Selecting-the-Right-Dispensing-Equipment-for-Your-Process.pdf - 第2页

widely used in most bench top applications becau se it has proven to be the most accurate and repeatable in the market. Typical applic ations include: to disp ense solder paste on PCB for rework applications or to dispen…

SELECTING THE RIGHT DISPENSING EQUIPMENT FOR YOUR

APPLICATIONS

By Can La

Fluid dispensing has come a long way. In the early days, the only dispensing option is to

use a plunger to manually push fluid out from a syringe. With the rapid advancement of

technology, today there are various dispensing techniques available and within each

technique there are quite a few types of dispensing equipment options. They range from

simple manual dispensing device to sophisticated positive displacement pump. Facing

with too many choices to choose from, selecting the right dispensing equipment can be a

daunting task if you are not familiar with the dispensing technology. The following

information will assist you selecting the right dispensing equipment for your applications.

Manual Syringe System

The rudimentary way to dispense fluid is through manual dispense process. A typical

manual system includes a plunger with piston and a syringe filled with fluid. The

operator just simply pushes the plunger down until enough fluid is dispensed on the work

piece. Repeatability rate is depends entirely on how consistent the operator can repeat the

process. This method is suitable for low volume applications where product yields below

80% are acceptable.

Time/Pressure Dispensing (TPD)

The next step up in process improvement is the Time/Pressure dispensing (TPD)

technique. As the name indicates, this technique utilizes air pressure couple with a

control timing circuit to help increase product throughput and yields. Because of the

simplicity and decent repeatability rate, over 60% dispensing applications is done by this

technique. OK International offers a complete range of TPD equipment and accessories

that includes: syringes, needles, receiver heads, pressure containers, valves and

controllers.

Time/Pressure Syringe System

The basic system TPD syringe system, shows in figure 1.0, starts with a dispenser, air

power syringe and receiver head assembly.

The syringe, filled with fluid to be

dispensed, is connected to the time/pressure

dispenser via the receiver head assembly.

Appropriate air pressure and dispense time

is then set according on the dispenser.

After the setup is done, the operator can

activate the dispense cycle by simply

pressing the foot switch or finger switch.

During operation, the dispenser delivers a

measured amount of air pressure for a pre-

set dispense time to a syringe. The

controlled shot of air forces the fluid out of the syringe, through the dispensing needle

and onto the work surface. The DX series dispenser from OK International has been

Figure 1.

A precision dispenser

that features a digital

timer to dispense small

dot of fluid accurately

from a syringe

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