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UV glue dispensing studieson the Dima Dotmaster

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The study involves the testing of 3 UV cure glues.The candidates are:

•Loctite 3525•Dymax 6-621•Dymax 3013

Two dot pattern types have been defined, a five dot pattern and a one dot pattern.Due to time constraints this study will focus on the five dot pattern only.

There are several steps to the testing procedure, they are:

•Dispensing feasibility. Can the glue be dispensed and what setup is required for theDima Dotmaster glue dispenser?•Dispensing timing step test. Stepping through different dispense times until a target dispense time duration is indentified which gives the correct mass of glue.•Dispensing consistency for the identified fixed dispense time.

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Dispensing feasibility

The Dima Dotmaster comes with two glue dispensing configurations.First test is the Archimedes screw feed system. The current setup uses 1ml syringes with a brass plunger which uses a very low air pressure (0.1 bar) to feed the glue through the Archimedes screw.

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All the UV cure glues have a very low viscosity which results in the brass plunger sinking through the glue. Rubber plungers are also unsuitable as these plunger types need to be lubricated which introduces a contaminant into the glue.

On to the next method...

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Second test uses a simple configuration. The syringe that comes with the glue is clamped into a support block and the dispensing needle is attached directly to the syringe.

Success! This dispensing method works well; it is also the easiest to set up. 5

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Glue dispense testing system

A specially designed glue testing system is used for qualifying glues. This consists of a jig designed to hold three glass slides, one slide for a pre-test glue purge and two glass slides for dispensing test dots.The method is as follows:•Weigh the glass slide.•Dispense test dots on to the test slides.•Weigh glass slide to determine mass of glue dispensed.

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Dotmaster parameters for dispensingNeedle selection

Pick a needle size that won’t drip (too big) or prevent dispensing (too small)If a glue has particles in it (such as silver loaded epoxy), then these particles can

block the needle if the needle diameter is too small. Some trail and error is required. 7

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Dotmaster parameters for dispensingNeedle tip distance from surface

Too high.Glue will stay

on needle tip and build up.

Too low.Needle may be too close to surface and reduce

glue flow or even become blocked.

Correct.Glue will transfer

from tip to surface during dispensing.

General rule: The lower the viscosity the smaller the gap required.Check variation of surface height. e.g. A glass slide varies in thickness by 0.2mm

Some trial and error is required. 8

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Dotmaster parameters for dispensingDispense time

This is the important parameter for testing and is the most time consuming.The goal is to find the upper and lower dispense times to find the target massand the time step increments that work with the glue. Also expect variations.This can involve a lot of failed testing before the correct results are found. So take a firm grip on your sanity!

The three UV glues trialled took two weeks to get to this Power point document.First results are as follows...

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Loctite 3525Dispense time step test, 5 dots per slide. Increments are in 0.05 and 0.1 seconds. Target mass is 2.0 mg. 1 bar pressure. 0.6mm distance. Blue 22 gauge needle.

Target dispense time is around 1.60 seconds.10

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Average 1.90Std Dev 0.29

Average 2.32Std Dev 0.31

Average 2.27Std Dev 0.23

Loctite 3525Variation in weight for fixed dispense times

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Dymax 6_621Dispense time step test, 5 dots per slide. Increments are in 0.1 seconds. Target mass is 2.0 mg. 1 bar pressure. 0.3mm distance. Red 25 gauge needle.

Target dispense time is around 0.5 seconds.12

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Average 1.79Std Dev 0.45

Average 2.07Std Dev 0.32

Dymax 6_621Variation in weight for fixed dispense times

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Dymax 3013Dispense time step test, 5 dots per slide. Increments are in 0.05 seconds. Target mass is 2.0 mg. 1 bar pressure. 0.2mm distance. Clear 27 gauge needle.

Target dispense time is around 0.225 seconds.14

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Average 2.12Std Dev 0.44

Due to the very low viscosity of this glue there is a dispensing problem. There is a method to fixing this...

Dymax 3013Variation in weight for fixed dispense times

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Prior to dispensing the glue will drip slightly. This will form a ball of glue at the tipwhich will spread up the outside of the dispensing needle and stabilise - even with a gap of only 0.2mm. (The glass slides have a thickness variation of 1mm to 1.2mm). Once dispensing takes place this ball will continue to increase in size upward and will notdispense on to the glass slide.

Solution: Pre-dip the needle as part of the purge process; then dispense straight away!

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Conclusions

•All three glues can be dispensed with the Dotmaster but the Dymax 3013 looks the least suitable.

•The trial demonstrates all three glues require differentparameters which is most likely caused by the differencesin viscosity.

•The trail also demonstrates the variations in accuracy of the Dima Dotmaster glue dispensing system. This variation couldbe used as a guide for buying an up to date dispensing system. Newer dispensing systems may be more accurate.

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