Critical Care in Dispensing

Originally Published MDDI May 2003

COVER STORY: Pumps and Valves

Recent advances in pump technology can optimize the delivery of adhesives in medical device assembly applications.

by Jerry Drake

Today's medical device assembly applications require top-quality performance from the adhesives and application equipment involved in the manufacturing process. More than that, the entire package, including the substrate and surface treatment, requires that critical care be taken during the entire assembly process.
To achieve optimum performance from an adhesive, device makers must create partnerships with both the adhesive manufacturer and the dispensing equipment supplier. It may also be necessary to enlist the aid of automation or integration experts to ensure a firm “handshake” between all components of the assembly system being used.

Consider the Application

A careful analysis of the application must be made to determine the parameters upon which the entire process will be based. It has not been uncommon to see an engineer examine other products thought to have similar assembly needs. After asking a few questions, the same adhesive used to join, pot, or otherwise assemble an existing device was selected for use in a totally different scenario. Sometimes this was satisfactory but in other cases, variations in thermal stresses, strain, or durability, among other factors, were substantially different and the adhesive failed. It was not because it was a poor adhesive. Rather, the adhesive had been misapplied. 

This is where a partnership with the adhesive manufacturer or supplier is critical. Let the supplier earn your business by providing you with test data and samples to prove the viability of its adhesive selection.

Although cost is a key consideration, it must take a back seat to the performance characteristics of the adhesive. Certainly one of the driving factors will be production speed. This will govern many of the decisions to be made in fulfilling the process definition. Can the rate of production be achieved with simple manual techniques, or will more-elaborate automated work cells be required? The production speed can ultimately dictate the type of adhesive, the type of dispenser, and the postcure handling that must be employed.

Environmental Factors

Medical assembly applications typically require certain degrees of cleanliness to be maintained in assembly areas. The level of cleanliness is designated by a classification number defined in Federal Standard 209E.

For example, cleanrooms are identified by a class number such as Class 10,000. This indicates the maximum particulate level, measured in parts per million, that is allowed within the cleanroom.

An air-filtration system designed to meet the required particulate-level limits must be installed and monitored on a continuous basis to ensure compliance. In addition, worker clothing, eye protection, and facial coverings are generally required to avoid contaminating the assembly area. Temperature and humidity levels are also tightly controlled to minimize or eliminate the potential for product failure. 

Dispensing equipment may be powered by pneumatic cylinders or electric drives. In the case of pneumatic systems, care must be exercised in exhausting air from the cylinder to prevent lubricating oils or other contaminants from having a negative impact on assembly area air.

Dispensing Equipment Selection

Key to selection of a suitable dispensing device for a medical application is having knowledge of every aspect of the material to be dispensed. Also, rules governing the construction and cleanability of such devices must be observed. 

Compliance with U.S. Pharmacopeia Class VI may be called for. Meeting Class VI standards, for example, requires use of low-carbon stainless-steel components, as well as other construction materials that will withstand sterilization by irradiation after the device is assembled.

Other factors affecting valve or pump selection are the dispense pattern to be used (such as beads or dots) and 
the speed at which the adhesive must be delivered. There are numerous physical properties of these materials that can steer one toward or away from a particular type of dispenser.

Filler content present in the material can lead to viscosity levels that make it difficult to move or deliver through various sizes of hoses, tubing, needles, or check valves. Filler particle size will present similar challenges and, depending on whether they are crystalline or amorphous, the particles can range from nonabrasive to very abrasive. The hardness of the filler, as measured by the Moh's scale from 1 to 10 (10 being diamond and 1 being talc), greatly influences the construction materials used for a pump or valve. 

In addition, corrosive properties must be known for metal and seal selection. Material viscosity often necessitates the use of a particular style of dispenser. For example, a simple time-and-pressure valve with disposable pinch tubing may be satisfactory for dispensing various low-viscosity (<5000 cp) fluids or adhesives. The disposable tubing is especially well suited for maintaining cleanliness in medical applications.

High-viscosity materials (>5000 cp) may work well with rod-displacement-type pumps or valves; however, this style then presents more of a cleaning challenge. Other dispense valves have been designed with the medical market in mind. Features such as long-wearing components, no threaded fittings that can trap material or harbor bacteria, and ease of assembly and disassembly for cleaning are incorporated in systems intended for medical applications.

From Cans, Pails, and Drums to Medical Parts

The materials to be dispensed may be supplied in any of a number of shipping containers. Some present only minor difficulties when the material is transferred to the dispenser and from there to the part.

Others may present some of the most perplexing problems imaginable. Filler can settle out of suspension, requiring the material to be stirred or agitated before and after it is removed from the container. Protection of the material with an inert gas such as nitrogen or argon to prevent contamination by moisture may be required, while in other cases, a simple desiccant device may do the trick. Urethane technology may have such requirements for use. In addition, certain amine curing agents for epoxies, as well as certain silicone formulations, may have similar constraints.

Pump Evolution 

Seven basic pump types are typically employed to process or deliver adhesive materials to device parts. These are 
designated as piston metering, rod metering, ball-check, peristaltic, gear, and progressive cavity. Each has its merits with regard to the materials to be pumped. Most engineers are familiar with the more commonly used pump designs—piston, rod, gear, ball-check, and diaphragm. 

Positive-displacement piston pumps were derived from the rod-displacement-style pump, with a metal or engineered plastic piston affixed to the end of the pump shaft. Piston pumps are best suited for shot or short-duration bead applications. They have the ability to tolerate filled resins or materials far better than gear pumps, but cannot provide long-duration continuous flows. Although very accurate, this pump style is viscosity-cycle sensitive. In other words, if the material viscosity is close to water thin, the pump can cycle in the vicinity of 30 times per minute. Conversely, if the material is more like molasses or heavyweight oil, the cycle rate may be reduced by more than half. Piston pumps must be pressure fed or reloaded using a pressure pot or transfer pump. 

A variation of the piston-style pump has been developed, which is a metering pump that has no ball-check valves on the inlet and uses a poppet-type valve on the discharge. This pump has only one moving part, is insensitive to viscosity changes with regard to metering accuracy, and can be gravity fed with materials in excess of 40–50,000 cp. 

The ball-check style of pump is typically referred to as a double-acting ball-check pump because of its ability to pump in both directions. Ball-check pumps have traditionally found use as a means for transferring fluids from one point to another. Although they can cycle rapidly, these pumps were not designed to provide accurate metering, because the rate at which the check-balls unseat and seat is viscosity dependent.

Medical applications frequently use a peristaltic design because of the ready disposability of such pumps' wetted portion, which consists of various grades of flexible tubing. The pumps operate on the theory of peristalsis as is commonly seen in the human digestive system. While they are easy to clean, their durability and accuracy, with no means of in-line monitoring, somewhat limit the use of pumps of this style. 

Gear-style pumps work well for continuous-flow or bead applications—barring the use of filled materials. Fillers reduce gear life, which can affect pump accuracy. Not all gear pumps possess the same degree of accuracy owing to the slippage factor. The thinness of the material to be metered, as well as open tolerances between gear teeth and back pressure, can adversely affect the repeatability of this pump style. Certain close-tolerance gear pumps, while costly, provide a high degree of accuracy. Other, more-open designs can have significantly less accuracy (see Figure 1). 

Back pressure caused by in-line valves, mixers, or hose restrictions can influence pump accuracy unless continuous-loop recirculation with pressure-balancing adjustments is provided. The recent revival of an older metering principle has been incorporated into the design of the progressive-cavity pump. This pump features a lobed rotor, which rotates within the walls of a stator seated within the pump housing. The rotor, made of any one of several different metals and driven by electrical drive motors, turns at a controlled speed, moving material longitudinally through the stator. The stator is generally 
constructed of an engineered plastic or rubber material selected for its resistance to attack by certain ingredients, such as solvents, found in the material. This style of pump is well suited to handling high-viscosity, abrasive-filled materials, and doing so with a good degree of accuracy in a continuous mode. This same pump technology has been reduced in scale and utilized in 
dispensing-valve design. 

Hybrid Meter/Mix Systems

By combining pump styles in a single machine frame and using state-of-the-art electronic drives and controls, an advanced metering, mixing, and dispensing system can be developed. Such a system enables a high-viscosity, abrasive resin to be accurately metered with a progressive-cavity pump on a continuous basis while a close-tolerance gear pump made of stainless steel meters the necessary water-thin catalyst.

Benefits of Robotics 

Smaller, faster, smarter all describe where today's medical device assembly programs are headed. Some applications may never be able to make use of robots or work cell hardware. Where such systems can be implemented, however, they can yield significant savings in production costs in the form of reject reduction and increased throughput. Robot-mounted dispense heads can achieve a high degree of accuracy and increase production for bead and shot applications down to 0.005 cc or less.

Conclusion

Recent advances in pump technologies have given rise to a new generation of adhesive dispensing systems for medical device applications. Manufacturers still struggling with equipment problems should perhaps consider the evolution occurring in the industry. 

Copyright ©2003 Medical Device & Diagnostic Industry