Gravimetric Blenders for Plastics
series WB

They are modular systems constituted by more dosing zones, each one of them dedicated to respective ingredients in the recipe.

They are used to feed the extrusion lines in the plastic industry.

The products that can be dosed with the gravimetric blenders WB are granules, pellets or lentils with dimensions that vary from 1mm up to 6-7 mm. These products are constituted most of all by thermoplastic polymers. Their prerogative is to be very free flowing and so easily dosable, due to their features.

The gravimetric blenders WB are not suitable to dose powder products, flakes or fiber, that usually present fluency issues. For these kind of products are suggested the powder dosers CH, CHW, CHY, BH, BHW,BHY, DFV, DFW and DFY or dosers for flakes VF, VFW and VFY.

The gravimetric blenders WB can be installed directly on the extrusion line and off line as well. In this specific case they can be dimensioned also to serve more extrusion lines.

In case the blender WB is installed off line it is necessary to provide a proper conveying system to transfer the dosed products to the extrusion lines.

The most used conveying systems are pneumatic type. During the transport the products may separate due to their different density and geometries. To avoid these problems, it is better to provide a mixing hopper ( MXV ), to be installed directly at the extruder inlet , and suitable to receive the conveyed products. The mixing hopper MXV feeds the extruder with an even blend and with the correct proportion of the ingredients. For the pneumatic conveyors it is possible to use vacuum loaders series ATS, AJS, ATSY and AJSY with their blowers and vacuum pump series UATC, UATV and UAVL.

The mixing hopper MXV has level sensors that allow to manage the transfer cycles automatically. The most used sensors are capacitive, rotary levels and vibrating level switches to choose according to the product characteristics.

The gravimetric blenders WB are constituted by a dosing module installed above a box inside which a weighing hopper and a mixing device are installed.

The dosing module consists in a group of electro-pneumatically controlled slide gates installed at the bottom of as many storage hoppers.

The dosing module has a geometry which allows to place the automatic gates nearby and opposed to each other in a way that the products fall at the centre of the weigh hopper below. The storage hoppers are independent of eachother, even of different capacities, installed next to eachother in a compact way.

At the controlled opening of every dosing gate each product falls by gravity in the weighing hopper below. The products are dosed by weight one at a time according to the sequence expected by the recipe.

The weighing hopper has two side bars that slide on two rails hung on load cells. In this way it is possible to remove the weighing hopper in a simple and quick way to accomplish the usual cleaning and maintenance. The load cells are off-center or flexion type, made in aluminium or in stainless steel with protection grade up to IP67, available also with Atex certification when requested.

The weigh hopper has a discharge outlet provided with a flap valve electro-pneumatically operated.

The gravimetric blenders WB have an horizontal mixer electrically driven and installed just under the weigh hopper. The mixer is constituted by a rotating shaft with suitably shaped blades connected to the transmission unit. On the opposite side the shaft is supported by an end support with ball bearing.

The transmission unit which operates the mixer is constituted by a worm type gearbox and a three-phase electric motor.

The mixing chamber is in the shape of two opposing semi- cones which allows to properly homogenize products with different density and shapes, even with percentages much different from each other. The particular geometry of the mixing chamber and the mixer make converge the products in the centre where the discharge outlet is placed. Under the mixing chamber, in correspondence of the discharge outlet, there is a flanged socket by which the gravimetric blender WB can be installed on feeding inlet of the extruder.

The logic functioning of the gravimetric blenders WB is based on consecutive cycles constituted by a dosing phase in the weighing hopper and a mixing phase. The mixing of the products dosed in the previous cycle lasts for a programmed time during which the following dosage is done. The dosed products are unloaded from the weighing hopper only after the mixing phase is completed . In this way it is possible to feed the extruder constantly with a cyclical programmed sequence of batch dosages. The gravimetric blenders WB are dimensioned to work at higher capacities than the extruders ones on which they are installed; so the mixing chamber is never empty and contains always a minimum volume of mixed product. In the mixing chamber it is installed a level sensor which enables the discharge of the weighed products only when the necessary volume to receive them is again available. In this way the gravimetric blender WB works practically like a continuous dosing system.

Between the flanged socket of the gravimetric blender WB and the feeding inlet of the extruder it is possible to install a slide gate which allows to hold the product inside the mixing chamber for a programmed interval time. This option is particularly suggested to avoid the products to enter directly into the extruder without being properly blended during the first cycle. In the following cycles it is the product itself present in the mixing chamber which acts as a barrier.

The dimensions of the gravimetric blenders WB are very small, even for high flow rates.

The entire family of gravimetric blenders WB consists in 5 standard sizes that allow to cover a capacity range from a minimum of 180 Kg/h to a maximum of 2.000Kg/h.

According to the sizes we have different dimension dosing gates and storage hoppers and weigh hoppers with different volumes.

For the components in the recipe whose dosing percentages are very low, under 1-2% of the total, it is possible to provide screw dosing devices as an alternative to the slide gates. The small dosing screws can ensure very low ranges and very high accuracy.

The most common arrangement provides 4 dosing zones but versions with 2 zones up to a maximum of 6 zones are also available.

The gravimetric blenders WB are easy to maintain because they are modular and made of parts easy to disassemble.

The storage hoppers are normally reversed pyramid shaped. They have removable lids with an inlet for the automatic replenishment. In this last case vacuum loaders are frequently used. They are installed directly on the lids of the hoppers with flanged coupling.

The storage hoppers have wide transparent polycarbonate sight glasses and eventual level sensors to detect the presence of product and allow the automatic management of replenishing cycles. The most used sensors are capacitive type.

On the front side of the gravimetric blenders WB there is a wide transparent door which allows to have access to the inner parts of the machine for cleaning and maintenance. There is a safety micro-switch which inhibits the devices to operate when the door is open.

The most used construction materials are stainless steel Aisi 304 and 316L.

The surfaces in contact with the product can be simply pickled or mirror-polished if required.

The outer surfaces are satined 180 mesh. It is possible to have them also glass bead blasted or electro-polished on request.

The gaskets are made in dust-tight rubber , such as SBR or NBR. They are available also with FDA certification on request.

Electrical control panels can also be supplied including software specifically designed accordingly to the requested functionality.The electric panels and the junction boxes are normally installed directly on the machine.