Transcript An Overview of Dispensing Techniques

Precision Dispensing
Webinar
Peter Swanson
& Paul Whitehead
INTERTRONICS
Wednesday
May 2, 2012
11:00 AM
You are familiar with dispensing!
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Success!
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Failure!
Ideal application process
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Pre-determined volume
Applied to the same location
Accurately
 Repeatedly
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Medical device manufacturing
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Micro-electronics
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Validated repeatability
Precision
But all engineers want to remove variability in their process
Material examples
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Adhesives
2 parts
 Thermally or electrically conductive
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Coatings
Oils
Inks
Paint
Greases
Solder paste
Enzymes or catalysts
Part of the design process
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The process of material or adhesive specification must include
consultation with manufacturing & processing engineers
How will it be dispensed/applied?
 Can it be dispensed/applied?
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How the material is purchased makes a
difference
Why use a dispensing system?
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Measured amount
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Faster than a human
Removes skill factor
Reduces waste & mess
Health & safety
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Fewer rejects
Speed
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No more, no less
Consistency & repeatability
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Reduced exposure to chemicals
Operator fatigue, RSI
Can’t be done manually
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Result?
Precision, gaskets
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Quality
Economy
Throughput
Factors affecting dispensing technique
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Packaging
1 part, 2 part, multi-part
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Hot melt
Stability & reactivity
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Abrasiveness
Settling
Compressibility
Behaviour with temperature
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Mix ratio
Viscosity
Thixotropy
Fillers
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Cure mechanism
Pot life or working life
Dispense pattern
Corrosiveness
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Solvents
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Metal parts
Seals?
Hazards
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Toxicity
Flammability
2k – two or multi-part systems
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Complex subject!
Time/pressure dispensing
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Simple time/pressure
dispensing from barrel (syringe)
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Few £100s
Very popular
Control of time/pressure
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Pressure of air pulse
Time of air pulse
Size of needle
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Diameter of 0.25mm up
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0.06 possible
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Rheology of material
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Suitable for manual use or as
part of automation
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Reasonably accurate
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+/- 1-10%?
Digital timers
Issues with time/pressure
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Air is compressible, liquid is not
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Repeated compression of air
(pulsing) causes heat
Issues with time/pressure
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Delay in dispense whilst waiting for air compression
As barrel empties
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Less liquid, more air
More compression time needed
Dot size decreases
Issues with time/pressure
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Material viscosity can change
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With temperature
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Pulsing
Ambient temperature
With time
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Cure
Dispensing valves
Why use a dispensing valve?
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Accuracy
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Remove the variable of air volume increase as the dispensing
barrel empties
Mitigate the effects of temperature induced viscosity changes
Challenging materials
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Liquids with characteristics at the extremes
Cyanoacrylate adhesives
(very low viscosity)
 Thermally conductive RTV silicone adhesives
(very high viscosity)
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Why use a dispensing valve?
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Volume
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Largest dispensing barrel typically 55ml
Dispensing cartridges available with a capacity of 400ml or more
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Not suitable for handheld use?
Barrels and cartridges need to be filled off-line, and changed
when empty.
For higher daily volumes some form of larger reservoir is needed
Automation
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Valves can be fixtured to a robot and provide better stability than
a dispensing barrel
Valves – material feed
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Reservoir
Pressurised
 Gravity feed
 Extrusion pump
 Cartridges and barrels
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Valves - types
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Pneumatic control most popular
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Robust
Accurate
Can be cycled quickly
Often implemented at low cost
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Pinch tube
Diaphragm
Spool
Needle
Poppet
and
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Spray!
Valves - operation
True volumetric dispensing
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True, volumetric dispense has led to the development of valves
which work on a “positive displacement” principle
1)
2)
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Within the valve, a cavity of the desired volume is filled with
the liquid, and then this volume is ejected – mechanically
or pneumatically
Auger screw inside a tube which can be driven by a motor
These valves are usually specific to a limited range of deposit
size or to certain viscosities
Positive displacement valve
Progressive cavity pump
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Single-helix metal rotor and a double-helix hole in an
elastomeric stator
Rotor seals against the stator, forming a series of spaces or
pockets, which translate along as the rotor rotates, keeping their
form and volume
Pumped material is moved inside the pockets
Pockets are shaped such that they taper and overlap
The output is
Continuous
 Even
 Non-pulsing
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Progressive cavity pump
Endless-Piston-Principle®
Imagine a piston pushing on a liquid
Or several pistons in sequence
Make the piston thickness zero
Change the piston shape
A continuous or “endless” piston
Positive displacement dispensing
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Flow rate is directly proportional to the rate of rotation
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Can be reversed
Volumetric output of the pump is directly proportional to the
number of rotations
Due to the rotor/stator seal, input pressure has no effect on the
pump
Able to pump at very low rates
Low levels of shear are applied to the pumped fluid
True positive displacement dispensing/dosing
Practical application
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Controller allows programing of the motor
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Speed and number of rotations
Dots or deposits of specific volumes, or
continuous beads
Motor can be reversed briefly to prevent
stringing or dripping
Dispensing needle is fitted to the end of
the pen using a standard luer fitting
Practical application
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Once material characterised - desired
volume can be selected on the controller
Dispensed regardless of material viscosity
changes
 Independent of ambient temperature
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Viscosities from water up to very high
viscosity pastes
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Including abrasive, filled or shear-sensitive
media
Flow rates range from ~ 0.1 to 60
ml/minute
Can be hand-held or fitted to automation
Capabilities
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Highest precision of at least ± 1% and reproducibility over 99%
of the medium
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With automated handling
Dispensing a bead is accomplished by setting one parameter the volume flow; matched to the speed of a robot or other
automation, possible to dose coherent beads down to a width of
250 µm and at traverse speeds of up to 300 mm/s
Metering & mixing
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Metering and mixing two
component materials
Two pumps supply the
components to a static mixing
nozzle in the correct volumetric
ratio
Contact dispensing
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Practical limitations
Speed
 Volume - 0.1 µl minimum?
 Release of material from needle
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Have to touch material to receiving surface
Stamping or pin-transfer
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Leads us to non-contact dispensing
Jetting dispense valves
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Fast-pneumatic or piezo driven
From ~1nL
Up to ~1000Hz dispensing frequency
Up to ~2,000,000 mPa.s viscosity
Material dependent
 Valve technology used
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A word on needles
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Smooth flow for filled materials
Avoids agglomerations
Thin rigid metal construction
Accuracy
ID from 0.06mm to 1mm
Summary
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Many applications are relatively simple
Single part
 Not extreme viscosity
 Non-curing, only cure with outside energy or catalyst, long pot life
 Not hazardous, corrosive
 Modest outputs
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Volumes, speeds
BUT many applications fall outside this
Can get quite complicated
 Consult with both your material supplier and your dispensing
supplier
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Our Products
Adhesives
Dispensing
Robotics
UV & Light
Curing
Metering &
Mixing
Sealants
Potting
Gaskets
Coatings
Masking
Cleaning
Thank you
Contact
[email protected]
+44 1865 842842
www.intertronics.co.uk
Version 1.0
April 30 2012