Sealing a solution

Seals are some of the most important components in many medical devices. While small in cost, seals have a profound effect on the function of a medical device and the outcome of a medical procedure.

Engineered sealing solutions have advanced to meet the new medical device designs due to new materials and to new processes for producing seals. Understanding the fundamentals of seal design, the tools available to assist in manufacturing, and pitfalls to avoid will help in achieve a successful seal and medical device outcome.
 

Classifying seal designs

When approaching a new seal design, it’s important to classify the seal based on its intended function. All seals fall into one of three distinct groups. While certain applications may combine more than one group, one is always dominant. The three basic seal designs are:

• Static – seal applications where there is no movement
• Reciprocating – seal applications where there is linear motion
• Rotary – seal applications where there is rotation

Static seal applications are the most common and include those that prevent fluids and drugs from escaping into or out of a medical device. The seal design can range from basic O-rings to complex shapes. Static seals can be found in the broadest range of medical devices from pumps and blood separators to oxygen concentrators.

A reciprocating seal application with linear motion would include endoscopes that require trocar seals. These trocar seals are complex in design and allow the surgeon to insert and manipulate instruments to accomplish the medical procedure. All of these minimally invasive surgeries employ endoscopes with seals that rely on seal stretch, durability, and the ability to retain shape during lengthy and arduous procedures. This particular seal application combines both reciprocating and rotary motion with the main function being linear motion.

A rotary seal application most commonly includes O-rings used to seal rotating shafts with the turning shaft passing through the inside dimension of the O-ring. Systems utilizing motors, such as various types of scanning systems, require rotary seals, but there are many non-motorized applications that also require these types of seals. The most important consideration in designing a rotary seal is the frictional heat buildup, with stretch, squeeze, and application temperature limits also being important.
 

Function of design, environment

What is the function of the seal? It is important to identify specifically if the design must seal a fluid and be impermeable to a particular fluid or if the seal will transmit a fluid or gas, transmit energy, absorb energy, or provide structural support of other components in the device. All of these factors and combinations need to be thoroughly examined and understood to arrive at successful seal design.

In what environment will a seal operate? Water, chemicals, and solvents can cause shrinkage and deformation of a seal. It is important to identify the short and long term effects of all environmental factors including oxygen, ozone, sunlight, and alternating effects of wet and dry situations. Equally important are the effects of constant pressure, changing pressure cycle, and dynamic stress that can cause potential seal deformation.

There are temperature limits in which a seal will function properly. Depending on the seal material and design, a rotary shaft seal generally would be limited to an operating temperature between -30°F and +225°F. To generalize even further, the ideal operating temperature for most seals is room temperature.
 

Expected seal life

What is a reasonable life expectancy for a particular seal? To determine a logical answer, one must also determine factors such as stretch before breaking (high ultimate elongation). High modulus or resistance to deformation condition to estimate. Seal squeeze is another factor to consider for both function and life expectancy. For most rotational applications, O-ring squeeze should be kept to as little as 0.002" using an O-ring outer diameter at least 5% larger than the gland. The less squeeze applied minimizes potential heat build-up and prolongs seal life.

Another factor to consider is resistance to set under extensive loads. In addition, dimensional changes over time, and embrittlement in the presence of heat or fluids, can impact performance and seal life.
 

Interrelated factors

All three seal types are subject to multiple factors with interrelationships becoming important and often quite complex. Any combination of the factors discussed above will affect performance and can be influenced even more by such conditions as the surface finish on metal parts, use or absence of lubrication, pressure, shock and reciprocating loads within the system, and system cycling speed.

Because of these complex interrelationships, it is important to seek out experienced help when designing a new medical seal application. Successful seal design is a constantly evolving technology with many tradeoffs and innovations.
 

Material selection

In approaching a new design, material selection is key to product performance. There is no substitute for experience when making compound evaluations, and both custom molders and material suppliers can provide invaluable help early in the design process.

Dozens of compounds are available for a seal design, not all of which are FDA compliant. All compounds are identified by three classifications. The first is by chemical term, second by an ASTM designated abbreviation, and third by a polymer trade name. An example of these three descriptions is the widely used compound, silicone. Silicone is the material’s chemical name; its ASTM designated abbreviation is VMQ, PMQ, and PVMQ, and its trade name among others are Plioflex and Stereon (see chart above.)

The many other available seal materials, such as ethyhlene acrylic and polyubutadiene, are similarly identified and classified. All compounds can be modified with the addition of other materials and/or with changes in the molding and manufacturing process to enhance a particular desired feature.

Certain compounds that are USP Class VI, ISO 10993, and FDA compliant may be required for specific medical applications. The supplier chosen for a particular seal should be certified to supply that compound to qualify for a particular seal project. Typical medical seals that require ISO 10993 and FDA compliant compounds for medical applications include medical valves, medical pumps, medical connectors, diaphragms, plunger tips, medical disposables, lab equipment, medical diagnostic products, and surgical instruments.
 

Tools to use

One of the most important tools for designers is the use of finite element analysis (FEA). Unlike frictional analysis, where everything must be empirically tested, FEA can accurately predict deformation and ultimate failure of a material. Although FEA is a common tool, it is most often used in the analysis of stiff materials such as metal or plastics. Seal applications are different. Seal designs use rubber where extreme elongation, deformation, and bounce back are the most important element of the part design.

This requires the use of a special type of FEA called non-linear FEA, where the seal designer creates a series of iterative seal designs that can be tested quickly. A typical FEA output appears as a video. The seal, its housing, and the instrument are all simulated to show what will happen to the seal in a working assembly. After a series of iterations are tested with FEA, it is customary to confirm the final output with prototype seals for further evaluation.

Minnesota Rubber and Plastics
www.mnrubber.com

About the author: Ted Ahrenholtz is the engineering design manager at Minnesota Rubber and Plastics and can be reached at tahrenholtz@mnrubber.com or 952.927.1400.

Part 2 of Sealing a Solution will be in the April issue of TMD, covering testing, seal differences, and successful applications.