The melt pump (or gear pump) is known as an accurate metering device that eliminates surging of the extruder screw, reduces back pressure and increases output of the complete line (up to 20%). It is used in a wide range of applications from PET sheet and rubber extrusion to recycling and compounding. The Swiss company Eprotec Extrusion is a leading manufacturer of gear pumps for extrusion with more than 15 years of experience. Here the company addresses the eight most frequently asked gear pump questions.
- Are there any different melt pump designs used in extrusion?
In principle, all extruder melt pumps have the same design: two externally toothed gear wheels mate with each other when they rotate (see also ‘The working principle of the melt-x gear pump‘). There are differences when it comes to the gear design itself: helical, spur and herringbone are the most common types. Eprotec uses the helical type as this design is suitable for most applications and ensures a continuous and smooth process. In addition, the helical gear design features less friction and has better self cleaning characteristics.
There are also special executions of gear pumps for applications where frequent colour changes are required, for example in TPE compounding. Here the gear pump has a so-called “bleeding system” for drainage and the specific discharge of melt which is used for the lubrication of the bearings.
- How to choose the right melt pump?
The most important criteria to chose the right melt pump are:
– Throughput (kg/hr)
– Application (sheet, compounding, recycling, etc.)
– Type of polymer (PET, PE, etc.)
– Filler content (kind of filler and percentage)
– Pressure range
- How melt pumps are lubricated?
Basically all gear pumps used in the extrusion process are lubricated by the polymer. Therefore they are called “melt pumps”. Some time ago external geared pumps for abrasive materials were introduced to the market. However, they have not been able to establish themselves mainly due to the fact that only comparable low pressures could be realized.
- How melt pumps are controlled?
Since more than 35 years gear pumps with associated control system are state-of-the-art. The most common control mode is P1-inlet pressure control in auto-loop. This means that the pump inlet pressure (P1) is measured by a pressure sensor. The controller evaluates it with the set pressure value and regulates the speed of the extruder screw (sheet extrusion) or the speed of the gear pump (compounding).
- Can a gear pump be used with very high filler content?
The use of a gear pump in connection with high filler content is a demanding task. Usually the high-filler content is not only highly abrasive but also increases the viscosity of the material which in turn increases pressure and power consumption of the drive unit. Here special focus must be given to coatings in order to protect the tool steel against wear and corrosion. The melt pumps of Eprotec have a special PVD coating and can process a maximum filler content of up to 83% CaCO3, for instance. Also compounds with high amount of glass fibre (≤40%), carbon black CB, TiO2 and WPC (Wood Plastic Composites) can be processed.
- Are there any materials which are NOT suitable to be processed with a melt pump?
For example, PLA is extremely corrosive, and PVC, in addition, also heat-sensitive. But both materials can be processed with Eprotec melt pumps. In general, if corrosive, heat-sensitive or easily degradable materials are intended to be processed, detailed clarification and communication with the gear pump manufacturer is required. Beside thermoplastic materials, gear pumps are also commonly used with rubber (NBR, SBR) and TPE’s, SEBS and SBS.
- Is the gear pump suitable for all extrusion processes?
From the technical point of view there is basically no extrusion process where the melt pump cannot be employed. It is commonly used in sheet extrusion (PET, PP, PE, PS, HIPS), compounding, pelletizing (strand and underwater), coating, pipe, profile, blown film and cast film extrusion. Melt pumps can also be found in the production of wires, cables, textiles (fibers and nonwovens), EVA, hot melt adhesives and PSA.
- How to protect the gear pump against over pressure?
Each gear pump has a stated, maximum operating pressure. If this pressure is exceeded serious damage can occur. A typical scenario is high discharge pressure due to a ‘cold start’, meaning that down stream adapters, screen changers or die heads are still too cold, or that solid (unmelted) material blocks the way.
To protect the pump from over-pressure either rupture discs and/or pressure transducers should be located directly before and after the melt pump. The pressure transducers are typically connected to a control system with dual alarm function (warning and shut down). Eprotec integrates pressure sensor threads in the housing of the gear-pump as standard. Therefore no additional holes in the adapters prior or after the melt-pump are necessary.
Eprotec Extrusion is specialised in the manufacturing of high-precision melt pumps for throughputs from 30 to 8,000 kg/h. The company places particular attention to service and retrofitting of melt pumps, and also offers spare parts for other brands.