Selecting-the-Right-Dispensing-Equipment-for-Your-Process.pdf - 第3页

Diaphragm Valve One draw back of the pinch valve is that so me applications might required frequent pinch tube replacement. In such cases, the Diaphragm valve, shows in figure 4.0, is a perfect replacement. Due to the re…

widely used in most bench top applications because it has proven to be the most accurate

and repeatable in the market. Typical applications include: to dispense solder paste on

PCB for rework applications or to dispense all types of adhesive to bond parts together.

The disadvantage of the syringe system is that it depends on too many variables to

maintain good repeatability rate for the entire dispensing process. One of the variables is

the internal diameter of the syringe. If the internal diameter of the syringe is tapered too

much then the shot size will decrease as the fluid travels down toward the bottom. To

address this problem, OK International offers various series of TPD valve systems. A

typical TPD valve system includes a pneumatic valve, a controller and a pressure

reservoir, show in figure 2.

Pinch Tube Valve

The simplest and most inexpensive TPD valve is the disposable pinch tube valve, shows

in figure 3. It is designed to dispense small dots or continuous beads of low viscosity

fluid. This air-actuated valve is operated by

automatically opening and closing (pinching) a

molded plastic tube assembly. The only part of

the valve making contact with the fluid is the

disposable pinch tube assembly. This unique

feature making the valve perfectly suited for

applications that use quick curing adhesives or

highly aggressive fluids that might damage the

valve body. This valve is commonly used to

dispense Cyanoacrylate (CA) for general bonding

applications, or dispense

two-component epoxies for potting applications.

Benefits include disposable path, light weight, compact size and simple operation.

Figure 2

A typical setup for a

pneumatic valve system

Figure 3

Pinch Tube Valve

Diaphragm Valve

One draw back of the pinch valve is that some applications might required frequent pinch

tube replacement. In such cases, the

Diaphragm valve, shows in figure 4.0, is a

perfect replacement. Due to the resilient of

the diaphragm material, it can last for

millions of cycles before needing

replacement. This pneumatic valve uses a

diaphragm to open and close the fluid path.

During the dispense cycle, the air pressure

signal forces the diaphragm to pull back

creating an opening for the fluid to flow

through. When the air signal stops, the

internal spring pushes the diaphragm down

to the closed position. The unique component of this valve is the diaphragm. It is a

solid barrier made from Ultra High Molecular Weight Polyethylene (UHMWPE) to

prevent moisture from the air cylinder to coming contact with the fluid. This attribute

makes the valve an excellent choice to dispense moisture sensitive fluid such as

Cyanoacrylate. Other attributes that make this valve widely acceptable in the industry

are: compact size/light weight, simple and low maintenance, adjustable stroke control

that can produce shot size as small as 0.002 CC. Virtually any low to medium viscosity

fluid can be dispensed using one of the diaphragm valve variations. Applications of this

valve are ranging from dispense moisture sensitive adhesives for general bonding

applications, to dispense UV cured adhesives in the medical device industry, to spin

coating application in the DVD manufacturing industry.

Needle Valve

For applications that required micro dot size (less than 0.002CC), the microshot needle

valve is an ideal choice. Shot size as small as 0.0005 CC can be easily achieved thanks to

the unique designed of the shut off mechanism located inside the dispense tip that

virtually eliminate the “dead fluid” volume.

Smallest needle size of 32 gauge can be used

without dripping between dispense cycles. The

needle valve is a pneumatically triggered single-

acting valve. Air pressure signal lifts the piston-

driven needle away from its seat, allowing fluid

under pressure to flow through. When the cycle

is completed, the internal compression spring

pushed the needle back in the closed position.

Higher viscosity fluid, up to 100,000 cps, can be

dispensed accurately with this valve. However

moisture sensitive fluid and quick curing

adhesives are not to be used. Some recommended fluid are UV adhesive, solvents, inks,

oils, primers, activators… Typical applications include: dispense small amount of UV

adhesive to bond speaker of the hearing aid device, dispense a small dot of ink for

marking application.

Figure 4

A diaphragm

valve designed

to dispense

consistent

deposits of low

to medium

viscosity fluid

Figure 5

A microshot

needle valve

designed to

dispense 0.0005

cc shot without

dripping

Spool Valve

Valves described above have one thing in common is that they are not designed to

dispense high viscosity fluid in high volume applications. To fill this gap, OK

International offers the spool valve series. High

viscosity fluid, up to 3 million cps is dispensable

through a heavy duty spool valve version. The valve

uses an air-actuated piston to opens and closes the

fluid’s path. However, contrary to the needle valve, the

air pressure signal drives the piston down to open the

fluid path. When the dispense cycle is done, the

internal compression spring lift the piston up to keep the

valve in the closed position. The pull back action of the

piston in the closing cycle creates a natural “suck-back”

action. This feature is very effective for preventing

drips when dispensing high viscosity fluid such as

silicones, greases and sealants. Most use application of

this valve is to dispense sealant for form-in-place gasket

application. Other commonly applications that well

suited are: dispensing grease for automotive application, filling material into syringes or

cartridges for the material packaging industry.

Rotary Auger Valve

The common weak spot among pneumatic valves is that they are susceptible to fluid‘s

viscosity fluctuation. When the fluid become thicker or thinner, the fluid pressure needs

to be adjusted accordingly to keep the dispense repeatability rate stable. One way to

minimize this problem is to use the Rotary Displacement Dispensing technique.

Developed in 1987 by Techcon Systems, a division of OK International, this method uses

a rotary feed screw (auger) coupled with a precision DC motor to dispense fluid with a

rotary displacement action. The unique design ensures fluid

is constantly present at the feed screw inlet while the

controlled rotation of the feed screw dispenses fluid from the

feed point out to the dispense tip. Dot size of 0.010” (0.25

mm) in diameter can be achieved with high percentage of

repeatability rate.

Due to its versatility, high accuracy and easy to setup, the

TS5000 series rotary valve is the prefer choice in the

electronics industry. It can dispense virtually any fluid that

has viscosity of 30,000 cps or higher, including difficult

material to dispense like solder paste. In fact it is the only

valve that can dispense solder paste consistently.

Applications in electronics industry such as dispensing

surface mount adhesives, silver filled epoxies, encapsulant

materials, solder pastes… are excellent choice for the rotary

valve.

Figure 6

High pressure

spool valve is

an ideal choice

for high

viscosity fluid

Figure 7

A TS5000

Rotary Valve is

the favorite

valve in the

electronic

applications