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Automated Premix Manufacturing

Plastics are mixed with fillers and additives specific to each application for limitless variety. This variability leads to increasing need for flexible automation in manufacturing premixes in order to achieve the properties of the compounds to be produced.

Coperion combines every step, from ingredient delivery to supplying premixes via the extruder, within the concept of Multi-Ingredient Handling (MIH). Along with automated and manual feeding, mixing and in-house transport are also included.

As the name states, MIH systems handle a variety of components. With the growing number and increasing variety of products used, the scope of the feeding stations in conventional feeding systems increases significantly, since each component requires its own feeder. To counteract this increase, the MIH concept relies upon interchangeable bulk containers (IBCs) that are only used when the product they contain is required for the current recipe. Unneeded receptacles can remain stored outside of the feeding area, reducing the space required in the active feeding area as well as the number of feeders needed for the required components.

The transport containers can be used to some extent directly at the various feeding locations (IBC, big bag) or they are emptied in equipment designed for that purpose into standardized containers (sacks, octabins). Storage container exchange, as well as transport of premix IBCs, represent critical points for the level of overall system automation. One approach to automated transport is classical conveying technology using floor-level conveying means, such as roller or chain conveyors, that are primarily suited for high transport throughput. In contrast, driverless transport systems (DTS) made up of various transport vehicles, depending upon the scenario and task at hand, are a convincing solution.

Coperion ADD-A-LOT
ADD-A-LOT is a modular system for fully automated gravimetric feeding.

Automatic Dosing
Gravimetric dosing forms the core of an MIH system; it can work automatically or manually. The system controls transport of centrally stored components in both directions as well as precise dosing and mixing of additives and supplements in even minute quantities. Use of mobile containers provides additional flexibility. Thanks to its modular construction, the ADD-A-LOT (AAL) system can be expanded and modified as desired to meet specific customer needs; depending upon the design, up to 8 storage containers in the AAL can be assigned to one reception bin. The discharge station connects storage and reception bins and is used for both discharge and feeding bulk material using a feeder screw.

With the bulk material-specific selection of screw geometry and a pneumatic butterfly valve or a slide valve is provided an additional barrier at the outlet, continuously optimal feeding results can be guaranteed. Depending upon the product, standard discharge aids are planned for the AAL to prevent bridging and enable complete container unloading. The most commonly used discharge aid is the pneumatic knocker; vibrator use is an option as well. During weighing, the system differentiates between additive (weighing the premix container) and subtractive (weighing the storage container) weight recording.

Manual Dosing
If one component is used less often and demands placed upon the feeding process are higher due to product properties and feeding precision, oftentimes manufacturers will fall back on manual feeding, whereby the operator is supported on the machine by a control system in the form of a guidance system which enables reliable recipe traceability and a simultaneous reduction of error batches.

Depending upon requirements, Coperion offers the option of static (stationary storage containers) or dynamic (container boxes in a shelf warehouse) storage. Dynamic storage’s advantages include very good use of space and minimal operator effort as any component can be withdrawn at the same spot. Moreover, differing quantities of individual products can be kept in chaotic storage. On the other hand, low investment costs and safe handling of hazardous products are strong arguments for static solutions.

Inline Mixing
Mixing is an integral part of an MIH system that should not be neglected. Mixing, like the entire feeding process, is usually carried out offline, decoupled from the actual compounding. The MIX-A-LOT (MXL) represents an alternative in this case, as batchwise mixing can be carried out inline and thus in direct connection to the extruder. With the MXL, three tasks can be accomplished with one machine. First, the mixing container acts as a polymer separator following pneumatic transport. The container’s operational and design specifications enable safe operation in an overpressure as well as underpressure range up to -0.5 bar (g). The second task is the precise weighing in of each recipe component.

Coperion MIX-A-LOT
The MIX-A-LOT enables batchwise mixing inline as well and this in direct connection to the extruder.

Each individual raw material is first added in the coarse stream until the weight detection reaches the variably adjustable switchover point, when the system changes to fine stream. A preset weight value initiates closing of the devices in the product-conveying pipeline while concurrently opening the suction valve to convey the small quantity of remaining product into the mixing container. The control system identifies this residual amount and corrects the preset value corresponding to the actual weight achieved for the next charging cycle.

In the MXL’s final task, polymers are homogeneously mixed in batch operation, whereby the machine demonstrates a unique selling point: The integrated mixing rotor achieves outstanding results even at mixing times between 90 and 180 seconds. The low circumferential velocities and optimal design of the gap between the mixing rotor and housing prevents particle destruction or product heating to the greatest extent possible, making product handling very gentle.

The premixed polymer batch is then finally completely discharged to the high-accuracy loss-in-weight feeder located directly below. While the highly accurate continuous feeding is running in the extruder, the MXL is already being filled with the next polymer batch and is mixing. The pneumatic conveying systems, as well as the weighing and mixing tasks, are designed such that a seamless transition from batch to continuous operation is ensured.

This article was also published in CAV 10/2020.

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