Tolerance Stack-Up Analysis in Medical Device Packaging
Design requirements play a significant role in facilitating the functionality of a medical device package, whether meant for device containment, retention, or protection. The question posed here is a very basic one: How do you ensure that the produced component will meet the design requirement in the long term?
The intent of designing a "good" medical device package is not met in entirety by meeting all functional requirements. In my article, Packaging Critical Dimensions – Afterthoughts or Necessary?, I mentioned the importance of critical dimensions. In the same article, I stated considerations for focusing on tolerance and tolerance stack-up.
Providing a solid foundation for confidence in critical dimensions can be achieved by tolerance stack-up analysis. This short piece is not an attempt to create an all-inclusive discussion about best methods of performing tolerance stack-up, but to create awareness and discuss couple of popular methods for performing the analysis.
The goal of performing the tolerance stack-up analysis is to understand the overall variation of a given packaging system to determine whether the fit-functionality of the design will exist at each end of the spectrum, low and high. Before an accurate tolerance stack-up can be performed, it is very important for us to understand supplier process tolerances.
The tolerance for any given manufacture will vary depending on the material, equipment, and production process. Certain processes such as injection molding may have tolerance as low as 0.002 inches while processes such as die-cutting may be as high as 0.25 inches. Depending on the "know-how" that exists with your supplier you may already have a pretty good idea of the process tolerance ahead of time.
In cases where I have struggled to understand the tolerance for an existing process at a supplier, I have requested quality assurance lot release measurement data and performed simple statistical calculations (distribution analysis, standard deviation etc.) to get a rough idea for the process tolerance. For a new technological process, you may have to perform process development and run feasibility validations to determine the process tolerance. Either way, knowing what the process tolerance ahead of time is quite essential.
The analysis may be performed in many ways, the most common methods being worst-case analysis and statistical analysis. For purposes of a focused discussion, we will start with the simple worst-case analysis.
The worst-case analysis entails summing up the worst-case dimensions, at lowest and highest, to determine whether the design intent (desired fit) can be met.
Let's take a simple example of vacuumed formed tray that needs to be packaged in a shelf carton and in a corrugated shipper. The design intent here is that each component must fit within the other without any interference, some clearance is acceptable. Is it correct to assume that designing the tray, shelf carton, and shipper with the same dimensions will allow for each component to fit without interference?
Yes, maybe in a world where process variation does not exist. But that is not the case and therefor the answer is No. Consider the following:
- Tray dimensions (outer dimensions) > L = 5.00±0.040 inches, W = 3.00±0.040 inches, D = 1.00±0.020 inches
- Carton dimensions (inner dimensions) > L = 5.00±0.125 inches, W = 3.00±0.125 inches, D = 1.00±0.125 inches
- Shipper dimensions (inner dimensions) > L = 5.00±0.250 inches, W = 3.00±0.250 inches, D = 1.00±0.250 inches
- Tray at largest worst-case -- 5.40 x 3.40 x 1.20 inches
- Carton at smallest worst-case -- 4.875 x 2.875 x 0.875 inches
- Carton at largest worst-case -- 5.125 x 3.125x 1.125 inches
- Shipper at smallest worst-case -- 4.750 x 2.750 x 0.750 inches
A quick glance clearly indicates that the tray at its worst-case largest will not fit in the carton and therefore the carton will not fit in the shipper. It is apparent that for the smallest worst-case, the carton dimensions should be able to accommodate for largest worst-case tray dimensions. Similarly for the smallest worst-case, the shipper should accommodate for carton largest worst-case.
The example presented is rather a simple one but can be applied to the most complex packaging systems to avoid design conflicts. A simple way of avoiding such a conflict is to design the packaging system inside out staying clear of tolerance stack-up issues. In many instances, I personally do not finalize the outer component dimensions before finalizing and validating the inner component dimensions.
As I mentioned, there are multiple methods of performing tolerance stack-up and another popular way is to use statistics. I will discuss more in my next post about tolerance stack-up. I am interested to know if you consider tolerance stack-up analysis during the design and development process.
Abhishek Gautam, Packaging R&D Manager, ConMed