Medical devices frequently require coatings that can range from lubricants that keep parts moving smoothly, to reagents on point-of-care test strips, to coatings on membranes and non-woven materials. Common coating methods include dipping, airbrushes and spray valves. While these methods can produce acceptable results, device manufacturers often encounter one or more of the following problems:
Inconsistent process control. Coating placement, volume and thickness are critical on many devices. Manual processes or spray systems with coarse adjustments may not provide the controlled, consistent coverage needed to achieve predictable process results and reliable device performance.
Slow production speeds. When prototypes or short production runs are made in a laboratory or R&D environment, extra time can be spent fine-tuning the coating equipment to produce acceptable results. This is not the case in high-volume production environments, where coating processes need to be fast, precise and consistent.
High reject rates. Uncoated or partially coated areas, variations in coating thickness, or overspray onto nearby components can all result in costly rejects, lower yields and reduced profitability.
Waste and downtime. Over-application of expensive materials can increase production costs and require extra cleanup time. Standard coating systems can be difficult to set up and adjust, may not produce uniform results on every device, and often contain seals and O-rings that wear out and leak. With many spray systems, frequent clogging can also be a problem. Any of these issues can result in time-consuming, non-value-added activity.
LVLP coating systems
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Low volume low pressure (LVLP) systems, as seen in Figure 1, can provide an accurate, precise, and cost-effective coating method with the flexibility to handle many different coating materials and viscosities.
LVLP coating systems typically consist of a precision spray valve, a microprocessor-based valve controller and a fluid reservoir. They operate on compressed air and electricity, and regulate the amount of material applied to the substrate through a combination of valve open time, fluid pressure and stroke setting.
The pneumatic spray valve has two separate air inputs – one for valve actuation, and one for fluid application. A low fluid flow rate is used, so that very low air pressure (typically just 1–3psi) is needed to apply the fluid as a soft, controlled spray. This approach ensures high transfer efficiency (the amount of material deposited on the substrate, compared to the amount of material leaving the nozzle) and minimises the possibility of overspray or mist. Different spray
patterns can be produced by changing the valve nozzle and air cap.
The valve controller simplifies valve setup, regulates valve operation and permits nozzle pressure to be adjusted for optimal results with different coating materials.
During system operation, actuating air lifts a piston inside the valve, retracting the needle from the nozzle seat and allowing fluid to flow to the nozzle. Low-pressure air is simultaneously forced through an annulus around the nozzle, creating a pressure drop that causes the fluid to break into fine droplets.
At the end of the dispensing cycle the actuating air is shut off and the needle moves down into the nozzle seat to stop the flow of fluid. Nozzle air is then applied for a few milliseconds longer to ensure a clean cutoff and prevent clogging between shots.
The high transfer efficiency of LVLP coating systems ensures that material is applied only where it is needed, without waste or unwanted application on surrounding areas. Valve operation can be either timed or continuous. For timed application, spray time can be adjusted in increments as small as 0.001 seconds. In addition, nozzles are available in a variety of configurations to accommodate different substrates and spray pattern requirements.
A new spin on cylinder coating
Many medical devices contain cylindrical components whose interiors need to be coated with silicone oil or other lubricants to ensure reliable operation. The tendency of these fluids to creep or 'migrate' makes accurate placement and controlled application especially important.
The low volume low pressure systems described above are a good choice for most coating applications, due to their tightly controlled spray patterns and high transfer efficiency. Some devices, however, require material to be kept within a more well-defined area than even the tightest spray pattern can produce.
Radial spinner systems, as shown in Figure 2, are well suited to this type of application because they are capable of applying low-to medium-viscosity fluids as neat, controlled bands (rather than a continuous coating) on the interior walls of cylinders from 0.4in to 5in in diameter.
These systems include an air motor that drives a small, shaft-mounted spinner disk, a precision dispense valve fitted with an angled stainless steel dispensing tip, and a microprocessor-based controller that regulates the operation of the motor and the valve.
The spinner/disk assembly consists of a small tapered disk on the end of a shaft secured in a chuck on the air motor. The dispensing tip has a 30° bend that positions the tip orifice just above the surface.
Dispensing needles and disks are available in several configurations to accommodate different fluid viscosities and cylinder diameters.
With the air motor and dispense valve fixtured in a machined bracket, the air motor is activated as the valve dispenses a precisely metered amount of material onto the spinning disk. When the fluid reaches the edge of the disk, it spins off to form a neat band on the inside wall of the cylinder. Additional bands can be applied by raising or lowering the disk within the cylinder. The system can be used on automated lines as well as benchtop processes in which the operator manually presents the cylinder for coating.
Dedicated valve controllers
While dispensing valves may be a very small component of an overall assembly line, they can have a very big effect on performance and productivity. Choosing the right valve technology for a particular coating application will increase yields, reduce rejects and lower overall assembly costs.
Once the correct valve has been selected, the best and easiest way to integrate it into the assembly line and achieve maximum performance is with a dedicated valve controller that simplifies valve setup and operation.
Manufacturing personnel new to automated dispensing often question why a dedicated valve controller is recommended when the valve could be actuated by indexing or some other mechanical means, or linked to a programmable logic controller (PLC). The simplest explanation is that a microprocessor-based valve controller provides the most accurate, efficient way to adjust valve open time and puts this capability where it needs to be – at the dispensing station. By supplementing rather than replacing a PLC, a valve controller provides several benefits:
Precise control of deposit volume. A valve controller provides an easy starting point for determining shot volume and requires less programming than a PLC. If valve open time needs to be increased or decreased after the initial deposit size has been established, the user can simply scroll up or down in 0.1, 0.01, or 0.001 second increments. Once the desired open time is known, it can be entered directly the next time that particular device is produced.
Higher output with less downtime. Most automatic assembly machines use a PLC to sequence machine functions, but in many cases the PLC will not have on-line, 'on-the-fly' valve programming capability. Even if a PLC does have this feature, the valve may not be within sight of the person attempting to make the adjustment, making it necessary for them to travel back and forth between the PLC and the dispensing station. A dedicated controller installed near the valve makes it easy to perform any necessary adjustments and observe the results immediately, thereby eliminating the need to shut down the assembly line.
Greater convenience and efficiency. A controller with a purge function provides an easy way to clear the dispensing system of any air introduced while filling the fluid reservoir. Purging is best done at the valve, where the operator can confirm that all air bubbles have been removed.
A controller with an end-of-cycle feedback circuit and I/O connector will permit easy interfacing with a PLC, and is useful for assembly processes where it is necessary to detect the end of a dispensing cycle. In these situations the feedback circuit can be used to signal back to the PLC, start another device in sequence, or initiate other operations dependent on completion of the dispense cycle.
This range of innovative new coating dispensers offers device manufacturers a more simple, cost-effective way of preparing their products for the market. Improved performance means increased sales and a better reputation, while also allowing manufacturers to focus on continued innovation.