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Moulding
machines need feeding
Our capacitives
are used plastics ancilliary equipment, in automated feeding, often on
hopper loaders to detect level of powders, granules, regrind or
particulates to either call for more material or to stop filling. They
are fitted to vacuum loaders, one shot loaders, colour blenders, and all
sorts of ancillary processing equipment.
If you mould plastic parts or run many moulding machines, you
probably automate the thankless task of machine feeding. You may be
using loaders to draw material from a bin and need warning when the bin
needs re-filling. |
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| Sensing
through non metallic materials |
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method of sensing is sometimes described as "non-invasive" and
often as "non-contact" because the sensor need not pierce the
side of your container or physically contact your target material to
function reliably. Capacitive sensors detect plastic materials through
non metal containers, sight windows etc. |
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Plastic
loading tube or sight glass
Here our sensor
is fitted on the outside of a loading tube. Granules are loaded via the
tube into the top of the moulding machine. This particular tube is clear
such that material is visible to the naked eye. The tube does not
however need to be clear for our capacitive switch to detect material
through it. |
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Viewed through the perspex loading tube
The sensor is
fitted close to or touching the wall of the tube through which it will
detect the target material.
In this case when there is no material present in the tube the
sensor would switch state and "call" the loader to feed more
plastic. |
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An adjustable bracket
Mounting the
sensor on a bracket adjustable for height allows you to change the level
at which it calls for more material. Moulders can therefore adjust when
changing shot size so the loader works efficiently, or for "one-shot"
loading to minimise material changeover times. |
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| Direct
sensing in a plastic hopper |
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Mounting through a hopper wall
In some cases
it is mechanically convenient to mount the sensor though the container
wall. If you mould very low dielectric plastics or if the wall is likely
to become coated in a residue of clinging material or crazed or damaged
it may also assist long term repeatability for sensor switching points.
[See notes at the bottom of the page]. |
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Quick disconnect (QD) connectors
QD connectors
allow fast change of sensor, power cable or complete hopper if required,
without disconnecting wires. Reduction of set-up or change-over time is
vital in a batching environment. Our sensors can assist you. How do you
achieve fast colour or material changes? |
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Mounting
with a rubber gasket
This sensor has
been mounted through a hole in a plastic part of the hopper sealed with
a rubber washer to prevent dust escape at the curvature of the tube.
Note: Twin LEDs for "output-made" & "power-on"
clearly indicate your sensor's status. |
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| Direct
sensing in a metal hopper |
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Direct
sensing
Capacitive
sensors cannot detect plastic through a metal wall so must be mounted
through the wall.
This is therefore "direct sensing". In this case the
sensor is mounted detecting down onto the top of material. This can
avoid material piling up on the sensor head. |
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Non
or Flush-fitting
You could also
mount the sensor lower in the hopper and level with the wall or just
slightly into the hopper. The sensor can be adjusted accordingly.
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Low
level in a hopper
Positioned as
at left allows detection of "low material level". You may
start the filling process using a timer actuated loading device or a
high level cutoff sensor. |
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Notes:
Material factors:
Nominal sensing range is specified with a standard 1 mm thick
square target of earthed mild steel having sides equal to 3 X the
nominal sensing distance. For capacitive sensors and switches this
target is not earthed. If the target is of another material, or of a
different size, there will be variation in the sensing distance
achieved. Our rough guide to "material factors" below
indicates for PVC the nominal range of our capacitive switches should be
halved.
MATERlAL FACTORS = Mild Steel 1.0 - Stainless Steel 1.0 - Water 0.9
- PVC 0.5 - Glass 0.5 - Ceramics 0.4 - Beer 0.9 - Lubricating Oil 0.1 -
(We hope you will not be trying to mould Beer or Lubricating oil!)
So for example our capacitive switch CPO040VFSN with a nominal
range of 40mm would see a 120mm x 120mm x 1mm PVC target at 20mm range,
our CPO020VDTN with a nominal range of 20mm would see the corresponding
60mm x 60mm x 1mm PVC target at 10mm range. To see our part selection
guide click here.
Adjusting to detect through a sight-glass, container wall or
window:
Setting up to see through a sight glass, window or into a non
metallic container. Mount the sensing face of the Proxistor as near as
possible to or touching the face of the sight glass or window.
Effectively the material of the window now becomes an extension of the
housing of the capacitive switch itself and you now adjust the switch to
compensate for this. Now you can follow the procedure to set up for
direct sensing of a target. (to see installation and set-up guide for
capacitive switches click
here). If there is a problem with the sensor achieving the “no
target present state” follow the procedure for detecting targets
with very low dielectric.
A small point on this aspect, the temperature stability of the
material you are using as a sight glass, window or container wall may
now itself affect the temperature stability of the switch. (this does
not normally present any problem). However sensing non invasively allows
you to slightly isolate the sensor from a target material at a high
temperature. Various of our standard sensors work from -30 to +100 Deg C
or -25 to + 75 Deg C, slightly isolating them from a higher temperature
target material allows you to avoid having to buy an expensive high
temperature version.
Dielectric values:
Dielectric is a measure of the actual value that the capacitive
switch is detecting. For most materials think of a combination of the
mass and electrical conductivity of the target material. Thus while
water and mild steel are relatively easy for a capacitive switch to
detect, fine dust, flour or dry grain is a little harder. Basically the
lower the dielectric of the target material the more of it there needs
to be, or the closer the sensor needs to be for reliable detection.
Electrical earths and static:
In some instances the plastic granule loading application described
on this page can generate a static charge. Sometimes this is more than
20KV generated in the actual granules being conveyed. This can be
exacerbated if there is no suitable path to earth for the voltage to
discharge. In most applications, especially where suitable paths to
earth have been designed in, there will be no problem. However while our
standard capacitives withstand many thousands of volts of discharge onto
their sensing face for example in blow moulding applications, an
extremely large static charge can kill sensors. If you feel you have had
problems with sensors from any manufacturer being damaged by static
discharge call us for our special static resistant capacitives and see
below.
Specials for plastic sensing:
For most plastic level sensing (indeed level sensing in general)
our standard capacitive switch range including PNP, NPN, AC, normally
open or closed and cable or connector versions, which are widely
stocked, will work fine.
We do however make specials optimised for plastics which feature
adjustment pre-tuned for the range of low dielectric target materials
used in plastic moulding, have semi flush fitting allowing you to embed
them in a wider variety of installations and have increased resistance
to electrical static discharge.. |
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