A device’s functionality is central to a successful product launch. However, other design requirements must perform equally well, such as how an enclosure satisfies an 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 incur significant expense, production delay, and redesign efforts to remedy and ensure compliance with industry standards, such as with the ingress protection (IP) code of IEC 60529.
Ideally, with a thorough product development process, this type of failure is identified and adequately vetted well ahead of manufacturing.
Water Ingress Challenge: Redesign to Solve IPX7 Waterproofing Reliability Issues
One of our customers brought us a challenge to solve a recurring waterproofing reliability issue with their handheld medical device.
Although they had a specification for IPX7 fluid ingress protection, their original design had several manufacturing and field failures. Vacuum leak testing by the manufacturer produced periods of reduced first-pass yield, resulting in excessive rework and scrap costs.
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.
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?
Let’s look at several terms to understand the mechanical design of a gasket seal used in this type of waterproof enclosure.
Design Terminology
| Design Term | Fig. Ref. | Function | Design Note |
|---|---|---|---|
| Gasket | – | A mechanical seal that fills the space between two objects | This seal prevents leakage between two objects. |
| Gland Depth | A | Distance between the two faces that are compressing a gasket or o-ring; in the direction of compression | |
| Groove Width | B | Distance between the faces that contain the gasket; in the direction normal to the direction of compression | |
| Gland Fill | – | Percentage of the gland that is occupied by the gasket | This percentage is calculated by taking the cross-sectional area of the gasket divided by the cross-sectional area of the gland. |
| Face Seal (Axial Seal) | C | 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) | B | 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. |
Mechanical Design Analysis of the Leaky Gasket Seal
We started our analysis with one of the critical areas for water sealing: the gasket between the front and rear housings.
Our customer’s design employed a face seal using a gasket with a round cross-section diameter of 1.2mm. However, 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.
Additionally, 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 the existing gasket did not have adequate tolerance to consistently meet the client’s IPX7 waterproofing specification.
Failure Modes Analysis
Next, as we began our analysis to redesign the gasket, we examined the potential failure modes to identify how the existing face seal design could 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 rule of thumb is that the gasket cross section area should not exceed 95% of the gland cross section area at worst-case conditions. |
Gland Fill Percentage Constraint
Next, we examined the gland fill percentage of 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, parts are never perfect. So, we 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. 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, making the gland significantly wider was impossible. 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
Clearly, we needed to change from a circular cross-section to a design that allows more margin in the axial (gland depth) direction. Critical to its success, it would need to work 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 to deliver as much compression as possible for a given clamping force.
| Design Consideration | Redesign to Improve the Gasket Seal | Performance Objective |
|---|---|---|
| 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. For example: > 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. |
| Mechanical advantage | 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. |
| Stability | 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. |
Clearly, we needed to change from a circular cross-section to a design that allows more margin in the axial (gland depth) direction. Critical to its success, it would need to work without taking up more space in the radial (groove width) direction.
Material Solutions: Design Accuracy through FEA Simulations
Our solution required further analysis to develop an improved gasket and gland shape to resolve the IPX7 waterproof failures. To achieve this, we regularly conducted a nonlinear finite element analysis. FEA enabled us to explore how design decisions impacted the parts’ forces, deformation, and stresses.
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. 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. It provides 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. This is better represented by using nonlinear FEA instead of linear FEA. |
Redesigned Gasket FEA Simulation
Solution for IPX7 Waterproofing: Hybrid Gasket Seal with Axial Compression & Radial Support
After a thorough process of mechanical design review, failure analysis, and material stress simulations, we decided on a hybrid shape (shown below). Our solution was a gasket that was 1.5mm wide (just a little bit wider than the old 1.2mm diameter circular cross-section) and 4.35mm tall (about 3.6 times the height).
The 50° angle interface between the gasket and rear enclosure creates a hybrid axial-radial seal. Clamping the case screws directly creates one part of the compression force (axial). Then, the case part assembly creates another compression force (radial).
Adding the relief points allowed the material to deflect into the relief area when compressing the gasket. The relief points kept the required clamping force low. Additionally, they created a focused contact area for the remaining material to press against in the rear case groove.
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.
- Stereolithography allowed us to produce parts with smooth surfaces on the gasket contact areas.
- Carbon DLSTM allowed for the direct printing of elastomeric parts with properties similar to silicone rubber.
3D-Printed, End-Use Quality Parts Pass Submersion Tests
We used testable 3D-printed parts to confirm our redesign solution. Even though end-use quality parts are not as good as production parts, they passed enough submersion tests to give us the confidence to proceed with production tooling. Ultimately, to everyone’s satisfaction, the redesign successfully passed our client’s IPX7 waterproofing requirement.
Solutions through Mechanical Design, Analysis, & Testing
The solution to this design challenge required a comprehensive mechanical design review of the materials.
We gained insights into design profiles and performed rapid prototyping to produce testable parts.
Our team also conducted a Finite Element Analysis to build accurate linear and nonlinear material simulations to vet design solutions.
How can we help solve your design challenge?
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?
Product innovators turn to AC for end-to-end product development and individual design challenges.
AC engineers help you:
- 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
What challenges are holding you back? Let’s talk.
