Control and programmed interventions can keep the doser for plastics efficient

The gravimetric doser is an extremely useful machine in processing of plastic materials as it automatically doses and releases the pre-established quantity of material inside the extruders or injection presses.

The gravimetric dosers can work with both granules, ground and powders, allowing precise behavior inside of the compound to be prepared.

But, being a mechanical system, it is subject to normal wear and, therefore, it is necessary to program the intervals precisely maintenance and control of its parts, to avoid breakage or incorrect weighing, which would involve a high economic outlay if the final product turned out to be non-compliant.

Furthermore, a malfunctioning gravimetric mixer can lead to wasteful use and consumption of additives for no reason.

In general we can say that the maintenance intervals should be scheduled at a maximum of one year apart from each other, even if the machine is functioning correctly.

What are the main control interventions?

Let's start with the valves and dosing shutters which allow the dispensing of the materials to be mixed, checking the correct synchronism set, the run that must not have obstacles and the speed of movement. It is also necessary to check the sliding shutter tracks, the position of the cylinder and the correct closure of the doors.

The closing movement should be fast and should not be strained, furthermore the closing limit must not go beyond the furthest edge in order not to create the possibility of jam with the material.

It is also advisable to check the pin connecting the pneumatic cylinder that it is not worn, broken or malfunctioning.

Checking that the air pressure is correct, that the closures are tightened and that the supply pipes are intact and perfectly functional, are important tests.

As far as the load cells are concerned, an inspection is recommended to remove any material residues, accumulated over time, through the use of compressed air.

Depending on the exposure to dust of normally processed materials, contamination of the load cell can be a permanent problem for the mixer, and closure may be required of the most refined load cell.

As regards the containers of the material to be weighed, it is necessary to check the functioning of the discharge valves and the opening and closing doors, taking care to check that the rotation points of the mechanisms are always free and efficient.

The valve mechanism must accommodate the static accumulation of pellets without interfering with stopping material flow.

Also carefully examine all parts of the weighing pan and its support bracket, to ensure that nothing is touching any fixed part of the mixer, and that its weight is fully supported by the load cells, as expected.

A light press on the container should show a change in the weight reading on the display. Removing that pressure should return the screen to exactly the same number, plus or minus 1 or 1/10 gram.

As regards the mixing chamber, it is necessary to check the metal blades that mix the material, in order to verify that they are not bent or worn due to abrasion from the products used. In fact, working with worn blades could increase the risk that they could come off and damage the screw.

Machine translation. We apologize for any inaccuracies. Original article in Italian.