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Top left the melting module is shown, while top right it can be seen a cascade combination of a melting module followed by a mixing module. |
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| On the right side is visible the schematic section of the Melting Unit. We observe the plastic solid particles falling down into SCP (Shearing Channel Pump) feed port. In SCP a grooved rotor and a smooth rotor cooperate to in-take the particulate resin and press against the hot rotor of the processor, which rotates clockwise. The static channel encased into the bush is tapered thus promoting compaction and melting. As soon as enters in forced contact with the hot rotor wall, the resin melts quickly while dragged toward the exit, over the pumping section.
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The melting module is able to receive any type of solid provided in particulate form from powder up to about 10 mm size, thanks to SCP. SCP plays more functions at the same time:
a) Presses the material against the main hot rotor. The hot rotor rubs and melts the material into a thin film which is removed in continuous by drag.
b) Provides instantaneous increase of the feedstock density. Therefore the melting rate is relatively insensitive to the bulk density at feeding.
c) Due to friction, heats up the material prior melting. By this way the thermal gap occurring between room and melting temperature reduces and melting occurs in a shorter time.
d) Prevents the solid plastic particles to rotate in the melting chamber. Sofar the mechanical energy discharged on the material is maximized, providing for very effective melting. |
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Nexxus Melting is also ideal for melting recycled plastic. To face hard contaminants occurrence, a special cleaning device to catch hard contaminants up to 3-4 mm is working in the bottom of the melting channel. Such device can be operated either manually or automatically. |
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The rotor speed may be adjusted from 0 to 4 m/s (for a 200 mm diam. it corresponds to 0-380 rpm). Depending on the rotor speed, the channel width, the material physical condition and the polymer nature throughputs in the range of 5 and >1000 kg/h can be achieved.
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The specific energy consumption (SEC) approaches to the specific enthalpy of a given polymer under given processing conditions. |
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A top melt quality, i.e. without gels or unmelted particles as visible by naked eye, (e.g. less than 1 micron in a thin film) is a close function of the applied counter-pressure at the discharge. When the discharge pressure is zero then the throughput is maximum but rarely the melt quality will be acceptable. Viceversa when the discharge pressure is increased then the melt quality raises up to the desired level. The simplest way to adjust the throughput vs melt quality in Nexxus is to design a thicker discharge gap and regulate the counter-pressure at the discharge port. For example, If the most important issue is the throughput, then a rather large gap (2-4mm) can be designed while the discharge pressure is reduced to a minimum. By the other hand if the melt quality is the issue, then the gap will be considerably thinner (0.2-0.5 mm) and the discharge pressure will be set rather high. The most simple way to modulate the discharge pressure is a gear pump mounted downstream the discharge port. For this reason the gear pump is offered like a standard equipment in Nexxus melting.
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The gear pump will be used as feed rate controller.
The working principle of the gear pump in combination with the rotor speed will be: |
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- A volumetric dosing unit maintains filled the SCP hopper (level sensor)
- SCP rotates at constant speed to feed and pre-heat the feedstock
- A closed loop between pressure detected upstream of the gear pump and the rotor motor is arranged in the PLC to keep the pressure value within a preset window.
- By fine rotor speed adjustments, within preset limits, the system does provide a uniform melt conveying rate.
- The program keeps constant the melt pressure upstream of the gear pump, within a pre-set window, continuously adjusting the rotor speed.
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The dosing system assisted by a gear pump not only is more reliable than a standard gravimetric dosing unit but is more robust as it makes possible to feed any type of particulate material including poorly flowing feedstock which faces serious troubles in standard dosing equipment. |
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Both the cylindrical bush as well as the rotor are properly heated. The rotor temperature is also controlled by a thermoregulation unit. The thermal control of the rotor is very accurate and makes possible to achieve a uniform temperature distribution over the rotor surface. The accurate rotor temperature control makes also possible to set and keep constant the radial temperature gradients, between rotor and bush, which are fundamental to optimize and keep constant the melt conveying rate. |
