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Achieving 100% with momentum

One major reason for the success of plastics is that their properties can be varied in manifold ways through the selection of the original material and the manufacturing technique, which predestines them for use in manifold areas of application. PMMA, for example, is a synthetic plastic that is often used for acrylic glass especially in the field of optics because of its transparency. As a high-quality raw material, it is processed in the form of pellets or granules for the manufacture of diverse plastics products and must be made available in flawless quality for these applications. This is why the separation of dust, agglomerates and overlong pieces, which can adhere to the granules, play a more significant role. In this, the capability of the product to be screened and the integration of it into existing production systems and production processes pose the highest demand on screening technology, which then requires the use of special machine solutions.

The first cast slabs of acrylic glass were produced in Germany as early as the 1930s. Today it is used in large quantities for a broad range of applications such as contact lenses, lenses for eyeglasses or in industry for vitrification in construction or in horticulture such as for the construction of greenhouses. One characteristic of PMMA is that it is less prone to scratches than other thermoplastics. It transmits light better than glass, is easy to dye and is particularly resistant to weather and aging.

Electrostatic charge makes the screening process more difficult

Before PMMA is ready for delivery for the processing industry, dust particles, agglomerates and overlong pieces adhering to the cylindrical granules are sifted off in a special screening process in order to attain the desired particle size and/or quality of the granules. The properties of plastics in their primary form, however, make the screening process more difficult and make severe demands on the machines in use. The phenomenon of electrostatic charge, which especially occurs in plastics in the form of powder or granules, involves problems during screening time and time again. If, for example, dust particles are separated from the granular particles, these usually remain stuck to the granular particles due to electrostatic charge. Furthermore, screening action that is especially gentle on the product with minimum abrasion to the granule particles has to be guaranteed in order to ensure high quality of the goods, i.e. the product to be sold.

It was a leading PMMA manufacturer that supplies the processing industry with acrylic glass that was faced with this problem. Until that point, it had used vibration machines for separating out overlong pieces, multiples (pieces of granule that are attached or stuck together) and dust in every production line, i.e. the various types of PMMA manufacture. Disadvantages with using the vibration screening machines which came up over the years, however, were the low degree of filtration and the insufficient throughput capacity when compared to current models. Even the power consumption of the machines used was higher than that of the conventional screening machines. Moreover, problems with dust particles adhering to the product because of the electrostatic charge of the granule were a problem time and time again. Because of this, the decision was made to replace the vibration screening machine of one production line with a new machine. The new screening technology needed to have the highest possible filtration rate, i.e. selectivity and guarantee a screening action that is gentle on the product in order to ensure that the sales goods have the highest quality possible. Furthermore, the problem of electrostatic charge had to be solved reliably, the consumption of power used until that point had to be reduced significantly and the new machine had to be integrated with the existing in-flows and out-flows for product on the current system on site as well.

Thanks to their extensive experience with complex screening tasks, the screening technology specialists at J. Engelsmann AG in Ludwigshafen, Germany, already knew about the problem of screening plastics. After their first meeting for determining the task and the exact requirements, thorough series of tests lasting several days were conducted using the original product and various screening machine types from the Engelsmann range were tested according to the existing requirements. Some of the experiments in the Engelsmann technology center were conducted with the customer present to ensure their involvement in the decision-making process when selecting the right type of machine. In these tests, the JEL cantilever long-stroke screening machine provided the best screening results, which were analyzed by the customer in their own laboratory. This means that the experiments in the technical center not only delivered important insights for the right machine solution, they also provided the assurance for all involved parties that they would work impeccably under the prevailing conditions later in live production.

100% filtration degree with screening action that is gentle on the product

The long-stroke screening machine in the cantilever series was specially designed by Engelsmann for screening pourable bulk solids. Because of its high throughput capacity of up to 250 metric t/h and its especially gentle screening action with a selective filtration degree, the cantilever model is used in all areas of production. For the special case of screening in this application, the basic model of the cantilever long-stroke screening machine in its model for granules was selected as the appropriate machine solution. The JEL Freischwinger for granules was designed by the Engelsmann builders to separate overlong particles of all conventional types of plastic granules reliably. It is for this purpose that the cantilever screening machine for granules was equipped with three decks of sieves arranged in a row to screen the granules multiple times in this special case. After the product is fed into the machine, the granules are screened from fine to coarse on the individual sieve decks thereby attaining a filtration degree of one hundred percent. The first two sieves are covered with a fine wire screen with 1.5 mm mesh width which serves to de-dust the PMMA granules (separating the undersized particles). The third sieve contains a round hole perforated sheet and takes care of sifting off overlong particles and multiples more than 5.5 mm in size (separating the oversized particles). In order to ensure the best possible degree of sifting off, a distributer sheet with an overflow edge is installed under the inlet connection on the sieve which serves to distribute the material to be screened optimally over the sieve. In order to prevent the screen of the sieve from becoming clogged with near-mesh-sized particles, a cleaning system using balls was installed underneath every sieve in order to keep the screen permeable thereby attaining the required throughput capacity of 2.0 t per hour. With this system, the surface of each sieve is cleaned by the action of firm balls being tossed against the mesh with the horizontal vibrations of the machine. In order to keep the sieve-cleaning process as gentle on the product as possible and to prevent contaminating the PMMA with material abrasion, silicon balls were used. This is also why there are no seals or any other plastics in the interior of the sieve trough. Moreover, all parts which come into contact with the product are made of high-quality stainless steel V4A (1.4571). The in-flows and out-flows were arranged exactly the same as on the vibration screening machines already in use in order to avoid having to adapt the in-flows and out-flows for the product on site. This means the in-flow of the cantilever screening machine is oval so that it fits the out-flow of the upstream granulator. This cuts up (granulates) the cylinders of granules and dull blades or tolerances can result in particles that are too short or too long. These enter the screening machine directly before arriving at the out-flow of the granulator. This screening machine also has three out-flows for the fine material (undersized particles), standard material (sold product) and coarse material (oversized and overlong particles). After the PMMA is sifted off, the material is fed into the packaging system and filled into octabins, big bags or barrels ready to be sold. For monitoring or inspecting the screening process, the long-stroke screening machine has a viewing window as well as rotation monitoring and feed monitoring which are displayed using the process control system.

In this case, the problem with electrostatic charge was solved with the use of ionizers. Placed in the product in-flow of the machine, the ionizers remove the majority of the product’s electrostatic charge before the screening process so that the granule falls into the screening machine in a virtually neutral state. The ionizers generate an electric field using a high-voltage generator. This field splits the air around the ionization spray spikes into positive and negative ions. As soon as the charged granule product passes through the field, the ions are exchanged and it is then no longer charged. Because of the electrostatic charge of the plastic product, the machine was also grounded throughout.

Efficient use of machinery thanks to modular concept and momentum drive

Thanks to the modular construction of the granule screening machine, after removing the machine lid and with just a few manual steps, the standardized sieves held in place with pressure plates can be changed individually using the attached quick release thereby refitting the specialized cantilever machine to different granule colors and sizes. Because the sieves are easy to access and dismount, it is easy to clean the product space of the machine spotlessly. Furthermore, parts in the interior can be accessed and replaced easily. The quick release clamps seal the sieve trough with lids so that it is dust-proof thereby preventing dust particles from escaping during the screening process.

To operate the cantilever granule screening machine in a manner that is gentle on resources, it is equipped with a specially developed momentum drive. Driven by an electric motor which delivers its power using a V-belt on an oscillating weight equipped with a thrust rod, it is tuned exactly to the mass of the sieve trough. This means the drive train (shaft and disc flywheel) weigh about as much as the trough and the product together. The electric motor propels the mass for approx. 15 seconds with full nominal current and then keeps it in motion using momentum with only 10-20 percent of the power of the nominal current. In this process, the electrical energy is transformed into kinetic energy of rotation, is saved in the disk flywheel and then dispersed gradually. Thanks to its drive based on the principle of balancing mass, the machine requires only a small amount of energy. In contrast to screening machines with motor control gear lubricated with oil, this momentum drive is only subject to a small degree of wear and tear, which involves less material usage and less maintenance work.

After a four-month period of planning and production, it was possible to deliver the special construction cantilever screening machine two weeks earlier than planned and have it assembled and put into operation by Engelsmann technicians. The investment amount required for acquiring the new screening machine entirely fulfilled the expectations of the PMMA manufacturer. In addition to sifting off granules that are out of place in a manner that is more gentle on products and more selective, the quality of the ready-to-sell product has also been optimized and the power requirements have been reduced by more than 30%. Because of these results, the customer has decided to replace all of the vibration screening machines the company uses with the long-stroke model bit by bit.