What is an Investment Casting?

Investment Casting is a foundry process known for producing highly accurate, near-net-shape, castings.  Once known as the “Lost Wax” process, much as Sand Castings are produced from sand molds, and Die Castings are manufactured from metal dies, Investment Casting derives its name from its ceramic molding material that is known as “Investment”. 

A technical definition of the Investment Casting process is “A foundry process by which a metal part is produced from a ceramic (Investment) mold that was formed by a heat-disposable (wax or plastic) pattern.  The earliest known evidence of the Investment Casting process was more than 5000 years ago in ancient China.  Other early civilizations in Asia, Africa, Europe and the Americas are also found to have utilized the process. 

Through most of history Investment Casting was largely used for the manufacture of custom jewelry, sculpture and art.  However, in the 1930’s the Investment Casting process was rediscovered when its ability to cast near-net-shape parts was found to be advantageous for the manufacture of components for aircraft & arms during World War II.

Following the War, the use of Investment Casting began to expand into broader markets.  In 1953 a coalition of Investment Casters, and their suppliers, formed the Investment Casting Institute which then established design guidelines to aid Engineers to better design for the process

Today, Investment Casting foundries exist in many countries around the globe, serve in a vast array of markets and are manufactured in a stunning variety of metals.  O’Fallon Casting itself specializes in the Investment Casting of Aluminum, Brass, Bronze and Silicon Carbide / Metal Matrix Composite alloys. 

Engineers continue to discover the ability of the Investment Casting process to manufacture highly detailed and accurate shapes that reduce weight, improve functionality, or reduce part count. 

If you have any questions, please contact us at:   sales@ofalloncasting.com

Characteristics of Casting Design – AFS Video

O’Fallon Casting was pleased by a video that has been released by the American Foundry Society on their MCTV – Metalcasting Television.  In the video design engineering consultant and author, Jiten Shah, a Product Development Analyst expounds on design elements of O’Fallon Casting’s “Casting of the Year”.

Jiten Shah skillfully promotes aspects of “Good” casting design such as the combining multiple pieces into one cast structure.  He also hits upon some of the distinct advantages of the Investment Casting process such as surface finish and zero draft.

O’Fallon Casting is delighted to have been selected as winner of the AFS / Metal Casting Design & Purchasing 2013 Casting of the Year award.  However, winning the award first required that a Design Engineer envision the cast part and then “put the black lines on the white paper”. 

To promote good casting engineering and design, O’Fallon Casting offers its Investment Casting-101 class to its customers.  IC-101 explores, in detail, the fundamental principles and considerations for successful casting engineering.

We thank Jiten Shah for his kind and incisive comments about the casting, and the American Foundry Society for sponsoring the video.

http://www.afsinc.org/multimedia/MCTVDetail.cfm?ItemNumber=15468#.UkGrj-riZWo.email

Nanocomposite Aluminum Alloys

On Dec 12 2012, Vince Gimeno and Tim Hill of O’Fallon Casting, attended a meeting at the University of Wisconsin-Madison on the ultrasonic dispersion of nanoparticles for strengthening of cast aluminum alloys.  The technical presentation was made by University Professor Xiaochun Li, supporting staff and students.

The purpose for the meeting is to establish an industry-university partnership to advance technical and scientific knowledge of large scale manufacturing of bulk nanomaterials for practical applications. O’Fallon Casting, as an industry leader in the manufacture of Aluminum/Silicon Carbide Metal Matrix Composite Alloys, is following the development of these new materials very closely.

Cast nanocomposite aluminum alloys provide much higher strength and ductility to those currently available for “structural” applications, and hold great promise in automotive and aerospace applications.  The industry-university partnerships are seeking solutions for issues, such as the uniform dispersion of the nanoparticles, to unlock the promise of casting these new materials.

 For further information, the following articles might be of interest:

 Theoreticaland experimental study on ultrasonic dispersion of nanoparticles for strengtheningcast Aluminum Alloy A356  Authors Xiaochung Li, Yong Yang University of Wisconsin-Madison & David Weiss, Eck Industries, Manitowoc, WI

 MetallurgicalScience and Technology Vol. 26-2 – Ed 2008

Investment Casting 101

O’Fallon Casting offers Investment Casting-101 classes to its customers to help educate their Engineers on the Fundamentals of Investment Casting design.   More than twenty “IC-101” classes were held at customer sites in 2012.  IC-101 is intentionally non-promotional and taught in a classroom environment from an industry prospective.

A typical IC-101 class begins with an in-depth review of how an Investment Casting is manufactured, providing a foundation for the more technical discussion that follows.  Once these fundamentals are taught, the Advantages & Disadvantages of the process are discussed at length, so that students may grasp how Investment Casting fits with their needs, and also if Investment Casting might offer a better option relative to alternative processes. 

For example, one of the advantages of Investment Casting is that it can produce complex configurations with high accuracy.  This capability presents Engineering an opportunity to design one-piece castings instead multi-piece fabrications (assemblies, weldments or dip-brazings).  A one-piece casting can provide a more rigid and lightweight structure that eliminates assembly time and also the many “soft” costs associated with procurements of multiple pieces. 

The second phase of an IC-101 class is a review of Drawing Notes and Specification requirements, and commences with a discussion of minimum information that needs to flow to a foundry by a customer. This is followed by a review of an actual customer drawing, and accessing the adequacy of the notes and the cost of “overkill”.  Following the Drawing Note discussion, an in-depth review of Material and NDT Specifications is conducted again identifying their cost implications and noting any “hidden” requirements.

For Example, unless otherwise overwritten by a drawing, some commonly employed specification, such as AMS 4218, require the certification of physical properties be determined from test bars excised from a casting (a relatively expensive method of test that also adds the cost for a destroyed part).  When, however, a drawing is employing this material specification also includes a note to determine physical properties from Separately Cast test bars, the recurring cost for physical property testing is significantly reduced.

An IC-101 class concludes with recommendations and detailed discussion of Good Investment Casting Design Practices.  Topics covered include:   tolerancing, datum selection, datum point placement, fillets & edge radii, etc.  Also discussed is the current state of technology in the industry for Rapid Prototypes, Digital Radiography, Non-Contact Metrology, etc.

Students learn, for example, not only the recommended sizes for fillet radii but also explained is why they are necessary to casting production and the cost implications when over/under specified.

A “normal” IC-101 class, allowing for discussion and Q&A, requires approximately three hours.  Shorter sessions, of 1 – 2 hours, are conducted for Technical and Administrative staffs to cover the general topics with less specific detail.  Each student is given a textbook to be retained for their future reference.

O’Fallon Casting provides IC-101 as a free service to its customers that they might be better users of Investment Castings and to enhance the affordability of their products. 

For more information, please see the Investment Casting-101 tab of the O'Fallon Casting website.

Casting Datum Points – Avoiding potential pitfalls of Datum Point selection

Castings provide a cost effect method to provide the Near Net Shape of a configuration, but critical details are inevitably machined.  The machining first cut should always be from the Datum Points of the casting, and just in case something goes wrong, it is important to retain the original casting datums.  So it is important that the Casting Datums be located on surfaces that are not subsequently machined.  Where the available datum surfaces must be machined, it might sometimes be necessary to create additional features that serve no other purpose than to preserve the Casting Datum Points.  If the Casting Datum Points are preserved through subsequent manufacturing, the integrity of the original casting measurements will remain intact.

I also recommend that all Datum Points be co-planer with each other.    The basic philosophy of using Datum Points is to establish an arbitrary “Zero Plane” from which to measure.  As there is allowable variation between the Zero Plane and other cast features, it is a poor practice to designate features that are not a part of the Zero Plane as also being at “Zero”.  Any dimensional variation from nominal, between non-coplanar datum points will skew the measurements of an entire part.   I recommend, that if a configuration does not readily offer coplanar datum points, then coplanar features should be added to the configuration such as raised / recessed pads or lugs. 

I also recommend that a part should balance on the Primary Datum Points without clamping.  Although not always possible, when a part will balance on the Datum Points it helps to prevent any miss-positioning of the casting on the Datum points. 

The final point I’d make is to encourage the centering of the datums on the part configuration.  Casting variation is cumulative, so the longer the dimension being measured the greater the amount of tolerance should be allowed.  Especially when living in the Model Based Definition world, Engineers are being compelled to allow profile tolerances that accommodate the most distant features.  Centering the three datums effectively shortens the longest dimensions of the part and lessens that amount of tolerance needed to accommodate the configuration.

I want you to avoid selecting theoretical centerlines as datums as they increase both part and tooling expense while decreasing the accuracy of the part measurements.  Datum points need to reside on hard features on the Zero Plane.  If there are no hard features, we recommend that you either create them or shift the datum to an adjacent surface.

For additional information about our recommendations for Datum and Datum Points, please see our blog posts:

• Casting Datums and Datum Points – Optimizing their selection
• Casting - Datum & Datum Point Basics

Casting Datums and Datum Points – Optimizing their selection

Let us further consider the selection of the datum surfaces.  If we visualize a Shoe Box as an example, we may choose either the inner, outer or a combination of each to serve as datum surfaces.  My recommendation is that Exterior surfaces should be selected in preference to interior.  Consider our Shoe Box and the physical volume that a Tool Point Fixture, built for internal Datum Points, will consume.  The presence of the Tool Point Fixture, within the box interior, will constrain our ability to inspect the interior of the casting.  A better situation exists when the Datum Points are located on the casting exterior and where the Tool Point Fixture will not impede the instruments needed to measure part features.

I have several thoughts to help select the best locations for the three Primary Datum Points.  First, it is important that the Datum Points be spread to the extents of the surface as distant from each other as possible.  Remembering the castings a never perfectly flat, square or perpendicular, spreading the Datum Points helps to minimize the multiplying effect that surface variations can impart to measurements.  Let’s consider our Shoe Box again.  The walls and floor of most Shoe Boxes are distorted, bowed outward, or “oil canned”.   If a Datum Point were placed onto this distortion you can n appreciate the potential affect this “bow” might have, on measurements of the box, especially if the Datum Points are in close proximity to each other.  When, however, the datum points are spread to the corners or edges of the Shoe Box, the wall & floor distortion will have little effect on the measurements of the Box. 

This also brings us to a second recommendation, to locate the Datum Points onto “Stable” areas of the casting.  In the previous example the “bow” in the base of the Shoe Box that exists because the floor is free to flex.  However, areas of the floor, that are adjacent to the corners and edge of the box are constrained by the perpendicular wall from flexing.  Areas of the Shoe Box, or any part, less susceptible to distortion are considered “Stable” areas.  When the Datum Points are located on Stable areas, a part it is easier to measure and also to identify, and correct, areas with distortion. 

For additional information about our recommendations for Datum and Datum Points, please see our blog posts concerning: Casting - Datum & Datum Point Basics

Casting - Datum & Datum Point Basics

Recalling basic Geometry, three points are required to define a plane (-A-), two additional points to define a perpendicular, secondary (-B-), plane to the primary and one additional point to define a tertiary plane perpendicular to the first two (-C-).  As castings are never perfectly flat, straight or square, this Geometric fundamental is often used by Casting Designers to establish a datum structure from which to obtain a consistent measurement of cast parts.  Moreover, Datum Points allow a casting to be machined from the same datum structure.

The selection of datum surfaces, and assignment of the datum point (also called Tool Points) locations, is a critical consideration for good casting design, so here are a few helpful tips:

Generally the longest & largest surface of a part is designated as the primary (-A-) datum surface.  The second largest perpendicular surface would then be then designated as the Secondary (-B-) and the Tertiary (-C-) a surface perpendicular to the first two.  For example envision a Shoe Box, with the floor of the Box being Primary Surface the longest side as being Secondary and the short end as being Tertiary.  Although most castings are not as straightforward as a Shoe Box, the same logic applies. 

Once the datum surfaces are determined, Datum Point locations, with three points on the Primary, two points on the secondary and one on the tertiary need to be designated on each of the datum planes.  By placing the part against these six datum points, generally a Tool Point Fixture, a casting then can be measured from the theoretical planes using either a height gage or CMM probe.

For additional information about our recommendations for Datum and Datum Points, please see our next blog posting.