Proxistor capacitive switches detect plastic materials in hopper loaders August 2000.

Plastic materials sensing


Our capacitive switches detect plastic particulate level in hopper loaders, colour blenders, vacuum loaders and in-line drying used in moulding.


		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.


Sensing through non metallic materials
This 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.

		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.

		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.

		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.


Direct sensing in a plastic hopper

		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].

		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?

		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.


Direct sensing in a metal hopper

		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.

		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.

		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.

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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