Investment castings are desired for their tight tolerances, smooth surfaces, and the ability to cast structurally thinner parts than sand or permanent mold casting parts or other design flexible manufacturing processes. However, the current technology process has some drawbacks relative to alternatives including a slower shell production cycle, less reuse of consumed materials in the molds/cores, and lesser defined rules for risering castings in the full array of alloys used in investment castings. Minimizing these drawbacks is likely to yield even greater interest in investment castings as the optimum solution for various military and commercial machines and building part design communities. 
To capture the opportunity to overcome these drawbacks, it is helpful to understand the impacts each of these limitations has on part design, cost, quality, and lead time.

The long production lead times for the shell building part of the process result in longer delivery times, and more required space which can be a limitation on the total production of a facility.

Similarly, the inability to reuse spent shell materials requires additional cost to ship and dispose of these engineered materials plus it requires the re-order of replacement materials, which requires additional expense and resource allocation time. Fortunately, several investment casting foundries in opportune geographic locations have identified entities interested in securing these spent materials so disposal charges can be avoided. However, most investment casting foundries benefiting from this arrangement still incur shipping costs and pay for replacement materials. Hence, these additional costs result in higher casting prices. 

Risering refers to the act of creating a reservoir of metal capable of taking the place of metal volumetrically shrinking in the casting cavity during cooling and solidification. Simulation inputs and risering rules have long existed and were fine-tuned in high-volume alloy and molding media combinations; however, these rules are still largely evolving in those alloy and molding media combinations with less history and scope. Examples include some of the newer Ni-base, Cr-base or even steel and aluminum-based alloys. Furthermore, as shell materials have evolved, the unfinished rules that previously existed need to be adjusted for subtle thermal conductivity and performance changes. The importance of risering and the scatter in results from these rules are exaggerated in investment casting due to the notoriously slow cooling rates caused by the preheated shell. Improper risering will yield lower quality castings, higher percentage scrap, lower yields, and accordingly a higher cost for the castings.

In response to the opportunity created by overcoming the aforementioned process limits, a co-sponsored project was devised to research options for overcoming some limitations caused by the three thrusts of shell productivity, reuse of spent ceramic, and improved risering knowledge and tools. A literature search established a list of previous efforts to improve upon these industrial thrusts.

Experimental Procedure

An online survey was conducted to establish baseline metrics for the current practices of the domestic investment casting industry. The online Survey Monkey platform was used to transact the survey with the questions programmed with logic so the number of questions to be answered varied between 24 and 26 based on the answers provided to each question. Most of the questions were multiple choice but with the option to select “other” if other selections than the preselected multiple-choice options were possible. When “other” was selected, an open field was made available for respondents to type in an answer. Three questions were open-text fields for typed-only responses.

The survey was preliminarily drafted by the project team and refined by the AFS Investment Casting Committee for full applicability to the industry. The survey was sent to nearly 500 verified email addresses from active investment casting foundries incorporated in the U.S. Although a few dozen locations had the survey sent to multiple employees, the survey requested that members coordinate internally to combine responses into a single survey. 

A total of 37 surveys were submitted from February 29, 2024 until mid-May 2024. Although several respondents exercised the option to remain anonymous, a glance at the survey suggests that only a single operation submitted a duplicate location survey suggesting 36 unique operations were surveyed. Survey Monkey allows the user to export the results as both tables and bar charts in the form of a .pdf to facilitate analysis. When helpful, Acrobat Reader was used to convert the .pdf files into .xlsx files for sorting and additional viewing options.

Results

Several survey questions will be presented with the results and interpretations from these. Some questions have been skipped in pursuit of brevity but will appear in the full paper in April.  Note that in some instances the totals will add up to more than 100, as some foundries will meet more than one category, such as pouring multiple alloy families.

Conclusions
Several conclusions were drawn from the results of this survey and include:

1. Industry-wide yield is a mixed bag and seemingly supports the need for improved risering tools in alloys and molding criteria where they do not exist.
2. High-scrap and or re-work due to shrinkage-related defects are not systemic in the industry but comprise portions of the industry; a portion that hopefully can be improved with assistance.
3. Industry lead times are quite scattered regardless of whether foundries need to create new tooling or not. Interestingly, a survey question prompting respondents to report on the time specific to shell manufacturing received very few responses suggesting it is possibly unknown or too volatile to confidently determine.
4. The 3D-printed wax-like patterns are slowly proliferating within industry. The predominant reasons for their use are for prototyping, low-volume tooling less production, or as an interim solution while awaiting production tooling. Nearly 70% of respondents report using 3D wax-like patterns, less than 5% hinting that there may be a need for further research and technology transfer before comprehensive industry adoption.
5. As expected, industrial shell recycling remains in its infancy per the survey responses. A literature search confirmed efforts are in place globally to overcome the remaining hurdles for implementation. Zircon and fused silica remain the biggest opportunities due to largest use, followed by alumino-silicates, alumina, and cobalt-aluminate. A wide spectrum of mesh size requirements in the industry will require solutions tailorable to each facility’s needs.