Analysis of dispensing application

1. Common issues with dispensing

Common problems with dispensing include nozzle dripping, hanging, scattering, bubbles, and inconsistent dispensing sizes during valve dispensing. There may be residual glue, misalignment, and loose wrapping on the product. The quality of the adhesive, the dispensing process, temperature, viscosity, air pressure, back pressure, and the configuration of the needle all affect the dispensing quality. For the adjustment of the above parameters, it should be done from point to surface, as any change in one parameter will affect other aspects. The occurrence of simultaneous defects may be caused by multiple factors, and possible factors should be checked item by item to eliminate them. In short, in production, various parameters should be adjusted according to the actual situation to ensure production quality and improve production efficiency.

1.1 Common problems with dispensing products: Bubbles (using UV glue on spray valves as an example)

A common problem with spray dispensing is the tendency to produce bubbles. The generation of bubbles can affect the adhesion, fatigue resistance, and tensile strength of the adhesive, and can also splash onto surrounding devices.

2) Bubble solution

Try to reduce the stroke of the firing pin as much as possible and minimize the generation of vacuum zone (choose a reasonable firing pin and nozzle; replace the integrated nozzle with a split nozzle, increase the length of the nozzle's slender flow channel, so that even if air enters, it will only exist in the slender flow channel and cannot enter the vacuum zone; increase the feeding pressure to quickly fill the vacuum zone with glue, reducing the possibility of air entering the vacuum zone.

1.2 The effect of back pressure on spray dispensing

Excessive fluid pressure (inside the hose) combined with too short valve opening time may allow air to seep into the liquid. The solution is to reduce the fluid pressure and use a conical oblique needle.

1) Excessive back pressure can easily cause glue overflow and excessive amount of glue; If the pressure is too low, intermittent dispensing and leakage may occur, resulting in defects. The pressure should be selected based on the same quality of adhesive and the working environment temperature. A high ambient temperature will reduce the viscosity and improve the fluidity of the adhesive. In this case, it is necessary to lower the back pressure to ensure the supply of adhesive, and vice versa.

2) Replacing a larger nozzle can improve the back pressure problem, but the process requirements of the product do not allow the use of large-diameter nozzles. Therefore, optimizing the nozzle's structural shape was adopted to improve this problem. The conical oblique needle produces the least back pressure and the smoothest liquid flow.

1.3 Inconsistent adhesive size

When the size of the adhesive is inconsistent, it is mainly caused by the unstable air pressure inside the hose. Please check in a timely manner whether the inner sealing piston inside the rubber hose is suspended due to detachment from the adhesive. Glue is a paste like fluid, and when using a white piston, it often rebounds and hangs in the air. It is better to use a safety piston in the industry.

1.4 Glue temperature

Various fluid materials have corresponding storage temperatures, flow temperatures, and curing conditions (UV curing/temperature curing, etc.). Generally, epoxy resin adhesive should be stored in a refrigerator at 0-5 ℃. When using, it should be taken out 1-2 hours in advance to ensure that the adhesive is fully compatible with the working temperature. The usage temperature of glue should be between 23 ℃ and 25 ℃; The ambient temperature has a significant impact on the viscosity of the adhesive. If the temperature is too low, the adhesive point will become smaller and wire drawing phenomenon will occur. A temperature difference of 5 ℃ in the environment can cause a 50% change in the amount of glue applied. Therefore, the environment temperature should be controlled, and the humidity of the environment should also be guaranteed. Small humidity can easily dry the glue points, affecting the bonding strength.

The valve body can be equipped with constant temperature heating, which can effectively adjust the temperature of the adhesive.

The viscosity of 1.5 glue

The viscosity of glue directly affects the quality of dispensing. If the viscosity is high, the adhesive point will decrease, and even wire drawing may occur; When the viscosity is low, the adhesive point will increase, which may lead to infiltration of solder pads. During the dispensing process, choose a reasonable back pressure and dispensing speed for adhesives with different viscosities.

1.6 Case: Leakage, Hanging, and Scattering of Rubber Valve (Taking Pneumatic Spray Valve as an Example)

1. The quality of the glue, such as whether the glue is overdue and whether there are residual bubbles (glue in the glue bucket).

2. Check if there is a tip (needle tip) on the nozzle and if there are any notches on the needle tip (* * under a microscope). Normally, the nozzle is transparent and the tip is not worn.

3. Check for wear on the O-ring and Seal (sealing ring). Normal is: none of them are worn.

4. Check for wear on the tip of the needle, carefully check for cracks and burrs on the sealing ring. Normal is: no wear. If there is wear and tear, the adhesive may lack power or become unstable, resulting in poor adhesive release such as hanging or scattering.

5. Finally, check for severe wear and aging inside the Jet body.

6. Check if the solenoid valve has worn and aged due to prolonged use.

7. Setting of dispensing valve dispensing parameters, such as the combination of nozzle firing pins, stroke size, glue pressure, and glue temperature.

2. Glue dispensing implementation method - contact and non-contact

2.1 Process selection conditions - Line width requirements

Basically, the minimum point diameter or line width of needle dispensing can be slightly larger than the inner diameter of the needle, depending on the distance between the needle head and the substrate (basically equivalent to the inner diameter) and the appropriate movement speed (not too low)

JETTING: Due to the loss of constraint on the inner wall of the nozzle when droplets fly in the air, the droplets will accumulate and increase due to surface tension when they fly out of the nozzle. The theoretical minimum point diameter sprayed will be greater than twice the nozzle diameter (actual value is larger)

2.2 Process selection conditions - flow rate and efficiency

For the contact needle dispensing method, keeping the height of the needle to the substrate constant is a key factor in ensuring dispensing quality. Without considering the influence of height differences on fluid resistance at the outlet of the needle, that is, under constant flow rate and movement speed, it can be seen that the cross-sectional area of the adhesive strip is almost equal. When the height from the board is low, the adhesive strip will obtain a larger width; When the height from the board is higher, its cross-section is closer to a circle. When it is higher than a certain range (approximately the outer diameter of the needle), stable dispensing effect cannot be achieved.

For some adhesive applications with narrow slits or borders, although the needle method can achieve smaller line widths, the side walls of the needle have thickness, and its outer diameter may be larger than the adhesive area, or there is a risk of component collision. In this case, the JETTING method will be more advantageous.

2.3 Process Selection Conditions - Key Parameters

3. Selection of dispensing needles

Needle size: In practical work, the inner diameter of the needle should be 1/2 of the diameter of the dispensing point, which can ensure the quality of the dispensing point and improve production efficiency.

There are four criteria for selecting a dispensing needle:

Small point - small needle, low pressure, short time

Larger - larger needle, higher pressure, longer duration

Concentrated glue - oblique needle, high pressure, set time as needed

Water based liquid - small needle, lower pressure, set time as needed

2. Fluids that require special settings:

(1) Instant glue: For water-based instant glue, use a safety piston and Teflon lined metal needle. For thick instant glue, use a conical oblique needle. If flexibility is required, use a PP needle.

(2) UV glue: Use an amber syringe, white piston, and angled needle (which can block ultraviolet rays).

(3) UV curable adhesive: Using a black opaque needle, white piston, and UV shielded needle tip.

(4) Sealant and paste like fluid: If the white piston rebounds severely, please switch to a safety type active type and use an angled needle.