statistical process control applied to additive manufacturing enables series production of orthopedic implants.

by:INDUSTRIAL-MAN     2019-09-18
1.
Statistical process control and process window statistical process control are the monitoring of the application of statistical methods to manufacturing processes such as EBM to ensure their full potential.
The focus of statistical process control is to ensure that the final product reaches the required quality at the lowest scrap rate possible.
Statistical process control can be used to digitally check a process and the source of changes in the process to provide a complete understanding of the advantages and disadvantages of the process.
In addition, statistical process control is a tool to increase productivity by identifying process improvements in order to improve efficiency.
Combining EBM with a reduction in build time, it has been demonstrated that statistical process control is a valuable tool to reduce costs.
Product inspection is used to ensure the final quality of the production parts.
It is essential to maintain the required production quality, but it cannot provide the necessary help for improvement.
Statistical process control is a supplement to and beyond the scope of product inspection, because its purpose is to improve the production process, to a certain extent by systematizing the inspection results, however, in particular, by linking these results with changes in the production process, the coupling between process parameters and product quality is revealed.
When these links are displayed and presented in the control diagram, it is possible to systematically improve the process.
It is important to distinguish between common and special causes in the production process.
All processes have inherent statistical variability and can be evaluated by statistical methods.
A complex process may be affected by a large number of individual parameter changes.
Common cause changes in these parameters follow a normal distribution, so not only can the mean and standard deviation of these parameters be determined, but also the combined results of the parameters can be determined.
This makes the process predictable under the supervision of statistical process control, just as it can foresee the quality changes and rejection rates of the produced goods.
The change for special reasons was not observed before, nor was itquantifiable.
For example, this may be due to failure or degradation of system functions, changes in skill levels when replacing system operators, changes in raw materials or external conditions with lower quality, such as power stability.
Statistical process control can not only determine whether the quality of the product is declining, but also effectively identify the root cause so that action can be taken to quickly return the process to its normal state. 2.
Part verification and process verification of critical life-
Maintenance components, such as orthopedic implants and aerospace components, are carefully focused on the quality of the components.
In worstcase, material quality such as tensile strength, hardness and fatigue may endanger the service life of the implant or the safety of passengers.
Therefore, these parts need to be checked after careful production.
Although a large amount of information about the quality of materials can be
Destructive testing, this part validation does not fully cover all aspects of part quality control, whether it is a manufacturing method.
Therefore, for these key parts, it is necessary to carry out quality control based on the connection between process stability and part quality.
This process verification is done through statistical process control, in which the process needs to comply with the process window.
One way to define and implement the process validation isto method :(1)
Develop optimal system parameters through optimization combined with product quality assessment. (2)
Using fixed system control parameters to make a statistical evaluation of the quality changes of parts in a sufficient number of production operations, use destructive testing of production parts to verify that common cause changes are always within the process window that meets the product. (3)
Use statistical process control to ensure that there are no changes for special reasons in each production process. (4)
Implement strict quality control for all conditions other than system control parameters, such as raw material quality, operating procedures, system maintenance and support systems.
Geometric dependence is an important variable for additive manufacturing.
At present, the quality of parts in the additive manufacturing system is essentially dependent on the part geometry, and the way to ensure the process verification required for multiple geometries is in principle the process verification method defined above for each individual geometric part in production.
However, for certain changes in the part geometry, it is clear that you can define a geometry class that is validated.
In order to determine the effectiveness of process validation, it is necessary to combine representative samples of various geometries under the statistical evaluation of Process Validation methods.
With the development and maturity of additional manufacturing technology, the robustness of the validation process will become possible, as validation is effective for geometric changes of large span. 3.
EBM process stability EBM has developed into additive manufacturing technology mainly for titanium parts.
The first commercial EBM system was delivered to customers in 2003.
Initially, most EBM users came from academia or research departments in business organizations.
EBM has been focusing on production since the beginning, high build rate and excellent material quality for manufacturing parts have become the main goal of EBM development.
The focus is still valid.
At the end of 2006, the first EBM system was delivered to a manufacturing company in the implant industry for mass production of hip implants.
Subsequently, several systems for the same market application were also delivered quickly.
Since the start of serial production of EBM, statistical process control has been implemented to evaluate the yield and process quality of EBM processes.
In addition, it has been used since then for continuous improvement of ebm technology.
R & D department of Arcam
Guided by the statistical process control results of the system in serial production, the focus of development is to improve the reliability of the system to improve the system yield, and ensure that the quality requirements of the hip implants produced meet the specifications without exception.
At present, the reliability of EBM system in implant production exceeds 95%.
System reliability that enables high system utilization and predictable production volumes has been greatly improved, which has been a key factor in the successful development of this additive manufacturing application into serial production.
Further evidence of EBM reaching production value is that there is now a hip and spine cage on the market.
Both products benefit from engineered surface pores and are implanted regularly in patients. 4.
EBM technology development carried out statistical log file evaluation, evaluated the main cause of system failure in production, and provided valuable insights for the main contribution of EBM system reliability.
These main reasons are usually the type of change for special reasons, which have been identified and basically eliminated through specialized R & D work and system upgrades.
One reason for this discovery is the repeated failure of linear bearings in the powder distribution system. This sub-
The results show that in a vacuum environment, the risk of failure of the system combined with titanium powder is greatly increased, which leads to early failure of the bearing.
A dedicated project has found remedial measures in optimizing the use of vacuum grease for bearings and determining sufficient intervals of use.
Since the implementation of this improvement on the production system, the occurrence of linear bearing faults has almost disappeared.
This is a good example of the substantial improvement of ebm system reliability through the combination of statistical process control and R & D efforts.
In the past few years, the use of statistical process control has identified, investigated and corrected quality problems for many other special reasons.
Another development to improve system reliability is interesting because it addresses a common type of cause change.
This means that it does not correct the problem that directly causes the system failure, but rather corrects the changes that cause the process window to decrease and thus affect the system yield.
The physical effect of using an electron beam as an energy source during heating is to generate local surface charges near the electron beam interaction area.
These charges are distributed on a limited number of metal powder particles during a highly dynamic physical process.
As the charge of metal powder particles continues, there is a limit on the charge density, where the gravity between charged particles can overcome the gravity that usually holds them in the powder bed.
When this happens, the result is a quick re-
Distribution of a large number of powder particles in the powder bed.
This in turn increases the risk of system failure due to powder contamination of the electron gun.
Traditionally, this effect is avoided by continuously increasing the power of the electron beam, while rapidly scanning the electron beam on the surface to promote the pre-
Sintering of molten powder, thereby increasing the conductivity of the powder bed, thus avoiding the re-sintering of the powder
Distribution when fusion is started.
The R & D project in response to this issue developed a \"controlled vacuum\" function that allows inert gas to control leakage to the vacuum Construction Chamber of the EBM system.
When the inert gas interacts with the electron beam in the powder bed, positive ions are generated.
These ions undergo an attractive force that scales the strength of the powder particles with negative electricity, and the strength of the force with the amount of charge of the powder particles.
Therefore, when ions come into contact with the surface of the powder bed, they are accelerated to the surface and negative electricity is eliminated.
This is a repetitive process because the inert gas atoms produced can go through the same ionization and powder bed charge disappearance process over and over again.
Since this feature is introduced on the EBM system, this not only increases productivity, but also reduces the demand for time consumption and gradually increases the intensity of the electron beam, but it also provides an expanded process window, which greatly increases production. 5.
Although EBM has made some strong improvements in promoting serial production, it will focus on further improvements in the future.
Projects to improve the beam are in progress
Material Interaction and electron beam scanning strategies.
Another area where EBM can still be improved is process monitoring.
Since EBM is a hot process, thermodynamics strongly affects the quality of the process in decision-making strategies. IR-
Camera integration opens the door for advanced layers
Check the heat distribution and defect control wisely during construction.
The extended process monitoring will further effectively improve the capability of statistical process control, making EBM a more mature serial production technology.
As EBM is also about to become a manufacturing technology for turbine blades of commercial aircraft engines, the integration of extended process monitoring and statistical process control will be the most important.
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