Engineering a device’s overall functionality is always central to a successful product launch, but other design requirements must perform equally well, such as how an enclosure satisfies a IPX7 waterproofing requirement. It can be easy to forget that just one failure area, like water ingress protection, can render the overall medical device non-compliant.
A single design flaw can also incur significant expense, production delay, and redesign effort to remedy and ensure compliance with industry standards, such as with the ingress protection (IP) code of IEC 60529.
In today’s Tech TalkTM, of one our most experienced project leaders, Dan Canfield, a Principal Mechanical Engineer with the AC Product Development group, shares some details on how the team solved a water ingress design problem.
Our solution to this design challenge required a comprehensive mechanical design review of the materials, insights into design profiles, and rapid prototyping to produce testable parts. The team also leveraged Finite Element Analysis to build the most accurate linear and nonlinear material simulations to vet their designs solutions.
So, let’s dive into this product development case study and learn how the team solved this redesign challenge to achieve IPX7 waterproofing of a leaky seal.
Devices with an IPX7 rating can endure 1-meter water immersion for 30 minutes without harm, making them fully submersible.
What’s in a Gasket? | Design Terminology
As we move forward in our case study, here are a few terms to be aware of which help our understanding of gasket seal mechanical design.
|What does it mean?
|How does it work?
|A mechanical seal that fills the space between two objects
|Prevents leakage between two objects
|Distance between the two faces that are compressing a gasket or o-ring; in the direction of compression
|Distance between the faces that contain the gasket; in the direction normal to the direction of compression
|Percentage of the gland that is occupied by the gasket
|Calculated by taking the cross-sectional area of the gasket divided by the cross-sectional area of the gland
|Used for a gasket or o-ring sandwiched between two parts
|The force required to compress the gasket must be fully provided by the fasteners holding the two parts together.
|Piston Seal (Radial Seal)
|Used for a gasket or o-ring captured between parallel faces; the direction of compression is perpendicular to the assembly direction instead of parallel
|Once the parts are assembled, the force required to keep the gasket compressed is provided by the stiffness of the parts and does not apply any load to the fasteners after the parts are assembled together.
Challenge: Redesign to Solve IPX7 Waterproofing Reliability Issues
When a MedTech industry customer asked us to redesign the enclosure of their handheld medical device, they were seeing recurring reliability issues with the waterproofing performance.
Although they had specified that their device required a fluid ingress protection specification of IPX7, they had been experiencing several failures in manufacturing and in the field.
Failures Identified During Manufacturing
During the manufacturing process, a vacuum leak test checked whether the device was assembled properly with all of the gaskets in place. The vacuum leak testing produced periods of reduced First Pass Yield, resulting in excessive rework and scrap cost in the manufacturing process.
Ideally, a thorough product development process seeks to identify and adequately vet this type of failure well before the manufacturing stage.
Aware of this recurring leak test failure, we were eager to support them and develop a new design that would consistently meet their IPX7 waterproofing requirement.
Mechanical Analysis of Gasket Seal
We started by analyzing one of the critical areas for water sealing: the gasket between the front and rear housings.
The existing design employed a face seal using a gasket with a round cross section with a diameter of 1.2mm. But the gasket was placed into a groove with a gland depth of 0.9 mm. So, even under perfect conditions the gasket would be compressed by 0.3mm in the axial direction.
And, because the width of the groove was 1.4mm, it left only 0.1mm of radial clearance on each side of the gasket.
It was clear there was not adequate tolerance for the existing gasket to consistently meet the client’s IPX7 waterproofing specification.
Gasket Seal Analysis | Failure Modes
As we began our analysis to redesign the gasket, we examined the potential failure modes to identify how the existing face seal design can fail:
|Potential Failure Mode
|Potential Failure Effects
|Not enough engagement
|If the gasket is too small for the groove, or if the gasket is too soft, then the normal force between the gasket and the enclosure will be too low and the fluid can push through the seal.
|Too much engagement
|If the gasket is too large for the groove, or if the gasket is too hard, then the normal force between the gasket and the enclosure will be too high. It may be impossible for the fastening features to hold the enclosure parts together, or the enclosure parts may bend, compromising the seal in the areas.
|Too much gasket volume
|No matter how soft it is, solid gasket material is virtually incompressible. Soft gaskets can change cross-sectional shape easily, but the cross-sectional area will remain the same… if you squeeze them in one direction they will spread in the other direction. So, if the gasket spreads out against the sides of the groove as it is being compressed, the compression force will increase rapidly. For static seals, a general rule of thumb is that the gasket cross section area should not exceed 95% of the gland cross section area at worst-case conditions.
Gasket Seal Analysis | Gland Fill Percentage
Next, we needed to examine the gland fill percentage for the existing design. Nominally, this 0.9mm X 1.4mm gland and 1.2mm diameter gasket give a gland fill percentage of 90%.
But in real life, because parts are never perfect, we also needed to assign allowed tolerance ranges for the part measurements:
> If we assign a tolerance of just ±0.05mm to the diameter of the gasket and the dimensions of the groove, the maximum gland fill percentage would be 115%.
> At that calculated percentage, the gasket would be 15% larger than the gland.
> That meant we should anticipate that the case would be impossible to close completely.
Running the numbers confirmed our suspicions that we needed to reduce the maximum gland fill percentage.
Unfortunately, for a gasket with a round cross-section, our options were limited to some combination of:
> Reducing the diameter of the gasket
> Increasing the depth of the gland
> Increasing the width of the gland
Because we were constrained to use the same design envelope, it wasn’t possible to make the gland significantly wider. And reducing the gasket diameter or increasing the gland depth would cause reduced engagement and possibly compromise the seal if tolerances ran on the small side.
Design Considerations: Taller Gasket Advantages
It was clear that we needed to change from a circular cross section to a design that allowed more margin in the axial (gland depth) direction without taking up more space in the radial (groove width) direction.
We were also concerned about the pull-out strength of the screw features holding the enclosure parts together. We wanted a design that would get as much compression as possible for a given amount of clamping force.
|Redesign to Improve the Gasket Seal
|Tall aspect ratio
|A taller gasket allows for more compression distance without increasing the percentage compression.
|Keep the percent of compression low to keep the required clamping force low as well.
> Compression at 0.3mm on a 1.2mm tall gasket means it gets 25% compressed.
> But if the gasket is 3.6mm tall, then you can compress it 0.9mm with only 25% compression.
|Use geometries that have the gasket press on a ramped surface on the opposing part instead of a flat surface.
|Reduce the amount of clamping force required to create a given amount of surface pressure on the gasket interface.
|Use the groove to support the tall gasket on both sides.
|Protect the gasket from tipping over or buckling.
|Maximum gland fill percentage
|Keep the gland fill percentage below 95% in worst case conditions.
|Keep the gasket smaller than the gland to ensure case fully closes.
It was clear that we needed to change from a circular cross section to a design that allowed more margin in the axial (gland depth) direction without taking more space in the radial (gland width) direction.
Material Solutions: Design Accuracy through FEA Simulations
As we explored the options for developing an improved shape for a gasket and gland capable of resolving the IPX7 waterproof failures, our solution required further analysis.
To achieve this, we regularly used nonlinear finite element analysis to explore the impact of design decisions on the forces, deformation, and stresses experienced by the parts.
Nonlinear Finite Element Analysis is ideal for situations like this where there are large deflections and use of hyperelastic materials like silicone rubber that don’t have linear stress-strain properties.
|Linear FEA Modeling
|Nonlinear FEA Modeling
|In standard linear FEA, the stiffness matrix of the part is calculated at the beginning of the analysis and the behavior of the part is predicted using only that initial stiffness matrix.
|Nonlinear FEA applies the load or deflection in steps and recalculates the stiffness matrix at each step, providing a much more accurate model of the behavior.
|For parts with large deflections, it is not realistic to use the same linear FEA stiffness matrix as the shape of the part changes.
|Hyperelastic materials like silicone rubber also have a nonlinear relationship between stress and strain, which is better represented by using nonlinear FEA instead of linear FEA.
Solution for IPX7 Waterproofing: Hybrid Gasket Seal Delivering Axial Compression & Radial Support
After a thorough process of mechanical design review, failure analysis, and material stress simulations, we decided on the shape shown below. Our solution was a gasket that is 1.5mm wide (just a little bit wider than the old 1.2mm diameter circular cross section) and is 4.35mm tall (about 3.6 times the height).
The 50° angle interface between the gasket and the rear enclosure makes this a hybrid between an axial seal and a radial seal, meaning that part of the compression force is provided directly by clamping of the case screws (axial) and part is provided by the stiffness of the assembled case parts (radial).
We found that the relief points shown above would allow material to deflect into the relief area when the gasket is compressed, keeping the required clamping force low, and creating a focused contact area where the remaining material was pressed against the groove in the rear case.
We calculated the new gland fill percentage for the redesigned gasket seal and found that the worst case gland fill was 95%.
Validation: Prototyping & Testing with Carbon DLSTM 3D Printed Parts
Next, we needed to test parts that would achieve the IPX7 waterproofing specification. We prototyped the redesigned gasket using stereolithography case parts and gasket seals using Carbon DLSTM 3D printing.
Production Tooling from Testable, End-Use Quality Printed Parts
Once we had testable 3D printed parts, although we knew the quality wouldn’t be as good as the production parts were going to be, we were pleasantly surprised when the 3D printed parts passed some of the submersion tests. That gave us the confidence to go ahead with the production tooling, which ultimately passed the IPX7 waterproofing requirement.
How can we help solve your design challenge?
AC’s Product Development team developed a successful solution to enable the enclosure of our client’s medical device to consistently meet their IPX7 waterproofing specification.
Product innovators and MedTech industry developers turn to AC engineers for end-to-end product development. AC can also support individual design challenges encountered along the product development journey, helping to:
> Identify failure points through analysis
> Solve design flaws with improved materials selection and mechanical engineering features
> Vet product performance to specifications through appropriate testing
> Ensure compliance and V&V testing for relevant medical device standards
Are you seeing failures with your launched product and in need of a knowledgeable engineering partner to help with analysis and testing to identify the root cause?
Or perhaps you’re ready for a significant design improvement that will take your product’s performance to the next level?
We’re here to help! Let’s talk.