Dispensing plays an important role in medical device manufacturing. There are many stringent requirements for accuracy, reliability, repeatability, dispensing speed, and throughput. In addition, the dispensing processes require a wide variety of fluids with a broad range of viscosities. With all the complexities of medical device manufacturing, the performance of something as simple as a dispense valve often gets overlooked.
Here are some questions to consider when evaluating medical technology dispensing processes.
Are You Using the Best Dispense Valve for Your Fluid Application?
For many dispensing applications, a well-designed general-purpose diaphragm or piston valve can handle a range of fluid viscosities. In most situations, however, best results will be obtained with a valve style and configuration carefully matched to the specific properties of the fluid being dispensed.
Thick Fluids: Thick materials like RTV silicone or heavy grease, for example, pose different challenges than thinner fluids like adhesives or threadlockers. When using thick fluids, a high-pressure valve with a balanced spool design will provide good control. Look for a snuffback feature. It will prevent drooling and tailing and help reduce the rework and cleanup.
Thin to Medium Fluids: Thin fluids like solvents and watery adhesives, especially when very small deposits are needed, work best with needle valves because shutoff occurs close to the valve outlet or dispense tip. This is an important design feature because it minimizes dead volume that can cause dripping or oozing. For critical applications, there is a needle valve that seats the needle in the dispense tip instead of the valve body. By virtually eliminating dead volume, this design makes it possible to produce even smaller and more consistent micro deposits. (See Figure 1)
Tricky Fluids, like Cyanoacrylates: Wetted internal parts, as well as any fittings and tubing that come in contact with the fluid, should always be carefully chosen for compatibility with the fluid being dispensed. When working with cyanoacrylates (CAs), for example, wetted parts made of inert, ultra high molecular weight (UHMW) polymer are a good choice because they will not react with the fluid. Nylon or metal fluid fittings should never be used with CAs because they absorb moisture and will promote premature curing. Use polyethylene or polypropylene fittings instead. Chemically inert, polyethylene-lined or polytetrafluoroethylene (PTFE) fluorinated ethylene propylene (FEP) tubing are good choices for fluid feed lines.
Are You Using a Dispense Valve System or Just a Dispense Valve?
Taking a system approach to fluid dispensing and carefully evaluating all the details, even something as small as a fluid fitting, will help prevent many problems on your device assembly line. A dispense valve system has four main components:
- The dispense valve,
- A precision dispense tip,
- A means of initiating the dispensing cycle, and
- A fluid reservoir.
The greatest accuracy, reliability, and production yields will be obtained when all four components are engineered to work together as an integrated system. This approach will also simplify qualification and validation processes. A valve paired with a dedicated valve controller will typically provide faster response time than a valve triggered by mechanical means or a remote programmable logic controller (PLC).
Is Your Dispensing Line Running as Fast as It Can?
If your dispense valves are not cycling fast enough, the valve control system may not be compatible with the dispense valve. Most automatic assembly machines use PLCs to sequence machine functions, but a PLC’s primary purpose is not to control dispense valves. When faster cycle times and more precise control of deposit size are required, a dedicated valve controller with a fast-acting solenoid and a digital timer can be a simple and cost-effective way to achieve these objectives. The controller can also be interfaced with the PLC, if desired.
However, a PLC may or may not offer online programming of dispensing functions. Without this capability, entire production lines have to be shut down just to make simple adjustments to deposit size. Even if a PLC can program valve functions, the valve may not be within the line of sight of the engineer or operator trying to adjust it. A dedicated controller mounted at the dispensing station will simplify initial setup, make it faster and easier to purge the valve after refilling the fluid reservoir, and allow adjustments to be made and checked “on the fly” without shutting down the production line.
Do Your Valves Leak and Drip?
Leaking is a common problem with valves that have complex designs or seals and O-rings that wear out over time. The most reliable diaphragm designs entirely eliminate the need for seals and O-rings. The best valves will easily handle many different fluid applications and provide tens of millions of cycles without maintenance. Carefully choosing the valve seat materials will also prevent many problems.
UHMW polyethylene, for example, provides exceptional wear characteristics and chemical compatibility with a wide range of assembly fluids, keeping the valve system working longer without downtime or maintenance.