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.
Using valves small enough to be mounted at the point of fluid application reduces the risk of drooling. (See Figure 2)
Is Your Current Valve Setup Trapping Air?
Entrapped air can cause oozing and variations in shot size. Be sure to purge all air and fluid lines whenever setting up a system, refilling the fluid tank, or performing maintenance. Also be sure to note:
- Keeping air lines shorter than five feet reduces the risk of trapped air and improves valve response time.
- The proper tip can help prevent air entrapment. When using metal tips, use 21 gauge (0.020") or larger if the application permits, as they will allow small air bubbles to purge through.
- Tapered polyethylene tips are good in any size. They allow fluid to flow freely through the tip to purge and prevent air bubbles from collecting. Tapered tips typically range from 14 gauge to 27 gauge.
- Use a valve controller with a purge function that allows the user to bleed any air in the system quickly and easily.
- Install a filter/regulator between the plant air supply and the dispense valve to remove any residual moisture from the system—this is especially important when working with cyanoacrylates.
Is It Difficult to Produce Consistent Shots?
Valve open time is the most precise way to adjust shot size. A dedicated controller is an efficient approach to establish shot size and regulate valve operation. Open time can be adjusted in increments as small as 0.001 seconds for exceptional control over the amount of material applied. On production lines with multiple dispensing stations, using a dedicated valve controller at each station can make it simple to adjust each valve's open time independently and obtain an identical shot from each valve.
How Often Do Your Valves Require Maintenance?
All valves require maintenance, but some designs require more frequent repair than others. Here are some things to consider:
- How many cycles can it go without degradation? A well-engineered valve design will go tens of millions of cycles without any degradation in performance or accuracy.
- Can maintenance be performed on site, or does the valve have to be returned to the manufacturer?
- If service can be performed on site, how complicated is it? Can the fluid head be removed without dismounting the valve or does the valve have to be removed from the mounting fixture and taken apart? With some high-performance designs, routine maintenance is as simple as replacing the dispense tip.
Are You Cutting Corners on Your Dispense Tips?
Correct tip selection is very important to dispense valve performance. The best choice is using a tip with the largest possible internal opening for the intended application. This will prevent air bubbles from forming. Tip quality has a surprisingly large effect on the accuracy and uniformity of fluid deposits, especially in critical applications where very small deposits are required. Even the most precise dispensing system will not produce consistent results if the tip—the last path the fluid travels before it reaches the part—is obstructed by debris from the molding or machining process.
Would High-Speed Jetting Fit Your Application Needs?
Non-contact jetting systems are capable of dispensing a wide variety of fluids at speeds of up to 500 shots per second. By combining high speed with exceptional accuracy, these systems allow products to be built more cost-effectively with consistently high quality. Additionally, since jet valve systems are non-contact, it is possible to apply fluid in hard-to-access areas or onto uneven or delicate substrates where dispensing needles cannot be used. Jetting can be used with a wide range of fluids. (See Figure 3)
If your dispense valve system is not giving you accurate deposits with minimal maintenance or you are applying inconsistent amounts of fluid and wasting too much time and money on downtime, rework, and cleanup, valve performance might be the culprit. Re-evaluating the valves used in your medical device manufacturing processes could help you achieve substantial material waste reduction, higher productivity, and better final product quality.
The article was written by Claude Bergeron, Global Product Line Manager–Valves, Nordson EFD, East Providence, RI. For more information, Click Here . MD&M West, Booth 2835