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Whether PC or PMMA – Separating Is Easy

Raw material manufacturers deliver PC polycarbonate and PMMA polymethylmethacrylate as pellets or granules which are further processed to produce the end products. In order to guarantee high material qualities, the granules are to be supplied in the best possible quality for further processing. The separation of dust, agglomerates and oversize pieces plays an important role in this production. Here, the properties of the plastic pellets are highly demanding on the screening technology, which requires use of special machine solutions.

PC polycarbonate is a material used for CDs, DVDs, drinking water containers or eyeglass lenses. PMMA polymethylmethacrylate is used for the production of reflectors, discs or illuminated signage.

Before the plastic granulates are ready to be delivered, the individual particles are put through a special screening process where adherent dust particles, agglomerates and oversized pieces are sieved out in order to attain the desired grain size or granulate quality. However, the properties of unprocessed plastics make the screening task difficult and impose the highest demands upon the machinery. Screening operations are continually faced with problems caused by phenomena such as electrostatic charge, especially when handling plastics in powder or granulate form. When trying to separate dust from a granulate fraction for example, in most cases, the dust remains stuck to the larger grains because of their electrostatic charge. Furthermore, in order to secure the highest product quality, a particularly gentle screening process with minimal abrasion of the individual particles has to be achieved.

Specialist screening machines guarantee a selectivity rate of 100%

There are specialist screening machines for these specific requirements such as the granulate screening machines of J. Engelsmann AG, the screening technology specialists from Ludwigshafen. Their JEL Freischwinger Granulat model was designed for the perfect separation of oversized particles of all common plastic granulates in production. For this purpose, the free-swinging granulate screening machine is equipped with up to five sieving decks and a sieving area of up to 20 m² per deck as well as three outlets for oversized/multiple products, good and fine parts. The sieving decks are arranged in levels above one another, ensuring multiple screenings of the granulates. After being fed into the machine, the granulate particles are sieved from fine to coarse on the individual sieving decks, thereby attaining a perfect separation of oversized particles.

Depending on the requirements, the problem of electrostatic charge can be tackled in two ways: One is to use antistatic agents that increase the conductivity of the product, and the other is to use ionizers. Inline installation of ionizers into the product feed prevents the product from being charged even before it enters the screening machine, so that the product falls into the screening machine with practically no charge. The way ionizers work is by producing an electric field, using a high voltage generator. Air is split up into positive and negative ions around high voltage emitter points. As the charged product passes through this field, ion exchange takes place removing the charge from the product.

In order to achieve a high throughput of up to 20 tonnes per hour, a ball cleaning system is installed on the sieving fabric below the wire mesh, keeping the mesh permeable during the screening process. The impact of bouncing rubber balls cleans the individual sieving areas, where they are thrown against the wire mesh by the horizontal swinging motion of the machine. The sieving area's modifiable angle of inclination influences the granulate product's rate of flow and retention time on the sieving material. This allows the machine to be adjusted to various screening and product requirements.

Cost-effective use of machines through the principle of modular design and swing drives

Thanks to the modular design of the specialist screening machine, after easily removing the aluminum lid, the standardized pressure-bar locked sieve inserts can be changed with the help of the fixed quick releases, thereby allowing the specialized Freischwinger machine to be transferred to other granulate colors and sizes. Due to the free access to the sieve inserts and their simple disassembly, the product compartment of the machine can be cleaned quickly without leaving any residues. In addition to this, internal components can be easily accessed and replaced. The quick release latches seal the lid, making the sieve basket dust-proof, preventing dust particles from escaping during the screening process.

The Freischwinger granulate screening machine is equipped with a specially developed swing drive for a resource-saving operation. The screening machine is driven by an electromotor which transfers its power via a V-belt onto a flywheel equipped with an actuating rod, which in turn is exactly adjusted to the mass of the sieve trough. The powertrain (shaft and flywheel) weighs almost as much as the trough including the product. The electric motor drives the mass for around 15 seconds with full nominal power and then keeps running with only 10–20 percent nominal power. In this way, electrical energy is converted into rotational energy, saved in the flywheels and released gradually. Thanks to the mass-balancing drive system, the machine does not require much energy (7.5kW at a throughput of 20 t/h). In contrast to motor-driven screening machines lubricated by oil, swing drive motors are subject to less wear and tear which results in less material being needed as well as far less maintenance work being required.

Various delivery options and application areas

Engelmann's granulate screening machines come in 110 different sizes and emphasize the versatility of the specialist screening machines. The machines can be easily adjusted to the customer's individual processes and granulate specifications, as two current practical examples illustrate. A leading chemical group is using the JEL Freischwinger granulate screening machine to sieve out polycarbonate pellets on one of its production sites near Shanghai, where the removal of dust as well as the cooling of pellets is also taking place. The air flow against the screening direction carries the dust particles to the downstream filter systems, cooling the polycarbonate pellets from 60 °C down to 40 °C. In the second case, a leading manufacturer of specialist chemicals uses a free-swinging granulate screening machine for the removal of dust and separation of oversized particles in PMMA polymethylmethacrylate. Here the removal of dust and separation of overlength particles takes place on a screening deck. In both projects, the customers took over the assembly and set-up of the granulate screening machines where neither special tools nor special settings are required.

Overview of properties JEL Freischwinger granulate screening machine

  • Versatile for use with various granulate products, pellets
  • High selectivity (up to 100% selectivity between good products and overlengths or multiple particles)
  • Dust-proof sieving
  • Low energy usage through swing drive system
  • Quick retrofitting to other granulate colors through easy sieve changes
  • Operating smoothness < 74 dB(A)
  • Low operating costs
  • Low energy consumption
  • Low maintenance and spare part requirements