The customer, a globally active plastics producer, manufactures polymers for application areas in the dental and sanitary ceramics segment. In the production of PMMA, however, agglomerates, overlengths or faulty grains have no place.

Grain sizes outside a defined grain spectrum have a negative effect on the quality of the resulting end product. They must be avoided at all costs. However, the customer’s internal quality controls repeatedly detected fluctuations in the grain spectrum. The reason: the electrostatic charging of the PMMA granules.

To ensure consistent product quality, an additional control sieve is therefore to be integrated before filling into the sales containers in order to optimise the quality of the PMMA granulate ready for sale – without any conversion work on the existing plant.

Target throughput: 300 kg PMMA granulate per hour

Control sieving with mesh size of 200 µm for a homogeneous raw material, i.e. PMMA granulate in the defined particle size spectrum

Separation of coarse grain or agglomerates outside the defined grain spectrum

Conveying of the good grain, i.e. the goods ready for sale, to the filling and filling of the sales containers

The tendency of PMMA granules to become electrostatically charged must be resolved throughout the entire process.

Limited space at the installation site, with plate dryer and the supply lines to the packaging

Various types of screening machines from the Engelsmann portfolio were tested with the original product in the technical centre. Always together with the customer, who was thus able to make his decision on a sound data basis. The choice fell on the JEL Freischwinger vibrating screen.

The JEL-Freischwinger was specially designed for screening of free-flowing bulk materials. With a throughput capacity of up to 250 t/h, a screening process that is gentle on the product and separation efficiencies of up to 99%, it is the ideal vibration screen for applications in the chemical, pharmaceutical and food industries.

Electrostatic charge complicates the screening task: dust and fine particles of PMMA adhere to the granulate due to the charge

The problem of electrostatic charging can be countered in two ways: by using antistatic additives or – as in this project example – by using ionisers in the screening process.

For the special application of screening PMMA granulate, the JEL Freischwinger vibration screen was chosen as the basis.

The two screen feeders are covered with a mesh of 200 µm. The screened PMMA granulate is discharged via two outlets: one outlet for the coarse material and the unwanted agglomerates. And a second outlet for the good granulate or good product. Arranged in the product feed of the screening machine, ionisers eliminate the electrostatic charge of the product before the actual screening process. The screenings thus fall into the screening machine with virtually no charge. The ionisers generate an electric field via a high-voltage generator that breaks down the air around the ion spray tips into positive and negative ions. As soon as the statically charged PMMA granulate passes the field, an ion exchange takes place and it is discharged.

Not like this: Electrostatic charge makes screening of plastic powders and granulates such as PMMA difficult

Much better: Free screen mesh thanks to ionisation. Only when the meshes are free is the full throughput achieved

Due to the electrostatic charge of the plastic granulate, the vibrating screen was additionally earthed throughout. The machine is designed for operation in ATEX zone 20 (inside) and zone 22 (outside).

Two different screen cleaning systems are integrated to ensure that the screen mesh does not become clogged with grit during the screening process and that the required throughput of up to 300 kg per hour is achieved.

A ball cleaning system is installed below the screen feeder to keep the screen mesh mechanically free during the screening process. An additionally installed ultrasonic cleaning system can be switched on at intervals or permanently – all ATEX-compliant.

The screen feeders of the JEL Freischwinger screen are equipped with impact balls that rest on a grid directly under the screen mesh. The movement of the screening machine during the screening process also sets the cleaning balls in motion. They hit the screen mesh from below and loosen particles stuck between the meshes.

The second cleaning system, an ultrasonic cleaning, can be switched on as required. Depending on the properties of the PMMA granulate of the respective batch, it can be switched on either at intervals or permanently. The ultrasonic cleaning generator is temperature-resistant up to 80°C, the two ultrasonic converters up to 135°C. The flow rate or dwell time of the plastic granules on the screen fabric is controlled by the angle of inclination of the screen feeders (between 1° and 3°). This enables adaptation to the respective screened product.

PMMA granulates with different grain specifications are screened on the JEL Freischwinger. This requires screen feeders with different mesh sizes

In order to keep the downtime associated with changing the screen feeders as short as possible, free access and easy changeover of the screening machine are the be-all and end-all.

As a modification from the standard, a lightweight stainless steel cover has been fitted, which significantly simplifies handling for the operator. Equipped with BFM nozzles and quick-change design, sieve inserts can be changed in no time at all.

After loosening the quick-release clamps, the lid is simply folded up. The sieve inserts are now easily accessible. Pressure bars that securely fix the sieve inserts during the sieving process are removed and the sieve insert can be taken out of the sieve shaker. The new sieve inserter is inserted, clamped in place and production resumes after a few minutes. Also included: Time saving during the cleaning of the product area without leaving any residues.

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