MEDICAL
PRECISION IN
small DOSES
Here Paul Whitehead, key account manager
at adhesives specialist Intertronics, shares
his thoughts on the precise metering,
mixing and dispensing of small
amounts of two-part materials.
32
www.fastening-solutions.co.uk
AUTUMN 2019
When dispensing small amounts
of two-part adhesive or
other materials, two key
considerations include whether the
material is mixed correctly, and the
accuracy of the amount dispensed.
There is a growing demand for the
precise metering, mixing and dispensing
of small amounts of two-part technical
materials like adhesives, potting compounds,
encapsulants and sealants — including
electrically or thermally conductive variants.
This is driven by manufacturers in the
electronics, medical device and automotive
industries requiring increased functionality
in seemingly ever smaller formats. In
parallel, these engineers are looking for high
accuracy material application in a robust
and consistent process. They want precise
quantities, uniformly mixed in the correct
ratio, applied in exact locations.
Two-part formulations can offer superior
cured performance but can be difficult to
process. A typical method involves weighing
out the constituent parts, mixing in a
container with a spatula and loading into a
dispensing syringe barrel for application.
There may be a vacuum degassing or
centrifuge step to remove entrapped air.
Some suppliers offer respite from this
procedure by providing their materials in a
pre-mixed, degassed and frozen packaging.
However, there are extra costs associated
with shipping and storage, which are partly
offset by the material being ready to use
after thawing.
Once the material is mixed (or thawed),
curing starts and the material’s viscosity will
start to increase. The amounts dispensed
directly from the syringe or through a
dispensing valve using air pressure will vary
with viscosity change and it is difficult to
maintain an accurate deposition. Moreover,
the mixed material will have a limited
working life, and this can lead to waste as it
cures too much to still be usable.
One solution is to use a metering and
mixing machine, where measured amounts
of material are fed through a mixing nozzle
and can then be dispensed directly to the
part. Systems operating with gear pumps or
piston pumps are readily available but are
more suited to larger material quantities,
such as potting transformers.
If the requirement is for the application
of a few millilitres or even fractions
of a millilitre, then another technology
is considered — metering and mixing
based on progressive cavity pumps. The
pump comprises a specially shaped rotor
inside an elastomeric stator, forming a
series of tapering and overlapping spaces
or pockets which progress through the
pump as the rotor is turned, producing a
pulseless material flow. Flow rate is directly
proportional to the rate of rotation. Input
pressure and material viscosity have no
impact on the output of the pump and the
system will achieve true volumetric output
based on positive displacement. Dispensing
at the point of mixing precludes variability
due to viscosity change and reduces
wastage. Materials may be purchased in
larger package sizes, which can reduce
purchasing costs.
One example of such technology is
the preeflow eco-DUO, a high precision
volumetric metering, mixing and dispensing
unit, which integrates two preeflow eco-
PENs through a manifold and static mixing
nozzle. Suitable for a wide range of materials
with viscosities from water up to pastes,
it enables flow rates from 0.1 to 32 ml/
minute. It also offers a mixing ratio of up
to 10:1, which can be changed dependent on
the project. The variable mix ratio means
that unlike many other systems, the user
can reconfigure it for different materials,
rather than sending it back to the system
manufacturer for modification.
The operator can control the flow to fine
limits and calibrate the controller to the
system for a repeatable output. It is readily
integrated into automation to ensure that as
well as dispensing volume, positioning is
extremely repeatable.
One customer success we have seen is
with BPR Medical, which manufactures a
/www.fastening-solutions.co.uk