Presentation on theme: "Precision Dispensing Webinar Peter Swanson & Paul Whitehead INTERTRONICS Wednesday May 2, 2012 11:00 AM."— Presentation transcript:
Precision Dispensing Webinar Peter Swanson & Paul Whitehead INTERTRONICS Wednesday May 2, 2012 11:00 AM
You are familiar with dispensing! Success! Failure!
Ideal application process Pre-determined volume Applied to the same location Accurately Repeatedly Medical device manufacturing Validated repeatability Micro-electronics Precision But all engineers want to remove variability in their process
Material examples Adhesives 2 parts Thermally or electrically conductive Coatings Oils Inks Paint Greases Solder paste Enzymes or catalysts
Part of the design process 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?
How the material is purchased makes a difference
Why use a dispensing system? Measured amount No more, no less Consistency & repeatability Fewer rejects Speed Faster than a human Removes skill factor Reduces waste & mess Health & safety Reduced exposure to chemicals Operator fatigue, RSI Cant be done manually Precision, gaskets Result? Quality Economy Throughput
Factors affecting dispensing technique Packaging 1 part, 2 part, multi-part Mix ratio Viscosity Thixotropy Fillers Abrasiveness Settling Compressibility Behaviour with temperature Hot melt Stability & reactivity Cure mechanism Pot life or working life Dispense pattern Corrosiveness Metal parts Solvents Seals? Hazards Toxicity Flammability
Time/pressure dispensing Simple time/pressure dispensing from barrel (syringe) Few £100s Very popular
Control of time/pressure Pressure of air pulse Time of air pulse Size of needle Diameter of 0.25mm up 0.06 possible Rheology of material Suitable for manual use or as part of automation Reasonably accurate +/- 1-10%? Digital timers
Issues with time/pressure Air is compressible, liquid is not Repeated compression of air (pulsing) causes heat
Issues with time/pressure Delay in dispense whilst waiting for air compression As barrel empties Less liquid, more air More compression time needed Dot size decreases
Issues with time/pressure Material viscosity can change With temperature Pulsing Ambient temperature With time Cure
Why use a dispensing valve? Accuracy Remove the variable of air volume increase as the dispensing barrel empties Mitigate the effects of temperature induced viscosity changes Challenging materials Liquids with characteristics at the extremes Cyanoacrylate adhesives (very low viscosity) Thermally conductive RTV silicone adhesives (very high viscosity)
Why use a dispensing valve? Volume Largest dispensing barrel typically 55ml Dispensing cartridges available with a capacity of 400ml or more 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 Valves can be fixtured to a robot and provide better stability than a dispensing barrel
Valves – material feed Reservoir Pressurised Gravity feed Extrusion pump Cartridges and barrels
Valves - types Pneumatic control most popular Robust Accurate Can be cycled quickly Often implemented at low cost Pinch tube Diaphragm Spool Needle Poppet and Spray!
True volumetric dispensing True, volumetric dispense has led to the development of valves which work on a positive displacement principle 1) Within the valve, a cavity of the desired volume is filled with the liquid, and then this volume is ejected – mechanically or pneumatically 2) 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
Progressive cavity pump 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
Endless-Piston-Principle 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 Flow rate is directly proportional to the rate of rotation 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 Controller allows programing of the motor 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 Once material characterised - desired volume can be selected on the controller Dispensed regardless of material viscosity changes Independent of ambient temperature Viscosities from water up to very high viscosity pastes 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 Highest precision of at least ± 1% and reproducibility over 99% of the medium 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 Metering and mixing two component materials Two pumps supply the components to a static mixing nozzle in the correct volumetric ratio
Contact dispensing Practical limitations Speed Volume - 0.1 µl minimum? Release of material from needle Have to touch material to receiving surface Leads us to non-contact dispensing Stamping or pin-transfer
Jetting dispense valves 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
A word on needles Smooth flow for filled materials Avoids agglomerations Thin rigid metal construction Accuracy ID from 0.06mm to 1mm
Summary 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 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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