Many medical devices require some type of coating, ranging from lubricants that keep parts moving smoothly to reagents on point-of-care test strips. While a device may be cutting edge, the method to coat it may be less so. This is especially true in benchtop processes, where it's common to dip parts or apply material with swabs or brushes. Aside from there being no way to guarantee the same amount of fluid is always applied to every device, manual coating methods are typically accompanied by significant rework and waste, because excess fluid must be wiped off and applicators and containers discarded when they become contaminated. If rework is too extensive, the device may end up as a reject.

A new spin on coating cylinders

Medical devices often contain cylindrical components that must be internally 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.

Low volume, low pressure (LVLP) spray valves are a good choice for most coating applications due to their controlled spray patterns and high transfer efficiency. Some devices, however, require keeping material within an area so precise, even the tightest spray patterns do not work well.

For these applications the Radial Spinner System applies neat, controlled bands of low to medium viscosity fluids to the interior walls of cylinders between 0.4 and 5-in. diameter. The system includes an air motor that drives a small, shaft-mounted spinner disk, a precision dispense valve with an angled stainless steel dispensing tip, and a microprocessor-based controller that regulates operation of the motor and valve.

The air motor and dispense valve are fixtured in a machined bracket included with the system. The spinner-disk assembly consists of a small tapered disk on the end of a shaft that is secured in a chuck on the end of the air motor. The 1.5-in. long dispensing tip has a 30-degree bend that positions it parallel to the spinner shaft, with the tip orifice just above the surface of the disk. Dispensing needles and disks are available in several sizes to accommodate a variety of fluid viscosities and cylinder widths.

Activating the air motor lets the valve dispense a precisely metered amount of material onto the spinning disk. As the fluid reaches the edge of the disk it spins off to form a neat band on the cylinder's inside wall. Additional bands can be applied by raising or lowering the disc within the cylinder. The system can be used on automated lines as well as benchtop processes in which the operator presents the cylinder for coating.

Spray patterns down to 0.0625-in.

As medical devices continue to pack more features and capabilities into smaller packages, the size of the fluid deposits needed to build them also decreases. This can make consistent application with existing tools difficult. Coating and spraying can be especially challenging, since they usually involve thinner materials applied over a wider area than adhesives, solder pastes, or other materials typically applied as dots or thin stripes.

The original 781 Series valve uses a nozzle, air cap, and LVLP air to produce spray patterns between 0.17 and 2.0-in. The 787MS-SS uses a small gauge (0.013 to 0.004-in internal diameter) dispensing tip which protrudes through the air cap. This directs the LVLP air to atomize the coating in a tight pattern, producing deposits between 0.0625 and 0.75-in. wide. A ValveMate 8040 controller controls operation, making it simple to adjust nozzle air pressure for different materials and viscosities. Digital readouts permit documenting settings for process control.

Spinner system eliminates lubrication rework

The Radial Spinner System helped a manufacturer of single-use medical devices eliminate rework and cut production time by almost 70% in a critical assembly operation. The process requires applying a food-grade silicone oil inside a 10-mm diameter plastic cylinder. A small spring-loaded piston inside the cylinder needs the air to move freely.

The maintenance technician assigned to the project says the controlled lubrication is critical for the device to work properly. “If we use too little oil, the piston will stick. If we use too much, the extra oil migrates,” he says.

Until recently, operators used swabs to apply the silicone oil inside the cylinder. The problem was that the results depended on who was doing the application. “The swabs gave us no control over the amount of lubricant we applied,” he says, “and we would sometimes have some oil on the exterior of the product. We were probably reworking 30% of whatever we made, which involved removing a press-fit cap from the end of the cylinder, taking the piston out, applying more silicone, then reassembling and testing the device all over again. Even then, we still had a few rejects.”

The device manufacturer chose the Radial Spinner System for its ability to dispense a line of fluid in a specific location. In addition, the consistent fluid volume let the company perform extensive testing to determine just the right amount of silicone needed to ensure that the device operates properly.

In the company's new setup, the operator places the cylinder in a fixture and pulls a lever to raise it under the spinner. As the cylinder moves into position, an optical sensor signals a controller to actuate the valve and spinner and apply a line of silicone oil in the middle of the cylinder. Testing the device lets the piston evenly distribute the oil inside the cylinder.

The technician says the system has nearly eliminated rework and rejects, and reduced process time by almost 70%. “The system has given us total control of the lubrication process, and lets us test and refine our process to the point that when we test each unit, we know it's going to work perfectly,” he concludes.