From Casting to Milling
The revolution of precious metal manufacturing in the dental industry
Scott A. Mappin
From pure gold foil balls peened into tooth preparations as fillings to castable gold dental alloys, gold is the only restorative material with a track record spanning more than 4,000 years.1 Nearing the end of 2011, as dental laboratories increasingly moved more of their manufacturing to a digital workflow, one area of the analog world was slower to achieve a fully digital process: the precious alloys casting department. Would this medium find a path to a digitized workflow?
One stepping stone in this industry's evolution was the advent of CAD. It provided the ability to design then mill or print a castable material. The ability to streamline design to mill addressed front-end manufacturing issues, providing a level of consistency in design and manufacturing that previously did not exist. Milling softer castable materials did not require the use of more robust and expensive milling platforms. Printing carries with it the cost of the printer plus the printing medium, attaching a cost factor the laboratory had to acknowledge. Despite these positive advances, the negative issues related to casting still existed. These issues pertain to the physics of melting and cooling metal alloys, which arguably can never be overcome without deviating from the lost wax process.
When manufacturers considered what pieces of the puzzle were needed to make the leap to milling, the development of a blank or puck of dental alloy quickly came to mind. Research and development of a solution was very expensive on many levels. While some processes and metallurgical tests could be performed with small volumes of alloy, the scale of the research and development must, at some point, match the intended scale of production for validation purposes.
Accurate inventory management combined with intense process development were critical to successfully move from pre-alpha testing to a full product release to the market. All the while, pressure to reach the first sale loomed.
Beginning with dental alloy's unique traits such as density, hardness, elongation, elasticity, etc, experienced strategy developers employed comparable, less expensive aluminum alloys to understand how this metal would cut. After many test cycles, as the process became about 85% understood, moving to a gold puck was necessary for fine tuning and validation of the finishing detail.
What about the milling platform? Once a mill that satisfied business plan and technical needs was determined, a test facility was located that used the same milling platform. The mill requirements included a spindle capable of cutting metal; a high degree of stability, repeatability, and onboard self-diagnostics to stay within the specifications; and the ability to cut the restoration free from the puck and identify it somehow. The chosen mill also had to be capable of operating 24/7/365, hold a high number of tools and manage their lifespan, be accessible remotely, and present a simplistic opportunity for the recovery of milling debris. It needed mill the part to a required refinement in a predetermined timeframe to satisfy the quantity and quality demanded by production goals as determined in the business plan. Failure of any of these aspects could prove the process would deem the business plan undesirable. Altogether, this is a lot to ask of a mill.
A validated gold puck was brought to the test site. The mill manufacturer sent a leading service technician and an experienced strategy developer to support testing, since no mills would be purchased until the method was validated. The first round was filled with anticipation, exhilaration, and concern. About a year of research and development had gone into this moment. The amount of discovery that took place was considerable. This test phase exposed issues with the puck itself, tooling, strategies, and material recovery. Not surprisingly, it drove the process back to the home facility to address the many details in need of tweaking in order to reach the desired result. Further cycles of adjustments and testing in all the suspect areas were necessary. A second visit to the test site was conducted and this time the results were excellent. Mills were purchased, business plan items were showing promise, adjustments made to puck development were positive, and the previous tool and sequence issues were put to rest.
Upon the first mill's arrival to the brand-new facility, two days were spent ramping up and calibrating these sophisticated milling machines by factory technicians. "Final" testing of the complete manufacturing process and production workflow began under one roof. From incoming alloy documentation to puck manufacturing, from milled part inspection to a packing and shipping process, the steps of the theory became reality. A real workflow began to take shape, bringing the project to life.
During the following months, beta tests began as restoration files were cut over and over. Pucks were made, milled through, and a material recovery process was developed, then refined. Every aspect of the process was repeatedly re-evaluated; changes were made and documented; and the results were shared with metallurgists. Tooling suppliers observed cutting results and suggestions were offered for improvement, which in turn demanded repeated integration with the milling strategy developers. Customer feedback identified other small adjustments which were addressed at the same time.
Orchestrating these many aspects distilled the initial theory, replacing it with reality. This process adjustment cycle never ends; rather, it slows down, exposing finer details requiring adjustment over time.
"Good enough" has never been considered acceptable. Internal process development continues today. Mill manufacturers continue to refine operating software and improve mill efficiency. Tool vendors develop new tool geometry and coatings, increasing tool life and improving surface texture. CAM software is always improving, with software developers creating new sequences to enable the milling of unique parts at the client's request. Puck manufacturing is constantly evaluated, documented, and minute discoveries are made leading to small adjustments driving the mission of constant improvement. Mastery of all these aspects drives the day to day efforts in support of maintaining a successful precious metal milling operation.
Keeping precious metals in play as restorative materials while the price of elements continues to rise increases the value of this process for laboratories. Lean manufacturing principals suggest that purchasing alloy in the shape desired rather than an ingot is ideal, eliminating the many laborious steps which increase the chance of failure. The digital process allows the laboratory to buy precisely what it needs, in the desired shape, just before delivery; no more, no less. When it comes to filling a prescription for a precious-metal-based restoration, outsourcing for milled precious metal prosthetics is the most efficient way to achieve this. Milling at this level produces the most refined and desired result. Being first in this space drew attention from researchers who wanted to learn just how good milling gold could be, results can be found in two published white papers related to this specific milling process.2,3
Laboratories can now focus on maximizing their design capacity while minimizing their effort, equipment, time, consumables, and expensive inventory. Additionally, this development allows a technician or laboratory that has little to no metal manufacturing experience or equipment to provide this premium product to their clients and ultimately the patients.
Developing the platform, processes, and team of people to manage all the above aspects is a serious hill to climb. The result is a consistent, reliable restoration and service worthy of any patient. It is both a great responsibility and honor to have achieved this goal.
References
Helmut K, Holliday RJ, Corti CW. Gold in dentistry: Alloys, uses and performance. 2003, https://link.springer.com/content/pdf/10.1007/bf03215496.pdf
2. Afify A, Haney S, Verrett R, Mansueto M, Cray J, Johnson R. Marginal discrepancy of noble metal ceramic fixed dental prosthesis frameworks fabricated by conventional and digital technologies. https://strategygoldmilling.com/documents/Marginal-discrepancy-of-noble-metaleceramic-fixed-dental-prosthesis.pdf.Published 2017. Accessed October 27, 2019.
3. Johnson R, Verrett R, Haney S, Mansueto M, Challa S. Marginal gap of milled versus cast gold restorations. https://strategygoldmilling.com/documents/Johnson_et_al-2016-Journal_of_Prosthodontics.pdf. Published 2016. Accessed October 27, 2019.
