions on the cost of a product. Had my product design competed with the Five Part Solution in the “Real World” one manufacturer (his) would hold a significant price advantage and so enjoyed much greater success in the marketplace.

The story of the Five Part Solution demonstrates why Castings and in particular Investment Castings are such an effective manufacturing method.  Castings offer a unique capability to combine multiple features into a unitized structure that reduces the total part count of an assembly and improves the overall affordability of an engineered product.  We encourage that Engineers take advantage of the opportunity that castings present and to write their own Five Part Solution.