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Zeroing in on Quality (PMMA)

PMMA polymethyl methacrylate, a high quality raw material, is processed for the production of an extremely wide range of end products which need to fulfill the highest quality standards. Whether for use in dental or sanitary ceramics technology, a precise product consistency is indispensable in almost every area of application. Agglomerates, excess lengths or defective particles thus have no place in the production of the polymer. In order to ensure the highest possible degree of purity in this free-flowing raw material, any possible defective particles which do not meet the desired particle specifications are screened out before it is filled into sale containers. The integration of the required screening technology into the existing system, as well as the screening properties of the product, must be taken into account in the development of the machine solution, as demonstrated by an application example from the screening technology experts, J. Engelsmann AG.

A globally active supplier to the plastics industry has production facilities in 25 countries around the world and produces a variety of PMMA polymers, which distinguish themselves with consistent quality within strict specification limits and outstanding pourability, for the demanding dental and sanitary ceramics industries.

Partially significant fluctuations in the particle spectrum were detected during internal quality controls on the finished products in the South Hesse factory in Darmstadt. In order to reduce the proportion of defective particles in the finished batches as much as possible and ensure a consistent product quality, the company decided to install a protective screening and grading system before filling of the polymer product into the sale containers. Oversize particle separation with a mesh width of 200 µm should ensure that unwanted agglomerates and defective particles outside the specified particle spectrum are screened out. The correct size particles, on the other hand, should be fed directly into the filling process. However, the properties of plastics in primary form make the screening job harder and make the most stringent demands on the machinery. In addition, the machine solution needed to fit well into the existing production system and be adjusted to the actual conditions on site. A drying plate installed upstream in the screening process and the installation location in a corner of the construction hall thus takes care of the restricted space.

Series of joint tests creates clarity and trust

Thanks to extensive experience with complex screening tasks, the people at Engelsmann know all about the problems in the screening of plastics. A wide variety of types of screening machines from the Engelsmann portfolio were tested against the present requirements in extensive tests with multi-day test series using the original product. The tests at the Engelsmann technical center were performed in the client's presence so that they were involved in the decision-making process when selecting the appropriate machine type. Here, the JEL long-stroke screening machine achieved the best screening results which were analyzed by the client in their in-house laboratory. The testing at the technical center thus not only provided important insights into the right machine solution, it also provided everyone involved with the certainty that this would also function perfectly in actual operations later on under the specified conditions

Engelsmann designed the Freischwinger series long-stroke screening machines especially for the screening of free-flowing bulk solids. Thanks to its high throughput of up to 250 t/h and the particularly gentle screening process with a selective screening level of up to 99%, the Freischwinger models are used in all areas of production. For the special screening application, the basic model of the free-swinging long stroke screening machine was equipped with a screening deck and two sieve inserts with a 200 µm mesh and a screening surface of 0.75 m² as well as two outlets for course product with a diameter of 100 mm and the product and fine product with a diameter of 150 mm.

Static electricity makes the screening task harder

The phenomenon of static electricity, which is particularly common with plastic in powder or granulate form, causes problems in screening time and again. If, for example, dust should be separated from granulate particles, in most cases it remains stuck to the particles owing to the static electricity. The problem of static electricity can be solved in two ways, depending on the requirements: firstly, through the use of anti-static additives or, as in this case, through the use of ionizers. Arranged in the product supply for the machine, the ionizers remove the static electricity from the product to a large extent before the screening process so that the screening material enters the screening machine with almost no charge. Using a high-voltage generator, the ionizers generate an electrical field which separates the air around the ion spray spikes into positive and negative ions. As soon as the charged granulate product passes into the field, an ion exchange occurs and the charge is removed from the granulate. Owing to the static electricity in the plastic product, the machine is also continuously earthed. In addition, the long-stroke screener is designed without the electric motor for operation in ATEX zone 20 (interior) and zone 22 (exterior).

Constant screen cleaning with two independent systems

So that the screening mesh doesn't get clogged up with near-size particles (clogging material) during the screening process, the special screening machine was equipped with two screen cleaning systems to keep the screening mesh open for as long as possible in order to achieve the required throughput of up to 300 kg per hour. A ball dedusting system, which keeps the mesh clear during the screening process, is installed beneath the sieve insert. An additionally installed ultrasound cleaning system has an ATEX compliant ultrasound screen generator with temperature resistance of up to 80°C as well as two ultrasound converters with a temperature resistance of up to 135°C. It can be switched on as pulses or continuously in order to stick to the scheduled production time. Equipped with an adjustable screen angle, the flow speed for the product and thus its retention time on the screening mesh can be influenced with a modified angle of inclination of between 1°-3° for the screening deck.

Since there are two different particle specifications being screened on the screening machine, the sieve inserts with the different mesh sizes must be changed after each screening process. In order to keep the resulting downtime as short as possible, the screening machine was fitted with a hinged cover made from stainless steel with quick release closures and BFM sockets upon customer request, allowing for quick access to the sieve inserts. With the available quick-change model, the standardized sieve inserts, held in place by pressure bars, can be changed quickly and easily with just a few movements once the cover is open. The free access to the sieve inserts and their easy removal makes it possible to clean the product area within the machine with no residue in a very short time.