In part 3 of our blog series, Resin Designs explored the various types of gel seal connectors. This week, we’ll be discussing connector design considerations.
Connector Design Considerations
At Resin Designs, we help customers create new connectors with low wire insertion forces using silicone gel grommet seals. Along with this, existing connectors can be modified to accept gel seals with slight tooling changes. The goal is to ensure proper initial compression on the gel seal and held in place with latching caps to retain internal pressure within the connector.
Take a look below at an example of a simple rectangular connector with two rows of contacts.
Prior to the insertion of a contact, gel seals are pre-slit at the contact locations, re-heal and are sealed. The gel seal will re-heal again and seal the cavity once the contact has been removed. It is required that the contacts are passed through the gel on insertion and extraction. This process allows the contact geometry to have a major impact on seal performance.
Since gels re-heal quite nicely, they are not affected by features that cut through the gel. Monoblock grommets, however, do not re-heal when cut. Seal performance is affected by excess removal of gel seals. Some gel removal is anticipated on insertion and extraction but excessive removal of gels may affect performance. Because of sharp edges and scoop-like features, the contact feature that reduces the sealing effectiveness of the gel most is one that can remove a majority of gel.
Typically, a smaller cavity size is better for supporting the gel seal. Contact cavities with .6 mm and 1.5 mm contacts are generally compatible with the standard gel grommet configuration.
Contact to Contact Cavity Relationship
In gel seal designs, the fit of the contact in the contact cavity is crucial. If the frontal area of the contact is less than or equal to 55% of the cavity area, low contact insertion forces will result. Insertion force increases as the percentage increases. When the area percentage increases over 85%, it becomes possible to shear off pieces of gel during contact insertion; this degrades long-term sealing performance.
Center-to-center spacing that is accomplished in a connector design is a mixture of the contact cavity size and an insulating wall between adjacent cavities. The contact design determines the contact cavity size. The size of the insulation wall is measured by molding limitations or restrictions of the sealing system.
Since gel grommets don’t restrict the size of the insulation wall, contacts can be placed at minimum center-to-center spacing. However, there are two issues to consider.
- Thin wall sections of less than .5 mm offer support for the gel grommet.
- A very thin layer of gel is sufficient to create a seal.
Large wires can be placed on minimum spacing while still providing sufficient gel between adjacent wires to create a seal.
The recommended edge distance for a .66 mm and 1.5 mm contact is 2.0 mm. Distances with smaller edges are acceptable in connector configurations. Contacts with an edge distance over 2.0 mm do not significantly improve sealing performance.
In order to enhance sealing performance, sharp corner radii should be avoided. We recommend a minimum of .5 mm radius.
Seal Cavity Depth
The gel grommet seal must be contained and put under pressure to create a seal. The seal cavity is required at the rear of the connector in order to hold the gel seal. The seal cavity should also contain the sides of the gel seal because it is the primary sealing interface. To reduce assembly issues, the depth of the cavity should exceed the free height of the gel grommet by 0.5 mm to 1.0 mm.
The support structure is the combination of the above parameters. This can be viewed in Figure 1. As the size of the holes in the support structure increase, modulus will decrease. Alterations in the apparent modulus need changes to the final compressed height of the grommet to accomplish the initial pressure on the gel.
Gel grommet compression data is typically defined in terms of pressure. The total area of the grommet should be utilized when converting from pressure to force for any specific configuration.
Figure 1: (Length x width) – (area of the corner radii)
When put in compression, the grommet is not supported over the entire area. The modulus of the material will change as a function of the support structure.
The apparent modulus of the gel grommet is the observed stress-strain relation when the grommet is compressed in the connector between two support structures with the sides fully contained. When compressed with a different support structure, the material will appear to have a different modulus.
For applications, contact Resin Designs for the correct stress-strain curves. Figure 2 displays representative stress-strain curves for 0 .6 mm and 1.5 mm contact support geometry.
Below is a visual representation of pressure vs. compressed height data based on contact size and grommet thickness.
Initial design pressure for gel grommets will be 10 N/cm2
Within an hour, the initial pressure will decay to an equilibrium pressure around 60% of its initial value. Pressure is generally stable after that time.
In the next blog, you’ll learn about grommet compressed height, tolerances, compression cap and more. Make sure to follow our entire blog series!