Four Critical Operations in Custom Rubber Molding for Miniature Pneumatics Applications

Medical devices are in the forefront of the use of miniature pneumatic parts made from custom rubber molding, although other industries, such as process control, are also incorporating miniature pneumatics. For portable medical devices, parts such as miniature pneumatic solenoid valves are used where space and power are limited.

According to a white paper from the National Fluid Power Association, “Fluid power technology is adapting to the latest trends in medical device design by focusing on lightweight, portable, low power solutions and more precise control. Reducing power consumption, particularly for battery-operated equipment, has become an important factor, along with the ability to leverage more precise control and innovations in electro-pneumatic control.”

• Anesthesia
• Ventilators
• Hematology
• Urinalysis
• Laboratory and analytical instruments
• Miniature pumps
• Dental equipment
• Air logic and control
• Gas detection
• HVAC systems
• Test equipment
• Process control

• Integrated components of rubber bonded to metal or plastic inserts
• Poppets
• Valves
• Seats
• Diaphragms
• Gaskets
• Check valves

Miniature pneumatic parts must hold up under punishing environmental conditions, such as contact with hydrocarbons and other chemicals, high pressures and extreme temperatures. As a result, the materials that can be used in custom rubber molding for miniature pneumatics are typically limited to three families of elastomers: fluorocarbons, silicones and EPDM.

Fluorocarbons (Viton® and other FKMs)

• Excellent chemical resistance to anesthesia gases and other hydrocarbons
• Good durability and toughness with a long history in sealing applications
• Excellent high temperature resistance to + 400 to 450F
• General purpose fluorocarbons are not as strong at low temperatures but will function at temperatures as low as – 20F
• Specialty and exotic FKMs can be compounded to perform in temperatures as low as – 40F but can be considerably more expensive

Silicones

• Excellent high temperature resistance to + 400 to 450F
• Excellent low temperature resistance to as low as – 60F
• Poor resistance to hydrocarbons and noxious chemicals and may swell in those environments, but can be used in applications involving air
• Can be compounded to remain soft, which allows seals to work with a very low amount of force or pressure
• Wears well but has poor tear and abrasion resistance

EPDM

• Temperature resistance ranging from – 60F to + 300 F
• Poor resistance to hydrocarbons
• Can be compounded to remain soft, which allows seals to work with a very low amount of force or pressure
• Tougher than silicone

With a temperature range of – 55F to + 225 F, nitriles (NBR) are also used, although less frequently, for miniature pneumatic applications that do not require extremely high temperature resistance or excessive hydrocarbon resistance. Nitriles have the advantage of a relatively low cost.

Additives, such as Teflon, molybdenum disulfide (MoS2 ) or graphite, may be used to address issues such as tackiness and the co-efficient of friction of a given rubber.

Custom rubber molding firms develop proprietary compounds of the proper elastomer and necessary additives for the application at hand and provide in-house mixing and compounding to ensure material quality and consistency.

Components for miniature pneumatics are both small and complex, requiring a custom rubber molder to be able to design and produce the precision molding tools that can handle extremely tight tolerances. The rubber molder also must have the technology to control and meter small amounts of rubber during the molding process.

Integrated components of rubber bonded to metal and rubber bonded to plastic are frequently used in miniature pneumatics. Rubber may be bonded to inserts made of specialty metals. For example, 430 FR steel has magnetic properties that allow it to be used in devices actuated by electro-magnetic induction. Rubber bonded to plastic parts are often used in applications that need to be lighter weight, such as portable devices.

For more information on the bonding process in custom rubber molding, please see our earlier blog article: Six steps to ensuring bond strength in rubber to metal bonding.

In miniature pneumatics, molded rubber is often used as a seal. Typically, the goal is to create a seal that will hold at the lowest sealing force. To do so, the molded rubber part often must be finished to create a very true, flat surface and to improve the surface texture for better sealing.

For some applications, the rubber molder can achieve the right surface in the precision molding tool itself via a highly polished cavity surface. In other applications, secondary operations for finishing after molding are required, including grinding and lapping. Grinding involves applying an abrasive such as sandpaper directly to the surface of the molded part. Lapping is a more elaborate process involving a steel turntable and a diamond slurry upon which the molded part rotates, which provides an extremely true, flat seal.

Once the part has been finished, the molder will clean the part as needed to remove contaminants, with cleaning methods ranging from water and a mild surfactant to ultrasonic cleaning.

Custom rubber molding firms rely on high-magnification inspection of parts to ensure quality. Often, manual inspection is more effective than automatic inspection technology, as inspectors are trained to identify defects in line with the client’s specifications, while automated technology primarily tracks measurements.

Depending on the client’s requirements, the molder’s team can inspect 100% of parts or conduct inspections on selected samples. The rubber molding technical team will establish the appropriate inspection plan with the client upfront.

Because miniature pneumatic parts operate in difficult and even dangerous environments, ensuring the quality and durability of the custom rubber molded part helps to ensure the safety of the end product and the safety of the end-user as well.

Viton® is a registered trademark of DuPont.