Tech Briefs
Gaskets

A variety of edge profiles are available. The standard edge that can be achieved with Select-a-Seal is shown in green. The wrapped edge is shown in red, and the intruded edge is shown in blue.

Agasket can be defined as a material, combination of materials, or a device used to create and maintain an impervious barrier against the transfer of fluids across the separable surfaces of a mechanical assembly (joint) that do not move relative to one another. Over the past 100 years, gasket materials have ranged from simple paper and sophisticated polymers to liquid sealant applied by robots. Continued research in the area of gaskets has brought about a new family of gaskets know as Select-a-Seal from Interface Solutions, Inc.

According to Louis J. D'Annibale, Executive Vice President, Sales and Marketing, the new gasket uses a three-part barrier against leaks:

• Polymer edge - A custom engineered polymer edge that is taller than the thickness of the carrier material is the first barrier against leaks. The edge conforms to flange irregularities when compressed, which results in permeation resistance to fluids and gases.
• Intrusion zone - The polymer edge is applied directly to the pores of the structural carrier and creates a narrow band of high-density material between the edge and carrier structure. This intrusion of the edge provides a high stress concentration under compression, making it an effective second sealing barrier. This also results in an edge that is strongly bonded to the structural carrier.
• Structural carrier - The third barrier is the structural barrier that is responsible for locating and adapting the polymer edge to the joint and carrying the compressive and dynamic loads created by the joint. The carrier provides compressive strength, shear strength, and high surface friction resulting in long-term durability. Dr. Shelly N. Garman, Executive Vice President, Interface Solutions, explained that one of the interesting parts of these new gaskets is that they employ a material science solution using the attributes of a custom engineered polymer for the sealing edge. The manufacturing process is such that a stack of gaskets is placed into a machine that rotates the stack so that the polymer material is allowed to flow over the appropriate edges. After the polymer is cured the stack is separated into individual gaskets. The top and bottom gasket of the stack are discarded. Once separated from the stack the polymer material, no longer under compression, expands to a predetermined height above and below the structural carrier. The manufacturing process is such that three different polymers can be applied to a single gasket.

There are a number of concerns that the new gasket system design addresses:

• conforming to flange irregularities without damaging the polymer
• maintaining sufficient sealing stress after compressive stress relaxation
• accommodating shear stresses without surface abrasion or structural damage.

It addresses these concerns through a patented sealing mechanism. The edge geometry and polymer formulation allow for high compression without cracking. The geometry of the standard edge exhibits a very low spring rate at the tips of the oval, which allows it to accommodate rough and irregular flanges. When the edge is compressed to the thickness of the structural carrier, the polymer-to-carrier attachment point and intrusion zone add stiffness and generate considerable sealing force.

Compressive stress relaxation is addressed in three ways. First, the structural carrier is compressed rather than rigid. This actually improves the polymer edge compression over time. Second, the sealing mechanism of the polymer edge is a combination of sealing stress and surface adhesion to the flanges. Therefore, as stress is reduced over time, the seal mechanism is not necessarily affected. Third, the wrapped edge design concentrates the available clamping load on the polymer edge, creating sealing stresses up to 10 times higher than on typical rubber seals.

Shear stresses are accommodated by the structural carrier that can flex up to 20% of its thickness in shear without sliding on the flange surfaces. Surface abrasion and polymer-to-carrier delamination are avoided by designing the correct carrier thickness into the gasket.

There are four standard polymers currently used in the new gasket process:

• two types of polyacrylics are used with automotive and diesel engine lubricants
• SBR (styrene butadiene rubber) polymers are used with coolant, water, and low pressure steam applications
• NBR (nitrile butadiene rubber) formulations can be used with low temperature applications.

Jeffrey L. Barrall, General Manager, Strategic Business Analysis and New Market Development, Interface Solutions, explained that the new gasket system was developed to have a turnaround time of 10 days—from the time the CAD files of the mating flanges are received, a gasket can be designed, the sheets can be cut, the polymer edge can be applied, and the gasket can be tested. The Select-a-Seal production tooling costs are said to be $5000 vs. $40,000 for rubber-to-metal systems and $100,000 for rubber-edged metal systems. The design freeze lead times for rubber-coated metal is 60 days and 120 days for rubber-edged metal.

During a recent press conference at the company's Lancaster facility, editors were requested to design a gasket by sketching it on a sheet of paper. In three hours, the sketch was turned into an actual gasket with two types of polymer applied to two of the sealing surfaces. The process created more than 20 useable gaskets in less than three hours.

Dan Holt