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Electrostatic discharge - Part 1 of 2
Electrostatic discharge in a gravimetric micro-feeding system
Occasionally in the plastics industry we encounter a raw material that is easily charged with static electricity. Raw material that is charged with static electricity tends to stick to the walls of hoppers, to flow unevenly and to jump randomly. If the material in question is the main material, which enters the production machine by the force of gravity, the problem is not noticeable. But when you try to dose a charged additive the task is not so simple.
In the two parts of this article we'll explain about charged raw material and the problems it may cause in gravimetric feeding and even more in micro-feeding, that is feeding at low throughput, and we'll also present the way we at LIAD Weighing and Control Systems solved the problem in our gravimetric micro-feeder, ColorSave-Micro.
The creation of electrostatic charge
Electrical charging of raw materials in the plastics industry results from the transfer of electrons between two granules as a result of friction between them. In one a surplus of negative ions (negatively charged atoms) is created and in the other a surplus of positive ions (positively charged atoms). Friction is generated when the raw material is transported from place to place, whether it's when a container of raw material is taken from place to place (from friction among the granules themselves) or when the raw material is transported by air pressure so the granules rub against each other and against a flexible plastic tube. The faster the speed of the friction, the higher the accumulated electrostatic charge (voltage), and the raw material can quickly develop charges of thousands or tens of thousands of volts. Inside the doser as well there is friction of the granules as they move to the production machine.
The raw material in the plastics industry is insulated material. In contrast with a charged conductor that can be discharged by connection to the ground, the raw material in the plastics industry can't lose its static electricity by grounding and can remain charged for hours.
The problems with feeding charged material
Proper feeding is first of all uniform feeding, but an additive that is charged with static electricity tends to clump, so there will probably be spaces between the clumps instead of feeding at a uniform rate. The additive is also liable to get stuck somewhere in the feeder and to cause a clog, or part of it may remain stuck to the feeder at the outlet and not fall into the production machine.
With regard to a gravimetric feeder, the task is especially difficult, since it's hard for the control system to carry out proper control when the feeding is in clumps with spaces between them. Also, the static electricity may cause the particles to jump and reach unwanted places, such as the load cell, thus disrupting the weighing process.
If gravimetric feeding of a charged additive is a difficult task, then the feeding of a charged material in a gravimetric micro-feeder, that is gravimetric feeding at very low throughput, is ten times harder.
It's important to note that in most cases the additives don't tend to get charged with static electricity to the extent that might cause the malfunction of gravimetric dosers and of the ColorSave-Micro. However, there are indeed cases in which there is a problem of static electricity, and it's important to deal with them as well.
Gravimetric micro-feeding by the ColorSave-Micro
The ColorSave-Micro is the first gravimetric feeder in the world for additives in the plastics industry that is designed for low throughput. In a continuous process, the ColrSave-Micro can easily feed uniformly and accurately at a low throughput of 50 g/hour and even less. In injection molding, this means a small amount in each shot, to the point of a single-digit number of granules.
In the ColorSave-Micro there's a hopper with a special dispenser that, using a vibration mechanism, feeds the granules uniformly into the production machine. The system is on a small load cell, so uniform gravimetric feeding is facilitated.
In the next section we'll talk about the results of an experiment we did with an especially charged additive in the ColorSave-Micro. In addition, we'll see how static electricity is discharged, how we chose to implement the discharge in the ColorSave-Micro, and what the results were.