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Dispensing the Facts on Valves

Fri, 12/05/2008 - 7:03am
Steven Young
Dispense valves can be a vital tool in the assembly and manufacture of medical devices. However, choosing the correct type to use for a particular application may not always be abundantly clear. This article reviews the factors that an engineer must consider and highlights the main attributes of the most common dispense valves being used by OEMs.



Steven Young is a technical specialist with I&J Fisnar Inc. He is responsible for technical client assistance and can be reached at 201-796-1477 or syoung@ijfisnar.com.

Most industrial valves used in dispensing liquids and pastes are pneumatically operated. The overwhelming reason is reliability and high cycle performance.

Operation of a Pneumatic Valve
High Press Needle Valve
By actuating a timed air pulse, originated from a controlled air source, an air solenoid in the valve opens a seal (gate), which allows material to flow unrestricted for the duration that the seal is opened. A return-spring returns the seal to its closed position, preventing fluid from flowing. For fast cycling, a four-way action is recommended. This requires an initial air pulse to open the seal and a final pulse to snap the seal shut after a timed interval. Pinch-tube valves simply pinch a tube, in the normal or closed state. When an air pulse is received, the pinch is released when the solenoid is actuated.

Controllers can be stand-alone bench systems with timed pulse outputs, integrated robotic timed pulses, or PLC actuated air solenoids. In the case of spray valves, a controller ensures that the atomizer commences before fluid flow and stays on momentarily after the fluid shuts off.

The precision of the pulse timed material deposit depends upon the construction and quality of the valve.

Fluid and Purpose
Plunger Valve
There are two considerations in selecting a valve—fluid and purpose.

Fluids can be described as watery, creamy, gel, treacle, or paste. Terms that further define the fluid are:

The condition of the fluid also has an effect on valve selection. For example, it needs to be determined if the fluid is corrosive, abrasive (fillers), or anaerobic, and whether or not its cure rate and condition of cure impacts the selection. Knowing the viscosity and condition will assist in selecting the right valve. The following examples show the effect of viscosity and condition.

In the first example, the fluid is a paste-like material (200,000 cps) with corrosive and abrasive fillers that will require a valve of heavy duty construction with resistant seals. The valve will require regular maintenance due to the fluid's abrasive and corrosive content.

The second example describes a light fluid of 100 cps that cures quickly, such as a cyanoacrylate. A suitable valve will separate the wetted parts from the air-actuated moving parts. A further issue in this case is the likely necessity to purge or park the tip in a material that prevents clogging during dispense cycles.

Once these variables are determined, the actual valves can be reviewed so the selection of an appropriate valve type can be made.

Spray Valves
Spray Valve
Obviously if the application requires spraying, only a spray valve will be suitable. The material must be low-viscosity—typically less than 1,000 cps. The criteria to ensure proper selection is the angle of spray permitted by the valve and that the control module ensures that atomization takes place prior to material flow, then remains on momentarily after the fluid has ceased to flow, thus preventing clogging, common with conformal coating products.

Needle Valves
 
Mini Valve
A needle valve is ideal for single micro-shot deposits of all types of fluids, provided the construction is suitably configured to handle the condition of the fluid. These valves are often adjustable and, if rated for high-pressure working, will handle high-viscosity (unfilled) material. As they can be opened for any determined time, needle valves are also often suitable for applying beads of low to medium viscosities. They are not recommended for abrasive materials, which can clog. Needle valves are also available with a replaceable cartridge seal between the wet-chamber and moving parts; this enables a set maintenance schedule to be implemented based on cycling of the valve.

Diaphragm Valves
A diaphragm valve is chosen for adjustable high cycling dispensing of many low to medium viscosity fluids including adhesives, solvents, and corrosive agents, provided the valve is configured. They are light-weight and easily maintained.

Spool Valves
 
Mini Poppet Valve
A spool valve is suitable for high-viscosity pastes, treacle, and gel-type fluids. Spooler valves can operate at high material feed pressures necessary for high-viscosity fluids and they also include a suck-back or snuff-back feature inherent in their design. Ideal for applying beads and with proper automated control, this valve can seamlessly complete a beaded path of fluid as in a gasket.

Poppet Valves
 
Poppet Valve
A poppet valve is similar to a spooler valve in features. For example, it offers suck-back but is used with low to mid-high viscosities. It can apply a smaller shot size than a spooler valve.

Pinch-Tube Valves
 
Pinch-Tube Valve
A pinch-tube valve is a common solution for aggressive solvent base fluids, with the valve having fully disposable wetted parts. It can be used with moderate precision and repeatability within the confines of a pinch-tube design. The pinch-tube valve is often used as a valve for two-part, meter, and mixed applications where the material will cure and disposing of the tube is a low cost solution.

Positive Displacement Valves
 
Positive Displacement Valve
A positive displacement valve is used when highly accurate micro deposits are required. The choice is to either use a pneumatic or an electrically controlled auger/screw valve, determined by viscosity and shot size. Pneumatic valves can positively displace lower viscosities than auger valves and their shot size is less in range with a higher minimum deposit. Pneumatic positive displacement valves are less expensive than electronically controlled auger systems.

Conclusion
From this summary of dispensing valves, a suitable choice with the proper construction and suitability of purpose can be determined.

Online
For additional information on the technologies and products discussed in this article, see MDT online at www.mdtmag.com or I&J Fisnar Inc. at www.ijfisnar.com.

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